Through-Tubing Sand-Control Techniques Reduce Completion Costs H.L. Restarick Jr., SPE, S.H. Fowler Jr., SPE, and W.P. Sedotal, Halliburton Energy Services
Summary
Sand-control problems in existing wells typically result from improper completion techniques or changes in reservoir properties. While recompletion of many of these wells with conventional sand-control methods and workover rigs is economically unfeasible, enhancements to gravel-pack fluid systems, downhole equipment, and service capabilities have increased success and reduced costs in through-tubing recompletions, providing new options to the operator for successful sand control in existing wells. Introduction
The most effective sand-control techniques are those implemented early in the life of the well before sand production becomes a problem. These techniques are carried out before the onset of water production or before formation damage occurs from formation disturbance or subsidence. High production rates cause excessive stress on weakly consolidated formations and exceed the capability of the cement material to bond the sand grains together. Once sand is produced as a result of formation damage, effective sand-control methods become more difficult and harder to justify. Marginal wells producing sand with poor reserves may not support the cost of a major workover program. Remedial options include sand bailing with wireline and sand washing with coiled tubing, but these only provide temporary solutions to sand-production problems. Although a low authority-for-expenditure budget, limited reserves, and a sanded-up well can limit the feasibility of a major workover, a number of products and services are available to the industry today that increase the success of through-tubing sand-control techniques. There are two categories of through-tubing sand control: mechanical methods, which include the use of small-diameter gravel-pack screens, and chemical methods, which bond the formation sand in place. The success of these methods has been aided by advances in wellbore-cleaning techniques that use high-pressure-fluid jets in conjunction with coiled tubing to clean tubulars, liners, perforation tunnels, or existing screens to prepare for the through-tubing sand-control operations. 1 Small through-tubing and prepack screens are manufactured today with precision quality-control procedures. Wireline techniques and a wide range of various wire sizes can be selected, depending on the well application. New advances in equipment design have enhanced coiled-tubing services, and new heavy-duty injectors with larger and stronger tubing sizes have made pumping heavy slurries and running through-tubing bottomhole gravel-pack assemblies into the wellbore easier. Today, fluid technology concerning retained permeability of the formation is extremely good and getting better. In fact, many initial completion designs of non-gravel-packed wells are now designed for future through-tubing gravel-pack contingencies in anticipation of sand problems that could develop during production. A combination of these new advances in products and services will increase the success ratio of through-tubing sand control. Method Selection
As is often the case before any decision is made on remedial maintenance to put the well back on production, the tubing has already sanded up and the well has been shut in for some time. Once the decision has been made to clean up the well and perform through-tubing sand control, the job method and design will depend on the mechanical configuration of the well bore. Factors to consider are tubCopyright 1994 Society of Petroleum Engineers Original SPE manuscript received for review Feb. 11, 1992. Revised manuscript received April 12. 1994. Paper accepted for publication June 1.1994. Paper (SPE 23130) first pre· sented at the 1991 SPE Offshore Europe Conference held in Aberdeen, Sept. 3-6.
236
ing and casing sizes, minimum restriction in the wellbore, type and locations of landing nipples, packer-setting depth, tail pipe below the packer, length of interval to be recompleted, location of interval in relation to production tubing, length of rat hole below interval, formation type, and type of well production (oil or gas). In some cases, through-tubing sand-control techniques are used to "repair" an existing, but damaged, gravel-pack screen. Mechanical Methods
Mechanical methods of through-tubing sand control involve the use of gravel-pack screens designed to be deployed through tubing, then set inside tubing, casing, or even another larger gravel-pack screen. In addition to the use of screens, a sand medium is often used to help keep the formation sand in place. Through-tubing gravel packing is not a new process; however, several advances in surface and downhole equipment and in fluid systems have made this process a popular alternative to a full-scale workover operation. 2 Three mechanical methods are commonly used. 1. The packoffmethod uses a through-tubing gravel-pack screen with a blank spacer pipe and packoff seal assembly. This can be placed inside casing or exiting gravel-pack screen and spaced up and packed off inside the production tubing (Fig. la). 2. The dual-screen method uses two screen assemblies separated by blank pipe placed and packed in the casing; production enters the lower screen and exits the upper screen section (Fig. Ib). 3. The wash-down method uses a ceramic bead prepack with the gravel-pack screen "washed" into place and packed off. This method is applicable for both casing and tubing (Fig. Ie). Each method will be discussed; however, several procedures are common to all three methods. I. The well bore and perforations or existing gravel-pack screen must be clean to perform the through-tubing gravel pack. All produced formation sand, scale, etc., should be removed and a clean completion fluid should be left in the well bore. This can be accomplished effectively with a coiled-tubing unit. Foam washing may be required on low bottomhole pressure (BHP) wells. 2. If the zone is reperforated, the fluid across the interval at the time of perforating should be a clear fluid, containing no undissolved solids. Solid particles in dirty completion fluid may be driven into the formation by the force of the perforating charge and impair permeability. This interferes with injection of treating fluids and with production after sand control is established. The practice of filtering workover and treating fluids is becoming more and more prevalent. 3. Perforating and stimulations are performed according to well requirements. 4. An injection rate is established into the formation. 5. The formation is prepacked with gravel-pack sand by use of coiled tubing or a small concentric workstring. 6. The maximum screen OD will be determined by the nipple ID's, and in some cases, by restrictions resulting from tubing damage. If the screen is to be run through the tubing and set in casing, the screen/casing ratio will always be out of proportion. A I-in. screen inside 7 -in. casing is not unusual with a 23/s-in. production string. In several completions, a I-in. screen even was used inside 9 5/s-in. casing. Because of the disproportional screen/casing ratio, a flowing pressure drop will occur and should be calculated during the preplanning stage. Table 1 shows the maximum screen OD's and ID's available vs. tubing and nipple sizes for several different types of screens. 7. Screen length should be calculated to extend at least 5 ft above and below the perforated interval. 8. The length of the rat hole below the perforations is especially important. If the length is excessive, setting a "bottom" < 10 to 15 ft below the perforations will be necessary. In most cases, this is SPE Drilling & Completion, December 1994
PRODUCTION TUBING
PACK OFF ANCHOR STOP
HYDRAULIC DISCONNECT PRODUCTION TUBING
PRODUCTION TUBING
CENTRMJZED SEAL RECEPTICAL
PRODUCTION PACKER
PRODUCTION PACKER
WASH PIPE RUNNING NECK/PLUG
Fig. 1-Mechanical methods: (a) packoff, (b) dual-screen, and (c) wash-down.
done by circulating sand through coiled tubing to establish a sand bottom. If the through-tubing gravel pack is done completely within the production tubing, a wireline plug can be set and used as a base. The base is critical to the job to eliminate sand from settling out past the bottom of the screen into the rat hole, thus creating a void in the screen/casing annulus. The use of an inflatable packer provides another method for setting a base in the casing. This method is wellsuited for isolating a lower set of perforations that have already started producing water in the casing below the production tubing. 9. A gauge run should be made before running the screen into the wellbore. Not only will the gauge run establish how much spacer will be needed below the screen, it will also drift (check) the tubing for restrictions and determine the amount of drag before going into
the wellbore with the through-tubing gravel-packing assembly. The drag and calculated weight of the bottomhole assembly (BHA) will determine the running procedure. When possible, the gauge run should be made during the pre planning stage to determinc the well conditions and the best method of running the screen assembly. An early gauge run will help reduce the chance of improper decisions that could result from well problems.
PackofT Method. Generally, when the tops of the perforations are < 100 ft below the end of the production tubing, the through-tubing screen assembly is attached to a blank pipe of sufficient length to allow the screen to be placed at total depth (TD), with the blank pipe extending up into the production tubing where it can be packed off
TABLE 1-SCREEN SELECTION BASED ON TUBING AND NIPPLE SIZE
Tubing Size WeighV 00 Length (in.) (Ibm/tt) 2.375 4.6 to 4.7 2.875 6.4 to 6.5 3.5 9.3 to 10.3
Standard Nipples Packing Bore 10 (in.)
1.875 2.313 2.813 to 2.750 4.0 10.9 to 11.0 3.313 4.5 3.813 12.75 5.0 13.00 4.313 5.5 15.50 to 17.00 4.562
Maximum Screen Sizes Wire-Wrapped All-Welded and Special-Clearance Perforated Pipe Prepack Dual-Screen Prepack Screen Prepack Screens Standard Standard Standard No-Go Standard Standard No-Go Standard Standard No-Go No-Go Nipples Nipples Nipples Nipples Nipples Nipples Nipples 10 00 10 00 10 00 10 00 10 00 10 00 10 (in.) (in.) (in.) (in.) (in.) (in.) (in.) (in.) (in.) (in.) (in.) (in.) (in.)
1.791 2.205 2.635 to 2.635 3.135 3.725 4.455
SPE Drilling & Completion, December 1994
1.735 2.100 2.550
1.750 1.049 2.100 1.380 2.630 1.751
1.460 1.750 2.550
0.824 1.735 0.720 1.049 2.100 0.720 1.610 2.550 0.824
1.995 2.441 2.992 2.992
2.970 3.480
1.995 2.550 0.824 2.550 2.441 3.480 1.610 3.480 4.130 1.995 2.992 4.130 1.995 4.130
2.970 3.480 4.130 4.130
4.130
1.660 1.900 2.375
0.720 0.720 0.824
0.824 2.875 1.049 2.875 1.610 3.500 1.610 3.500 4.000 1.995 1.995 4.000 1.995 4.000
1.049 1.610
0.720 1.660 0.720 0.720 1.900 0.720 0.824 2.375 0.824
1.995
237
OVERSHOT TYPE RUNNING TOOL SHEAR PIN RETRIEVABLE RECEPTACLE PLUG CENTRALIZED SEAL RECEPTACLE SUB
Attached to "G" packott assembly
0>
o
o
8Z
U'-..J.._/
(c) o o
SINTERED SCREEN
~
o o
(a)
r
r
Z u'--__ '--_...J...J
(b)
Fig. 2-Through-tubing gravel-pack tool: (a) centralized seal receptacle sub, (b) overshot-type running tool, and (c) retrievable receptacle plug.
(Fig. la). Blank pipe lengths of > 100 ft can create excessive pressure loss through the small diameter and are difficult to deploy. BHA's can be run into the wellbore by several methods. Most assemblies consist of a bull plug, screen, blank pipe, centralizers, and some type of combination running/releasing tool. The combination tool is designed to run the BHA by means of wireline or coiled tubing. Either method has its pros and cons, but wireline would be recommended in most cases because of its capability to jar and shear off the assembly being placed on bottom. Generally, if the total BHA weight is < 400 pounds, 0.092-in. wire can be used. If the BHA is heavier, 0.108-in. or 3/ w in. braided line is recommended. The standard through-tubing gravel-pack tool, which allows the screen to be run and set in the hole, consists of several parts. A centralized seal receptacle (Fig. 2a) is made up on the top section of blank pipe and is carried into the hole by an overshot-type running tool (Fig. 2b). The seal receptacle also houses a retrievable bull plug with a fishing neck. The bull plug (Fig. 2c) keeps the slurry from entering the screen and liner during the pumping operation. The seal receptacle, running tool, and receptacle plug are all pinned together and connected to the blank pipe and screen. The BHA is then run into the wellbore to bottom (Fig. 3a). The running tool is released while the receptacle plug is left pinned in the seal receptacle in preparation for the slurry pack. The coiled tubing is rigged up and run to 30 ft above the screen assembly. The injection rate is established; then, the sand slurry is pumped at a concentration of 0.5 to I Ibm/gal. Case histories have recorded 4- to 7-lbm/gal sand concentrations being pumped through I-in. coiled tubing. When screenout occurs, the excess sand is circulated out. On completion of the pumping phase, the slurry is given a predetermined time to settle out around the screen and liner; then, any sand left above the receptacle plug is circulated out of the hole with coiled tubing. The receptacle plug is then pulled with wireline and replaced with a small O-ring stinger run below a packoff-element assembly (Fig. 3b). Coiled tubing can now be used to jet the well onto production. Not all through-tubing gravel packs are set on bottom. Today, more operators are asking for prepack screens to be run without
I
(a)
o o o Z ()==-...:w (b)
<.)
Fig. 3-(a ) Complete running assembly and (b) O-ring seal stinger.
gravel packing. The BHA is simply run on wireline or coiled tubing and landed in a tubing nipple by use of a locking mandrel to seal off the blank/casing annulus. However, the life of the screen will be very limited if this option is used because turbulent flow cuts the BHA. This technique also leaves the formation sands unstabilized and the screen susceptible to plugging during production. In many cases, a "no-go" nipple is located at the bottom of the production tubing. This allows one of the best and easiest ways to hang off a through-tubing screen assembly before gravel packing. By use of either wireline or coiled tubing, a fluted hanger with sufficient bypass area through the flutes simply is landed in the nipple profile (Fig. 4). On completion of the gravel pack, the receptacle plug is pulled and replaced with the production assembly. In cases with the option of either hanging off the screen from the end of the tubing with a nipple profile or simply setting the screen on bottom and allowing the upper end of the tubing assembly to extend up into the production tubing, the latter is recommended. This will allow for a greater flow area around the top of the liner during the pumping stage, thereby decreasing the chance of a sand bridge at the nipple. If the nipple used to hang off the through-tubing assembly is below the production packer, the production tubing or pup joint between the packer and nipple can be perforated just above the nipple to increase the flow area during the pumping stage. On gravel packing, the tubing perforations can then be isolated with a short spacer between the seal assembly and the packoff element (Figs. Sa and 5b). For new non-gravel-pack completions that may require future through-tubing sand control, the tail pipe below the production packer is initially installed with a ported pup joint above the lower landing nipple and a second nipple below the packer (Fig. 5c). This tail-pipe configuration also provides an excellent location for landing BHP and temperature gauges. By setting the gauges in the lower landing nipple, production restrictions are eliminated during tests.
IM---+t--
I~,_c I.
ANCHOR
II
,,~PACKOFF
'~ =
IIO-I+--+f--~
c- -
STINGER SEAL
FLUTED RECEPTACLE
J
Fig. 4-Fluted-hanger assembly.
Because of the configuration and length of the through-tubing gravel-pack assembly, the well must be dead to rig up and run the assembly into the wellbore. The screen section and centralizers may make the assembly difficult to run under pressure. This is not to say it is impossible; in some applications, the screen and blank can be run under pressure in short sections and connected downhole with a series of seal subs and receptacles. This method is not recommended in the casing but should be limited to within the production tubing where alignment of the assemblies is possible. Sliding-Sleeve Method. For wells that start producing sand from an alternative non-gravel-packed zone through a sliding sleeve, prepack screens can easily be set across the sleeve (Fig. 6). If more than one sleeve is run, the upper through-tubing screens can be packed off above and below the sleeve ports to allow production in and through the upper screen assemblies. Other designs that anticipate sand production include placing an all-welded screen jacket or prepack screen over the sliding sleeve. This method does not restrict the tubing ID with a through-tubing assembly, and the sleeve can be opened and closed easily during the life of the well. Dual-Screen Method. When the tops of the perforations are located > 100 ft below the end of the production tubing, a unique and simple option that uses a combination of two gravel-pack screens is available. Production through the gravel-pack medium is estimated to be limited to =60 ft. With this theory, a unique flow path for production can be formed. A screen assembly is used that consists of a primary production screen long enough to cover the perforations, 60 to 100 ft of blank spacer pipe, and a short upper section of screen bull plugged on top. This double-screen assembly is run into the wellbore with wireline or coiled tubing and set in the casing on bottom. A double-screen through-tubing assembly does not require the top of the blank liner to be extended up into the production tubing (Fig. Ib). After the assembly is properly set on bottom, the slurry is pumped completely over and around the screen, covering the entire SPE Drilling & Completion, December 1994
screen assembly. In most cases, the slurry is pumped through the coiled tubing. In some cases where no gas-lift mandrels are in the tubing string, the slurry is simply pumped down the production tubing. Then, the slurry is allowed to settle out around the bottom section of the screen. After the slurry settles, the sand is washed from around the upper screen section with coiled tubing. The production is now directed through the lower screen section, up the blank pipe, and out the top section of the screen. The well now can be jetted onto production with coiled tubing and nitrogen. This technique offers several advantages. First, drawdown pressures are kept to a minimum because of a shorter length of blank pipe run in the BHA. Second, the lower landing nipples are kept open and can be used for setting different flow-control devices because the blank pipe is not extended up into the production tubing. By leaving the entire screen assembly in the casing, several assemblies can be eliminated. The packoff, seals, and seal receptacles are omitted, reducing the cost and time required to run them. The last advantage of double-screen setting is that fewer trips are required to recomplete; in most cases, it can be accomplished in a single trip. We recommend the use of centralizers for this type of recompletion to centralize the gravel-pack assembly in the casing and to allow placement of a tight gravel pack without voids. The type of centralizer will depend on where the through-tubing gravel-pack assembly will be set. If the BHA is run through tubing and set in the casing below the tubing string, some type of spring centralizer will be required; severa types are available. A standard spring type, which wraps around a collar or limit ring that allows both ends to move, enables the blades to be pulled in either direction, creating less drag. Fig. 7 shows a centralizer that is designed with an adjustable tension ring. This type of centralizer has been run through 23/ g-in. tubing and set in 9 5/ g-in. casing. We recommend that a spring centralizer be run below the production-screen section. If the through-tubing gravel-pack assembly is to be left inside the production tubing with standard ID landing nipples, a rigid weld-on-type centralizer is recommended. Wash-Down Method. The wash-down method consists of depositing spherical beads or gravel to a predetermined height above perforations, then running the screen-and-liner assembly with a wash pipe and a circulating-type shoe on coiled tubing. The screen is washed down through the gravel. When the shoe is on bottom, the beads are allowed to fall back around the screen and liner; then, the coiled tubing is released from the screen assembly and tripped out of the hole. 3 A packoff device is installed, and the well jetted onto production. Using a spherical material, such as sintered bauxite or ceramic beads, in the prepack will help hold formation sand outside the casing. Owing to the spherical nature of the beads, an absolute flow space is created between the beads, allowing the hydrocarbons to flow through.4 Spherical beads allow the screen to be washed into place easily without compaction of the filter media. The maximum length of intervals that can be treated with this method is = 50 to 60 ft because keeping the spherical bead prepack fluidized during the washdown process on longer intervals is difficult. As mentioned earlier, the wellbore must be properly prepared by washing and displacing with a clean, filtered fluid. A gauge run is made, a bottom is set, if required, and an injection rate is established. The prepack slurry is mixed and pumped through coiled tubing, which is located above the perforated interval. The slurry is spotted at the end of the coiled tubing, and the annulus valve closed while the slurry is squeezed into the formation until sandout occurs. Formation fracture pressure should not be exceeded. The coiled tubing is picked up, and time is allowed for the gel to break and the gravel to settle. The coiled tubing is then run back down to tag the prepack top. If the prepack top is too low, an additional slurry pack will be required. The coiled tubing is then removed from the wellbore. Then, the wash-down gravel-pack assembly is connected to the end of the coiled tubing and includes (in ascending order) jet-wash shoe with check valve, wash-pipe-seal sub, gravel-pack screen, blank spacer pipe, locating mandrel (optional), and hydraulic disconnect. An internal wash pipe is connected to the hydraulic release and run through the screen assembly to allow circulation through the jet wash shoe (Fig. Ic). Circulation through the jet-wash shoe should be established before running into the wellbore. The wash239
UPPER NIPPLE
----J1"'lrr- UPPER NIPPLE
If~~:~_c9_ I d - - UPPER NIPPLE
ANCHOR REELED TUBING
PACK OFF RECEPTACLE PLUG PORTED PUP JOINT NIPPLE LOCATOR IN LOWER NIPPLE
SHORT SPACER j---- PORTED PUP
PORTED PUP JOINT LOWER NIPPLE
JOINT
II\;~::;;:_=:-:::/~tt--
LOWER NIPPLE
~
Fig. S-Completions: (a) tail-pipe through-tubing, (b) tail-pipe through-tubing with packoff, and (c) new non-gravel-pack.
down screen assembly is run while slurry is pumped slowly until it reaches = 30 ft above the top ofthe prepack. The pump is stopped, and the top of the prepack tagged; then picked back up. Circulation is established at 5/S to 1/4 bbUmin, and the screen is slowly washed down through the filter medium until TD is reached. The ball for the hydraulic releasing tool is placed into the reel and pumped until it is past the wellhead. The pump is then shut down, and the ball is allowed to gravitate down to the hydraulic release tool. The screen assembly is pressured up and released, then picked up and circulated from the bottom up. It is then pulled out ofthe hole with the hydraulic release tool and wash pipe. Wireline is used to run the packoff and anchor assembly. The well is jetted onto production with coiled tubing and nitrogen. If a uniform gravel or bead size is not used, some segregation of the gravel sizes during the wash-down process may occur. Screen Types
All-Welded Screen. Fig. 8a shows the all-welded screen that has been a standard in sand control for several decades. This screen is wire-wrapped with a keystone-shaped wire on ribs and welded onto a base pipe. Keystone-shaped wire is used to decrease the chances of plugging the screen. If sand is smaller than the slot size, it will flow through because the perforations on the pipe behind the screen jacket are much larger. Slot size of the screen is determined by the gravel size used to pack around the screen. As long as the slot opening is smaller than the smallest gravel size used in the pack, no problems should occur. Typically, a stainless-steel screen is used on a carbon-steel pipe. This configuration has been used successfully for many years worldwide. However, depending upon well environments, other alloys should be used at times. 240
Dual-Screen Prepack. The dual-screen prepack (Fig. 8b) has a high inlet area with a "double" screen design that allows maximum radial placement of gravel between screen jackets. These screens are available in 1.735 and 2.100-in. OD's and an ID of =0.720-in. 'for through-tubing applications. The inner screen is a "rod-based" design without an inner perforated pipe base, which allows maximum gravel thickness. The advantages of the prepack are that gravel packing is not required, the outer screen jacket is more flexible than that of a perforated prepack screen, and the outer screen jacket provides more flow area than a perforated prepack screen. The disadvantages are that the screens could become plugged with fines or mud; the dualscreen prepack has a large OD and a small ID, which can restrict production rates; and the outer screen jacket may become damaged if run through a tight spot. Perforated Prepack Screen. The perforated prepack screen (Fig. 8c) allows flexibility in through-tubing applications for running in areas of damaged casing or tubing. The minimum OD pipe available is 1.660-in., with a rod-based screen designed for internal gravel retention. Gravel thickness (radial placement) is measured through the perforation to the actual pipe OD; the gravel is an epoxy-coated, thermally set material. Advantages ofthe perforated prepack screen are that gravel packing is not required and perforated pipe case is rugged. The disadvantages are that the screen could become plugged with fines and mud, resin-coated gravel could break and fall out through the perforated pipe case, and flow area through the perforated pipe case is less than that of other designs. Special-Clearance Prepack Screen. Fig. 8d shows the special-clearance prepack screen. This screen has a high inlet area with flow chanSPE Drilling & Completion, December 1994
SLIDING SLEEVE
LOCKING MANDREL
EQUALIZING SUB
OPEN PORTS
PREPACK SCREEN
BULL PLUG
Fig. 7-Centralizer with tension ring. Fig. 6-Sliding sleeve with prepack screen (sintered-metal screen).
nels on an inner retention screen, which allows maximum flow potential. The design accommodates the maximum ID and the screen OD is the same as that of the standard all-welded design with a thin wall of gravel pack available between the screen jackets for added sand control. It is available in sizes as small as 1.3IS-in.-OD base pipe. Screen advantages are a large flow area and the same OD and ID as regularnonprepack screens. Its disadvantages are that it has minimal prepack gravel and is recommended only for wells that will be gravel packed. Sintered-Metal Screen. The sintered-metal screen (Fig. Se) embodies a technology proved in the industrial filtration markets. Sintered stainless steel (or other applicable high-nickel alloys) is bonded under extreme temperature and pressure and shaped into a cylindrical body (tube), producing a porous metal filter medium whose performance compares favorably with that of such devices as prepack well screens. This cylindrical body provides a porous medium capable of = 100-,um absolute filtration. This tolerance can be adjusted downward by reducing the individual sintered-metal diameters. The advantages are that one size sintered-screen design fits most gravel-pack sand sizes vs. requiring a specific wire-wrapped-screen gauge size, the sintered screen design is tougher than the wirewrapped design, its wall thickness allows for a minimum OD with a maximum ID, and it allows 100% inlet flow from any direction. In addition, the sintered-metal screen has less flow restriction than current prepack screen designs; has more flexibility to pass short radial bends compared with standard prepack-screen designs; is impervious to acid, which will dissolve the sand in current prepack screens as well as resin; and in some applications, will allow the sintered screen to be run without the base pipe, lowering the cost and increasing the ID. Its disadvantages are that the current maximum limit is 100-,um absolute filtration and certain high-nickel alloys currently cannot be sintered. SPE Drilling & Completion, December 1994
Slotted Liners. Slotted liners (Fig. Sf) have been manufactured in a number of ways. The simplest are made of oilfield tubular goods that have been slotted with a precision saw or mill. Their advantages are that they cost less than wire-wrapped screen and offer a large ID for production. Disadvantages of slotted liners are that they have a low inlet area and are susceptible to slot erosion encrustation (closing of slot). Gel Systems
Through-tubing gravel packing may be performed with a number of carrier fluids. Each fluid fits an application and should be selected carefully. The objective is to use a fluid that will effectively carry the gravel or sand at pump rates low enough to prevent scouring, abrasion, and intermixing of pack and formation sand. As a rule, most through-tubing gravel packs must be bullheaded into place. No tell-tale screen or wash pipe is required because no returns are taken during the procedure. Because viscous systems require less fluid owing to high gravel concentrations, use of the thickened-fluid system is preferable to use of thin fluids, which would result in high fluid losses to the formation and possible formation damage. Experience has shown that simple gel systems are suitable for gravel packing most wells that need sand control. However, if the well has a highly deviated interval of ~ 60 from vertical, simple gel systems may not support the pack sand completely. The problem is not in the coiled tubing, which has a small pipe diameter resulting in high fluid velocities, but in the slurry exiting the coiled tubing into the casing where the velocities are slow; this results in the sand settling out to form a bridge. One method for improving this situation is to crosslink the gel system. Crosslinked gels are much more viscous than simple gels and can fully support a high concentration of sand, even in a static state. Because no settling occurs, the chance of a premature sandout is reduced greatly. Crosslinked gels are formed by mixing a low concentration of simple gel, then adding a crosslinking agent. This crosslinking agent 0
241
Resin Method. Water-compatible furan resins have been used effectively for near-wellbore sand consolidation. Resins have been used to "repair" damaged gravel-pack screens in place. An average wellbore radius of > 3 ft can be consolidated. 5 The process involves cleaning up the well bore and preparing the treatment interval as discussed earlier, then pumping a leading load of salt water into the formation to prepare the sand surfaces so that they will provide a site for the chemical reaction needed for the resin to absorb the sand. The resin is then pumped, followed by a saltwater spacer to separate acid and resin, to remove excess resin from the pore spaces, and to flush the resin further into the foundation. The acid is then pumped to catalyze the resin. A final brine flush is injected to enhance displacement of the acid catalyst. Nitrogen is often commingled with the injected fluids to act as a diverter and to aid in uniform placement of the resins in the formation. Coiled tubing has proved beneficial in placing these chemicals uniformly over long intervals by use of specially designed nozzles and by manipulating the tubing through the entire interval during treating. After the resin hardens, a permeable, but solid, sand filter is formed. Typically, 85% to 90% of the original formation permeability will remain. Resin-Coated-Sand Methods. For any type of sand control, having sand packed very tightly behind every perforation to compress the formation to its "natural" state is considered very desirable. Packing presumably replaces formation sand in the perforations with higherpermeability pack sand. 5 Studies have shown that the resin-coated sands can achieve very high compressive strengths and remain highly permeable even after a flow of 30 million PV. 6 The procedure for resin-coated sand packs is very similar to the resin procedures. Typically, resieved Ottawa 20/40- or 40/60-mesh sand is used; this is batch-mixed with the resin and carrier fluids at a ratio of 1 Ibm sand/gal carrier fluid and 1.5 gal resin/per sack sand. The system can either be catalyzed internally or externally. Using an externally catalyzed system will allow placement of the pack into the perforations and formation, with any excess sand washed out of the wellbore before the acid to set the resin is pumped. An internally catalyzed system, which requires fewer steps for pack placement because catalysts are mixed with the resin-coated sand mixture before pumping, can be used. The treatment is pumped until sandout occurs in the perforations, meaning that the resincoated sand will extend above the producing formation. This will form a sand column that will have to be removed after the resin cures. 6 Removal can be accomplished with mills and under-reamers on concentric or coiled tubing. Case Histories Fig. 8-Screens and slotted liner: (a) all-welded screen, (b) dual prepack screen, (c) perforated prepack screen, (d) special-clearance prepack screen, (e) sintered-metal screen, and (f) slotted liner.
forms a chemical bond between the polymer molecules of a simple gel and increases its viscosity dramatically. Gelling and breaking agents similar to those used in the regular gravel-pack fluids can be used in the crosslinked systems. Another advantage to the use of crosslinked gels is that they add stability to a completion. A disadvantage is that they provide some fluid loss to the formation, and therefore, may not form as "tight" a gravel pack as simple gel systems. Chemical Methods
Chemical sand-control methods use chemicals and resins injected into poorly consolidated formations to provide in-situ grain-tograin bonding. Two common methods are consolidation of formation sand with neat resins by use of brine placement fluids and packing of formations with resin-coated sands. 2 These methods can be catalyzed either internally or externally and offer economical options for solving sand-production problems in wells with the following conditions: (1) relatively long intervals, (2) minimal wellbore ID's, (3) static bottomhole temperatures between 60 and 400°F, and (4) BHP gradients of < 11.6 Ibm/gal for brine-compatible resins. 242
Case History I-PackoffMethod. An offshore U.S. Gulf of Me xico well was originally completed by the operator as a single gravelpack completion. This well was a leaseholder and was sanded up; average production had been 160 BOPD, 55 BWPD, and 1.28 MMcflD gas. The last test yield showed 13 BOPD, 7 BWPD, and 935 McflD gas with a 1,624-psi flowing tubing pressure (FTP). The original gravel pack consisted of 180 ft of 27/g-in. 0.006-gauge screen set across the perforations at 11,164 to 11 ,346 ft. An "S-I" nipple was 268 ft above the top perforations in the 27/g-in. tubing. The average deviation through the completion interval was 40° . The job was designed with a through-tubing prepacked screen assembly run on 1.25-in. coiled tubing and located in the nipple profile. Procedure. The following was the gravel-packing procedure. 1. Surface equipment was rigged up and tested. 2. The hole was entered and sand was washed to 11,350 ft with 8.6-lbm/gal salt water. Then, we checked for fill. 3. The well was killed with 13.2 Ibm/gal CaBr2, and we pulled out of the hole. 4. Braided line was rigged up, and a gauge run was made. 5. Screen assembly was run into the hole on coiled tubing and located in S-I nipple. The screen assembly consisted of a bull plug, 198 ft of prepacked screen, 270 ft of centralized blank pipe, and an S- I locator with upper seal receptacle and milled bypass. 6. With the end of the coiled tubing 5 ft above the locator assembly, the tubing was flushed with two tubing volumes of filtered fluid and the following pack performed: (a) 6 bbl 10% HCl with 7 Ibm of SPE Drilling & Completion, December 1994
citric acid and 0.2% organic inhibitor, (b) 3 bbl3% ammonium chloride water, (c) 3 bbl gel pad without gravel, (d) 5 bbl sand slurry containing 50nO-mesh resieved sand, (e) 3 bbl3% ammonium chloride water, and (t) filtered 13.2-lbmlgal completion fluid . 7. Acid and slurry were squeezed into the formation until a 2,500-psi sandout was reached. 8. 13.2 Ibmlgal fluid was displaced with 8.6-lbmlgal seawater, and we pulled out of the hole. 9. Braided line was used to pull the receptacle plug and install the packoff. The well was placed back on production at 82 BOPO, 92 BWPO, and I MMcflD gas. The cost of a rig-type workover was = $1.5 million, while the cost of the through-tubing gravel pack was =$150,000. Case History 2-Dual-Screen Method. The operator's well was on inland water and completed as a single gas well with 23!s-in. tubing to 11,510 ft and 95!s-in. casing to a plugged-back TO of 11,683 ft. The average production was 2.8 McflD gas with 80 BWPO at a 2,400-psi FrP. The job was designed to run a dual screen through the tubing, which had a 1.7I -in. no-go nipple, and set it in the casing. Procedure. The procedure used follows. 1. I-in. coiled-tubing unit was rigged up and tested. Tubing was pickled. 2. The wellbore was entered and foam washed to TO. 3. The injector rate was checked with filtered fluid. 4. Fill was checked for, and we pulled out of the wellbore. 5. Oual screen was run on wireline with ajar-down-release running tool consisting of a bull plug, centralizer, 16 ft of O.OOS-gauge screen, centralizer, 132 ft of blank pipe, and 5 ftofupper screen with a running-neck bull plug. This was set and released on bottom. 6. Coiled tubing was into the wellbore to the top of the liner at 11,495 ft, and the injection rate was established. 7. A preacid treatment was performed. 8. A gravel-pack slurry was pumped that consisted of (a) 3 bbl of gel, (b) 5 of bbl gel containing 1,000 Ibm of 40/60-mesh gravel, and (c) 3 bbl of gel. 9. The slurry was displaced into the formation until sandout occurred. 10. The slurry was washed down past the upper screen to a maximum of 11,593 ft. 11. The well was jetted in with nitrogen. The well was placed back on production at 2.1 McflD gas and 70 BWPO with a 2,1 OO-psi FrP. Estimated cost of a rig-type workover was $250,000, while the dual-screen sand-control job was performed for =$15,000. Case History 3-Resin Method. This offshore U.S. Gulf of Mexico well was originally completed as a single completion with a selective alternative, or "stack pack," where two reservoirs were gravel packed individually with the upper sand blanked off with an isolation string. The lower zone watered out and was isolated with a tubing plug set in a landing nipple. The upper zone was accessed through wireline with a tubing punch to perforate the isolation string. Initial gas production was 14 MMcflD, but the well test indicated an I8-MMcflD potential. An acid stimulation job was performed; however, the well started to produce sand and flow rates had to be reduced to 2.5 MMcf/ O. A resin treatment was designed to consolidate the sand behind the 32 ft gravel-pack screen with nitrogen and a I-in. coiled-tubing unit. Procedure. In this well, we used the following procedure. 1. Surface equipment was rigged up and tested. 2. Coiled tubing was pickled with acid, and the acid was displaced with neutralizer. 3. Sand was washed to 11,499 ft with 2% KCI water. 4. Coiled tubing was positioned across interval, spotted, and squeezed with following treatment at V4 to V2 bbl/min: (a) 65 bbl 15% NaCI water with 0.25% surfactant and 600 scf/bbl nitrogen; (b) 25 bbl externally catalyzed furan resin with 600 scf/bbl nitrogen; (c) 24 bblI5% NaCl water with 0.25% surfactant and 600 scf/bbl nitrogen; and (d) 83 bbllO% HCI catalyst (mixed with NaCI water) with 0.25% surfactant, 0.3% acid inhibitor, and 600 scf/bbl nitrogen. 6. Coiled tubing was displaced with filtered 2% NaCl water. 7. The well was shut in for 8 to 12 hours while the resin cured. SPE Drilling & Completion, December 1994
8. Production was resumed at 4 MMcflD until the load water was recovered. The well was stabilized at a flow rate of 13 MMcflD gas with no sand production. The cost of performing a conventional rig workover on this well had been estimated at $1.3 million; the resin job to repair the screen was performed for = $90,000. Conclusions
New downhole completion techniques that include a family of coiledtubing-conveyed through-tubing tools and fluid systems have been developed to work with wireline and hydraulic workover services. These fully coordinated methods reduce the exposure time when incompatible fluids are in contact with the formation, allowing a higher percentage of regained permeability. These services have proved their success as a means of well maintenance with the onset of sand production, even in deviated wellbores. However, the considerable reduction in well maintenance costs that these services can offer compared with the cost of a workover rig is of special significance in light of the prevailing climate of the oil and gas industry. References I. Fowler, S.H.: "A Reeled-Tubing Downhole-Jet Cleaning System," paper SPE 21676 presented at the 1991 SPE Production Operations Symposium. Oklahoma City, April 7-9. 2. Shurtz, G.C., Breiner, WG .. and Comeaux, B.G.: "New Through-Tubing Gravel-Pack Techniques," paper SPE 5660 presented at the 1975 SPE Annual Technical Conference and Exhibition, Dallas, Sept. 28-Oct. I. 3. Otis Sand Control Customer Manual (1991) . 4. Caillier. M.: "Process for Washing Through Filter Media in a Production Zone With a Pre-Packed Screen and Coiled Tubing," U.S. Patent No. 4,856,590 (Aug. 15 1989). 5. Murphey, J.R., Bila, V.J., and Totty, K. : "Sand Consolidation Systems Placed With Water," paper SPE 5031 presented at the 1974 SPE Annual Meeting, Dallas, Oct. 6-9. 6. Stutz, L., Cavender, T., and Murphy, J. : "Epoxy-Coated Sand Taps New Gas in Old Wells," Oil & Gas J. (March 4, 1991) l.
SI Metric Conversion Factors
bbl x 1.589 873 E-Ol =m 3 ft x3.048* E-Ol =m ft 3 x 2.S31 685 E- 02 = m3 OF CF-32)/1.8 = °C gal x3.785 412 E-03 =m 3 in. 2.54* E+OI =mm Ibm x4.535 924 E-OI =kg psi x 6.894 757 E+OO =kPa SPEDC
' Conversion factor is exact.
Henry L. Restarick is sand control manager of Halliburton Energy Services in Houston. He has been with Halliburton for 22 years in the U.S. gulf coast area and Dallas, where he has held positions of completion specialist, sales manager, senior sales conSUltant, and specialized completion manager. He holds 10 oilfield patents. Restarlck holds a degree from Kemper C. S. Hampton Fowler has been with Halliburton Energy Services for 13 years and is currently global manager of Coiled Tubing & Nitrogen Services. During his carreer with Halliburton , he has held positions of field engineer in Louisiana , where he worked with coiled tubing, nitrogen, snubbing units, gate-valve drilling equipment and freeze services; as service development manager for Coiled Tubing & Nitrogen Services in Dallas; and as operations manager for Coiled Tubing & Stimulation Services for the U.K. and North Sea area. He holds four U.S. and foreign patents associated with coiled-tubing tools and processes. Fowler holds a BS degree in agricultural engineering from Texas A&M U. Biography and photograph for W.P. Sedotal are unavailable.
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Fowler
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