Permeability variation law of porous media during heating process under effect of initial geostress

Key words:  temperature stress  porous media  permeability  initial geostress  thermo-fluid-solid coupling

Abstract:

To reveal the rule of permeability variation during thermal recovery of oil and gas reservoirs under different ground stresses, a thermo-hydro-mechanical coupled experimental system was developed, and non-adsorbed helium was used in the experiments. The change of helium gas viscosity with temperature and pressure was considered to eliminate the influences on the infiltration experiments. A series of infiltration experiments were conducted for coal and sandstone under different temperature conditions and different initial stresses. The results indicate that with the increase of temperature, the permeability of porous medium increases firstly, and then decreases in the form of nonlinearity and non-monotone. There is a temperature of inflection point related with the initial effective stress. The mechanism is described as the following:when temperature is lower than the temperature of inflection point, temperature stress is less than the initial effective stress, pore space increases because of the outward expansion of solid skeleton, so the permeability will increase. When temperature is higher than the temperature of inflection point, temperature stress is bigger than the initial effective stress, pore space decreases because of the inward expansion of solid skeleton. In addition, the temperature of permeability variation inflection point decreases with the increase of initial volume stress, the increasing rate of temperature stress is higher when the size of the initial volume stress is bigger.