Leakage detection and characterization through pressure monitoring

Abstract

Geological storage of carbon dioxide provides the possibility of maintaining access to fossil energy while reducing emissions of carbon dioxide to the atmosphere. One of the essential concerns in geologic storage is the risk of CO2 leakage from the storage formations. CO2 may leak through various pathways in the cap-rock overlying the storage aquifer. Characterization of the CO2 leakage pathways from the storage formations into overlying formations is required. We present a flow and pressure test to locate and characterize the leaks. The flow test is based on the injection (or production) of water into (or from) a storage aquifer at a constant rate. The pressure is measured at one or several monitoring wells in an aquifer overlying the storage aquifer, which is separated by an aquitard. The objective of the test is to locate and characterize any leakage through the separating aquitard. We present an inverse procedure to obtain the leakage pathway transmissibility and location, based on the pressure measurements in the presence of noise. A single monitoring well allows good determination of the leak magnitude but provides limited constraints on location. Adding a second monitoring well provides two-dimensional location of the leak location in the presence of noise/uncertainty in pressure measurements. It seems plausible that the use of multiple monitoring wells could enable cost-effective and sensitive detection of leakage over a large area. Unlike seismic imaging which only detects leakage when CO2 penetrates the leak, these methods are able to test for leaks before CO2 injection, or during injection but before the CO2 plume reaches the leak.