A Novel Technique to Investigate Effects of Thermal Shocks on Cement under In-situ Conditions for CCS Well Integrity

During carbon capture and storage (CCS), the periodic injection of pressurized CO2 leads to thermal cycling and shocks in the subsurface, due to the endothermic expansion of pressurized CO2 upon injection. Under these temperature variations, micro-annuli between wellbore casing and cement, and cracks in the cement can develop. They impede the safe geological storage of CO2. Therefore it is of significance to understand how the sealing ability of the cement sheath of CCS wells is affected by thermal cycling or shocks. This paper reports a novel technique to investigate cracking in cement by thermal shocks under in-situ conditions. We use a triaxial deformation apparatus capable of mounting a cement sample in a vessel at a confining pressure of up to 70 MPa, with axial stress up to 26 MPa. An internal furnace is used to achieve an elevated temperature in the vessel. Pore fluid lines are fitted in upper and lower axial pistons to allow water injection. During the experiments, the triaxial vessel is filled with heat-resistant oil which provides the confining pressure. We load the sample at different in-situ states of hydrostatic stress and heat the sample assembly to various elevated temperatures (60 - 120ºC). We then inject cold water (20ºC) through the sample using two high-pressure syringe pumps at a designated flow rate for a given time. To study how thermal shocks affect cement, we measure permeability with a differential pressure transducer measuring the difference between the up- and down-stream pore fluid line, and we use micro-computed tomography to characterize the microstructure of the cement sample before and after the experiments.