Interface Light Scattering Measurement of Low Interfacial Tension on a Gas Condensate System at High Pressure and Temperature
Authors: Lindeberg E.G.B. , Bjorkvik B.J.A. , Strand K.A. 
Published: December 2007

Abstract
A laser light scattering technique was used to measure simultaneously accurate values for the interfacial tension (IFT) and the sum of gas and liquid viscosities in a low IFT (&lt; 1 mN/m) real gas condensate system at high pressure (30-41 MPa) and high temperature (124 C) reservoir conditions. The measured IFT values were found to be three to five times as large as estimated values obtained by a pVT simulator using compositional data and the Weinaug-Katz correlation. thus demonstrating the need of good experimental IFT data on low IFT gas condensate systems.

Introduction
Accurate measurement of low interfacial tension (IFT) (&lt;1 mN/m) in oil/gas systems at high pressure and temperature reservoir conditions represents a considerable experimental challenge. Two techniques have proved successful for accurate IFT measurement under these conditions. i.e. the optical technique of interface laser-light scattering (ILLS) spectroscopy and the drop shape technique using pendant drops. Of the two the ILLS method is the one best suited for the low IFT range. partly because the laser beam represents a non-contact and low-perturbative probe, partly because the signal strength increases with decreasing IFT. Whereas the pendant drop technique has been used for accurate measurement of oil/gas IFT down to approximately 5.10-2 mN/m, the ILLS method has been used down to 10-3 mN/m, and has the capability of reaching even lower values (&lt;10-4 mN/m), as shown in measurements on microemulsion systems. (The only alternative technique in this IFT range, the drop shape technique using spinning drops, is not as convenient at elevated pressures and temperatures.)
The present paper demonstrates accurate ILLS measurement of low IFT on a real light brown gas condensate system (composition in Table 1) at high pressure (30-41 MPa) and high temperature (124 C) reservoir conditions. The ILLS instrument has been described in detail elsewhere, excepting the optical high-pressure cell, which was specially designed and constructed for the present work. (The high-pressure cell used previously for IFT measurement on a synthetic transparent gas condensate system, had not sufficient strength for the present pressure and temperature conditions).
The main difficulty of applying the ILLS method to real reservoir fluids is that such fluids in greater or less degree absorb light. When a laser beam of sufficient intensity is incident upon the surface of a photoabsorbing liquid. the laser can cause surface heating. This generally results in surface deformation and defocusing of the laser beam, which is seriously detrimental to ILLS measurement. A prime requirement for the successful ILLS measurement on a photoabsorbing system is therefore that the incident laser intensity be kept low.
This low intensity constraint has hitherto restricted ILLS measurement on photoabsorbing fluids to very small scattering angles (0.1). (The reason is that the intensity of light scattered at a fluid interface is very weak and diminishes rapidly in strength with increasing scattering angle.) The main disadvantage of working at such small scattering angles is that the experimental data have to be corrected for effects of instrumental resolution.
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