Experimental investigation of interfacial tension in two-phase system involving methane and water

The understanding of the physical properties of reservoir fluids such as viscous and interfacial forces is critical in many oil and gas, petrochemical, and other related industries. In oil and gas industry, the production, transportation and processing of hydrocarbon fluids involve dealing with the multiphase system. Viscosity and interfacial tension (IFT) are among the fluid properties that affect fluid behaviour. These properties have significant effects on fluid flow characteristics, and thus their importance in oil and gas production and processing aspects from the reservoir to other surface facilities. Hence accurate determination of these fluid properties plays a vital role in predicting multiphase flow and managing associated problems faced by the industry such as hydrates, scale and corrosion. Surfactants are added along the line, to prevent hydrate formation, by depressing the formation temperature, and transport it as a hydrate slurry when formed to avoid pipe blockages. While significant research has been conducted in reservoir conditions, but Pipeline system, the presence of salt and polymer-surfactants which tend to alter the IFT behaviour has not been well studied. This study investigates methane/water interfacial tension experimentally in the presence of salt (NaCl) and polymer-surfactants (PEG 8000-SDS) at operating conditions suitable for upstream and midstream industry application. Also, experimental investigation of pH, resistivity, conductivity, viscosity and density of the liquid phase (brine + polymer-surfactant) was conducted to characterises the liquid phase. The results show that the electrical resistivity and conductivity depend significantly on the ionic concentration of NaCl presents. Resistivity decreases with increasing NaCl concentration from 0.22 Ω-m at 2.9 wt% NaCl to 0.075 Ω-m at 10.7wt% NaCl, which shows the possibility of corroding the pipe in the case of the pipeline system. The electrical conductivity measured shows an increase with increasing the concentration of NaCl from 4.55 to 13.33 S/m at 2.9 wt% and 10.7 wt% NaCl respectively. This increase in conductivity affirmed the strength of ionic concentration in the system, which could cause scaling and corrosion in the system. High electrical conductivity in the system also signifies a potentially harmful accumulation of solids in cooling towers. Conductivity was also investigated as a function of temperature. The results show that in all the concentration of NaCl studied, increasing temperature (298.15, 303.15, 308.15 and 313.15 K) led to a corresponding average increase in 0.74 S/m in the values of conductivity which shows an increase in ionisation. Results obtained also show that NaCl concentration affects the pH of the solution. The pH value was observed to increase from 6.5 to ≈ 7.0 at 8.2 wt% NaCl and 40 wt% PEG 8000 and decreases at 10.7 wt% NaCl at same PEG 8000 concentration to ≈ 6.9. Knowing the electrical resistivity, conductivity and pH of the aqueous solutions, it is critical to accurately determine the viscosity of the solution as it determines the fluid flow and much other application. The viscosity of PEG 8000 and SDS solution were presented at 298.15 – 313.15 K. The shear stress obtained at 10 – 40 wt% PEG 8000 was found to increased from 0.14 Pa at 28.1 s-1 to 7.6 Pa at 340.5 s-1 indicating Newtonian fluid behaviour. While the apparent viscosity curve obtained, show that the viscosity increased from 3.7 cP at 10 wt% PEG 8000 to 58.1 cP at 40 wt% PEG 8000. However, at each concentration, the viscosity was found to be decreasing with an increase in shear rate. Results on interfacial tension at two-phase involving methane and water shows that the IFT is a function of pressure, temperature, salinity, and surfactant/polymer. Both increase in pressure and temperature decreases the IFT working at CH4-H2O. Presences of 2.9, 5.6, 8.2 and 10.7 wt% NaCl resulted in an average rise of 1.46, 2.57, 3.51 and 4.24 mN.m-1 respectively, in IFT. This increase in IFT due to presence of NaCl signifies the formation of corrosion in to the pipeline. Further, active presence of PEG 8000 and SDS reduces the IFT, with critical micelle-like behaviour observed at 30 and 40 wt% PEG 8000 and with more significant effect with the addition of 0.5 wt% SDS. Therefore, these percentage could be a good combination in hydrate prevention along the pipeline. Some of the results obtained were evaluated against independent experimental data gathered from open sources to validate the method and IFT data at the methane-water interface as a function of PEG 8000, and PEG 8000 + SDS were here reported for the first time.