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Role of Capillary Forces in Determining Microscopic Displacement Efficiency for Oil Recovery by Waterflooding

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Role of Capillary Forces in Determining Microscopic Displacement Efficiency for Oil Recovery by Waterflooding ENHANCED RECOVERY Role of Capillary Forces in Determining Microscopic Displacement Efficiency for Oil Recovery by Waterflooding J. C. Melrose and C. F. Brandner, When displacement is carried out by flooding with Mobil Research and Development Corporation, an immiscible fluid such as water or gas, the surface Dallas, Texas forces (or, more accurately, the capillary forces) are responsible for trapping a large portion of the oil Abstract phase within the interstices or pores of the rock. For typical, homogeneous sandstone rocks, under usual waterflooding conditions, trapping will occur within The factors controlling the distribution of two immiscible fluids, SUJch as oil and wa,ter, within the interstices of a an appreciable fraction of the total number of pores porous solid are reviewed. It is shown that an improved constituting the pore space of the rock. The fraction understanding of these factors forms the basis of much of the pore volume which is involved may range from Tecent work aimed at the development of new methods fOT 10 to 50 per cent. Thus, capillary forces playa major reducing capillary forces and, consequently, eliminating residual oil saturations. role in limiting the recovery efficiency of such dis­ placement processes. Capillary forces will clearly be eliminated if the Introduction drive fluid is preceded by a slug or bank of fluid which is miscible both with the reservoir oil and with the THE PETROLEUM INDUSTRY has for many years devoted displacing phase(4l. However, the costs of the solvent much research effort to the development of new pro­ materials required in processes based on this principle cesses for achieving improved oil recovery efficien­ may be prohibitive. Alternatively, water-soluble sur­ cies. The current status of the most promising of these factants, such as petroleum sulphonates, can be em­ new processes has been reviewed by Elkins(l) and by ployed to eliminate the tendency of the oil phase to ArnoldC'). For the most part, the research leading to remain trapped in the pores of the rock. Details of these processes has developed along lines pointed out several processes based on the use of such surfactants by Muskat(3) in a review published about 20 years ago. have been recently reported<5-9J. In addition to thermal processes, which are used In this paper, the nature of the surface or capillary primarily to overcome adverse crude oil viscosity forces encountered in petroleum reservoirs, together characteristiCs, a variety of processes are designed with the microscopic features of the associated trap­ to eliminate or reduce the so-called "surface forces" ping mechanism, will first be reviewed. Some fea­ within the crude-oil-displacing fluid-reservoir rock tures of a new recovery process based on the use of system. surfactants, i.e., the low-tension waterflooding process(7-9J, will then be described. This discussion will emphasize one of the principal microscopic mechan­ James C. Melrose is a senior research isms on which the process is based. An analysis of this associate with the gxploration and mechanism indicates that ultra-low values of the oil­ Production Research Division of Mobil water interfacial tension are required in order to Research and Development Corpora­ tion in Dallas, Texas. He holds an achieve improved recovery. S.B. degree from Harvard University and a Ph.D. degree from Stanford University in physical chemistry. He joined Mobil in 1947 and has worked Hydrostatics, Capillarity and in the areas of drilling fluid rheology, clay chemistry, reservoir fluid prop­ Residual Saturations erties and the application of surface chemistry to improved oil recovery processes. HYDROSTATIC PRINCIPLES IN MULTIPHASE FLUID SYSTEMS Carl F. Brandner is a research as­ sociate with the Production Research Two or more fluid phases confined within a porous Division of Mobil Research and Devel­ opment Corporation in Dallas, Texas. solid phase of very small average pore size will gen­ He holds a B.S. from Iowa State Uni­ erally be microscopically commingled. The fluid-fluid versity and a Masters degree from interfacial areas associated with fluid distributions University of Minnesota in physics. of this type are large and characterized by high cur­ He joined the Field Research Labora­ tory in 1949 and has worked in the vatures. The precise configurations of the individual rheology of drilling fluids, formation segments of the total fluid-fluid interfacial area in evaluation, core analysis, reservoir such a system are described by two equations derivable description, and the physics and chem- from the fundamental principle of fluid hydro­ istry of fluid flow in porous media, especially as applied to improved oil recovery processes. statics<lOl. For simplicity, in what follows, only the case of two immiscible and relatively incompressible 54 The Journal of Canadian Petroleum fluids, e.g., oil and water, will be explicitly discussed. The extension to cases involving three fluids presents / / / / / / / / // / '// / some complexities(ll), but, in principle, can be carried ///// /SOLI D PHASE / ~ /// out without recourse to any additional fundamental / / ~ / / // / / // / / / / considerations. ~ The first of the two hydrostatic equations is the Yos Laplace equation; Yow WATER OIL = Yow Po -P w Jow. ........................... .. (1) PHASE PHASE Here, P denotes pressure in a fluid phase, ')I the in­ Pw Po terfacial tension and J the curvature of the fluid­ radius of fluid interface (c.f. Fig. 1). The subscripts 0 and w cu rvatu re refer to oil and water, and the double subscript ow refers to the corresponding interface. The thermo­ dynamic derivation of Equation (1) provides a con­ text within which the effects of temperature, pressure and interfacial curvature on the interfacial tension can be interrelatedOo,121, These considerations also apply, of course, to interfaces for which one of the FIGURE 1 - Hydrostatic equilibrium two fluid phases in contact with a solid phase. contiguous fluids is a gas (denoted by the subscript g). In this case, the quantities ')IgO and ')Igw are referred to as surface tensions. within porous solids presents difficulties not readily The second hydrostatic equation is the Young apparent from the usual textbook treatments of the equation, problem. Because the average pore size is assumed to be small, the pressures in the two fluid phases may Yos = Yws + Yow cos 60ws · .. .. ..... (2) be taken as constant over distances comparable to the Here, (Jows represents the angle of contact, as measured dimensions of an average pore. Also, the interfacial through the water phase, which the oil-water interface tension may be taken as independent of the magnitude OO (fluid meniscus) makes with the solid surface, denoted of the curvature ). According to Equation (1), then, by the subscript s (c.f. Fig. 1). The contact angle, the interfacial configurations will correspond to sur­ hereafter denoted simply as 0, is clearly a property faces of constant curvature. Even under this simplifi­ characterizing the three-phase line of contact. Meas­ ~ation, the algebraic expression for the curvature, J, urements of this property typically display hysteresis IS not a simple one. The expression includes terms effects(12). It should also be pointed out that a more which involve the various first and second partial rigorous version of Equation (2) will include a term derivatives of the function representing the shape of accounting for the line-tension in the three-phase line the interface. In fact, the expression corresponds to of contact°l). This "line property" is the analogue of a second-order non-linear differential equation for the interfacial tension for surfaces. which general solutions in terms of known functions are not available. The problem is generally circum­ The contact angle provides the only direct and un­ vented by the use of highly simplified geometrical ambiguous specification of the so-called "wettability" models to represent the shape of the actual pores property characteristic of a given oil-water-reservoir involved. The necessary boundary conditions are then rock system. If the angle (J is small, say less than 40 provided by the assumed configuration of the pore degrees, the system is usually said to be water-wet. walls and by the contact angle, 0, defined by Equation Similarly, an oil-wet system is characterized by an (2) . angle 0 greater than 140 degrees. For angles of inter­ mediate magnitude the wettability is best described as For example, in the case of a porous solid formed intermediate. by a random packing of uniform spherical particles, the pore openings may be approximated by the inner The measurement of the interfacial properties de­ surface of a torus(20, 21). In such a model, the radius of fined by Equations (1) and (2) under the tempera­ the torus ring corresponds to the radius of the ture and pressure conditions typical of petroleum re­ particles forming the packing. The equatorial distance servoirs presents a number of difficulties. Even for from the axis of symmetry to the ring surface is systems of relatively simple composition, such data then adjusted to provide pore openings of the ap­ are only gradually being reported in the literature. propriate sizes. Recent studies of this type which should be noted are those of Jennings(13, 14) and McCafferyOS-l7). An earlier A further problem arises in connection with the (8 stability of the fluid-fluid interfacial configuration study by Hocott ) reported surface and interfacial tension data for several field systems. Measurements which results from the solution of the differential of contact angles for crude oil - reservoir water sys­ equation represented by Equation (1). For typical tems on flat, polished surfaces of quartz and calcite pore models, such as packings of spherical particles have heen carried out by Treiber, Archer and of uniform size, it is easily shown(22,23) that a limited (9 range of such configurations are stable (c.f. Fig. 2). Owens ).
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