Characterization of pore structure and dynamic seepage characteristics of sedimentary rocks determined by nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques

Understanding the pore structure and dynamic seepage features of fine-grained sedimentary rocks is necessary for the secure and efficient exploitation of coalbed methane resources. This research assessed the pore structure and dynamic seepage features of the sandstone and shale samples were investigated through innovative, dynamic nuclear magnetic resonance (NMR) T2 spectrum and magnetic resonance imaging (MRI) measurements during a series of specially designed seepage experiments with two respective fluids of different wettability (i.e., distilled water and kerosene). The seepage T2 spectra and MRI images at various times during the seepage process are discussed. Results show that sandstone and shale possess remarkably different T2 spectra. Sandstone samples' T2 spectra reveal a unique peak distribution with a range of 0.3-1000ms. The T2 spectra of shale samples exhibit a bimodal distribution, with most micropores spanning between 0.1 and 5 ms. Sandstone possesses significantly greater pore connectivity and fluid mobility than shale. The dynamic seepage experiments showed that the seepage T2 spectra of distilled water and kerosene seepage in sandstone present a bimodal distribution, and those of kerosene seepage in shale show a trimodal distribution. Meanwhile, the relationship between seepage time is sandstone kerosene seepage<sandstone water seepage <shale kerosene seepage, and the relationship between fluid volume is shale kerosene seepage <sandstone kerosene seepage <shale kerosene seepage. In addition, the fluid volumes of water and kerosene in sandstone have a strong linear relationship with time, while those of kerosene in shale have a power-function relationship.