Experimental study and theoretical simulation of dynamic shear modulus hardening in saturated tight sandstone

According to Gassmann theory, the shear modulus of rock remains unchanged before and after saturation, which is widely used in conventional reservoirs with high porosity and high permeability. However, tight sandstone and other unconventional reservoirs usually have low porosity, low permeability and complex pore structure, therefore the applicability of Gassmann fluid substitution theory in such reservoirs is not clear. In order to solve this problem, ultrasonic P-wave and S-wave velocities of dry and saturated tight sandstone samples were measured under the effective pressure of 1 similar to 60 MPa, and the variation characteristics of shear modulus of sandstones before and after water saturation were analyzed. The results show that the shear modulus of tight sandstone may weaken or harden before and after water saturation. Through the quantitative analysis of the pore aspect ratio of tight sandstone samples, it is found that samples of micro fractures mainly distributed between particles and within particles, with wide distribution range of soft pore aspect ratio and high content of soft pore are more likely to show the characteristics of shear hardening. The general squirt flow model can be used to simulate the shear hardening phenomenon, but the influence of micro crack closure on the input parameters should be considered. Shear hardening is mainly due to the dispersion of squirt flow excited by high frequency elastic wave. Considering the frequency dispersion may also occur in seismic and logging frequency bands, it is necessary to realize that the shear modulus may change during fluid replacement in unconventional reservoirs such as tight sandstone. Blind using of the Gassmann theory with assumption that shear modulus does not change in fluid replacement may cause large prediction error.