Interfacial tension between water and pentane saturated with methane

The plugging of natural gas pipelines caused by hydrate formation has been recognized as a problem in the oil and gas industry for approximately 90 years. Interfacial tension is an important physical property to understand the dynamics of multiphase flow, namely, natural gas, condensate, and water, inside the pipelines. This paper describes our measurements of the interfacial tension between water and pentane saturated with methane as a model for the fluids in the pipelines transporting gas and condensate. The measurements were conducted by the pendant drop method in the temperature range between 278.2 K and 293.2 K, and the methane pressure range between 1.1 MPa and 13.7 MPa. The uncertainty of the measurements is typically below 1.5 mN center dot m- 1. The interfacial tension decreased with increasing temperature at 2 MPa. As for the pressure dependence at 288.2 K, with increasing pressure, the interfacial tension decreased from 49.12 mN center dot m- 1 at 1.1 MPa to 47.64 mN center dot m- 1 at 4.0 MPa. The interfacial tension remained roughly constant until 8.7 MPa with the value being 48.02 mN center dot m- 1 and then increased to 59.81 mN center dot m- 1 at 12.9 MPa. The decrease of the interfacial tension between 1.1 MPa and 4.0 MPa could be attributed to the increase of the methane molecule adsorption at the interface. In the pressure range between 4.0 MPa and 8.7 MPa, it is likely that the methane adsorption at the water - pentane interface became saturated. The increase of the interfacial tension above 8.7 MPa is a novel phenomenon which requires further investigation.