A STUDY OF THE EFFECT OF PORE STRUCTURE ON CAPILLARY TRAPPING IN GEOLOGIC POROUS MEDIA

With a global shift in energy trends towards greener and more sustainable fuels, a rapid change is required in oil and gas sector to adopt practices such as carbon capture and storage, and enhanced oil recovery. These practices mainly aim at increasing the efficiency of oil and gas production while at the same time capturing CO2 from anthropogenic sources and trapping it in geological reservoirs. Mechanisms included in these processes are imbibition and capillary trapping that can mobilize or immobilize the desired non-wetting phase. Immobilization of a non-wetting phase by the wetting phase in porous media is called capillary trapping. Capillary trapping is dependent on wettability and pore geometry. Therefore, an analysis of effects of pore geometry and structure on the extent of capillary trapping is required. We present an analysis on the effect of pore structure on capillary trapping on four rocks: two sandstones (Berea and Clashach) and two carbonates (Estaillades and Ketton) with wide span of permeability and pore size distribution under strongly water-wet conditions. We first characterized the rock samples in terms of pore size distribution from the mercury injection capillary pressure (MICP) data. Then we analyzed network properties such as coordination number and aspect ratio from micro-computerized tomography (micro-CT) for all the scanned images obtained. Afterwards, we compared the network properties from the Pore Scale Model (PSM) with the MICP to guarantee consistency with the results. Published capillary trapping data in terms of coordination number, Z and aspect ratio, α were also analyzed. The degree of capillary trapping is determined by the parameter normalized non-wetting phase residual saturation, ς. We found out that as coordination number increases capillary trapping decreases and as aspect ratio increases, capillary trapping increases. Moreover, empirical relationships are presented that correlate rock physical properties such as porosity, ϕ and dynamic flow parameter, k/ϕ ; and pore geometry parameters, such as α and Z to the non-wetting phase residual saturation, ς. In order to determine the trapping mechanism for a given rock at a certain applied capillary pressure, a modified equation is also presented. This work contains a comprehensive set of results along with an in-depth analysis of capillary trapping in both sandstones and carbonates