Method to predict downhole reservoir fluids interfacial tension

A method, comprising:

• emplacing a downhole tool within a wellbore, wherein the downhole tool comprises a spectrometer having a dual measurement cell prism comprising first and second measurement cells having different cell lengths from each other, wherein the dual measurement cell prism comprises first and second sample windows having different lengths and widths from each other for sampling different fluid samples, and wherein the dual measurement cell prism comprises an optical guidance enhancement layer having a lower refractive index in a spectral region of interest than a base optical prism material of the dual measurement cell prism;
• sampling a fluid at a downhole location within the wellbore with the downhole tool;
• analyzing the fluid using the spectrometer;
• and
• calculating an interfacial tension (IFT) for the fluid according to the formula:
• γ=γHC+γAB
• wherein γAB is an acid-base IFT contribution and is equal to
• γAB=−RTΓm ln(1+KLC)
• wherein R is the gas constant, T is the temperature of a sample, Γm is the maximum adsorption at equilibrium, KL is the Langmuir adsorption equilibrium constant, and C is the bulk volume concentration of surface active species in the fluid, and γHC is a hydrocarbon fluid IFT contribution and is equal to
• (
• 1.58
• ⁢
• (
• ρ
• w
• -
• ρ
• h
• )
• +
• 1.76
• T
• r
• 0.3125
• )
• 4
• wherein ρw and ρh are the densities of water and hydrocarbon respectively, and Tr is the reduced temperature, wherein the reduced temperature Tr is a ratio of operating temperature to a pseudo-critical temperature of the water and hydrocarbon, and
• wherein calculating the acid-base IFT contribution comprises measuring the bulk volume concentration of the surface active species directly.