Simplified Model for CO2 Leakage and its Attenuation due to Geological Structures

For risk associated with storage of CO2 under the Earth's surface, the impervious cap is one of the most significant factors. Geological structures such as faults can provide conduits for CO2 to escape through the cap. When conductive faults intersect the storage formation and overlying permeable layers, CO2 leaks may exhibit three distinct behaviors: initial upward migration of fluids through the fault, lateral fluid movement through permeable layers, and continued movement of CO2 along the fault above the leakage pathways. To quantify this behavior, especially the attenuation, we develop a quasi-1D model for migration of buoyant fluid from a reservoir along a conductive fault. The fault can intersect multiple shallower formations. The model accounts for flow from the fault into a permeable formation using the concept of leakoff. The leakoff coefficient depends on the geometry and on the petrophysical properties of the formation. We use a commercial simulator (GEM from CMG) to run 2D verification studies of the 1D model. We present a series of examples that illustrate the controlling mechanisms for leakage rate from the reservoir and its attenuation by flux into shallower layers. Leakage flux and its attenuation vary nonlinearly with the permeability of the fault and the permeability of the shallower layers intersected by the fault. Permeable layers near the CO2 storage reservoir exhibit the greatest attenuation. While there is a nonlinear relationship between flux through the fault and the size of the leakoff coefficient, there is a linear relationship between the percentage which leaks off into neighboring formations and the ratio of fault permeability to leakoff coefficient. The rate of attenuation of CO2 leakage depends on geometric and/or petrophysical properties of highly permeable structures as well as reservoir properties. An analytical derivation of the leak-off coefficient based on Darcy's equation compares favorably with 2D simulations. On the other hand, in the case of tilted permeable layers, calculation of the leakoff coefficient is less straightforward due to preferential flow within the layer (lighter CO2 rising above heavier brine). © Elsevier Ltd. All rights reserved