The determination of surface relaxivity and application to coal spontaneous imbibition

Abstract

Experimental researches on spontaneous imbibition are of great significance for improving the recovery rate of coalbed methane and preventing gas disasters. The determination of microstructure is a prerequisite for studying pore seepage behaviors. The transverse surface relaxivity must be known first before obtaining the pore size distribution (PSD) by nuclear magnetic resonance (NMR). In this paper, a method for determining the transverse surface relaxivity of low-permeability coal was improved by correlating the NMR cumulative PSD curves and the corrected mercury intrusion porosimetry (MIP) capillary pressure curves. The MIP data was calibrated for the matrix compression effect in the high-pressure part only by MIP experiment and for the surface cavity effect in the low-pressure part. Then the transverse surface relaxivity obtained was applied to study the time and space evolution behaviors of water imbibition volume and water content in pores of different scales during spontaneous imbibition. The results indicate that the pore volume only accounts for 31.64%-66.47% of the measured mercury volume. Besides, in the rapid rising stage and the prior period of the slow rising stage of spontaneous imbibition, micropores dominate water imbibition and are the first to reach a water-saturated state. As time increases, the contributions of mesopores and macropores to water imbibition become significant gradually. After water enters coal samples, a small part of water advances along the connected path with low capillary resistance and reaches the outlet first, forming the dominant channel. However, most of the water diffuses laterally to the same horizontal plane. When the pores in this horizontal plane approach a saturated state, water migrates upward.