Pressure falloff testing to characterize CO2 plume and dry-out zone during CO2 injection in saline aquifers

The fluid mobilities and the lateral extent of the CO2 plume and the dry-out zone are among important unknowns required to effectively manage CO2 storage in saline aquifers projects. These unknowns can be derived from pressure falloff testing which has been widely used to characterize the subsurface. This work provides a three-region analytical solution for falloff pressure during the infinite-acting period of CO2 injection in saline aquifers. The solution is derived using Laplace transformation technique of the governing diffusivity equations and the accompanying initial and boundary conditions. The exact solution is validated against numerical results obtained from a black-oil reservoir simulator for two cases. The solution is approximated into simple forms in real-time domain considering different time periods. Then, a graphical semi-log analysis technique based on the approximate solutions is provided as an interpretation tool of falloff tests where the fluid mobilities and the extents of dry-out zone and CO2 plume can be inferred. The interpretation technique is applied to synthetic falloff test data to investigate its potential and reliability.