Downhole PV tests for bubble point pressure

• US 20040260497A1
• Filed: 06/21/2004
• Published: 12/23/2004
• Est. Priority Date: 06/20/2003
• Status: Active Grant

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: DiFoggio, Rocco, Michaels, John M., Lee, Jaedong
• Granted Patent: US 7,346,460 B2
• Time in Patent Office: Days
• US Class Current: 702/98
• CPC Class Codes: G01N 2013/0266 Bubble methods
• CPC Class Codes: G01N 33/2823 Raw oil, drilling fluid or polyphasic mixtures
• CPC Class Codes: G01N 7/00 Analysing materials by measuring the pressure or volume of a gas or vapour
• CPC Class Codes: G01N 9/28 by measuring the blowing pressure of gas bubbles escaping from nozzles at different depths in a liquid

Abstract

The present invention provides an objective bubble point pressure determination method enables new applications of downhole PV (Pressure Volume) test results such as in situ measurement of the bubble point pressure for use as a quality control parameter for sampling. The present invention provides a method and apparatus to fit data points to a curve and to smooth these points over an interval of N points. The derivative of the curve is determined to obtain peak acceleration. The peak acceleration represents the bubble point pressure. This bubble point pressure is verified by comparison to changes in fluid compressibility and visual observation of a pressure volume plot history.

Claims

1. A method for determining a property of a fluid comprising:

fitting a curve to a set of pressure versus volume data points for a fluid;

computing a second derivative of a function representing the curve; and

estimating a point where the slope of the second derivative is zero.

2. The method of claim 1 wherein the property of the fluid comprises the bubble point and further comprising:

determining the property of the fluid based on the point where the slope of the second derivative is zero.

3. The method of claim 2 wherein the step of determining the property comprises:

computing the pressure associated with the point where the slope of the second derivative is zero.

4. The method of claim 2 wherein the property of the fluid comprises the bubble point and further comprising:

determining the property of the fluid based on the point where the second derivative is an extremum.

5. The method of claim 1

wherein the property of the fluid comprises at least two bubble points, the method further comprising:

estimating a first bubble point from the point where the second derivative is zero and an extremum; and

estimating a second bubble point from a second point where the second derivative is zero.

6. The method of claim 1, wherein the step of estimating the point at which the slope is zero is done visually.

7. The method of claim 5, further comprising:

comparing the first bubble point pressure to the second bubble point pressure to estimate the accuracy of the first bubble point pressure calculation.

8. The method of claim 1 wherein the property is the presence of filtrate and wherein the fluid comprises a sample from a wellbore.

9. The method of claim 1 further comprising:

comparing the property the property as determined by direct testing.

10. The method of claim 1 wherein the step of fitting a curve further comprises:

using a rolling series of fits to at least one equation.

The method of claim 1 wherein the step of fitting a curve is performed as to an interval of N points.

11. The method of claim 1 wherein the step of fitting a curve further comprises:

selecting a first set of points;

fitting a curve to the first set of points;

selecting a second set of points inclusive of a subset of the first set of points; and

fitting a curve to the second set of points.

12. A method for determining a property of a fluid comprising:

fitting a set of curves to a set of pressure versus volume data points for the fluid;

computing the second derivative of a set of functions representing the set of curves; and

comparing the second derivative of the set of functions to determine the property of the fluid.

13. The method of claim 12 wherein the step of comparing comprises locating an extremum.

14. The method of claim 13 wherein the extremum is localized.

15. The method of claim 12 wherein the property is a bubble point and wherein the step of comparing comprises:

locating an extremum of the second derivative; and

determining the pressure associated with the extremum.

16. A computer readable medium containing instructions that when executed by a computer perform a method for determining a property of a fluid comprising:

fitting a curve to a set of pressure versus volume data points for a fluid;

computing a second derivative of a function representing the curve; and

estimating a point where the slope of the second derivative is zero.

17. The method of claim 16 wherein the property of the fluid comprises the bubble point and further comprising:

determining the property of the fluid based on the point where the slope of the second derivative is zero.

18. The method of claim 17 wherein the step of determining the property comprises:

computing the pressure associated with the point where the slope of the second derivative is zero.

19. The method of claim 16

wherein the property of the fluid comprises at least two bubble points, the method further comprising:

estimating a first bubble point from the point where the second derivative is zero and an extremum; and

estimating a second bubble point from a second point where the second derivative is zero.

20. A computer readable medium containing instructions that when executed by a computer perform a method for determining a property of a fluid comprising:

fitting a set of curves to a set of pressure versus volume data points for the fluid;

computing the second derivative of a set of functions representing the set of curves; and

comparing the second derivative of the set of functions to determine the property of the fluid.

21. The method of claim 20 wherein the step of comparing comprises locating an extremum.

22. The method of claim 21 wherein the extremum is localized.

23. The method of claim 20 wherein the property is a bubble point and wherein the step of comparing comprises:

locating an extremum of the second derivative; and

determining the pressure associated with the extremum.

24. An apparatus for determining a property of a fluid comprising:

a processor programmed to fit a curve to a set of pressure versus volume data points for a fluid, compute a second derivative of a function representing the curve; and

estimate a point where the slope of the second derivative is zero; and

an output device in data communication with the processors via which an estimate of the property is communicated.

25. The apparatus of claim 24 wherein the output device comprises a printer.

26. The apparatus of claim 24 further comprising:

a wireline tool comprising a fluid collector in data communication with the processor.

27. The apparatus of claim 26 wherein the wireline tool is a member of the set consisting essentially of a) a sampling tool and b) a testing tool.