Patent ID: 9568459
Date: 2017-02-14
CPC Classifications: E21B,G01N,G01V

Claim:
1. A method for determining asphaltene onset pressure of a formation fluid, the method comprising: (a) transmitting light through a sample of the formation fluid; (b) decreasing pressure of the formation fluid sample, wherein the rate of depressurization is less than 100 psi/second (c) detecting intensity of the transmitted light during depressurization; (d) identifying a change in intensity of the transmitted light during depressurization representative of asphaltene flocculation; (e) increasing pressure of the formation fluid sample to a fixed pressure; (f) detecting intensity of the transmitted light at the fixed pressure; maintaining the pressure of the formation fluid sample at the fixed pressure until the detected intensity of the transmitted light reaches a steady state; determining an equilibrium value by doing one of the following: allow the detected intensity of the transmitted light to reach a steady state and then identify the steady state value of the detected intensity of the transmitted light as the equilibrium value, or extrapolate the steady state value by applying a fit to the detected intensity of the transmitted light at the fixed pressure as function of time; (g) repeating processes (a) to (f) for a plurality of different fixed pressures; and (h) determining the asphaltene onset pressure of the fluid sample using (i) the intensity of the transmitted light from each process (c) and (ii) the intensity of the transmitted light from each process (f), wherein determining the asphaltene onset pressure of the fluid sample comprises identifying the intensity of the transmitted light during the depressurization at the fixed pressure and subtracting (i) the equilibrium value for process (f) at each different fixed pressure from (ii) the identified intensity of the transmitted light at the fixed pressure for each process (c) to determine difference values for each different fixed pressure, and wherein the asphaltene onset pressure is determined by identifying one difference value of the difference values representing at least one of (i) a value of zero and (ii) a steady state value of the difference values from a plot of the difference values vs pressure.