Pore connectivity and water accessibility in Upper Permian transitional shales, southern China

Pore connectivity of shale controls shale gas migration and production behavior. The pore connectivity and water accessibility in clay-rich Upper Permian transitional shales remain unclear. Contrast matching small-angle neutron scattering (CM-SANS) tests were used to determine the accessibility of water to pores in the transitional shales. Complementary analyses with SANS, gas (CO2 and N-2) physisorption isotherm, mercury intrusion capillary pressure (MICP), helium ion microscopy (HIM), as well as field emission-scanning electron microscopy (FE-SEM) were conducted to study the pore connectivity and distribution characteristics of closed pores. The results show that closed pores (inaccessible to nitrogen molecules and mercury) are mainly distributed in pore diameters < 10 nm and associated with organic pores and interlayer spaces of illite-smectite mixed-layer mineral. The pore volume values obtained from MICP and N-2 adsorption underestimate the large pores (pore diameters > 100 nm) in shales. Based on deuterated water CM-SANS tests, 87-98% of the pores (2-200 nm diameters) are water-connected in transitional samples. The low accessibility to water is at pore-sizes of 5-10 nm and 20-30 nm. Results from in-situ gas contents show that closed pores have a certain gas bearing capacity, but micropores (pore diameters < 2 nm) control the gas occurrence in transitional samples. The connectivity of the organic pore network and between organic pores and surrounding interparticle pores is directly supported by FE-SEM and HIM imaging. Overall, improving pore-fracture connectivity through effective fracturing techniques is a means of mitigating the rapid decline in shale gas production.