Semianalytical solution for CO2 plume shape and pressure evolution during CO2 injection in deep saline formations

Abstract

The injection of supercritical carbon dioxide (CO2) in deep saline aquifers leads to the formation of a CO2 rich phase plume that tends to float over the resident brine. As pressure builds up, CO2 density will increase because of its high compressibility. Current analytical solutions do not account for CO2 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 CO2 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 CO2 plume geometry and fluid pressure evolution, accounting for CO2 compressibility and buoyancy effects in the injection well, so CO2 is not uniformly injected along the aquifer thickness. We formulate the problem in terms of a CO2 potential that facilitates solution in horizontal layers, with which we discretize the aquifer. Capillary pressure is considered at the interface between the CO2 rich phase and the aqueous phase. When a prescribed CO2 mass flow rate is injected, CO2 advances initially through the top portion of the aquifer. As CO2 is being injected, the CO2 plume advances not only laterally, but also vertically downwards. However, the CO2 plume does not necessarily occupy the whole thickness of the aquifer. We found that even in the cases in which the CO2 plume reaches the bottom of the aquifer, most of the injected CO2 enters the aquifer through the layers at the top. Both CO2 plume position and fluid pressure compare well with numerical simulations. This solution permits quick evaluations of the CO2 plume position and fluid pressure distribution when injecting supercritical CO2 in a deep saline aquifer.