Stability determination of asphaltenes in oil fields, comprises use of calibrated equation of state and characterization of asphaltenes to verify stability of asphaltenes in relation to temperature and pressure of live oil

摘要

Determining the stability of asphaltenes comprises titrations of stock tank oil, with use of the values determined, and the physico-chemical analyses of the oil to thermodynamically characterize the asphaltenes and heavy oil fraction. The experimental data relating to the phase behavior of the live oil are interpolated to give an improved representation by means of an equation of state, and the stability is verified in relation to the temperature and pressure of live oil. Determining the stability of asphaltenes in oil fields comprises: (a) carrying out titrations of stock tank oil, diluted with good solvents, with 2-20C aliphatic hydrocarbons, to determine the precipitation threshold of the asphaltenes, at a temperature ranging from 20[deg]C to the field temperature, preferably the field temperature, and subsequently, by the use of equations (6), (2) and (3) (referring to stock tank oil), obtaining delta sto(T) and delta a(T); (b) obtaining, from the physico-chemical analyses of the stock tank oil and delta sto(T), the boiling point of the residue (Tbp) by means of an equation of state; (c) interpolating, from the Tbp of the residue and from physico-chemical analyses of the live oil, the experimental data relating to the phase behavior of the live oil, in order to improve the representation of the live oil by the above equation of state; (d) using the equation of state with the parameters in step (c) to determine vlo and delta lo values of the live oil under different T and P parameters of interest; (e) evaluating the x parameter for every condition, using: (i) delta a referring to the temperature T, this value being equal to that determined in step (a) of the measurement has been effected at the temperature T or, if the measurement in (a) has been effected at a different temperature, calculated from that obtained in (a) by means of equation (4); and (ii) vlo and delta lo obtained in (d), these parameters being used in equation (1); and (f) correlating the stability of the asphaltenes to the x parameter, whereby the asphaltenes are stable when x 0.5 and unstable when x 0.5. (vs/RT)(delta a - delta s)2 = x (I) vs = voxo + vgsxgs + vpxp (2) delta s = delta ophi o + delta gsphi gs + delta pphi p (3) delta a(T) = delta a(To) exp[-9.1 . 10-4 (T-To) (4) (vs/RT)((delta a - delta s)2 = 0.5 (6) In equations (1)-(6): delta s = solubility parameter of the solvent mixture, i.e. of everything but asphaltenes; delta a = solubility parameter of the asphaltenes; delta o = solubility parameter of the oil (delta sto for stock tank oil and delta lo for live oil); delta p = solubility parameter of the paraffin used in the titration; delta gs = solubility parameter of the good solvent optionally added to the oil; x = the asphaltene-solvent mixture interaction parameter; vs = molar volume of the solvent mixture; vo = molar volume of the oil (vsto for stock tank oil and vlo for live oil); vp = molar volume of the paraffin used in the titration; vgs = molar volume of the good solvent optionally added to the oil; R = universal gas constant; T = temperature in Kelvin; xo = molar fraction of the stock oil under threshold conditions; xgs = molar fraction of the good solvent under threshold conditions; xp = molar fraction of the paraffin under threshold conditions; phi o = volumetric fraction of the oil under threshold conditions; phi p = volumetric fraction of the paraffin under threshold conditions; phi gs = volumetric fraction of the good solvent under threshold conditions; and To = reference temperature, coinciding with the measurement temperature.