Numerical simulation of CO2 injection into deep saline aquifers

In the last two centuries, atmospheric concentrations of carbon dioxide (CO2) have augmented by about 35% as a result of increased anthropogenic emissions. To reduce these emissions, geologic Carbon Capture and Sequestration (CCS) has been advanced as an emerging technology. Current estimations report that 60% are of the total storage capacity under the subsurface is included in deep saline formations. Carbon geologic storage involves injection of CO2 in supercritical state into deep confined aquifers and can be modeled based upon the theory of two-phase flow in porous media. Supercritical CO2 is less dense and less viscous than brine, which causes gravity override. The mass conservation equations for the two phases constitute a non-linear system of partial differential equations (PDE). In this work, MFLOW3D, a numerical finite-element model, is used to solve this non-linear system of PDE’s and to test the simulation of CO2 injection into hypothetical and ideal confined aquifers. In MFLOW3D, the PDE’s are discretized in space using linear three-dimensional finite elements in order to obtain a nonlinear system of ordinary differential equations. The temporal discretization is implemented via a finite difference backward-Euler scheme. The non-linearity is solved by using Newton-like iterative methods. Several scenarios are here simulated in order to analyze the effect of different factors on the accuracy of MFLOW3D results. These factors are: resolution and non regularity of finite element mesh; presence of subsurface heterogeneities; lateral size of the domain. The reliability of simulations is checked for global accuracy in terms of the mass balance relative error for the injected CO2. The numerical tests offer an important feedback on the capabilities and limitations of the adopted numerical approach. Department of Civil and Environmental Engineering Colorado State University Fort Collins CO 80523 Tel: (970) 491-1470 e-mail: [email protected] 2 e-mail: [email protected] 3 e-mail: [email protected]