Multiphase Flow in Fractured Porous Media: Application to CO₂ Leakages From Natural and Stimulated Fractures

Abstract

Leakage to the atmosphere of a significant fraction of injected CO₂ would constitute a failure of a geological CO₂ storage project. We present a numerical model that simulates flow and transport of CO₂ into heterogeneous subsurface systems. The model, StoTran, is a flexible numerical environment that uses state-of-the-art finite element and AMR scheme implemented using MPI and OpenMP protocols. Multiphase flow equations and the geomechanical equations are implicitly solved and either fully or sequentially coupled. In the current study, StoTran has been used to simulate several scenarios spanning from a homogeneous single layered reservoir to heterogeneous multi-layered systems, which including cap-rock with embedded fractures, have been simulated under different operations of CO₂ injection and CO₂ leakages conditions. Results show the impact of the injection and leakage rates on the time evolution of the spread of the CO₂ plume, its interception of the fractured cap-rock and the risk associated with the contamination of the overlaying aquifer. Furthermore, several leakage scenarios show the intermittence behavior and development of the CO₂ plume in the subsurface. A remedy to CO₂ leakages from the reservoir using silica gels is presented to enhance and ensure a long term containment of the injected CO₂.

1. INTRODUCTION

Global temperatures are increasing. Atmospheric CO₂ concentration is increasing (Figure 1). New carbon management legislation and initiatives are based on the assumption that increased CO₂ levels are a major cause of global warming. Several ways of reducing CO₂ emissions and CO₂ levels in the atmosphere include: improved efficiency in power generation by upgrading existing plants, higher efficiency in all new plants, relying more on renewable energy and nuclear-generated power, distribution of energy consumption among other resources and finally, carbon capture and sequestration (CCS). Of all emissions reduction mechanisms, CCS is projected to provide the largest contribution to emissions reduction in the near-term (~decade time scale). CCS begins with capture of CO₂ at its source, such as a coal fired power plant, transporting the CO₂ to a location where it can be sequestered or stored safely away from the earth's atmosphere and oceans. Three types of CO₂ sequestration are under way: terrestrial sequestration, geologic sequestration, and mineralization. Geologic sequestration is storage of CO₂ within geological formations under the earth's surface. Oil, gas, nonmineable coal and saline water reservoirs are those best suited for CO₂ sequestration (DOE 1999).