A one-way coupled geomechanical model for CO₂ storage in faulted depleted gas reservoirs

Robust geomechanical models are critical for predicting and monitoring challenges associated with fluid injection and withdrawal in different operations, including underground gas storage. However, little attention has been given to geomechanical modeling and responses of faulted structures in the context of CO₂ storage. In this study, a one-way geomechanical model was developed for a depleted gas field in the Bredasdorp Basin in South Africa and coupled with a dynamic model to gain insights into the geomechanical challenges associated with CO₂ storage in faulted depleted gas fields. Three geomechanical deformation scenarios were formulated to evaluate the behaviour of the gas field. The results obtained indicated a substantial contrast in the stress state between the central and peripheral regions of the reservoir. Moreover, during the depletion phase, the reservoir experienced a subsidence of 3 cm, while 1.8 cm was observed during the injection phase. It was also found that the uplift of the underburden compensated for the loss of fluid support, enabling the reservoir to uphold the overlying overburden despite depletion. However, after 40 years of CO₂ injection, the caprock failed while the reservoir rock remained stable. Furthermore, depletion increased the likelihood of fault slip, while CO₂ injection significantly reduced deviatoric stresses and indirectly minimized the tendency for fault activation. It seems that the one-way coupled model can provide valuable insights into the poromechanical response of the faulted depleted gas field used for CO₂ storage, although the results presented in this study may need further calibration against the field data.