Lower threshold of pore-throat diameter for the shale gas reservoir: Experimental and molecular simulation study

Abstract

Low-field nuclear magnetic resonance (LF-NMR), high-speed centrifuge and low-pressure nitrogen adsorption (LPNA) experiments were conducted on shale samples from the Lower Silurian Longmaxi Formation to measure pore-throat parameters of the shale reservoir. We measured bound water saturation, helium porosity, bulk density, specific surface area, and pore-size distribution to evaluate reservoir quality. The thickness of the bound water film was calculated based on the equation established from these parameters, showing that its thickness ranges from 1.07 nm to 2.73 nm, with a mean of 1.72 nm. In addition, Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were carried out to estimate the methane adsorption capacity and the number of the adsorbed layers in pores with different sizes at the given temperature and pressure (393.15 K, 65 MPa). The simulation results demonstrate that methane is unanimously adsorbed into pores <1.52 nm in diameter. Shale gas was present in both free and adsorbed states when the pore size was larger than 1.52 nm, and two layers were adsorbed on each pore wall. By combining the thicknesses of bound water film and adsorbed layers, the pore-throat lower limit for the shale gas reservoir was calculated to be about 4.96 nm.