42

si-end,te-rrynt

ce-re-id- ofngce-fulndep

ryeteingesp-edus

ec-re-n-

s athtaln-al

ingndas

muds that achieve drilling goals, but do notimpede cementing success. Considerationmust be given to providing gauge holes thatallow casing centralization. It may be neces-sary to reduce rates of penetration—averageto high instead of very high—during drillingif that means improved borehole conditions,lower-cost primary cement jobs and reduc-tion or elimination of expensive repairworkovers. Necessary elements are avail-able and, in most cases, in place to do this;where efforts often fall short is in coordina-tion and management of the entire processto realize maximum benefits. Success interms of the final product—a safe, long-last-ing wellbore at the lowest possiblecost—should be an incentive to rethink andrestate fluid objectives.

Better understanding of annular displace-ment is a key element that is already inplace.2 By using physical and computermodeling, cementing criteria haveimproved. Simulation and design softwareallow the myriad of fluid factors and com-plicated interactions involved in primarycementing to be addressed qualitatively, andmost of the time quantitatively as well. Thetotal process (mud removal and cementplacement) including conditioning, annularflow regimes, spacer—a buffer betweendrilling muds and cement slurries—selec-tion and fluid displacement inside pipe cannow be evaluated in planning and designstages, during mud maintenance and condi-tioning, and before or after jobs.

Seamless Fluids Programs: A Key to Better Well Construction

New insights into displace nulus, combined with integrated

drilling and cementing flu This structured “fluids-train”

approach also optimizes o t lower cost for operators.

Lindsay FraserBill StangerHouston, Texas, USA

Tom GriffinSugar Land, Texas

Mourhaf JabriBalikpapan, Indonesia

Greg SonesAnadarko Petroleum CorporationHouston, Texas

Mike SteelmanCalgary, Alberta, Canada

Peter ValkóTexas A&M UniversityCollege Station, Texas

For help in preparation of this article, thanDominique Guillot, Dowell, Clamart, FranJonas, Dowell, Sugar Land.

In this article, CBT (Cement Bond Tool), C(Cement Evaluation Tool), DeepSea EXPREMUDPUSH, SALTBOND, USI (Ultrasonic WELLCLEAN are marks of Schlumberger.

Improvements in well construction are posble if long-standing boundaries betwedrilling and cementing can be eliminateand if mud removal and displacement criria are properly applied. Efficient sluplacement for complete and permanezonal isolation relies on effective displament of drilling fluids from the casing-bohole annulus—mud removal—and on avoing bypassing, mixing and contaminationfluids in the annulus and casing duricement placement. Understanding displament mechanics is essential to successcementing, but an integrated drilling acementing fluids approach is a first sttoward overall wellbore optimization.

The consequences of poor primacementing jobs can be severe. Incomplmud removal may leave channels, allowcommunication between subsurface zonor to the surface. Likewise, failure to proerly separate fluids as they are pumpdownhole can negate the most meticuloplans or the best designs and lead to inefftive mud removal or contamination that pvents cement from ever setting up (hardeing). Approaching well construction aseries of interrelated events in which bomud and cement play important roles—tofluids management—results in a more cotrollable, structured process with optimwellbores as the objective.1

Traditionally, drilling fluids and cementservices have been provided separately athe lack of stated, common objectives h

ment mechanics inside casing and in the an

id services, can improve primary cementing.

verall drilling and completion performance a

ks toce, and Jason

emCADE, CETS, EXPRES,Imager) and

Oilfield Review

been a roadblock to optimizing these opera-tions. Better management of fluid servicesrequires drillers and cementers to worktogether from well start to finish to select

High flow rates effectively displace mud ifturbulent3 flow is achieved around the entireannulus, but are viable only if casing andhole sizes are relatively small and casingstandoff4 from the borehole is adequate.Lower flow rates can also successfullyremove mud in many cases where higherflow rates are not practical, but more sophis-ticated designs and modified fluids are oftenneeded to achieve laminar5 displacements.Spacers with controllable properties—abilityto suspend weighting agents, reasonable tur-bulent rates, adjustable rheology, compatibil-ity, low fluid loss and a wide range of appli-cations—are needed to meet and betterapply mud removal criteria (see “Engineered,Fit-To-Purpose Spacers,” page 46).6

Finally, to close the fluids loop, displace-ments inside pipe must be understoodbecause density differences may cause mix-ing of fluids or bypassing of mud by spacers,spacers by cement slurries or lead by tailslurries.7 Better understanding and applica-tion of fluid flow and displacement mechan-ics are required along with more careful

43Summer 1996

1. Fraser L and Griffin TJ: “Economic Advantages of anIntegrated Fluids Approach to the Well ConstructionProcess,” presented at the American Association ofDrilling Engineers Drilling Fluids Technology Confer-ence, Houston, Texas, USA, April 3-4, 1996.

2. Lockyear CF and Hibbert AP: “Integrated PrimaryCementing Study Defines Key Factors for Field Success,” Journal of Petroleum Technology 41(December 1989): 1320-1325.Lockyear CF, Ryan DF and Gunningham MM:“Cement Channeling: How to Predict and Prevent,”SPE Drilling Engineering 5 (September 1990): 201-208.

3. Turbulent flow occurs at higher flow rates. Individualfluid particles swirl around, but their average velocityresults in what is considered a flat velocity profile.Momentum is constantly transferring from one regionto another, but overall flow is relatively constant.

4. Specification 10D, Specification for Casing Centralizers,2nd. Dallas, Texas, USA: American Petroleum Institute,1983.Casing standoff (STO) in percent is defined as STO =2w/D - d x 100 or w/R-r x 100, where D is hole diame-ter, d is pipe outside diameter (OD), R is hole radius, r ispipe radius and w is the smallest annular gap. STO is100% when casing is concentric—perfectly centered.

5. Laminar flow occurs at relatively low flow rates. Fluidparticles move parallel to the casing axis or annuluswalls along straight lines in the direction of flow, with a parabolic velocity profile. At the walls, where liquidswet the surface, fluid particles in contact with pipe orannulus walls are stationary and velocity is zero,increasing to a maximum—twice the average velocityfor Newtonian fluids—at the center of the flow channel.

6. Couturier M, Guillot D, Hendricks H and Callet F:“Design Rules and Associated Spacer Properties forOptimum Mud Removal in Eccentric Annuli,” paperCIM/SPE 90-112, presented at the International Tech-nical Meeting of the Petroleum Society of CIM/SPE,Calgary, Alberta, Canada, June 10-13, 1990.Tehrani A, Ferguson J and Bittleston SH: “Laminar Dis-placement in Annuli: A Combined Experimental andTheoretical Study,” paper SPE 24569, presented at the67th SPE Annual Technical Conference and Exhibi-tion, Washington, DC, USA, October 4-7, 1992.

7. Griffin TJ: Displacement Inside Casing. SchlumbergerDowell Report (January 3, 1995).

ers and cement. Ideal muds for efficient dis-placement are nonthixotropic8 and havereduced gel strengths, plastic viscosities andyield points; low density to facilitateremoval by buoyant forces; minimal fluidloss to prevent thick filter cakes and differ-ential sticking; and are chemically compati-ble with cements. Perfect muds, however,cannot be achieved in practice, so effortsmust be made to get close to ideal charac-teristics during selection, maintenance andprecementing circulation.

Drilling fluid density and rheology mustbe kept low to meet mud-removal require-ments. Displacing fluid weights and viscosi-ties become higher with each successiveinterface, which can lead to unacceptablyhigh cement densities and viscosities, andpossible lost circulation if initial mudweight is too high. Just circulating and con-ditioning mud before cementing is notenough; effective solids and chemical con-trol of rheology are required throughoutdrilling operations. If drilling fluids are notproperly designed or deteriorate duringdrilling or logging, gelled mud that is diffi-cult to remove may be left in washouts oron the narrow side of the annulus.

Fluids compatibility also impacts annulardisplacement. Fluid mixtures should have

design of mud systems, spacer fluids andcement slurries to avoid common cement-ing problems (above). This article gives anoverview of integrated fluids services, andreviews mud conditioning and removalfrom the annulus by turbulent and effectivelaminar flow (ELF). A Dowell and TexasA&M University study defining downwardflow in pipe and proposing methods toimprove cement placement without sacrific-ing effective mud removal is also examined.

The Case for Total Fluids Management In the past, drilling and cementing fluidswere often provided under individual ser-vice contracts, often by different companies.All too frequently, the attitude seemed to be,“drill as fast as possible and worry aboutcementing after reaching TD.” Other needsand intentions, and deleterious effects thatoccur when some fluids commingle wereoften ignored. In principle, instead of segre-gating drilling and cementing fluid services,operations can be unified in a single, inte-grated process. Isolated service-line mentali-ties are replaced by a common goal of pro-viding seamless fluids programs—”fluidstrains”—to optimize overall performanceand results. Territorial considerations are for-

44 Oilfield Review

gotten, and the two disciplines worktogether to maximize the efficiency andeffectiveness of all well-construction fluids.

Good communications and coordinationare a necessity. Cementing designs are per-formed before drilling is complete, sochoices about flow regime—turbulent orlaminar—and spacer properties are madeassuming hole size and mud characteristics.Last-minute changes or unexpected varia-tions in borehole conditions place cementersat a disadvantage. Irregular holes andwashouts hinder mud removal and casingcentralization, and may preclude use of pre-ferred turbulent flow. Low standoffs result inlarge radial variations in annular fluid veloc-ity around casing with higher velocity on thewide side and lower velocity on the narrowside. This leads to inefficient annular dis-placement and potentially poor cementbonds or channels. For cement jobs, casingOD to hole diameter ratio is close to unity,so annular flow can be calculated using abasic slot model (next page, top).

Drilling fluid designs also influencecement job quality. For example, zonal iso-lation cannot be achieved unless mud andcuttings are removed from the annulus.Drilling fluids must be designed, maintainedand treated to provide optimum final holeconditions, and ultimately be conditionedbefore cementing for easy removal by spac-

No bottomwiper plugs

Mud Mud

Chemicalwash

Weightedspacer

Immobile mudin narrow gap

Good Bad

Chemicalwash

Weightedspacer

Float shoe

Tailslurry

Zones ofinterest Inflow

Lostcirculation

Gelled mudchannel

Tail slurrybelow zonesof interest

Mud

Good

Weightedspacer

Leadslurry

Tailslurry Bypassed

lead slurry

Tail slurryahead oflead slurry

Cementmixes withspacer

Spacerbypassesmud

Bad

Top wiperplug

Bottomwiper plugs

Floatcollar Bypassed

or mixedfluids inshoe track

Float joints(shoe track)

Top of cement too high

Borehole Geometry and Mud Removal Displacements

■■Common cement-ing problems (red)related to drilling,mud removal anddisplacement.

drilled with bentonite mud and three withpartially hydrolized polyacrylamide (PHPA)fluids was $26,600/well, or $3.58/ft[$11.75/m] drilled. Average hole enlarge-ment was 113% by volume and typically 23days were spent drilling. Lost time due tohole problems and backreaming was about24 hr/well.

Some elements of drilling fluids perfor-mance were acceptable, but hole geometriesthat cementers had to address were not.Bentonite mud was not conducive to drillinggauge holes and a PHPA fluid failed to pre-vent washouts that were responsible formajor cementing cost over-runs. Enlarged

lower rheologies than the individual fluids,but because this is difficult to achieve formuds and spacers, designs need to mini-mize mixture viscosities. Problems also ariseif cement and mud mix inside or outsidecasing. Some drilling fluid additives acceler-ate or retard cement thickening times. Butmore commonly, cement-mud combina-tions result in high-viscosity mixtures andcorresponding friction pressure increasesthat lead to excessive surface pump pres-sures and premature job termination as wellas inefficient displacement. Washes andspacers isolate these potentially incompati-ble fluids, but unexpected variations incomposition leave cementers unprepared tomaintain this separation. This can beavoided by using bottom wiper plugs to sep-arate fluids inside casing and liners.

In addition to displacement considera-tions, cementing cost is an issue as holesizes increase from washout or enlargement.The cost of larger cement volumes is obvi-ous, but additional centralizer cost toachieve adequate standoff for effective mudremoval is often overlooked (right).

Spacer cost is also important. As hole sizeincreases, higher flow rates are needed forturbulent flow and spacer volumes must beincreased. For example, if hole diameterincreases from 6.5 to 8.0 in., the rate toachieve turbulent flow goes from 4 to 14bbl/min and cost of standard spacers goesfrom about $6500 to $15,500.

Workovers are another often overlookedcost component when drilling and cement-ing services are segregated. Typically, if aprimary cement job is unsuccessful and acement squeeze is necessary, more than oneattempt is needed to achieve zonal isola-tion. Remedial cementing costs, includingcement, perforating, packers and rig time,can be as much as, or more than, the pri-mary cement job.

Integrating Fluids Services in CanadaA managed fluids approach proved success-ful in western Alberta, Canada, where verti-cal wells are drilled to between 6888 and7544 ft [2100 and 2300 m] through uncon-solidated formations. Historically, drillingand cementing fluids had been provided byone company, but individual services werenot working to meet common goals. Drillingfluids services tried to minimize expendi-tures directly related to mud use, andcementers did the best job possible withresulting hole conditions. Managed sepa-rately, drilling fluids cost on four wells

45Summer 1996

8. Thixotropic fluids are highly viscous when static, butbecome more fluid-like and less viscous when dis-turbed or moved by pumping.

■■Flow velocity profiles around a 60% standoff eccentric annulus. For cement jobs,outside casing to borehole diameter ratio is close to unity, and annular flow condi-tions can be evaluated and calculated assuming flow through a slot (inset). If annu-lar flow is uniform, the ratio of local to average velocity is equal to one. For thinNewtonian fluids like water in turbulent flow, velocity profiles are relatively flatwith lower-than-average flow in the narrow gap and above-average flow in thewide gap. Viscous non-Newtonian fluids like polymers in laminar flow move mostlyon the wide side and can be static in the narrow annulus gap. Higher pump ratesor increased standoff improve flow velocity on the narrow side of the annulus.

■■Cementing costs versus hole size. The cost of additional centralizers to achieve adequate standoff is often overlooked.As hole size increases from 6.5 to 8.0 in., combined centralizerand cement costs to fill from 8000 ft [2440 m] total depth (TD)up to 5000 ft [1520 m] using a 16.45 ppg slurry with moderatefluid-loss control almost triples from $7850 to $22,500.

0

1

2

3

0° 90° 180°

Loca

l to

aver

age

velo

city

rat

io

Position around annulus

Narrow side (ns) Wideside (ws)

-90°-180°Wideside (ws)

Basic Slot Model

Polymer profiles

Water profiles

1 bbl/min3 bbl/min6 bbl/min

Pump rates

Concentric slot

Eccentric slot

nsws ws

0° 180°-180°

0

5

10

15

20

25

6.5 7.0 7.5 8.0

Cos

t, $1

000

Total

Cement

Centralizers

Hole size, in.

holes were compensated for by pumpingextra cement, knowing that there was risk ofchanneling due to reduced fluid velocities inwashouts. Cementing on these seven wellscost $103,750/well or $13.96/ft [$46/m]drilled, about four times drilling fluid costs.Total fluids averaged over $130,000/well, or$17.56/ft [$57.60/m] of hole.

The primary functions of spacers are fluid separa-

tion to avoid compatibility problems and ensuring

flow under a specific regime—turbulent or lami-

nar—while maintaining hydrostatic well control.

Improved mud removal guidelines require pre-

flushes for either turbulent flow or effective laminar

flow (ELF) techniques, so weighted MUDPUSH

spacers were developed for use with WELLCLEAN

optimal mud removal services (right). XT and XS

spacers are for turbulent flow. Viscous XL is used

with ELF. All three can be adapted for use with oil-

base muds—XTO, XSO and XLO spacers.

Turbulent spacers were designed to overcome

settling problems experienced with thin spacers.

Weighting agents are suspended at surface or bot-

tomhole temperatures under static and shear con-

ditions by a properly designed base-fluid rheology

that eliminates free water and particle settling over

a wide range of densities while allowing turbulent

flow at reasonable pump rates. The XT spacer is for

turbulent flow regimes in low-salinity environments

(fresh or less than 10% salt by weight of mix water)

and the XS spacer is for high-salinity applications

(30% salt by weight of mix water). Both can be for-

mulated at 10 to 19 lbm/gal [1.2 to 2.3 specific

gravity (SG)] densities.

Laminar-flow spacers have higher viscosities

than turbulent-flow spacers, so good particle-carry-

ing capacity ensures that weighting agents to

achieve required densities do not settle out. To

meet ELF friction-pressure hierarchy criterion,

spacer rheology can be adjusted so that apparent

viscosity across the range of pumping shear rates

falls between drilling mud and cement slurry

apparent viscosities. Spacer density can also be

designed halfway between mud and cement slurry

weights at any density from 10 to 20 lbm/gal

[1.2 to 2.3 SG].

In addition to proper spacer rheology and parti-

cle-carrying capacity, fluid-loss control and com-

patibility are important. Fluid-loss control must be

considered because water lost during displacement

increases the spacer solids-to-liquid ratio, density,

and to a greater extent, apparent viscosity. Exces-

sive fluid loss introduces the possibility of spacers

coming out of turbulent flow at design rates, which

can lead to channeling of spacer through the mud.

Fluid loss for these spacers is low and few compat-

ibility problems have been encountered. Some

mixtures of these spacers and cement slurries

develop weak gel strengths when left static at low

temperature, but these gels are broken by shear

rate or small temperature increases.

Consistent performance under field conditions is

also an advantage in effective mud removal. Spac-

ers must perform under variable conditions from

low-quality barite and brackish or high-salinity

water to low-shear mixing without major changes

in properties and effectiveness. Spacers should

also have adequate viscosity and fluid-loss control

at field conditions. MUDPUSH spacers perform

successfully under a wide range of operational con-

ditions, and rheological properties are consistent

with laboratory measurements made prior to jobs.

These spacers are limited to maximum bottom-

hole circulating temperatures of 300°F [149°C], but

the new XEO spacer, a polymer-modified, oil-in-

water emulsion spacer, extends applicability to

450°F [232°C] for oil-base mud removal only. The

WHT spacer is a water-base spacer developed for

these same higher temperature applications and

oil- or water-base mud removal to complement the

XEO spacer. However, it exhibits less fluid-loss

control, especially when seawater is used as mix

water. MUDPUSH spacers can also be used for

other cementing applications where weighted spac-

ers are needed, such as plug or squeeze cement-

ing, even when WELLCLEAN services are not

directly applicable.

Reasonable turbulent flow pump rates

Excellent ability to suspendweighing agents

MUDPUSH Spacer Properties

Adjustable viscosity and densityfor laminar flow

Cement, oil- and water-base mudcompatibility

Good fluid-loss control

Applicable for a wide range of fluidweights and salinities

Engineered, Fit-to-Purpose Spacers

46 Oilfield Review

Overall improvement was the goal of aunified fluids approach on two subsequentwells. Total fluids costs were targeted to bereduced by improving hole gauge andreducing cement volumes. Unconsolidatedformations in these wells were identified asthe cause of washouts, so because of thelack of success with even a moderatelyinhibitive PHPA system, mixed-metal-hydroxide (MMH) mud with unique fluidrheology was chosen to minimize holeenlargement.

After the revised fluids program wasimplemented, gauge holes allowed for bet-ter casing centralization and improved dis-placement designs—a laminar flow regimewas chosen for these wellbore geometries.Spacers effectively removed MMH fluidsfrom the annulus and logs indicated goodcement placement and successful zonal iso-lation. Cement returns compared to cementvolume pumped in excess of caliper holevolume indicated minimal if any channelingin both the wells drilled with MMH fluid.But severe channeling was likely in three ofthe previous seven offset wells, and one hadsignificant losses during cement placement.

Water flow—the first in this field—occurred while drilling the initial test well.Although most of the 57% washout wasover the interval where flow occurred onthis well, this still compares well with over100% average washout on offsets. Drillingfluid cost exceeded average offset costbecause dilution, borehole instability andthe need to increase density resulted inexcess product use that skewed cost. Posi-tive results, however, were seen in improvedhole gauge and cement cost, which fell to64% of the average.

The second test well had no losses or flowand was drilled in the least number of days,despite moderate rates of penetration. Lostdrilling time on this well was the lowest forthis field and washouts were reduced to29%. Drilling fluid cost at $43,000 wasabove the $25,000/well average, butcementing costs of $45,000 were less thanhalf those of previous wells.

Total fluids cost was the lowest on recordfor this field—a 32% savings over the aver-age for offsets. The objective of reducingoverall well construction fluid costs wasachieved by reducing washouts, and higherdrilling fluid costs to minimize hole enlarge-ment were more than offset by cement sav-ings. Proper drilling practices cannot assurecementing success, but poor drilling prac-tices may make cementing successunachievable.

1. Courturier et al, reference 6, main text.Tehrani et al, reference 6, main text.

47Summer 1996

18

16

14

12

10

8

6

4

2

00 10 20 30 40 50 60 70 80 90 100

API standoff, %

Flow

-rat

e ra

tio

R

rD d

w

STO, % = or x 100R-rw

D-d2w

Adjust rheology if necessary

Adjuststandoffor flow rate

Eccentered Flow Screen

Evaluate flow regimes and range offlow rates versus hole size; selectflow regime and standoff.

Centralizer Calculation

Select centralizers appropriate forhole dimensions and desired standoff.

Pump Rate Selection

Select pump rate that meets criteriafor the chosen flow regime, holesize and standoff.

U-Tube Calculation

Evaluate U-tubing that occurswhile pumping at the selected rate.

Evaluate Mud Removal Criteria

Determine if mud removal criteriaare met across all zones of interest.

■■Turbulent flow-ratecorrections versuscasing eccentricity.The critical flow rateto achieve turbulentflow completelyaround a casing-borehole annulusdoubles as casingstandoff (STO)decreases from 100to 70% and there isalmost a tenfoldincrease if standoffdrops to 30%.

■■Optimizing mudremoval. In the early1990s, pipe eccen-tricity was first takeninto consideration indesigns and in thefield by using WELL-CLEAN optimal mudremoval service inCemCADE cement-ing design and evaluation software. This comprehensivesoftware is used toevaluate all wellparameters, includ-ing casing standoff, and to recommendflow regimes, preflushes and volumes, and pump-rate sequences for optimum fluid displacement.

annulus because of distorted velocities,lower frictional pressure drops and unevenwall shear stress distribution (left ). This isundesirable because stationary mud may gelor dehydrate by static filtration at permeablezones and be difficult to mobilize duringmud removal and cement placement.

Conditions leading to zero flow in narrowannular gaps need to be defined by account-

■■Cementing versus drilling geometries: the importance of standoff. At lower stand-offs, the decrease in frictional pressuredrop in a cementing geometry—large cas-ing in open hole—is significantly greaterthan in a drilling geometry— smaller drillpipe in open hole. Standoff, therefore, hasa double effect on annular displacement ina cementing geometry. Both wall shearstress and pressure drop are lower for poorstandoffs in an eccentric annulus, whichfurther compounds mud removal andcementing problems. In the past, mostcementing designs used drilling simulatorsthat assumed a concentric annulus.

2000

1500

1000

500

0 20 40 60 80 100

Pipe standoff (STO), %

Fric

tiona

l pre

ssur

e dr

op, P

a/m

0

Cementing geometry:0.81 diameter ratio

Drilling geometry:0.55 diameter ratio

9. Howard GC and Clark JB: “Factors to be Consideredin Obtaining Proper Cementing of Casing,” in Drillingand Production Practices. Dallas, Texas, USA: Ameri-can Petroleum Institute (1948): 257-272.Haut RC and Crook RJ: “An Integrated Approach forSuccessful Primary Cementations,” paper SPE 8253,presented at the 54th SPE Annual Technical Confer-ence and Exhibition, Las Vegas, Nevada, USA,September 23-25, 1979.

Circulation: Mud ConditioningPrimary cementing operations often havemultiple objectives. On long intermediatecasing strings, a complete cement sheathfrom bottom to top is preferred, but a goodseal near the bottom of the string andaround the casing seat is all that may berequired, making the casing seat the primaryand the full cement sheath the secondaryobjectives. For liners, isolation away fromthe shoe (bottom) may be important as wellas a seal at the liner-casing overlap (top).Cementing goals dictate job designs. Tosolve cementing problems, better under-standing and application of fluid flow, dis-placements and placement are requiredalong with careful design of mud systems,spacer fluids and cement slurries. Cementplacement is important in most cases; mudremoval is critical on all cementing jobs.

The accepted procedure is to circulate andcondition before cement jobs.9 However, inthe past, there were few guidelines for theseprocedures, except generally to reduce mudviscosity, gel strength and fluid loss; maxi-mize standoff—casing centralization; usepreflushes—chemical washes and spacers toseparate mud and cement; move thepipe—rotate or reciprocate; circulate a min-imum of two hole volumes and pump at

high rates. Also, until a few years ago, criti-cal flow-rate calculations assumed that cas-ing was perfectly centered in the hole. How-ever, the critical flow rate correction toaccount for casing eccentricity is significantand must be taken into consideration (top).In the early 1990s, eccentricity was firsttaken into consideration in designs and inthe field by using WELLCLEAN optimal mudremoval service in the CemCADE software(above).

Gelled mud must be removed from theannulus before placing cement, but mud inthe narrow side of an eccentric annulus isoften difficult to move. Casing standoff fromborehole walls is less than 100% even invertical wells, and frequently no higher than85%. At low flow rates, drilling mud withhigh yield stress and gel strength can bestatic in the narrow gap of an eccentric

48 Oilfield Review

■■Annular flow regimes. Fluids calculated to be in turbulent flow, assuming perfectly cen-tered casing, are now known to be turbulent only in part of the annulus. In fact, threeflow regimes—no flow, laminar and turbulent—can coexist in an annulus, which meansthat mud may be removed effectively on the wide side, while on the narrow side mud isstatic, resulting in a channel. Between the extremes of no flow on the annulus narrowside and full turbulent flow around the annulus, mud removal may be poor, unless lami-nar flow displacements are properly designed.

Increasing flow rate

Decreasing standoff

Turbulent flow

A B

Flow Regimes

Laminar flow

No flow A B

ing for casing eccentricity. In the absence ofpipe movement, frictional pressure drop anddensity differences are the only forces actingto move mud. Mud yield strength must beless than the wall shear stress generated byfrictional pressure drop from viscous forcesfor mud to flow in narrow gaps. Wall shearstress can be increased by higher flow rates,improved standoff and increasing density dif-ferences, or mud gel strength can bereduced before casing is run.

Another consequence of uneven velocityprofiles is coexistence of different flowregimes. In an eccentric annulus, mixedflow regimes are possible if critical flow ratefor turbulence is calculated, as in the past,based on a concentric annulus, a commonassumption in drilling hydraulics models.For fluids exhibiting yield stress and gelstrength like muds and cements, it is possi-ble for three annular flow regimes to coex-ist—no flow if wall stress is less than fluidyield strength on the narrow side of theannulus, turbulent on the wide side andlaminar in between (right).

Dis

tanc

e fro

m s

hoe,

m

0

1

2

3

4

5

6

10

7

8

9

MudCement Spacer

ExperimentTheory

Effi

cien

cy, %

100

75

50

25

00 1 2 3 4 5 6 7

Hole volumes pumped

STO = 75%Displacement Efficiency

STO = 50%

STORate

40%8 bbl/min

60%2 bbl/min

60%5 bbl/min

wsnsws wsnsws wsnsws wsnsws wsnsws

40%2 bbl/min

50%8 bbl/min

■■Mud, spacer and cement distribution forvarious displacement rates, standoffs andspacer properties. In the base case (far left),mud and spacer channels were left alongthe length of a simulated annulus in thisfull-scale flow loop. As displacement ratewas increased, mud was displaced fromthe annulus narrow side, but full cementplacement did not occur because interfa-cial velocity was low. Increasing standoff(STO) had a dramatic effect on mud dis-placement and cement placement (middleand bottom), but further rate increaseunder these conditions did not significantlyimprove cement placement. Rate is, there-fore, important in mud displacement, butless influential in cement placement. Bet-ter standoff, higher rate and a thin spacerfor more effective turbulent flow also had apositive impact on cement placement,highlighting the importance of proper fluidrheology designs, especially for spacers(far right). (From Lockyear and Hibbert, refer-ence 2 and Tehrani et al, reference 6.)

49Summer 1996

The Annulus: Removing Mud, Placing CementA better understanding of annular displace-ment emerged in the late 1980s and early1990s.10 Previously, casing eccentricity, orstandoff, was not considered in designs,even though it was known to be a factor inchanneling and primary cementing failures.Competent cement sheaths and a good sealdepend on effective mud removal by turbu-lent or, under certain conditions, laminarflow. But fluids calculated to be in turbulentflow assuming perfectly centered pipe mightactually bypass mud in an eccentric annulusbecause fluid velocities vary radially aroundeccentric casing. Now CemCADE cement-ing design and evaluation software can beused to make mud circulation, annular dis-placement and cementing recommenda-tions based on actual well geometry, casingstandoff and fluid rheologies (right).

Even if mud gel strength is broken duringcirculation and conditioning, the questionof whether cement will flow into the narrowannulus gap needs to be answered. Ifcement flows primarily on the annulus wideside and leaves a slow-moving mud orspacer channel in the narrow side, goodcement placement and zonal isolation willnot be achieved. Cementing, therefore, canbe considered in two parts: mud removaland cement placement—uniform cementflow without channeling—which bothdepend on proper displacements up theannulus and down casing. Increasing stand-off improves mud displacement and cementplacement; displacement rate is importantfor effective turbulent mud removal (previ-ous page, bottom).

Displacing mud with spacers in turbulentflow is one of the most effective and widelyaccepted cementing techniques. Turbulent-flow mud removal dates back to the 1940s.It was subsequently recognized that turbu-lent scavenger displacing fluids—pre-flushes—placed in contact with formationsfor about 10 minutes improved mudremoval.11 Increasing displacement rateimproves turbulent mud removal. And thin,less viscous spacers like water and surfac-tants that can easily be placed in turbulentflow at low pump rates work best, probablybecause of combined drag, erosion and

CemCADE Design and Evaluation

Not met

Prepare customerreports, printouts

and plots

View “EfficientTime” or “Efficient

Volume” Plots

Enter pumpingschedule andrun simulation

Acceptable rate

Select pumpingrate using

“design rateselection”

Rate notacceptable

If standoff OK

Designcentralizersbased on

eccenteredflow analysis

Centralizer data fromdata base or userenters vendorcentralizer information

If standoffnot OKIf OK

Pressure margins

If notOK

Foamed cementplacementPPA-gas migration

Enter allsequences

Enter fluids

Enter well data Administration, well, casing,caliper, survey and formationFluid editor: rheologies,

slurry design, APIdata, spacer design,wash design, chemicalsand materials

Evaluate displacementcriteria using “EccenteredFlow” screen: Turbulent orEffective Laminar Flow (ELF)versus hole size, standoffand rheology

Mud removal criteria met

3D survey (if significant),Efficient Time/Volume,well security andcontrol, andsurface pressure plots

■■Computer-assisted cement job designs. CemCADE software can be used to make mudcirculation, annular displacement and cement placement recommendations based onactual well geometry, casing standoff and fluid rheologies.

10. Bittleston S and Guillot D: “Mud Removal: ResearchImproves Traditional Cementing Guidelines,” Oilfield Review 3, no.2 (April 1991): 44-54.

11. Brice JW and Holmes BC: “Engineered CasingCementing Programs Using Turbulent Flow Tech-niques,” Journal of Petroleum Technology 16 (May 1973): 503-508.Clark CR and Carter LG: “Mud Displacement WithCement Slurries,” Journal of Petroleum Technology25 (July 1973): 775-783.

dilution at interfaces due to turbulent eddies(below left ). Chemical washes shouldalways be used, but weighted spacersdesigned for turbulent flow—low rheologiesand temperature stability—can be usedunder some conditions if required. The max-imum wash or spacer volume without com-promising well control should be recom-mended or the 10-minute annular contacttime should be used. Even moderate chemi-cal wash volumes used with spacers reducemud viscosity and are preferable to spacersalone.

Pump rates to achieve turbulence on theannulus narrow side depend on hole dimen-sions and casing standoff. However, achiev-ing turbulence around the entire annulus,even on the narrow side, requires highpump rates in large casing that may not bepractical because of surface equipment lim-its or fracture gradients. Achieving mudremoval by turbulent flow becomes harderas hole sizes get larger and standoffdecreases, and is even more difficult whenweighted spacers are used. Turbulent flowcriteria for annular mud removal require tur-bulence around the entire annulus, includ-ing the narrow side, thin preflushes in con-tact with formations for 10 minutes, andsimilar displacing and displaced fluid densi-ties (above).

50 Oilfield Review

Position around a 50° STO annulus0° 90° 180°Narrowside

Wideside

Loca

l to

aver

age

velo

city

ratio

0

1.0

2.0

0.5

1.5

2.5Velocity Profile

3-lbm/bbl xanthan polymer2-lbm/bbl xanthan polymer0.6-lbm/bbl xanthan polymerWater

Displacing fluids:

■■Various viscosity fluids displacing a 3-lbm/bbl xanthanpolymer. Thin fluids like water displace thicker, more viscousfluids because of increasing turbulence.(From Lockyear, Ryanet al, reference 6.)

Turbulent FlowDisplacement Criteria

Preflushes in turbulence allaround the pipe

+

+

Preflushes in contact withzones of interest for 10 min

Similar displacing and displacedfluid densities

When turbulent flow is not an option, thereis a need for properly designed mud dis-placements with spacers and cement in lam-inar flow. These designs are more compli-cated, but criteria have been established toensure displacement efficiency (below right).Effective laminar flow requires positive den-sity contrasts—10% is recommended when-ever possible—a minimum pressure gradient(MPG) to overcome mud yield stress andpositive rheological hierarchies to maintainincreasing friction pressure and minimizedifferential velocity between fluids. Positivedensity differential, which is independent ofhole geometry, helps generate a flatter, morestable interface and is the first condition tocheck. In cases where cement slurry density

is close to mud density and mud weight can-not be modified, spacer density range is lim-ited and it may not be possible to meet thiscriterion.

Yield stress of fluids being displaced mustbe exceeded by wall shear stress. Minimumpressure gradient defines the force neededto move drilling fluids in the annulus narrowgap and should also be applied prior tocementing during mud circulation to ensurethat all the mud is moving and recondi-tioned. Below this force some mud remainsimmobile on the narrow side of the annulus.When mud is displaced by heavier fluids inlaminar flow, a density differential helpsmeet this condition by contributing to wallshear stress (next page, top left ). MPG veri-fies fluid mobility and defines a lower flow-rate limit to ensure that flow occurs allaround the annulus.

The differential between frictional pres-sures generated by fluids should be at least20% to increase interfacial stability. Other-wise the displacing fluid tends to bypassfluid ahead. Under laminar flow, spacerswith higher rheologies—thicker or moreviscous than the mud—are most effective(next page, top right). This is equivalent tohaving apparent mud viscosity lower thanthat of the displacing fluid for a given flowrate and annular geometry. The frictional

Effective Laminar Flow(ELF) Displacement Criteria

Minimum pressure gradient (MPG)

Positive density hierarchy

Positive frictional pressure hierarchy

Minimum differential velocityat interfaces

+

+

+

■■Recommendations for ELF displacements.These conditions should be applied to bothmud-spacer and spacer-cement interfacesthroughout the zone of interest. The differen-tial velocity criterion is optional because it is difficult to achieve, but should be appliedwhenever possible to get good displace-ment up to the designed top of cement.

51Summer 1996

ExperimentTheory

Effi

cien

cy, %

100

75

50

25

00 1 2 3 4 5 6 7

Annular volumes pumped

∆ρ = 16%Displacement Efficiency

∆ρ = 2%

1.61.291.161.0

Position around a 60% STO annulus

Loca

l to

aver

age

velo

city

rat

io

00° 90° 180°

Velocity Profile

Narrowside

Wideside

1.0

0.5

1.5

2.0

2.5 Displacing fluid specific gravity (SG):

ExperimentTheory

Position around a 50° STO annulus

Loca

l to

aver

age

velo

city

rat

io0

1

2

0° 90° 180°

Velocity Profile

Narrowside

Wideside

Displacing fluid:3-lbm/bbl xanthan polymer

3-lbm/bbl xanthan polymer2-lbm/bbl xanthan polymer0.6-lbm/bbl xanthan polymerWater

Displaced fluids:

Case 1

Case 2

Effi

cien

cy, %

100

75

50

25

00 1 2 3 4 5 6 7

Annular volumes pumped

Displacement Efficiency

pressure criterion is important and an ini-tial check should be always be made. Ifthere is not at least a 40% friction pressuredifferential between mud and cement, bothspacer and cement cannot meet this condi-tion and rheological properties must bechanged by reducing mud yield point, den-sity and solids contents to a minimum dur-ing mud conditioning prior to cementing orby increasing spacer and cement rheology(plastic viscosity and yield point). Improv-ing casing standoff and increasing densitydifferentials also helps satisfy this criterion.Friction pressure hierarchy and MPG estab-lish minimum flow rates.

Differential velocity around the annulus atfluid interfaces must be minimized to estab-lish a relatively flat interface. The combina-tion of density and frictional pressure differ-entials helps generate a relatively flat and

■■How differential velocity affects laminardisplacements. Friction pressure developsfaster on the annulus narrow side becauseof the smaller flow area (effective slot size),so the two friction pressure curves cross,since displaced and displacing frictionpressures increase at different rates. Tomaintain a stable interface between fluids,velocity must remain below the criticalvalue (Vc) represented by the intersectionof the two curves. And displacing fluidvelocity must be less than displaced fluidvelocity.

V2 V1 VcVelocity

Pre

ssur

e dr

op

Displaced mudor spacer (1)

Displacing spaceror cement (2)

<

■■How density (ρ) affects laminar flow displacements. Positive density hierarchies—increasing the density of eachsuccessive displacing fluid—greatly improve mud removaland minimize channeling because of buoyancy effects. Thegreater the differential density, the better the displacementefficiency (top left). Like the classic example of communicat-ing vessels from basic physics where liquids come to thesame level regardless of container size or shape, denser displacing fluids try to equalize in an eccentered annulus (top right). Increasing displacing fluid density greatlyimproves the interfacial velocity profile and displacementefficiency as shown by various specific gravity (SG) fluids dis-placing a 1.0 SG fluid (bottom). (From Lockyear, Ryan et al, ref-erence 2 and Tehrani et al, reference 6.)

■■How viscosity affects laminar displacements. A positive rheological hierarchy between displacing and displaced fluids at a low flow rate (Case 2 top) results in more efficient displacement than displacing and displaced fluids of similar rheologies at a high flow rate (Case 1 top) Thick fluids displacethin fluid more uniformly than the reverse. Interfacial velocityon the annulus narrow side improves as displacing fluid plas-tic viscosity and yield point increase—higher rheologies—because of the large frictional pressure drops generated bymore viscous fluids (bottom). (From Lockyear, Ryan et al, refer-ence 2 and Tehrani et al, reference 6.)

stable interface and reduce the possibility ofone fluid fingering or channeling throughanother. The sum of gravitational and fric-tion forces for displacing fluids in the wideside must be greater than those of the fluidbeing displaced on the narrow side of theannulus to balance forces so flow is uniformaround the annulus. This condition can besatisfied if annular flow rate is below a criti-cal value (right).

Annular velocity differential can be mini-mized by reducing mud yield point duringconditioning, maximizing standoff, meetingdensity and friction pressure heirarchy con-ditions by using viscous weighted spacers,displacing at low pump rates and movingthe pipe. When displacement rates are toohigh, displacing fluids tend to flow faster inthe wide side of the annulus, regardless ofgravitational effects that tend to flatten theinterface. Therefore, differential velocity cri-

teria establish maximum annular flow ratesand contradict “pump-as-fast-as-you-can”philosophies.

Unlike turbulent displacements in whichannular flow is maintained above a criticalrate, displacements by ELF must be main-tained between maximum and minimumrates. In turbulent flow, preflush volume isdetermined from the 10-minute contact timeat a critical rate. For ELF displacements,spacer volumes should be at least 500 ft[150 m] of annular fill, with a 60 bbl [10 m3]minimum. Increased wellbore inclinationreduces displacement efficiency by decreas-ing gravitational effects, but this reductioncan be compensated for by optimizing pumprates and fluid rheologies. Complicated lami-nar displacements highlight how properlydesigned spacers are essential in annularmud removal.

Down Casing: Displacing CementMuch effort goes into selecting proper flu-ids, flow regimes and displacementmechanics to remove mud from the annulusand place cement. This usually meanspumping fluid stages with increasing densi-ties. For downward flow inside pipe, how-ever, a positive density hierarchy is counterto effective displacement. Mixing and con-tamination occur when interfaces betweenfluids are unstable or displacing fluidsbypass—fall through—fluids ahead, prob-lems that can be overcome by using wiperplugs for mechanical separation. Sometimesonly one bottom wiper plug is run, but moreoften, none is used.

After investigation of primary cementingfailures in which fluid mixing inside casingwas a possible cause, P. Valkó performed anin-depth study of frictional and gravitationalforces on fluids flowing downward inpipe.12 The mechanics of heavier fluids dis-placing lighter fluids down casing whenwiper plugs are not used were defined, andmethods were developed to calculate dis-placement efficiency and interfacial bound-ary shapes. This project was based on ear-lier work involving upward flow in annuli

52 Oilfield Review

completely effective, demonstrating theneed for mechanical separation—bottomwiper plugs.

Incomplete fluid displacement inside cas-ing is likely to mean an unsuccessfulcement job (left). The tendency for upwardflow at interfaces can cause spacer orcement leading edges to be contaminated orcomplete mixing of mud, spacer andcement, leading to inefficient mud removal.Extreme viscosity increases and correspond-ing high pump pressures can also result ifslurries and muds are incompatible. Fluidmixing can have disastrous results, includ-ing appearance of premature set if incom-patibility is severe enough. It is also possiblefor displacing fluids to bypass fluids thatwere pumped ahead. This is often evidenton pressure charts in the form of early liftpressure and from returns at the surface asheavier fluids bypass lighter fluids and “turnthe corner”—U-tube—from the casing intothe annulus sooner than expected.

Cement contamination by spacer or mudcan change slurry rheology or retard thick-ening time, as evidenced by friction pres-sure increases during displacement orapparent lack of set cement on evaluationlogs. In some cases, mixing may be only atthe slurry leading edge and result in lowerthan expected cement tops or low-strengthcement up hole. It is also possible for tail

Subsequent work with this software showsthat there may be three forms of displace-ment inside pipe (next page, top). Fluidinterfacial boundaries may form smoothparabolas with moderate displacement effi-ciency or there may be an outer cylinder ofthe first fluid that is not moving, so effi-ciency is lower. It is also possible to have aregion where the first fluid tends to moveupward, in opposition to primary flow, sodisplacement efficiency is quite low. Incementing applications it is not possible forfluids in the casing to flow up because ofthe cementing head, but this force can leadto a high degree of mixing at fluid inter-faces. As expected, displacement is never

Spacer

Mud MudInterfacialboundary

■■Velocity profiles for displacement inside pipe. Overall flow direction was defined to bedownward, but allowed to be locally positive (down) or negative (up). Arrows representvelocity relative to radial position at an axial location. To compute interfacial boundaryshape, computations are made along the entire length of the pipe. A software calledMathematica (version 2.2.3, Wolfram Research) derived displacement calculation rou-tines for displacement efficiency versus time and interfacial boundary position at varioustimes during displacement, using fluid density, yield stress, plastic viscosity, pipe lengthand diameter, and pump rate.

Retarded (delayed) cement set time

Poor zonal isolation

Unset cement at liner tops

Lack of hard cement in “shoe tracks”

High displacement pressures fromviscous incompatible fluids mixtures

Problems associated withincomplete casing displacement

and packed, fluid-filled columns (above).13

The software to make these calculationsuses fluid densities and rheologies alongwith gravitational effects, assuming vertical,laminar flow and no mixing.14 This softwareis only qualitative and not a simulation, andcannot determine when bottom wiper plugsshould not be run.

53Summer 1996

slurries to fall through lead slurries; in thiscase, cement evaluation logs may showgood cement bond across most of the inter-val, but poor cement at the bottom, wheregood, strong tail cement should be. Theremay also be spotty occurrences of good andbad cement. In some cases, no evidence ofcement may be found even after severaldays because of complete mixing and retar-dation of cement by spacer.

Two common problems are failure ofcement to provide a seal at the shoe andlack of hardened cement in shoe tracks(float joints) during drill out. Shoe failuremay be related more to formation character-istics where casing is set than to cement jobquality, but there are cases when slurriesbypass spacers and the cement seal is actu-ally being tested.

Displacement efficiency also affectscement quality in shoe joints. If bottomplugs are not run and cement bypassesspacer or mud, the top wiper plug can pushbypassed spacer and mud into the shoejoint. Since wiper plugs stop at float collars,there may also be low-quality cement ormixed fluids between the float collar andfloat shoe. Even when bottom plugs are run,cement may bypass other fluids in the shoetrack. Also, float collar outlet orifices estab-lish a thin fluid jet through casing or liner

12. Valkó P: Fluid Displacement in Pipe. College Station, Texas, USA: Texas A&M University, October 30, 1994.

13. Flumerfelt RW: “Laminar Displacement of Non-Newtonian Fluids in Parallel Plate and Narrow GapAnnular Geometries,” SPE Journal 15 (April 1975): 169-180.Beirute RM and Flumerfelt RW: “Mechanics of theDisplacement Process of Drilling Muds by CementSlurries Using an Accurate Rheological Model,”paper SPE 6801, presented at the 52nd SPE AnnualTechnical Conference and Exhibition, Denver, Col-orado, USA, October 9-12, 1977.Hill S: “Channelling in Packed Columns,” ChemicalEngineering Science 1, no. 6 (1952): 247-253.Flumerfelt RW: in B Elvers, ed: Ullman’s Encyclope-dia of Industrial Chemistry, vol. B1. Cambridge, Eng-land: VCH Publishing (1990): 4-35.

joints below float collars, compounding adifficult situation.

Sensitivity analyses using this new soft-ware indicate that effective displacementinside casing cannot be achieved by modi-fying fluids without adversely affectingannular displacements. Properties that mightinfluence interface shape and displacementefficiency include average velocity, yieldpoint, density, plastic viscosity and pipesize. Displacement efficiency improves asflow velocity and yield point differencebetween bottom and top fluids increase.Efficiency decreases as fluid-density differ-ences increase; even at similar densities,displacement is only 70% after a pipe vol-ume of fluid is pumped. Differences in plas-tic viscosity have little effect on displace-ments in the range of geometries and shearrates studied. As pipe sizes increase, dis-

placements become more inefficient, and inlarger pipe sizes, reverse flow of lighter flu-ids causes unstable conditions.

Although there are often acceptable resultswhen bottom plugs are not used, theory andfield data indicate that mechanical separa-tion at each interface is the only way toensure that competent fluids leave the cas-ing and enter the annulus. This work sug-gests that bottom plugs should be usedwhenever possible and that many undesir-able results can be explained by the phe-nomenon of heavier fluids “falling through”or mixing with fluids being displaced aheadin the casing. Running bottom wiper plugsis strongly recommended and, in criticalcases, bottom plugs should be run at eachinterface (see “Using Multiple Wiper Plugs,”next page).

Increasing casing size or density difference between fluids

0

60

80100

20

40

0 1 2 3 4 5 6

Dis

plac

emen

tef

ficie

ncy,

%

Normalized time, t

0

60

80100

20

40

0 1 2 3 4 5 6

Dis

plac

emen

tef

ficie

ncy,

%

Normalized time, t Normalized time, t

00 1 2 3 4 5 6

Dis

plac

emen

tef

ficie

ncy,

%

60

80100

20

40

t=1 t=1 t=1

■■Displacement efficiencies (top) and fluid interfacialboundary shapeswhen the leading edgereaches the end ofpipe (bottom). Depend-ing on fluid properties,pipe (wireframe) diameter and flowvelocity, the interfacebetween fluid stagesmay be stable andapproach the shape ofa parabola (left). Theremay be a region inwhich the lighter bot-tom fluid is static andthe heavier top fluid isflowing down throughthe middle of the pipein an internalparabola (middle).Or there may be aregion where thelighter fluid is flowingupward, counter to theprimary downwardflow direction (right).

14. Wolfram S: Mathematica‚ A System for Doing Mathematics by Computer, 2nd ed. Reading, Mas-sachusetts, USA: Addison-Wesley Publishing Com-pany, 1993.

Use of the EXPRES Extrusion Plug Release

System, a next generation cementing head,

continues to expand. This innovative design

automates release procedures and gives a

positive indication of plug launch. Plugs are held

in a basket below the head and inside casing so

that cementing fluids—chemical washes,

spacers and cement slurries—can flow around

the basket (right). Over 2000 lb of force from a

hydraulic ram launches the plugs, minimizing

chance of premature or accidental release.

Mechanical stops in the launcher provide an end

to each phase of the job. An oil-level gauge

indicates launcher-rod position and gives a clear

indication of plug departure. Top plug departure

is verified by sensors mounted on the casing that

detect drillable magnets in the plug, sounding a

horn and sending a signal to the cementing unit.

Modular design, quick-latch connectors and

remote operating capability save rig-up and job

execution time. This means better mud

conditioning prior to cementing and the unique

ability to launch plugs on the fly—without

interrupting pumping—which reduces U-tube

effects and improves mud removal. High

pressure ratings allow pressure-integrity testing

immediately after cementing, saving rig time and

reducing possibility of forming a microannulus.

An exclusive wiper plug fin design ensures

complete fluid separation and effectively wipes

casing walls, so cement slurry reaches the float

collar without being contaminated. Exposure to

high pressure is minimized by remote control and

light, well-balanced modules make the EXPRES

system easy and safe to handle.

The concept, developed several years ago, of

preloading plugs in a basket has been expanded

from two plugs to three plugs and to subsea

cementing using a Surface Dart Launcher (SDL)

and Subsea Tool (SST) (next page). The first

DeepSea EXPRES prototype was used off the west

coast of Africa in mid-1994 and two other

prototypes were placed in service in the Gulf of

Mexico earlier this year. Over 28 jobs have been

performed with these tools. The SDL holds

identical darts, which are individually released

from surface during cementing jobs. These darts

launch the wiper plugs when they reach the

downhole SST, but unlike free-falling balls, are

pumped down drillstrings to separate fluids and

wipe pipe walls. Other advantages over dropping

balls include positive fluid displacement and

elimination of the time and uncertainty of waiting

for balls to reach bottom.

The heart of DeepSea EXPRES, the downhole

SST, allows use of high-performance, easily

drillable EXPRES plugs with simplified designs

that eliminate problems associated with pumping

fluids through wiper plugs. The tool retains wiper

plugs, preloaded in a basket with over 2000 lb

force, until they are launched by arrival of a dart

from the SDL. Friction holds plugs in place during

pumping operations. The current design accepts

up to three 8 5/8- to 13 5/8-in. plugs, or two 16- to

20-in. plugs that are under development. During

circulation, mud flows down the drillpipe,

through a sliding sleeve and out two orifices into

the casing-SST annulus. When a dart reaches the

tool, drillpipe pressure forces the sliding sleeve

down, ensuring that each dart travels a full

length. Continued pumping forces the dart and

rod down, pushing a plug out of the basket. After

a dart reaches its final position, a spring retracts

the sliding sleeve to ensure complete,

unobstructed flow through the orifices. Darts

remain in the holder and are retrieved with the

tool after the job.

Rod travel is slowed by a shock absorber filled

with hydraulic oil that flows past a small gap

Clamp

2-in.inlet

Casingadapter

Casingcollar

Wiperplugs

Plugbasket

Casing

Hydraulic launcher

Wiperplugfins

■■EXPRES cementing head. The automated ExtrusionPlug Release System improves mud circulation andconditioning, and cement job quality in addition toreducing high-pressure hazards. Plugs are held in abasket below the head and inside casing that cement-ing fluids can flow around. Over 2000 lb of force froma hydraulic ram launches plugs, minimizing chance ofpremature or accidental release. A safety latch pre-vents top plug release until the hydraulic ram beginsits final stroke.

Using Multiple Wiper Plugs

54 Oilfield Review

between the rod piston and bore. The resulting

pressure differential resists rapid movement

and stops the rod after plugs are released.

Combined with plug friction, this causes a

1500 psi [10,350 kPa] pumping pressure increase

and provides a positive indication of plug launch.

Three brass shear pins increase top-plug release

pressure to 3000 psi [20,700 kPa]. A sleeve

holding these pins slides down, but remains

inside the basket after the top plug leaves the

tool. Spacers that keep plugs from sticking

together also slide down the basket and are

retrieved with the tool.

Systems are also available to improve liner

cement jobs. In the past, one pump-down plug

and a top plug were used, but new top and

bottom, four-plug systems prevent cement

contamination inside liners. Spacer is pumped

down drillpipe followed by a pump-down plug,

cement slurry, another pump-down plug and

displacement fluid. The first pump-down plug

passes through the top wiper plug and into the

bottom wiper plug at the top of the liner where it

latches into a catcher. Pressure shears pins

attaching the bottom wiper plug to a mandrel and

the plug is pumped down the liner to the float

collar. A further increase in pressure shears the

catcher from the bottom wiper plug, allowing it to

move into a circulating tube, which permits

cement slurry to pass through float equipment

into the annulus. The second pump-down plug

latches into the top wiper plug, which is

displaced through the liner until it reaches the

bottom wiper plug where it forms a seal.

■■The heart of DeepSea EXPRES. The downhole Sub-sea Tool (SST) allows use of high-performance, easilydrillable EXPRES plugs with simplified designs thateliminate pumping fluids through wiper plugs. Duringpumping operations, wiper plugs, preloaded in a bas-ket with over 2000 lb force, are held in place inside abasket until they are launched by arrival of a dart fromthe Surface Dart Launcher (SDL).

Spring

Orifice

Rod

Hydraulicshock

absorber

Bottomplug being

released

Sliding sleeve

First dart

Dart holder

Hydraulic oil

Shear pins

Plug basket

Top plug

Plug spacers

1. Drelkhausen H: “Quality Improvement of LinerCementations by Using Bottom and Top Plugs,” paper SPE/IADC 21971, presented at the SPE/IADCDrilling Conference, Amsterdam, The Netherlands,March 11-14, 1991.

Cement and Spacer MixingA mixed 95/8- by 97/8-in. intermediate cas-ing string was set at 12,673 ft [3863 m] inthe Gulf of Mexico by Anadarko PetroleumCorporation. A bottom wiper plug was runbetween mud and spacer. From all indica-tions, pipe was cemented normally and thejob was successful. On surface, full returnswere taken and samples for quality controlset up as expected. However, two days aftercementing, while testing casing to 5000 psi,pressure dropped to zero. After casingintegrity was checked with a packer andfound to be intact, the float shoe wasdrilled, but no cement was found. After pri-mary cementing, the well circulated aroundthe intermediate casing annulus during acement squeeze. Evaluation with CBTCement Bond Tool, CET Cement EvaluationTool and USI UltraSonic Imager logs indi-cated no cement with strength.

Common problems with cement harden-ing and over-retardation by cement addi-tives were ruled out as causes, but tests oncement-spacer mixtures indicated that mod-erate amounts of spacer could cause longsetting times. A total of 382 bbl [60.6 m3] ofcement and 80 bbl [12.7 m3] of spacer wereused. If these two fluids mixed completely,the ratio of spacer to cement would beabout 17%. Cement contaminated by 20%spacer attained a compressive strength ofonly about 25 psi in 48 hours, whichmatched the actual behavior observed in thefield. Cement-mud mixtures were evenmore retarded.

Software to evaluate casing displacementswas not available during this investigation,but mixing due to poor rheological dis-placement and cement retardation byspacer were suspected. Later, displacementcalculations using these well conditionsshowed that the spacer-cement interfacewas unstable and displacement efficiency

55Summer 1996

was well below 50% (above left). Running abottom wiper plug only between mud andspacer allowed cement to fall through andmix with spacer.

Tail Bypassing Lead SlurryIn Balikpapan, Indonesia, Unocal cementeda long, 7-in, liner with two slurries—12.5ppg lead and 15.8 ppg tail. The liner topwas at 2240 ft [683 m] and the bottom wasat 9844 ft [3000 m]. In liner applications, ofcourse, an added difficulty is dropping bot-tom plugs, and in this case, the problemwas compounded because viscosities had tobe kept low to avoid fracturing the well dueto high friction pressures. During displace-ment, high frictional pressures resulted inthe premature termination of the job, leav-ing cement in the liner. Evaluation of dis-placements for this liner cement job indi-cated that lead slurry fell through spacerand tail slurry fell through the lead.

Interfacial boundary shapes betweenspacer and lead slurry, and lead and tailslurries show a tendency for reverse flow oflighter fluids at the interface in both cases,indicating high likelihood of fluid mixingbetween stages. Calculations also showlow displacement efficiencies—10 and20% (above right ). Tests on cement andmud mixtures resulted in high viscositiesthat correlated with high displacementpressures during the actual job.

Integrating Fluid Services Quality cement jobs depend fundamentallyon the ability to predict and manage fluidsand displacement performance over a widerange of conditions. Personnel training,from management through engineering tofield operations, is high on the list of issuesthat must be addressed to properly integratedrilling and cementing fluids and imple-ment total fluids management. Mud engi-neers do not have to run cement pumpsand cementers do not have to supervisedrilling fluids programs, but it is helpful ifeach understands the other’s needs. If theentire fluids process is to be optimized,cooperation must develop through appreci-ation of needs and intentions of the otherdiscipline. Formal crosstraining must besupplemented by practical experience, withthe goal of establishing wellsite “fluids-engineering” teams dedicated to optimizingall fluid operations.

Rather than view other services from afar,drilling fluids engineers and cementers need

to cooperate in designing structured fluidsequences—fluids trains—for wells. Atwellsites, cementers should gain hands-onfluids experience as backup mud engineersand act as mentors to mud engineers duringcementing operations. At offshore andremote locations where engineers reside onlocation, this approach can be formalizedwith one service-line specialist acting asteam leader in addition to performing pri-mary product-line responsibilities. Effectiveteam leaders must be experts in their pri-mary field, familiar with other disciples andbe good communicators. With availablefluids technology, efficiencies can be foundin cooperation and interfacing between flu-ids services, and between fluids teams andoperators. By restructuring the approach towell construction fluids, savings are avail-able with no up-front increase in either costor risk. —MET

56 Oilfield Review

95/8-in. casing

MUDPUSH XS/SALTBONDCement Slurry

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60

80100

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7-in. liner

MUDPUSH/Lead Slurry

Lead/Tail Slurries

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ficie

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%

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40

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80

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60

■■Gulf of Mexico case history. On this intermediate-casing primarycement job, a bottom plug was run between mud and spacer.Since there was no plug between spacer and cement, cement couldmix with spacer while flowing down casing. Displacement effi-ciency is less than 50% when cement reaches the bottom of thestring. The interfacial boundary shape highlights the magnitude ofthe problem. There is a region of no spacer flow around the insidediameter of the pipe as cement flows down through the center. Thisplot assumes no interfacial mixing, but in reality, there is probablya high degree of interfacial mixing between the two fluids.

■■Balikpapan, Indonesia case history. Efficiency plots show verylow displacement—10 and 20%, respectively—for interfacesbetween lead cement and spacer, and lead and tail slurries forcementing operations on this long liner. Interfacial boundaryplots also show a region of negative velocity, indicating highlikelihood of interfacial mixing between fluids.