Assessing Caprock Integrity Through Thermo-Hydro-Mechanical Modeling of CO2 in a Deformable Chalk Depleted Gas Reservoir
Authors: Hosseinzadeh B. , Amour F. , Abdollahi C. C. A. , Hajiabadi M. R. , Ferreira C. A. S. , Nick H. M. 
Published: August 2024

ABSTRACT:
This paper presents a hydromechanical simulation of large-scale analysis to assess the risk of CO2 leakage through faults within the reservoir and caprock as potential pathways. The study involves the evaluation of slip tendency and distance to failure of existing faults in both the caprock and reservoir, which are crucial parameters influencing the potential for induced seismicity. To that aim, thermo-hydro-mechanical simulation of cold CO2 injection close to the cap rock and within a high-temperature (110°C) depleted gas reservoir is carried out to quantify the additional stresses and strains caused by the thermal expansion/contraction of the rocks. Since preferential fluid flow is more pronounced through faults and fractures experiencing high slip tendency, here the assessment of slip tendency shows no concerns regarding fault seal and the behavior of faults as pathways for the migration of CO2 injection. In addition, this study utilizes distance to failure analysis by driving data from analyses of stress orientation, magnitude, fault geometry, and temperature. The findings indicate that there are no significant concerns regarding the integrity of fault seals or the behavior of faults as potential routes for CO2 injection migration.

1. INTRODUCTION
The injection of CO2 alters both the reservoir pressure and the distribution of effective stress within the reservoir and caprock formations. The primary mechanism influencing hydromechanical behavior is the overall reduction in effective stresses, which is induced by the high-pressure injection of CO2 around the injection well. However, injecting CO2 into depleted gas reservoirs requires further analysis. During gas production, the reservoir pressure significantly decreases, leading to an increase in effective stresses. This study presents a numerical simulation of large-scale coupled analysis to evaluate the risk of fault reactivation within the reservoir and caprock.
Rohmer & Seyedi (2010) conducted an analysis of fault slip potential during CO2 injection, accounting for factors such as fluid pressure, stress fields, and poro-elastic effects. Through coupled simulation, they demonstrated that the maximum sustainable overpressure is strongly dependent on the in-situ stress state. Soltanzadeh & Hawkes (2008) explored how changes in fluid pressure within porous reservoirs can lead to stress alterations in surrounding rocks, potentially triggering the reactivation of existing faults. The findings indicate that, under fluid injection into a reservoir experiencing an extensional stress regime, fault reactivation tendencies are observed solely in rocks situated above and below the reservoir. Conversely, within a compressional stress regime, fault reactivation is more likely to manifest within the reservoir itself. Ferrill et al. (2020) explored the interconnected concepts of failure mode and resolved stress analysis, offering examples of fault geometry in normal and reverse faulting stress regimes. These analyses predicted deformation behavior and elucidated how variations in fault geometry and associated deformation mechanisms influence fluid transmission.