Long-term Redistribution of Residually Trapped Gas due to Diffusion

Abstract:

Residually trapped gas can be found in numerous geological systems such as oil and gas reservoirs and geological CO2 storage sites. In such a geological system where the gas density is smaller than that of reservoir brine, a continuous gas plume migrates upward toward the reservoir top driven by buoyancy forces. At the bottom of the plume, gas ganglia are disconnected from the plume and are residually trapped in pore spaces by capillary forces. For example, in a geological CO2 storage reservoir where residually trapped CO2 typically occupies 10-30% of pore spaces, aggregation of residually trapped CO2 may potentially cause CO2 to remobilize and thus increase the risk of CO2 leakage. However, the long-term fate of residually trapped gas remains unverified so far due to lack of field observations. Previous studies reveal that capillary heterogeneity can create diffusive fluxes in the aqueous phase to redistribute the gas ganglia, despite them remaining trapped in pore spaces. In this work, we develop a novel and comprehensive theory to evaluate the redistribution of gas ganglia by taking spatial variation of reservoir conditions into consideration, e.g., pressure and temperature. It is shown that this spatial variation can induce inter-ganglion diffusion to redistribute gas ganglia in addition to capillary heterogeneity. Based on the mathematical model derived from this theory, we develop a continuum-scale model to demonstrate the evolution of gas saturation and the time scale for the system to approach the steady state. It is demonstrated that the time scale of gas redistribution can be approximately tens of thousands of years, which is roughly at the same order of other trapping mechanisms such as mineral trapping. This study provides a novel perspective on evaluating the long-term stability of residually trapped gas and thus can be of great relevance to long-term gas migration in natural geological systems.