A Decoupled CFD-Mechanical Model for Estimating the Deflections from CO2 Sequestration in Low & High Buoyancy Flow Regimes
Authors: Sarris Ernestos , Papaloizou Loizos , Gravanis Elias 
Published: September 2023

ABSTRACT
This work evaluates the caprock deflections at various timeframes induced by underground injection of CO2 in all flow regimes for cap integrity considerations in CO2 sequestration. The pressure profiles in the various regimes are not known a-priori and depend strongly on the pumping scenarios followed and are hence obtained numerically via CFD numerical simulations that were performed with the software Ansys-Fluent. Following a decoupled approach for time effectiveness, pressure profiles are approximated by logarithmic functions and integrated into a mathematical model assuming elastic medium storage, cap and surface rocks while the formulation is in the spirit of the Germain-Poisson-Kirchhoff thin plate theory. Elastic solutions are developed for the axisymmetric flexure deflections of the cap layer during CO2 internal pressurization of the aquifer for the low buoyancy and high buoyancy regimes. The novelty of the model lies in the fact that is capable for estimating fast and accurately the deflections created in the various flow regimes with a simplified approach, effectively avoiding the time consuming fully coupled FEA-CFD and could be used as a convenient tool for evaluating cap rock integrity. The results of the analysis indicate that for the set of data used, most uplift magnitudes are not capable for creating fractures on the cap rock compromising the cap integrity when the solution is mapped in the high-buoyancy regime. On the other hand, if the solution is mapped in the low-buoyancy regime the pressure-induced, could cause poro-elastic deflections over time which could lead eventually to fracture creation.

INTRODUCTION
Deep geologic storage of hazardous and toxic fluids, by injection into confined formations for isolation has always been an option for environmental management (Raza et al., 2019). Saline aquifers have proved to be very promising geologic formations for CO2 storage. The hydromechanical contribution in the coupled process involved in CO2 sequestration have received scientific attention because the hydrodynamic trapping of CO2 is associated with the pore space of storage rocks (Bachu and Adams, 2022; Bachu, 2003). The hydrodynamic process of injection leads to the development of an evolving plume within the storage rock confined by a very low permeability cap/seal rock. The advancing plume displaces the resident water (brine) acting as immiscible fluids (Nordbotten and Celia, 2006; Houseworth, 2012). The type of evolution/propagation is controlled by the relative strength of the injection rates and buoyancy effects, as well as the relative mobility of CO2 plume to brine, during sequestration of CO2 under supercritical conditions. This plume propagates as large volumes of CO2 are injected into the aquifer over time, creating overpressure in the formation and perturbs the stress field which extends both vertically and laterally to the cap rock, possibly compromising its integrity (Nordbotten et al., 2005; Dentz and Tartakovsky, 2009; Sarris et al., 2014; Guo et al., 2016a). Currently, there are several on-going CO2 storage projects at various scales, e.g. pilot scale: Ketzin, Cranfield, Frio, demonstration scale: Decatur and large commercial scale: Sleipner, Shohvit, Gorgon and In-Salah. The latter, the In-Salah injection site is a deep saline project in Algeria (Guo et al., 2016b). Stratigraphically, the aquifer is overlaid by an impermeable caprock which is necessary for preventing CO2 from escaping to the atmosphere. It is then understood that the cap uplift and/or integrity over time is a significant issue (Sarris and Gravanis, 2019; Shukla et al., 2010).