oilfield review winter 2012

5/25/2018 Oilfield Review Winter 2012

1/60

Winter 2012/2013

Downhole Debris Recovery

Asphaltene Science

Fishing Techniques

Carbon Capture and Storage

Oilfield Review


2/60

13-OR-0001


3/60

Widely recognized in some quarters as a means to reducecarbon dioxide [CO2] emissions to the atmosphere, thepractice of carbon capture, utilization and storage remainslargely unfamiliar to the general public. The utilizationcomponent of the technology is familiar to those who have

worked in enhanced oil recovery (EOR) operations, but theconcept of deep-well injection and storage of dense-phaseCO2is unknown to many regulators, elected officials andthe population at large. In addition, these practices pres-ent many unanswered questions, including those address-ing how such practices affect the environment andpersonal property or whether current understanding of thescience also predicts future CO2storage behavior.

As a consequence, the road to popular acceptance andwidespread use of carbon capture and storage (CCS), inde-pendent of the oil field, requires that demonstration fieldtests be performed over time. It is also imperative thatfindings from those tests be presented honestly and clearlyto the community. Proving the capacity for safe CO2stor-age and containment and the effectiveness of geologic res-ervoirs and seals may be accomplished through thedevelopment of test projects that are scalable to the vol-umes of CO2emitted from commercial power plants. Earlyprojects such as those in Japan, Germany and Australiainjected up to about 100,000 metric tons [110,000 tonUS]of CO2using truck or pipeline delivery.

However, larger demonstration projects require pipelinedelivery of CO2to an injection well at rates of about 0.25 to

1 million metric tons [0.28 to 1.1 million tonUS] or moreper year. The objective of these larger projects is to createa CO2plume in a target formation that may be monitoredeffectively through well logging, chemical sampling, pres-sure and temperature monitoring, geophysical surveyingand other means.

In the US, the US Environmental Protection Agencyunder the Underground Injection Control program, is pro-mulgating regulations and associated guidance for the newClass VI operational classification. The US Department ofEnergy has had a program of multiple field tests and dem-onstrations in place since 2003 under the Regional Carbon

Sequestration Partnership program, with several demon-stration projects currently underway, including the 1 mil-lion metric ton demonstration at Decatur, Illinois, USA,described in this issue (see CO2SequestrationOneResponse to Emissions,page 36).

Regional geologic screening followed by careful sitecharacterization and selection constitute the foundation ofsafe and effective CO2storage. The objectives in this explo-ration process are to find a porous and permeable reservoirand a competent reservoir seal. Geophysical tools provide

The Future of CCS

the initial look at a subsurface volume, while drilling,logging and coring confirm expectations of a suitablereservoir-seal system.

Project operators and technical staff members havegained considerable knowledge from the demonstration

projects underway today and have shared much of thisknowledge through conferences and peer reviews.However, as regulators ask future-oriented questions aboutreservoir simulation, groundwater flow modeling and otherpredictive approaches, the industry will need a compre-hensive knowledge-sharing framework that will answersuch questions and allow larger, commercial-scale projectsto proceed.

The public also has a role in the future of CCS, but insome instances, project developers have not gained thecommunitys confidence. There is an ongoing need for openand complete communication with the general public in away that understandably conveys the technical basis forCCS. This consideration should not be overlooked duringproject development and operation.

Although the portfolio of projects is not as diversetoday as may have been envisioned five years ago, thenext generation of projectsassociated with hydrocar-bon production, power generation and natural gas pro-cessingis being developed now. Implementation of CCSin saline reservoirs is still in its infancy, but the resultsto date are encouraging.

Oil and natural gas production industry technologies and

the experience of its personnel are essential for the suc-cess of CO2injection and storage projects. Successful dem-onstration projects will apply and even advance oilfieldindustry technologies, and knowledge acquired duringproject development and ongoing monitoring must betransparent to the public. This transparency will gain theconfidence of stakeholders outside the industry and is keyto ensuring the popular understanding that CCS is a viabletool for climate change mitigation.

Robert J. Finley

Director, Advanced Energy Technology Initiative

Illinois State Geological Survey

Champaign, Illinois, USA

Robert J. Finley is a Director of the Advanced Energy Technology Initiative with

the Illinois State Geological Survey in Champaign, Illinois, USA. He has worked

in reservoir development for unrecovered oil and natural gas, with coalbed

methane and tight gas sandstone reservoir development in Texas and the Rocky

Mountains in the US and in reservoir development for carbon sequestration in

the Illinois basin. Robert earned a BS degree from City University of New York,

an MS degree from Syracuse University, New York, USA, and a PhD degree in

geology from the University of South Carolina, Columbia, USA.


4/60

www.slb.com/oilfieldreview

Schlumberger

Oilfield Review

1 The Future of CCS

Editorial contributed by Robert J. Finley, Director, Advanced Energy Technology Initiative,

Illinois State Geological Survey

4 Specialized Tools for Wellbore Debris Recovery

Wellbore completion operations often generate downhole

debris, including sand, perforating gun residue and metal

particulates. In addition, drillers frequently discover assorted

nuts, bolts, tools and other materials that have been acciden-

tally dropped in the wellbore. Unless these materials areremoved, optimal well productivity may be compromised.

This article describes new tools and techniques for efficient

wellbore debris recovery.

14 Revealing Reservoir Secrets ThroughAsphaltene Science

By combining downhole fluid analysis with advances inasphaltene science, oil companies are gaining a better

understanding of reservoir architecture. Downhole analysis

of asphaltenesthe heaviest components of petroleum

can help geoscientists determine asphaltene concentration

gradients, which in turn, can help operators ascertain the

presence of sealing barriers and assess the communication

and equilibrium of fluids in complex reservoirs. Examples

from the Gulf of Mexico and the Middle East show how com-

panies are using asphaltene gradient techniques to learn

more about reservoir connectivity and fluid distribution.

Executive EditorLisa Stewart

Senior EditorsTony SmithsonMatt VarhaugRick von Flatern

EditorRichard Nolen-Hoeksema

Contributing EditorsDavid AllanGinger OppenheimerRana RottenbergDon Williamson

Design/ProductionHerring DesignMike Messinger

IllustrationChris LockwoodTom McNeffMike MessingerGeorge Stewart

PrintingRR DonnelleyWetmore PlantCurtis Weeks

Oilfield Reviewis published quarterly andprinted in the USA.

Visit www.slb.com/oilfieldreview forelectronic copies of articles in English,Spanish, Chinese and Russian.

2013 Schlumberger. All rights reseReproductions without permission astrictly prohibited.

For a comprehensive dictionary of oiterms, see the Schlumberger OilfieldGlossary at www.glossary.oilfield.slb

About Oilfield ReviewOilfield Review,a Schlumberger journal,communicates technical advances infinding and producing hydrocarbons toemployees, customers and other oilfieldprofessionals. Contributors to articlesinclude industry professionals and expertsfrom around the world; those listed withonly geographic location are employeesof Schlumberger or its affiliates.

On the cover:

Carbon sequestration is one approachto managing greenhouse gas emissions.Here, a derrickman handles drillpipe asa rig captures cores of the Maquoketa

Shale in Illinois, USA. The shale is acontainment barrier within the IllinoisBasinDecatur Project carbon sequestra-tion verification well. During constructionof a verification well in the nearby IllinoisBasin Carbon Capture and Sequestrationproject (small inset), all casing stringsare cemented to the surface. Verificationwells are equipped with monitoringsystems to track carbon dioxide as it isbeing injected underground during carbonsequestration operations.Rig photographs by Daniel Byers forthe Midwest Geological SequestrationConsortium.

2


5/60

Winter 2012/2013

Volume 24

Number 4

ISSN 0923-1730

49 Contributors

51 Coming in Oilfield Review

52 New Books

54 Defining Testing:Well Testing Fundamentals

This is the eighth in a series of introductory articles describing basic concepts of the E&P industry.

56 Annual Index

26 Landing the Big OneThe Art of Fishing

Second only to blowouts, one of the worst situations a

driller may encounter is the loss of equipment downhole.

Fishingthe art of recovering lost, damaged or stuck objects

from the boreholedraws on the experience, imagination

and innovation of the fishing expert. This article describes

tools and strategies developed for dealing with items lost in

the wellbore.

36 CO2SequestrationOne Response to Emissions

One response to concerns that human activity is influencing

climate has been to remove the CO2from emissions created

when carbon-based fuels are burned and sequester it deep

underground. Upstream oil industry experts are uniquely

qualified to manage the selection, construction and monitor-

ing of these complex injection projects.

Hani ElshahawiShell Exploration and ProductionHouston, Texas, USA

Gretchen M. GillisAramco Services CompanyHouston, Texas

Roland HampWoodside Energy Ltd.Perth, Australia

Dilip M. KaleONGC Energy CentreDelhi, India

George King

Apache CorporationHouston, Texas

Advisory Panel

Editorial correspondenceOilfield Review5599 San FelipeHouston, TX 77056United States(1) 713-513-1194Fax: (1) 713-513-2057E-mail: [email protected]

SubscriptionsCustomer subscriptions can be obtainedthrough any Schlumberger sales office.Paid subscriptions are available fromOilfield ReviewServicesPear Tree Cottage, Kelsall RoadAshton Hayes, Chester CH3 8BHUnited KingdomE-mail: [email protected]

Distribution inquiriesTony SmithsonOilfield Review12149 Lakeview Manor Dr.Northport, AL 35475-3850United States(1) 832-886-5217Fax: (1) 281-285-0065E-mail: [email protected]

Oilfield Reviewis pleased to welcom

Hani Elshahawi to its editorial advispanel. Hani is Deepwater TechnologAdvisor at Shell in Houston. Previouhe led FEAST, Shells Fluid Evaluatioand Sampling Technologies Center oExcellence, where he was responsibfor the planning, execution and analof global high-profile formation testand fluid sampling operations. With than 25 years of experience in the oindustry, he has worked in both servand operating companies in more thacountries in Africa, Asia, the Middleand North America and has held varpositions in interpretation, consultinoperations, marketing and productdevelopment. Hani has lectured widin various areas of petrophysics, geoences and petroleum engineering, h

several patents and has written morthan 100 technical papers. He was t20092010 president of the SPWLAand a distinguished lecturer in 2010and 2011. Hani attained a BS degreemechanical engineering and an MSdegree in petroleum engineering froThe University of Texas at Austin.


6/604 Oilfield Review4 Oilfield Review

Specialized Tools forWellbore Debris Recovery

In the late 1700s, Giovanni Battista Venturi, an Italian physicist, described a

reduction in pressure when fluid flows through a restriction. Now, engineers

are using this principle to design specialized wellbore cleaning systems capable

of performing critical debris recovery operations in some of the worlds most

challenging subsurface environments.

Brian Coll

Graeme Laws

M-I SWACO

Aberdeen, Scotland

Julie Jeanpert

Ravenna, Italy

Marco Sportelli

Eni SpA E&P Division

Ravenna, Italy

Charles Svoboda

Mark TrimbleM-I SWACO

Houston, Texas, USA

Oilfield ReviewWinter 2012/2013: 24, no. 4.Copyright 2013 Schlumberger.

For help in preparation of this article, thanks to KennethSimpkins, M-I SWACO, Houston.

FRAC-N-PAC, PURE and QUANTUM are marks ofSchlumberger.

HEAVY DUTY RAZOR BACK CCT, MAGNOSTAR, RIDGE BACKBURR MILL, WELL PATROLLER and WELL SCAVENGER aremarks of M-I SWACO LLC.

Debris removal is a vital step in assuring the suc-cess of drilling or completion operations. Debrisremoval involves the extraction of junk andunwanted materials from a borehole or com-pleted wellbore. Junk typically consists of smallpieces of downhole tools, bit cones, hand tools,

wireline, chain, metal cuttings from milling oper-ations and an array of other debris. Although notgenerally considered junk, sand and other mate-rials used during completion, stimulation andproduction operations often require removalfrom the wellbore prior to production.

Because there are many types of debris, engi-neers have developed a variety of tools and tech-niques to facilitate debris removal from a

wellbore. This article focuses on the postdrillingphase of well construction and issues related toridding the borehole of relatively small fragmentsof debris such as metal cuttings, perforating gundebris, small hardware and sand. The articlebegins with a discussion on the sources of smalldebris and then reviews various techniques avail-able to remove these materials from the wellbore.Case studies demonstrate how operators areapplying these new technologies in a variety of

completion environments to reduce risks, mini-mize downtime and improve well productivity.

Sources of Small Debris

The drill floor is a busy place, providing numer-ous opportunities for small items to inadvertentlyfall into an open hole. In deepwater operations,the surface opening at the riser pipe may have adiameter of 1 m [3 ft], creating opportunities forlarger items to fall to the depths.

Debris is also generated downhole by variouwell operations. Often, drillers must mill hard

ware such as packers, liner tops and equipmenwithin the wellbore (above).1Metal cuttings fromthese operations are among the most commontype of debris found downhole. Circulation odrilling, milling or completion fluid transportmuch of the metal debris to the surface. Howeversome metal cuttings may still be left in the holefrequently in locations that cause problems during the completion or production process.2

1. Milling is the process of using a downhole tool to cut,grind and remove material from equipment or tools in thewellbore. Successful milling operations require selectionof milling tools, fluids and techniques that are compatible

with the fish materials and wellbore conditions.2. Connell P and Haughton DB: Removal of Debris from

Deepwater Wellbores Using Vectored Annulus CleaningSystems Reduces Problems and Saves Rig Time, paperSPE 96440, presented at the SPE Annual TechnicalConference and Exhibition, Dallas, October 912, 2005.

>Typical junk mill. Junk mills are designed to grinany type of material encountered downhole,including bit cones, drillpipe, bridge plugs orother objects. Wear pads provide stabilizationwhile the mill is grinding. Drillers deploy a varietyof grinding faces or tool configurations,depending on the type of material to be milled.

Wear pad

Grindinface


7/60Winter 2012/2013Winter 2012/2013


8/606 Oilfield Review

During well completion, cased wells may beperforated using an array of specialized explo-sive charges mounted on perforating guns.

When perforating guns are fired, shaped chargespierce the casing, cement sheath and formation.

A shot density of 33 shots/m [10 shots/ft] acrossa producing zone may create hundreds of perfo-ration tunnels; this perforation process gener-ates a considerable amount of metal and

formation debris that needs to be cleared fromthe wellbore.

Historically, fragments from explosivecharges, the casing, the cement and the forma-tion were left in perforation tunnels, which maycause a reduction in production efficiency.Postperforation analysis often showed that manyperforation tunnels were plugged and nonpro-ductive. Developments in perforating technol-ogy, such as the PURE perforating system forclean perforations, in conjunction with shapedcharges that generate minimal debris, allowengineers to reduce this type of perforation tun-

nel damage.3Although less debris remains in theperforation tunnels using these techniques,more debris may be deposited in the wellbore,potentially fouling latching mechanisms onretrievable bridge plugs or impeding the perfor-mance of completion hardware.

Certain materials are sometimes deliber-ately introduced into the wellbore, only to beremoved during subsequent cleanout opera-tions. Stimulation operations typically use sandto cover the top of temporary packers and openperforations to protect them from damage while

drillers work in other locations within the well-bore (left). Once these operations are complete,the sand must be removed before productioncan commence. Other stimulation activities,such as those used in conjunction with theFRAC-N-PAC proppant exclusion system, inten-tionally place sand and synthetic proppant inthe wellbore to aid production.4 In all cases,excess sand and proppant must be removedprior to producing a well.

Regardless of precautions taken to keep awellbore and associated production equipmentfree of debris, unwanted materials often find

their way to problematic locations and increasethe risk of damaging completion equipment,reducing production efficiency and jeopardizingthe long-term viability of a well.5

Complexity of Design

Oil and gas wells are becoming more complexand expensive to construct. To drill wells characterized by remote locations, deepwater settings or great drilling depths, operationaspread rates often reach US$ 1 million per dayIn the face of such increasing complexities andto hold costs down, operators must make critical drilling and completion decisions. Risk analy

sis costs, as a result, are now considered on a peminute basis, rather than per day.

With wellbore geometries and completiondesigns becoming increasingly sophisticatedengineers recognize that risk managementimproved efficiency and optimized productionmay require removal of debris that might havonce been considered inconsequential. Evensmall amounts of debris have the potential tlimit production and cause completion failureJunk and small debris may create difficultie

when operators run long and complex completion assemblies in deep and deviated wellbores

In advanced completion designssuch as thoswith production sleeves that selectively isolatproducing intervalssmall debris, includinmetal fragments and sand, may plug or other

wise render production sleeves difficult to accesor operate.

Wells with tortuous trajectories are hard tclean using conventional methods. Determininoptimal circulation rates is difficult when engineers must consider varying deviation, equivalencirculating density (ECD) limitations, telescoping casing sizes and pump capacity limitation

(next page, top left). Even modest circulationrates, in combination with viscous fluids, risk loscirculation from elevated ECDs. These comple

well environments demand new approaches.

3. Berss K, Stenhaug M, Doornbosch F, Langseth B,Fimreite H and Parrott B: Perforations on Target,Oilfield Review16, no. 1 (Spring 2004): 2837.

4. Gadiyar B, Meese C, Stimatz G, Morales H, Piedras J,Profinet J and Watson G: Optimizing Frac Packs,Oilfield Review16, no. 3 (Autumn 2004): 1829.

5. Haughton DB and Connell P: Reliable and EffectiveDownhole Cleaning System for Debris and JunkRemoval, paper SPE 101727, presented at the SPE AsiaPacific Oil and Gas Conference and Exhibition, Adelaide,South Australia, Australia, September 1113, 2006.

6. Visual Physics, School of Physics, University of Sydney,

Australia: Fluid Flow, Ideal Fluid, Bernoullis Principle,http://www.physics.usyd.edu.au/teach_res/jp/fluids/flow3.pdf (accessed September 16, 2012).

>Protecting open perforations. To isolate openperforations, which may be damaged by debrisfrom ongoing well operations in zones above theperforations, drillers place sand on top of atemporary packer. After operations in the upperzone are completed, the sand and debris fromuphole are removed from the top of the packer;the packer is then released and retrieved fromthe wellbore.

Perforating gun

Newperforations

Sand

Temporary packer

Openperforations


9/60Winter 2012/2013

Old ConceptNew ApplicationOne approach to overcoming the risks of highcirculation ratesthe venturi vacuumhasexisted for centuries. In the late 1700s, GiovanniBattista Venturi, an Italian physicist, describedthe effect that came to be named after him. Heand Daniel Bernoulli, a Swiss mathematician

who worked in fluid mechanics, are known fordiscoveries that led to the development of the

venturi vacuum pump. Engineers and develop-ers have used the venturi vacuum pump designin many applications, from fluid mixing systemsto health care and home maintenance equip-ment such as the common garden hose sprayer.Today, engineers are applying this fundamentalprinciplethe venturi effectto design spe-cialized wellbore cleaning systems capable ofperforming debris removal operations in diffi-cult subsurface environments.

The venturi effect can be described as ainduced vacuum. The laws of fluid dynamdescribed by Venturi and Bernoulli dictate flow velocity increases with a constriction o

flow path diameter, satisfying the principlcontinuity, while a corresponding decreaspressure occurs, satisfying the principle of servation of mechanical energy. A concurdrop in localized static pressure creates a uum (above).6

Venturi vacuum systems have numeadvantages over conventional mechanical puConventional mechanical vacuum systems cally have moving parts that can be troubles

Valves may become stuck, intake filters become clogged and motors are subject to fai

Venturi pumps, by contrast, have few or no moparts and thus require little maintenance.

Debris from the Deep

Recently, engineers have used venturi vacpumps to remove debris from difficult-to-reand problematic areas of wellbores. Multdesigns have been developed, each with unfeatures to meet an array of operational reqments. Several service companies, incluM-I SWACO, a Schlumberger company, downhole debris recovery tools based on theturi effect; some are configured to be used

coiled tubing and others to be used on contional workstrings.

The WELL SCAVENGER tool offers a moddesign that provides application flexibility. upper module contains a single-nozzle fldriven engine designed on the venturi princPressure from surface pumps generates an cient, localized reverse-circulation flow achieves optimal lifting velocities without

, Annular flow rate and cleaning capacity. Mostwells use consecutive strings of casing, witheach subsequent string smaller in diameter thanthe previous one, creating a telescoping effect.In offshore deepwater wells, multiple strings ofcasing are required to control subsurfacepressure and formation stress. The ability tomove debris from the bottom of the hole to thetop by circulation alone is a function of the fluidscarrying capacity and is directly affected by thefluids annular velocity and viscoelasticproperties. However, as the fluid moves uphole,its velocity slows with each increase in casingsize and effective hydraulic diameter. This placesgreater demand on the viscosity characteristicsof the fluid to carry debris. Compensating for lossof carrying capacity by increasing the viscosityor velocity of the carrier fluid may result inincreased equivalent circulating density, whichplaces greater hydraulic force on the formationand may promote lost circulation. Achievingsatisfactory carrying capacity uphole whilekeeping the well within ECD limitations downholeis the drillers challenge. Because of thisproblem, debris removal by conventionalmethods can be difficult.

36 in.

28 in.

22 in.

Casing size

Flow

Cement

Casing shoe

18 in.

16 in.

135/8in.

95/8in.

75/8in.

Open hole

113/4in.Increasinghydraulicdiameter

>Venturi effect. As fluid passes through a flow constriction at high velocity,it generates a localized pressure drop, thus creating suction, which can beharnessed to vacuum debris.

Fluidinput

Fluidoutput

Jet

Suction

Pressuredrop area



Engine module

Debris-screening

module

Debriscollection

modules

Filtrationscreen

Low pressurearea generated bythe venturi effect

Jet

Normal circulationreversed

down the tool

Reverseflow

Conventionalflow

Engine

Mixedflow

Magnetassembly

Debriscollection area



Debris andfluid flow path

Lower debriscollection chamber

Debrisdeflector

pump rates. This reverse flow causes debris tflow up the inside the lower tubular and into thcollection chambers before it reaches the ferroucollection chamber and then flows through thfiltration screen (left). The basic three-modulsystem can be augmented with an array of ancillary tools such as the MAGNOSTAR magneassembly, WELL PATROLLER downhole filtetool, RIDGE BACK BURR MILL device and singl

action bypass sub (SABS) to expand the scope owork (next page).

Because debris removal tools are oftendeployed in brine fluids that inherently have limited solids carrying capacity, conventional techniques typically require high circulation rates o

viscous carrier fluids to lift debris into capturinbaskets or chambers. These measures are not necessary with the WELL SCAVENGER tool. Whenperforations are open and subject to lost circulation or damage, when pressure sensitive downholhardware is in place or when surface equipmenlimitations make it impossible to achieve high

pump rates, the newer generation tools, such athe WELL SCAVENGER device, offer engineers significant advantage. M-I SWACO engineers usproprietary flow regime software to determine thsurface pump rate required to recover thexpected debris without affecting downhole hard

ware or open perforations.Depending on the volume of debris antici

pated, engineers configure one or more debricollection modules at the lower end of the workstring. Each module is designed with a debris collection area, a flow diverter and an inner flow

tube equipped with an internal centralizer tprovide strength and stability. The inner flowtube provides the path for the reverse flow, andthe diverter encourages debris to fall out of thfluid and into the collection area as the fluidflows through each chamber.

The screening unit is fitted above the debricollection modules and below the engine. Fluidflows up through the tool, passes over a magneassembly and then through a filter before exitinthe tool. The screen and magnet assemblies arinternally centralized for stability in deviate

wells. After cleanup, or when the system become

filled or plugged, the SABS tool can be openedallowing higher annular circulation rates, whichelp clean residual debris located above the tooThe WELL SCAVENGER tool is able to remove

wide variety of debris types from wellboresincluding milling debris, bit teeth and conessand, small hand tools and debris from perforating guns.

>WELL SCAVENGER tool module configuration.Fluid flowing through the WELL SCAVENGERengine (top left) takes the following path:Fluid flowing from the surface through the jet(downward green arrow) generates a low-pressure zone. The vacuum effect resultingfrom this localized pressure drop causesfluid and debris to be pulled up through theWELL SCAVENGER tool and then through the

center of the engine (upward red arrow). Thefluid passes around the perimeter of the engine,reverses direction proximal to the jet (curved redarrows) and then flows out of the tool (blackarrows). Upon exiting the tool, a portion of thefluid travels up the hole to the surface (upwardgreen arrows), while the remainder travels back.

Prior to reaching the engine, debris-laden fluidpasses through the lower collection chamber(right). Once inside the tool, moving debrisinteracts with the tools deflector elements,promoting settling into collection chambers.When one chamber is full, the debris flows tosubsequent chambers. As debris-laden fluidpasses up through the WELL SCAVENGER tool,not all debris settles in the collection chambers.

Some debris passes on to the screening module,where a magnet assembly attracts and collectsferrous materials; the fluid then passes through afilter that removes residual nonferrous materials.


11/60Winter 2012/2013

At the surface, safe handling of the recoverytools loaded with debris is essential, especially

when they have been exposed to zinc bromideand other completion fluids characterized byelevated HSE risks. To address these concerns,the WELL SCAVENGER tool modules are fitted

with sealed lifting caps designed to containrecovered materials during tool extraction atthe surface.

Sand and Gun Debris Removal

Operators typically set temporary bridge plugsabove productive zones while performing opera-tions such as reperforating upper zones. In addi-tion, sand or ceramic proppant is typically placedon top of temporary plugs to provide additionalprotection to upward facing latching mechanismsthat release and retrieve the temporary plugs.

In 2011, Eni SpA used QUANTUM gravel-BA packer plugs to carry out multizone grapack completion operations in a series of welthe Adriatic Sea offshore Italy. After the p

were set, drillers spotted sand on top of eachto protect the plugs from gun and formadebris generated while perforating the zabove. On completion of perforating operat

>Wellbore cleanup tools. The MAGNOSTAR tool is a magnet assembly that collects ferrous debris as the debris stream passes thetool. The vanes on the magnet assembly housing create a flow area for fluid bypass around the tool while providing standoff fromthe casing wall. The WELL PATROLLER tool is a downhole filter device that runs in the cleanup string. This device helps clean thewellbore when running in the hole. The tool then filters any remaining debris from the annulus through a wire screen filter as theassembly is pulled from the hole. The RIDGE BACK BURR MILL tool is a casing cleanup tool for perforated casing or liners. The toolremoves perforation burrs to ensure unrestricted passage of completion equipment downhole. Users have the ability to turn off theRIDGE BACK BURR MILL after the milling and cleaning operation is complete. The driller circulates an actuation ball down to thetool; the ball shifts an internal support sleeve to remove the expanding force on the milling ribs. The single action bypass sub (SABS)allows drillers to boost the flow velocity in a casing string above a liner or casing crossover. The tool is run in the hole with itscirculating ports in the closed position (second from right). The driller drops an actuation ball to open the circulating ports (right).This action redirects and reverses fluid flow from down the toolstring to bypass the string, thus removing flow restrictions, allowingan increase in pump rate and establishing higher annular velocity. To close the ports, the driller drops a second actuation ball.

Fluidflow

path

Actuationball

MAGNOSTARtool

WELL PATROLLERtool

RIDGE BACK BURR MILLtool

SABStool

Vane

Retractablemillingribs

Stabilizersleeve

CirculatingportsFilter

Fluiddiverter

Ports closed Ports open

Magnetassembly



the WELL SCAVENGER tool was run in the holeand successfully cleaned the sand and the gundebris from the top of each packer.

M-I SWACO engineers in Aberdeen workedwith Schlumberger engineers in Ravenna, Italy,to carefully plan each completion. The operatorused 1.3 g/cm3 [10.8 lbm/galUS] of calciumchloride [CaCl2] completion fluid in the wellboreand spotted 20 liters [5.3 galUS] of 2.7-g/cm3[22.5-lbm/galUS] ceramic proppant on top ofeach temporary packer prior to perforating shal-lower zones. The first well, which was vertical, wasperforated with 39 shots/m [12 shots/ft] (above).

After each zone was perforated, the driller rana WELL SCAVENGER tool and washover shoe inthe hole to remove excess ceramic proppant andclear the packer retrieval latching mechanism.

On the first run, the top of the debris waslocated with the WELL SCAVENGER tool; no cir-culation was initiated, thus allowing the wash-over shoe to slide over the debris and land on the

packer plug. The sand and debris were success-fully removed and the temporary plug retrieved

without incident. However, to reduce the risk ofthe tool becoming stuck in the sand or damagingthe packer, engineers chose to initiate circula-tion approximately 30 m [100 ft] above the antici-pated top of the sand pill on future runs.

In each well, after positioning the washovershoe on the packer plug, the driller circulatedone and one-half to three annular volumes toassist in overall debris cleanout. The WELLSCAVENGER tool cleared each sand pill in anaverage of 25 minutes. Based on total nonfer-

rous debris recovery, 16 kg [35 lbm] wet weight,or approximately 65% of the ceramic sand, waspumped through the filter screen and out of the

wellbore. Gun debris and larger sand particleswere retained in the collection chambers, andferrous materials were collected on the filtermodule magnet assembly (below). The crewshandled, cleaned, inspected and prepared

debris chambers for rerun in subsequent bottomhole assemblies (BHAs).

Similar operations were conducted on twsubsequent wells; the third well was deviate24. Using the WELL SCAVENGER tool, drillersuccessfully removed the sand and the gundebris in all 12 runs, allowing each packer to bretrieved without incident.

Debris in Pressure Sensitive AreasAccumulations of sand and other small debrion top of packers can make the packers difficulto retrieve. Similarly, these materials can interfere with the operation of other downholmechanical hardware such as formation isolation

valves (FIVs). Because these valves are pressuractivated, debris removal techniques musensure minimal localized pressure changes. Th

WELL SCAVENGER single-nozzle venturi enginprovides debris removal at low circulation ratesthus minimizing pressure changes near an FIV. Ina typical FIV cleanup operation, the BHA com

prises WELL SCAVENGER system componentand one or more complementary wellborcleanup tools such as the MAGNOSTAR tool andthe WELL PATROLLER tool (next page, left).

In 2012, a major international operator in thUK sector of the North Sea planned a targetedcleanup above an FIV. Conventional tools tharequire high flow rates may cause problem

when they clean the area near the FIV. Thesconditions increase the risk of accidental valvactuation or damage to components of the completion assembly.

For optimal tool performance, the bullnose othe bottom of the WELL SCAVENGER tool shoulbe placed 0.3 to 1 m [1 to 3 ft] above the FIV actuation ball. In this case, a 7 1/8-in. landing suachieved this spacing, thus reducing the risk odamage to the FIV from accidental contact.

In this operation, the WELL SCAVENGERtool was run in the hole until the bullnose waapproximately 6 m [20 ft] above the FIV actuation ball. The driller began pumping at a predetermined rate of 4 bbl/min [0.6 m3/min] whilslowly running the tool in the hole. When thbullnose was approximately 0.75 m [2.5 ft

above the FIV actuation ball, the engineeincreased the pump rates slightly to 6 bbl/min[0.95 m3/min], which ensured optimal cleaninaround the FIV ball area without risking damagto the downhole hardware.

After pumping for 30 minutes, the rig crewretrieved the tool to the surface. The debris chambers had collected a total of 11.8 kg [26 lbm] o

Depth, topZone Depth, base Zone length Shots

1,782 m [5,846 ft]1 1,794 m [5,886 ft] 12 m [39 ft] 472

1,640 m [5,381 ft]2 1,648 m [5,407 ft] 8 m [26 ft] 315

1,522 m [4,993 ft]3 1,546 m [5,072 ft] 24 m [79 ft] 964

1,471 m [4,826 ft]4 1,480 m [4,856 ft] 9 m [30 ft] 354

>Collecting wellbore debris in the Adriatic Sea. The WELL SCAVENGER magnet assembly attractsferrous debris, which has circulated up through the WELL SCAVENGER tool (A). Ceramic debris (B)and perforating gun residue (C) were recovered from the debris collection chambers.

A B

C

>Intervals perforated in an Adriatic Sea well.


13/60Winter 2012/2013

assorted nonferrous debris consisting mainly ofsand and small pieces of rubber. Crews recoveredan additional 0.91 kg [2 lbm] of ferrous debrisfrom the internal magnet section of the tool.

The operator originally intended to operatethe FIV within a relatively short period aftercleanup. However, the well was temporarily sus-pended. Although final confirmation of cleanupcannot be verified until the valve is operated, the

successful placement of the WELL SCAVENGERtool close to the FIV, combined with the amountof debris recovered, implied a successful opera-tion. The company intends to return to this wellin the near future.

Gravel-packed wellbores, particularly thosewith low reservoir pressure and that are subjectto lost circulation, may also be easily damaged bydebris removal techniques. Sand and other smalldebris may accumulate inside the gravel-packscreens and impede production. In recompletionoperations, operators often need to remove thesematerials from the inside of delicate screens to

improve production rates.For completion engineers, the inability to cir-

culate completion brine in low-pressure reser-voirs limits debris recovery options. One of theunique features of the WELL SCAVENGER tool isits ability to recover downhole debris at low cir-culation rates, making it an ideal solution forthese difficult applications.

This was precisely the situation in 2012, whenan operator working on the North Slope of Alaska,USA, needed to recomplete an openhole gravel-packed well that began experiencing production

declines. Engineers theorized that sand buildingup inside the gravel pack screens was choking offproduction. But when the well was reentered, lowreservoir pressures resulted in loss of returnsas workover crews attempted to circulate with1.02-g/cm3[8.5-lbm/galUS] filtered water. Engineersat M-I SWACO recommended cleaning the 9 5/8-in.casing to the top of the gravel-pack assembly ataround 4,300 ft [1,300 m] and then running the

WELL SCAVENGER assembly into the openholegravel-pack assembly to clean out debris to a totaldepth of approximately 5,000 ft [1,500 m].

To protect the openhole gravel pack while

cleaning and logging the upper 9 5/8-in. casing, atemporary packer was placed just above thelower completion assembly. Next, 1,000 lbm[454 kg] of sand was placed on top of the packerto protect the release mechanism from fallingdebris during upper casing cleanout. After thecasing was cleaned and the well logged, the sand

was circulated to the surface and the temporarypacker was successfully retrieved.

The M-I SWACO crew ran WELL SCAVENtools in the hole at 3 ft/min [1 m/min] wpumping at 4 bbl/min [0.6 m3/min] (aboSurface pump rates were maintained at the

>Configuring the WELL SCAVENGER tool forformation isolation valves debris removal. Toolsmay be configured to clean in sensitive areasnear FIVs. In this case, a WELL PATROLLER tool,MAGNOSTAR magnet assembly and SABS toolwere run above the WELL SCAVENGER tool toensure debris removal from the wellbore. Ano-go locator limits downward movement of theworkstring into the completion assembly.

Debris collectionchambers

SABS tool

Workstring

No-golocator

Wash pipe

Mule shoe

WELL PATROLLERtool

WELL SCAVENGERengine and debris-

screening module

MAGNOSTARtool

>Cleaning the inside of gravel-pack screens. TWELL SCAVENGER assembly is configured to rinside gravel-pack screen assemblies. Four decollection chambers and 21 joints of workstringare assembled below the engine; thesecomponents are small enough to be insertedinside the gravel-pack screen assembly. In thiscase, the engine and debris collection chambesit above the top of the gravel-pack screens dudebris removal. After the tool removes the debthe driller pulls the tool assembly to the liner toand the SABS tool is opened, which allowsincreased annular circulation rates and ensure

that any residual debris in the annular space isremoved to the surface.

Workstring

SABS tool

Debris collectionchambers

21 jointsof workstring

Mule shoe

WELL SCAVENGERengine and debris-screening module



end of the tools optimal range, minimizing loss ofreturns. After the driller circulated down eachstand, the pump rates were increased to 7 bbl/min[1.1 m3/min] for five minutes. The tool reachedthe targeted depth in one run. The workover crew

recovered 14.5 lbm [6.6 kg] of formation sand,rubber and metal debris from the gravel-packscreens (below). Following successful debrisrecovery from inside the gravel-pack screens, theoperator continued well recompletion operations.

Milling Debris Removal

Drillers use milling techniques for various weloperations such as cutting windows in casingsmoothing burrs and edges on the top of tools angrinding plugs and packers into small pieces sthat they can be circulated out of the wellbore.

In 2010, a major operator working in the Gulof Mexico planned to remove a cast-iron bridgplug (CIBP) from the wellbore. Before the CIBP

could be milled, the operator had to remove 200 f[60 m] of cement that had been placed on the topof the plug. The driller ran into the hole with an81/2-in. roller cone bit and located the top of thecement at approximately 800 ft [240 m]. Durindrilling operations, a bit cone was lost in the hole

The driller pulled the damaged bit from the holand then ran back in with a mill but was unable tgrind up the errant bit cone. To minimize additionalost rig time, the operator sought a solution thacould remove the bit cone and mill the CIBP in single trip. M-I SWACO engineers recommendethe WELL SCAVENGER tool with a special BHA t

meet the companys objectives in a single trip.The BHA comprised two pieces: a washove

shoedressed with a smooth exterior, roughinterior and rough leading edgeand a washpipe extension dressed with two rows of fingebaskets. Cable fingers were inserted to help capture the bit cone. The BHA allowed 16.5 ft [5 mbetween the bottom of the WELL SCAVENGERtool and the leading edge of the shoe.

The driller tripped into the hole and locatethe top of the CIBP, broke circulation and beganmilling the plug. Operating the mill at 80 rpm

the fishing supervisor milled the CIBP in aboufive hours with 1,000 to 6,000 lbf [4,450 t26,700 N] of weight on the tool and 1,000 t3,000 lbf.ft [1,356 to 4,067 N.m] of torque. Whenthe total interval of 2.0 ft [0.6 m] was milled, thrig crew pulled the BHA to the surface. The toohad collected between 12 and 15 lbm [5.4 and6.8 kg] of metallic debris. Larger items that coulnot enter the WELL SCAVENGER tool were foundinside the cable fingers and below the finger basket. These included the bit cone, cone ringspacker rubber and other CIBP componentsBased on the amount of accumulated material

technicians determined that most of the debrihad been removed from the wellbore.

Despite the inferior lifting properties of thseawater-base drilling fluid used in the wellborethe WELL SCAVENGER debris recovery systemremoved the bit cone and debris associated witmilling the CIBP. Drillers successfully trippeinto the hole and retrieved the remaining tooelements with no interference from debris o

junk, thus avoiding the cost of additional trips.

>Assorted debris removed from the depths. Drillers sealed the WELL SCAVENGER debris chambers asthe tool was removed from a well on the North Slope of Alaska. When opened later at the M-I SWACOfacility, the four collection chambers contained various materials, including a mix of formation sand,rubber pieces and ferrous material. A pen, not retrieved from the hole, illustrates relative size.

Recovered Debris Close-Up View


15/60Winter 2012/2013

Removing Stuck Packers

Drillers and engineers make every effort to mini-mize operational risks. Despite these efforts,

drillstrings become stuck, completion assembliesfail to reach their objectives and junk winds up inthe wellbore. A major operator working on theNorth Slope of Alaska recently experienced suchan event.

While the operator was running a packer in9 5/8-in. casing, the packer set prematurely at8,184 ft [2,494 m]. Previously, the operator hadset a packer with a stinger assembly attached atapproximately 10,100 ft [3,078 m]. Once thestuck packer was drilled out, the wellbore had tobe cleaned down to the top of the deeper packerbefore the driller could resume further recomple-

tion operations.Debris removal was complicated by the wells

80 deviation from approximately 2,500 ft [762 m]to total depth. After a competitors boot basketretrieval tool yielded very little debris in tworuns, engineers from the M-I SWACO specializedtools group in Alaska and Houston recommendeda specially modified BHA combined with the

WELL SCAVENGER tool and several high-capacity MAGNOSTAR tools.

The BHA included 90 ft [27 m] of wash pipe, aHEAVY DUTY RAZOR BACK CCT casing scraper,the MAGNOSTAR tools, the WELL SCAVENGER

tool and the SABS circulating sub. After the toolsreached a depth of 6,200 ft [1,890 m], a largeaccumulation of debris on the lower side of the

wellbore hindered progress. Through continuouscirculation and near-constant pipe movement,the driller was able to push the tool assembly to6,280 ft [1,914 m]. The tools were then pulledfrom the hole. Once the tools were on the surface,technicians recovered 184 lbm [83 kg] of ferrousdebris from the MAGNOSTAR tools (above).

While technicians cleaned the MAGNOSTARtools, the driller ran back into the hole with a com-petitors boot basket fishing tool and magnet

assembly. When the tool was pulled from the hole,technicians recovered a packer slip and 20 lbm[9 kg] of ferrous debris. A second run of the

WELL SCAVENGER assembly included threeMAGNOSTAR tools. This run yielded an additional287 lbm [130 kg] of ferrous debris on theMAGNOSTAR tools and 1,033 lbm [469 kg] of sandand silt along with 168 lbm [76 kg] of ferrousdebris recovered from the WELL SCAVENGER toolcollection chambers.

A final run made with the three MAGNOStools yielded an additional 145 lbm [66 kg] ofrous debris. After clearing most of the defrom the wellbore, the driller was able to ruthe hole with a polish mill to clean the lopacker bore. M-I SWACO tools removed a tot1,817 lbm [824 kg] of ferrous and nonferdebris from the wellbore.

Rapidly Evolving Tool DevelopmentComplicated completions, complex borehole figurations and high rig-time costs are leaengineers to identify new applications for

WELL SCAVENGER assembly and associdebris removal tools. Because of new derecovery tools and techniques, drillers are better able to remove materials intentionplaced downhole or items accidentally lost in

wellbore. Tool combinations are evolvinaddress a broader array of completion and derecovery needs. The evolution in debris recotool designs is reducing risks, cutting costs

improving well productivity.Ongoing design work further enhances

range and scope of debris recovery tools usegreat depths. Given the increasing cost of rig tespecially in deepwater settings, engineersfocusing on the development of systems that adebris recovery to be combined with other operations in a single tool run. For example, tests have shown that debris recovery and mitools can be combined with packer retrieval h

ware to deburr casing perforations, recovergenerated debris and remove a temporary pa

all in a single tool run, thus improving operatiefficiency and reducing costs. Other devements are underway to help operators recdebris in low-pressure, lost circulation envments, setting the bar for successful completin increasingly challenging situations.

>Recovering ferrous debris. The vanes of the MAGNOSTAR tool are covered with ferrous debris,which has been recovered from the well after milling operations. Debris removed from the tool ( inset)is laid out for inspection on the drill floor. A ruler, not recovered from the borehole, shows the scale ofthe debris.



Revealing Reservoir Secrets ThroughAsphaltene Science

Downhole fluid analysis of the heaviest components of petroleum can help unlock

information about reservoir structure. Understanding how asphaltenes associate in

oil columns permits scientists and engineers to use asphaltene concentration

gradients to determine the presence of sealing barriers. Production results have

confirmed the validity of this approach, which is being extended to address the

structure and dynamics of fluids in complex reservoirs.

A. Ballard Andrews

Oliver C. Mullins

Andrew E. Pomerantz

Cambridge, Massachusetts, USA

Chengli DongHani Elshahawi

Shell Exploration and Production

Houston, Texas, USA

David Petro

Marathon Oil Corporation

Houston, Texas

Douglas J. Seifert

Saudi Aramco

Dhahran, Saudi Arabia

Murat Zeybek

Dhahran, Saudi Arabia

Julian Y. Zuo

Sugar Land, Texas

Oilfield ReviewWinter 2012/2013: 24, no. 4.Copyright 2013 Schlumberger.

For help in preparation of this article, thanks to John Mainstone,The University of Queensland, Brisbane, Australia.

InSitu Fluid Analyzer, LFA and MDT are marks ofSchlumberger.

INTERSECT is a joint mark of Schlumberger, Chevronand Total.

Long before scientists grappled with the heaviestcomponent of petroleumasphalthumans

were putting it to use. In the ancient world,Babylonians used asphalt as mortar, andEgyptians employed it for mummification.1

Asphalts ability to preserve and bind has beencarried through the intervening centuries to ahost of current applications that include paving,roofing, waterproofing and insulation.

In the oil field, the utility of asphalt is lesclear. Asphaltenes, the primary component oasphalt, tar or bitumen, can create flow assurance problems in the formation, production tubing and pipeline.2Additionally, crudes with highasphaltene levels are less valuable on world markets; their hydrogen deficiency limits their yielof liquid hydrocarbons and their sulfur and metacontent creates problems for refining.3

1. Yen TF and Chilingarian GV (eds): Asphaltenes andAsphalts, 2. Amsterdam: Elsevier Science BV,Developments in Petroleum Science, 40B,2000.

2. Kabir CS and Jamaluddin AKM: AsphalteneCharacterization and Mitigation in South Kuwaits MarratReservoir, paper SPE 80285, presented at the SPEMiddle East Oil Show and Conference, Bahrain,February 2023, 1999.

3. Allan D and Davis PE: Refining ReviewA Look Behindthe Fence, Oilfield Review19, no. 2 (Summer 2007): 1421.

4. Elshahawi H, Mullins OC, Hows M, Colacelli S,Flannery M, Zuo J and Dong C: Reservoir Fluid Analysisas a Proxy for Connectivity in Deepwater Reservoirs,Petrophysics51, no. 2 (April 2010): 7588.

5. This classification is typically labeled a SARAanalysissaturates, aromatics, resins and asphaltenes.For more: Akbarzadeh K, Hammami A, Kharrat A,Zhang D, Allenson S, Creek J, Kabir S, Jamaluddin A,Marshall AG, Rodgers RP, Mullins OC and Solbakken T:AsphaltenesProblematic but Rich in Potential,Oilfield Review19, no. 2 (Summer 2007): 2243.

6. Black oil is used in reservoir modeling to describe oil inplace. The conventional black oil model uses threecomponents: water, oil and gas. For more on black oilmodeling: Huan G: The Black Oil Model for a Heavy OilReservoir, paper SPE 14853, prepared for presentation at

the SPE International Meeting on Petroleum Engineering,Beijing, March 1720, 1986.

7. Mullins OC: The Physics of Reservoir Fluids. Sugar Land,Texas, USA: Schlumberger, 2008.

Zuo JY, Freed D, Mullins OC, Zhang D and Gisolf A:Interpretation of DFA Color Gradients in Oil ColumnsUsing the Flory-Huggins Solubility Model, paperSPE 130305, presented at the CPS/SPE InternationalOil and Gas Conference and Exhibition, Beijing,June 810, 2010.

>Reservoir gradients. Measurements on a condensate oil from a reservoirin Norway show that the formation pressure and temperature at the gas/oilcontact zone lie on an equation of state (EOS)generated bubblepoint linedividing the liquid and two-phase regions. Composition data on reservoirfluids from this field show large gradients. Composition gradients in thereservoir depend on fluid conditions, and as the reservoir temperature andpressure approach the bubblepoint line and critical point, largecomposition gradients develop.

Pressure,

bar

Temperature, K

Increasingcompositiongradients

Liquid VaporTwo-phase region

700600500400300200100

100

200

300

400

500

0800 900

Formation conditions

Critical point

Bubblepoint

Dewpoint


17/60Winter 2012/2013

The high cost of offshore operations and thetrend toward deeper wells worldwide haverenewed the imperative for understanding reser-

voir fluids at a molecular level. Operators can nolonger afford to view reservoirs as homogeneoustanks of oil and gas. In addition to knowing fluidcomposition, they must also be able to assess res-ervoir connectivity, particularly when costs dic-tate a limited number of wells. Imaging and

pressure surveys are often insufficient to com-pletely assess oil drainage patterns, so operatorsare turning to downhole fluid analysis (DFA) andasphaltene science to better understand reser-

voir structures.4

In the recent past, operators characterizedoil in reservoirs with a few parameters such asspecific gravity, gas/oil ratio (GOR) and a simplechemical classification of the bulk oil.5However,

DFA measurements on oil columns from aroundthe world reveal that reservoir fluids present amuch more complex picture, both vertically inthe oil column and laterally across the field. Suchresults, coupled with decades of analyticalresearch, are yielding a more complete picture ofasphaltene physical forms in the reservoir. Theseresearch advances explain how and under whatconditions asphaltenes associate with each other

and allow all components of the reservoir fluidmixgas, liquids and solidsto be described byequations based on thermodynamic principles.The end result of this work enables use of pre-dicted and observed asphaltene concentrationgradients to confirm or disprove fluid drainageconnectivity in an oil column.

This article focuses on new asphaltene sci-ence and covers its origins, development anduses. Cases from deepwater Gulf of Mexico and

Middle Eastern fields illustrate how these dopments are helping oilfield scientists and eneers learn more about connectivity in reservand the distribution of gases, liquids and solithe fluids contained therein.

Reservoir FluidsA Complex Picture

A beaker of petroleum on a laboratory bencan open hatch on a stock tank presents a de

tively simple view of underground fluidsthaentire reservoir consists of only black oil and Fluid property gradients, where present becof reservoir conditions, may appear to affect the GOR. However, this view is inaccubecause at actual reservoir conditions, comption gradients can exist not only for the GORalso for asphaltenes and the individual comnents of the oil (previous page).7

N



Asphaltenes in petroleum have been a focusof study by oilfield engineers and scientists fordecades. Much about asphaltenes has seemedcomplex and inconclusive. Interest in thesecompounds has taken on several dimensionsover time. In the early years of the industry,downstream research was centered on optimiz-ing uses for the asphalt by-products from refin-ing operations. In the last half of the twentieth

century, that focus turned toward efficient con-version of heavy ends and their asphaltene com-ponent as refiners sought to maximize theproduction of transportation fuels. In upstreamexploration and production, the focus onasphaltenes has almost always been on mitigat-ing and avoiding their negative impacts. Theseimpacts include formation plugging because ofprecipitation and the effects of high viscosityduring production and transportation (below).8However, new science developed over the last

decade has shown that asphaltene gradients inthe reservoir can provide valuable insightsabout reservoir structure.

Asphaltenes found in reservoir fluids are acomplex molecular mixture of particles colloi-dally suspended in oil that have no single chemi-cal identity. They are usually defined as a solubilityclassthat is, those molecules that are insolublein n-heptane but soluble in toluene. Asphaltenemolecules are typically condensed aromatic ringsthat can contain heteroatoms such as nitrogen

and sulfur as well as metals such as nickel andvanadium. Almost every chemical property ofasphaltenes has been the subject of significantdebate, except for their elemental composition.

An early controversy centered on the nature ofthe covalently bound chemical groups versusthose that are associated in noncovalent aggre-gates.9 The wide range of molecular weightsobtained at that time1,700 to 500,000 g/mol

was attributed to varying aggregate sizes. Ovethe last decade, research on asphaltenes haencompassed multiple branches of analyticachemical science to produce a much clearer picture of asphaltene properties and how individuaasphaltene molecules associate to form largeparticles (above).10

Downhole Fluid Analysis

Downhole fluid analysis helps scientists and engineers examine reservoir fluids in their nativ

environment. The DFA concept has evolved froma technique for fluid identification via openholsample acquisition to a means of analyzing reser

voir fluids and their spatial variations at formation conditions in real time. The concept isimple: Following drilling, a cylindrical samplinand analysis module is lowered into a well on

wireline, and fluids are collected from the formation. This tool, the MDT modular formation

8. Mullins, reference 7.

Edgeworth R, Dalton BJ and Parnell T: The Pitch DropExperiment, European Journal of Physics5, no. 4(October 1984): 198200.

9. Dickie JP and Yen TF: Macrostructures of the Asphaltic

Fractions by Various Instrumental Methods, AnalyticalChemistry39, no. 14 (December 1967): 18471852.

10. Mullins OC: The Modified Yen Model,Energy & Fuels24 (January 2010): 21792207.

11. Creek J, Cribbs M, Dong C, Mullins OC, Elshahawi H,Hegeman P, OKeefe M, Peters K and Zuo JY:Downhole Fluids Laboratory, Oilfield Review21, no. 4(Winter 2009/2010): 3854.

>Asphaltene viscosity. In 1927, researchers atThe University of Queensland, Australia, heateda sample of pitch, or asphalt, and placed it in afunnel that was subsequently sealed (inset). Theasphalt was allowed to settle for three years atroom temperature before researchers cut thefunnel stem. Since that date, the asphalt hasdripped from the funnel, averaging one dropevery nine to ten years. In 2002, the ninth dropwas starting to form. While the viscosity ofheavy oils is not nearly as high as that ofasphalt, viscosity rises sharply with increasingasphaltene content. Data on asphaltenes anddeasphalted oil from several crudes show arapid increase in viscosity with rising hexaneasphaltene content that spans six orders ofmagnitude in viscosity. These data arerepresented by a Pal-Rhodes viscosity model.(Photograph courtesy of JS Mainstone, TheUniversity of Queensland).

Hexane asphaltenes, wt %

Visco

sity,

Pa.sat60C

0 10101

102

103

104

105

106

107

108

109

20 30 40 50

Asphaltenes,deasphalted oilsPal-Rhodesviscosity model

>

Asphaltene properties. During the past decade, advances in analyticalscience have allowed a more consistent picture of asphaltene structure toemerge. Estimates for the mean asphaltene molecular weight have beenreduced by several orders of magnitude and are now about 750 g/mol, andthe range is significantly tighter. Similarly, scientists now know the mediannumber of fused rings per asphaltene polyaromatic hydrocarbon (PAH) isabout seven, with one PAH per molecule dominating. In addition, thenumber of PAH stacks in an asphaltene nanoaggregate, unknown a decadeago, is one. All of these developments have allowed researchers toestablish consistent physical models regarding asphaltene molecules andto show how they associate with one another in reservoir fluids.

Property

Mean asphaltene molecular weight

Number of PAHs per asphaltene

Number of fused rings perasphaltene PAH

Number of PAH stacksin a nanoaggregate

1 to 20

2 to 20

unknown 1

7 (average)

1 dominates

750 g/mol103 to 106 g/mol

Reported Values, 1998 Reported Values, 2009

12. Optical density, measured by MDT spectroscopy, iscalculated from the degree of absorption in the visibleand near-infrared portion of the frequency bandfromwavelengths of about 400 to 2,000 nm. Components ofreservoir fluids, such as asphaltenes, have

characteristic absorptions in this range that reflect theimolecular structures. Optical density gives adimensionless numerical value to the colorcharacteristics of these fluids. For more on downholeoptical density applications: Creek et al, reference 11.

13. Mullins OC, Andrews AB, Pomerantz AE, Dong C, Zuo JYPfeiffer T, Latifzai AS, Elshahawi H, Barr L and Larter SImpact of Asphaltene Nanoscience on UnderstandingOilfield Reservoirs, paper SPE 146649, presented at theSPE Annual Technical Conference and Exhibition,Denver, October 30November 2, 2011.


19/60Winter 2012/2013

dynamics tester, contains a probe for samplingreservoir fluids and an array of sensors for analyz-ing the sampled fluids on a real-time basis(above). An MDT tool configured for DFA can pro-

vide a long list of reservoir data ranging from gen-

eral properties such as GOR and pressure andtemperature at depth to specific attributes suchas density, composition and miscible sample con-tamination by nonaqueous drilling fluids.11 Inaddition to determining GOR and other proper-ties, the MDT tool uses spectroscopy to measureoptical densityessentially oil colorwhich isdirectly proportional to asphaltene concentra-

tion.12Fluid property variations interpreted fromDFA measurements made at several depth sta-tions in a well can sometimes indicate nearbysealing barriers (right).13

Identifying compartments in a reservoir is not

as challenging as assessing oil drainage connec-tivity within those compartments, especiallybefore production. Static pressure surveys mayfail to find hard-to-image sealing barriers beforeproduction starts because pressure equilibriumand composition equilibrium are achieved overdifferent time scales. Composition equilibrium isachieved slowly, and the difference between the

>Modular formation dynamics tester. The MDT tool (above) contains a complex array ofinstrumentation for downhole sampling and analysis. In a typical configuration (right), the MDT toolcomponents include a section for storing samples in addition to an InSitu Fluid Analyzer system andLFA live fluid analyzer system for real-time downhole fluid analysis. Reservoir fluids enter the tool atthe formation probe and are pumped in two directionsupward toward the InSitu Fluid Analyzer tooland downward toward the LFA module. The InSitu Fluid Analyzer tool contains two spectrometers anda fluorescence detector for analysis of hydrocarbons, CO2, pH and fluid color; it also containsinstruments for measuring density, resistivity, pressure and temperature. Reservoir fluid from thesampling probe that is pumped downward passes through the LFA module. This device employs anabsorption spectrometer to quantify and monitor the amount of reservoir and drilling fluids that arepresent. A gas refractometer (not shown) in the tool differentiates between gas and liquids.

Sample modules

InSitu Fluid Analyzersystem

LFA live fluid

analyzer system

Focused probe

Pump 2

Pump 1

>

Sealing barriers. Using DFA to reveal thepresence of fluid density inversions cansometimes help identify sealing barriers in areservoir. GOR data for two depth zones in ancolumn illustrate this concept. Using GOR as proxy for density in this column, scientists foulow-GOR, high-density fluid at Point A (left), aa high-GOR, low-density fluid at Point B (righThis finding indicates the possible presence osealing barrier between the two zones.

Verticaldepth,

m

GOR, ft3/bbl

Possible sealing barrier

A

X,800

X,700

1,000 2,000 3,000 4,000

X,600

X,500

X,400

X,300

X,200



time to reach pressure equilibrium and that toreach composition equilibrium for the heaviestfraction of crude can be several orders of magni-tude (above).14Massive fluid migration in the res-ervoir is required to achieve compositionalequilibration, and for this to occur, there must begood reservoir connectivity. In contrast, pressureequilibration can be achieved with very small

mass transfer, which can occur through leakyseals. Consequently, pressure equilibration is a

necessary but insufficient condition to establishconnectivity in the reservoir.

Nearly equilibrated asphaltene concentrationgradients between two zones are indicative ofconnectivity. However, before that concept canbe implemented on a practical basis, it is neces-sary to have a model for asphaltenes thataccounts for their thermodynamic characteris-

tics and how they associate with each other deepin the reservoir.

Modeling Asphaltenes

Since 2000, advances in analytical instrumentation and science have allowed a much clearepicture of asphaltene structure to emerge. Suchadvances have narrowed the knowledge gapabout their properties and have led to a morrefined description of asphaltene science aembodied in the modified Yen model.15 Thimodel was later renamed the Yen-Mullin

model.16It envisions asphaltenes in crude oil aexisting in three distinct and separate formsaasphaltene molecules, as nanoaggregates of indi

vidual asphaltene molecules and as clusters onanoaggregates (below). The number of analytical methods employed over the last decade tresolve the molecular weight, size and aggregation parameters in this model is extensive andincludes time-resolved fluorescence depolarization and laser-based mass spectrometry fomolecular and aggregate size and weight determination. For most model parameters, such aasphaltene molecular weight, scientists mus

apply several techniques to reduce thuncertainty.

The asphaltene molecule is at the first level othe Yen-Mullins model. The typical asphaltenmolecule consists of several fused aromatic ring

with peripheral alkane substituents, often witscattered sulfur and nitrogen heteroatomsThis molecule has a mean molecular weight o750 g/mol with most of the population ranginfrom 500 to 1,000 g/mol and a length of abou1.5 nm. In this model hierarchy, the asphalten

>The Yen-Mullins model of asphaltene nanoscience. At low concentrationstypical in condensates and volatile oilsasphaltenes are predicted to exist as a solution of molecules that measure about 1.5 nm (left). At higher concentrationsfoundin black oilsasphaltenes are dispersed as 2-nm nanoaggregates (center). At still higher concentrations, such as those found inmobile heavy oils, asphaltenes are dispersed as clusters of 5 nm (right).

Molecule Nanoaggregate Cluster

~1.5 nm ~2 nm ~5 nm

N

>Reservoir equilibration. Reservoir modeling gives insight to the timerequired to reach equilibration. Modeling of a tilted sheet reservoirwith a low-permeability zone in the center shows that fluid compositionequilibrationmeasured by density, methane or heavy fractionis seven to eight orders of magnitude slower than the corresponding

pressure equilibration.

Timetoreac

hequilibration,

years

Black oil Volatile oil Condensate Gas0

1

101

102

103

104

105

106

107

108

109

Pressure

Fluid density

Methane

Heavy fraction


21/60Winter 2012/2013

14. Pfeiffer T, Reza Z, Schechter DS, McCain WD andMullins OC: Determination of Fluid Composition

Equilibrium Under Consideration of AsphaltenesASubstantially Superior Way to Assess ReservoirConnectivity than Formation Pressure Surveys, paperSPE 145609, presented at the SPE Annual TechnicalConference and Exhibition, Denver, Colorado, USA,October 30November 2, 2011.

15. Mullins, reference 10.

Mullins et al, reference 13.

16. Sabbah H, Morrow AL, Pomerantz AE and Zare RN:Evidence for Island Structures as the Dominant

nanoaggregate is the next structure in size. Theseparticles represent an aggregation of about sixasphaltene molecules in a single disorderedstack about 2 nm in size. The asphaltenes innanoaggregates are tightly bound, and exterioralkanes on the nanoaggregate particle projectoutwardly. The largest particle in the Yen-Mullinsmodel is the cluster, which represents a group ofabout eight nanoaggregates. Clusters, which are

loosely bound, are about 5 nm in size.Although all of the forms envisioned by the

Yen-Mullins model may occur in any oil column,the specific form depends largely on the asphal-tene concentration. In wells that produce volatileoils and condensates with high GOR, the asphal-tene concentration will be less than 0.5 wt % andthe asphaltene particles will be 1 to 1.5 nm insize. At higher asphaltene concentrations, suchas black oil columns with moderate GOR values,the asphaltene concentration will usually be lessthan 5 wt % and the asphaltene particles will beprincipally 2-nm nanoaggregates. In even higher

asphaltene concentrations, as seen in mobileheavy oils that have low GOR, asphaltene levels

will range from 5 to 35 wt %, with 5-nm clusters asthe primary asphaltene particle.

Tar mats may occur in formations with signifi-cant levels of mobile heavy oil and are areas ofnearly immobile asphaltenes usually found at thebase of an oil column near the oil/water contact.There are two predominant forms of tar mats. 17One type occurs at the base of a mobile heavy oilcolumn as a result of seemingly continuousextension of a large asphaltene concentration

and viscosity gradient. The other type of tar matoccurs at the base of a lighter oil column and isdiscontinuous in asphaltene concentration.

The first type of tar mat results from a subtledestabilization of asphaltenes at the top of the oilcolumn followed by transport of asphaltenes tothe base of the oil column to form a mat. The sec-ond type of tar mat may occur when there is a sig-nificant gas charge into the top of a reservoircontaining black oil. As the gas diffuses down thecolumn, the GOR increases and causes asphal-tene molecules and nanoaggregates to form clus-ters. These clusters descend ahead of the diffusive

gas front, which moves lower in the column withtime. When the gas front reaches the bottom ofthe column, the asphaltenes are expelled fromthe oil to form the tar mat (above right).18

>Tar mat formation. One mechanism for tar mat formation (top) envisions astable black oil column (A) in which biogenic methane moves beneath anupper seal (B). As the methane slowly diffuses down the oil column, largeGOR and asphaltene gradients are formed (C). These gradients can becomelarge enough that a dense, asphaltene-rich tar mat may form at the bottomof the column (D). A thin section from a tar mat formed at the base of a high-GOR column shows tar on the grains of a cemented sandstone ( bottom).

Aquifer Aquifer

Aquifer Aquifer

Black oilcolumn

Methanecharging updip

A B

C D

Methanediffusing downdip

Tar mat

Seal

Tar

Architecture of Asphaltenes, Energy & Fuels25, no(2011): 15971604.

17. Mullins OC, Zuo JY, Dong C, Andrews AB, ElshahawPfeiffer T, Cribbs ME and Pomerantz AE: DownholFluid Analysis and Asphaltene Nanoscience forReservoir Evaluation Measurement, Transactions SPWLA 53rd Annual Logging Symposium, CartagenColombia, June 1620, 2012, paper CCC.

18. Zuo JY, Elshahawi H, Mullins OC, Dong C, Zhang D,Jia N and Zhao H: Asphaltene Gradients and Tar MFormation in Reservoirs Under Active Gas ChargingFluid Phase Equilibria315 (February 15, 2012): 9198



Correctly modeling asphaltenes requires atwo-pronged approach. The Yen-Mullins modelprovides the solution to the first challengeauseful framework for the asphaltene particlesthat form in an oil column along with estimates of

particle size and molar volume. The second partof the problem is to mathematically describe theasphaltene concentration gradients for the vari-ous asphaltene physical states as predicted bythe Yen-Mullins model.

In thermodynamic systems, a state variable isa parameter such as temperature, pressure or

volume, which depends on the state of the systembut not the path used to get to that state. Themathematical equation that relates state vari-ables is called an equation of state (EOS). In1834, Benoit Paul mile Clapeyron, a Frenchengineer and physicist, developed the ideal gas

law, an EOS that relates pressure, volume andtemperature. The ideal gas law is a first-orderequation that ignores molecular volumes andforces and is accurate only for weakly interactinggases at moderate conditions. In 1873, van der

Waals developed a cubic EOS that approximatesthe behavior of gases and liquids by taking intoaccount molecular forces and the size of mole-cules. Since that time, many variants of the clas-sic cubic EOS have been developed, and these

equations have been used for decades to modelfluid behavior in oil columns. However, usingthese equations for black oil modeling in reser-

voirs containing significant levels of asphaltenesis not satisfactory. Because asphaltenes lack a

gas phase or a critical point, they must be treatedas a pseudocomponent and handled empirically.Although this approach is adequate to modelhydrocarbon gas-liquid equilibria and determineparameters such as GOR, it is inadequate formodeling molecular and colloidally suspendedparticles such as asphaltenes, asphaltene nano-aggregates and clusters of nanoaggregates.

The need to model solution behavior of mix-tures containing solvents and large moleculessuch as asphaltenes has existed for decades.Much research in the 1940s focused on the ther-modynamics and solution behavior of polymer

compounds and resulted in the Flory-Hugginstheory.19More recently, the Flory-Huggins approachhas been used to examine asphaltene instabil-ity.20Recognizing the need for a first principlesapproach to describe asphaltene concentrationgradients in oil columns, scientists have devel-oped the Flory-Huggins-Zuo EOS for this pur-pose.21This equation incorporates a gravity termfor asphaltenes using their known size. This grav-ity term is essential for modeling asphaltene

gradients. The equation was developed startinwith the free energy of a mixture of asphalteneand solvent as a function of the free energieassociated with gravity, solubility and entropy omixing. At equilibrium, the derivative of the fre

energy sum is zero, and the solution of the resulting partial differential equations yields the FloryHuggins-Zuo EOS. In its original form, thiequation expresses the asphaltene concentratiogradient as a volume fraction of asphaltenes a

various depths in the oil column. Since oil color idirectly proportional to the asphaltene concentration, the optical density ratio is usually substituted for the volume ratio for a more practicameasurement. The resulting equation gives thasphaltene concentration in terms of optical density and is an exponential function of severaparameters (above).22

The first term in the Flory-Huggins-Zuo EOSaccounts for the effect of gravity and is thmost significant term for asphaltenes in an oicolumn for low-GOR oils (next page, top right)Gravitational effects cause asphaltenes to accumulate at the base of a column, although thermaenergy counteracts gravity to some extent. Thifirst term expresses gravitational effects as thbuoyancy of an object in a liquidthe graviteffectdivided by a function of the tempera

>Asphaltene equation of state (EOS). The Flory-Huggins-Zuo EOS (top) predicts asphaltene gradients in an oil column. Opticaldensity at two depths is predicted as an exponential function of three termsgravity, entropy and solubility. The gravity termdepends primarily on asphaltene particle size and depth. The entropy term is a measure of molecular randomness and dependson molar volumes. The final term in this equationsolubilitydepends on GOR, density and composition.

OD hi( ( optical density at depth hi

a hi( ( asphaltene concentration at depth hi

va asphaltene molar volume

v oil phase molar volume

g gravitational constant

density difference between asphaltenes and oil phase

T temperature

R ideal gas constant

a asphaltene solubility parameter

oil phase solubility parameter

Fluid color Gravity term Entropy term Solubility term= +

RTRTexp==

h2 h1

h1h2

v

va va vav

+aOD gva

2 2

a

a( ( ( (

aOD

( ( ( ( ( (

( ( ( (

h2 h2 h2

h1 h1

h1


23/60Winter 2012/2013

turethe thermal effect. For large physicalforms of asphaltenes, such as clusters found inheavy oils, the gravity term is significant andgives rise to high concentrations of asphaltenesnear the base of the oil column.

The remaining two terms in the new asphal-tene EOS are similar to the original Flory-Huggins terms for entropy and solubility. Theentropy is stated in terms of ratios of molar vol-

umes of asphaltenes and solvent at two depths.The entropy effect tends to randomize theasphaltene distribution and counteract gradi-ents, but is usually not large for asphaltenes incrude oils. The other factor in the Flory-Huggins-Zuo EOS that essentially corresponds to theoriginal Flory-Huggins work is the solubilityterm. For asphaltene gradients, this term isexpressed in solubility parameters that are cal-culated from GOR or mass densities. This termaccounts for changes of asphaltene solubility inthe liquid phase and is important for high-GORoil that produces a low-density liquid, rich in

paraffinic alkanes that decrease asphaltenesolubility. For low-GOR oils, however, the solu-bility term is usually not significant.

The end result of this new equation of statefor asphaltenes is the prediction of asphalteneconcentrations, directly proportional to fluidcolor, at any depth in the oil column. Almost all ofthe parameters may be measured or estimatedfrom downhole fluid analysis results of the bulkoil at various depth stations. Those parametersnot directly measuredsuch as the solubilityparametersmay be obtained from correlations

of known properties.The only adjustable parameter in the Flory-Huggins-Zuo EOS is the asphaltene molar vol-ume, which is related to particle size. The

particle size cannot be determined directly

from the downhole data, but there are otherways to find it. The first method is to tune theunknown size of the asphaltene particles tomatch the downhole fluid color data frommeasurements at different depths. This size isthen checked against the Yen-Mullins modelparticle types to ensure it is within the boundar-ies described by the model. The second methodis to assume that heavy ends in the oil column

are either asphaltene molecules, nanoag

gates or clusters. In this case, the assumedis used to predict the downhole asphaltene dients in the oil column, which can be checagainst the actual data. If there is consistethen the data can be used to assess connectand other reservoir properties. Analysis ofdata may not always suggest a single asphaltparticle type because multiple particle tmay be involved (below).

19. Flory PJ: Thermodynamics of High Polymer Solutions,Journal of Chemical Physics10, no. 1 (January 1942):5161.

Huggins ML: Thermodynamic Properties of Solutions ofLong-Chain Compounds, Annals of the New YorkAcademy of Sciences43, no. 1 (March 1942): 132.

20. Buckley JS, Wang J and Creek JL: Solubility of theLeast Soluble Asphaltenes, in Mullins OC, Sheu EY,Hammami A and Marshall AG (eds): Asphaltenes,Heavy Oils, and Petroleomics, New York: SpringerScience+Business Media (2007): 401438.

21. Zuo JY, Mullins OC, Freed D, Elshahawi H, Dong C and

Seifert DJ: Advances in the Flory-Huggins-Zuo Equationof State for Asphaltene Gradients and FormationEvaluation, Energy & Fuels(in press).

22. Freed DE, Mullins OC and Zuo JY: Theoretical Treatmentof Asphaltene Gradients in the Presence of GORGradients, Energy & Fuels24, no. 7 (July 15, 2010):39423949.

Zuo et al, reference 21.

>Gravity effects. The effect of gravity depends on which asphaltenephysical form predominates in the well. For a 100-m [328-ft] oil columncontaining mostly asphaltene clusters (black), gravity effects are large, asevidenced by the dramatic increase of asphaltene content with depth. Theintermediate size nanoaggregates (blue) show a much more gradualchange, while the asphaltene molecules (red) show only a small changefrom top to bottom of the column.

Verti

caldepth,

m

0 0.2 0.4 0.6 0.8 1.0100

80

60

40

20

0

5.0-nm clusters

2.0-nm nanoaggregates

1.5-nm molecules

Asphaltene concentration at depth

Asphaltene concentration at 100 m

>Multiple particle types. A black oil column that was subjected to a late gasand condensate charge shows evidence that more than one asphalteneparticle type is present in the column. Analysis of DFA data using theFlory-Huggins-Zuo EOS indicates that nanoaggregates alone would notaccount for the increase in asphaltene concentrationas measured byoptical densitywith depth (left). In this example, the late gas chargedestabilized the asphaltenes, causing clusters to form; these clusters settledtoward the bottom of the oil column because of gravity (right). The presenceof large viscosity and asphaltene gradients characterized this oil column,and production of this well proceeded with no significant problems.

Vert

ica

ldepth

,m

Oilco

lumn

Optical density

Nanoaggregatesand clusters

Nanoaggregates

X,Y00

X,Z00

X,X50

X,Y50

X,Z500 0.5 1.5 2.51.0 2.0 3.0

EOS models

DFA data

Nanoaggregate Cluster



Downhole fluid analysis, the new Yen-Mullinsmodel and the Flory-Huggins-Zuo EOS can be used

together to model asphaltene gradients in actualoil columns. The first step is the use of DFA to giveexperimental data on asphaltene concentration

via fluid color, GOR and other physical parametersat several depth stations in a well. The Yen-Mullinsmodel then provides a physical picture of theasphaltene entities that may be present and allowsthe operator to make reasonable assumptions onparticle size. That size is then used in the Flory-Huggins-Zuo EOS to predict the asphaltene con-centration gradient in the well. If this gradientmatches the experimental data, it can be used tofurther assess reservoir connectivity. This analysis

is not a mere curve-fitting exercise. The matchingof sizes computed by the new EOS and the Yen-Mullins model gives the operator confidence thatthe system is in equilibrium.

Asphaltene Science and Complex Reservoirs

An example from a complex field in the Gulf ofMexico illustrates how asphaltene science isused in answering practical questions. This field,operated by Marathon, included an area produc-

ing intermediate-GOR black oil that consisted ofsix sand layers spanning 1,000 ft [300 m] of depth

and intersected by multiple wells.

23

The chal-lenge for the operator was to develop an accuratedescription of reservoir fluid properties andunderstand connectivity among the various sandlayers. The reservoir fluids were analyzed by mul-tiple methods. DFA was employed using the MDTtool both to gather real-time information andobtain samples for further PVT analysis in thelaboratory. Using advanced gas chromatographicanalysis, the operator also performed geochemi-cal fingerprinting on collected samples. Althoughthe data covered multiple wells in the area ofinterest, not all analyses were performed at all

depth stations; the most complete dataset camefrom two wells in one of the sands. These dataand their analyses show how connectivity ques-tions can be viewed through the lens of the newasphaltene science.

Prior to use of asphaltene gradients to giveclues to connectivity in a reservoir sand layer,operators often used data from bulk oil samplingand formation pressure at several depths to make

judgments on connectivity. Data on GOR, stock-

tank oil density and formation pressure from th

two Marathon wells spanning about 500 f

[152 m] of depth in Sand A show differences thasuggest barriers to connectivity. In particular, th

pressure gradients from the two wells do no

appear to coincide, which is indicative of a seal

ing barrier. However, these differences ma

reflect either measurement imprecision or differ

ences in the way the data were collected (above)

>Fluid properties and formation pressure in a field in the Gulf of Mexico. DFA data on GOR ( left) and density (center) from two wellsin Sand A show variability that lies either within or very close to the measurement error bands; scientists can draw no definitiveconclusi

oilfield review winter 2012

Documents

larger demonstration

larger projects

metric tons

thepractice of carbon

storage ccs

storage of dense

demonstration fieldtests

oil field