Penberthy Jr., W.L., SPE-AIME, Penberthy Jr., W.L., SPE-AIME, Exxon Production Research Co. Shaughnessy, C.M., SPE-AIME, Exxon Production Research Co. Gruesbeck, C., SPE-AIME, Exxon Production Research Co. Salathiel, W.M., SPE-AIME, Exxon Production Research Co. For effective sand consolidation, resin must wet the surface of sand grains. When plastic resins do not have this ability, preflushing is essential. Model studies demonstrated that preflushing effectiveness depended on preflush volume, viscosity, and sand permeability. Results indicated that an optimum volume of 100 gal/ft was required for an effective preflush. Introduction Experience with sand consolidation for the past 30 years has shown that candidate wells should have relatively thin, clean, homogeneous, undamaged sand zones. Proper preflushing also is essential for effective sand Proper preflushing also is essential for effective sand consolidation. A variety of aqueous and organic preflushes have been used to remove formation fluids ahead preflushes have been used to remove formation fluids ahead of sand consolidation resins. Proper preflushing can contribute significantly to the strength of the consolidated sand by improving the adhesion between the resin and sand matrix. Because of increased emphasis on sand consolidation performance and lifetime, a considerable incentive exists for improving preflush selection and volume. To be effective, a sand consolidation resin first must wet and then must adhere to the surface of the sand grains. Because the sand grains in most reservoirs are water wet originally, it is critical for the resin to replace water on the surface of the grains. Fig. 1 shows the effect of residual water saturation on the strength of sand consolidated with an epoxy resin. As water saturation increases, compressive strength decreases. At 6-percent water saturation, resin is prevented from wetting the sand matrix and consolidation has little compressive strength. While studying all available sand consolidation processes, laboratory tests showed that some resins were processes, laboratory tests showed that some resins were able to displace water by themselves. Others depended heavily on preflushing for water removal. Although oil removal appears desirable, most sand-consolidation resins exhibit good sand-grain wetting in the presence of oil. Consequently, mutual solvents that preferentially remove water are more desirable for sand consolidation preflushing, particularly where epoxy resins are preflushing, particularly where epoxy resins are concerned. Preflush Study Preflush Study A series of tests identified solvents that preferentially remove water in the presence of oil. Solvents were characterized on the basis of their phase behavior with brine and oil. Fig. 2 illustrates four possible types of phase behavior for the preflush-brine-oil system. Dashed phase behavior for the preflush-brine-oil system. Dashed lines represent tie lines connecting equilibrium phases in the two-phase region. Of the four types of phase behavior, Type 2 solvent is the most desirable because it preferentially removes water and also removes oil. Type preferentially removes water and also removes oil. Type 1 solvent results in a residual oil saturation, Type 3 solvent preferentially removes oil, and Type 4 solvent has no water miscibility. Most tests were conducted with 6-percent NaCl brine and diesel oil. Promising candidates were studied further using combinations of twine, 15-percent HC1, spent mud acid, and diesel and crude oil. Tests were conducted on three classes of compounds - alcohols, glycol ethers, and glycol ether acetates. Results showed that, of the alcohols, only isopropyl alcohol demonstrated mutual miscibility for brine and oil. The glycol ether acetates were all oil miscible. Many glycol ethers, however, were mutually miscible with brine and diesel. JPT P. 845

When liquid sodium silicate boils, it forms a rigid foam on the heated surface. It is an effective and relatively inexpensive means of insulating steam-injection wells, and might also be useful for preventing paraffin deposition and hydrate formation. Introduction Thermally induced stress that causes casing failure has been a problem in oilfield steam-injection operations for a little more than a decade. Heat transfer in a well has been described analytically, and a number of methods have been devised to reduce wellbore heat losses so that lower casing temperatures can be maintained and higher steam qualities attained at the sand face of a reservoir. The methods include using insulated tubing, pumping a low-thermal-conductivity liquid into the annulus, and coating the tubing with aluminum paint. Insulated tubing is expensive and economics in many instances do not justify its use. Low-thermal-conductivity fluids when placed in a packed-off annulus and subjected to high temperatures packed-off annulus and subjected to high temperatures may gravity segregate, solidify, or become so viscous that the removal of a packer and injection tubing is often difficult. The primary draw-back to the use of aluminum-painted tubing is that it is difficult to prevent oil or other high-emissivity materials from prevent oil or other high-emissivity materials from clinging to its low-emissivity surface when it is being handled and lowered into the well. Such high-emissivity materials destroy its thermal effectiveness. A new insulating material is now available and a technique for its use in steam-injection wells has been developed. The insulating material, silicate foam, is formed by boiling a sodium silicate solution. The foam is an excellent insulator, having a thermal conductivity of approximately 0.017 Btu/hr-ft. degrees F. Fig. 1 is a photograph of the foam's structure. Its physical properties are given in Table 1. The Insulation Process In a field operation, a solution of sodium silicate is placed in a packed-off annulus, and then steam is placed in a packed-off annulus, and then steam is injected down the tubing. The hot tubing causes the silicate solution to boil, leaving a coating of insulating foam, usually about 1/4 to 1/2 in. thick, on the hot tubing surface. Since the foam immediately becomes an effective insulation, none is deposited on the inner wag of the casing. Silicate solution that remains in the annulus after steaming for several hours is removed from the annulus by displacing it with water (if the solution is not removed, it may solidify in the annulus). The water is removed by gas-lifting or swabbing. Fig. 2 is a schematic showing steps of the insulation process. Once the insulation is formed, heat loss is reduced and lower casing temperatures and higher sand-face steam qualities are the result. A comparison showing the foam's effectiveness is presented in Fig. 3, which illustrates the calculated maximum casing temperature in a well with packed-off tubing. The three cases show the relationship between casing temperature and steam-injection time for uninsulated tubing, commercially available insulated tubing, and tubing with a 1/4-in.-thick coating of silicate foam. The calculated casing temperatures are considerably lower for the insulated cases; however, there is not a great deal of difference between the two insulated cases. JPT P. 583

Field-scale gravel-pack studies have shown that openhole gravel packs have higher productivity than cased-hole gravel packs; however, in the latter, well productivity can be improved by increasing the size and number of perforations. Results also show that cased-hole gravel-pack productivity can be enhanced by prepacking gravel outside the perforations. Introduction Gravel packing to exclude formation sand from produced fluids has been used as a completion technique for oil and gas wells for the past 40 to 50 years. However, the technology used in early gravelpack work was borrowed from the water-well industry, which for many years had been applying several types of mechanical sand retention devices to prevent sand production from produced groundwater.Early oil and gas completions, similar to those used in water wells, were what normally is referred to as openhole gravel packs (Fig. 1). Accurately sized gravel was placed around a slotted liner to act as a filter for formation sand. Subsequent completions sometimes consisted of cementing casing through the formation and perforating to communicate with the oil reservoir. This technique became common with the advent of gun and jet perforating, because the exclusion of water or gas was sometimes difficult to achieve in openhole gravel packs.If fluid production also resulted in sand production, cased-hole gravel packs were performed inside the well's casing (Fig. 1). The well productivity of these packs, however, quickly was recognized as being far below that experienced with the openhole packs. Field experience showed that, in general, the productivity of prepacked cased-hole completions was sometimes only 25 to 33% of that of the openhole pack, whereas the lack of prepacking could cause productivity losses of greater than 95%, as shown in Table 1.As a result of these problems, a whole new dimension in gravel packing arose with the cased-hole gravel-pack completion, since substantially different techniques and procedures are required to perform these completions properly in comparison with openhole gravel packs.The productivity limitations encountered in cased-hole gravel packs prompted numerous engineers and researchers to begin work to define and solve problems related to them. Early work on the proper gravel size required for effective sand control indicated that gravel/sand ratios of from 5:1 to 13:1 should be used. Formation sand design points anywhere from a cumulative of 10 to 70% based on a representative formation sand sample were recommended.Most of the early research was conducted in linear-flow packs operated under single-phase flow conditions. Additional work indicated that the main restriction to flow was sand-filled perforations and that for maximum productivity, plugging in the perforation with formation sand must be avoided. The remedy for this situation has been to pressure-pack the gravel through the perforation so that the gravel in the perforation is of the highest permeability capable of preventing the production of formation sand. JPT P. 1679＾

Gravel Placement Through Perforations and Perforation Cleaning Perforations and Perforation Cleaning for Gravel Packing Summary. In large-scale model testing, the effectiveness of gravel prepacking under four different conditions was studied: when there had been prepacking under four different conditions was studied: when there had been no previous sand production, after sand production had occurred, after perforation washing, and after perforation surging. In the case of no perforation washing, and after perforation surging. In the case of no previous sand production, pressure parting occurred in the unconsolidated previous sand production, pressure parting occurred in the unconsolidated formation when a critical injection pressure level was exceeded. The orientation was normal to the least principal stress. In those cases where earlier fluid production caused the removal of formation sand. prepacking produced multiple pressure parts in the low-stress-state sand in the vicinity of the perforation because stable cavities had not tonned. The orientation of these pressure parts was also normal to the least principal stress. pressure parts was also normal to the least principal stress. Gravel prepacking after perforation washing and surging resulted in gravel-placement geometries that were controlled by the stress state and geometry of the formation sand around the perforation after these operations. Prepacking after perforation washing generally resulted in placement that resembled the geometry of the washed region, which was usually a void. Prepacking after surging resulted in pressure parting and considerable mixing of the prepack gravel with the formation sand because stable cavities had not formed. Perforation washing results demonstrated that washing should be conducted at the maximum practical pump rate with water used as the wash fluid. Perforation surging tests indicated that the amount of formation sand removed was proportional to the surge pressure. pressure. Introduction Gravel packing has proved to be a viable well-completion technique to exclude formation sand from the produced fluids. The state of the art has steadily improved during the past 50 years to the point where, in certain oil- and gas-producing region, the majority of wells are gravel packed. Although gravel packing can be performed with either openhole or cased-hole techniques, cased-hole gravel packs are more commonly used. Reasons for this choice include packs are more commonly used. Reasons for this choice include fewer complications with drilling and completion operations, as well as reservoir and workover considerations. Openhole gravel packs are usually selected when completion and reservoir conditions are ideal. Because such conditions seldom prevail, cased-hole gravel packs have been the usual choice. packs have been the usual choice. Yet laboratory and field data have shown that cased-hole gravel packs generally have lower productivity indices than openhole gravel packs generally have lower productivity indices than openhole gravel packs. This fact relates to the entry of fluid through packs. This fact relates to the entry of fluid through perforations, which exposes a small fraction of the reservoir sand to the perforations, which exposes a small fraction of the reservoir sand to the well. Added to this problem is the fact that about two-thirds of a perforation's cross-sectional area is filled with either gravel or the perforation's cross-sectional area is filled with either gravel or the reservoir sand. Lower productivity can result if proper completion procedures are not implemented. Research and field experience procedures are not implemented. Research and field experience have shown that the best plan is to perforate the well properly and to prepack the perforations with gravel. Failure to take these steps will lessen fluid inflow because the perforations tend to fill with reservoir sand. Not only will perforations tend to fill with reservoir sand. Not only will productivity be low, but high completion pressure drawdown and short productivity be low, but high completion pressure drawdown and short completion life will result as well. To improve cased-hole gravel pack productivity, large-diameter, high-density perforating pack productivity, large-diameter, high-density perforating programs have been initiated. In addition, such perforation cleaning programs have been initiated. In addition, such perforation cleaning methods as washing or surging have been used to enhance well productivity. These measures are designed to remove plugging productivity. These measures are designed to remove plugging material and formation sand from the perforations so that highpermeability gravel can subsequently be packed into the perforations- The benefits of this practice are higher productivity, lower perforations- The benefits of this practice are higher productivity, lower pressure drawdown, and longer completion life. pressure drawdown, and longer completion life. When no perforation cleaning has been conducted, prepacking the perforation tunnels is theoretically the optimum procedure Possible without resorting to high pump rates and pressures. On the Possible without resorting to high pump rates and pressures. On the other hand, should workover operations be conducted to gravel pack, a well that was initially completed by perforating only, any prepacking operations would have to be Performed through the prepacking operations would have to be Performed through the perforations from which sand production has occurred. Here the prepack perforations from which sand production has occurred. Here the prepack geometry should depend on the size and shape of any void around the perforation, as well as the stress state of the formation sand. The Model To study the effects of perforation washing, surging, and prepacking under various conditions, a physical model was designed and constricted. The model consisted of a thick-walled 42-in. 11.07-m] -ID pressure shell rated at 500 psi [3.4 MPa] and an inner pressure shell rated at 500 psi [3.4 MPa] and an inner formation-sand container with a 36-in. [91 -cm] ID and 42-in. 11.07-m] length. The container was a wire-wrapped screen (slot width of 0.002 in. [0.05 mm]). Because the screen was used as a container. it was cut longitudinally and reverse-rolled so that the "keystones" were along the ID of the screen rather than in the normal position, toward the outside. A simulated wellbore was at one end of the model. It was arranged so that the size and number of perforations could be adjusted to meet test conditions. A floating piston was positioned at the opposite end of the container to relieve internal forces when gravel was pumped into the model. The laboratory model was designed to approximate the boundary conditions that exist around a well-, a photograph is shown in Fig. 1. Fig. 2 illustrates the interior of the model and shows the wire-wrapped screen/sand container and the wellbore section. Fig. 3 is a schematic of the model packed with sand. The design of this particular model permitted three-dimensional leakoff through permeable sands. Also, the end effects were sufficiently removed from the vicinity of the perforations that their effects on gravel-placement geometry were minimal. perforations that their effects on gravel-placement geometry were minimal. It was believed that this model could simulate field conditions to the extent that a reasonable understanding of near-wellbore placement of gravel was possible. With a pneumatic tamper. the model was packed in a vertical position with 13 ft3 [0.37 M3) of Brazos River sand to a packed position with 13 ft3 [0.37 M3) of Brazos River sand to a packed height of- 18 in. [46 cm). Sands ranged in permeability from 140 md to 3.7 darcies and contained about 6% clay. The average grain diameter of the sands at the 50 percentile point was 0.0042 in. [0.11 mm). The uniformity coefficient averaged about 2.3. P. 229

Summary Long, horizontal gravel packs are viable completions that have been placed successfully in more than 80 wells. Extensive field-scale testing has significantly aided the development of field procedures and operating guidelines. Software has also been developed to assist with horizontal gravel-pack design. To be performed properly, these completions require a systems approach for their implementation because drilling and displacing the completion interval, maintaining hole stability, selecting and running equipment, and maintaining returns during the gravel-packing operation must be integrated into the completion strategy. Field experience suggests that, in some cases, gravel packs maintain productivity better than prepacked screens or slotted liners. Introduction Gravel packing is a well-completion technique used to exclude formation sand from produced reservoir fluids and extend completion longevity. It is performed by placing a gravel-retention device (such as a wire-wrapped screen) in a well opposite the completion interval, and subsequently circulating gravel around the gravel-retention device to create a permeable filter. Gravel packing has been applied in vertical and deviated wells that were completed either cased or openhole. Transport fluids used to gravel pack wells have been either polymer-viscosified brines or unviscosified brines. Of the two choices, unviscosified brines are usually preferred because they exhibit lower-porosity, void-free packs. Several authors have investigated the factors affecting gravel placement, productivity, and completion longevity.1–17Fig. 1 illustrates an example of the gravel placement at well deviations of 0 and 45° using water as the transport fluid.16 Note that the gravel is initially placed at the bottom of the well and packs sequentially upwards. At well deviations in excess of 60°, the angle of repose of gravel is exceeded and the gravel initially settles on the low side of the well and remains at rest, unless sufficient flow is available to move it forward. The 60° well deviation represents a transition in the placement of gravel, because at lower well deviations the gravel will settle to the bottom of the well. Placing gravel at the bottom of a highly deviated well requires higher pump rates and a slightly different completion geometry than in vertical wells. A large-diameter wash pipe can be used inside the screen to enhance placement.2 It forces a larger fraction of the fluid to flow along the outside of the screen, rather than being diverted into the screen/wash-pipe annulus. The higher screen-casing annulus flow rate assists in transporting the gravel and prevents a premature stall of the gravel pack before it reaches the bottom of the completion. When the proper screen and wash-pipe size are used, the gravel-deposition process consists of a primary or "alpha" wave that packs from the top to the bottom of the completion, leaving an open void over the gravel dune. Upon reaching the bottom of the completion, a subsequent secondary or "beta" wave deposition proceeds in the opposite direction of the alpha wave (towards the top of the well) until the interval is completely packed. The packing process in a highly deviated well is portrayed in Fig. 2.16 This technology has been used to successfully gravel pack many conventional wells with deviations as high as 80° or more and completion lengths of several hundred feet. Until recently, gravel packing long, horizontal wells has not been used as a completion technique, apparently because the technology was not thought to be in hand. Instead, prepacked screens and slotted liners had been used as stand-alone sand-control equipment, even though their performance in conventional wells had been disappointing. Their improved performance in horizontal wells is probably attributed to much lower flow rates per foot of completion interval than when used in vertical wells. In horizontal service, the initial productivities of screens and slotted liners usually have been good, but their tendency is to plug and restrict productivity, particularly when completed in dirty sands, when high water or gas/oil ratios occur, or when placed in viscous-oil service. Not surprisingly, screens and slotted liners seem to perform best in clean, high-permeability sands. Hence, how well screens and slotted liners perform tends to be site-specific. Gravel packing horizontal wells has been successfully demonstrated in field-scale studies and actual completions. Gravel has been placed in over 80 wells, with intervals ranging from 600 to 3,300 ft. The test data and field experience suggest that packing even longer intervals is possible. The following discussion reviews testing and field experience gained with gravel packing long horizontal wells. It also presents guidelines for completion operations. Field-Scale Testing Most field-scale testing for simulating gravel packs has been performed with clear plastic models. Until recently, the gravel-pack-model lengths were less than 100 ft, and most were about 25 ft long. The implications of testing in these models were that they were too short to provide meaningful results for long, horizontal wells. While numerical simulation was also an alternative, designing completions using unverified simulators was believed to be risky and unreliable. For these reasons, a field-scale model was designed and constructed to simulate gravel packing horizontal wells. Description of the Model. The horizontal model is 1,500-ft long with a 4 1/2-in. outside diameter (OD) [4-in. inside diameter (ID)]. It is equipped with a centralized 2 1/16-in. screen (0.006-in. slot openings). The wash-pipe diameter is 1.315 in. The wash-pipe OD to screen ID ratio is 0.75. The model is equipped with clear, high-strength plastic windows capable of operating at pressures of 1,000 psi to allow the visual observation of the packing process at six locations along its length. Sliding sleeves are also located along the model to allow the visual observation of gravel deposition in the model at that location after pumping has ceased. Fluid loss is simulated with 400 perforation tubes that are l-ft long, 1/2-in. diameter pipe filled with resin-coated 40/60 U.S. mesh gravel. Five pressure sensors are installed at strategic locations. Inlet and exit flowmeters measure entrance and return flow. Pumping is performed with a field unit. Data acquisition consists of recording rates, pressures, and gravel-mix ratio as a function of time. Fig. 3 illustrates a schematic of the model while Figs. 4 and 5 show the windows and the perforation arrangement. On the basis of previous experience, water was chosen as the carrier because viscous fluids have a strong tendency to bridge and dehydrate the gravel prematurely in long, highly deviated wells.16