Effect of stress cycling on the deformation and AE characteristics of reservoir rock from the perspective of energy storage: An experimental study

A successful energy transition requires effective storage options. Using underground energy storage (UES), such as depleted porous reservoirs, can help balance the production and demand for renewable energy. Because of the production and injection operations sequence, all underground energy storage systems, including compressed air and hydrogen energy storage, are subject to cyclic loading. To design and operate underground storage facilities, it is essential to understand the geomechanical behavior of porous reservoir rock under cyclic loading. We present the results of the triaxial cyclic laboratory experiments conducted on Red Phaelzer sandstone at 10 MPa confining pressure. By considering three frequencies (F1=0.014 Hz, F2=0.0014 Hz, and F1=0.0002 Hz), two amplitudes (A1=20 MPa and A2=5.9 MPa) and two stress regimes (38 MPa and 85 MPa), 12 triaxial cyclic tests were performed. In total, eight triangular stress cycles were applied for each test, and at the same time, six piezoelectric sensors recorded the acoustic emission (AE) activities during these experiments. Results showed that the total axial inelastic deformation increases when the stress regime and amplitude of cycles are increased. However, this parameter reduces by increasing the frequency of cycles. In addition, Young’s modulus computed from the loading ramps of the cycles increased significantly from the first cycle to the second cycle for all the tests. For tests in the brittle regime, the relation is that the larger the amplitude of cycles, the lower the increase in Young’s modulus. In addition, the AE analysis showed that major events were recorded in the first cycle. By increasing the number of cycles, the number of events, the maximum AE, and the average AE amplitude decreased. Our experimental results highlight that major mechanical changes and AE activities occur during the first cycle, and the stress regime influences the intensity of AE and mechanical changes. These outcomes can benefit studies about subsidence, uplift, fault reactivation, and other physical phenomena impacting the reservoir’s storage capacity, which is affected by cyclic sandstone deformation.