Abstract
Gas driving water can decrease the gas-phase effective permeability at the displacement front and block the coalbed methane (CBM) output during CO2-ECBM. However, for low-permeability coal reservoirs, there remain a lack of systematic and in-depth understandings of the inhibition process and quantification. In this work, based on a numerical model of an exploited low permeable coal reservoir, the impact of the injection rate and time on CO2-ECBM was investigated. The CH4 output inhibition mechanism in poorly permeable reservoirs was revealed, the inhibition process was quantified for the first time, and the influence of injection-drainage elements on the process was examined. Eventually, specific findings were confirmed in a field trial. The results demonstrated that (1) CO2 injected into the exploited reservoir could further enhance the CH4 output. The CH4 output rate significantly increased with increasing injection rate and time. (21 For poorly permeable reservoirs, the inhibition effect became more notable because a water-rich bank seemed to be a barrier, which trapped the injected and displaced gas, and free water contained in such media cannot be drained efficiently in a relatively short period. Conversely, it was the barrier that shielded the efficient CO2 storage. The storage efficiency reached as high as 99.9%. (3) The effect was only manifested at a specific stage, and the inhibition duration (T_I) and inhibition level (Dj) could be considered to characterize the effect. Increasing the injection rate, enhancing the bottom-hole pressure (BHP) drop, and prolonging the injection duration could promote fluid migration (especially fracture water) and further mitigate the effect, with the former two better than the latter one. (4) The hypothesis of homogeneous permeable reservoirs might result in an overestimate of the inhibition duration and inhibition level. A field trial confirmed the existence of the staged inhibition effect, with the inhibition time of three months and the inhibition level of roughly 5%. These findings pave the way to study the complex fluid migration for CO2-ECBM.
NSTL