Stiff cement, soft cement: Nonlinearity, arching effect, hysteresis, and irreversibility in CO2-well integrity and near-well geomechanics

During geologic CO2 storage, integrity of injection wells is of utmost importance. Tensile radial stresses at casing-cement and cement-rock interfaces caused by relatively cold CO2 flowing down the tubing may induce debonding and create microannuli. Microannuli represent potential leakage paths. Thermal stresses can be reduced by using softer cement (“flexible cement”). Numerical simulations show that benefits of soft cement are more pronounced in relatively hard (stiff) rocks. In a very soft rock, there might be as good as no effect of lowered cement stiffness on thermal stresses, while tensile strength of such cement might be severely reduced. Thus, to fully benefit from soft (flexible) cement in CO2-injection wells, such cement should be set against sufficiently stiff rock. Effect of rock stiffness on tensile radial stress build-up during cooling is stronger than that of cement stiffness. As a result, the benefits of carefully adjusting the cement stiffness might be offset by natural variation in the rock properties. Soft cement also reduces the stress build-up in the cement sheath caused by far-field in-situ stress variation during injection. The mechanism here is the arching effect in the near-well area: the rock effectively shields the cement sheath from in-situ stress changes. In order to fully exploit benefits of soft cement here, the rock again must be sufficiently stiff.