Correlations for the Interfacial Tension Between Supercritical Phase CO2 and Equilibrium Brines at In Situ Conditions

Abstract

Abstract The modeling of CO2 sequestration in saline aquifers is becoming increasingly important as this method emerges as the prime technology available for the disposal of large volumes of anthropogenically-generated CO2, thus reducing atmospheric CO2 emissions. The interfacial tension between the saline brine in the aquifer and the injected CO2 phase has a strong effect on the capillary pressure and relative permeability characteristics of the CO2-brine displacement, and proper understanding of the IFT level is necessary for accurate modeling and evaluation of such a process. This paper provides a summary of 168 brine-CO2 interfacial tension measurements conducted using a drop pendant interfacial tension apparatus at temperatures ranging from 41 to 125°C, pressures from 2,000 to 27,000 kPag and salinities from 0 ppm (distilled water) to over 334,000 ppm. The dataset was regressed to develop an empirical correlation to predict the brine-CO2 IFT over the range of conditions evaluated in this work. The correlation, including temperature, pressure and salinity dependence, fits the measured data with an overall regression coefficient in excess of 0.94. In addition, a very strong relationship was found between computed gaswater ratio (dissolved CO2) and interfacial tension. An additional correlation was developed to model this effect, also with a high regression coefficient of 0.92. Since the gas-water ratio is a much easier parameter to measure than IFT, this provides a rapid and inexpensive method to estimate in-situ IFT from available or CO2 solubility data. Introduction Interpretation of the historical temperature record on a scale of centuries to millennia indicates a slight increase in global average annual temperatures in the last 150 years, in the order of 0.76ºC, and predictions are that if this trend continues unabated, humankind is facing significant climate change by the end of this century, resulting in an average increase in global temperature in the 1.1–6.3ºC range depending on the greenhouse gas emission scenario considered.1 It is generally accepted that the main cause of the observed global warming is the increase in atmospheric concentrations of greenhouse gases, such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).1 Of all the greenhouse gases, CO2, whose atmospheric concentrations have risen from pre-industrial levels of 280 ppm to 380 ppm in 2005, is the most important greenhouse gas, believed to be responsible for approximately two thirds of the enhanced 'greenhouse effect'.1,2 A major challenge in mitigating climate change effects is the reduction of CO2 emissions to the atmosphere, which hopefully will lead to a stabilization of CO2 concentration to no more than double the pre-industrial level at around 550 ppm (i.e., double the preindustrial level), for which a corresponding average global warming of between 2 and 4.5°C is likely.1 In the broad portfolio of measures and actions that are envisaged for reducing anthropogenic CO2 emissions into the atmosphere, CO2 Capture and Storage (CCS), which entails CO2 capture from large industrial processes, mainly power generation based on fossil fuels, and injection into deep geological formations, plays an important role.3,4