Effect of cyclic loading on Anhydrite mechanical and petrophysical properties: an Iranian Underground Gas Storage Caprock

Underground gas storage (UGS) in depleted reservoirs holds the highest amount of working gas. In this study, Anhydrite caprock of an Iranian depleted reservoir was studied due to an increase in the number of seismic activities around this field. Anhydrite mechanical properties degrade due to cyclic loading effects resulted from the natural gas injection-production mechanisms. Anhydrite specimens were cored from the outcrop samples. The similarity of Anhydrite outcrop and drilling cuttings was confirmed using X-ray diffraction and SEM EDAX mineral analysis. The effect of cyclic loading on Anhydrite specimens has  not been studied previously. The specimens loaded 10, 20, and 30 times in the range of 18, 21, 24, and 25 MPa respectively. Anhydrite unconfined compressive strength (UCS) reduced between 3.5 to 23.9 percent under the cyclic loading experiments. The UCS is more sensitive to the intensity of loading rather than the cyclic times. The specimens CT images were obtained before and after the cyclic loading tests. Also petrophysical properties of the specimens were measured. The study of the specimens’ damage indicated that the anhydrite loading threshold was greater than 40 percent of the original UCS.