Thermo-mechanical simulation of CO2 injection in a North Sea depleted gas condensate reservoir

Denmark aims to progressively reduce its greenhouse gas emissions within the next decades with a specific target that is becoming
carbon neutral by 2050. Depleted oil and gas fields represent an economically viable solution to store a large amount of CO2 over a
relatively short period of time thanks to the existing infrastructures and subsurface data that ease the implementation of safe transport and storage of gas. However, the CO2 injection in depleted oil and gas fields, where the average reservoir pressure drops from fluid production, may lead to several operational challenges. More importantly for depleted gas fields, liquid injection of CO2 causes a considerable temperature drop owing to CO2 expansion, a phenomenon refers to as the Joule-Thomson effect (Bonto et al. 2021). This implies that when CO2 enters the depleted gas reservoir, the temperature drops to some extent as pressure drops away from the well, (Vilarrasa et al. 2017). Therefore, CO2 reaches the depleted reservoir at a colder temperature compared to the original reservoir temperature (Vilarrasa et al. 2014). Moreover, for depleted gas condensate reservoirs, where the pressure is below the dew point due to depletion, the liquid phase is shaped into a condensate ring around the wellbore reducing the well injectivity index. On the other hand, the temperature change induced by CO2 injection will influence stress and strain (Rutqvist 2012). The stress state can shift closer to the failure conditions of the rock (cap and reservoir rocks), thereby increasing the risk of CO2 leakage. It is thus crucial to accurately predict and monitor the phase behavior, the temperature distribution, and the building-up pore pressure during CO2 injection to avoid compromising the integrity of the storage site and to ensure safe long-term storing of CO2.

Coupled multiphysics CO2 injection simulations have not been investigated yet in detail for chalk reservoirs due to the high complexity of thermo-hydro-mechanical-chemical processes. In this paper, we focus on a coupled thermo-hydro-mechanical simulation of CO2 injection in a depleted gas condensate reservoir in the Danish sectors of the North Sea to obtain a realistic injection condition plus storage capacity. To that end, we develop a model in Petrel and Ocean that combines Eclipse thermal reservoir simulator and Visage geomechanics simulator to capture the induced mechanical alteration of chalk and its effect on porosity and permeability over time.