The economic performance of a waterflood or a water disposal project can be significantly affected by suspended solids in the injection water. Here are methods and a theory that can be used to interpret water quality data obtained with membrane filters or cores and to predict well impairment caused by suspended solids. predict well impairment caused by suspended solids. Introduction In a waterflood or a water disposal project the possibility exists that suspended solids will cause the possibility exists that suspended solids will cause the injection wells to become impaired. Filtration can usually reduce the concentration of suspended solids; however, the cost of water treating should be balanced against the cost of other alternatives, such as periodic stimulation or replacement of injection wells. In some cases extensive water treating can be justified, but under other circumstances it will be more profitable to inject untreated water. Water quality is affected by several types of contaminants, including suspended silts, clays, scale, oil and bacteria. Any of these may be the predominant source of improvement in a particular injection water and environment. Formation cores, artificial cores. and membrane filters have been used in the industry to monitor suspended solids and to evaluate water quality. Some studies have defined water quality in terms of filtration rates or other experimental data. The disadvantage of these empirical definitions is that they cannot be directly related to well impairment. This paper proposes a measure of water quality that is defined as the ratio of the concentration of suspended solids to the permeability of the filter cake formed by those solids. The water quality ratio can be obtained directly from membrane or core filtration data and can be used to calculate the rate of formation impairment. Formation Impairment from Suspended Solids In considering the effects of suspended solids, some measure of the rate of impairment is needed. A convenient way to estimate how long an injector can be used before stimulation is required is to calculate its half-life. The half-life is defined as the time required for the injection rate to decrease to 50 percent of its initial value. The time required to reach some other fractional reduction in rate can also be calculated. Impairment from suspended solids is thought to occur by one of the following mechanisms (see Fig. 1):The solids form a filter cake on the face of the wellbore (wellbore narrowing);The solids invade the formation, bridge, and form an internal filter cake (invasion);The solids become lodged in the perforations (perforation plugging); andThe solids settle to the bottom of the well by gravity and decrease the net zone height (wellbore fillup). Each of the four basic impairment mechanisms is modeled in Appendix A for a constant-pressure-drop process, Equations are derived that express the time process, Equations are derived that express the time required for the injection rate to decline to some fraction a of its initial value. For each mechanism, this time can be expressed as the product of the two function, F and G. P. 865
Water injection operations associated with oil-field, geothermal, and disposal processes may be subject to formation damage by particulates. Over long periods of time it is unlikely that sufficient water treatment can be obtained to prevent injectivity declines. Selecting water sources and alternatives to water treatment, such as well stimulation or replacement, requires an assessment of the rate and location of formation damage. AS part of this evaluation, core tests and analysis of solids movement in porous media were used to study the mechanism of particle invasion of formations. Introduction The flow of particulates in a porous media is a complex process, which in an oil reservoir is further complicated by the presence of multiple fluid phases (oil, water, and/or gas). The final disposition of injected solids is related to both geometric and fluid dynamic factors. This paper is concerned with assessing the effects of pore velocity, particle size, pore size, and the method of core particle size, pore size, and the method of core testing (constant rate or pressure drop) on solids deposition in pores. Doscher and Barkman and Davidson discussed the use of membrane filtration tests to evaluate formation damage. Although Barkman and Davidson proposed a method of quantifying rates of impairment, the analysis did not project how far particles might be expected to penetrate into the formation. Examples of particulate invasion have been reported for both laboratory and field tests. Champlin demonstrated that the ionic composition of the liquid alters the electrostatic particle-sand matrix interaction, which in turn affects the penetrating distance of submicron particles into a penetrating distance of submicron particles into a porous media. Abrams used a radial sand model to porous media. Abrams used a radial sand model to simulate a 4-to-1 reduction in wellbore velocity and showed that at injection rates approximating 7 B/D/ft of interval, particles ranging in size from 1/2 to 12 microns (median 3 mircrons) could penetrate 40-micron pores up to 3 in. from the wellbore and reduce the pores up to 3 in. from the wellbore and reduce the local permeability by a factor of 10-to-1. Subsequent backwashing did not raise permeability significantly. Ogletree discussed economic alternatives associated with injection of waters of different quality and also presented field data that indicated that particulate invasion had led to formation damage at particulate invasion had led to formation damage at significant distances from the wellbore. Mitchell, evaluating water injection into the Forties reservoir, noted core and millipore filter tests correlated. McCune discussed the use of mobile field core-testing facility to evaluate all aspects of water quality on permeability damage. Additional work is required to expand upon the data obtained from membrane filtration and core tests to quantify depths of invasion and rates of formation impairment by particulates. Testing principles and procedures must account for the principles and procedures must account for the large flow variations that exist between cores and the field, and this requires the impact of pore velocity on impairment. Core tests can be designed to simulate the dynamics of impairment at specific locations in the formation, by changing flow rates, but scale-up procedure must be developed to project overall formation damage. DISCUSSION OF INVASION MECHANISM Particles that are smaller than rock pore sizes will penetrate into the interior of the rock. The rate of impairment and depth of penetration are determined by the forces acting on the particle as it moves through the rock. Surface and gravitational forces cause particles to be separated from the fluid and deposit in pores, thus increasing the resistance to flow. The deposition process can be retarded by a force generated by the pore velocity. Two possibilities exist with respect to the final change in pore dimensions (see Figure 1).An equilibrium state is reached with no net deposition.
This paper presents a review of materials considerations associated with in situ mining processes for copper and uranium. Because the lixiviants employed are corrosive to ordinary oilfield equipment, special materials of construction for both surface and subsurface hardware are required. Surface equipment may be subjected to either low or high pressures. Low pressure equipment may be constructed of polyvinyl pressure equipment may be constructed of polyvinyl chloride or chlorinated polyvinyl chloride and other materials. High pressure will require the use of equipment constructed of stainless steels, fiberglass reinforced plastics (FRP), etc. Down-hole equipment presents similar problems. Depending upon depth of the hole plastic, FRP, or combination carbon steel/stainless steel tubulars are required. Cements for corrosive environments are available and are reviewed. Other down-hole hardware such as packers, tailpipes, seating nipples, liners, etc., must be fabricated from suitable materials. Equipment for use in production wells will be similar to that used in injection wells. In addition, materials of construction for lift systems are discussed. Various types of materials are reviewed including steels, plastics, elastomers, coatings, liners, and lubricants. Some suggestions for testing these materials in both the laboratory and the field are presented. presented. Special problems arise because oxidants are often incorporated into the leach solution. The oxidants may be hydrogen peroxide, sodium chlorate, oxygen or others. The concern for handling fluids containing oxidants is indicated. Introduction In-situ mining and in-situ leaching are synonymous processes employed to extract mineral values from the earth without removal of the ore body. These processes are, in fact, a combination of petroleum processes are, in fact, a combination of petroleum technology, ground water hydrology, corrosion chemistry, and hydrometallurgical engineering. The operational aspects of the process involve injection of a suitable fluid into the ore deposit in order to dissolve the valuable mineral and lifting the mineral laden fluid to the surface for extraction. The petroleum engineer is already familiar with most of petroleum engineer is already familiar with most of these activities, since they involve drilling and completion of wells and evaluation of the fluid flow characteristics of the ore deposits. In short, the major part of in-situ mining depends on the design of well fields, injection, and lift equipment. These activities are not unlike those used in waterflooding. A major element that makes in-situ mining distinctly different from waterflooding is that highly corrosive fluids, often called lixiviants, are injected. These lixiviants are chosen to dissolve portions of the ore body. Any lixiviant capable of reacting with and dissolving copper or uranium minerals usually is very corrosive to conventional oilfield hardware. This suggests that the in-situ mining engineer must understand not only borehole technology and flow of fluids through porous media, but also hydrometallurgy. In addition, because corrosive fluids are employed, the engineer must select appropriate materials for constructing the system and fabricating equipment. The system can be divided as follows:Processing Plant A. Lixiviant Preparation B. Metal ExtractionInjection FacilitiesWell FieldProduction Facilities This paper will address only the latter three elements of the system. CHEMICAL NATURE OF FLUIDS Aqueous fluids are employed in the leaching process for recovery of both copper and uranium. These process for recovery of both copper and uranium. These fluids may be either acidic or basic and may contain an oxidizing agent. The choice of fluid depends primarily upon the mineralization and the nature of primarily upon the mineralization and the nature of gangue. Oxidants are incorporated in the mineral to be leached must first be oxidized.
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