Method and system for determining asphaltene onset pressure

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. detecting an intensity of the light transmitted through the sample of the formation fluid;

c. decreasing a pressure of the sample of the formation fluid at a first controlled depressurization rate while monitoring the detected intensity of the light transmitted through the sample of the formation fluid;

d. using results from (c), determining a first pressure at which the detected intensity of the light transmitted through the sample of the formation fluid changes due to asphaltene flocculation;

e. repressurizing the sample of the formation fluid to, or introducing a second sample of the formation fluid at a pressure above the first pressure determined at (d);

f. repeating (a) to (d) at least once using at least a second, different depressurization rate to determine a second pressure at which the detected intensity of the light transmitted through the sample of the formation fluid changes due to asphaltene flocculation;

g. fitting at least the determined first and second pressures as a function of the first and second depressurization rates to a curve;

h. determining the asphaltene onset pressure of the sample of the formation fluid from said curve.

2. The method of claim 1 , further comprising:

positioning a wellbore tool within a wellbore;

drawing the formation fluid into the wellbore tool; and

performing (a) to (f) within the wellbore.

3. The method of claim 2 , wherein said repressurizing the sample of the formation fluid or introducing the second sample of the formation fluid comprises introducing the second sample of the formation fluid.

4. The method of claim 1 , wherein the sample of the formation fluid comprises a volume equal to or less than 1 mL.

5. The method of claim 1 , wherein the transmitted light travels along a path length through the fluid sample that is less than 2 mm.

6. The method of claim 1 , further comprising:

determining whether a change in the intensity of the light detected at is a result of flocculation of asphaltenes in the sample, wherein said detecting the intensity of the light transmitted through the sample of the formation fluid comprises detecting the intensity of the light transmitted through the sample of the formation fluid at a first and a second wavelength.

7. The method of claim 6 , wherein said determining whether the change in the intensity of the light detected at (c) is a result of flocculation of asphaltenes in the sample comprises:

determining a wavelength dependent signal using the change in the intensity of the light at the first wavelength and the change in the intensity of the light at the second wavelength.

8. The method of claim 1 , wherein said determining the first pressure at which the detected intensity of the light transmitted through the sample of the formation fluid changes due to asphaltene flocculation comprises:

determining a pressure at which the detected intensity of the light transmitted through the sample of the formation fluid changes past a predetermined threshold value.

9. The method of claim 1 , wherein said determining the first pressure at which the detected intensity of the light transmitted through the sample of the formation fluid changes due to asphaltene flocculation comprises:

plotting the detected intensity of the light transmitted through the sample of the formation fluid as a function of the pressure of the sample of the formation fluid;

fitting a first curve to the plotted detected intensity for a first range of the pressure of the sample of the formation fluid;

fitting a second curve to the plotted detected intensity for a second range of the pressure of the sample of the formation fluid;

extrapolating the first fitted curve and the second fitted curve; and

determining the pressure at which the extrapolated first fitted curve intercepts the extrapolated second fitted curve.

10. The method of claim 1 , wherein said fitting at least the determined first and second pressures as a function of the first and second depressurization rates to a curve comprises:

fitting the determined first and second pressures as a function of the first and second depressurization rates to a power law or other mathematical function.

11. The method of claim 10 , wherein said power law or other mathematical function includes at least one variable and said at least one variable is set to a constant value.

12. The method of claim 10 , wherein said power law or other mathematical function includes more than one variable and said more than one variable are set to constant values.

13. The method of claim 1 , wherein said fitting at least the determined first and second pressures as a function of the first and second depressurization rates to a curve comprises:

fitting the difference between the determined first and second pressures and the actual asphaltene onset pressure of the formation fluid determined from conventional laboratory measurements to a power law or other mathematical function.

14. The method of claim 13 , wherein said power law or other mathematical function includes at least one variable and said at least one variable is set to a constant value.

15. The method of claim 13 , wherein said power law or other mathematical function includes more than one variable and said more than one variable are set to constant values.

16. The method of claim 1 , wherein said determining the asphaltene onset pressure of the sample of the formation fluid from said curve comprises:

extrapolating said curve to a depressurization rate not directly measured in the sample.

17. The method of claim 16 , wherein said extrapolating comprises:

extrapolating to a depressurization rate of 0 psi/sec.

18. The method of claim 1 , wherein:

said decreasing the pressure of the sample of the formation fluid at the first controlled depressurization rate comprises:

decreasing the pressure of the sample of the formation fluid from an initial pressure value; and

said repressurizing the sample of the formation fluid to the pressure above the first pressure comprises:

repressurizing to said initial pressure value.

19. A system for determining asphaltene onset pressure of a fluid sample, the system comprising:

a fluid chamber;

a light source that generates light that is transmitted through the fluid sample;

a detector that detects light transmitted through the fluid sample;

a pressure control unit configured to vary the fluid sample pressure at a plurality of controlled depressurization rates; and

a controller configured to:

determine, for each of said plurality of controlled depressurization rates, a pressure at which an intensity of the light transmitted through the fluid sample and detected by the detector changes due to asphaltene flocculation;

fit the determined pressure for each of said plurality of controlled depressurization rates as a function of depressurization rate to a curve; and

determine the asphaltene onset pressure of the fluid sample from the curve.

20. The system of claim 19 , wherein the transmitted light travels along a path length through the fluid sample in the fluid chamber that is less than 2 mm.

21. The system of claim 19 , wherein at least said fluid chamber, said light source, said detector and said pressure control unit are incorporated into a wellbore tool.

22. The system of claim 21 , wherein the wellbore tool further comprises a probe for withdrawing the fluid sample from a formation and into the wellbore tool.

23. The system of claim 19 , wherein said fluid chamber has a volume equal to or less than 1 mL.