Microscopic Distribution of Wetting and Nonwetting Phases in Sandstones During Immiscible Displacements

Abstract

Summary. A novel technique has been developed and used to study the microscopic distribution of wetting and nonwetting phases in reservoir rocks during immiscible displacements. The underlying principle is the us of appropriate fluids, serving as the wetting and the nonwetting phases, that can be solidified in situ, one at a time, without altering to any significant extent the position and orientation of the phases acquired at capillary equilibrium conditions. After both phases are solidified, the rock matrix is etched and replaced by a resin to enhance the quality of polishing. Polished sections are then serially made at each 5- to 10-mu m depth and photomicrographed, wherein the three phases are distinguishable from each other. Experimental results have been obtained with Berea sandstone and the phase distribution is being analyzed, Relative permeabilities measured with the other phase solidified compare very well with conventional results obtained by the Penn State steady-state method, using Soltrol- and brine. Wettabilities of the fluid pairs were also visualized directly by this new technique. Introduction To model multiphase flow and to predict ultimate oil recoveries from reservoir formations more realistically, it is essential to study flow behavior and fluid distributions in the pore matrix on the pore level at various relative saturations. The microstructure of reservoir rocks strongly influences the transport of mass, momentum, and energy in one pore space. Both the pore geometry and pore topology dictate the motion and distribution of fluids within porous media. In addition, such factors asfluid/ fluid properties-interfacial tension (IFT), viscosity ratio, density difference, phase behavior, and interfacial mass transfer,fluid/solid properties-wettability, ion exchange, adsorption, and interaction; andmagnitude of applied pressure gradient, gravity, and agings play roles in entrapment, distribution, and mobilization of oil in petroleum reservoirs. Some interesting literature on the trapping of nonwetting phase and on blob mobilization includes data on sizes, shapes, populations, and distributions. Very little has been said however, about the location and distribution of the wetting phase at or near the so-called irreducible saturation, despite the fact that many reservoirs are known to be oil-wet and that "irreducible saturation" has been shown to be a misnomer. Recently, Dullien et al. observed that the irreducible wetting-phase saturation can be reduced by a so-called leakage mechanism if high capillary pressures are maintained, owing to the hydraulic continuity of the wetting phase in pore wedges and microgrooves in the pore walls. Irreducible wetting-phase saturations depend on specific surface area, surface texture, small scale heterogeneity in microstructure, and pore size and shape distributions. Wardlaw and Cassan generalized the recovery efficiencies in terms of pore connectivities and pore sizes. To date, experimental findings correlating macroscopic flow phenomena to pore-space morphology are sketchy and principally qualitative. Although considerable efforts have been made to incorporate pore-level phenomena and pore structure data into the network models of pore space for predictions of macroscopic flow behavior, there has been a dearth of experimentally determined quantitative information on fluid distributions and pore topology for testing the validity of the theoretical models. Such parameters as pore-body and pore-throat size distributions, and pore and individual fluid-phase coordination numbers as a function of saturation, which are incorporated in the mathematical models, have not been determined experimentally. This lacuna can be attributed partly to the lack of good experimental techniques required for three-dimensional (3D) visualization of microstructures in natural porous media while the original pore and phase geometry are maintained. SPERE P. 137^