LAUSR

Abstract The movable-water saturation (MWS) of coal is critical for the flow of coalbed methane (CBM), however, there is a relative lack of research on it, especially of different-scale pores. The relative permeability of coal is another crucial parameter for CBM development due to its strong control on the characteristics of both gas and water production, but it also remains relatively unexplored. Naturally, little attention has been paid to the connections between the MWS and the relative permeability. In this study, an innovative experiment combining water-saturated coal core displacement test with low-field nuclear magnetic resonance (LNMR) was designed to obtain the MWSs of different-scale pores and gas-water relative permeabilities of coals simultaneously. The relative permeability results show that low-medium coals from the Southern Junggar Basin (SJB) were characterized by low MWSs, narrow spans of two-phase flow, and low end-point relative permeabilities to water (Krw*) and gas (Krg*), which were all disadvantageous for CBM development. The calculated MWSs of different-scale pores through the amplitude of T2 spectrum show that “macropores and fractures (MF)” have the highest MWS, with mesopores following next, “micropores and transition pores (MT)” possess the lowest MWS. Correlation analyses show that the total MWSs and movable porosities both present strong positive correlations with the Ka, Krw*, and Krg*. The irreducible water causes a greater obstacle to gas flow than to water flow. The MWSs of different-scale pores all exhibit positive impacts on the coal permeabilities. However, compared with MWSs of MT and MF, the MWSs of mesopores exhibit the strongest correlation with Ka, Krw*, and Krg*, indicating the critical role in affecting the fluid flow of coal. Besides, two significant parameters, cleat tortuosity (η) and cleat size distribution index (λ) of SJB coals, were determined. η and λ were rank-dependent. Coal macerals also have impacts on η and λ.