New Method for Assessing Microfracture Stress Sensitivity in Tight Sandstone Reservoirs Based on Acoustic Experiments

Abstract

Research on the evaluation of microfractures in tight sandstone reservoirs is a frontier topic worldwide. A marine tight sandstone reservoir is present in the Silurian strata in the northern Tazhong area of the Tarim Basin, Northwest China, at a depth of more than 5, m. The study of the regular opening and closing of microfractures can provide guidance for the exploration and development of tight sandstone reservoirs. In this paper, a triaxial acoustic experiment was designed, and a new method for assessing the microfracture stress sensitivity of tight sandstone reservoirs was proposed by combining the Biot-consistent and the differential equivalent medium (DEM) theory models. By using the coupled model, the rock microfracture density ε and the pore aspect ratio were calculated. The pore-fracture morphological evolution and regular distribution within the rocks were discussed, and the microfracture stress sensitivity was evaluated. This method could effectively overcome the drawback of using a single method. The results showed that the microfracture density ε decreases and the pore aspect ratio increases with an increase in the confining pressure, which is mainly due to the closure of microfractures. According to the decreasing magnitude of microfracture density values under different confining pressures, the proportion of microfractures that stayed open or had closed could be determined. The stress sensitivity of the samples that had greater microfracture density under higher confining pressure was weak. In contrast, the stress sensitivities of the other samples were strong. According to the change rule of the rock pore aspect ratio, a critical pressure was defined. It was found that the rock microfracture density values corresponding to the critical pressures for all samples were relatively close. This ε value could be treated as a common feature of tight sandstone reservoir microfractures, and it could also be used as a contrast indicator to quantitatively characterize the degree of development of various tight reservoir microfractures. The correlations between the microfracture density ε under a confining pressure of 65 MPa, the defined critical pressure, and the rock porosity indicated that the advantageous types of microfractures were mainly influenced by the rock physical properties. There was a close correlation between the microfracture development degree and the rock physical properties. For the tight sandstone reservoir of this study, the degree of microfracture development with weak stress sensitivity was significantly higher in feldspar than in quartz. This method could be used for the quantitative evaluation of microfracture properties.