Characteristics of pore structure of tight gas reservoir and its influence on fluid distribution during fracturing

Micro-pore structural characteristics are complex in tight gas reservoirs, and it affects the fluid flow. Hydraulic fracturing is the key technique for economic gas production from tight reservoirs. In the fracturing process, fracturing fluid imbibes into reservoir matrix driven by capillary force and affects the dynamic distribution of reservoir fluid. It is thus of great interests to investigate the micro-pore structural characteristics, and its relationship with the dynamic distribution of fluid. In this study, cores from the Yanchang tight sandstone reservoirs in the Ordos basin were selected to evaluate the micro-pore structural characteristics and dynamic distribution of fluid. High-pressure mercury injection (HPMI), constant-rate mercury injection (CRMI), and low-pressure nitrogen gas adsorption (LP-N2GA) are used to evaluate the structural characteristics of micro-pores. The results show that average pore radius of samples ranges from 131.25 μm to 191.10 μm, with an average value of 170.42 μm, and the average throat radius ranges from 1.15 μm to 4.07 μm, with an average value of 2.489 μm. The pores are mainly closed small pores, and some H3-type crack-type pores are present. Imbibition, centrifugation experiment and nuclear magnetic resonance (NMR) techniques were used to study the dynamic distribution of fluid. The results show that imbibition recovery ranges from 40.04% to 87.29%, with an average of 62.14%, and movable fluid saturation ranges from 48.20% to 94.74%, with an average of 76.61%. The fluid during imbibition and the movable fluid during saturation were primarily distributed in mesopores and macropores. Results of statistical analysis suggests that the pore-throat structure is among the key factors impacting the imbibition recovery and movable fluid saturation.