A Novel Experimental Approach for Dynamic Quantification of Liquid Saturation and Capillary Pressure in Shale

Abstract

Abstract Determination of dynamic fluid intrusion in tight oil reservoirs requires a very careful and precise estimation of changes in fluid volumes during spontaneous and forced imbibition. Because of the extremely low permeability of these mudrocks, only very small volumes of fluid can flow through the matrix. To overcome this limitation, the pressure decay method commonly used to estimate permeability and porosity in tight rocks is adapted to determine liquid saturation in tight rocks using a slightly compressible fluid as the confining medium. This adaptation uses the transient pressure drop within the bulk liquid surrounding the rock during confinement, to dynamically estimate the volume of fluid intrusion within the tight rock. Pressure decay data for shale samples from the Eagle Ford, Utica and Bakken formations, each of varying particle diameter were obtained experimentally from batch tests. The total pressure drop observed for each shale type was used to measure oil saturations and a pseudo capillary pressure curve was derived from the pressure and saturation data. These measurements were confirmed with NMR measurements made on the same samples (before and after oil saturation). The results show that shale formations with larger permeability tend to absorb more incremental oil during forced imbibition than that which it absorbs during spontaneous imbibition. Furthermore, it was observed that the duration of oil intrusion at lower initial pressures during forced imbibition was longer than the time interval observed for oil intrusion at higher initial pressures for the same particle size. Also, in relation to particle size, it was observed that a significant amount of oil intrusion occurred at lower pressures for large shale crushed samples during forced imbibition, in contrast to smaller oil intrusion observed for small sized particles at lower pressures. The reason for this is because larger sized samples possess micro cracks which serve as extra channels through which oil can penetrate.