Abstract

Quantitative characterization of pore structure in shale can provide basic parameters for evaluation of the shale-gas reservoir quality. However, it is difficult to use conventional methods to accurately and comprehensively characterize the pore structure parameters. We take shale samples from the Longmaxi Formation in the Sichuan Basin as the study object, and we use the high-pressure mercury intrusion, nitrogen adsorption, and carbon dioxide adsorption methods to characterize the whole aperture distribution. We found that the pore size in shale is positively related to the transverse relaxation time (T2 value) and there exists a conversion coefficient. We have developed a new method combining nuclear magnetic resonance (NMR) with hybrid detection methods for testing the pore size distribution, and we optimized the conversion coefficient between pore size obtained by a hybrid detection method and the T2 value. NMR can then characterize the pore size distribution by conversion coefficient. This method can effectively make up for the deficiency of conventional methods for pore size distribution characterization by a single method. Our results indicate that the macropore, mesopore, and micropore in shale are very developed, and the pore shapes are ink bottle and slit-like. Shale pores mainly consist of mesopore and micropore, contributing to approximately 74.33% of pore volume, whereas micropore contributes approximately 70.18% of specific surface area (SSA). Therefore, the macropore has a limited effect on the pore volume and SSA. In addition, the establishment of whole aperture distribution characterization by the new method can more comprehensively reflect the actual pore distribution in shale.

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