The Effect of Interfacial Tension Upon Gas-Oil Relative Permeability Measurements: Interpretation Using Pore-Scale Models

Abstract

The effects of interfacial tension (IFT) upon gas-oil relative permeabilities are of considerable interest in the reservoir engineering of gas flooding processes. Several papers have presented experimental data on this matter, although there is some conflict among the findings of different authors. This paper describes the development and implementation of an unsteady-state pore-scale process simulator, which has been utilised to interpret a number of related laboratory gas-oil displacements performed at different values of interfacial tension. The experimental work was carried out at this laboratory and the range of IFTs considered covered almost three orders of magnitude - from 9.76mN/m down to 0.019mN/m. However, more importantly, all of the experimental data was available to the authors including the oil/gas viscosity ratio, M (= oil/ gas), as a function of IFT. This proved to be essential in the modelling and interpretation of the experimental results and it has rarely been reported by previous experimentalists. Both constant rate and constant pressure drop simulations were carried out, using pore-scale models that had been suitably anchored to the experimental samples. The resulting production data were analysed using a modified JBN technique and the calculated relative permeabilities were found to exhibit the same IFT sensitivities as the experimental datasets - viz. a marked increase in the gas curve and very little change in the oil curve with decreasing IFT. This behaviour is subsequently explained by considering both the viscous/capillary force balance and the viscosity ratio operating during each displacement. These vary considerably over the experimental range considered, with different displacements effectively lying in different flow regimes. In addition, results demonstrate that it is extremely difficult to predict a priori the directional trend exhibited by relative permeability curves with varying IFT, unless all of the experimental quantities are reported. The fact that both capillary number and viscosity ratio play a role in determining the phase distributions during a displacement means that such trends can only be properly understood in light of appropriately-scaled network simulations that incorporate the underlying pore-scale physics. Introduction Variations in gas-oil relative permeability as a function of interfacial tension (IFT) are of particular importance in the area of compositional simulation, where oil and gas compositions can vary significantly both spatially and temporally. This is especially significant in the context of volatile oil and gas condensate reservoirs, where phase compositions do not generally pass through the critical point and miscibility is consequently avoided. At present, many standard compositional simulators utilise a model first proposed by Coats, which employs simple linear interpolation to modify the relative permeability curves at different values of IFT. The interpolated relative permeabilities take the form: (1) P. 791^

Modeling Relative Permeability Effects in Gas-Condensate Reservoirs With a New Trapping Model

“…The gas relative permeability is a function of the interfacial tension ͑IFT͒ between the gas and condensate among other variables. For this reason, several laboratory studies [3][4][5][6][7][8][9][10][11][12][13][14] have been reported on the measurement of relative permeabilities of gas-condensate fluids as a function of interfacial tension. These studies show a significant increase in the relative permeability of the gas as the interfacial tension between the gas and condensate decreases.…”

Relative Permeability Normalization – Effects of Permeability, Wettability and Interfacial Tension

“…As a general observation, as the system approaches the miscibility (i.e., IFT approches zero), relative permeabilities increase and their curvature reduces. Some researchers [Bardon and Longeron (1980)] concluded that the non-wetting phase relative permeability, increases only slightly as the IFT is reduced while others [Harbert (1983), McDougall et al (1997), Henderson et al (1997Henderson et al ( ) (1998] concluded that the wetting phase relative permeability is less affected than non-wetting phase relative permeability or sometimes remains completely unaffected by IFT reduction. We used separate values of nl for gas and oil in the weight factors and for calculation of oil and gas relative permeability, respectively.…”

Effect of IFT Variation and Wettability on Three-Phase Relative Permeability

“…Because the IFT between oil and displacing fluid is an important parameter for most enhanced oil recovery techniques, there has been much interest in the effect of IFT on oil and displacing fluid relative permeabilities [1][2][3][4][5][6][7][8][9][10][11][12][13] . It has been shown experimentally that residual oil and relative permeability are strongly affected by the variations in IFT.…”