Characterization and Control of Formation Damage During Waterflooding of a High-Clay-Content Reservoir

Abstract

Summary. Permeability damage was suspected as a factor contributing to postcompletion production declines observed during waterflooding of a friable to unconsolidated sandstone. A loss in water injectivity also indicated that near-wellbore damage may have been occurring. This paper summarizes the laboratory studies undertaken to evaluate and understand permeability damage in this formation. The experimental program consisted of primarily coreflooding experiments to evaluate the influence of injection fluid composition, temperature, mineralogy, and flow velocity on permeability damage. The significance of damage during brine injection was observed to be a strong function of mineralogy and injection rate. Permeability losses of >60% were observed in cores from some sand intervals, while little or no damage was apparent in cores from a second zone in identical wells. In the sands that were susceptible to brine damage, a critical velocity below which permeability damage did not occur was repeatedly established. The occurrence of a critical velocity, along with other observations, indicated that the primary damage mechanism was fines migration. Damage could be controlled by maintaining velocities below the critical value or reducing the pH of the injected brine. Introduction Secondary and tertiary recovery processes can lead to severe and permanent reductions in permeability because of interactions between injected fluids and the reservoir rock. This is especially true in high-clay-content, low-permeability, poorly consolidated reservoirs. Near-wellbore permeability damage was suspected as a factor contributing to the rapid postcompletion production declines observed in a friable to unconsolidated sandstone. A loss in water injectivity in some zones also could have been the result of permeability damage. In response to these problems, a laboratory program to understand permeability damage in this reservoir was initiated. Coreflooding experiments with preserved (frozen) core samples were used to evaluate the characteristics of permeability damage during water injection. The influences of brine composition, temperature, and flow velocity on permeability damage were evaluated in cores from two mineralogically distinct zones. Experiments were designed to distinguish between likely damage mechanisms such as fines migration and clay swelling and dispersion. Brine and mineral analyses were used to understand the primary damage mechanisms. Background Permeability damage resulting from the interaction of injected fluids and reservoir rock usually involves the movement of fine particles. The specific mechanism leading to fines mobilization, however, depends strongly on the recovery process under consideration. Temperature, injection fluid composition, injection rate, pH, and roc mineralogy are among the variables that affect the nature and extent of rock/fluid interactions. There are actually numerous mechanisms by which rock/fluid interactions can lead to permeability damage. For convenience, we can group these damage mechanisms into three general categories.Mechanical fines migration. Permeability loss by mechanical fines migration occurs when loosely attached particles are mobilized because of drag forces exerted by the flowing fluid(s). If these particles are mobilized in sufficient quantities, they can collect at pore throats and reduce the permeability of the formation. Mechanical-fines-migration damage may be caused by the movement of man different types of fines including clay minerals, quartz, amorphous silica, feldspars, mica, and carbonates.Brine sensitivity. Some clay minerals are very sensitive to the composition and ionic strength of the surrounding water. For example, when contacted with fresh water, smectite will swell to many times its normal size and kaolinite will disperse. A loss in permeability can occur because the swollen clay occupies more of the pore space, but more often occurs because of fines released by the swelling. Fresh water is not the only cause of brine sensitivity problems. For instance, it has been demonstrated that fines can be released when the salinity of the injected water is abruptly increased.Geochemical transformations. Dissolution, precipitation, and chemical reaction can occur when foreign fluids are injected into a reservoir. For example, mobile fines can be liberated when carbonate cements dissolve in C02 injection processes. Further problems can occur if the dissolved carbonates precipitate near producers as C02 leaves solution because of wellbore drawdown. Another example of this type of formation alteration occurs in high-temperature processes where problem minerals, such as smectitecan form by chemical reaction. Mechanical fines migration is frequently caused by a chemical or brine sensitivity effect. For example, a change in the pH of an injection brine may alter the surface chemistry of clay fines (chemical effect), leading to a fines migration problem (mechanical effect). A review of the literature indicates that a great deal of attention has been focused on brine sensitivity problems. A significant portion of this work has dealt with clay stabilizers and other chemical treatments aimed at preventing freshwater damage in high-clay- content reservoirs. Comparatively little work about mechanical fines migration and geochemical transformations can be found. A brief review of the literature pertaining to each of these areas is presented below. Mechanical Fines Migration. Research in this area is described in Refs. 1 and 5 through 8. Mueckel used a micromodel to study the fundamental nature of fine particle movement in porous media. The micromodel consisted of a monolayer of 200-/Am glass chips sintered between two flat glass plates. The movement of calcium carbonate particles (2- to 15-Am diameter) through the porous media was followed with an optical microscope. Muecke's work resulted in a much better understanding of fine particle movement through porous media. He concluded that during single-phase flow the fines are wet by that phase and move along with the flowing fluid. SPERE P. 1279^

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