Nano-Pore Structure and Fractal Characteristics of Shale Gas Reservoirs: A Case Study of Longmaxi Formation in Southeastern Chongqing, China

Pore structure and fractal dimensions can characterize the adsorption, desorption and seepage characteristics of shale gas reservoirs. In this study, pore structure, fractal characteristics and influencing factors were studied of the Longmaxi formation shale gas reservoir in southeastern Chongqing, China. Scanning electron microscopy was used to describe the characteristics of various reservoirs. High pressure mercury intrusion and low temperature liquid N-2 and CO2 adsorption experiments were used to obtain pore structure parameters. V-S model, FHH model and Menger sponge model were selected to calculate the micropore, mesopore and macropore fractal dimensions, respectively. The results show that organic matter pores, inter-granular pores, intra-granular pores and micro-fractures are developed within the shale, and the pore morphology is mostly ink pores and parallel plate pores with aperture essentially in the 1-2 nm and 2-50 nm ranges. Moreover, macropores are the most complex in these samples, with mesopores being less complex than macropores, and the micropores being the simplest. D-1 (micropore fractal dimension) ranges from 2.31 to 2.50, D-2 (mesopore fractal dimension) ranges from 2.74 to 2.83, D-3 (macropore fractal dimension) ranges from 2.87 to 2.95, and D-1 (comprehensive fractal dimension) ranges from 2.69 to 2.83 of fractal characteristics. D-1 and D-2 are mainly controlled by TOC content, while D-3 and D-t are mainly controlled by brittle and clay mineral content. These results may be helpful for exploration and the development of shale gas in southeastern Chongqing, China.