Interfacial Tension Data and Correlations of Brine-CO2 Systems under Reservoir Conditions

Abstract

Abstract It has been long recognized that interfacial interactions (interfacial tension, wettability, capillarity and interfacial mass transfer) govern fluid distribution and behavior in porous media. Therefore the interfacial interactions between CO2, brine and reservoir oil and/or gas should have an important influence on the effectiveness of any CO2 storage operation. As a model, the interfacial tension of the pure water-CO2 system has been studied intensively. Nevertheless, to our knowledge, no interfacial tension (IFT) equilibrium data for brine-CO2 systems are available at reservoir conditions for different salinities, temperatures and pressures. In this paper, we present experimental IFT brine-CO2data obtained at high pressures (45 to 255 bar), high temperatures (27 to 100°C) and different salt concentrations (5,000 to 150,000 ppm of NaCl) using the axi-symmetric drop shape analysis technique (ADSA) for a rising drop case. Special attention was paid in developing a procedure to achieve true thermodynamic equilibrium. The themodynamic conditions were selected in order to cover the most practical CO2 storage cases of interest, liquid and supercritical CO2. A correlation was developped on the basis of the Parachor model, the salt effect and a regression fit of more than a hundred IFT experimental values obtained in this study. This correlation yields a Brine-CO2 IFT prediction at reservoir conditions with a mean absolute deviation of 2.5%. We also present correlations to determine the IFT increase due to salt concentration. The existence of a plateau in the brine-CO2 IFT values, independent of the temperature and the pressure and only dependent on the salt concentration, has been demonstrated from the experimental data for temperatures between 27 to 71°C and pressures above 150 bar. These pressure and temperature values can be easily found in many geological sites considered as prospects for CO2 storage. The linear dependency of the IFT increase with molal NaCl concentration has also been demonstrated. Introduction Large-scale subsurface storage of anthropogenic carbon dioxide is considered as a potential technology for greatly stabilizing greenhouse gas concentration in the atmosphere.1 At least three options exist for geological storage of CO22: oil and gas reservoirs, deep saline aquifers and unmineable coal beds. Because of the expertise and knowledge of many geological sites considered as prospects for CO2 storage, the oil industry is uniquely positioned to sequester CO2, no matter the source. Successful CO2 sequestration in deep saline aquifers and different types of hydrocarbon reservoirs is largely governed by the fluid-fluid and fluid-rock interfacial interactions. One of these fluid-fluid interactions is the interfacial tension. The interfacial tension of water-hydrocarbon systems is required in many reservoir engineering studies. The quality of the interfacial tension data used in these studies is particularly important in CO2 storage. On the one hand it highly influences the flow process and on the other hand it controls the capillary-sealing efficiency (see Equation 1) ..................... (1) where p.. is the threshold capillary pressure in the saturated brine caprock, which characterizes the ability of a porous medium saturated with a wetting phase (brine) to flow of a non-wetting phase (CO2); R is the largest connected pore throat in the caprock and ? is the contact angle. Because there is not an agreement concerning the use of the interfacial tension or surface tension terms under reservoir conditions, a preliminary clarification of vocabulary should be first made. Throughout this paper we use the term "surface tension" only for the case of pure compounds and gas/liquid systems at ambient conditions and the term "interfacial tension" in all other cases.