Capillary Pressure Discontinuities and End Effects in Homogeneous Composite Cores: Effect of Flow Rate and Wettability

Abstract

Abstract The optimum construction methodology of composite cores for multiphase flow tests is an important practical petrophysical concern. The critical practical petrophysical concern. The critical feature of construction is the minimization of saturation disturbances which occur due to capillary pressure discontinuities at individual core segment pressure discontinuities at individual core segment interfaces. The effect of matrix discontinuities on two-phase flow in homogeneous composite cores was studied as a function of flow rate and wettability. Five composite cores, representing a range of wettabilities, were prepared. The composite cores were constructed by splicing multiple core segments with bridging materials of differing wettabilities. Saturation profile data were measured with a microwave saturation scanner during imbibition and drainage floods. The interface between core segments had a strong effect on the saturation distributions when capillary contact was not maintained between contiguous segments. Under drainage conditions at low flow rates, each core segment behaved as though it were an independent core experiencing a strong exit end effect. In steady-state fractional flow experiments this occurred only at fractional flows close to unity. The most effective bridging materials for water-wet Berea were thin paper sheets. No bridging material was found to be completely adequate in an oil-wet environment. In the absence of good capillary contact, increasing the flow rate is a practical remedy for the saturation disturbances. practical remedy for the saturation disturbances Introduction Petrophysical flow measurements made on reservoir Petrophysical flow measurements made on reservoir core should be as accurate and as representative of the reservoir as possible. These requirements are nearly always compromised by the bedding planes being along the small dimension of the whole core so that only short cores are available for study. Short cores result in poor material balance accuracy, more susceptibility of results to infrequent flaws in the core, and a greater influence of unwanted end effects. To avoid these problems, composite cores are constructed by problems, composite cores are constructed by stacking end-to-end several short core plugs drilled parallel to the bedding plane. parallel to the bedding plane. The proper construction of composite cores requires care in both the selection of the individual core plug segments and in the manner in which the are plug segments and in the manner in which the are put together. The assembly procedure influences the put together. The assembly procedure influences the subsequent maintenance of capillary pressure contact between segments during two-phase flow experiments. A breakdown in capillary pressure continuity, even between well matched segments, is known to cause an undesirable saturation profile disruption. This paper presents a study of the techniques necessary paper presents a study of the techniques necessary to minimize capillary pressure discontinuities and/or alleviate the resulting saturation disruption. Previous Work Previous Work There are few published studies on the practical problem of maintaining capillary contact between problem of maintaining capillary contact between segments of a composite core. Osoba and coworkers made some observations in this area while studying the merits and deficiencies of the Penn State relative permeability method. They found that when capillary contact was lacking between the test section and the end effect absorbing sections of the apparatus, oil relative permeabilities showed a pressure (flow rate) dependence. Loosely pressure (flow rate) dependence. Loosely consolidated samples were noted to be especially susceptible. Limestone cores with tissue inserted between the segments were found, in contrast, not to show this dependence. Richardson also reported the use of tissue paper (2 layers) as a bridging material in a composite core. In addition, this group carefully machined the segment faces.