GRAVEL PACKING FEASIBLE IN HORIZONTAL WELL COMPLETIONS

June 11, 1990
Theodore E. Zaleski Jr., Jefferson P. Ashton Baker Sand Control Houston Successful completion of horizontal wells in unconsolidated formations depends on proper equipment selection and installation method balanced with reservoir objectives, formation parameters, and costs. The guidelines for designing these completions are based on generalized field experience, including horizontal cases where applicable.
Theodore E. Zaleski Jr., Jefferson P. Ashton
Baker Sand Control
Houston

Successful completion of horizontal wells in unconsolidated formations depends on proper equipment selection and installation method balanced with reservoir objectives, formation parameters, and costs.

The guidelines for designing these completions are based on generalized field experience, including horizontal cases where applicable.

HORIZONTAL WELLS

Horizontal wells are being widely applied throughout the oil and gas industry to enhance project economics and develop reservoirs that would otherwise not be commercially viable. Well productivity can be increased with horizontal wells and many fields have been developed for this reason.

Recently, horizontal wells have been completed in high productivity reservoirs for the purpose of reducing gas and/or water coning, thereby improving drainage efficiency and ultimate recovery.

Although originally drilled in carbonate and consolidated sandstone reservoirs, operators are now successfully drilling and completing horizontal wells in poorly consolidated formations that typically have high permeability and high-production potential. These formations are often incapable of producing without some type of sand-control technique.

The vast majority of normal wells utilize gravel packs for this purpose. For horizontal wells in unconsolidated reservoirs, gravel-packed completions are also a logical solution to sand-control problems.

However, the technical difficulties and costs often prove difficult to justify and alternative sand-control techniques become viable.

COMPLETION DESIGN

Several design philosophies are presented in this article along with case histories where available. Completion recommendations are given for each type of design based on specific reservoir parameters. The scope of the recommendations is limited to open-hole completions or those not having a cemented production casing or liner to effect zonal isolation.

Because many of the reservoirs meeting the criteria of unconsolidated and high productivity are located in environments favoring the application of medium and long-radius drilling technology (e.g., offshore locations), the discussion is further limited to wells with a minimum 300-ft radius of curvature or maximum 30/100 ft build rate.

The decision on how to complete a horizontal well in an unconsolidated formation depends largely on the cost of the completion relative to tangible and intangible benefits of the design compared to the cost of failure.

In normal wells, the cost of gravel packing is usually less than 10% of the total well cost. Because the benefits of a successful gravel pack include unrestricted productivity, long-term performance, and selective production capability, the decision to gravel pack is normally made without much difficulty. However, faced with productive intervals from 10-30 times the typical completion length, operators must give extra consideration in horizontal wells where the completion cost could equal or exceed the drilling cost.

Perhaps the single most important decision to be made concerns the type and size of sand exclusion device that will be used. Fig. 1 shows commonly used alternatives.

SLOTTED LINERS

Slotted liners (Fig. la) are fabricated by cutting narrow openings around the circumference of the pipe, parallel to the longitudinal axis of the pipe. Typical slot widths of 0.5 mm (0-020 in.) are spaced to provide inflow area equal to 2-3% of the pipe body surface area.

Slotted liners are the least expensive alternative and are not normally used in gravel-packed wells except in special instances. Because most formations requiring sand control have median formation grain sizes of less than 0.1 mm (0.004 in.), the slots do not directly prevent the entry of formation sand into the well bore.

Instead, sand grains must form stable bridges across the slots that in turn stop movement of the formation. The problems associated with slot plugging and insufficient flow area are well documented in testing performed by Shryock and Millhone (1980).

However, in horizontal wells, some amount of slot plugging may not be noticeable because of the unusually long completion interval lengths and associated flow area. Also, horizontal wells have much lower flowing velocities and pressure differentials outside the liner and thus will be less susceptible to instability of the formation sand bridges.

WIRE-WRAPPED SCREENS

Wire-wrapped screens (WWS) generally have 20+ times the flow area of comparably sized slotted liners (Fig. 1 b). The gauge or slot width in a WWS is normally determined by the size of the gravel-pack sand to be used. However, direct exclusion can easily be obtained with a WWS down to median formation grain sizes of 0.15 mm (0.006 in.) or less.

Compared to slotted liners, WWS's have a lower probability of failure due to erosion, although both are susceptible if the formation grains are small. The extra flow area makes the WWS more tolerant of plugging. Although the extra flow area dramatically reduces fluid velocity passing through the screen, the velocities are very low anyway in most horizontal wells because of the interval length.

Therefore, pressure losses due to turbulence should be negligible in both cases unless the production profile is very irregular. WWS's cost 23 times as much as slotted liners and are not typically used without gravel packing.

PREPACKED LINERS

Prepacked liners (PPL) are available in many design permutations that all have the same fundamental operating principle (Fig. 1c). A layer of uniformly sized gravel-packed sand is contained between concentric screen and/or tubular members such that fluid flow must pass through the gravel to enter the well bore.

The layer of gravel is normally from 3 to 10 mm (0.1 to 0.4 in.) thick and may be loose or resin coated (consolidated). By installing this type of device, sand control is affected much as with a gravel pack, i.e., formation sand bridges on the gravel, not the slot or screen, and all but the finest formation sand grains can be kept out of the well bore.

Prepacked liners can be classified based on their exterior construction and gravel type. In order to provide uniform entry into the liner, the exterior component of a PPL is usually a WWS or predrilled tubular member. Both offer good inflow area and mechanical strength although some feel the screen external construction is more likely to be damaged in severe applications.

A tubular external PPL must contain a resin consolidated gravel for obvious reasons. A screen external PPL may contain either loose or consolidated gravel because it is completely contained. The consolidated systems offer more protection from erosion because the sand grains cannot settle, reorient, or move which could allow formation sand to penetrate into the well bore.

The cost of a PPL can range from 2 to 3 times the cost of a WWS. In horizontal wells, many operators feel the added insurance of positive sand exclusion ability and durability sufficiently justifies the increased expense. Also, the cost of failure is likely to be a complete redrill or sidetrack in a horizontal well while a less expensive workover could be done in a normal well.

GRAVEL PACKING

The decision to gravel pack should be based on long-term well performance and design flexibility. Gravel packs can be placed around any of the previously discussed sand exclusion devices with equal success. The cost of the screen or liner relative to the cost of failure must be the deciding factor in most sequences.

By placing gravel-pack sand in the open hole screen-to-liner annulus, the formation is prevented from collapsing around the screen. This is desirable in those instances when formation collapse will considerably affect production, and improved productivity is the main objective of the well.

Many normal open hole gravel packs are under-reamed to increase the gravel volume around the screen or liner. Since gravel permeabilities are 2-4 orders of magnitude higher than formation permeabilities, the pressure losses in the near well bore area are negligible by design.

If the well bore is allowed to collapse around the screen/liner, the resultant permeability in the near well bore area may be considerably lower than the original formation permeability. The degree to which permeability is reduced is dependent on grain sizes, uniformity, and clay materials present.

Calculations based on a range of permeability reductions in the near well bore area indicate that productivity losses could range from 1 0 to 50%. Since many horizontal wells produce 5-10 times that of a vertical well, such losses might not be significant, especially if the well is being drilled solely to reduce coning.

Gravel-packing technology has evolved considerably in the last few years and should not be eliminated from consideration for purely technical reasons.

The additional cost of the slurry, pumping services, and rig time is highly variable but is typically much less than the PPL and accessory equipment.

SIZING SCREEN/LINER

The selection of screen/liner size should be based on completion type, expected well performance, and cost. In nongravel-packed applications, it is generally advisable to run the largest screen that will fit into the well to minimize the amount of formation collapse and subsequent productivity loss.

Another benefit of a large screen is that there will be less pressure loss from the far end of the well to the near end.

This will result in more uniform influx of fluid along the well bore length. The relative importance of this phenomenon depends on total flow rate, fluid viscosity, and other easily determined parameters.

Other considerations include the radius of curvature in the cased part of the well and the potential borehole irregularity in the open hole interval. Both would favor the use of a smaller screen as would economic considerations.

A starting point in screen selection for nongravel-packed open hole horizontal wells would be to size the screen OD approximately 40 mm (1.5 in.) less than the smaller of the nominal casing ID or borehole size.

Gravel-packed completions require considerable hydraulic analysis and need to have clearance around the screen/liner OD as well as a large ID to minimize circulating back-pressures.

Where possible, a minimum radial clearance of 40 mm is desirable and 50 mm (2 in.) preferred.

Because the borehole size depends on the casing size, under-reaming may become necessary where 178 mm (7 in.) OD casing and smaller is set.

With larger casing programs, the length of the horizontal section and formation parameters must be considered to determine if under-reaming is necessary. PPL's provide the same benefits in both gravel packed and nongravel-packed wells.

However, their dimensional considerations are critical in gravel-packed wells where borehole size is limited.

Special clearance PPL's have been designed and installed specifically for this purpose and also provide design flexibility in nongravel-packed wells. A guideline for screen/liner sizing is shown in Fig. 2.

INSTALLATION OF EQUIPMENT

Horizontal wells present some unique problems with equipment installation. In addition to having to pass through the curved casing in the build section, excessive frictional drag is likely to be encountered once the assembly enters the horizontal open hole section.

Rather than risk not getting to bottom and having to lay down a long assembly to make a conditioning trip, two methods can be used to increase the probability of a successful first trip.

ROTATABLE ASSEMBLIES

When frictional drag exceeds the force available to push the assembly around a turn or into the open hole, the assembly stops. Frictional drag is also supplemented by forces from loose formation material in the borehole which can accumulate ahead of the assembly.

By rotating the workstring and screen/liner assembly, frictional forces are greatly reduced and settled debris can be pushed aside. To do this, however, certain design modifications are necessary.

First, the torsional strength of all components must be considered and strengthened where applicable. The joint connections are normally the weak points and fortunately are easy to modify.

Screen and liner connections are normally API-type tapered connections, 8 or 10 round, on plain end (nonupset) tubulars. A number of alternative threaded and coupled connections are available, such as API buttress, that offer improved performance.

The liner hanger or packer that will be left in the well must also be capable of transmitting torque from the workstring to the screen/liner. Liner hangers are suitable for nongravel-packed applications and are usually not relied upon to provide a high-pressure seal between production tubing and annulus.

Production packers are normally run above liner hangers for this purpose. For gravel-packed wells, a rotatable production packer has been developed and used to facilitate screen installation and four position crossover tool operation in one trip.

Because there is no need to rotate and reciprocate once on bottom, this packer design has also been used in nongravel-packed horizontal wells as a combination liner hanger and production packer.

Two additional considerations for rotatable assemblies are screen centralization and bull-plug design. Centralizers are needed for gravel-packed completions to allow gravel to be placed around the entire screen. Centralizers are not needed otherwise, but may be desirable in nongravel-packed wells to lift the screen and minimize plugging on the trip to bottom.

If centralizers are used, they must not be damaged when the pipe is rotated. Conventional bow spring centralizers have proven acceptable for this purpose provided they are free to rotate on the pipe.

Finally, bull plugs that incorporate blade-type fins on the end can help move settled debris that could otherwise accumulate in front of the assembly.

Depending on hole conditions, rotation may not be needed to get to bottom. However, in wells with severe build rates, doglegs, ledges, or very long intervals, the ability to rotate should be incorporated, since the extra cost is insignificant.

WASH-DOWN ASSEMBLIES

In situations where well bore instability may result in excessive debris on the low side of the hole, the ability to circulate on bottom provides a means to condition the well without tripping the pipe. Wash-down assemblies typically consist of a concentric tubing string or tailpipe, extending through the entire length of screen/liner, and sealed near the end of the assembly with a check valve-type shoe or flapper.

Circulations can be performed as needed, and the tailpipe is removed after the hanger or packer is set. These assemblies can be used regardless of whether or not gravel packing is desired.

Although wash-down assemblies are useful for hole conditioning and cleaning, the problems of frictional drag, tight spots, and ledges can still prevent equipment from reaching bottom. For this reason, the best alternative combines the ability to circulate on bottom with rotating capability.

ADDITIONAL CONSIDERATIONS

The long-term performance of a horizontal completion is difficult to predict. Reservoir performance, premature gas or water breakthrough/coning, and nonuniform drainage along the well bore length are all viable reasons to isolate selected intervals for production or abandonment.

To accomplish this, inflatable external casing packers (ECP's) can be spaced out along the horizontal section.

Because ECP's prevent channeling in the original borehole, they can also be used to place multistage gravel packs where single-stage packers are considered too risky.

CASE HISTORIES

Five wells illustrate some of the completions that have been run.

WELL A

Well A (Fig. 3a) was completed with a 153-m (502 ft) long, under-reamed 280-mm (11 in.) well bore at 78 from vertical. The well bore was under-reamed with a xanthanderivative polymer brine. External casing packers were used to isolate water-bearing formations, and a rotatable bottom hole assembly (BHA) was used to push the WWS to bottom.

The well was gravel packed in two stages of 73 m using a hydroxyethylcellulose (HEC) viscosified brine and 480 kg/cu m (4 Ibm/gal) gravel.

WELLS B AND C

Wells B and C were completed in the fall of 1988 in the Giant Beaver and Santa Clara offshore fields west of Ventura, Calif. Both wells had 178-mm (7 in.) casing set in 280-mm under-reamed open hole intervals in the Hueneme Sespe and Upper Repeto sands.

Well B had a maximum well bore deviation of 87 but intersected the pay sand at 68. A 140 mm (51/2 in.) OD by 76 mm (3 in.) ID single-screen prepacked liner was placed in the open hole interval beneath a rotationally locked retrievable packer.

The 176-m (576 ft) interval was then gravel packed in one stage using an HEC viscosified brine to transport 7.3 cu m (256 cu ft) of 12-20 U.S. mesh gravel around the prepacked liner. This well has been flowed at 200,000 cu m/day (7 MMcfd) gas and 320 cu m/day (2,000 b/d) total fluid.

Well C (Fig. 3b), completed in the upper Repeto sand, is presently producing 40 cu m/day (250 b/d). It was gravel packed similarly to Well B. This well was a recompletion that previously produced 23 cu m/day (145 b/d) from an open hole gravel-packed completion having an average deviation of 50.

The recompletion of the well included the lower horizon of the Repeto sand and, therefore, had a total length of 450 m (1,475 ft) at a maximum deviation of 84.

The successful completion of these wells was considered by the operating company's management to be proof that horizontal wells can be successfully gravel packed using available technology.

WELLS D AND E

Two wells were drilled and completed using retrievable rotationally locked liner-hanger packers. These wells were not gravel packed because the reservoirs in which they were completed were considered to be somewhat consolidated.

Well D (Fig. 3c) is located in Lake Maracaibo, Venezuela, and was the first horizontal well drilled in that country. A 178-mm slotted liner having a total length of 316 mm (1,037 ft) was hung in a 216-mm (8 1/4 in.) open hole.

The total length of the completion setting was 344 m (1,128 ft) because a casing shoe and collar having a 73-mm (2.875 in.) sealbore were used beneath the slotted liner to permit the liner setting to be rotated and washed into place.

This setting proved necessary for complete placement of the liner. The liner setting had to be rotated and simultaneously washed into place for the last 105 m (350 ft) of open hole.

Well E had 140-mm casing set with a horizontal section of 44 m (1 43 ft). The BHA was installed in the 121-mm (4 3/4 in.) open hole using a rotatable packer.

The production liner setting consisted of three 3-m (10 ft) long sections of 75-mm (2.97 in.) OD prepacked screen, spaced between 73-mm (2 7/8 in.) OD tubing, such that the prepacked screen was located in the top, middle and bottom of the producing interval.

No specific problems were encountered during the placement of this liner setting, Production data are not available for these last two wells.

RECOMMENDATIONS

Table 1 compares the completion methods for controlling sand.

Because every reservoir is different, the following recommendations should be tailored to fit individual cases and not applied universally.

  • Slotted liners should be used in formations with coarse-grained formations, low production rates, and formations whose sand-producing tendencies are low.

  • Wire-wrapped screens should be used in lieu of slotted liners for the same applications with medium to coarse-grained formations

  • Prepacked liners are preferred for most formations, flowrates, and formation strengths. Optimized design and performance are obtained with thin protective-gravel layers that are resin consolidated.

  • Gravel packing should be performed in reservoirs with high-flow capacity when seeking maximum performance (when flow along the outside of the screen is not desirable) and in formations with substantial clay materials or formation fines. Generally, prepacked liners are recommended to protect against minor pack imperfections.

  • Screens or liners should be sized larger in nongravel-packed completions. Diametrical clearance should be approximately 40 mm (1.5 in.). Centralizers are not needed but are not considered detrimental.

  • In gravel-packed wells, radial clearance should be approximately 40 mm. Centralizers are desirable. If this requirement causes excessive circulating back-pressure due to concentric tailpipe ID, borehole under-reaming should be performed to allow the use of larger equipment.

  • First trip installation of downhole equipment is more likely where rotation of the BHA is possible. The ability to circulate on bottom may also be useful.

BIBLIOGRAPHY

  1. Ashton, J.P., Liput, J.C., Lemons, R.W., and Summerlin, J.W., "Gravel Packing Horizontal and Highly Deviated Openhole Completions Using Single Screen Prepacked Liners in Offshore California Fields." paper SPE 19718, SPE Annual Technical Conference and Exhibition, San Antonio, Oct. B-11, 1989.

  2. Shyrock, S.G., and Millhone. R. S., "Gravel-Packing Studies in a Full-Scale Vertical Model Wellbore-Progress Report," Journal of Petroleum Technology, July 1980, pp. 1137-43.

  3. Stewart, C.D., and Williamson, D.R., "Horizontal Drilling Aspects of the Helder Field Redevelopment," paper OTC 5792, 20th Annual Offshore Technology Conference, Houston, May 2-5, 1988.

  4. Zaleski T.E.. Jr., "Innovations in completion technology for horizontal wells," Offshore, February 1989, pp. 34-37.

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