Regulatory mechanism of microscopic pore structure on anisotropy of gas multimodal seepage in original coals

Abstract

Geological constraint of massive gas production is the realistic technical predicament faced by coalbed methane (CBM) recovery worldwide. The primary impact factor that induces geological adaptation obstacles of CBM seepage is coal pore structure. This research investigates the regulatory effect of coal microscopic pore structure on gas anisotropic seepage via photoelectric radiation technique, fluid penetration method, digital reconstruction technique and hydromechanic simulation. Emphasis was placed on revealing the competitive mechanism between space and topology of coal microscopic pores occurs in the regulatory process. Pore morphology and skeleton have consistent dominant orientation, serving as the spatial foundation for the anisotropic seepage. Pore space is more anisotropic in high metamorphic coals than in low metamorphic coals, while pore topology follows the opposite tendency. Anisotropic discrepancies in pore structures trigger a competitive mechanism: in high metamorphic coals, anisotropy of gas seepage is mainly dominated by pore topology, but by pore space in low metamorphic coals. Competitive mechanism caused by the disadvantaged pore space severely restricts the directional selection of gas seepage, resulting in enhanced anisotropy. Accordingly, pore closure may dramatically impair CBM production efficiency in severe geological circumstances. Research results are crucial for technically facilitating the CBM recovery enhancement under severe geological environments.