Coupled Fluid Flow-Geomechanics Modelling for CO2 Storage Simulation with Thermal Effects in a Depleted Gas Reservoir
Authors: Guerra J. , Golaco C. , Abdur Rahman S. , Povstyanova M. , Robert M. J. , Stashevsky V. , Rat C. , Hasan H. , Shady M. , Jain S. , Stojic S. 
Published: November 2024

This case study describes the workflow implemented to model and assess the CO2 site storage integrity, using coupled thermal dynamic-geomechanics modeling of supercritical CO2 in the study field and its impact on the reservoir, caprock, fault deformation, and eventual failure. The One-way coupled fluid-flow geomechanics simulation examines the impact of reservoir re-pressurization and thermal cooling in the caprock deformation, reservoir integrity and fault stability, associated to physical phenomena such as Joule-Thomson expansion, heat diffusion, and undrained deformation processes. The reservoir model was augmented by adding the overburden, sideburden and underburden to not only impose the boundary conditions but also for capturing the impact of the thermal changes in the caprock. The simulation is conducted by bringing the pressure and temperature changes into the geomechanics simulation to compute rock deformation and eventual failure. Rock yielding was assessed in all the materials to determine critical zones and pressure thresholds that should be avoided during CO2 injection.

The dynamic simulation and well modeling showed a large temperature reduction at the bottomhole due to CO2 expansion effects (Joule-Thomson). These effects were reduced as the reservoir repressurized. A correlation of bottomhole pressure and temperature with reservoir pressure was developed to capture the impact of changing temperature over time with reservoir re-pressurization. The coupling of these dynamic changes over time with geomechanics demonstrated that caprock integrity was not compromised, but certain locations posed more risk than others. The fault integrity showed locations with potential for reactivation, depending on the fault type and maximum injection pressures. These limits should be considered during the injection plan. In general, thermal effects play the most critical role in the risk of rock failure and fault reactivation, and this risk is enhanced by reservoir re-pressurization (playing a secondary role), especially in proximity to the injection wells. Both shear and tensile failures are mainly influenced by severe cooling around the injectors. This cooling generally extends 3-4 cells around the wellbore (150-200m), sometimes even extending &gt;400m. The simulation shows the Nahr Umr Formation caprock is competent enough, only slightly affected by thermal effects when exposed to the main injection borehole with a very low risk of hydrofracturing. In general, injection pressures for all the injection scenarios fall below the fracturing pressure across the reservoir; nonetheless, the fracturing pressure might be reduced drastically due to the severe cooling around some injector wells, which tends to reduce the stresses and create tensile failure. The study demonstrated that the potential integrity problems can be mitigated by using an adequate injection strategy, selecting injection and monitoring wells, staging the injection rates, and limiting the cooling effects around the injectors.

A coupled fluid-flow geomechanics simulation must be standard practice to study the complex physical phenomena involved in CO2 injection properly, especially in heavily depleted reservoirs containing compositional fluids and scenarios containing injection mixes (CO2 and H2S) The coupled injection allowed the simulation of complex phenomena, including the Joule-Thomson effect in the reservoir, the heat transfer in the caprock (diffusion), and the undrained deformation process in the caprock.