Pore-scale simulations of gas storage in tight sandstone reservoirs for a sequence of increasing injection pressure based on micro-CT

Abstract

Gas storage and movement in porous media can give valuable insight into the gas accumulation at the macroscopic. Until now, there are few research reports on how the injection pressure affects the gas storage in tight sandstone reservoir at microscopic scale. We choose a Bashijiqike tight sandstone core to conduct a series of pore-scale simulations of gas storage for a sequence of increasing injection pressure, and with the aid of micro-CT techniques, distributions of fluid phases, characteristics of gas clusters, and change laws of gas storage have been studied. Results show that after gas injection pressure improved, there is an obvious rise in the gas saturation, at the same time, for gas clusters, the quantity and volume are growing, the shapes deviate a significant degree from sphere, and the connectivities are getting better. Three different occupation patterns of gas clusters have been identified-new birth, augment and connection, which can occur separately or together in the process of gas displacing aqueous. In the rapid injection process (RIP) gas clusters are more likely to be discontinuous droplets trapped by snap-off, while in the slow injection process (SIP) gas clusters tend to be continuous phase. The shapes and connectivities of small and large gas clusters are controlled by the pore structure characteristics of small and large pores, respectively, which have different fractals. Even the proportion of small gas clusters is overwhelming, the acquisition of gas saturation is attributed to the large gas clusters with a small quantity. Again, because the volume of gas cluster is limited to pore volume, so the gas storage capacity of tight sandstone reservoir is depending on the large connected-pore, linked by passable throat (reflection of injection pressure), but it has nothing to do with the dispersed small pores. These results have important implications for further effective exploitation of tight sandstone reservoirs.