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. Introduction The degree of overpressure in excess of the discovery pressure that a gas storage reservoir can withstand is determined by the ability of its caprock to contain gas, providing the pressure does not exceed the structural limit of the reservoir. The retention of gas in a reservoir by a caprock saturated with water is a result of the capillary forces acting at the gas water interface. Without the presence of water in a caprock, gas would leak out of the reservoir at a rate determined by the permeability of the caprock to gas. The ability of a caprock to contain gas is expressed in terms of its threshold displacement pressure, which is defined as the minimum pressure needed to initiate the displacement of a wetting phase by a nonwetting phase from a porous medium 100 percent saturated with the wetting phase. The gas industry must be able to predict how much "overpressure" a gas reservoir can withstand before leakage occurs by gas displacing water from caprock overlying the reservoir. This is of economic importance since the storage capacity of a gas reservoir is proportional to its pressure. The discovery pressure for most reservoirs is found between the upper and lower limits shown in Fig. 1. Discovery pressures in excess of their hydraulic pressure gradient are attributed to the compaction of an isolated shale formation by the overburden. The lower limit of 0.433 psi/ft corresponds to the weight of the overburden, the gradient of the rock itself.