Effective pressure law for permeability of E-bei sandstones

Laboratory experiments were conducted to determine the effective pressure law for permeability of tight sandstone rocks from the E-bei gas reservoir, China. The permeability k of five core samples was measured while cycling the confining pressure p(c) and fluid pressure p(f). The permeability data were analyzed using the response-surface method, a statistical model-building approach yielding a representation of k in (p(c), p(f)) space that can be used to determine the effective pressure law, i.e., p(eff) = p(c) - kappa p(f). The results show that the coefficient kappa of the effective pressure law for permeability varies with confining pressure and fluid pressure as well as with the loading or unloading cycles (i.e., hysteresis effect). Moreover, kappa took very small values in some of the samples, even possibly lower than the value of porosity, in contradiction with a well-accepted theoretical model. We also reanalyzed a previously published permeability data set on fissured crystalline rocks and found again that the kappa varies with p(c) but did not observe k values lower than 0.4, a value much larger than porosity. Analysis of the dependence of permeability on effective pressure suggests that the occurrence of low kappa values may be linked to the high-pressure sensitivity of E-bei sandstones.