An investigation of the variation in the sweep and diffusion front displacement as a function of reservoir temperature and seepage velocity with implications in CO2 sequestration

Abstract

The steady accumulation of greenhouse gases resulting from the combustion of fossil fuels has led to an increase in the amount of solar radiation trapped between the atmosphere and earth. This increased radiation raises the temperature of the earth's atmosphere and ocean systems. It is believed that continuing increases in temperature will lead to catastrophic changes in weather conditions around the globe. With carbon dioxide (CO2) being the most abundant greenhouse gas, many efforts are underway to reduce the level of CO2 entering the atmosphere. One promising technology involves the sequestration of CO2 in deep geologic formations. CO2 is first separated from flue gas expelled by coal fired power plants, compressed to a supercritical phase (ScCO2), and injected into underground formations such as exhausted gas reservoirs and deep brine aquifers. Flue gas normally contains 10% to 15% CO2 by volume. It is believed that CO2 can remain permanently sequestered in such formations, depending on the chemical and mechanical characteristics of the underground resident water and rock constituents. However, uncertainties remain in several areas such as the chemical and physical effects of CO2 injection on subsurface rocks which may introduce unwanted side-effects that could hamper long-term sequestration, such as induced seismicity. Risk estimation of short- and long-term geologic storage of CO2 can only be addressed through numerical modeling and simulation. In this paper we examine a short-term side-effect of CO2 injection in which an acidic fluid region develops ahead of the main CO2-rich water injectant. It is shown that the length of this low pH region varies with injectant velocity and reservoir temperature. A leading fluid region of low pH could have an effect on the wetability of formation minerals and the capillarity of the moving effluent. An advancing low pH front may increase the wetability of mineral surfaces which would improve CO2 sequestration efficiency and aid in enhanced oil recovery operations.