Thermo-Poro-Mechanical Analysis of the Effects of Low-Temperature CO₂ Injection on Caprock Integrity

For any CO₂ sequestration project, a competent caprock acts as the primary and immediate trapping mechanism that prevents the injected fluid to leak out of the reservoir. This paper reviews the results of a thermo-poro-mechanical model developed to study the effects of low-temperature CO₂ injection on the caprock integrity for a hypothetical CO₂ storage project. A detailed study was conducted to characterize the behaviour of the model and to identify the influence of interactions between fluid flow, heat transfer, and geomechanics in the model. The results show that pressure and temperature distributions in the model are dissimilar and, consequently, their effects on the geomechanical response of the model are significantly different. It was observed that the low temperature of CO₂ has a dominant effect on the geomechanical response of the model. In general, a temperature decrease leads to severe reduction in the values of in-situ stresses and, consequently, it increases the potential of fracturing in the aquifer and the caprock. Nevertheless, this potential is smaller in the caprock than in the aquifer as a result of a lower temperature decrease in the caprock. This contrast is accentuated by the pore pressure drop caused by the temperature decrease in the low-permeability caprock with undrained flow behaviour. It is also shown that the behaviour of the model may be approximated by using analytical solutions. The paper also reviews the results of a series of analyses conducted to evaluate the sensitivity of tensile fracturing potential in the caprock to several different modeling parameters. These sensitivity analyses showed that the variation in some parameters such as Poisson’s ratio, heat capacity, thermal conductivity, relative permeability, and permeability of the underburden have modest effects on the value of effective stresses in the caprock, while variations in Young’s modulus and the thermal expansion coefficient can significantly influence these stresses and the potential for tensile fracturing in the caprock. Sensitivity analyses also show that injection of CO₂ with a temperature closer to the temperature of the aquifer reduces the potential for tensile fracturing in the caprock and aquifer to a great extent. The potential of fracturing in the caprock is also highly reduced by injecting CO₂ at greater depths within the aquifer and farther away from the caprock.