Analytical models for determining pressure change in an overlying aquifer due to leakage

Abstract

Abstract Various methodologies are proposed to reduce CO2 emissions that are believed to be the main drivers of the climate change. CO2 capture and storage in deep underground formations is one of the promising methods that allow reducing the emissions while continuing the use of fossil fuels. Injection of immense quantities of CO2 is required to make a reasonable cut of the emissions. Deep saline aquifers can provide the capacity to accommodate the storage of such huge amounts of CO2. However, one of main challenges in deployment of CO2 storage is the risk of CO2 leakage through pathways in the cap-rock overlying the target aquifer. The sealing capacity of the cap-rock must be evaluated to ensure the safety of the storage. Therefore, characterization of the cap-rock is required to find the potential leakage pathways even before the CO2 storage begins. Methods to characterize the leakage pathways are proposed at two different scales: 1- By point sampling of the cap-rock and testing the potential pathways such as abandoned wells, 2- by analysing geophysical (e.g. 3-D seismic) data to estimate paths of upward migration of the injected CO2. Flow based methods have the potential for bridging the large gap that exists between the length scale of these two approaches. The aquifer could be tested for the leakage pathways before CO2 storage. This will allow finding proper storage aquifers and locations for the injection wells. In this work we present an analytical model to evaluate the pressure variation in the overlying aquifers due to leakage from the storage aquifer. In a companion paper, this model will be used along with an inverse modelling approach to locate and characterize the leakage pathways based on pressure data. This paper introduces two new analytical solutions: 1- Exact solutions for the pressure variation in an overlying aquifer due to leakage (obtained in Laplace-transformation domain), 2- time-domain approximations for the exact solutions to make the inversion possible. In deriving the analytical solutions two aquifers are considered: Storage and monitoring. The aquifers are separated by an aquitard and are in communication through a leakage pathway. In departure from previous works the leakage pathways are not required to be line source/sink. Such consideration allows incorporation of large pathways such as stratigraphic and structural heterogeneities in the cap-rock. We consider a single-phase 1-D radial flow system in the storage and monitoring aquifers. Both of the aquifers are considered as homogeneous, isotropic, and infinite-acting with constant thickness. The injection (or production) rate is taken as constant. The analytical solution are applied to a base case and corroborated versus numerical solution.