Microscopic occurrence characteristics and seepage law of water bodies in gas reservoir under stress: a case study of tight reservoirs in the eighth member of Permian Shihezi Formation, Shenmu Gas Field, Ordos Basin

In order to solve the problem that the pore structure and the microscopic occurrence characteristics of fluid change with stress in real time during the depletion development of tight gas reservoirs, the pore structure, water production and irreducible water distribution characteristics of tight reservoirs in the eighth member of Permian Shihezi Formation, Shenmu Gas Field, Ordos Basin under different stress conditions were quantitatively characterized, and the influence of stress on gas-water relative permeability was analyzed by using nuclear magnetic resonance (NMR), high-pressure mercury injection (HPMI) and displacement experiments. The results show that when effective stress increases from 7 MPa to 17 MPa, reservoir porosity decreases from 8.39% to 7.65%, and the maximum pore radius decreases from 38.3 μm to 35.2 μm among which, the number of pore throats in the range of 0.3-18.9 μm decreases the most. The number of pores involved in seepage decreases due to the decrease of pore throat size, the bound water saturation increases from 51.3% to 59.7%, and the distribution range of the size of bound water increases from 0.012-17.4 μm to 0.012-20.1 μm. The changes of pore structure and bound water distribution characteristics further reduce the relative permeability of both gas and water phases, the relative permeability of gas phase decreases from 0.481 to 0.283, and that of isotonic point decreases from 0.157 to 0.09.