Best Completion Practices

AbstractOver one hundred Gulf of Mexico completions have been analyzed over a period of two years, resulting in the best practices, as outlined in this paper. The completions have been categorized into ten different types, and optimized methods for implementing each type are listed. Completion engineers, production engineers, rig foremen and many service company personnel have contributed to Best Completion Practices. The work, however, has mainly been driven by an Alliance Process Improvement Team (APIT) composed of the authors, thus shared visions and solutions are the means by which common completion objectives have been reached.

IntroductionThe number of possible completion configurations for oil and gas wells is infinite; nevertheless this paper endeavors to classify completions into three basic categories: non-sand control, sand control and horizontal. This classification is based on experience from over one hundred Gulf of Mexico wells. Several conventions are adopted in order to define these three major completion categories: (1) a non-sand control completion is any non-horizontal completion without a gravel pack screen, (2) a sand control completion is any non horizontal completion with a gravel pack screen, and (3) a horizontal completion is one with deviation greater than eighty-five degrees. For example, a well with seventy-degree deviation and no gravel pack screen would be termed non-sand control. Within these three categories there are ten individual completion types, which are diagrammed in Figure 1 (located in the appendix).

Figure 1 titled “Single, Flowing Completions,” depicts each of the completion types discussed in this paper; it also contains some completion guidelines and decision aids. An AllianceProcess Improvement Team (APIT) designed this schematic as a reference for basic decisions concerning completions. The APIT’s purpose is to implement process improvements, reduce completion costs and increase overall efficiency by merging the knowledge and experience of operating and service company personnel; the completion practices outlined in this paper are focused toward this goal. It should be noted that all of the completion types discussed in this paper are flowing, and have only a single tubing string.Some completions such as cased hole horizontal, furan resin consolidation and screen less frac packs did not exist in the set of wells examined, so they have been omitted from discussion.

Common DecisionsIn the Gulf of Mexico it is common to encounter multiple pay sands with commercial potential in a single well. This can prove very helpful to the economics of a newly drilled well, but increased planning is required to provide the operator with an optimal completion.A number of different completion configurations exist for wells with multiple pay zones; sometimes only minor changes are made for different well conditions. Once it has been determined whether sand control and fracturing are necessary, the process of equipment selection can begin. Pertinent reservoir information is also necessary to determine working pressures for equipment. The completions discussed below can be greatly affected by the lack of a satisfactory primary cement job across the pay intervals. Methods to handle secondary cementing will be mentioned at the end of the discussion on completion types.Nodal analyses and economic evaluations are performed to determine whether to complete and produce multiple zones simultaneously or independently. Mechanically, it is less risky to produce the lowest zone to its economic limit and then recomplete in the

upper zone. However, because it is more economical, the upper zone is often selected as the initial completion. In this case, a sliding sleeve, or some other type of isolation device which can be operated with slickline or coiled tubing, can be used. The major disadvantage in producing the upper zone first is reliance on isolation devices that can later fail. Sand production, corrosive well fluids, or the inability to operate the device can lead to costly remediation. On the other hand, this type of completion is more economical because it affords future access to a lower zone.

Non-Sand Control CompletionsWithin the context of this paper a non-sand control completion is one that has less than eighty-five degrees deviation, and does not require sand control screens. These completions are generally confined to land, inland waters or deep, consolidated offshore reservoirs.

Type 1 – Open Hole Single – There are many variations of open hole single completions, but this type is not often used in the Gulf Coast because of poor consolidation of many of the formations. However, open hole single completions should not be eliminated from consideration, as they are inexpensive and quite versatile. However for well consolidated formations, open hole single completions should be considered because of their low cost and versatility.

Type 2 – Cased Hole Single – Unlike open hole completions, cased hole single completions guarantee hole stability.Furthermore, a successful primary cement job will isolate the productive interval from water zones. This type of completion is typical of land and inland waters wells, but it is also found offshore when sand control is not required. Cased hole single completions are more expensive than open hole completions due to the cost of production casing, but they are still relatively inexpensive due to the limited hardware and number of services required for their installation. Aside from the cost of setting production casing, completion costs include tubing, nipples, packer, fluids and perforating (normally done with wireline). Shallow dry gas wells (1,400 to 3,000 feet) may frequently be produced in this manner, whereas oil zones at similar depths require sand control.

Type 3 – Single Packer with Plug Back(s) – The cost and mechanics of this completion type are essentially the same as the previous one. The difference between the two is that Type 3 allows for a future plug back to a secondary zone located between the primary zone and the production packer. Multiple wireline plug backs (WLPB’s) can be performed under a single packer as long as the zones are reasonably close together, and meet the same conditions as the primary zone, such as not requiring sand-control or artificial lift. In some cases, WLPB’s can be performed with a single packer even when there are large distances between zones

Type 4 – Multiple Packers with Plug Back(s) – This type of completion is used when two productive zones that are separated by a significant distance need to be completed. The difference between Type 4 and Type 3 is that two additional packers are required for Type 4. This configuration allows block squeeze perforations to be isolated between the two packers. Placing a plug in the tubing between the lowest two packers is the easiest way to perform the plug back to the upper zone.

Perforating – Both tubing- and wireline-conveyed perforating methods are used in non-sand control completions, but the wireline-conveyed option is more common and more economical, as it does not require a rig. Shot density, entrance hole diameter, and perforation tunnel length should be considered in order to prevent sand production in non sand control completions. Large diameter, deep penetrating charges in densities greater than six shots per foot (twelve shots per foot preferred) can prevent or defer

sand production. This technique allows the production of marginally economic zones not suitable for sand control completions.Tubing-conveyed guns may be used as zone length increases.For long zones, which would require multiple wireline trips, tubing conveyed perforating (TCP) is likely the best option.Another benefit of TCP is that it allows significantly higher underbalance pressures than the wireline method. Perforating underbalanced is important in order to create open, undamaged perforations so that productivity is not hampered, but it creates the added risk of sticking the guns in the hole. Using an exploding firing head to drop the guns upon detonation can minimize this risk. This technique works very well when the guns are run in the hole below the production packer.Other factors to be considered are shot density and phasing.Many times zero degree phasing and low shot densities are sufficient, but when there are heterogeneities such as anisotropy or laminations that require higher shot densities,TCP is the method of choice. However, when fracture stimulation is to be performed, wireline-conveyed methods are most often used because fracturing sufficiently addresses the heterogeneities.

Fracturing – The second and third completion types described above may be candidates for a fracture stimulation that is pumped down the production tubing. Wells that are to be fractured should be designed so that (1) a permanent, seal bore type packer is used, and (2) the production seal assembly has adequate length to accommodate shrinkage during fracturing.One additional concern is the tubing size: 2 7/8 inch tubing is generally the smallest size that can accommodate a fracture treatment, but the pump rate is limited to about twenty barrels per minute.

Sand Control CompletionsThere are many completion types commonly considered to be sand control completions; only those with sand control screens are addressed here. Four major types of sand control completions are defined below, and their relative strengths and weaknesses are discussed. Issues such as sand control pumping techniques and perforating techniques are discussed later in this section. While sand control can significantly increase completion costs, the life of the completion is also greatly increased. Proper hardware selection insures completion reliability. Some general rules, which contributed to the success of the studied wells, include:1) Maintain 0.85 inch minimum standoff from gravel pack screen to casing inner diameter.2) Use ninety feet of blank above gravel pack screen whenever possible.3) Keep draw down pressure less than 1000 psi for gravel packs and 500 psi for non-sand control completions.4) Restress gravel packs at pressures ranging from 1000 to 1500 psi.5) Insure that gravel pack assembly inner diameter is at least 1.875 inches.6) Test cement squeezes to within 80% of fracture gradient.7) Allow no more than 10 feet between the bottom perforation and the top of sump packer or bridge plug.8) Always use P-110 grade pipe in order minimize the risk of blank pipe collapse.

Type 5 – Single Gravel Pack Completion –The most basic and least likely to fail sand control completion, the single gravel pack is widely used, but is limited by the need for a rig workover if other zones are to be produced. This completion type may have a sump packer or a bridge plug below the gravel pack screen. A sump packer should be used (1) if an open sump is required in order to accommodate production logging tools, or (2) to allow debris to fall through the completion. Bridge plugs or

cement retainers (if squeeze cementing is necessary) are generally used in conjunction with a bull plugged lower screen. This method saves time and money during completion operations, but does not allow complete evaluation of the perforated interval with production logging tools.

Type 6 – Single Selective – This type of completion is gravel packed with an internal sliding sleeve, which allows a nonsand control completion to be produced through the gravel packed completion. However, if the sliding sleeve fails, secondary reserves may be lost if a rig workover cannot be justified. For this reason, the zone with larger reserves is generally produced first. A single selective completion differs from a single gravel pack in two ways: (1) the presence of a productive zone below the primary completion, and (2) a sliding sleeve that allows the production of a secondary zone after depletion of the primary zone. A major limitation of single selective completions is their dependence on a sliding sleeves.

Type 7 – Single Selective with Multiple Options – The single selective with multiple options is perhaps the most common completion in the Gulf of Mexico offshore. This configuration allows for great versatility and minimizes the need for rig workovers, but its success depends on the reliability of more mechanical components than the types discussed above. Great care must be taken in the design of these completions to insure their prolonged success.

Type 8 - Stacked Selective – This is the most complex of the single tubing completions and is often avoided for this reason.It requires the use of no fewer than three gravel pack packers, and usually a sump packer. These completions are quite expensive and require several weeks to install, but these drawbacks are offset by the ability to produce three or more zones without a single workover. Usually these are installed in marginally economic zones, and where future workover costs can be prohibitively high, such as offshore. If significant reserves exist, performing a future workover to produce uphole zones is more attractive than the mechanical risk of a complex completion.

Perforating – High shot density guns (casing guns with twelve shots per foot or more) using big-hole perforating charges are commonly used to maximize completion efficiency.Underbalanced perforating is the best technique for removing debris from the perforation tunnels; consequently, it contributes greatly to the productivity of the completion.6Debris can mix with the gravel pack sand, reducing the its permeability. However, often it is not possible to perforate at the optimum underbalance pressure due to the following limitations: (1) the danger of casing collapse, (2) the production of large quantities of formation sand, (3) the potential for sticking the pipe or guns, and (4) movement of the perforating packer.TCP systems offer the benefits of longer gun length and greater underbalance pressures, but they are prone to sticking.Care must be taken in gun selection to insure annular clearance (usually one-half to one inch) for two reasons. (1) The clearance is necessary should guns become stuck and require fishing. (2) Entrance hole diameter is highly sensitive the annular clearance. This problem is magnified in deviated wells where the gun system lies on the bottom of the hole causing an absence of perforations in the upper portion of the hole.Another concern in deviated wells is the cleaning of the perforation tunnels on the lower side of the hole. Gravity prevents removal of the debris and thus the tunnels cannot be properly gravel packed. This makes them suseptible to collapse which drastically reduces the productivity of the completion. Perforation cleaning is also

hampered by the presence of the guns, so for the above reasons, perforating the lower portion of the hole in deviated zones is often avoided.Furthermore, the upper portion of the hole in deviated zones is sometimes not perforated due to the difficulty of placing gravel pack sand in the perforation tunnels in this portion of the hole. This perforating method also minimizes the possibility of water production through an inadequate cement sheath on the lower side of the hole.

Single Trip Perforating and Gravel Packing –The practice of single trip perforating and gravel packing involves combining the tools and hardware necessary for both perforating and gravel packing, and running them into the hole simultaneously.This method saves many hours of rig time, and prevents large fluid losses after perforating. 7, 11 These two benefits lower completion cost and enhance well productivity; they should be used whenever possible. Some factors that limit the use of single trip perforating and gravel packing systems are inadequate sump space for perforating guns, productive zones beneath the zone of interest, and old perforations above the zone of interest.4 (The term “single trip perforating and gravel packing” does not imply that frac packing is not an option with these systems.)

Sand Control Pumping Techniques – The sand control completion types discussed above require pumping, normally one the following techniques: (1) a gravel pack using a mildly viscous carrier fluid, (2) a high rate water pack (HRWP) using a completion brine carrier fluid, or (3) a frac pack, which uses a highly viscous carrier fluid.Frac packs generally provide good completion efficiency with a slightly negative skin. They minimize the mobility of fine grained sediments, and thus reduce the necessity for future acid treatments.5 Drawback of frac packs are that they sometimes require long clean up periods and they can foul the production facilities with guar gel. These problems can be minimized by using visco elastic surfactant (VES) fluids. The viscosity of VES fluids is broken upon contact with hydrocarbons, thus causing less proppant pack damage, and providing faster well clean up.Another common use of VES fluids is in gravel packing. They can be used effectively to reduce pumping costs when the alternative carrier fluid is heavy completion brine. This VES method is the middle ground between frac packs and gravel packs using completion brine. Since they are slightly viscous, VES fluids allow typical proppant concentrations of three to four PPA, and proppant placement is usually at least 150 pounds of proppant per foot of perforations. Normally, pump rates are about 8 barrels per minute (BPM). These treatments are pumped in the circulating position with the annulus closed.Near the end of the treatment, 25% of the pump rate is allowed to return through the annulus to insure a good pack. This method is also applied to gravel packs using completion brines, commonly referred to as high rate water packs (HRWP).Most current sand control pumping techniques fall into one of the above categories. Frac packs are almost always considered to be the best method unless (1) the reserves do not justify the expense, (2) there is excessive zone length in a highly deviated well, or (3) there is significant risk of connecting a nearby water sand with the frac pack.

Remedial Cementing – In circumstances where the primary cement job does not meet acceptable standards, secondary cementing techniques are sometimes necessary. The most common method is a “block squeeze.” Cement block squeezes are performed in an effort to isolate productive zones and prevent extraneous water production. They can also substantially drive up the cost of the initial completion.Determining whether a block squeeze is necessary is usually left up to the completion engineer based on his interpretation of the cement bond log. In the past, if there was doubt about the interpretation, a block squeeze was usually performed. To prevent an excessive number of block squeezes, thereby reducing costs, the APIT recommends the following guidelines for sufficient bond:

1) 5 feet of 60% bond in 5 inch casing2) 6 feet of 60% bond in 5 ½ inch casing3) 10 feet of 60% bond in 7 inch casing4) 12 feet of 60% bond in 7 5/8 inch casingThese guidelines promote a “lean forward” approach to reducing the number of block squeezes, resulting in three main benefits, significantly reduced completion costs, less rig time and elimination of complications resulting from the squeezes.

Horizontal CompletionsA horizontal completion offers several advantages over a high angle or vertical completion: (1) it can eliminate water or gas coning problems, (2) it increases the area open to flow thus reducing reservoir fluid velocities and fine grained sediment mobility, and (3) it may allow for more efficient reservoir drainage. These benefits make horizontal wells very attractive, but there are also limitations, including increased drilling and completion cost, poor completion reliability and very limited intervention options. In order to optimize completion design, it is necessary to compare the costs and benefits of several viable completion options.For discussion purposes, horizontal completions have been divided into two main types, supported and unsupported open hole. In supported completions, the support is in the form of a slotted liner or a shrouded screen. A shrouded screen refers to the open hole version of the shunt tube technology where integral joints of perforated casing and gravel pack screen are made up and run in the hole simultaneously. It should be noted that screen alone is not considered adequate support for the borehole.

Type 9 - Open Hole Unsupported – This type of horizontal open hole completion may be without screen, or it may have any type of sand control screen with or without a gravel pack.This type of completion is divided into three sub-categories: barefoot, screened, and gravel packed. Barefoot completions are those without screen or gravel pack, and are the least expensive of horizontal completions, but they are limited to competent rock that does not produce sand. Screened and gravel packed completions are effective in controlling sand production, and reduce the migration of fine grained sediment into the wellbore. It should be noted that none of the horizontal completion types discussed above are recommended in formations with poor hole stability. Experience has shown that open hole, unsupported horizontal completions fail if they are placed in formations with an unstable borehole.

Type 10 - Open Hole Supported – If the competency or stability of the hole is questionable, an alternative horizontal completion is the open hole supported type, in which mechanical borehole support, in the form of a slotted liner or shrouded screen, is added. In one field where seven horizontal wells were completed, six were Type 9 and one was Type 10. After slightly more than one year of production, all but two of the unsupported variety had failed. The supported well was, in fact, the most trouble-free horizontal in the field.Note that the supported, horizontal completion type does not include cemented, cased hole horizontal completions. These have been omitted from this discussion because none existed in the wells that were studied.

Summary and RecommendationsFor competent formations, non sand control completions are best because of fewer components, little risk and low installation cost. In poorly consolidated formations, sand control completions are required. Since they are more prone to failure and intervention is expensive, mechanical risk should be minimized. Horizontal completions are the best alternative in reservoirs where a fluid interface such as an oil-water contact is present and the vertical and horizontal permeability are similar.

Communication and flexibility are keys to designing and performing the optimum completions. This communication should begin long in advance of the drilling phase of the well.Expected flowrates should be identified by the production engineers and confirmed with the geologist’s data. Nodal analyses should be performed over the entire range of possible outcomes, and the most likely production tubing size should be selected. The actual reservoir often differs from initial expectations, so changes to the completion design are common. It is imperative have sufficient to incorporate these changes both from a cost and equipment standpoint. Wells with deviations greater than sixty-five degrees may require additional planning since equipment such as electric line cannot be used. In these wells, the number of trips should be minimized whenever possible to avoid increased cost and reduce the chance sticking. Logging and slickline work should be minimized or eliminated.Completions with multiple pay zones require more equipment and hardware (packers, gravel pack settings, etc.) than single zone completions. Because of the difficulty and expense of removing this equipment, designs should attempt to minimize risk whenever feasible. Sliding sleeves have less chance of failure if not relied upon to produce upper sands prior to lower sands. Another danger lies in using unproven technology and equipment with inadequate caution. However, the rewards of using new systems such as single trip perforating and gravel packing have been well worth the risk. The overall goal in multiple zone completion design should be to achieve a trouble-free completion that will provide a production outlet for the life of all zones in the well.The importance of post-completion bottom hole pressure data can not be overemphasized. Tracking completion performance and determining the extent of formation stimulation or damage is essential in designing and developing new completion procedures. However, without drawdown and buildup test data, the validity and efficiency of these methods cannot be accurately assessed.