A Simple Approximate Semi-analytical Solution for Estimating Leakage of Carbon Dioxide Through Faults

Abstract

Abstract Assuring CO2 storage security is essential for the widespread implementation of carbon capture and sequestration. Appraising the potential for leakage through faults in seals is an important component of site screening, assessment, and selection. The focus of this study is to understand and quantify the potential rates of CO2 leakage via faults and fractures which could provide fluid migration pathways from the storage reservoir to overlying aquifers. Several analytical solutions exist for estimating rates of fluid migration between reservoirs via faults or leaky wells [1] , [2] , [3] , [4] , [5] , [6] , [7] , [8] . However, there is little focus on leakage up finite length faults. Here we present a new semi-analytical approximate solution for CO2 leakage through a finite length fault zone that relies on a derivation similar to that of calculating the single phase flow rate through a series of units. Under many conditions, this solution provides a good first order estimation of the amount of CO2 that leaks into the overlying aquifer relative to the amount of CO2 injected into the system with only basic knowledge of system geometry and permeability values. Detailed sensitivity analysis of simulation models was performed in order to understand which fault and reservoir parameters most strongly influence leakage rates of CO2 from storage reservoirs. Based on this analysis the three most important parameters were, in order of sensitivity, reservoir permeability, fault permeability and aquifer permeability. With these results, a semi- analytical approximation was developed which relies almost entirely on these permeabilities and the geometry of the system (ie. reservoir and aquifer height, fault thickness, etc.) While this solution does not incorporate multiphase fluid flow properties, it still provides a good approximation for CO2 leakage from a saline aquifer especially when the relative permeability characteristic curves result in mobility ratios near one for typical CO2 saturation values in the plume, which is common for Brooks-Corey relative permeability curves and viscosity ratios for supercritical CO2 and brine at reservoir conditions. Results from this semi- analytical solution are compared to over 50 different numerical models with different fault geometries and locations and a wide range of permeability values for the reservoir, fault and overlying aquifer. Overall, leakage predictions from the analytical solution compare very well with the numerical simulations. The approximation improves when faults are assumed to have no capillary pressure however many cases with capillary pressure are examined. Finally, the approximation is more accurate at lower leakage rates (leakage