Analytical model for CO₂ geo-sequestration in deep saline aquifers considering the effect of heterogeneity and matrix-fracture interaction

Deep saline aquifers are considered to be the best storage option for long-term CO₂ geo-sequestration because such geological formation has the largest identified storage potential that can securely trap CO₂ under its low-permeability caprock layers. Many analytical models have been proposed to rapidly detect the CO₂ leakage from the storage aquifers. However, the heterogeneity and matrix-fracture interaction of storage layers have not been taken into consideration in previous studies. In this paper, we develop an analytical model to determine the dimensionless leakage rate and pressure build-up due to CO₂ leakage from the heterogeneous, dual-porosity and multi-layer bounded storage aquifer. Based on the model description and assumptions, the analytical solution is obtained by means of dimensionless transformation, Laplace transform and numerical inversion. Afterwards, our model is verified by comparing with existing analytical model. In order to determine the effect of heterogeneity and the matrix-fracture interaction on the dimensionless leakage rate and pressure change, we conducted sensitivity analysis taking account of the heterogeneous factors. Based on the obtained results, increasing layer number and layer thickness from the bottom layer to the upper layer will delay the CO₂ leakage. Furthermore, each layer in storage aquifer with greater fluid mobility is not conductive to CO₂ storage but the overall effect is not obvious. As for the matrix-fracture fluid transfer processes, when the ratio of the matrix permeability to the fracture permeability is small, the fast flow of CO₂ through the fracture system without significant storage in the matrix. Obviously, the resistance along the leakage path is decreasing with the decrease of the caprock thickness, which leads to greater leakage between the two aquifers. The derived analytical solutions considering heterogeneity and dual media is practical and helpful for CO₂ geo-sequestration site selection and storage risk assessment.