Interpretation of above-zone pressure influence time to characterize CO2 leakage

Pressure transient analysis has long been used to characterize underground geological formations. Pressure is monitored for evaluating the containment of injected CO2 in the target formations in CO2 geological storage. Pressure interference testing in a permeable zone overlying the storage zone can determine migration of CO2 from the target reservoir. This study focuses on interpretation of interference tests in the above-zone to obtain information on the plume size from pressure influence time. We define a conceptual model with idealization of the CO2 leakage scenario into a 3-region linear composite system. For each region of this system, the governing pressure diffusivity equations and corresponding initial and boundary conditions are presented. An analytical solution is derived by sequentially transforming the equation system into Laplace and Fourier domains. The analytical solution is verified by comparing to numerical simulation results and a limiting analytical solution. Next, the ability of the analytical solution to predict the pressure influence time for an interference test is evaluated. It is shown that the pressure influence time is independent of the plume shape and depends only on the plume size on the line connecting the two interference test wells. This suggests that the measured pressure influence time can be inverted using our analytical solution to obtain the leaked plume size. The influence time is used for an example field application to obtain the plume volume percentage. (c) 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.