Threshold pressure phenomena in porous media

Threshold displacement pressures are needed to determine how much over-pressure 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 8 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 were also measured and correlated with air- water threshold pressures. A study of the effect of time on threshold pressures show that they are independent of time. An aerial photography of gas emerging from the top of a core shortly after its threshold pressure has been exceeded shows that the gas bubbles are 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. (18 refs.)