Analysis of downhole OBM-contaminated formation fluid

• US 6,350,986 B1
• Filed: 04/27/1999
• Issued: 02/26/2002
• Est. Priority Date: 02/23/1999
• Status: Expired due to Term

• Current Assignee: Schlumberger Technology Corporation (SLB)
• Original Assignee: Schlumberger Technology Corporation (SLB)
• Inventors: Shroer, Jon J., Mullins, Oliver C.
• Primary Examiner(s): Hannaher, Constantine
• Application Number:
  US09/300,190
• Time in Patent Office: 1,036 Days
• Field of Search: 250/269.1
• US Class Current: 250/269.1
• CPC Class Codes: E21B 47/113 using electrical indications; using light radiations
• CPC Class Codes: E21B 49/005 Testing the nature of borehole walls or the formation by using drilling mud or cutting data
• CPC Class Codes: E21B 49/088 combined with sampling
• CPC Class Codes: E21B 49/10 using side-wall fluid samplers or testers
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 21/3577 for analysing liquids, e.g. polluted water
• CPC Class Codes: G01N 21/359 using near infrared light
• CPC Class Codes: G01N 21/534 by measuring transmission alone, i.e. determining opacity

Abstract

A method and apparatus is provided for determining a quality of downhole fluid. A series of measurements are taken of at least one parameter of borehole fluid that is indicative of OBM filtrate contamination. By curve-fitting, the series of the measured parameter values are used to create an asymptotic curve indicative of the quality of the downhole fluid. One embodiment determines OBM filtrate fraction in a borehole fluid sample. One embodiment is used when there is significant difference between the coloration of formation fluid and the coloration of OBM filtrate. Another is used when there is little or no difference between the coloration of formation fluid and the coloration of OBM filtrate. Another determines GOR of formation fluid corrected for OBM filtrate contamination. Another determines OD of formation fluid corrected for OBM filtrate contamination. Another determines conditions that would render optical density measurements invalid and sample capture premature. Another predicts the reduction of filtrate fraction for a specific extended pumping time. Another initiates sample capture when computed contamination fraction exhibits stable asymptotic convergence. Another compensates for wavelength-independent scattering. Another compensates for varying pump rate. Another reduces the effect of wavelength-dependent scattering.

Claims

1. A method for determining a quality of downhole fluid, comprising the steps of measuring at least one parameter of borehole fluid that is indicative of OBM filtrate contamination to produce at least one series of parameter values at intervals of time;

and using the series of the parameter values to create an asymptotic curve indicative of the quality of the downhole fluid.

2. A method according to claim 1, wherein the quality of downhole fluid is OBM filtrate fraction of borehole fluid, the method further comprising the steps of:

a) pumping borehole fluid through a downhole analyzer;

b) measuring OD of borehole fluid to produce a series of OD values at intervals of time; and

c) calculating an OD asymptotic ratio indicative of OBM filtrate fraction of borehole fluid.

3. A method according to claim 2, wherein calculating the OD asymptotic ratio includes solving a first mathematical function for coefficients by fitting the series of OD values to the first mathematical function, then using at least one of the coefficients in a second mathematical function to determine OBM filtrate fraction.

4. A method according to claim 3, wherein the first mathematical function expresses OD as a function of time, the first mathematical function having one coefficient representing an unknown asymptotic value, and at least one term which decreases with time.

5. A method according to claim 4, wherein the first mathematical function includes OD(t)=m₁+m₂t⁻

x, in which m₁ is a first coefficient representing the unknown OD asymptotic value, m₂ is a second coefficient, and x is a selected decay value.

6. A method according to claim 5, wherein x is approximately {fraction (5/12)}.

7. A method according to claim 5, wherein x is within the range 0.2 to 0.8.

8. A method according to claim 5, wherein the second mathematical function includes Fraction=(m₁−

OD)/m₁, in which m₁ is an OD asymptotic value determined by solving for coefficients, and OD is an OD value derived from the series of OD values.

9. A method according to claim 8, wherein the second mathematical function includes Fraction=|(m₁−

OD)|/m₁.

10. A method according to claim 5, further comprising the step of testing a computed coefficient to verify asymptotic convergence of OD values.

11. A method according to claim 10, further comprising the step of testing for m₂<

1.

12. A method according to claim 10, further comprising the step of testing for |m₂|<

1.

13. A method according to claim 5, further comprising the steps of:

repeating steps a) to c) to produce a series of asymptotic values; and

testing the series of asymptotic values for stability.

14. A method according to claim 13, further comprising the step of testing for (m₁−

m_1prev.) /m₁<

0.05.

15. A method according to claim 13, further comprising the step of testing for |(m₁−

m_1prev.)|/m₁<

0.05.

16. A method according to claim 2, wherein measuring OD includes illuminating borehole fluid with light of wavelength in the visible spectrum selected in accordance with coloration contrast between formation fluid and OBM filtrate.

17. A method according to claim 16, wherein the wavelength is selected as being the shortest wavelength that yields an OD in the range 0.05 to 2.0.

18. A method according to claim 17, wherein the first wavelength selected is approximately 537 nm.

19. A method according to claim 2, wherein measuring OD includes illuminating borehole fluid with light of wavelength selected in accordance with contrast between the OD of condensate dissolved in the formation fluid and the OD of OBM filtrate.

20. A method according to claim 19, wherein the condensate includes methane.

21. A method according to claim 20, wherein the wavelength is proximate to a methane peak.

22. A method according to claim 21, wherein the wavelength is on a lower wavelength shoulder of a methane peak.

23. A method according to claim 2, further comprising the step of testing for scattering, including testing to determine if OD is less than a predetermined value.

24. A method according to claim 23, wherein testing for scattering includes testing for OD<

0.2.

25. A method according to claim 23, wherein testing for scattering includes testing at wavelength 1600 nm.

26. A method according to claim 1, wherein the quality of downhole fluid is OBM filtrate fraction of borehole fluid, further comprising the steps of:

a) pumping borehole fluid through an analyzer;

b) illuminating the borehole fluid with light in the visible spectrum and with near infra-red (NIR) light at a wavelength associated with gas;

c) detecting optical absorbance in the visible spectrum to produce a visible spectrum optical density value and NIR absorbance to produce an NIR optical density value;

d) calculating GOR as the ratio of the NIR optical density value to the visible spectrum optical density value;

e) repeating steps a) to d) to produce a series of GOR values at intervals of time; and

f) calculating a GOR asymptotic ratio indicative of OBM filtrate fraction of borehole fluid.

27. A method according to claim 26, wherein calculating the GOR asymptotic ratio includes solving a third mathematical function for its coefficients by fitting the series of GOR values to the third mathematical function, then using at least one of the coefficients in a fourth mathematical function to determine OBM filtrate fraction.

28. A method according to claim 27, wherein the third mathematical function expresses GOR as a function of time, the third mathematical function having one constant coefficient representing an unknown asymptotic value, and at least one term which decreases with time.

29. A method according to claim 28, where in the third mathematical function includes GOR(t)=r₁+r₂t⁻

y, in which r₁ is a first constant coefficient representing the unknown GOR asymptotic value, r₂ is a second constant coefficient, and y is a selected decay value.

30. A method according to claim 29, wherein y is approximately 0.5.

31. A method according to claim 29, wherein y is within the range 0.2 to 0.8.

32. A method according to claim 27, wherein the fourth mathematical function includes Fraction=(r₁−

GOR)/r₁, in which r₁ is the asymptotic value determined by solving for coefficients, and GOR is a GOR value derived from the series of GOR values.

33. A method according to claim 1, wherein the quality of downhole fluid is GOR of formation fluid, further comprising the steps of:

a) pumping borehole fluid through an analyzer;

b) illuminating the borehole fluid with light in the visible spectrum and with near infra-red (NIR) light at a wavelength associated with gas;

c) detecting optical absorbance in the visible spectrum to produce a visible spectrum optical density value and NIR absorbance to produce an NIR optical density value;

d) calculating GOR as the ratio of the NIR optical density value to the visible spectrum optical density value;

e) repeating steps a) to d) to produce a series of GOR values at intervals of time; and

f) calculating a GOR asymptotic value from the series of GOR values.

34. A method according to claim 33, wherein calculating the GOR asymptotic value includes solving a third mathematical function for a coefficient by fitting the series of GOR values to the third mathematical function.

35. A method according to claim 34, wherein the third mathematical function expresses GOR as a function of time, the function having one coefficient representing an unknown GOR asymptotic value, and at least one term which decreases with time.

36. A method according to claim 35, wherein the mathematical function includes GOR(t)=r₁+r₂t⁻

y, in which r₁ is the unknown asymptotic value, r₂ is a constant, and y is a selected decay value.

37. A method according to claim 36, wherein y is approximately {fraction (5/12)}.

38. A method according to claim 36, wherein y is within the range 0.2 to 0.8.

39. A method according to claim 1, wherein the quality of downhole fluid is optical density of formation fluid, further comprising the steps of:

a) pumping borehole fluid through an analyzer;

b) measuring optical density of borehole fluid to produce a series of optical density values at intervals of time; and

c) calculating an OD asymptotic value indicative of optical density of formation fluid from the series of optical density values.

40. A method according to claim 39, wherein calculating the OD asymptotic ratio includes solving a first mathematical function for a coefficient by fitting the series of OD values to the first mathematical function.

41. A method according to claim 40, wherein the first mathematical function expresses OD as a function of time, the first mathematical function having one constant coefficient representing an unknown asymptotic value, and at least one term which decreases with time.

42. A method according to claim 41, wherein the first mathematical function includes OD(t)=m₁+m₂t⁻

x, in which m₁ is a first constant coefficient representing the unknown OD asymptotic value, m₂ is a second constant coefficient, and x is a selected decay value.

43. A method according to claim 42, wherein x is approximately {fraction (5/12)}.

44. A method according to claim 42, wherein x is within the range 0.2 to 0.8.

45. A method according to claim 1, wherein the step of using the parameter values to identify an asymptotic curve includes curve fitting.

46. A method according to claim 45, wherein the step of using the parameter values to identify an asymptotic curve includes curve fitting on a time axis.

47. A method according to claim 45, wherein the step of using the parameter values to identify an asymptotic curve includes curve fitting on a volume axis, whereby errors due to varying pump rate are reduced.

48. A method according to claim 1, wherein the quality of downhole fluid is one of a group of qualities consisting of OBM filtrate fraction of downhole fluid, GOR of formation fluid, and optical density of formation fluid.

49. A method according to claim 1, wherein the parameter is optical density of downhole fluid.

50. A method according to claim 49, further including the step of subtracting, from a first signal associated with an optical density measurement at a first wavelength, a second signal associated with an optical density measurement at a second wavelength, the second wavelength longer than the first wavelength, to produce a difference signal for use as the function of the parameter values, whereby an unwanted effect of wavelength-independent scattering is reduced.

51. A method according to claim 50,

wherein the step of using the parameter values to identify an asymptotic curve includes curve fitting;

and

wherein curve fitting is delayed until the difference signal is increasing with time;

whereby an unwanted effect of wavelength-dependent scattering is reduced.

52. A method according to claim 1, further comprising:

a) pumping borehole fluid through a downhole analyzer;

b) measuring a parameter to produce a series of parameter values at intervals of time; and

c) calculating a parameter asymptotic ratio indicative of the quality of borehole fluid.

53. A method according to claim 52, wherein calculating the parameter asymptotic ratio includes solving a first mathematical function for coefficients by fitting the series of parameter values to the first mathematical function, then using at least one of the coefficients in a second mathematical function to determine the quality.

54. A method according to claim 53, wherein the first mathematical function expresses the parameter as a function of time, the first mathematical function having one coefficient representing an unknown asymptotic value, and at least one term which decreases with time.

55. A method according to claim 54, wherein the first mathematical function includes parameter(t)=p₁+p₂t⁻

x, in which p₁ is a first coefficient representing the unknown parameter asymptotic value, p₂ is a second coefficient, and x is a selected decay value.

56. A method according to claim 55, wherein x is approximately {fraction (5/12)}.

57. A method according to claim 55, wherein x is within the range 0.2 to 0.8.

58. A method according to claim 55, wherein the second mathematical function includes

59. A method according to claim 58, wherein x is approximately {fraction (5/12)}.

60. A method according to claim 58, wherein x is within the range 0.2 to 0.8.

61. A method according to claim 55, wherein the second mathematical function includes

62. A method according to claim 1, wherein the quality is OBM filtrate fraction.

63. A method for validating initiation of sample capture of borehole fluid, using a borehole tool having a pump, a flowline, an optical analyzer, and means for capturing a sample, the method comprising the steps of:

a) pumping borehole fluid through the analyzer;

b) measuring OD of borehole fluid to produce a series of OD values at intervals of time;

c) testing for scattering; and

d) initiating sample capture if scattering is less than a predetermined value.

64. A method according to claim 63, wherein the predetermined value is 0.2.

65. A method according to claim 63, wherein measuring OD of borehole fluid includes illuminating the borehole fluid at a wavelength of approximately 1600 nm.

66. A method for validating initiation of sample capture of borehole fluid, using a borehole tool having a pump, a flowline, an optical analyzer, and means for capturing a sample, the method comprising the steps of:

a) pumping borehole fluid through the analyzer;

b) measuring OD of borehole fluid produce a series of optical density values at intervals of time;

c) calculating an asymptotic value indicative of optical density of formation fluid from the series of optical density values;

d) repeating steps a) through c) to produce a series of asymptotic values; and

e) initiating sample capture if asymptotic values are monotonicaly changing at less than a predetermined rate.

67. A method for predicting OBM filtrate fraction of borehole fluid after a predefined second period of pumping, using a borehole tool having a pump, a flowline, and an optical analyzer, the method comprising the steps of:

a) pumping borehole fluid through the analyzer;

b) illuminating the borehole fluid with light in the visible spectrum and with near infra-red (NIR) light at a wavelength associated with gas;

c) detecting optical absorbance in the visible spectrum to produce a visible spectrum optical density value and NIR absorbance to produce NIR optical density value;

d) calculating GOR as the ratio of the NIR optical density value to the visible spectrum optical density value;

e) repeating steps a) to d) during a first period of pumping to produce a series of GOR values at intervals of time; and

f) calculating a GOR asymptotic value indicative of predicted OBM filtrate fraction, including solving a third mathematical function for coefficients by fitting the series of GOR values to the first mathematical function, then using at least one of the coefficients in a fifth mathematical function to determine predicted OBM filtrate fraction.

68. A method according to claim 67, wherein solving a third mathematical function for coefficients includes fitting the series of ratio values to a third mathematical function of the form GOR(t)=r₁+r₂t⁻

y, in which r₁ is the unknown asymptotic value, r₂ is a constant, and y is a selected decay value, to solve for r₁ and r₂; and

wherein using at least one of the coefficients in a fifth mathematical function includes solving equation FRACTION=[r₂T_Pm<sup>−

y</sup>]/r₁, where T_Pm is the predefined second period of pumping.

69. A borehole apparatus, comprising:

a borehole tool including a flowline with an optical cell, a pump coupled to the flowline for pumping borehole fluid through the cell, and an analyzer optically coupled to the cell and configured to produce OD values; and

control means, coupled to the borehole tool, including means for accepting OD values from the borehole tool and calculating from the OD values an asymptotic value.

70. A borehole apparatus according to claim 69, wherein the asymptotic value is an OD asymptotic value indicative of OBM filtrate fraction.

71. A borehole apparatus according to claim 69, wherein the asymptotic value is a GOR asymptotic ratio indicative of OBM filtrate fraction.

72. A borehole apparatus according to claim 69, wherein the asymptotic value is a GOR asymptotic value indicative of GOR corrected for OBM filtrate fraction.

73. A borehole apparatus according to claim 69, wherein the asymptotic value is an OD asymptotic value indicative of OD corrected for OBM filtrate fraction.

74. A borehole apparatus according to claim 69, wherein the control means further includes means for testing OD values to validate measurement by confirming absence of scattering.

75. A borehole apparatus according to claim 69, wherein the control means further includes means for testing OD values to validate measurement by confirming asymptotic convergence.

76. A borehole apparatus according to claim 69, wherein the control means further includes means for testing OD values to validate measurement by confirming stable asymptote.

77. A borehole apparatus according to claim 69, wherein the asymptotic value is an OD asymptotic value indicative of OBM filtrate fraction associated with a selected additional pumping time.

78. A computer usable medium having computer readable program code thereon, the medium adapted for use with borehole apparatus, the program code including code structured to

(a) accept a series of borehole fluid parameter values indicative of OBM filtrate contamination;

(b) calculate from the parameter values an asymptotic value; and

(c) calculate from the asymptotic value a quality of a downhole fluid.

79. A computer usable medium according to claim 78 wherein the quality is OBM filtrate fraction of borehole fluid.

80. A computer usable medium according to claim 78 wherein the quality is GOR of formation fluid.

81. A computer usable medium according to claim 78 wherein the quality is OD of formation fluid.