Experimental study: Effect of pore geometry and structural heterogeneity on the repetitive two-phase fluid flow in porous media and its implications to PM-CAES

Abstract

Compressed air energy storage in porous media (PM-CAES) has recently been suggested as a promising alternative to existing CAES plants. PM-CAES incurs repetitive two-phase fluid flow caused by the cyclic injection and withdrawal of air to/from the porous medium that is initially saturated with the formation water during the operation. Therefore, predicting the overall macro-scale performance of porous media for energy storage requires a better understanding of repetitive two-phase fluid flow in the pore network at the fundamental pore-scale level. To answer this need, we conducted an experimental study using the microfluidics technology; we constructed polydimethylsiloxane (PDMS)-based micromodels with two different geometries (Type I: circular solids and Type II: square solids) and three different structural heterogeneities (coefficient of variation: COV=0, 0.25 and 0.5). Then, we applied a total of ten injection-withdrawal cycles to each micromodel (i.e., ten cyclic drainage-imbibition processes) at different flow rate conditions (Q=0.01 and 0.1 ml/min). It was observed that the displacement patterns of the initially residing fluid (wetting fluid; oil in this study) and the injected fluid (non-wetting fluid; water in this study) were greatly influenced by the geometry and heterogeneity of the pore structure, and imposed flow rate. Results such as the effective sweep efficiency and residual saturation of the non-wetting fluid were analyzed at each drainage-imbibition cycle to aid in understanding the impact of repetitive fluid flow. The experimental observations imply that the flow rate and structural heterogeneity may influence the efficiency of PM-CAES more than the pore geometry does.