Basic Downhole Config

8/13/2019 Basic Downhole Config

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Openhole Completions

In an openholeor barefoot completion, the production casing is set in the caprock above orjust into the top of the pay zone, while the bottom of the hole is left uncased (Figure 1 , (a)

openhole completion; (b) uncemented liner completion; (c) perforated completion).

Figure 1

Often, the final drilling of the pay zone is carried out with special non-damaging drillingfluids or an underbalanced mud column.

This form of well completion dates back to the days of cable tool drilling, but is rarely used

today. Nonetheless, openhole completions offer certain advantages in thick, relativelycompetent formations:

Exposure of entire pay zone to the wellbore;

No perforating expense;

less critical need for precise log interpretation;

Reduced drawdown because of the large inflow area;

Slightly reduced casing cost;

Ease of deepening the well;
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relative ease of converting the well to a liner completion;

no risk of formation damage resulting from cementing casing.

Unfortunately, the disadvantages and limitations of openhole completions outweigh thesebenefits in most cases. Some of these disadvantages are as follows:

inability to control excessive gas-oil and/or water-oil ratios (except in the case of bottom water); need to set casing before drilling or logging the pay;

difficulty of controlling the well during completion operations;

unsuitability for producing layered formations consisting of separate reservoirs with incompatiblefluid properties;

inability to selectively stimulate separate zones within the completion interval ;

need for frequent clean-outs if the producing sands are not completely competent or if theshoulder of the caprock between the shoe and top of the pay is not stable.

Liner Completions

To overcome the problems of collapsing sands plugging the production system, the early oil

producers placed slotted pipe or screens across the openhole section as a downhole sandfilter (Figure 1 (a) openhole completion; (b) uncemented liner completion; (c) perforatedcompletion).


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Figure 1

The simplest and oldest liner completion method involves running slotted pipe into the open

hole interval. The slots are cut small enough that the produced sand bridges off on theopening rather than passing through. This method is till used in some areeas today, but

because it entails many of the same disadvantages inherent in openhole completions (i.e.,lack of control), its use is not widespread.

For very fine sands, wire-wrapped screens or sintered bronze are used in place of machine-cut slots. This technique is a reasonably effective sand control method in uniform coarsesands with little or no fine particles (e.g., in California). Sometimes this is the only sand

control system that can be used because of pressure loss and placement considerations(e.g., in unconsolidated heavy oil sands).

In general, however, the uncemented liner completion is no longer recommended for thefollwoing reasons:

Sand movement into the wellbore tends to cause permeability impairment by the intermixing ofsand sizes, and of sand and shale particles.

Fine formation sands tend to plug the slots or the screen.

At high rates, the screen often erodes as formation sand moves into the wellbore.

Poor support of the formation can cause shale layers to collapse and plug the slots or screen.

Formation failure can cause the liner itself to collapse.

To overcome these problems, operators have resorted to more effective sand controlmethods such as gravel packing, in which the annulus between the screen and the openholeis filled with coarse, graded sand, or the use of pre-packed screens. In some cases, even

where sand control is planned, it may be best to employ a cased and perforated completion

with an external gravel pack--this configuration has become the norm for light iol and gasdevelopments because of the flexibility it provides.

Cemented and Perforated Completions

By far the most common type of completion today involves cementing the production casing(or liner) through the pay zone, and subsequently providing communication with theformation by perforating holes through the casing and cement (Figure 1 : (a) openhole

completion; (b) uncemented liner completion; (c) perforated completion).


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Figure 1

Ideally, perforations should penetrate any damaged zone around the original wellbore andcreate a clean conduit within the undamaged formation. If the well is cased and

unperforated during the early stages of the completion operation, well control is easier andcompletion costs may be reduced.

Using various depth control techniques, we can select which sections of pay should be

perforated and opened to flow, thereby avoiding undesired fluids (gas, water), weak zonesthat might produce sand, and unproductive sections or shale barriers.

This selectivity, which is completely dependent on a good cement job and adequate

perforating, also allows a single well-bore to produce several separate reservoirs without

their being in communication. This is done by setting isolating packers within anunperforated section of the pipe. Selective perforation can also be used to control the flow

from, or stimulation of, various parts of the pay. By shutting off or partially pluggingselected perforations, injected fluids (water, stimulation fluids, or cement) can be divertedinto less permeable zones.

Cementing casing at TD rather than completing the well openhole can reduce the likelihood

of well control problems. Moreover, the decision to set production casing can be deferred


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until the openhole logs of the prospective pay zone have been evaluated, substantiallyreducing the dry-hole costs if the hole is dry.

In summary, the advantages of cased and perforated completions include:

Safer operations

More informed selection of the zones to be completed Reduced sensitivity to drilling damage

Facilitation of selective stimulation

Possibility of multizone completions

Easier planning of completion operations

This type of completion is generally used unless there is a specific reason to prefer an

openhole or uncemented liner completion. Even where sand control is planned, perforatedcompletions with internal gravel packs have become the norm for light oil and gasdevelopments because of the flexibility provided.

Single-String Completions (Single Zone)

Producing a well through a tubing string protects the casing from formation fluids andmaximizes flow efficiency. The tubing also provides a means of circulating fluids in the

well.Single-tubing-string completions may or may not use a packer, depending on the wellconditions and the completion method used. Some examples of single-string completionsare shown inFigure 1 ,


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Figure 1

Single-string flowing wells: (a) temporary; (b) tubingless gas well; (c) simple low cost; andFigure 2,(d) high pressure; (e) high-rate liner completion).


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Figure 2

The complexity of tubing and packer installations is driven by functional requirements andeconomic considerations. Since a number of useful features can be installed at very lowincremental cost, the designer should consider these options and possibilities:

Simplification of the completion and future workover operations

Optimum tubing size for maximum long term flowrate

Future artificial lift needs

A "bomb" well for future bottomhole pressure surveys

Use of a permanent packer and tailpipe to protect the formation during workovers or to facilitate"killing" the well;

The need for moving seals and/or a slip joint to accommodate tubing elongation and contractioncaused by thermal stresses

Anchoring the tubing to the packer

Availability of a downhole sliding sleeve for removing or adding fluid to the tubing (kick-off orkilling operations)

The need for downhole corrosion inhibitor injection

An additional packer and nipple between sets of perforations for future recompletion operations;

Use of tubing-conveyed perforating gun and/or through-tubing guns for underbalanced perforatingto improve completion efficiency.


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For single-tubing-string, liner completions, a polished bore receptacle in the liner hanger is

often used in place of a packer. This is simply a polished internal section at the top of the

liner, into which the tubing string is inserted, much as it would be into a packer. This isuseful both for deep wells where tubing/casing clearances are often small and for very high

productivity wells where the use of a packer might cause a restriction on well productivity.In this type of completion it is useful to incorporate a landing nipple in the liner string for

future isolation of the producing zone via wireline.

Tubingless completions are a particularly low-cost installation method used in marginal flowoperations, such as low rate gas development. While they are also used in high gas-oil ratiooilfields, problems develop when artificial lift is required. Hollow sucker rod pumps or small

diameter "macaroni" gas-injection tubing strings must be installed. Minor liquid buildup ingas wells can usually be easily blown out of the well with flexible small diameter tubing.

Single String Completions (Selective)

Selective completions include both single-stringconfigurations, such as those shown inFigure 1 (Multiple-zone completions: (a) tubingless; (b) low rate, single string; (c) high

rate, single string)and multiple-string arrangements such as those shown inFigure 2(Multiple-zone completions: (d) dual string (parallel gas lift); (e) concentric).

Figure 1
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Figure 2

Single string completions are often preferable to multiple-string completions because the casing size in multiple-stringcompletions limits the diameter, which, in turn, limits the flow rate obtainable through each string. Single-stringcompletions may also be used where segregation is required purely for reservoir control (e.g., in a case where zones willbe commingled at some stages, but shut in during other periods because of high gas-oil ratio, high water cut, or for someother reason). These completions may also be used to minimize completion costs, which is also often the reason for

limiting the size of the production cMultiple String Completions

Multiple tubing string completions are generally more expensive than single-stringcompletions, and more complicated to install and service. They do, however, offer the abilityto simultaneously produce from or inject into different zones, and to allocate production orinjection for each zone.

Dual-string completions may be parallel or concentric. Where artificial lift may be required,

parallelstrings are usually used. Concentric strings require less clearance and can oftenachieve a higher overall flow capability.


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Triple-string completions have also been used in some areas, but they are usually too

restrictive of well capacity to be economically attractive as a conventional completion. The

difficulty of future remedial workovers of wells thus completed also prevents theirwidespread use.

Multistring tubingless completionsare sometimes used for completing stacked deltaic

reservoirs (e.g., the U.S. Gulf Coast) that have low individual reserves and normalpressures. These completions are particularly attractive for depleting small oil accumulations

below a large gas reservoir and for low cost gas developments. The improvement in thedesign and equipment quality of more conventional casing completions has resulted in adecrease in the popularity of this last type of installation.

asing.

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Self Assessment

1. What is the most popular type of completion?

(A) Openhole

(B) Uncemented liner

(C) Perforated

(D) Gravel pack

(E) Slotted liner

2. Gravel packing has for the most part replaced the uncemented liner completion.

(A) True

(B) False

3. Gravel packs are only used in openhole completions.

(A) True

(B) False

4. An openhole completion provides more flexibility because it allows you time to study the entire paysection and then set a liner and perforate.

(A) True

(B) False

5. Concentric tubing installations in multiple string completions provide an avenue for higher flowatesthan separate parallel strings.

(A) True

(B) False

6. What is the chief weakness of tubingless completions?


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(A) Lack of corrosion protection for the casing

(B) Liquid loading

(C) Difficult to deepen the well

(D) None of these

Submit Your Answers

Note: Your answers CANNOT be changed after they have been submitted. Check your answers thoroughly BEFOREyou submit them.

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Course Scorecard

Student Name: 04761169 04761169

Date Started: 1/30/2014 1:05:27 PM

Date Completed: 1/30/2014 1:26:03 PM

Time Spent: 21 Minutes

Number of Questions: 6

Correct Answers: 4

Incorrect Answers: 2

Score: 67%

04761169 04761169's Self Assessment Answers

1. What is the most popular type of completion?

(C) Perforated

2. Gravel packing has for the most part replaced the uncemented liner completion.

(A) True

3. Gravel packs are only used in openhole completions.

(A) True

4. An openhole completion provides more flexibility because it allows you time to study the entire paysection and then set a liner and perforate.

(B) False

5. Concentric tubing installations in multiple string completions provide an avenue for higher flowatesthan separate parallel strings.

(A) True

6. What is the chief weakness of tubingless completions?

(D) None of these
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Basic Downhole Config

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