Measurement and modelling of interfacial tension in methane/water and methane/brine systems at reservoir conditions

It is well established that the presence of salt in the aqueous phase raises the interfacial tension (IFT) of gas-water systems when compared to the case of pure water under the same pressure and temperature conditions. However, experimental data for gas-brine systems is still scarce, in particular at high pressure and high temperature (HPHT) conditions. In this communication, IFT data for methane with distilled water were experimentally determined using the pendant drop and bubble rise methods for temperatures ranging from (311-473) K and pressures up to 92 MPa and the results compared against literature data. Moreover, for the first time, the effect of NaCl on the IFT of the methane-water system was investigated in solutions with a maximum salinity of 10 wt% at HPHT conditions.The Cubic-Plus-Association equation of state (CPA EoS) along with the Debye-Hückel activity model was used to describe the effect of salt in the phase behaviour of the methane-water system and bulk properties used to compute the IFT of the studied systems with the Density Gradient Theory (DGT). The modelling results showed that by using only bulk phase properties and one temperature independent binary interaction parameter adjusted to the methane-water system, the DGT was able to predict the impact of NaCl on the IFT with remarkably low deviations from measured values. Furthermore, the impact of the fluids microstructure and the distribution of molecules across the interface on the IFT were evaluated with the DGT.