Influence of the combined action of water and axial pressure on the microscopic damage and mechanical properties of limestone

Abstract

As one of the main methods, hydraulic fracturing method is widely used in underground energy exploitation. Groundwater not only restricts the energy exploitation efficiency, but seriously affects the safety and stability of underground energy storage structure. To study the influence of groundwater and geostress on hydraulic fracture propagation and rock failure mechanism, the water injection tests were performed on limestone under different constant axial loading (0, 1, 2 and 3 MPa). After water injection, the evolution law of the micropore structure in water-injection samples were studied through SEM and mercury-injection tests; Moreover, the mechanical properties of the water-injection samples were studied through laboratory loading crushing tests. During loading process, the acoustic emission (AE) and infrared radiation temperature (IRT) parameters were collected synchronously and nondestructively. The results indicated that the pore size and porosity of limestone considerably increased with increasing axial pressure, and obvious stage characteristics of the stress-strain curve, cumulative AE energy and IRT difference were determined. Furthermore, the strength, cumulative AE energy and cumulative IRT difference decreased continuously with increasing axial pressure. Compared to limestone subjected to only water injection, the cumulative AE energy of the specimens subjected to high-pressure water under 1–3 MPa axial pressure decreased by 11.31%, 24.71% and 25.56%, and the IRT difference decreased by 10.71%, 19.64% and 26.79%, respectively. In addition, the extreme values of both cumulative AE energy and IRT occurred earlier than the peak stress, which can be interpreted as a sign of structural failure in limestone.