Threshold Pressure Phenomena in Porous Media

Abstract

Threshold displacement pressures are needed to determine how much overpressure can be used in storing natural gas. An experimental technique for determining threshold pressures by displacing water with gas from samples saturated with water is presented. Threshold pressures for eight low permeability samples were measured. Threshold pressure data obtained in this work, plus data on higher permeability samples reported in the literature, are correlated with porosity, permeability, surface tension and formation resistivity factor, Mercury injection pressures also were measured and correlated with air-water threshold pressures. Threshold pressures were found to be independent of time. An aerial photograph of gas emerging from The top of a core shortly after its threshold pressure has been exceeded shows that the gas bubbles we uniformly distributed across the face of the core. Channeling will occur, however, when an increased gas phase permeability is reached. A porous medium can be resealed after its threshold pressure has been reached provided it has not been desaturated below a fixed saturation. Its new threshold pressure will be lower than when the sample is 100 percent saturated with water.