Effect of permeability and its horizontal anisotropy on enhanced coalbed methane recovery with CO2 storage: quantitative evaluation based on staged CH4 output inhibition

Abstract

The permeability and its horizontal anisotropy induce a critical influence on staged CH₄ output inhibition process. However, a quantitative evaluation of this influence has been rarely reported in the literature. In this work, the impact of horizontal anisotropic permeability on CO₂-ECBM was numerically investigated. The variation in the staged CH₄ output inhibition was analyzed. The ideal displacement profile of the CO₂-ECBM process was established for the first time. Moreover, the variation in CH₄ output of different wellbores was discussed. The results showed that 1) low-permeable or weak-anisotropic reservoirs were not conducive to enhanced CH₄ recovery owing to long inhibition time (> 1091 days) and high inhibition level (> 36.9%). As permeability and anisotropy increased, due to the accelerated seepage of free water, the hysteresis time and inhibition time could decrease to as short as 5 days and 87 days, respectively, and the inhibition level could weaken to as low as 5.00%. Additionally, the CH₄ output and CO₂ injection could increase significantly. 2) Nevertheless, high permeability and strong anisotropy easily induced CO₂ breakthrough, resulting in lower CH₄ production, CO₂ injection and CO₂ storage than expected. While maintaining high efficiency of CO₂ storage (> 99%), upregulating CO₂ breakthrough concentration from 10% to 20% might ease the unfavorable trend. 3) Along the direction of fluid flow, the ideal displacement profile consisted of CO₂ enriched bank, CO₂ and CH₄ mixed bank, CH₄ enriched bank, and water enriched bank, whereas a remarkable gap in the displacement profiles of the dominant and non-dominant seepage directions was observed. 4) The potential of CH₄ output might vary greatly among different wellbores. The producers along the dominant seepage direction held more potential for CH₄ recovery in the short-term, while those along the non-dominant seepage direction avoided becoming invalid only if a long-time injection measure was taken for the injectors. These findings pave the way to understand fluid seepage in real complex reservoirs during CO₂-ECBM and conduct further field projects.