Experimental Investigation of Pore Characteristics and Permeability in Coal-Measure Sandstones in Jixi Basin, China

Abstract

The research of pore and permeability characteristics of tight sandstone reservoirs in coal-measure is critical for coal-measure gas development. In this study, the pore systems of tight sandstones were studied based on low-field nuclear magnetic resonance (LF-NMR) data. The permeability of tight sandstones was obtained by the tester based on the pulse transient method. The permeability variation with the effective stress, grains, and pore characteristics was analyzed. The results show that the tight sandstone reservoirs in the coal-measure have low total porosity (2.80–4.14%), low effective porosity (0.51–1.56%), and low permeability (0.351 × 10−6–13.910 × 10−6 um2). LF-NMR T2 spectra of the testing sandstones show that the micropores are the most developed, but most of the micropores are immovable pores. The pore characteristics are significantly affected by the grain size of sandstones. The pore connectivity ranks from good to poor with decreasing sandstone particle size. The total porosity and effective porosity increase with the grain size. There is a near-linear negative relationship between permeability and effective stress when the effective stress is between 405 psi and 808 psi. The greater the number of movable pores and the larger the effective porosity, the bigger the permeability of the sandstone. The effective porosity of sandstones is a sensitive indicator for evaluating the permeability of tight sandstone reservoirs. The stress sensitivity coefficient of permeability (Ss) increases with the increase of the effective stress. The sandstone with lower permeability, smaller effective porosity, and finer grains has a higher Ss. The particle size of sandstone from coal-measure has a great influence on both permeability and Ss. The findings will provide a better understanding of the characterization of pore structure and permeability in the process the coal-measure gas extraction, which is useful for the efficient development of coal-measure gas.