Semianalytical Solution for CO_2 Plume Shape and Pressure Evolution During CO_2 Injection in Deep Saline Formations

摘要

The injection of supercritical carbon dioxide (CO_2) in deep saline aquifers leads to the formation of a CO_2 rich phase plume that tends to float over the resident brine. As pressure builds up, CO_2 density will increase because of its high compressibility. Current analytical solutions do not account for CO_2 compressibility and consider a volumetric injection rate that is uniformly distributed along the whole thickness of the aquifer, which is unrealistic. Furthermore, the slope of the CO_2 pressure with respect to the logarithm of distance obtained from these solutions differs from that of numerical solutions. We develop a semianalytical solution for the CO_2 plume geometry and fluid pressure evolution, accounting for CO_2 compressibility and buoyancy effects in the injection well, so CO_2 is not uniformly injected along the aquifer thickness. We formulate the problem in terms of a CO_2 potential that facilitates solution in horizontal layers, with which we discretize the aquifer. Capillary pressure is considered at the interface between the CO_2 rich phase and the aqueous phase. When a prescribed CO_2 mass flow rate is injected, CO_2 advances initially through the top portion of the aquifer. As CO_2 is being injected, the CO_2 plume advances not only laterally, but also vertically downwards. However, the CO_2 plume does not necessarily occupy the whole thickness of the aquifer. We found that even in the cases in which the CO_2 plume reaches the bottom of the aquifer, most of the injected CO_2 enters the aquifer through the layers at the top. Both CO_2 plume position and fluid pressure compare well with numerical simulations. This solution permits quick evaluations of the CO_2 plume position and fluid pressure distribution when injecting supercritical CO_2 in a deep saline aquifer.