Evolution of physical and mechanical properties of cementing materials during underground energy exploitation and storage

Abstract

The sealing problem of cement sheath often appears in gas wells for the increasing depth of underground energy exploitation and storage, especially when horizontal multi-stage fracturing technology is used in the shale gas industry. To investigate the effect of high temperature and high pressure (HTHP) on the evolution of the physical and mechanical properties of cement stone, conventional triaxial compression tests of cement stone at 5 different temperatures from 25 °C to 135 °C, with confining pressure varying from 5 to 45 MPa were tested. It was found that with increasing confining pressure and temperature, the mechanical properties of cement stone change from strain softening to ideal plasticity, finally, become strain hardening. The specimens show irregular longitudinal splitting under low confining pressure and room temperature. When the confining stress rises to 30 MPa and the temperature rises to 95 °C, expansive destruction occurs in the radial direction. In addition to the above, the failure mode shows single a shear plane or multiple splitting planes. Peak strength of cement stone is proportional to confining pressure and is inversely related to temperature. There is a negative correlation between elastic modulus and temperature. The effect of temperature on cohesion and friction angle is complex, and with the increasing temperature, the cohesion continuously decreases. The friction angle increases until the temperature reaches 60 °C, but then it starts to drop dramatically. The results of the physical and mechanical properties of cement stone guide the cementing design and construction of horizontal wells and the safety evaluation during underground energy exploitation and storage.