Basics Downhole Configurations

8/19/2019 Basics Downhole Configurations

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Openhole Completions

In an openhole or barefoot completion, the production casing is set in the caprockabove or just into the top of the pay zone, while the bottom of the hole is leftuncased ( Figure 1 , Completion categories based on the reservoir-wellbore interface:(a) openhole completion; (b) uncemented liner completion; (c) perforated

completion)

Figure 1

!ften, the "nal drilling of the pay sone is carried out with special non#damagingdrilling $uids or an underbalanced mud column %his form of well completion datesback to the days of cable tool drilling, but is rarely used today

!penhole completions o&er certain advantages in thick, relatively competentformations'

• eposure of entire pay zone to the wellbore

• no perforating epense

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• less critical need for precise log interpretation

• reduced drawdown because of the large in$ow area

• slightly reduced casing cost

• ease of deepening the well

• relative ease of converting the well to a liner completion

• no risk of formation damage resulting from cementing casing

*nfortunately, the disadvantages and limitations of openhole completions outweighthese bene"ts in most cases +ome of these disadvantages are as follows'

• inability to control ecessive gas#oil andor water#oil ratios (ecept in the

case of bottom water)

• need to set casing before drilling or logging the pay

• di-culty of controlling the well during completion operations

• unsuitability for producing layered formations consisting of separate

reservoirs with incompatible $uid properties

• inability to selectively stimulate separate zones within the completion

interval

• need for fre.uent clean#outs if the producing sands are not completely

competent or if the shoulder of the caprock between the shoe and top of thepay is not stable

Liner Completions

%o overcome the problems of collapsing sands plugging the production system, theearly oil producers placed slotted pipe or screens across the openhole section as adownhole sand "lter ( Figure 1 , Completion categories based on the reservior-wellbore interface: (a) openhole completion; (b) uncemented liner completion; (c)

perforated completion)



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Figure 1

%he simplest and oldest liner completion method involves running slotted pipe intothe openhole %he slots are cut small enough that the produced sand bridges o& onthe opening rather than passing through %his method is till used in some areeastoday, but because it entails many of the same disadvantages inherent in openholecompletions (ie, lack of control), its use is not widespread

For very "ne sands, wire#wrapped screens or sintered bronze are used in place ofmachine#cut slots %his techni.ue is a reasonably e&ective sand control method inuniform coarse sands with little or no "ne particles (eg, in /alifornia) +ometimesthis is the only sand control system that can be used because of pressure loss andplacement considerations (eg, in unconsolidated heavy oil sands) In general,however, the uncemented liner completion is no longer recommended because

• sand movement into the wellbore tends to cause permeability impairment by

the intermiing of sand sizes, and of sand and shale particles

• "ne formation sands tend to plug the slots or the screen

• at high rates, the screen often erodes as formation sand moves into the

wellbore


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• 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

%o overcome these problems, operators have resorted to more e&ective sand control

methods such as gravel packing, in which the annulus between the screen and theopenhole is "lled with coarse, graded sand, or the use of pre#packed screens In somecases, even where sand control is planned, it may be best to employ a cased andperforated completion with an eternal gravel pack##this con"guration has becomethe norm for light iol and gas developments because of the $eibility it provides

Liner Completions

%o overcome the problems of collapsing sands plugging the production system, theearly oil producers placed slotted pipe or screens across the openhole section as adownhole sand "lter ( Figure 1 , Completion categories based on the reservior-wellbore interface: (a) openhole completion; (b) uncemented liner completion; (c)

perforated completion)

Figure 1



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%he simplest and oldest liner completion method involves running slotted pipe intothe openhole %he slots are cut small enough that the produced sand bridges o& onthe opening rather than passing through %his method is till used in some areeas

today, but because it entails many of the same disadvantages inherent in openholecompletions (ie, lack of control), its use is not widespread

For very "ne sands, wire#wrapped screens or sintered bronze are used in place ofmachine#cut slots %his techni.ue is a reasonably e&ective sand control method inuniform coarse sands with little or no "ne particles (eg, in /alifornia) +ometimesthis is the only sand control system that can be used because of pressure loss andplacement considerations (eg, in unconsolidated heavy oil sands) In general,however, the uncemented liner completion is no longer recommended because

• sand movement into the wellbore tends to cause permeability impairment by

the intermiing of sand sizes, and of sand and shale particles

• "ne 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

%o overcome these problems, operators have resorted to more e&ective sand control

methods such as gravel packing, in which the annulus between the screen and theopenhole is "lled with coarse, graded sand, or the use of pre#packed screens In somecases, even where sand control is planned, it may be best to employ a cased andperforated completion with an eternal gravel pack##this con"guration has becomethe norm for light iol and gas developments because of the $eibility it provides

Cemented and Perforated Completions

0y far the most common type of completion today involves cementing the productioncasing (or liner) through the pay zone, and subse.uently providing communicationwith the formation by perforating holes through the casing and cement ( Figure 1 ,Completion categories based on the reservior-wellbore interface: (a) openhole

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



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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

• reduced dry#hole costs

• easier planning of completion operations

%his type of completion is generally used unless there is a speci"c reason to prefer an

openhole or uncemented liner completion 3ven where sand control is planned,perforated completions with internal gravel packs have become the norm for light oiland gas developments because of the $eibility provided

Single-String Completions (Single Zone)

4roducing a well through a tubing string protects the casing from formation $uids andmaimizes $ow e-ciency %he tubing also provides a means of circulating $uids inthe well+ingle#tubing#string completions may or may not use a packer, depending on

the well conditions and the completion method used ( Figure 1 ,



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Figure 1

Single-string owing wells: (a) temporary; (b) tubingless gas well; (c) simple low cost

and Figure 5 , (d) high pressure; (e) high-rate liner completion).



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Figure 2

%he compleity of tubing and packer installations depends on the functionalre.uirements and economic considerations +ince a number of useful features can beinstalled at very low incremental cost, the designer should consider these optionsand possibilities'

• simpli"cation of the completion and future workover operations

• the optimum tubing size for maimum long term $owrate

• future arti"cial lift needs

• a 6bomb6 well for future bottomhole pressure surveys

• use of a permanent packer and tailpipe to protect the formation during

workovers or to facilitate 6killing6 the well

• the need for moving seals andor a slip joint to accommodate tubing

elongation and contraction caused by thermal stresses


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• anchoring the tubing to the packer

• availability of a downhole sliding sleeve for removing or adding $uid to the

tubing (kick#o& or killing 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 andor through#tubing guns for

underbalanced perforating to improve completion e-ciency

For single#tubing#string, liner completions, a polished bore receptacle in the linerhanger is often used in place of a packer %his is simply a polished internal section atthe top of the liner, into which the tubing string is inserted, much as it would be intoa packer %his is useful both for deep wells where tubingcasing clearances are oftensmall 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 alanding nipple in the liner string for future isolation of the producing zone via wireline

%ubingless completions are a particularly low#cost installation method used inmarginal $ow operations, such as low rate gas development 7hile they are alsoused in high gas#oil ratio oil"elds, problems develop when arti"cial lift is re.uired8ollow sucker rod pumps or small diameter 6macaroni6 gas#injection tubing stringsmust be installed 2inor li.uid buildup in gas wells can usually be easily blown out ofthe well with $eible small diameter tubing

Single String Completions (Selective)

+ingle#string, selective completions ( Figure 1 ,



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Figure 1

ultiple-!one completions: (a) tubingless; (b) low rate" single string; (c) high rate"

single string and Figure 5 ,



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%riple#string completions have also been used in some areas, but they are usually toorestrictive of well capacity to be economically attractive as a conventionalcompletion %he di-culty of future remedial workovers of wells thus completed alsoprevents their widespread use

2ultistring tubingless completions are sometimes used for completing stacked deltaic

reservoirs (eg, the *+ 9ulf /oast) that have low individual reserves and normalpressures %hese completions are particularly attractive for depleting small oilaccumulations below a large gas reservoir and for low cost gas developments %heimprovement in the design and e.uipment .uality of more conventional casingcompletions has resulted in a decrease in the popularity of this last type ofinstallation

Basics Downhole Configurations

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