well completion introduction

TU PE 4063/6463 Well Completion Fall 2010

Dr. Evren M. Ozbayoglu, Tel: 918-631 2972, e-mail: [email protected] Chapter-1, 1/21

PE 4063 / 6463 Well Completion

CHAPTER 1 - Introduction

Schlumberger Oilfield Glossary:

The hardware used to optimize the production of hydrocarbons from the well. This may range

from nothing but a packer on tubing above an openhole completion ("barefoot" completion), to a

system of mechanical filtering elements outside of perforated pipe, to a fully automated

measurement and control system that optimizes reservoir economics without human intervention

(an "intelligent" completion).

A generic term used to describe the assembly of downhole tubulars and equipment required to

enable safe and efficient production from an oil or gas well. The point at which the completion

process begins may depend on the type and design of well. However, there are many options

applied or actions performed during the construction phase of a well that have significant impact

on the productivity of the well.

Introduction

The word "completion" means the conclusion of a borehole that has just been drilled. Completion

is, therefore, the link between drilling the wellbore and the production phase. Completion

involves all of the operations designed to make the well produce, in particular connecting the

borehole and the pay zone, treating the pay zone, equipping the well, putting it on stream and

assessing it. Pay zone is the reservoir rocks which contain oil and/or gas that can be recovered.

Generally speaking, certain measurement and maintenance operations in the well along with any

workover jobs that might be required also come under the heading of completion are considered.

Therefore, completion begins with well positioning and ends only at well abandonment.

Whatever the operational entity in charge of well completion and workover, its actions are greatly

influenced by the way the well has been designed and drilled and by the production problems the

reservoir might cause. The "completion man" will therefore have to work in close cooperation

with the "driller" (who may both work in one and the same department), and also with reservoir

engineers and production technical staff.



After a well has been drilled, it must be properly completed before it can be put into production.

A complex technology has evolved around the techniques and equipment developed for this

purpose. Consequently, the selection of materials, equipment and techniques should only be made

following a thorough investigation of the factors which are specific to the reservoir, wellbore and

production system under study.

Thus, completion engineer should be in coordination of many different professionals. As seen

from the following figure, the completion engineers should be in contact with drilling engineers,

reservoir engineers, production engineers, geologists, etc. Therefore, completion process required

a massive teamwork.

There are three basic requirements of any completion (in common with almost every oilfield

product or service). A completion system must provide a means of oil or gas production (or

injection) which is;

i) Safe

ii) Efficient

iii) Economic

Current industry conditions may force operators to place undue emphasis on the economic

requirement of completions. However, a non-optimized completion system may compromise



long-term company objectives. For example, if the company objective is to maximize the

recoverable reserves of a reservoir or field, a poor or inappropriate completion design can

seriously jeopardize achievement of the objective as the reservoir becomes depleted. In short, it is

the technical efficiency of the entire completion system, viewed alongside the specific company

objectives, which ultimately determines the completion configuration and equipment used.

Well completion processes extend far beyond the installation of wellbore tubulars and equipment.

Installing and cementing the production casing or liner, as well as logging, perforating and testing

are part of the completion process. In addition, complex wellhead equipment and processing or

storage requirements effect the production of a well so may have some bearing on the design and

configuration of the completion.

As the understanding of reservoir and production performance has evolved, so the systems and

techniques put in place as part of the completion process. Early wells were drilled in very shallow

reservoirs, which were sufficiently consolidated to prevent caving. As deeper wells were drilled,

the problems associated with surface water prompted the use of a casing or conductor to isolate

water and prevent caving of the wellbore. Further development of this process led to fully cased

wellbores in which the interval of interest is selectively perforated. Modern completions are now

commonly undertaken in deep, hot and difficult conditions. With the simultaneous improvement

in seismic interpretation and drilling technology, wellbores can be precisely placed to optimize

production and enable effective reservoir management. There are clear economic benefits to be

gained from reducing the number of wellbores required for any reservoir development. However,

fewer, but more efficient wellbores require a greater emphasis to be placed on the design,

selection and installation of the completion equipment. Horizontal wellbores, and the technology

associated with their completion are becoming common in many fields. Drilling extended reach

wells often means that well servicing and intervention options are severely restricted, further

emphasizing the importance of correct design and installation of the initial completion

equipment. In all cases, achieving the completion objectives, and subsequent production targets

are a result of careful planning and preparation.



The following figure shows the well cost analysis of a well drilled for 10,000 ft. It can be seen

that, "completion equipment" accounted for approximately 10% of the total cost for the well.

Overall approximate cost for such a well is estimated to be 2.5 MM $.

Other1%Rental Equipment

2%

Personnel Logistics1%

Supervision2%

Site Preparation2%

Cementing6%

Bits & Coring6%

Directional Services8%

Logging & Perforating8%

Completion Tubulars & Equipment

10%

Drilling Fluid12%

Drilling Rig13%

Casing13%

Mob/Demob15%

Camp1%

Well Completion Planning

Planning a completion, from concept through to installation, is a complex process comprising

several distinct phases. Many factors must be considered, although in most cases, a high

proportion can be quickly resolved or disregarded. Ultimately, it is the predicted technical

efficiency of a completion system, viewed alongside the company objectives, which will

determine the configuration and components to be used.

Although many wells (and fields) may be similar, the success of each completion system should

be closely based on the individual requirements of each well. Therefore, generic design or

installation procedures should be carefully reviewed and amended as required. The economic

impact of designing and installing non-optimized completions can be significant. Consequently

the importance of completing a thorough design and engineering process must be stressed.



Delaying the commencement of the wells payout period is one example of how non-optimized

completion design, or performance, can effect the achievement of objectives. However, while

reducing installation cost and expediting start-up are important objectives, further reaching

objectives, such as long-term profitability must not be ignored. A more complex and costly

completion may provide a greater return over a longer period.

Principal phases of the general sequence in which completion design and installation factors

typically studied can be summarized as follows:

Establishing the objectives and design basis Determining the optimum well performance Establishing the conceptual completion designs Reviewing the strategy for life of the well and field Developing detailed completion design Planning of associated components, and service activities Preparation of offsite and onsite Installation Evaluation

Data Sources

In order to select the suitable completion type as well as conduct a proper completion design,

information should be gathered from different possible sources. Following figure summarizes the

sources that are used for this purpose.



Reservoir Parameters

The information about the reservoir can be obtained by formation and reservoir evaluation

programs such as coring, testing and logging. Typically, such data will be integrated by reservoir

engineers to compose a reservoir model. The reservoir structure, continuity and production drive

mechanism are fundamental to the production process of any well. Frequently, assumptions are

made of these factors, which later prove to be significant constraints on the performance of the

completion system selected.

Physical characteristics of the reservoir, such as pressure and temperature, are used in describing

reservoir and downhole conditions. The effects of temperature and pressure on many other

factors can be significant. For example, corrosion rates, selection of seal materials and the

properties of produced fluids are all affected by temperature and pressure. When investigating the

reservoir rock characteristics, the principal concern is assessing formation behavior and reaction.

This includes behavior and reaction to the drilling, production or stimulation treatments which



may be required to fully exploit the potential of the reservoir. The formation structure and

stability should be closely investigated to determine any requirement for stimulation or sand

control treatment as part of the completion process. The reservoir characteristics effecting

completion configuration or component selections are best summarized by reviewing the

reservoir structure, continuity, drive mechanism and physical characteristics.

Reservoir Parameters

Reservoir Boundaries o Structural traps o Stratographic traps o Unconformities o Permeability contrasts

Reservoir Structure o Continuity o Permeability barriers o Isotropy

Production Mechanism o Water drive o Solution gas o Gas cap o Combination o Injection o Artificial

Physical Parameters o Size o Shape o Height o Pressure



o Temperature

Rock Properties o Porosity o Permeability o Pore size distribution o Fluid saturation o Grain size and shape o Wettability

Rock Composition o Composition o Contamination o Clay content o Moveable fines o Cementaceous material o Scale forming materials

Reservoir pressure is the key parameter in the well's natural flow capability. If the reservoir

pressure is or becomes insufficient to offset production pressure drawdown (particularly the

hydrostatic pressure of the fluid column in the well and pressure losses), it is then necessary to

install a suitable artificial lift system such as pumping the fluids or lightening them by gas

injection in the lower part of the tubing (gas lift). If a reasonably accurate estimate of future

requirements in this area can be made at the time of initial completion, an attempt is made to take

them into consideration when completion equipment is chosen. Such a procedure can make later

workover easier or unnecessary. The change in reservoir pressure is physically related to

cumulative production (rather than directly to time) and to the drive mechanism(s) involved.



Injection wells may supplement the action of natural drive mechanisms such as one-phase

expansion, solution gas drive, gas cap drive or water drive. The injected fluid maintains pressure

(or slows down the pressure drop) and in addition flushes out the oil. Although the two functions

cannot be dissociated in practice, one of them (maintaining pressure or oil flushing) can more

particularly justify this type of well. Mostly water is injected, but gas may sometimes also have to

be injected.

Rock characteristics and the type of reservoir fluids will directly influence completion, especially

with respect to the well's flow capacity, the type of formation treatments that have to be

considered and the production problems that have to be dealt with. Let us mention the following

parameters in particular:

The nature and composition of the rocks The degree of reservoir consolidation The extent of reservoir damage The temperature The fluid's viscosity The fluid's corrosive or toxic properties The fluid's tendency to emulsify or lay down deposits



Produced Fluid Characteristics

Two conditions, relating to the chemical properties of the produced fluid most affect the physical

qualities of completion components and materials. These are chemical deposition (scale,

asphaltenes etc.) and chemical corrosion (weight loss and material degradation). The ability of

the reservoir fluid to flow through the completion tubulars and equipment, including the wellhead

and surface production facilities, must be assessed. For example, as the temperature and pressure

of the fluid changes, the viscosity may rise or wax may be deposited. Both conditions may place

unacceptable backpressure, therefore causes a dramatic reduction the efficiency of the completion

system. While the downhole conditions contributing to these factors may occur over the lifetime

of the well, consideration must be made at the time the completion components are being

selected. Cost effective completion designs generally utilize the minimum acceptable components

of an appropriate material. In many cases, reservoir and downhole conditions will change during

the period of production. The resulting possibility of rendering the completion design or material

unsuitable should be considered during the selection process.

Produced Fluid Characteristics

Physical Properties o Oil density o Gas gravity o Viscosity o Pour point o Gas-oil ratio o Water-oil ratio

Chemical Properties o Composition o Wax content o Asphaltenes



o Corrosive agents o Toxic components o Scale

The existence of interfaces between fluids, in particular when they are not controlled, causes a

decrease in target fluid productivity at the same time as an increase in unwanted fluids (water

and gas for an oil reservoir, water for a gas reservoir). Additionally, since these unwanted fluids

get into the well, they must be brought up to the surface before they can be disposed of. They,

therefore, not only penalize well productivity, but also instrumental in decreasing reservoir

pressure. This interface problem is more particularly critical when the viscosity of the target fluid

is more or less the same (light oil and water) or even much greater (heavy oil and water, oil and

gas) than that of the unwanted fluid. The interfaces vary with time, for example locally around a

well, by a suction phenomenon causing a cone (coning) which is related to the withdrawal rate.

They can also vary throughout the reservoir depending on the amount of fluid that has already

been withdrawn, allowing a gas cap or an aquifer, etc. to expand.

Coning; a) stable cone, b) water encroachment

Wellbore Construction

Wellbore construction factors can be categorized in the following phases;

i) Drilling The processes required to efficiently drill to and through the reservoir



ii) Coring and testing The acquisition of wellbore survey and reservoir test data used to identify

completion design constraints

iii) Pre completion stimulation or treatment final preparation of the wellbore through the zone

of interest for the completion installation phase.

It is an obvious requirement that the drilling program must be designed and completed within the

scope and limits determined by the completion design criteria. Most obvious are the dimensional

requirements determined by the selected completion tubulars and components. For example, if a

multiple string completion is to be selected, an adequate size of production casing (and

consequently hole size) must be installed. Similarly, the wellbore deviation or profile can have a

significant impact. Drilling and associated operations, e.g., cementing, performed in the pay zone

must be completed with extra vigilance. It is becoming increasingly accepted that the prevention

of formation damage is easier, and much more cost effective, than the cure. Fluids used to drill,

cement or service the pay zone should be closely scrutinized and selected to minimize the

likelihood of formation damage. Similarly, the acquisition of accurate data relating to the pay

zone is important. The basis of several major decisions concerning the technical feasibility and

economic viability of possible completion systems will rest on the data obtained at this time. A

pre-completion stimulation treatment is frequently conducted. This is often part of the evaluation

process in a test treat-test program in which the response of the reservoir formation to a

stimulation treatment can be assessed.

Wellbore Construction

Drilling o Hole size o Depth o Deviation o Well path o Formation damage

Evaluation o Logging



o Coring o Testing o Fluid sampling

Pre-completion o Casing schedules o Primary cementing o Pre-completion stimulation

For a development well, the most important thing is to have a borehole with a big enough

diameter to accommodate the equipment that will be installed in it. In contrast, when the pay

zone drilling diameter is increased above and beyond what is required for the production

equipment, it does not boost the well's flow capacity very much. Since the diameter depends on

the initial drilling program, this explains the saying that is sometimes used: "Completion begins

with the first turn of the bit". As a result, the drilling and casing program must be optimized

taking both drilling and production requirements into account, without losing sight of the flow

capacity versus investment criterion.

From the time the drilling bit reaches the top of the reservoir and during all later operations,

reservoir conditions are disturbed. Because of this, problems may arise in putting the well on

stream. In particular, the pay zone may be damaged by the fluids used in the well (drilling fluid,

cement slurry, etc.), and this means reduced productivity. Depending on the case, productivity

can be restored relatively easily (generally true for carbonate formations: limestone, dolomites,

etc.). It may prove to be difficult or even impossible for sandstone formations. In any case, it

requires costly treatment in terms of rig time and of the treatment itself. Formation damage

should not be seen simply in terms of the cure but also in terms of prevention, especially when

formation plugging is very expensive or impossible to solve. As a result, the choice of fluid used

to drill the pay zone is critical.



Completion Assembly Installation

This stages marks the beginning of what is commonly perceived as the completion program.

Considerable preparation, evaluation and design work has been completed before the completion

tubulars and components are selected. With all design data gathered and verified, the completion

component selection, assembly and installation process commences. This phase carries obvious

importance since the overall efficiency of the completion system depends on proper selection and

installation of components. A visionary approach is necessary since the influence of all factors

must be considered at this stage, i.e., factors resulting from previous operations or events, plus an

allowance, or contingency, for factors which are likely or liable to affect the completion system

performance in the future. The correct assembly and installation of components in the wellbore is

as critical as the selection process by which they are chosen. This is typically a time at which

many people and resources are brought together to perform the operation. In general, completion

components are broadly categorized as follows

i) Primary completion, components

ii) Auxiliary completion components.

Primary completion components are considered essential for the completion to function safely as

designed. Such components include the wellhead, tubing string, safety valves and packers. In

special applications, e.g., artificial lift, the components necessary to enable the completion system

to function as designed will normally be considered primary components. Auxiliary completion

components enable a higher level of control or flexibility for the completion system. For

example, the installation of nipples and flow control devices can allow improved control. Several

types of device, with varying degrees of importance, can be installed to permit greater flexibility

of the completion. While this is generally viewed as beneficial, a complex completion will often

be more vulnerable to problems or failure, e.g., due to leakage. The desire for flexibility in a

completion system stems from the changing conditions over the lifetime of a well, field or

reservoir. For example, as the reservoir pressure depletes, gas injection via a side-pocket mandrel

may be necessary to maintain optimized production levels.



Completion fluids often require special mixing and handling procedures, since;

i) the level quality control exercised on density and cleanliness is high

ii) completion fluids are often formulated with dangerous brines and inhibitors.

The ultimate selection of completion components and fluids should generally be made to provide

a balance between flexibility and simplicity.

Completion Assembly Installation

Primary Components o Wellhead o Christmas tree o Tubing o Packer o Safety valve

Completion Fluids o Completion fluid o Packer fluid o Perforating fluid o Kick-off fluid

Auxiliary Components o Circulating devices o Nipples o Flow couplings o Injection mandrels o Tubing seal assembly



Initiating Production

The three stages associated with this phase of the completion process include;

i) Kick-off

ii) Clean up

iii) Stimulation

The process of initiating flow and establishing communication between the reservoir and the

wellbore is obviously closely associated with perforating operations. If the well is to be

perforated overbalanced, then the flow initiation and clean up program may be dealt with in

separate procedures. However, if the well is perforated in an underbalanced condition, the flow

initiation and clean up procedures must commence immediately upon perforation. While the

reservoir/wellbore pressure differential may be sufficient to provide an underbalance at time of

perforation, the reservoir pressure may be insufficient to cause the well to flow after the pressure

has equalized. Adequate reservoir pressure must exist to displace the fluids from within the

production tubing if the well is to flow unaided. Should the reservoir pressure be insufficient to

achieve this, measures must be taken to lighten the fluid column - typically by gas lifting or

circulating less dense fluid. The preparations for these eventualities are part of the completion

design process. The flowrates and pressures used to exercise control during the clean up period

are intended to maximize the return of drilling or completion fluids and debris. This controlled

backflush of perforating debris or filtrate also enables surface production facilities to reach stable

conditions gradually. In some completion designs, an initial stimulation treatment may be

conducted at this stage. An acid wash or soak placed over the perforations has proved effective in

some conditions. However, as underbalanced perforating becomes more popular, the need and

opportunity for this type of treatment has diminished.

Production initialization

Inducing Flow o Gas lift o Nitrogen kick-off o Light-fluid circulation



o Using completion components

Clean-up Program o Initial flowrate and rate of increase o Evaluation program o Test-treat-test

Stimulation

There are four general categories of stimulation treatment which may be considered necessary

during the process of completing a well

i) Wellbore cleanup

ii) Perforation washing or opening

iii) Matrix treatment of the near wellbore area

iv) Hydraulic fracturing. Wellbore clean up will not normally be required with new completions.

However, in wells which are to be reperforated or in which a new pay zone is to be opened, a

well bore clean up treatment may be appropriate. There are various perforation treatments which

may be associated with new or re-completion operations. Perforating acids and treatment fluids

are designed to be placed across the interval to be perforated before the guns are fired. Used in

overbalanced perforating applications, the perforating acid or fluid reduces the damage resulting

from the perforating operation. Perforation washing is an attempt to ensure that as many

perforations as possible are contributing to the flow from the reservoir. Rock compaction, mud

and cement filtrate and perforation debris have been identified as types of damage, which will

limit the flow capacity of a perforation, and therefore completion efficiency. If the objective of

the treatment is to remove damage in or around the perforation, simply soaking acid across the

interval is unlikely to be adequate. The treatment fluid must penetrate and flow through the

perforation to be effective. In which case all the precautions associated with a matrix treatment

must be exercised to avoid causing further damage by inappropriate fluid selection. Matrix

treatment of the near wellbore area may be designed to remove or by-pass the damage. Hydraulic



fracturing treatments provide a high conductivity channel through any damaged area and

extending into the reservoir.

Stimulation

Wellbore and Perforations o Wellbore clean-up o Perforating acid o Perforation wash

Near Wellbore and Reservoir Matrix o Matrix acidizing o Hydraulic fracturing o Non-acid treatments

Well Service and Maintenance Requirements

The term well servicing is used to describe a wide range of activities including

i) Routine monitoring

ii) Wellhead and flowline servicing

iii) Minor workovers (thru-tubing)

iv) Major workovers (tubing pulled)

v) Emergency response and containment.

Well service or maintenance preferences and requirements must be considered during the

completion design process. With more complex completion systems, the availability and response

of service and support systems must also be considered. Wellbore geometry and completion

dimensions determine the limitations of conventional slickline, wireline, coiled tubing or

snubbing services in any application.



Well Service and Workover

Completion System Function o Well testing and routine monitoring o Emergency kill and containment

Light Service Units o Slickline o Electric wireline o Coiled tubing o Snubbing

Heavy Workover Units o Drilling rig o Workover rig o Combined coiled tubing and snubbing unit

Logistics

Restraints imposed by logistic or location driven criteria often compromise the basic cost

effective requirement of a completion system. Special safety and contingency precautions or

facilities are associated with certain locations, e.g., offshore and subsea.

Logistics

Surface Facilities o Separator capacity o Export capability o Operational flexibility o Disposal facility



Location o Access to well o Weather conditions o Environmental constraints o Proximity of neighboring interests

How Completion is Designed

The Companys operations management and the reservoir engineering department generally

decide the main purposes of a well;

For exploration and appraisal wells which mainly involves the level(s) that are to be tested, and the type and duration of the tests that are to be run.

For development wells which basically involves the level(s) that are to be produced. The production or injection profile required for the wells.

Based on the above, particularly for development wells, the problem is to design the best possible

completion in order to;

Optimize productivity or injectivity performance during the well's complete lifetime make sure that the field is produced reliably and safely

Optimize the implementation of an artificial lift process Optimize equipment lifetime Make it possible to change some or all of the well's equipment at a later date without too

much difficulty so that it can be adapted to future operating conditions

Minimize initial investment, operating costs and the cost of any workover jobs.

This may mean a compromise in the drilling and casing program or in operating conditions or

even that the objectives have to be modified if they prove to be unattainable. The data required to

set up a completion system are very numerous. Some of the most important constraints and

parameters are listed below;

Local constraints (regulations, environment, etc.) The type of effluents and their characteristics



The reservoir and its petrophysical characteristics The number of producing formations, each one's expected productivity and the interfaces The available diameter and the borehole profile Whether it is necessary to proceed to additional operations (well stimulation, sand control,

etc.)

Whether it is necessary to implement techniques to maintain reservoir pressure (water, gas, solvent or miscible product injection) or to lift the effluents artificially (gas lift, pumping,

nitrogen or carbon dioxide injection) immediately or at a later date

The eventuality of having to do any work on the pressurized well during the production phase by wireline, or with a concentric tubular (coiled tubing or snubbing)

Completion design is based on this body of data, so every effort must be made to be sure no

important point has been disregarded, since incomplete or wrong data might lead to poor design.

The job is not an easy one since;

These data are very numerous and may be interrelated. Some of them are not very accurately known when completion is designed (sometimes not

even when completion is being carried out).

Some of them are contradictory. Some of them are mandatory, while others can be subject to compromise.

IntroductionData SourcesReservoir ParametersReservoir Parameters

Produced Fluid CharacteristicsProduced Fluid Characteristics

Wellbore ConstructionWellbore Construction

Completion Assembly InstallationCompletion Assembly Installation

Initiating ProductionProduction initialization

StimulationStimulation

Well Service and Maintenance RequirementsWell Service and Workover

LogisticsLogistics

How Completion is Designed

well completion introduction

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