Experimental investigation on the effective pressure coefficient for permeability and deformation in the tight sandstone

The effective pressure law (EPL), P_eff = P_c - α P_f, where P_eff denotes the effective pressure, P_c the confining pressure, P_f the pore fluid pressure and α the effective pressure coefficient, has been proved to be very useful in describing the effect of pore fluid pressure on rock properties. The coefficient α can be a constant (linear EPL) or, as has been previously reported for tight sandstones, a function of P_c and P_f (non-linear EPL). Besides, the coefficient α is different for different rock properties, such as permeability and deformation which are two most important properties for completion and production activities. Here, the permeability and deformation of tight sandstone from Chinese gas reservoir were measured at the same time while cycling P_f and P_c according to two different procedures. In the first place, P_f and P_c were varied simultaneously so as to keep the pressure difference, P_c - P_f, constant. Three cycles, each with a distinct value of P_c - P_f were performed. In the second procedure, we cycled the pore pressure between a value and one slightly lower than P_c, while keeping the confining pressure constant. Again, three cycles with fixed values of P_c were performed. We used Argon gas to minimize chemical effects. The results showed that under constant P_c, the permeability decreased with the decrease of P_f but the deformation decrease with the increase of P_f, which agreed with previous results. Under constant P_c - P_f, the permeability decrease with the increase of P_f but the deformation decrease with the increase of P_f. The permeability or deformation was not constant although the value of P_c - P_f was constant, indicating that the permeability or deformation of our tight sandstone was not controlled by the ordinary effective pressure law. We estimated the EPL coefficient α_k for permeability and α_ɛ for deformation using the method proposed by Xiao et al.[2017]. We found that the EPL coefficients α_k and α_ɛ appeared to be a well-defined decreasing function of Terzaghi's differential pressure α(p_c - p_f); the range[0.633, 1.015] of coefficients α_k was greater than the one [0.595, 0.724] of α_ɛ . Improving the accuracy of the EPL will provide a more realistic determination and prediction of the permeability or deformation of tight sandstones under varying loading conditions.