COMPLETION/STIMULATIONSand Control: Why and How? Jon Carlson Derrel Gurley George King Colin Price-Smith Frank Waters Chevron Services Co. Houston, Texas, USA Amoco Production Co. Tulsa, Oklahoma, USA BP Exploration Inc. Houston, Texas, USA Tulsa, Oklahoma, USA Houston, Texas, USA Sand production erodes hardware, Unconsolidated sandstone reservoirs with permeability of 0.5 to 8 darcies are most blocks tubulars, creates downhole susceptible to sand production, which may cavities, and must be separated start during first flow or later when reservoir pressure has fallen or water breaks through. and disposed of on surface. Com- Sand production strikes with varying pletion methods that allow sand- degrees of severity, not all of which require action. The rate of sand production may prone reservoirs to be exploited decline with time at constant production conditions and is frequently associated with often severely reduce production cleanup after stimulation. efficiency. The challenge is to Sometimes, even continuous sand pro- duction is tolerated. But this option may complete wells to keep formation lead to a well becoming seriously damaged, production being killed or surface equip- sand in place without unduly ment being disabled (left ). What constitutes restricting productivity. an acceptable level of sand production depends on operational constraints like resistance to erosion, separator capacity, For help in preparation of this article, thanks to: Bob ease of sand disposal and the capability of Elder, Chevron UK Ltd., London, England; David Wag- artificial lift equipment to remove sand- ner, Chevron Exploration and Production Services Co., laden fluid from the well.1 Houston, Texas, USA; Mike Mayer, Dowell Schlumber- ger, Montrouge, France; Roger Card, Loren Haugland This article reviews the causes of sanding, and Ian Walton, Dowell Schlumberger, Tulsa, Okla- and how it can be predicted and controlled. homa, USA. It will examine the four main methods of In this article, NODAL (production system analysis) and IMPACT (Integrated Mechanical Properties Analysis & sand control: one that introduces an artifi- Characterization of Near Wellbore Heterogeneity) are cial cement into the formation and three marks of Schlumberger; PacCADE, ISOPAC and that use downhole filters in the wellbore. PERMPAC are trademarks or service marks of Dowell Schlumberger. The article then focuses on gravel packing, by far the most popular method of complet- 1. Veeken CAM, Davies DR, Kenter CJ and Kooijman ing sand-prone formations. AP: “Sand Production Prediction Review: Developing an Integrated Approach,” paper SPE 22792, presented at the 66th SPE Annual Technical Conference and Causes of Sanding Exhibition, Dallas, Texas, USA, October 6-9, 1991. Factors controlling the onset of mechanical 2. Anderson R, Coates G, Denoo S, Edwards D and rock failure include inherent rock strength, Risnes R: “Formation Collapse in a Producing Well,” The Technical Review 34, no. 3 (October 1986): 29-32. naturally existing earth stresses and addi- tional stress caused by drilling or produc- tion.2 In totally unconsolidated formations, sand production may be triggered during nPerils of sand production. At worst, sand the first flow of formation fluid due to drag production threatens a well. Voids can form from the fluid or gas turbulence. This behind the pipe, causing formation subsi- detaches sand grains and carries them into dence and casing collapse. The well may also fill with sand and cease flowing. Or the surface equipment may be catastroph- October 1992 ically damaged by erosion or plugging. 41 fluctuations in produc. many factors including the weight of the duction by reducing capillary pressure overburden. a perforation earth stresses is created. earth stresses and the way the rock will fail. onset of water production. and field and operational parameters. Predicting Sanding Potential zones of some of the wells. Additionally. tunnel may be filled with pulverized models have been developed to cope with sand and shaped-charge debris. First flow may remove this debris. pre. USA. accurate estimates of horizontal perforation cavity and weaken the formation stresses are integrated with logs and. and with high allow measurement of some far-field earth circular cylinder of rock is compressed par- pressure differentials during drawdown. sand prediction or in-situ stress. which commonly causes sand pro. bore to laboratory. In a uniaxial compressive test. sanding may be Fracture Treatment Design. However. a sand flow test is derive the elastic properties of rock and pre- Fluid inflow Perforation tunnel often used to assess the formation stability. A stable arch is established between sand production. wireline formation testing tools or DSTs. in-situ stress nDoorway to the wellbore. A correlation may then be correlating logs and cores. This has given rise to inter- Cement involves observing the performance of pretation techniques that combine direct nearby offset wells. the in-situ cantly altered during the journey from well- bilizes again. Other causes of sanding include water Compacted zone The earth’s in-situ stresses are due to influx. for example. a fluid viscosity and flow rate. needed and this may be estimated using ened and likely to suffer tensile failure. Before cleanup stones in an arched doorway—that is stable using the laboratory core data.7 Young’s Modulus and Poisson’s ratio—and tion rate. Although these techniques provide direct uniaxial compressive strength may then be changes in gas/liquid ratio. compacted zone can remain around Reservoir pore pressure information is also the surface of the cavity that is weak. empirical at constant drawdown and flow rate. predicts formation sanding potential using tion is reached or the maximum drawdown values for formation strength obtained by is achieved.5 Cement Perforation tunnel choose from. formation’s mechanical strength. sand particles are dislodged by mated using bulk density logs. Weakened zones may then become nDebris and damage in the perforation geologic model.” page 4 ). rock strength measure- Laboratory measurements on recovered ments gathered from core tests are crucial to cores may be used to gather rock strength the IMPACT analysis computation of rock the perforations. This is not always a straightforward yields a direct measurement of rock strength task. After water break. a continuous profile of susceptible to failure at sudden changes in tunnel.6 Quantitative of Near Wellbore Heterogeneity. Triaxial tests make the same voir pressure or subsidence. Before cleanup. Houston. recently information may be acquired by gradually developed by Schlumberger Well Services. drawdown pressure that will induce sanding. tectonic forces and pore pres- After cleanup between sand grains. correlations are derived to obtain the venting sand movement (above ). no logging tool sand.3 stresses (see “Cracking Rock: Progress in allel to its longitudinal axis. While the vertical stresses may be esti- through. the arch col- lapses and a new one forms once flow sta. Texas. that employ log and microfracture data and perforation cavity. Downhole wire- Fluid inflow The completion engineer needs to know the line log measurements provide continuous conditions under which a well will produce profiles of data. Changes desired rock strength parameters from log- in flow rate or production shut-in may result derived elastic properties. This radial displacements are measured. An arch is a hemispherical Because there is no unifying theory that cap of interlocking sand grains—like the relates log measurements to rock strength. using a (right ). but a the different environments encountered. If these are altered. The IMPACT soft- in collapse of the arch. and axial and In better cemented rocks. The sparked by incidents in the well’s produc. measurements with sonic and density logs to In exploratory wells. of sand arches. perforating may stresses are more problematic. causing sand to be ware has several empirical correlations to produced until a new arch forms. The IMPACT analysis Fluid inflow the anticipated flow capacity of the comple. increasing flow rate until sand is produced. reduced reser. well parameters derived from geologic models believed to form around the entrance to a data. Field techniques like microfracturing strength. 42 Oilfield Review . At its simplest. sure. dict from these the sanding potential. This arch remains sta.8 A sand flow test involves sand production A example is IMPACT Integrated Mechan- being detected and measured on surface ical Properties Analysis & Characterization Formation sand during a drillstem test (DST). The effect grows with higher data. Accurately predicting sand production one of two rock failure models. Various geologic flow rate. In IMPACT reduce permeability around the surface of a processing. they are computed. measurement of critical input data. information may then be used to predict the dynamic elastic properties—in particular tive life.4 relatively expensive to acquire and are only measurements at different confining pres- Fluctuations in the production rate affect available for discrete depths—in some of the sures and give a more complete picture of perforation cavity stability and in some the rock’s failure envelope as a function of cases hamper the creation and maintenance Casing Debris Compacted zone confining stress. ble as long as flow rate and drawdown are potential requires detailed knowledge of the Despite the fact that cores may be signifi- constant. horizontal flow friction. in situ. December 9. Texas. USA. Pelgrom J and Wilson RA: “Completion Develop- development of consolidation strength and Tippie DB and Kohlhaas CA: “Variation of Skin Dam. Nygaard O and Morita N: “Gullfaks Subsea flushed with a catalyst. and some Gulf of coverage of intervals longer than about 20 ft and prepacked screens. about 10% of sand-control completions. 1 London. DR: “Applications of Polymers in Sand Con- April 4-5. vented if chemical injection is uneven and 8. Finally. up to 10 to 15 ft [3 to 4 m]. both options reduce production. Complete completion are resin injection. More this relatively low-permeability. with formation sand be drilled through the pack to provide a rate may be reduced to ensure that draw. Singapore. Ouadfel H and Zundel JP: “Optimizing the Completion Procedure to Minimize Sand Produc- some exposed sand is uncoated. OSEA 90123. Ehlig-Economides C and Joseph J: “Testing be reduced by 25% and productivity cut by Design and Analysis. September 4. September 23-26. presented at the 66th SPE Annual Tech- Arches. together creating a stable matrix of perme- ing to Avoid Formation Damage. Wells: An Operator’s Implementation of a New Sand trol. Regional Meeting. tion of Production Through High-Rate Gravel- Clay concentration can hinder the effec. may affect productivity. Davies. no.” SPE Production Engineering 4. strengthening Once it has been established that at planned are particularly suited for high-angle wells. or open hole. In cased hole. September 13-15. Dallas. 6. Production casing. the plug may be com- duced. 1991. A first option page 45 ). sand production will not be pre. USA. 1989. Further. December 4-7. USA fields where permeabilities [6 m] is difficult to achieve. If the drillout tech- The most popular options for completing Slotted liners and screens are best suited nique is employed to reduce drawdown. bility in Unconsolidated-Sand Reservoirs. This can be achieved using coiled formation grains become detached.” Oilfield Review 4. California. which cannot be easily completed other. Alternatively. North Sea. generally through perforations. tubing if a conventional rig is not available. USA. 3 (March 1988): 33-37. small amount of fines can plug many needing no special hardware. Whitfill DL. wise (see “Screening Horizontal Wells. Also. 1987. Packed Oil Wells.” paper tiveness of the consolidation process. They bind rock particles Texas.” paper SPE 8427. Las USA.” Slots are typically sized to cause bridging drilled out to the top of the perforations/ minimizing shocks to the reservoir by of the largest 10% of the formation particles.” paper SPE 16893. conduit to reduce drawdown through the down is below the the point at which the However. How.” paper SPE 19476. it accounts for no more than Stein N: “Determine Properties of Friable Formation Sands. Santarelli FJ.” paper SPE 22797. 1991. 1979. October 1992 43 . are greater than 1 darcy. Dallas. Sydney. The technique without screens and gravel packing. screens and prepacked screens Resin-coated gravel may be used as a when they are crushed. 1974. October 6-9. production can be restricted by pack. Morita N and Boyd PA: “Typical Sand Production 9. organized by the Offshore Engi- Morita N. subtly. 10. available systems employ phenolic. 27-30. USA. furan or Annual Technical Conference and Exhibition.” paper presented at Use of Polymers in Drilling tion strength and reducing permeability. They are mostly resulting pack may be fragile. short intervals. forations. ments in Unconsolidated Oil-Rim Reservoirs.” paper SPE of resin.” paper age with Flow Rate Associated With Sand Flow or Sta. slotted liners used in California. consolidated grains around the casing. England. presented at the 62nd SPE Annual Technical Confer- epoxy resins. presented at the 54th SPE nical Conference and Exhibition.” pletely drilled out to leave gravel-filled per- tion strategy to limit sanding. Massie I. USA. presented at the 66th this. Texas. no. Residual water may also interfere with the Vegas. and then 3. Annual Technical Conference and Exhibition. Complete coverage of larger zones is diffi. 1991. creating a filter for remaining particles. Bratli R K and Risnes R: “Stability and Failure of Sand SPE 22795. resin-coated gravel Mexico. Deruyck B. October 6-9. either inside casing or open hole and program predicts sanding potential. Gullfaks Field. creates significant additional drawdown that may adversely affect field economics. Tixier MP. Most commercially Problems: Case Studies and Strategies for Sand Con. Massie I and Knudsen TW: example. Winchester PH: “The Cardinal Rules of Gravel Pack. 2 (April up to 10%. Australia. October 6-9. the next step is to choose a comple. while screens prepacked with resin-coated then squeezed to form a plug across the sand offer the finest filtering.9 within a few hours of installation. particularly prepacked screens. the wrapped screens have smaller openings. onshore markets. trol. USA.” paper SPE 22739. Resin-Coated Gravel Without Screens: they are pulled apart or they fail in shear ted pipes. Although resin consolidation is used (March 1975): 283-293. the technique tends to be reserved for SPE Annual Technical Conference and Exhibition. successfully. all sand-prone reservoirs physically restrain to formations that are friable rather than perforations must be evenly packed and the sand movement. is resin treated to give a compressive Approach. Slotted liners and represents about 5% of sand-control treat- prepacked screens are used in only about ments. Loveless GW and Anderson RA: “Estima- tion of Formation Strength From the Mechanical Prop- cult unless selective placement tools are erties Log. 11. 1991. if an 8-darcy unconsolidated sand “Realistic Sand-Production Prediction: Numerical neering Group of the Plastics and Rubber Institute. presented at the 44th SPE Annual California 1990. so a 5. downhole filter without installing a screen.” World Oil 206. strength of up to 3300 psi. the pack may be is to treat the well with “tender loving care. Because of tion Risk. The four main classes of completely unconsolidated. IMPACT analysis offer the lowest-cost downhole filtering. presented at the 66th SPE Production Prediction Model. no. open hole so that hydrocarbons are pro- changing drawdown and production rate filling the annulus between the screen and duced through the pack. From this. presented at the Eighth Offshore South may necessitate use of increased quantities East Asia Conference. which screens. San Francisco. sand. reference 3. A narrow hole can slowly and in small increments. slurry. production rates sand is likely to be pro. ence and Exhibition.” Journal of Petroleum Technology 27 used. The gravel is circulated into position as a likely failure mechanism. the pack. Each type can production zone. enables the interpreter to pick the most Slotted liners have the largest holes. rocks either fail in tension when Slotted Liners and Prepacked Screens: Slot. Dallas. Texas.11 1992): 28-45. Unneland T and Waage RI: “Experience and Evalua- able. mainly concentrated on low-cost Resin Injection: To cement the sand grains 5% of sand-control completions.10 The quantity of resin injected is a 4886. For and Oilfield Fluids. clay stabilizer is often used as a preflush. Morita and Boyd. But the pack ever. Dallas. compromise between enhancing consolida. a resin is injected into the formation. presented at the SPE Asia-Pacific Conference. Nevada. selective perforation may avoid packed annulus. permeability may (February 1989): 15-24. wire. production of even a Resin-coated gravel has the advantage of zones where sanding is most likely. Adjacent particles are Completion Options be run as part of the completion string and bonded together by the resin. 7. 14 (continued on page 47) 12. it is the most effective method of multiple zones with gravel packs is difficult. times the median formation grain size but hole gravel pack. The resulting sand grains may then be dried and sieved. the pack in perfo. act as an effective filter. Singapore. A sophisticated using sieve analysis. Cement annulus Screen There are various methods for translating the formation sand size distribution into a design size for the gravel. production. 13. the gravel must be clean. the particles can enter the pack Gravel pack and seriously diminish pack permeability. no more (above ). Formation samples from cores are passed Screen through successively smaller sieves to sepa- Perforation rate particles into a number of size groups Gravel-packed perforations Formation sand that are then weighed and plotted. foration tunnels is vital to successful gravel very high permeability—about 120 dar. they need to be broken up before the analysis—clay and silt particles Perforated casing binding the rock together may be removed Formation sand Gravel-packed by washing with chemicals.J. If the sam- ples are aggregated. pre- sented at the Eighth Offshore South East Asia Confer- ence. 0 4 8 12 16 dominant method in use today and warrants Gravel/grain-size ratio a detailed examination. preventing pack is to construct well performance 1. The pack then tends to trap the way of establishing the viability of a gravel damage in the perforations. stopping sand movement and permitting . Saucier that recommends the median gravel size should be up to six nAnatomy of a cased. The oil production rate for natural comple.2 and almost all well repairs involve the production.Gravel Packing: Gravel packing has been Designing Gravel Packs Water breakthrough used by the oil industry since the 1930s.” Journal of Petroleum Technology 26. an intermediate-strength rock that is sensi. These accurately sized gravel in a carrier fluid is Water breakthrough requirements depend on the correct selec- pumped into the annular space between a tion of gravel. packing and this requires gravel that is as cies—but prevents formation sand entering tion—unstimulated and not gravel packed— large as possible. For a gravel pack to maintain long-term Today.13 Gravel of sand movement and how this affects packs reduce the operating wellbore diame. gravel packing is the pre. . They also rely on scrupulous casing or open hole. to be small enough to restrain formation backs. it is the most widely employed sand productivity. formation sand. usually necessitating artificial lift equip.8 Effective/initial may seriously affect productivity. Oil production rate control measure. accounting for about tightly packed and placed with the mini- three-quarters of treatments. But since the pack must is compared with that for a gravel pack in the well (below ). tive to water breakthrough. carrier fluid and placement centralized screen and either perforated Natural Gravel pack technique. Because of this. (February 1974): 205-212. pack by small particles that significantly carrier fluid leaks off into the formation or 0 5 10 15 reduce pack permeability. reference 4.12 A slurry of mum damage to the formation. Saucier RJ: “Considerations in Gravel Pack Design.4 ment to be set above the zone. Welling R and Nyland T: “Detailed Testing of Grav- elpacked Completions” paper OSEA 90121. which is usually measured age the reservoir permeability and restrict sive method of completion. the gravel also has Gravel packs are not without their draw. . Winchester. The technique is also a relatively expen. The gravel also enters completion cleanliness during placement operations to perforations if a cased-hole gravel pack is prevent the contamination of the gravel being performed. The Time.0 clean up.6 ter. One of the most widely used methods is based on work car- ried out by R. December 4-7. through the screen and back to surface. 0 removal of the screen and pack. When Cement the gravel size/grain-size ratio reaches about six. drawdown (above ). curves for a range of completion methods pack-permeability ratio ration tunnels increases drawdown which using a reservoir simulator and predictions . 1990. This depends on the size of the injected into the formation which may dam. carrier fluid is particles. 44 14. Completing backs. The ratio of the effective pack permeability and the initial pack permeability represents the effect of the formation sand particles as Casing they partially plug the gravel pack. Although gravel packing has these draw- . As pumping continues. yr Minimizing the pressure drop in the per- gravel pack creates a granular filter with nAssessing the viability of a gravel pack. Once in place. nChoosing gravel size range. albeit at a reduced rate. During installation. presented at the 65th SPE Annual Technical Conference and Exhibition. ated wells. Texas. USA. USA. Between the to combat this. Wilson DJ and Barrilleaux MF: “Completion Design and significantly affect inflow performance. production from the near end of the well. and plugs and October 1992 straddle packers to be set using coiled tubing. 8 1/2-in. so Scale Model Wellbore. Operational Considerations for Multizone Gravel Packs in Deep. casing. wrapped with two layers of screen with an out. In an effort Zaleski TE Jr: “Sand-Control Alternatives for Horizontal 5/8-in. 1200 ft MD/TVD will have horizontal sections of up to 2600 ft. not 2. casing set between: 2500 ft -4000 ft MD expected by the end of the first year. September 23-26. On the downside. 800 ft MD/TVD field which is operated by Chevron UK Ltd. casing. Prepacked screens cost signifi- cantly less than gravel packs and are simpler to install. afforded by an openhole completion. Larger ID reduces the pressure drop along the resin-coated gravel. The screens will be inserted 1.000 B/D using electric sub- mersible pumps. and horizontal wells is gravel packing. 1500 ft MD/TVD produce up to 30. the longevity of the screens the 23rd Annual Offshore Technology Conference. each well will 103/4-in. Forrest JK: “Horizontal Gravel Packing Studies in a Full- horizontal length of the well. pipe is uncertain and there is a lack of zonal isolation ton.8-cm]. 3 (March 1992): 45-49. casing. At worst. [11 cm] as 1000 ft–2600 ft opposed to the 2. nHorizontal well com- tive technique for excluding sand in high-angle pletion design for the Alba field.4 cm] of the planned gravel packs. High-Angle Wells. inflow distribution—when the pressure drop is there is a likelihood of sand sloughing around the New Orleans.2 A case in point in the UK North Sea is the Alba 30-in. 45 . Louisiana. Most of the field’s production wells 20-in. where pressure differential will not reduced permeability from 3 darcies to 1. [7. 1990. However. the company continued to 95/8-in.” World Oil 213. on permeability around the wellbore. blank sections with internal seals 509-511. Water breakthrough is expected after only two months of production and 40% water cut is 133/8-in. allowing fluids to be spotted. open hole with prepacked screen but the increased internal diameter (ID) afforded by the prepacked screens—4. screens. presented at will employ conventional gravel packs. What convinced Chevron was not the cost 81/2-in. screen will be a 1/2-in. Hous- The prepacked screens will comprise 5-in. 1991.” paper SPE 20681. high. The 350-ft [107-m] thick Eocene sand reservoir is completely unconsolidated and currently under development.” Journal of Petroleum Technology 43 (May 1991): side diameter of 6 [16. no. Initial plans called for horizontal cased-hole gravel packs.3 -cm] thick pack of will be deployed every 400 ft [120 m] of screen. casing set ± 200 ft into horizontal: study alternative solutions and concluded that 7000 ft–9500 ft MD Eocene 6200 ft–6400 ft TVD prepacked screens could successfully keep sand at bay (right). In the field’s conventionally devi. Screening Horizontal Wells Studies generally conclude that the most effec.9 in. [22-cm] diameter. leading to a better into open hole. When the field comes onstream. May 6-9.” paper OTC 6751. Chevron enough to significantly limit production. the technical complexities of high-angle gravel packing and its relatively high cost mean that alternative techniques are often considered. Wells. making sand control a major factor of the development plan. Early water 2500 ft -3500 ft TVD production will exacerbate sand production by reducing the interstitial tension between sand grains. Chevron tested the effects of sloughing Sparlin DD and Hagen WH Jr: “Gravel Packing Horizontal bore is favored. it and High-Angle Wells.4 in. [1.1 Although 430-ft water depth there have been some notable operational suc- cesses. the service tool seals into also pumped down the casing-screen longer than 25 ft [8 m]. pumped down the casing-screen annulus screen below—located below the main Reverse circulation involves pumping and the carrier fluid can be squeezed screen. casing annulus above the packer. In In lower circulating position. Reversing position Service tool Permanent-retrievable packer Ported housing Sealbore housing Locating collars Blank pipe Primary screen ‘O’ ring seal sub Lower telltale Sump packer Seal unit nThe four positions for gravel packing. up the screen/ through any part of the screen. slurry is tale—a sealbore with a short piece of encourage bridging off higher up the well. The aim is try to maintain flow in fluid through the washpipe. 46 Oilfield Review . annulus. the washpipe is sealed into the lower tell. but returns of carrier fluid have to the fluid may cause the fluid to bypass the When slurry is pumped in this mode. which may In upper circulating position. Upper circulating position 3. slurry is However. annulus below the screen. Lower circulating position 4. all pass through the bottom of the pack where pack and pass down the well via the the carrier fluid leaks off into the formation. washpipe at the bottom of the service tool there is not a void in the gravel in the and back to surface via the service tool. screen/washpipe annulus. into the the casing-screen annulus and ensure that washpipe annulus and back up to surface. 1. backpressure on the packer and does not allow circulation. Squeeze position 2. if the interval being packed is squeeze position. Cased-hole gravel However. Louisiana. This ceases placement may be time-consuming. Control Symposium. This usually causes fluid to be lost centration in the slurry increases. Formation fluid trip. Gurley DG and Hudson TE: “Factors Affecting Gravel Placement in Long Deviated Intervals. Pressure drop from the packer. Hainey and J. Impor- packs present the additional challenge of gravel-pack operations does not permit sin. washpipe are removed. If the well is perforated with Each time LCM is used. the hardware used in many energy required to move the slurry. the positions of the service tool run 5 ft [1. the gravel will tend to sink to the bottom of ids and reservoir pressure (see “Choosing a To reduce losses.to 20-power micro. the cal wells. productivity may be then the workstring must removed after apparent viscosity of the fluid when calculated using NODAL production system gravel packing so that the completion string pumped downhole.” paper not be achieved through perforation.” page 54). perforation guns are fired and lowered into when pumping stops and gravel in the mum pressure drop across the pack. Screens are normally sized. fluids pumped downhole. cosity when it comes to “turning the corner” Pressure drop in perforation tunnels is a meability formation to the hydrostatic pres. After the formation interface and held there as the size of the formation sand.1% should be larger than the 20-mesh sieve. At retain the gravel. The LCM plugs the completion likely to be carried to the perforation tunnel- restricts formation flow—and is fixed by the fluid’s flow path into the formation. the relative densities of the gravel and To check that a well is suitable for cased. has more time to settle and will not effec- fixed. Excess gravel is then reversed out. the more the pack each trip. USA. 1992. The gravel material (LCM) suspended in a viscous fluid by the rate the carrier fluid leaks off into the size range determines pack permeability— is commonly pumped downhole before formation. offering higher pack its simplest. USA. inflow area removable with acid). In verti- openhole. According to American Petroleum Institute recom- perforations. a flapper valve can be paper SPE 23777. pack packer. assembly determine the flow direction of m] to improve the chances of a consistent scope and comparing the shapes with a ref. The best The service tool is then used to set the to using screens with at least 1-in. presented at the SPE International may also be reduced by increasing the employed below the packer.W. rather than as individual grains. be smaller than the 40-mesh sieve and not more shot density guns.” next page ).16 stages. the hardware becomes a complex the pack thickness and the ability to fish the Average grain size is not the only determi. When the completion string is run. match the inflow performance of openhole the washpipe and workstring are removed 16. Hainey and with a washpipe inside are usually run in a common rule of thumb calls for the slots Troncoso argue that in some cases formation hole with a service tool. the hardware may be chosen. open. the flapper valve is opened—either mechanically. tant factors determining settling are pump properly packing the perforations. annular clearance. This normally leads nant of gravel-pack permeability. During these trips. [2. the designation 40/60 indicates that not more than 2% of the gravel should tubing-conveyed perforating (TCP). Recently. gle-trip operations. The most common way of estimating Thereafter. with wireline or using pressure. where voids below a bridge are likely deposited on the formation by drilling fluid either the upper or lower circulating posi. is to be placed. sand grains move as larger agglomerates multiple zones are to be completed in Screen diameter depends on the inlet area. otherwise permeability. Hainey BW and Troncoso JC: “Frac-Pack: An Inno- vative Stimulation and Sand Control Technique. circulating. when to be 75% of the smallest gravel diameter. Openhole gravel packs have no In a single-trip gravel-pack treatment. the LCM is removed. Lafayette. there is a danger mended practices (RP 58). the gravel viscosity and reservoir pressure are also include marble chips (calcium carbonate. high of incomplete removal damaging the reser. 1990. Selection of wire spacing of using larger gravel. However. viscosity of the produced flu. To avoid the need to pump LCM when than 0. pack. The perforations may be filled annulus collapses into the voids. perforation area.17 analysis. to remain. upper Transporting gravel into the perforations monitored by sieve analysis. This models the pressure drop as may be run. This valve is Symposium on Formation Damage Control. series of screens and packers. into the formation. Care with gravel with the packer in the squeeze to be the case in wells deviated more than must be taken to remove the filter cake position and the annulus is filled with it in 50°. gravel packs can nearly voir. If leakoff is slow. work by B. Once the method of completion is Wire-wrapped screens are usually used to Troncoso of ARCO points to the possibility selected. Gravel-size distribution can be tems have four positions: squeeze. preventing comple. For a cased-hole gravel rate.5 m] above and below the pro- roundness and sphericity is by examining in the packer and washpipe in the screen ducing zone and centralized every 15 ft [5 the gravel through a 10. October 1992 47 . lower circulating and reverse and annulus is the next consideration. in the annulus and entering perforations. February 22-23. a packer and screen assembly is not subject to any hard and fast rules. It closes after the service tool and SPE 19400. incomplete fill may be rectified perforations and therefore offer the mini. annulus. the packer. major impediment to production and varies sure of the completion fluid above the Fluid viscosity must increase if gravel con- with tunnel length. Packing efficiency is also affected Perforation Strategy. the gravel is the smaller the grains. If leakoff is rapid.5 cm] gravel-pack sands are round and evenly packer above the zone to be completed. may be raised by increasing perforation salt pills (see “Gravel Packing Forth Field diameter and/or increasing the number of Exploration Wells. washpipe to direct flow to the casing-screen 17. turely. Sophisticated sys. presented at the SPE Formation Damage hole completion is required. pension properties of the fluid and the contaminating the gravel. tion fluid from passing through the pack and into the permeable formation. screen out of the hole. the TCP guns must be retrieved and the carrier fluid. erence chart. gravel fill. Louisiana. leaving voids in the annulus. the service There is also a relationship between reservoir fluid flows through the perforations tool and the washpipe are withdrawn from gravel concentration and carrier fluid vis- into the completion hardware to surface. If sufficient inflow area can. 15.15 To explain this. diameter of casing in which the gravel pack capable of accommodating a large-diameter Lafayette. exposing the relatively high-per. gravel diameter and the hole gravel packing. To reduce pressure drop. particulate loss control the well. but permeability. But the rathole. February 26-27. Transport is a function of the sus- and to avoid abrading the formation and tions. The next decision facing the engineer is circulating and therefore allow single-trip Gravel can sometimes bridge off prema- whether the completion should be cased or treatments (previous page).C. Common LCMs fluid leaks off.” packs for many reservoirs. oil-soluble resins or tively pack the perforations. Gravel Packing Forth Field Exploration Wells There is no such thing as a typical gravelpack. the formation (next page. Any contaminants that may plug the spotted across the tun- gravel pack and decrease productivity must be nel entrances. tion hardware was run and the LCM dissolved. 48 Oilfield Review . lated into place. Tub- prising massive. dis. The The LCM pill was dissolved by circulating tial returns of seawater-contaminated brine were TCP guns were then dropped off. Aberdeen.1 Gravel slurry Loss control material into the perforations and Cleanliness is fundamental to gravel packing pill loss control material efficiency. This example is based on a loss control material from gravel-packing procedure used on several verti- entering the perforation cal appraisal wells in the Forth field in the UK tunnels. removed. Prepacking the perforations prevents ple operations. about 5500 ft [1675 m]. Perforation was carried out using tubing-con. rations. nels (above). into place and then squeezed into the perfora. This was repeated two or three times to Offshore Europe Conference. 1. but to further clean the wellbore. LCM pill of sodium chloride in xanthan gum and a modified starch was then spotted across the packed perforations to prevent loss of completion fluid while the tubing was pulled. Ini. Cement for the production casing was dis. left). An September 3-6. BP decided to unsaturated brine and the main gravel pack circu- discarded before the system was closed and sur. Permeability is 6 to 12 gravel was bullheaded millidarcies and porosity is 35%. Forth. it was applied sparingly to the pin Casing end only. The main packer. Cleanup pills of detergent. ensure that all the perforations were packed.” paper SPE 23128. the mud pits were cleaned and the mud changed Prepack gravel to brine completion fluid. presented at the ance of about 300 psi. The cement scours the cas- ing. A second LCM pill was then face filters employed to reduce the maximum ning the screen assembly. reduces damage. Gravel in gelled carrier fluid was circulated veyed perforating (TCP) guns with an underbal. Formation Loss control material Cement placed with seawater. A short flow of 2 ft3/ft of tions. Gilchrist JM and Gilchrist AL: “A Review of Gravel Pack- ing in the Forth Field. A sump packer was set below the zone to be completed and above the dropped TCP guns. scrapers Gun fish Settled excess gravel were run and seawater circulated at high pump rates. perforation was performed to remove debris. subsequent cleanup and covered in 1986. this improves North Sea operated by BP Exploration. Because the dope used to lubricate pipe joints is a serious contaminant. internally jetted and steam cleaned before being run in hole. Tubulars were exter- nally shot blasted. to limit formation damage and prevent loss con. clean sand located at a depth of ing-conveyed perforating guns were dropped. The well was then displaced to brine. Scotland. This strategy was used spotted across the screen to allow recovery of the particulate size to less than 2 microns [µm]. The final comple- there were fewer than 10 parts per million. scouring pills with gel spacers and flocculants were also circu- lated. 1991. service tool without losing completion fluid into Solids in the brine were monitored to ensure that trol material from entering the perforation tun. nPrepacking the perfo- each is a complex combination of relatively sim. service tool and screen assembly were then run and the packer set. has an Eocene reservoir com. prepack the perforations with gravel prior to run. In preparation for the gravel packing. The Netherlands. circulating gravel packs. Efficient packing may be achieved with a fluid of closely matched densities—not the density ratio between 1. For this purpose. carrier-fluid viscosity and gravel con- centration are both increased to create a Optimum Dp:Dc ratio using ISOPAC plug of slurry. But increased slurry viscosity particles raises friction pressure and may increase the Standard Dp:Dc ratio using gravel possibility of bridging in the annulus. This case when using conventional gravels or range may be designed using low-density low-density brines. slowed. polymer coating with a low-density ceramic core. 70 0. 1988. Conventional gravel pro- density. October 2-5. perforations (below and next page). improving tightness of the pack and tions are the most common: increasing the time available to pack all the •In conventional.” paper SPE 20984. 1990. more carrier fluid must be lost. 30 Gulf of Mexico and North Sea jobs since extension Gravel concentration can range from 0. increased viscosity slows the k 80 Wireline reentry rate of leakoff—a 250-cp fluid will leak off guide more than six times slower than a 40-cp fluid. The most of the carrier fluid squeezed out of reduced viscosity increases the rate of the slurry is circulated back to surface.4. Another way of reducing settling. tion damage. Because set. Gravel-Pack Material Improves Placement Effi- ciency (Part 2). leakoff and reduces the potential for forma- The slurry usually has a low-viscosity car. Texas.1 ft3/ft 50 cp and a gravel concentration of 7 to 15 lbm/gal.2 1. tling is not a major problem when the densi- ing choice of fluid viscosity and gravel con. in an effort to improve place.” paper SPE 18227. However.18 Increasing carrier-fluid viscosity may nDissolving the loss control material and circulating also increase formation damage. but the following three combina. are used ceramic core to resist acid for short intervals of less than 25 ft. The efficiency with with sliding sleeve to 15 lbm/gal depending on the carrier which perforations have been packed can- Sliding sleeve closed fluid viscosity and company preference. ISOPAC particle ‘O’ ring seal sub •Squeeze packs. Dowell ISOPAC particles. % Partic drawbacks. The and. To place an equivalent quantity Pa 90 rtic of gravel. presented at the 63rd SPE Annual Technical Conference and Exhibi- tion.05 and 1. in which all the carrier Low-density Polymer coating Tell tale screen fluid leaks off into the formation. October 1992 49 . presented at Europec 90. if the interval is 100 ft or so.25 ft3/ft of conventional gravel as may lead to bridging. Hudson TE and Martin JW: “Use of Low-Density. an average pack screen slurry usually has a viscosity of more than placement efficiency of only about 0. Packer rier fluid of less than 50 centipoise (cp) ISOPAC particles have been used in over Gravel pack and ungelled water is a common carrier.8 1.2 2. Dp:Dc ment. vides a ratio of about 2. USA. les Logging reference increasing the potential for formation dam- sin screen age.8 an annular gravel pack.8. ence consider the placement efficiency of Blank pipe tions are packed. Particle density/carrier fluid density. helping nEffect of particle-carrier fluid density ratio gravel to turn the corner and efficiently pack on perforation-pack efficiency—percent perforations is to use a gravel and carrier volume of perforation filled with gravel. Sometimes. The Hague. Fluid (ft3/ft). not be measured directly. being satisfactory for intervals of less than •High-density circulating gravel packs are 60 ft [18 m]. high-strength particles. ties are matched. One indirect diag- Crossover The technique is generally employed for nostic method is based on the average vol- intervals of more than 50 ft [15 m] and ume of gravel placed per foot of interval Washpipe deviated holes up to horizontal. but excessive leakoff about 0. 18. Rules of thumb derived from experi- leakoff is essential to ensure that perfora. Washpipe bottom The conventional approach to controlling 100 at settling—decreasing gravel concentration les flo Sump packer and increasing carrier-fluid viscosity—has Packing efficiency. Bryant D. ISOPAC particles have a Schlumberger has developed ISOPAC low. Houston. Hudson T and Hoover S: “The Use of Low-Density Particles for Packing a Highly Devi- ated Well.8 2. October 22-24. For longer intervals it is more used for medium to long intervals—25 ft difficult to fill all the perforations equally Main gravel [8 m] to more than 100 ft [30 m]. the pump rate can be centration. There is no industry consensus on govern.25 introduction in 1991. Treatment A Treatment B Downhole hardware Crossover Sump packer Crossover Sump packer Gravel deposition 1.24 26. Plots of gravel deposition time to pack. Dow- ell Schlumberger has developed PacCADE computer-aided design and evaluation software that can simulate gravel-packing opera- tions. To aid the design of gravel-pack treatments.50 −1.36 36 . ft Measured depth. % 0. the lowermost perforations have not been completely packed.00 0.67 13.50 0 −0.00 Efficiency. min 11. ft Gravel concentration.26 16. 50 Oilfield Review .00 Normalized radius 0.50 0 −0.00 8427 8460 8493 8526 8559 8592 8625 8658 8427 8460 8493 8526 8559 8592 8625 8658 Measured depth.50 −1.35 23.50 packing −1.25 17.26 16. final gravel concentration and final pack efficiency—all versus depth—may be used to compare proposed gravel-pack treatment designs.61 20. In treatment B using lightweight ISOPAC particles in a prepack.24 24 . good perforation packing efficiency has been maintained for the whole interval. In treatment A using conventional gravel.00 8427 8460 8493 8526 8559 8592 8625 8658 8427 8460 8493 8526 8559 8592 8625 8658 Measured depth.48 Packed Final pack efficiency 1.22 15.09 28.50 0 Annular Packing Perforation −0.00 8427 8460 8493 8526 8559 8592 8625 8658 8427 8460 8493 8526 8559 8592 8625 8658 Measured depth.27 14.96 31. ft Measured depth. ft Measured depth. ft nComparing conventional (treatment A) and ISOPAC particle (treatment B) placement. % 0-6 6 -12 12 . ft Time to pack. min Time to pack.83 Final gravel concentration Normalized radius 1. as ensures that more perforations are acidized developed by Dowell Schlumberger. prior to the gravel pack. PERMPAC fluid fluids. plain water + + + + - + + + + + + + is used.” paper SPE 19741.+ + + + + + + + + + + + also be chosen. 40 the gravel. conventional and high-density circulating Prepacking with the screen in place is car- gravel packs may be preceded by ried out with the service tool in the squeeze 19. probably because of pressure that San Antonio.” paper SPE 22793. The gravel slurry not only Gravel-Diverted Acid Stimulation Prior to Gravel prepacks the perforations but also acts as a Packing. the carrier fluid must . is used to limit the pene. 1989. Too much may necessitate a trip although the acid also stimulates the well. no. wide fractures by carrying vents. ml base fluids are gelled with familiar stimula. Penberthy WL Jr and Echols EE: “Gravel Placement prior to packing the casing-screen annulus. long-interval + + + + + Water + + + + + + gravel packs using ISOPAC particles have + + Hydrocarbon + + + . 1 (January with gravel either before the screen has lated into place. Volume depends on a treatments cannot deliver. and by crude oil or organic sol. PERMPAC fluid in brine environment PERMPAC fluid in oil environment ventional gravel.” Oilfield Review 3. right ).” page 18). In others. The concentration aqueous solution. all of which tend to increase as the of the formation.64 ft3/ft. fluid loss into the formation is finite. Determining the prepack primary aim of this treatment is to increase high rates of leakoff into the formation. fluids gelled 80 ity—were tested on Berea sandstone with these polymers are often sheared using cores with nominal air permeabilities HEC 40 lbm/1000 gal of 300 millidarcies. Sometimes acidization is carried out as a micelles that have a high viscosity in low.has been found to be common using con. October 6-9. a 400 ft [122 + + + ++ + . tration of LCM into the perforation tunnels separate stage. diverter. The leakoff for three fluids— 2.” page 48 ). in concentra- concentration of nonhydrated polymer that tions that give equivalent viscos- may damage the formation. been run in hole or as a separate operation than the alternative prescreen technique.+ + + + + m] interval was packed with an efficiency of + + + + + + + 0. the quality of the cement out a tip-screenout fracturing treatment fol- fluid penetrates deeper into the formation job. Xanthan polymer because contact with oil causes the Breaker is added to reduce fluid viscosity 36 lbm/1000 gal fluid’s micelles to break up (above).20 fluid leaks off into the formation. The PERMPAC a pump and filtered prior to blending with fluid shows an enhanced leakoff. uses outlined in the Forth field example (see and then prepacked than would normally PERMPAC viscoelastic surfactant-based car. + + + + + +. one alternative is and shear. core + + Activator . It shows during tripping. Water. Surfactant although oil-base fluids are used for Hydrophilic + Hydrophobic severely water-sensitive formations. - easily exceeded these figures. The process takes less time 1991): 18-26. Prepacking prior to running the screen. Time. However. October 8-11. Pulsinelli R. 1991. the design and size of the perforation lowed by a circulating gravel pack (see (above. + - in the Norwegian North Sea. and volume is important. + + + While gravel and placement technique + + . For example. additives are used to + + + -+ + + + increase carrier-fluid viscosity. such as the competence to create short. charges. 160 nLeakoff tests (left) for different carrier Volume of fluid through core. Oil + + ++ + + + + + . “Gravel Packing Forth Field Exploration be the case. Brown E prepacks—where the perforations are filled position before the annular pack is circu. Unlike HEC. This fluid forms rod-shaped Wells. the extent of cleanup flow after “Rewriting the Rules for High-Permeability To improve perforation packing. minimizing per. USA. The prepack may be pumped as several in Wells. number of factors. Hawkins G. October 1992 51 . Perforations can be prepacked effectively stages of gravel slurry interspersed with Dallas. High-viscos- ity fluids are commonly water-base.+ + + + + + + + + + are being selected. Gulbis J. stages of acid to clean up damage around 21. min the polymer of choice because it has low residue after breaking and does not build a filter cake on the formation. forations.19 HEC is normally 0 10 20 30 40 as contact with oil is reduced. PERMPAC fluids do not require a breaker to clean out the excess in the sump and When stimulation is required that matrix because they are thinned by temperature covering perforations. Stimulation.21 rier fluid. In some cases. + + + + + . USA. and Elphick J: “Taking the Breaks Off Proppant-Pack Conductivity. King M. off during the subsequent gravel pack. both perforation and the formation permeability. to conventional polymers. presented at the 66th SPE Annual Technical Conference and Exhibition. 0 Final leakoff rate becomes constant imize formation damage. using either water or gelled fluid provided results when the higher viscosity carrier meability damage.5% by volume containing respectively the PERMPAC tion chemicals like hydroxyethyl cellulose 120 system. presented at the 64th SPE Annual Technical Conference and Exhibition. Matherne BB and Hall BE: “A Field Evaluation of a the perforations. To reduce the and xanthan polymer. Too little gravel will the rate at which the carrier fluid will leak has good suspending capabilities compared result in the LCM penetrating unpacked per. Texas. once the job is complete and therefore min. Diversion A radically different type of gelling agent. Texas. 20. hydroxyethyl cellulose (HEC) (HEC) or xanthan polymer. 24 repairs to be undertaken. no. gravel may be coated with radioac. “Jim Carroll: The Gulf Coast WID Kid. Houston. ids in the gravel-pack pore space. USA. assessing gravel pack performance a num- usually inconsistent and may wash off. Ehlig-Economides C and Joseph J: “Test. left). there are two technique uses a pulsed-neutron logging rate (next page. casing. Packing placement efficiency nIsotope logging of Depth. be gauged using nuclear density logging to The initial slurry 5550 estimate the density of material in the annu.Evaluating the Gravel Pack be used. The neutron activation logging near-detector curve.” Oilfield Review 4. toward decreasing count With the gravel pack in place. not all changes in density are taining scandium related to changes in gravel-pack quality— tracer packed the three high-perme- changes in the screen. However. often This also offers increased subtlety through Areas of high porosity—poor pack—are indi. Carpenter WW. postpack evaluation is return to their natural stable state. In cate 100% perfora- these cases. sensitive to changes in pack porosity. The other main strategy for testing gravel ment. The num. September 3-6. requires a DST prior to packing. With these use of multiple isotopes. overlay in areas of low porosity—good pack. 1991. Aberdeen. High-permeability zones 52 Oilfield Review . and iridium— indi- density particles have been employed. curves of the two detectors are scaled to Differentiating between the effects of the lated within each particle’s resistant shell. In addition. Then a tubing and formation sand all affect the slurry with particles reading. a compensated neutron using well tests and production logging. Gilchrist JM and Gilchrist AL: “A Review of Gravel Packing in the Forth Field. The multirate flow tests. which agitates the bridged gravel until it set- essential to detect incomplete fill and allow ber of gamma rays is proportional to the tles into the void. In gamma ray logging. amount of silicon and aluminum activated. 24. pipe base. Unneland and Waage. no.22 the perforated inter- Density measurement is not appropriate val. wireline shaking device attached to the go. Carroll JF and Smith BC: “Gravel Pack Field Examples of a New Pulsed Neu- tron Activation Logging Technique. The pack is bom. reference 9. A base log run prior to the gravel incorporating iridium packing can iron out these discrepancies was pumped that filled in the zone at (next page. Deruyck B. Scotland. with particles con- lus. a been packed everywhere it was supposed to to be mounted below it. Cumulative Completion schematic Cumulative Scandium Iridium a prepack using gamma spectroscopy tracer log (right ).25 been manufactured with isotope encapsu. elements to be evaluated: that gravel has tool modified to allow a gamma ray device Once voids in the pack are identified. The Since voids in the pack may lead to early gamma rays are emitted as the elements shakes create local turbulence in the fluid completion failure. (April 1987): 19-26. Watson JT. barded with fast neutrons. 1990. ability zones. formation and the gravel pack. making it ber of diagnostics are available. 2 (April 1992): 28-45. May 7- 10. including ing quantitative analysis unreliable. Prior to place. and that the well is producing hydrocar. 5650 sented at the Offshore Europe Conference. However. 22. evaluation tools may be used to break up bons satisfactorily. the effectiveness of fill may dium and iridium. presented at the 22nd Annual Offshore Tech- nology Conference. Silicon and alu. 2.” paper SPE 23128. 26. a reference 5630 ft and diverted screen may be set below the sump packer to 5600 to the remainder of register zero pack response. 23. The skin factor (which measures formation dam- One way to improve the accuracy of such tool’s near and far detectors are used to partly age as a function of its permeability) and logs is to use ISOPAC particles that have eliminate the effects of hole conditions. neutron activation logging can 5630 tion packing over the entire interval. The perforations cated by a shift of the curves. minum in the gravel are activated and bridges and allow the pack to settle. The cumulative when the completion fluid has a high den. and pack quality may be inferred. 23 packs centers on assessing performance tive isotopes and the pack assessed using In openhole packs. the coating is log can be used to detect hydrogen-rich flu. especially the data it is possible to identify the pressure may be prepacked using particles contain- ing scandium followed by particles contain. mak. ISOPAC particles containing scan- Alternatively. ft can be monitored. Multiple Isotope Log ing iridium. using a multiple-isotope. pre.” paper OTC 6464. Texas. top right ). tracks—the superpo- sition of scandium sity (more than 14 lbm/gal) or where low. 5700 ing Design and Analysis.” The Techni- cal Review 35. 25. and in some cases the formation and its fluids. October 1992 53 . the casing. Today. The near detector is affected mostly by the screen and wellbore fluids. the successful accomplish- ment of a gravel-packing operation has often been the main criterion used to judge its success. Base run Top of screen 5800 nNuclear density logging of a gravel pack.drop caused by the gravel pack. In the past. This judgement often fails to consider that the treatment may have dam- 7-3/4 in. ft Gamma ray after 2000 CPS 4000 Screen 4600 5700 Top of sand 4700 nCompensated neutron log of a gravel pack using near and far detectors. and completion engineers are increasingly seeking ways of stopping formation sand without seriously restricting productivity. Gamma ray 75 Near detector 300 Pack % mance versus time is another indication of 25 API 125 26. Running a base log prior to gravel packing allows the density effects of the bottomhole assembly to be taken into consideration and the gravel pack to be evaluated. liner aged the well.667 CPS 3877 0 100 performance. more attention is 4400 being paid to performance. casing that fines like kaolinite have migrated into the pack and around the gravel or that 4300 unpacked perforations have collapsed. Pressure drop across the pack is one measure. An increase could indicate 9-5/8 in.26 Gravel-pack perfor. The far detector is affected After gravel pack run by the gravel pack. Production Compensated Neutron Log logging may be used to evaluate each layer Far detector in the formation assessing the flow profile across the interval. —CF 4500 Gamma ray before Top of partial sand pack Depth.