Gravel Pack Completion Research Articles

Optimization of Horizontal Well-Completion Design With Cased/Perforated or Slotted Liner Completions

Summary A well completion is a critical interface between the productive formation and the wellbore. An effective completion must maintain the mechanical integrity of the borehole without creating any significant restrictions on the flow capacity of the well. In this paper, a process is outlined to design optimal completions for horizontal wells by applying comprehensive skin-factor models that include damage and turbulence effects for all common types of completions. Slotted or perforated liner, cased, perforated, or gravel-pack completions have been used in horizontal wells for borehole stability and sand-control purposes. However, these completions may have lower productivity (as characterized by a positive skin) relative to an equivalent openhole completion, because the convergent flow to perforations or slots increases fluid velocity in the near-well vicinity. In addition, any reduced permeability zones (formation damage caused by drilling, completion, or other processes) magnify the convergent flow effects and therefore may result in substantially increased skin factors. Compound effects of formation damage around the well completion, a crushed zone because of perforating, plugging of slots, and turbulent flow, as well as interactions among these effects, are included in the model. This paper illustrates how to use skin factor models to screen the available completion types for cased/perforated and slotted liner completions. This screening approach considers reservoir permeability, permeability anisotropy, fluid properties, formation damage effects, and rock mechanical characteristics as the key parameters. The types of completion that yield the most productive well performance for this matrix of properties are presented. A more detailed completion design is then illustrated by showing how the skin-factor models were used to redesign the slot configuration of liner completions for viscous oil reservoirs on the North Slope of Alaska. Application of the slotted or perforated liner models to the readily available liners showed that the completion skin factor can vary by as much as 40%, depending on the detailed characteristics of the slots or perforations in the liner (slot or perforation size, density, and distribution). The example showed that optimizing the performance of the completion can increase well productivity at little or no cost and with no loss in liner mechanical strength.

Rigless Interventions in Failed Gravel-Pack Gas Wells Using New Resin Systems

SummaryWhen a gravel-pack completion fails, it usually results in loss of production for the interval until a workover rig is available to recomplete the well. This paper describes two recent successful rigless interventions completed in offshore multizone completed gas wells in the Adriatic Sea. A key factor in the success of these remedial treatments has been the application of two different chemical technologies applied to the proppant while it is being pumped into the well.In the first well, production loss was caused by fines migration (fine silt ≤44 microns) into the 40/60 gravel-pack sand, which completely plugged the screens and gravel. An intervention was performed which included sealing the existing completion and then reperforating the premium screen using wireline guns. The interval was then fractured with a tip screenout design and a through-tubing screen was placed across the perforated screen. Critical to this treatment was the use of a surface modifying agent to prevent fines migration and plugging of the gravel pack. Production results have confirmed the correctness of this technical choice.A different problem existed with the second well, which had a frac-pack completion; in this case, a screen failure resulted in formation sand and proppant being produced to the surface. The well was refractured through the hole in the screen and a newly developed resin was applied to the proppant to lock the proppant into place and thus repair the damaged screen without restricting the flow area with a secondary inner string. Unlike conventional resin-coated proppant at low temperatures, this new type of resin does not require confinement pressure and/or the use of chemical flushes to cause the resin to set. Since the intervention, the well has been producing sand-free at very close to the original productivity index (PI).In both cases, these treatments have put back into production wells that in the past could not have been restored to production except through recompletion using a workover rig. This paper will describe the treatment design, well operations performed, materials used, and production performance of each well, comparing each aspect with the case of a conventional recompleted well.

Gravel Packing Long Horizontal Wells With Low Fracture Gradients

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 96396, "The Dynamic Bottomhole Pressure Management: A Necessity to Gravel Packing Long Horizontal Wells With Low Fracture Gradients," by Z. Chen, SPE, and R.J. Novotny, SPE, BJ Services Co., prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9–12 October. Unconsolidated formations are commonly found in offshore reservoirs where long and ultralong horizontal wells are required to produce heavy oil economically. Gravel packing is an efficient and cost-effective sand-control practice for these high-permeability, large-porosity, low-fracture-gradient formations. Successful placement of the gravel-pack completion requires that these low-fracture-gradient wells not be fractured during gravel placement. A dynamic-pressure-management scheme is critical to maintain bottomhole pressures (BHPs) in the required range. Introduction Horizontal-well openhole screen completions have been performed for years and offer many advantages. These advantages can be offset quickly by failure if the job is not carefully designed and executed. This is especially the case for wells with long and ultralong horizontal wellbore sections in unconsolidated formations with low fracture gradients. Although horizontal openhole gravel-pack completions have continued to gain acceptance as reliable completion techniques, they are still under development and often encounter new challenges. In some cases, there may not be any safe operational windows under conventional operation procedures. Dynamic BHP Management During gravel placement in horizontal wells, BHP must be monitored constantly to avoid fracturing the formation. Any excessive leakoff will cause a screenout and, thus, an incomplete sand-control completion. To ensure a successful packing job, an effective BHP-management scheme needs to consider both job-design time and actual job execution. Gravel-Packing Model The gravel-packing physical phenomenon in a horizontal well can be described as alpha-/beta-wave packing. Several flow patterns can be encountered in a given section, affecting the pressure-gradient/flow-rate relation. During alpha-/beta-wave packing, a stationary bed is created at the bottom of the flow path and a heterogeneous mixture of solid particles and carrier fluid flows through the upper part (Fig. 1). The stationary bed at the bottom of the flow path builds up and grows forward like a wave; it is from this that the expression "alpha wave" is derived. The flow path or void above the alpha wave will be packed after the alpha wave packs forward to the bottom of the screen. In the alpha-/beta-wave model, it is assumed that the space above the alpha wave will be packed backward to the top of the screen. The packing process during this phase exhibits a pistonlike movement, packing all the space backward, and is named the beta wave (Fig. 2).

Overview: Reservoir Performance and Monitoring (September 2006)

Advances in technology enable improved reservoir characterization as well as improved measurement and allocation of relevant production and other data. The continuous advance in reservoir monitoring improves efforts in finding unrecovered hydrocarbons in fields with long production histories. However, as one of the featured papers shows, even in such fields, considerable production-forecasting uncertainties exist. Thanks to ever-increasing computing power, the uncertainties in production forecasting in these cases can be quantified and, thereby, managed. Various methods for assisted history matching have been used successfully to perform this task. Time-lapse (4D) seismic has proved very successful in reducing uncertainties and finding remaining hydrocarbons. The technology is still developing at a fast pace. The application of 4D seismic has expanded from offshore to onshore heavy-oil developments. Because this technology is one of the few sources of surveillance information away from the well, new ideas are evolving of how to advance the methodology further. Another featured paper shows how geomechanical data were incorporated with the 4D seismic analysis, which might open another field for application of this method. Geomechanical effects are important in various aspects of hydrocarbon recovery. They can be used in combination with 4D seismic, as mentioned above, and they are very important in water-injection and thermal-recovery processes. The third featured paper provides insight into coupling geomechanical effects and fluid flow. Reservoir Performance and Monitoring additional reading available at the SPE eLibrary: www.spe.org SPE 95419 "Production Monitoring Through Openhole Gravel-Pack Completions Using Permanently Installed Fiber-Optic Distributed-Temperature Systems in the BP-Operated Azeri Field in Azerbaijan" by G. Brown, SPE, Schlumberger, et al. SPE 96897 "Mapping Fluid Flow in a Reservoir Using Tiltmeter-Based Surface-Deformation Measurements" by J. Du, SPE, Pinnacle Technologies Inc., et al. Available at the OTC Library: www.otcnet.org OTC 18222 "Constraining Reservoir Uncertainty With Frequent 4D-Seismic Data at Valhall Field" by H.S. Lane, SPE, BP plc, et al.

A Comprehensive Skin-Factor Model of Horizontal-Well Completion Performance

Summary Horizontal wells or laterals are completed as openhole, slotted-liner, cased and perforated, or gravel-pack completions. We have developed a comprehensive skin-factor model to predict the performance of any of these completion types and have calibrated this model with extensive finite-element simulations of flow for a horizontal-well completion. This model can be used to predict the performance of virtually any horizontal-well completion. The new completion skin-factor model accounts for the effects of formation damage, convergent flow to perforations and slots, and flow through slots, with interaction among these effects. To account for formation damage, we extended our previous rigorous model of a damaged horizontal well to include the presence of perforations within, or extending through, the damage zone. The formation damage model is also integrated with the models of slotted-liner performance to model these completions. The model of slotted-or perforated-liner performance is made on the basis of the relationship between pressure drop and flow rate for turbulent flow in these geometries. The slotted-liner model accounts for partial plugging of the slots by grains of formation minerals or precipitates (scale). Turbulence effects are a major part of the apparent skin factor for these completion types. The model shows the recommended conditions to obtain high-productivity (i.e., low skin factor) completions in horizontal wells. In particular, the interactions among damage effects and skin effects caused by perforations or slots are shown to greatly affect horizontal-well completion performance. The models developed can be applied to design optimal completions for horizontal wells or laterals.

Optimized Drill-In Fluid for an Openhole Gravel-Pack Completion

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 82279, "Optimized Drill-In Fluid Leads to Successful Openhole Gravel-Pack Completion Installation in Unconsolidated Reservoirs - Case History," by S. Cobianco, SPE, Enitecnologie, and E. Pitoni, SPE, P.N. Ricci, and M. Galli, Eni E&P Div., prepared for the 2003 SPE European Formation Damage Conference, The Hague, 13-14 May.

Enhanced Gravel-Pack Completions Revitalize a Mature Sand-Producing Field

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 77919, "Enhanced Gravel-Pack Completions Revitalize a Mature Sand-Producing Field - A Case Study," by Mohamed Zaini B. Md Noor and Kasim B. Selamat, Petronas Carigali Sdn. Bhd., and Sharifudin Salahudin, SPE, Halliburton Energy Services Inc., originally presented at the 2002 SPE Asia Pacific Oil & Gas Conference and Exhibition, Melbourne, Australia, 8-10 October.

Achieving High-Rate Completions With Innovative Through-Tubing Sand Control

Summary Recently, an improved through-tubing sand-control technique was used to enhance reservoir recovery and improve the life-cycle economics of an offshore well in the Gulf of Mexico. This coiled-tubing-conveyed method used several technical innovations to improve the reliability of the vent-screen sand-control completion. The two most important technologies in the system are a coiled-tubing wireless depth-correlation device and a new surface modification agent (SMA) for the proppant/sand. These innovations can improve reserve recovery rate and completion efficiency, reduce mechanical risk and completion costs, extend the economic life of the well, and control fines migration. The Ewing Bank 305 field is located approximately 100 miles south-west of New Orleans. The current strategy for this field is to develop deep oil reserves and exploit shallow gas reservoirs. The hydrocarbons located in fault-trap reservoirs are normally pressured and produced by a waterdrive mechanism. In the past, the wells were completed conventionally with circulating gravel packs, generally with a dual-completion configuration. In the mid-1990s, a single-completion strategy with 3½-in. production tubing was adopted to increase production rates from new wells. The current program continues to employ gravel-packed completions with larger, 3½-in., single-production strings. In addition, because many of the new wells cut multiple pays, a completion configuration is used to provide an initial conventional gravel-packed completion and through-tubing access so that future recompletions can be performed without rig intervention. For example, the packer and tubing do not need to be pulled from the well with a rig to perform the recompletion on the next zone of interest. Rather, coiled tubing and/or electric line can be used to perform the next zone change by means of a platform or jackup boat. Future recompletions can be executed with or without gravel packs, depending on the best economic value. If no screens are installed, production rates may be limited to prevent the onset of sand production by controlling the amount of pressure drop across the perforations. In the past, through-tubing screens were deployed in the tailpipe below the packer with a packoff or lock to suspend the screen assemblies. These methods, however, do not permit uphole recompletions because they are expensive and risky to recover from the well. The vent-screen method is effective for rigless sand control while providing future recompletion options for shallower zones without the need for a rig. Extensive time and effort are devoted to optimizing each completion in the reservoir. Because of the rapid depletion of these faulted reservoirs, control of completion costs is critical to the economic success of each well. In considering which rigless completion method to use at Ewing Bank 305, it was critical that the process be economical compared to current rig rates, afford minimal mechanical risk, and not sacrifice the best practices developed for sand-control completions in the area.

Mechanisms of Mudcake Removal During Flowback

SummaryExternal filter cakes are used to minimize fluid loss and solids invasion to a formation from drilling and completion fluids. Subsequently, the cake must be removed to increase the flow area and minimize skins, especially for openhole and gravel-packed completions in horizontal and deviated wells. Experimental data is presented to show that the pressure required to initiate flow into the wellbore after building up a filter cake is affected by rock permeability; mud properties, such as particle size and cake yield strength; flowback velocity; and overbalance pressure.Mudcake liftoff tests, mud particle size, and rock pore-throat size distribution measurements were performed to understand the factors that contribute to cake removal. The mechanisms that control filter-cake removal are discussed.In this study, it is clearly shown that the flow-initiation pressure during flowback is controlled by solids and filtrate invasion (i.e., internal formation damage rather than by the external mudcake). Flow-initiation pressures show a minimum with increasing rock permeability caused by two competing effects. Larger pore sizes result in smaller flow-initiation pressures, ΔPfi; however, more solids invasion increases ΔPfi. Higher overbalance pressures also increase the internal formation damage and flow-initiation pressure. A simple model to calculate the flow-initiation pressure during flowback is proposed. The model correctly predicts the experimentally observed trends with rock permeability, mud particle size distribution, extent of solids invasion, and yield strength of the mudcake. The model provides a systematic method for designing fluids with low flow-initiation pressures. It is also shown in this study that low flow-initiation pressures do not imply complete cake removal. Cake removal is primarily controlled by the permeability of the cake and its mechanical properties.

A Unique Cost-Effective Technique for One-Trip Selective Gravel Packing Over Multiple Zones: Dacion Field Case Study

Summary Selective gravel-pack completions of multiple zones present the oil industry with major challenges and expenses. Current practices have two key problems associated with them: they are limited to zones separated by 30 to 50 ft (depending on the treatment type) because of the length of the assemblies, and they have proven to have a higher risk of installation failure. This paper describes equipment and procedures for gravel packing multiple zones in a single completion with total zonal isolation for spacing as small as 5 ft. Zones can be produced separately or comingled. Risk of execution failure is significantly lower than with conventional stacked packs. Production data is presented that verifies complete zonal isolation, improved productivity, and reduced costs from this technique.

Ram/Powell Deepwater Tension-Leg Platform: Horizontal-Well Design and Operational Experience

Summary A key focus of the Ram/Powell project is to utilize high-rate (>20,000 BOPD), high-volume wells (i.e., reserves several times greater than that of shelf wells) to enhance project profitability. Completions with large 5½-in. production tubing and horizontal openhole gravel packs (OHGP's) exceeding 2,000 ft in length have been implemented successfully. The horizontal OHGP design, the first implemented in the deepwater Gulf of Mexico (GOM), was selected as a means to improve well reliability over prior horizontal completion designs and sustain high production rates without plugging. A high rate of sand-control failure and plugging have plagued prior GOM horizontal non-gravel-packed completions. Achieving high-quality completions is a major project emphasis. Drilling, completion design, and operational experience of the horizontal wells are discussed. Production from the Ram/Powell tension-leg-platform (TLP) development, Shell's third TLP, began in September 1997. The TLP is located at Viosca Knoll Block 956 in 3,214 ft of water. Four wells were predrilled prior to TLP installation. Ten wells are completed. Five are horizontal OHGP's, one is a single-zone cased gravel pack, and the remainder are two- and three-zone-selective cased gravel-packed completions. Rig operations concluded in January 2000.

Screenless Completions: Cost-Effective Sand Control in the Adriatic Sea

SummaryMany gas reservoirs in the central Adriatic Sea, offshore Italy, are formed of laminated, low-permeability, dirty sandstones, with as much as 50% clay content. Initially, wells were completed with gravel packs, but the nature of the formations caused productivity to decrease in some gas fields because fines plugged the gravel packs. In the past few years fracpacks have been used to alleviate the fines migration problem. However, there are many gravel-packed wells which are either sanded up and shut in or underperforming because fines have plugged the gravel-pack screens. These wells do not currently justify the expense of an immediate full rig workover.A candidate zone was selected to evaluate the viability of a screenless completion technique for competent sand control. We placed a highly conductive tip-screenout fracture in the zone, using recent innovations in fracturing fluid technology and proppant flowback control additives, to eliminate sand production. Successful implementation of this technique was confirmed by posttreatment production more than 2.5 times the normalized rate of the initial gravel-pack completion. No formation sand or proppant was produced back to surface during production. The screenless completion technique provided a cost-effective and reliable rigless rehabilitation technique.

Analysis of Post-Audits for Gulf of Mexico Gravel-Packed Oilwell Completions Leads to Continuous Improvement in Completion Practices

Summary Final production rate alone is not an adequate measure of the success of a well completion. Rather, we must estimate the "po-tential" of a reservoir and judge the ultimate success of a completion on how close we come to achieving this potential. Specific productivity indexes (SPI's) [(BFPD/ft)/psi], specific injectivity indexes (SII'S) [(BFPD/ft)/psi], and completion efficiencies (CE's) (percent of Darcy radial flow) can be calculated at various times throughout a well completion. Analysis of these data quantifies the efficiency of the completion after each individual completion operation, allowing a determination of the effects of each completion practice to be made. In addition to completion efficiency data, a comparison of gravel placement volumes behind casing helps quantify optimum gravel packing procedures. Twenty-two Gulf of Mexico completions have been analyzed using this technique. In this paper we will give details of the results of this analysis, in particular the productivity effects of various methods of underbalanced perforating, gravel packing, and well control. Items of discussion include the effects of underbalanced perforating on well performance, the effects of flowback after perforating on perforation tunnel cleaning, productivity impacts of various types of well control methods following perforating and gravel packing, and comparisons of gravel pack design parameters and gravel placement behind casing.

Experimental Investigation of the Influence of Perforating on Gravel-Pack Impairment

Summary Analysis of the performance of perforated, gravel-packed wells compared with openhole gravel-packed wells has indicated that the perforation could be responsible for the low productivity of internally gravel-packed wells. A series of laboratory experiments on weak rock [unconfined compressive strength (UCS) = 1,200 psi] was conducted to quantify this. Castlegate sandstone rock samples, 7-in. diameter and 18-in. long, were perforated with shaped charges under downhole conditions. The perforations were gravel packed by circulating a gravel slurry past the tunnel entrance while applying a differential pressure. The pre-perforation, post-perforation and post-gravel-pack productivities of the core samples were monitored at different flow rates (5 to 20 B/D/perf). Sectional photographs of the core samples were taken at post-gravel-pack conditions for qualitative analysis. Performance evaluation of gravel-packed "realistic perforations" (debris and loose sand in the tunnel removed by drawdown-induced flow) vs. "ideal perforations" (debris and loose sand removed by external means) showed the post-gravel-pack productivities of realistic perforations to be much lower than ideal perforations in oil-phase as well as water-phase experiments. Injection of fluid after gravel packing only temporarily unplugged the perforation tunnel. The study shows the importance of fines- and debris-free perforation tunnels in maintaining the productivities of gravel-packed completions.

Horizontal Gravel-Packing Technique for Slim, Cased Holes in Highly Permeable Zones: A Single-Well Case History

Summary This paper describes a technique for horizontal gravel packing of slim, cased holes. The key to the technique is the use of horizontally oriented perforations as a means of providing self-diversion of the gravel pack regardless of fluid leakoff. Traditional horizontal gravel packing requires minimizing fluid leakoff to the formation; otherwise, premature bridging can occur during the gravel-packing process. The cased hole horizontal gravel-packing process presented here, however, does not require minimizing fluid leakoff to the formation, thus, allowing the possibility of placing a gravel pack in a horizontal casing with less resultant damage than is possible with traditional techniques. Gravel packing is becoming a common practice for controlling unconsolidated sands in horizontal wellbores. So far, horizontal gravel packing has been applied almost exclusively to open holes with screens in them. If a formation has a high permeability, and therefore, a high fluid-loss rate, placement of a gravel pack may be prevented by premature dehydration and bridging of the gravel-pack sand in the wellbore/screen annulus. The usual method of preventing these high fluid losses and the resultant bridging is to damage the formation face prior to gravel packing. The damage mechanism is usually salt, calcium carbonate, or other particles that seal off the formation face. Once gravel packing is completed, the formation damage is (hopefully) remediated using acid or undersaturated brines to restore the completion's productivity. There is, however, a risk that the damage cannot be successfully removed once it is gravel packed against the formation face. Acid diversion is also a problem when attempting to remediate damage in an openhole gravel-packed completion. This paper presents an alternative technique for performing a horizontal gravel pack in a high-permeability zone while reducing the risk of permanently damaging the formation's permeability. The technique involves cementing casing through the horizontal completion interval and then shooting perforations in a horizontal orientation. The perforations can then be acidized with better diversion techniques than is possible in an openhole completion. An uncentralized screen is then gravel packed. The horizontally positioned perforations provide a self-diverting gravel pack. The self-diverting nature of this technique makes it relatively insensitive to fluid loss; whereas openhole techniques are very sensitive to fluid loss. The uncentralized liner rests on the bottom of the horizontal to provide maximum standoff from the perforations. The above technique was applied at Thums Long Beach Company to a well with a 560-ft horizontal completion cased with a 412-in. liner (4.052-in. ID). The technique's acidizing phase, however, was not performed due to fishing concerns inside the slim liner. This paper presents a case history for gravel packing a 690-ft-long, 278-in.-slotted liner inside the horizontal 412-in. casing. To our knowledge, this is the slimmest horizontal gravel pack ever attempted. The gravel packing was performed successfully even though fluid returns were never established during the job. The lack of fluid returns is due to excessive fluid loss to the formation. The self-diverting nature of the horizontal perforations was the key to successful gravel packing. Production rates, on the other hand, were not as successful since we did not acidize the perforations prior to gravel packing. Comparisons between this well and openhole horizontal gravel packs are made.

Challenges, Accomplishments, and Recent Developments in Gravel Packing

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized to be experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction Techniques used for sand control include restricted hydrocarbon production, in-situ consolidation, gravel packing, high-rate water packing, frac packing, fracturing without screens, and drilling horizontal wells completed with screens. Frac packing, high-rate water packing, and horizontal completions have been the most widely used of these techniques in recent years. Fracturing without screens, frac packing, and horizontal drilling all aim at reducing drawdown and/or increasing production rates by decreasing velocities in the formation with a larger flow area. The increased flow area also makes these methods more tolerant to formation and completion damage. Achieving long-term sand control without impairing well productivity has been a challenging task from both a technical and an economic perspective for many years in almost all major oil- and gas-producing regions of the world. Because this article is a review of the challenges, accomplishments, and recent developments in gravel packing, we limit our discussion to treatments below fracturing pressure by assuming that exceeding the fracturing pressure is undesirable for technical reasons. It is not our intent to suggest that gravel packing is the preferred sand-control method. All methods for sand control have their own applications and value in the industry. We first discuss recent developments and challenges in gravel, screen, and carrier-fluid areas and follow this by a discussion of gravel packing in cased holes, where major accomplishments have been introduction of combined processes that not only reduce completion costs but also improve productivity. Next, were view openhole gravel packing, primarily for highly deviated or horizontal wells because this is where most openhole gravel-packing applications have been. Finally, we draw conclusions and highlight the accomplishments and remaining challenges. Ideal Gravel-Pack Completion From a technical standpoint, an ideal gravel-pack completion accomplishes the following objectives.Complete packing, with a properly sized high-permeability gravel, of allvoid spaces between the screen and casing and perforations or between the screen and the open hole.Clear interface between the formation sand and gravel, with nomixing/invasion of formation material with/into the gravel pack during placement and during production throughout the life of the well.No invasion of the matrix with damaging material (e.g., unfiltered or incompatible brines, polymers, crushed gravel).No reduced-permeability section between the formation sand and the gravelpack (e.g., drilling-fluid filter cake in open holes or crushed zone in cased and perforated holes).No residuals from the carrier fluid and/or fluid-loss pills (e.g., polymers and bridging agents in gravel pack). Particular attention also must be given to prevention of screen plugging while running the screens in hole (discussed later). Obtaining a technically ideal gravel pack as described has been a major challenge, and all objectives have not been achieved under all circumstances. Furthermore, achieving this type of completion may not always be necessary, depending on cost/benefit analysis. For example, openhole horizontal completions sometimes can tolerate high levels of damage with little impact on productivity. In such cases, incremental production resulting from a cleanup treatment needs to be weighed vs. cost. Regardless of the type of completion (open or cased), the first potential source of formation damage is the drilling fluid. In recent years, with the increased applications of horizontal wells, substantial effort has been devoted to formulating reservoir drilling fluids (RDF's) that form a tight filter cake with minimal particle and polymer invasion into the matrix and that contain soluble (in acid, water, or oil) bridging agents. Proper RDF selection typically is considered to be less of an issue in cased holes because the damaged region is believed to be bypassed by the perforations; however, this may not be true in unconsolidated sand formations because perforation collapse is highly likely.

Evaluating Completion Damage in High-Rate, Gravel-Packed Wells

Summary Conoco recently completed and tested a series of high-rate, gravel-packed oil wells. Analysis of the well test data for the first completion indicated that significant formation/completion damage had been caused during the gravel-packing process. Extensive laboratory testing designed to closely simulate the completion procedure was performed in conjunction with detailed completion pressure loss evaluation. The studies identified the loss control material (LCM) pill used during completion operations as the primary source of damage. A new, self-breaking, LCM composition was designed that provided fluid-loss control with minimum formation/completion damage. This LCM system and a number of other design improvements were used in the field's remaining gravel-packed completions. We describe the analysis of well test data and the application of this information to evaluate completion procedures for these high-rate, cased-hole, gravel-packed completions. The analytical technique used is unique in that it combines laboratory, analytical, and operational data to quantify individual components of the total measured pressure drop across the completion. This approach provides significantly better insight into completion performance than merely stating an overall completion pressure drop, skin value, or well flow efficiency. The result of this systematic analysis of pressure losses is a better understanding of formation damage effects and the factors that influence them during the completion process. This in turn allows completion procedures to be modified to target specific areas for improvement.

Field Experiences and Guidelines for Use of Nonviscous Fluids in Frac-Pack Operations

Summary In the Adriatic offshore area, conventional cased-hole gravel-pack completions have generally produced low flow efficiences (10 to 50%) from the multilayered reservoirs of low-to-medium gas permeability (1 to 50 md). This has been attributed to the poor communication between the wellbore and the producing layers, caused by limited exposure of the interval and poor filling of the perforation tunnels with gravel. This is confirmed by openhole gravel-pack (OHGP) completions where much higher flow efficiencies are observed. The frac-pack technique, which combines gravel-pack and fracturing technologies, can improve communication between the layers in perforated completions by creating a vertical fracture with high conductivity and the injectivity necessary to fill the perforation tunnels. In addition, any existing near wellbore damage will be bypassed. This paper reviews the field experience gained from over 80 frac-pack operations that have been performed during the last 2 years in the thin, interbedded layers of poorly consolidated sand and shale commonly found in the Adriatic area. Owing to their relatively low permeability, these formations can be fractured with nonviscosified fluids at pump rates less than 1 bbl/min. Consequently, completion brine was selected as the treating fluid. To reduce operational time and costs, the frac-pack treatments were carried out with the gravel-pack assembly in place and injection rates of up to 7 bbl/min were used with gravel concentrations staged from 1 to 4 lbm/gal. Fluid losses were encountered after the frac-pack treatments and were controlled by spotting a small pill of drill-in fluid (containing calcium carbonate) that was sized to bridge inside the screens. This paper presents the production performance of the frac-pack completions and establishes guidelines for future applications of frac-pack operations in similar reservoirs.

Well-Completion Audits To Evaluate Gravel-Packing Procedures

SummaryData collected during gravel-packed completions may be analyzed to evaluate the impact of specific completion practices on well performance. Operations in which gathering data is critical are during the initial flow and perforation cleanup after underbalanced perforating; during prescribed injection tests before gravel packing; during periods in which fluid is being injected into the formation (pre-packing perforations, fluid loss after perforating, fluid loss after gravel packing); and during stabilized flow after the well is brought on line. This data can be used to calculate an approximate value for kh (md • ft) and an instantaneous completion efficiency at different stages of the completion. By comparing these instantaneous completion efficiencies, the damage contribution of certain operations and the damage-prevention aspects of other operations may be quantified. This methodology should provide a valuable resource to those pursuing continuous improvement in well-completion operations, particularly those involved in sand control.

A Review of Completions and Gravel-Pack Techniques of the Xijiang Development

Summary The first gravel-packed completions have been run in the Pearl River mouth basin of the South China Sea in the Xijiang fields. The reservoirs in these fields are characterized by unconsolidated, friable, high-permeability sandstones, and, as such, presented many challenges. This paper addresses the development of these fields and how reservoir characteristics and parameters and unexpected deviations were experienced by the operator, as well as the impact these deviations had on completion designs. Important considerations included the need for sand control, zonal isolation, high-rate production, and reduced completion durations. Finally, the practical application and evaluation of pregravelpack stimulation techniques, gravel-pack carrier consideration, fluid-loss controls, and general completion procedures are presented, along with the innovative completion designs developed to manage these needs.