Microfluidic Study on the Two-Phase Fluid Flow in Porous Media During Repetitive Drainage-Imbibition Cycles and Implications to the CAES Operation

Compressed air energy storage (CAES) technology has been re-emerging as a viable energy storage option to address challenges coming from the mismatch between renewable energy sources and energy demands. Various geologic formations, such as hard rock caverns, depleted oil/gas reservoirs, and saline aquifers, have been considered as the alternative of salt dome caverns for CAES. Storing compressed air in either depleted oil/gas reservoirs or saline aquifers involves two-phase fluid flow in porous media. Moreover, the drainage-imbibition process is likely to be repeated numerous cycles during the CAES operation. In this regard, a thorough understanding of the two-phase fluid flow during the cyclic injection and withdrawal of compressed air is critical to predict the performance of CAES in porous media and to improve its efficiency. This study investigates the repetitive two-phase fluid (water/oil) flow using polydimethylsiloxane-based pore-network micromodels. Two different geometries, Type I with circular solids and Type II with square solids were prepared to represent an unconsolidated and/or partially consolidated sandstone and a fracture network of carbonate rock. During repetitive drainage-imbibition cycles, it was observed that the occupation efficiency of the non-wetting fluid (water) converged to a narrow range for Type I model, while it showed a pronounced fluctuation for Type II, which was partly due to the low residual saturation of the non-wetting fluid during the imbibition process. Besides, the prevalent displacement modes of wetting and non-wetting fluids at the pore-scale were noticeably different between the two pore structures, which were manifested in the unpredictable pattern of non-wetting fluid flow for Type II over the extended cycles. Sweep efficiency and residual saturation in Type I were greater than those in Type II; however, an analysis of effective sweep efficiency and effective residual saturation yielded an opposite result. It implies that the actual efficiency of non-wetting fluid invasion is higher in Type II, and the Type II geometry can accommodate more non-wetting fluid in a given reservoir volume during the charge period. Moreover, more non-wetting fluid can be discharged back for energy regeneration. In conclusion, the geometry of porous media has a great influence on the efficiency of repetitive drainage-imbibition cycles of two-phase fluid flow in porous media, and thus more elaborate study is needed to gain the confidence on the cyclic efficiency of CAES in porous media.