Coreflooding with sub-core scale characterization for analysis of water encroachment in a gas reservoir

Abstract

Understanding gas-brine two-phase flow and residual gas is essential for evaluating and optimizing reservoir gas recovery and storage. Coreflooding experiments and simulations offer insight into multiphase flow behavior, yet most studies focus on core-scale characterization, often overlooking smaller, sub-core scale phenomenon. This work investigates drainage and imbibition gas-brine flow and residual gas trapping in a core sample, focusing on the sub-core effects that influence fluid distribution. We conduct vertical coreflooding experiments on a sample from a gas reservoir, with the application of water encroachment in mind. Experimental and simulation results are presented considering multiple scales: voxel, slice, and core, using X-ray computed tomography (CT) to reveal the distribution of porosity and steady-state gas saturation inside the rock sample. It is shown that residual gas saturation is controlled by sub-core scale capillary heterogeneity, even within a core that appears homogeneous. Capillary pressure is immaterial in homogeneous models but becomes significantly impactful at the sub-core scale, where heterogeneities influence fluid behavior. A one-dimensional heterogeneous model incorporating spatial relationship between capillary pressure and permeability is used to estimate permeability distribution and found to be able to capture some of the gas variations in the core, which indicates that the presence of capillary heterogeneity effects are responsible for the sub-core saturation variations.