Mobility of connate pore water in gas shales: A quantitative evaluation on the Longmaxi shales in the southern Sichuan basin, China

Abstract

Flowback and produced water is causing water environment pollution and elevated disposal costs during the shale gas development. Due to high salinity and variously hazardous substances in the connate pore water (i.e., initial water storing in shale matrix pores), whether the connate pore water is produced in company with fracking water flowback has long been concerned. The key is to determine the mobility of connate pore water, which is closely associated with the occurrence of adsorbed and free water in shale pores. In this study, five fresh shale samples were obtained from the Longmaxi Formation shale gas reservoirs in the southern Sichuan basin of China to evaluate the mobility of connate pore water. Theoretical models of describing the non-equilibrium and equilibrium expulsion processes of connate pore water from shale matrix were proposed. On this of basis, the amounts and microscopic distributions of adsorbed and free water in the shales were determined by using centrifugation and nuclear magnetic resonance (NMR) tests under atmospheric pressure and 30 °C temperature condition. Results show that: (1) the amounts of adsorbed and free water in the shales varies from 5.9132 to 8.3442 mg/g (mean 7.3842 mg/g) and 2.1224–5.7931 mg/g (mean 3.9507 mg/g), respectively. The amount of free water (i.e., maximum mobile water amount) accounts for 23.69–41.37 wt% (mean 34.12 wt%) of total water, indicating that about one-third of the connate pore water in gas shales is potentially recoverable. (2) The connate pore water in the adsorbed and free states mainly distributes in the pores of 0.5–10 nm, of which the adsorbed water mainly distributes in the pores of 0.5–3 nm and the free water (i.e., mobile water) mainly stores in the pores of 3–10 nm. (3) The mobility index, that is the ratio of free water amount to mid-value pressure difference, was introduced to quantify the mobility of connate pore water by considering the equilibrium expulsion process of pore water. The mobility index of connate pore water in fresh shales is negatively related to the water-adsorbed ratio. This study contributes to identifying the source of flowback and produced water and is also significant to water environment.