NUMERICAL STUDY OF GEOMECHANICAL FAULT REACTIVATION AND CAPROCK SYSTEM INTEGRITY IN CO₂ STORAGE: A SENSITIVITY ANALYSIS USING FULLY COUPLED FLOW-DEFORMATION THREE-DIMENSIONAL DISTINCT ELEMENT MODELLING

Abstract

To examine fault reactivation and caprock seal integrity during CO₂ injection, a fully coupled flow-deformation model is used to evaluate geomechanical performance. The In Salah greenhouse gas storage project in Algeria is used as a field example to investigate fault slip and potential for induced seismicity across geomechanical and operational conditions. A sensitivity analysis is conducted with variations in fault geomorphology, in-situ stress ratio, injection pressure, and rate. Results indicate fault slip is sensitive to fault dip angles, where faults with 15° fault dips experience the largest shear displacement (1.07×10⁻² m) relative to higher dipping fault angles (60° faults experience maximum slip of 2.24×10⁻³ m).

In addition, variations in the in-situ horizontal-to-vertical stress ratio revealed that raising this ratio from 1.0 to 2.0 led to a 50% decrease in vertical displacement, meaning horizontal stresses are beneficial for caprock containment. The evaluated parameters of fluid injection revealed that the injection rate affects fault stability; increasing the rate from 5 l/s to 6 l/s increased fault slip by 55%, with the resultant maximum displacements of 1.34 mm and 2.08 mm, respectively. Yet, increasing fluid viscosity by 20% increased fault slip from 1.34 mm to 1.26 mm, meaning increased fluid viscosity equals reduced fluid migration and pressure stabilization, thus decreasing fault reactivation likelihood.

Furthermore, the results showed that caprock strength controls deformation; deformation occurred in the form of 0.0004 m vertical displacement up for weak caprocks, but no vertical displacement occurred for strong caprocks. Therefore, this shows caprock failure is dependent upon stress regime and fluid properties with a possibility of failure via leakage. Ultimately, this study contributes to the body of knowledge about the complex geomechanical and fluid interactions associated with injecting/sequestering CO₂ into/from subsurface geological formations. It highlights fault geometry as well as the relative location of injection, injection volume and pressure, and caprock strength as important factors of stress manipulation for successful sequestration safety over the long term