Abstract
Natural or injection induced fractures, and abandoned wells with no complete seals are the main sources for CO2 leakage after sequestration. Based on research from various scientific studies, pilot CCS programs and commercial CCS projects, formations and/or cap rocks with fractures may have significant leakage problems thus degrade CO2 sequestration results. Modeling and Simulation of CO2 flow in fractured systems is crucially important for CO2 storage location selection and leakage risk evaluation.
With continuous improvement in fracture characterization, Discrete Fracture Modeling (DFM) is becoming more and more widely used as it can effectively resolve the fracture geometry with much less cells. Recent works from Gong et. al have presented successful application of DFM to large-scale compositional simulations for a gigantic carbonate reservoir. In this work, DFM work flow is applied for simulations on CO2 sequestration for the first time. Fractures are represented explicitly and individually as planar surfaces under an unstructured grid system. Transmissibility between each pair of adjacent cells is calculated and flow simulations are performed by a general connection-list based reservoir simulator.
Several examples including several 2D systems with or without fractures and a system with strong capillary pressure effects are demonstrated. Simulations on systems with fractures in CO2 injection formation versus in cap rocks are compared. Results have shown that the existence of mudstone layers could prevent injected CO2 from leaking outside the aquifer when no fractures are present. While vertical fractures intersecting with mudstone layers will cause significant leakage increase as the fractures form extremely preferential pathways for CO2 transport. Capillary forces also enable us to store CO2 in target aquifer for a long timeframe as it could help trap the residual phase CO2. In comparison with conventional simulators using structured grids, this work provides typical speedups of 3-10.
