Compensation of errors in logging-while-drilling density measurements

• US 6,307,199 B1
• Filed: 05/12/1999
• Issued: 10/23/2001
• Est. Priority Date: 05/12/1999
• Status: Expired due to Term

• Current Assignee: Schlumberger Technology Corporation (SLB)
• Original Assignee: Schlumberger Technology Corporation (SLB)
• Inventors: Edwards, John E., Sloan, W. Robert, Adolph, Robert A.
• Primary Examiner(s): Hannaher, Constantine
• Assistant Examiner(s): Gabor, Otilia
• Application Number:
  US09/310,893
• Time in Patent Office: 895 Days
• Field of Search: 250/269.3, 250/254, 250/262, 250/264
• US Class Current: 250/269.3
• CPC Class Codes: E21B 47/08 Measuring diameters or related dimensions at the borehole
• CPC Class Codes: G01V 5/125 and detecting the secondary gamma- or X-rays in different places along the bore hole

Abstract

A system is disclosed for compensating well logs for adverse effects of the borehole and near borehole formation effects. The system is configured primarily for processing logging-while-drilling (LWD) density measurements, and includes means for generating a one-dimensional density log which is corrected for adverse effects of logging tool standoff and “dipping” beds penetrated by the borehole. The system is, however, applicable to any type of LWD or other type of logging system which requires borehole corrections, and which responds to variations in formation properties in a plane perpendicular to the borehole. The system can also be modified to include LWD apparatus using sensors that require no borehole corrections, and only require corrections for dipping beds. The system is ideally suited for logging equipment using two sensors, but can be modified for use with single sensor systems or systems using more than two sensors.

Claims

1. A method for a determining a property of earth formation penetrated by a borehole, the method comprising:

(a) conveying at least one sensor within said borehole, wherein said sensor is either rotating or stationary within said borehole;

(b) measuring a responses of said sensor in a plurality of azimuthal borehole segments;

(c) determining, for each said segment, a segment formation property from said sensor response within that segment;

(d) forming a two-dimensional image of formation property from a plurality of said segment formation properties; and

(e) using said image to identify segment formation properties of substantially constant value and combining these segments to form a one-dimensional log of said formation property with respect to an azimuthal reference vector.

2. The method of claim 1 comprising the additional steps of:

(a) correcting each said segment formation property for sensor borehole effects thereby forming said corrected segment formation property for each said segment;

(b) forming said image from a plurality of said corrected segment formation properties; and

(c) combining corrected segment formation properties of substantially constant value to form said one-dimensional log.

3. The method of claim 2 comprising the additional steps of:

(a) conveying and rotating at least two sensors within said borehole; and

(b) correcting said segment formation properties for borehole effects comprising time correlated standoff and for depth correlated standoff.

4. The method of claim 3 comprising the additional steps of:

(a) providing gamma ray detectors as sensors;

(b) axially spacing said detectors from a source of nuclear radiation at different distances; and

(c) wherein said corrected formation property is bulk density.

5. The method of claim 4 comprising the additional steps of:

(a) mounting said detectors and said source within the wall of a drill collar within a drill string;

(b) rotating said detectors within said borehole by rotating said drill string; and

(c) conveying said detectors along said borehole while advancing said borehole by the action of a drill bit attached to said drill string.

6. The method of claim 1 including the additional step of forming two or more one-dimensional logs of said formation property wherein each one-dimensional log is referenced to a different azimuthal reference vector.

7. The method of claim 2 comprising the additional steps of:

(a) conveying and rotating a plurality of sensors within said borehole, wherein the axial spacing of each sensor differs; and

(b) correcting said segment formation properties for borehole effects comprising time correlated standoff and for depth correlated standoff.

8. A method for a determining density of earth formation penetrated by a borehole while drilling said borehole, the method comprising:

(a) rotating and conveying a tool in said borehole on a drill string, wherein said tool comprises a source of nuclear radiation and a long spaced detector and a short spaced detector axially spaced at different distances from said source;

(b) generating a long spaced detector response and a short spaced detector response in a plurality of azimuthal borehole segments wherein each response is indicative of nuclear radiation from said source interacting with said earth formation;

(c) determining, for each said segment, a segment formation density by combining said long spaced detector responses and said short spaced detector responses within that segment;

(d) correcting each said segment formation density for tool standoff thereby forming a plurality of corrected segment formation density values;

(e) forming a two-dimensional image from formation density from said plurality of corrected segment formation density values, wherein said image encompasses the full periphery of said borehole; and

(f) azimuthally averaging corrected segment densities of substantially constant value determined from said image to form a first one-dimensional log of said formation density with respect to an azimuthal reference vector.

9. The method of claim 8 comprising the additional steps of:

(a) correcting each said segment formation density for time correlated standoff; and

(b) correcting each said determined formation density for each segment for depth correlated standoff.

10. The method of claim 9 wherein said time correlated standoff is made using a spine and rib correction method.

11. The method of claim 9 wherein said depth correlated standoff correction is made by depth shifting said long spaced detector response and said short spaced detector point to a reference measure point on said tool.

12. The method of claim 8 wherein said azimuthal reference vector is aligned with a low side of said borehole.

13. The method of claim 8 including the additional steps of forming two or more one-dimensional logs and referencing each said log to a different azimuthal reference vector.

14. The method of claim 8 including the additional step of performing all computations required to obtain said formation density log using computer means within said tool.

15. The method of claim 14 including the additional step of telemetering said formation density log to the surface of the earth.

16. The method of claim 14 including the additional step of storing said formation density log in a storage means within said tool, and subsequently retrieving said log when said tool is returned to the surface of the earth.

17. The method of claim 8 wherein each said segment is equal in angle.

18. The method of claim 8 wherein all said segments are contiguous and encompass the entire periphery of said borehole.

19. The method of claim 8 wherein said source comprises an isotopic gamma ray emitting material.

20. The method of claim 8 wherein said long and short spaced detectors each comprises a scintillation crystal optically coupled to a photomultiplier tube.

21. The method of claim 8 wherein said at least the first one-dimensional log of said formation density comprises said azimuthally averaged regions of constant corrected density plotted as a function of measured depth of said tool within said borehole.

22. A system for determining a property of earth formation penetrated by a borehole comprising:

(a) a borehole tool comprising at least one sensor wherein said tool is conveyed and rotated within said borehole; and

(b) a computer for combining responses of said at least one sensor in a plurality of azimuthal borehole segments to obtain a measure of said property so that;

(i) a segment formation property is determined, for each said segment, from said sensor response within that segment,

(ii) a two-dimensional image is formed from said segment formation property determined in a plurality of segments, and

(iii) segment formation properties of substantially constant value are identified from said image and combining to form a one-dimensional log of said formation property with respect to an azimuthal reference vector.

23. The system of claim 22 wherein:

(a) each said segment formation property is corrected for borehole effects with a borehole correction thereby forming corrected segment formation property for each said segment;

(b) said two-dimensional image is formed from a plurality of said corrected segment formation properties; and

(c) corrected segment formation properties of substantially constant value are identified from said image and combined to form a one-dimensional log of corrected formation property with respect to said reference vector.

24. The system of claim 23 comprising two sensors and wherein said borehole correction comprises a time correlated standoff correction and a depth correlated standoff correction.

25. The system of claim 24 further comprising:

(a) gamma ray detectors as said sensors; and

(b) a source of nuclear radiation, so that;

(i) said detectors are axially spaced from said source at different distances, and

(ii) said corrected formation property is bulk density.

26. The system of claim 25 wherein;

(a) said tool comprises a drill collar;

(b) said detectors and said source are mounted within the wall of said drill collar and within a drill string;

(c) said tool is rotated within said borehole by rotating a drill string; and

(d) said tool is conveyed along said borehole while advancing said borehole by the action of a drill bit attached to said drill string.

27. The system of claim 22 wherein two or more one-dimensional logs of said formation property are formed, wherein each one-dimensional log is referenced to a different azimuthal reference vector.

28. A system for a determining density of earth formation penetrated by a borehole while drilling said borehole, the method comprising:

(a) a tool mounted in a collar in a drill string, wherein said tool comprises a source of nuclear radiation and a long spaced detector and a short spaced detector axially spaced at different distances from said source and is rotating and conveying in said borehole by said drill string;

(b) said long spaced detector and short spaced detector forming a response in a plurality of azimuthal borehole segments wherein each response is indicative of nuclear radiation from said source interacting with said earth formation defined by said segment;

(c) a computer connected to said long and short spaced detectors for determining, for each said segment, said formation density by combining said long spaced detector responses and said short spaced detector responses within that segment wherein each said determined formation density for each segment is corrected for tool standoff thereby forming a plurality of corrected density values for each segment; and

(d) said computer forms a two-dimensional image of corrected formation density from said plurality of corrected density values and azimuthally averaging regions of constant corrected density determined from said image to form at least one one-dimensional log of said formation density with respect to an azimuthal reference vector.

29. The system of claim 28 wherein said computer corrects for tool standoff and comprises:

(a) means for correcting each said determined formation density for time correlated standoff; and

(b) means for correcting each said determined formation density for each segment for depth correlated standoff.

30. The system of claim 29 wherein said computer connects with a memory storing a spine and rib process instructions.

31. The system of claim 29 wherein said depth correlated standoff correction is made by depth shifting said long spaced detector response and said short spaced detector point to a reference measure point on said tool.

32. The system of claim 28 wherein said azimuthal reference vector is aligned with a low side of said borehole.

33. The system of claim 28 whereby two or more one-dimensional logs are formed and each said log is referenced to a different azimuthal reference vector.

34. The system of claim 28 further comprising a computer mounted within said tool and with which all computations required to obtain said formation density log are performed.

35. The system of claim 34 further comprising means for telemetering said formation density log to the surface of the earth.

36. The system of claim 34 further comprising a memory mounted within said tool and in which said formation density log is stored and subsequently retrieved when said tool is returned to the surface of the earth.

37. The system of claim 28 wherein each said segment is equal in angle.

38. The system of claim 28 wherein all said segments are contiguous and encompass the entire periphery of said borehole.

39. The system of claim 28 wherein said source comprises an isotopic gamma ray emitting material.

40. The system of claim 28 wherein said long and short spaced detectors each comprises a scintillation crystals optically coupled to a photomultiplier tube.

41. The system of claim 28 wherein said at least one-dimensional log of said formation density comprising said azimuthally averaged regions of constant corrected density is plotted as a function of measured depth of said tool within said borehole.