­­Vertically-integrated dual-continuum models for CO2 sequestration in naturally fractured aquifers

CO2 injection into saline aquifers can be modeled as a two-phase (CO2 and brine) flow system. One type of simplified model for this system can be developed by integrating the three-dimensional two-phase flow equations in the vertical dimension; this leads to a class of so-called vertically integrated models. Conventional vertically integrated models are based on the vertical equilibrium (VE) assumption, which assumes that CO2 and brine segregate rapidly due to strong buoyancy and are always in pressure equilibrium in the vertical direction. Recently, Guo et al. (2014) introduced the concept of dynamic reconstruction for the CO2-brine system for vertically integrated models. That dynamic reconstruction is a more advanced vertically integrated approach that includes vertical two-phase flow dynamics of both CO2 and brine as one-dimensional fine-scale problems within the vertically integrated framework. This approach relaxes the VE assumption while maintaining much of the computational efficiencies of the vertically integrated formulation. In this presentation, we apply these concepts associated with vertically integrated models to CO2 injection in naturally fractured aquifers. We treat the fractured aquifer as a dual-continuum domain, using both dual-porosity and dual-permeability formulations, and we develop a hybrid vertically integrated model using different vertically integrated approaches in the fracture and the matrix domains. The fracture domain has a high permeability and is likely to have rapid segregation of CO2 and brine; as such, the VE model is appropriate for the fractures. For the dual-porosity approach, flow in the matrix is represented only in the effective exchange term, but in dual-permeability approaches, flow in the matrix needs to be modeled explicitly. Because flow in the matrix is typically slow, the VE assumption is unlikely to be valid. Therefore, in the matrix domain we apply a dynamic reconstruction for which vertical equilibrium is not required. We do this for several different conceptualizations of the fracture-matrix system. Comparisons between the new hybrid model and a fully multi-dimensional model of the system show the accuracy and computational efficiency of the new approach.