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. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.
CO2 geological storage is a promising method to cut CO2 atmospheric emissions. Determining the extent of the injected CO2 plume within a target storage formation has direct implications for the safety of the CO2 storage project as it determines the area of CO2 exposure. In this study, a pressure interference test is introduced to characterize the CO2 plume in the reservoir. For a given CO2 plume, water is injected at the injection well and pressure interference signal (and its arrival time) is obtained at several observation wells inside/outside of the plume. We utilize an analytical expression to determine multi-phase diffusivity coefficient from numerical simulation results. The relationship between travel time and diffusivity coefficient is expressed as a line integral to obtain the pressure arrival time. We introduce a method to invert the arrival time in the synthetic pressure interference data in order to estimate the average gas saturation. It is shown that when inverting the arrival time from the numerical simulation to find the average CO2-rich (gaseous) phase saturation, the percent error was high, but the actual change in gaseous saturation was small, showing strong potential to be applied to the field. Complexities were added to the system such as anisotropy and heterogeneity in two- and three-dimensional systems. Observation wells were located co-linear to the injection well, so anisotropy had minimal effect on the permeability used for calculations.
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