Entrainment and redeposition of naturally occurring fine particles in porous media has been suggested as a mechanism leading to abnormal decline in productivity of producing wells. This paper describes the results of studies conducted to determine factors affecting this phenomenon. Experimental work done as part of this study provides the basis for a proposed phenomenological theory of entrainment and deposition. The central concept of this theory is representation of both particle and pore size distributions by partitioning the porous medium at any cross section into parallel plugging and nonplugging pathways. This simple model appears to be completely adequate for describing a broad class of filtration and entrainment phenomena. We have shown that fines entrainment and redeposition are mechanisms that can cause abnormal productivity decline and are phenomena restricted to the near-wellbore region.

This paper describes a new technique for improving the effectiveness of gravel placement between the screen and the wellbore in deviated wells. This technique can be used to predict whether a set of design conditions leads to a complete gravel pack that fills the entire annulus of a wellbore, or to an unstable, incomplete gravel pack that leaves voids in the annulus. The design procedure was used to gravel pack several high-angle wells successfully. Introduction Sand production from unconsolidated formations long has been a source of completion problems. Millions of dollars are spent throughout the world each year to achieve effective sand-control measures. Millions more axe spent each year to remedy many originally successful procedures that fail before the reservoir is depleted. procedures that fail before the reservoir is depleted. Maximum reliability of initial sand-control practices is essential, therefore, particularly offshore and in remote locations where operating costs are high. The most widely used sand-control technique involves placing a screen in the wellbore and packing gravel placing a screen in the wellbore and packing gravel around it. The screen is sized to retain the gravel and the gravel, in turn, is sized to retain the formation sand. This technique has proven effective, especially when formation properties allow open-hole completions. In spite of properties allow open-hole completions. In spite of this success, many gravel-packing procedures have less than desirable effectiveness because of unstable or incomplete placement of gravel around the screen. This is particularly true when the wellbores are deviated. As a particularly true when the wellbores are deviated. As a result, there is considerable incentive to develop gravel-packing techniques that have both high initial success ratio and the ability to provide sand-free production for sustained periods of time. Gravel packs in deviated wellbores need special design considerations. Recent gravel-packing studie's have shown that the wellbore angle has a dramatic effect on gravel placement - high angles can lead to unstable, incomplete gravel packs. In spite of the need to know the factors that control gravel placement in deviated wellbores, little has been published on the relationship between the fluid mechanics of gravel packing and the final distribution of gravel in deviated wellbores. This is a serious limitation because high-angle wells can be gravel packed effectively when the design conditions are packed effectively when the design conditions are selected correctly. Observations made while gravel packing a small-scale laboratory wellbore model are discussed in this paper. Then, the process by which gravel is transported in deviated wells is described. Finally, the development of a new design technique is reported that can be used to predict whether a set of design conditions can lead to a predict whether a set of design conditions can lead to a complete gravel pack that fills the entire wellbore-screen annulus, or to an unstable, incomplete gravel pack that leaves voids in the annulus. Gravel-Packing Apparatus The dynamics of gravel transport in deviated wellbores can be studied most effectively when the process can be visualized. The experiments described here were conducted in two transparent Lucite wellbore models: a small-scale, 3-in. (7.62-cm)-ID model that was 10 ft (3.05 m) long, and a full-scale, 5 1/2-in. (13.9-cm)-ID model that was 20 ft (6.09 m) long. Fig. 1 shows the details of the small-scale model. A 1.9-in. (4.83-cm)-OD wire-wrapped screen with 0.012-in. (0.305-mm) slots was centered inside the Lucite tube. A stainless-steel tube with 1/2-in. (1.27-cm) OD was placed inside the screen to simulate the tailpipe. JPT P. 109

This paper describes a new procedure for determining particle-transport efficiency through perforations. Transport efficiency is the mass fraction of particles that are transported through the perforations relative to the total mass of particles injected. Our conclusions are based on laboratory studies of the transport of solid particles carried through perforations by different fluids having a wide range of physical properties. Prediction of particle-transport efficiencies based on theoretical considerations described in this paper gives an accurate representation of experimental results.

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