Dependence on Temperature, Pressure, and Salinity of the IFT and Relative Permeability Displacement Characteristics of CO2 Injected in Deep Saline Aquifers

Abstract

Abstract Sequestration of CO2 in deep saline aquifers is a means with great potential for reducing emissions of this greenhouse gas produced from a wide range of industrial operations. The displacement characteristics of CO2 injected into deep saline aquifers are essential in that they control both the migration of CO2 and the available pore space in the reservoir at irreducible saturation conditions, yet very little information exists about these characteristics. This paper presents continuing results of a research study conducted by the authors to investigate the displacement characteristics of CO2-brine systems at reservoir conditions for cases specific to the Alberta basin and representative for intracratonic and foreland basins in North America. More specifically, the work reported here focuses on a controlled series of tests in which the effects of temperature, pressure and brine salinity were varied in a systematic manner to identify and quantify their effect on the interfacial tension between CO2 and equilibrium brine. The results indicate that the CO2-brine IFT decreases with increasing pressure, while increasing temperature and brine salinity have an opposite effect. These results were then extended under similar controlled conditions to consolidated porous media to investigate the effect of interfacial tension, as impacted by specific reservoir conditions of temperature, pressure and salinity, on the relative permeability character of CO2 displacing brine for a number of different interfacial tension conditions. The results are presented, and the significance with respect to the feasibility and optimization of CO2 sequestration in deep saline aquifers is discussed. Introduction Atmospheric concentrations of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) have increased since the beginning of the industrial revolution in the mid 19th century, and evidence supports the supposition that these increasing concentrations are affecting the Earth's climate1. This increase is due to deforestation, agricultural practices and mainly to the use of fossil fuels for power generation and transportation. The high use of fossil fuels will continue this century2, 3, thus, a major challenge in mitigating climate change effects is the reduction of CO2 emissions to the atmosphere. Among various mitigation measures, CO2 capture and geological storage (CCGS) will play an important role in reducing CO2 atmospheric emissions3–5. The technology for the injection of CO2 into deep underground formations is well developed and currently practiced mainly by the energy and petrochemical industries for enhanced oil recovery (EOR)6 and acid gas (CO2 and H2S) disposal7. Carbon dioxide can also be sequestered in various geological media, of which deep saline aquifers have the largest storage capacity and widest distribution, but oil reservoirs are the most likely to be used preferentially initially because of the positive economic potential for increased oil production through CO2 or acid gas based EOR4,6schemes. In both cases, the basic phenomenon is the displacement of a fluid (water and/or oil) by CO2 during injection and the displacement of CO2 by either water or oil after cessation of injection by influx of fluids from adjacent aquifers or hydrocarbon sources. In the case of CO2 being displaced, a significant amount of CO2 may be stored in the pore space8once the mobile oil/water phase has been displaced to its irreducible saturation. Even in oil and gas producing regions, such as Alberta in Canada and Texas in the U.S.A., it may be more economic in some cases to inject CO2 into a deep saline aquifer that underlies a major CO2 source than to transport it to oil and/or gas reservoirs some distance away, particularly if these are still producing and not available or suited for CO2 storage or injection.