Experimental Study of Tight Reservoir Rock Failure Process by Acoustic Emission
Shan WU1#+, Hongkui GE2, Tiantia LI3, Xiaoqiong WANG2, Ke GAO1
1Southern University of Science and Technology, China, 2China University of Petroleum, Beijing, China, 3Xi'an Shiyou University, China

A fundamental problem in hydraulic fracturing is to understand the mechanism of failure process of tight reservoir rocks. Rock failure is generally caused by the propagation of fractures under the influence natural cracks. Since acoustic emission (AE) technique has proven to be an effective tool to monitor the dynamical fracture propagation, here, AE is employed to investigate the different rock failure processes under uniaxial compression. The experiment rock samples are sampled from four tight reservoirs in typical oil and gas production fields in China. The C7 and LCG tight sandstone are respectively collected from the Ordos Basin and the Junggar Basin. Both the LJP shale and LMX shale are obtained from the Sichuan Basin but with different formations. We analyze the characteristics of acoustic emission data and discuss the relation between acoustic emission parameters and the rock failure process. The results demonstrate that on experimental scale the natural fractures contained in these rocks have distinct impacts on fracture propagation. The AE rate curves indicate different failure patterns, and the RA-AF (RA value and Average Frequency) values of AE reveal that the final shear failure is caused by tensile fracture accumulation. For the vertical-bedding samples, we barely observe shear phenomena accumulated by tensile cracks. While for the parallel-bedding samples, it is common that the tensile cracks could gradually accumulate into shear failure. Additionally, the b-value before the final failure is closely related to the natural crack activation, and the b-value during the final failure signifies the complexity of the fracture network. The b-value could reflect the formation of the fracture network under the joint effects of stress and natural cracks. Specifically, in stress dominated failure process, the b-value is high before the final failure and decreases thereafter. When the failure is controlled by both the natural cracks and stress, the b-value is low before the failure and increases after that. While when the natural cracks dominate the failure process, the b-value is generally within a low level during the whole procedure .