Dissipation of Overpressure into Ambient Mudrocks During Geological Carbon Dioxide Storage

Abstract

Geological carbon dioxide (CO2) storage in deep geological formations can only lead to significant reductions in anthropogenic CO2 emissions if large amounts of CO2 can be stored. Estimates of the storage capacity are therefore essential to the evaluation of individual storage sites as well as the feasibility of the technology as a whole. One important limitation on the storage capacity is the lateral extent of the pressure perturbation, the radius of review of the storage project. We show that pressure dissipation into ambient mudrocks retards lateral pressure propagation significantly and therefore increases the storage capacity. For a three-layer model of an aquifer surrounded by thick mudrocks the far-field pressure is well approximated by a single-phase model. Through dimensional analysis and numerical simulations we show that the lateral pressure propagation follows a power-law that depends on a single parameter M ∼ log10(RkRSR12), where Rk and RS are the ratios of mudrock to reservoir permeability and storativity and Rl is the aspect ratio of the confined pressure plume. Both the coefficient and the exponent of the power-law are sigmoid decreasing functions of M. The M-values of typical geological storage sites are in the region where the power-law is changing rapidly. The combinations of large uncertainty in mudrock properties and the sigmoid shape lead to wide and strongly skewed probability distributions for the predicted radius of review. Therefore, the determination of the mudrock properties is an important component of the site characterization, if pressure dissipation has the potential significantly increase storage capacity. After injection the pressure will continue to diffuse and the radius of review may continue to increase, which will emphasize the significance of monitoring post-injection overpressure to secure the stability of the storage formation.