6 526 354

6,526,354 Title:

Sonic well logging for alteration detection

A method for determining alteration of a region of an earth formation surrounding an earth borehole, comprising the steps of providing a logging device that is moveable through the borehole; transmitting sonic energy into the formation and receiving, at a plurality of transmitter-to-receiver spacings, sonic energy that has traveled through the formation, and producing signals representative of the received sonic energy for the plurality of transmitter-to-receiver spacings; determining sonic transit times and differential transit times for the respective transmitter-to-receiver spacings; deriving a test statistic from the differential transit times; and determining the presence of alteration of a region of the formations from the test statistic. An associated apparatus for carrying out the method is also described.

What is claimed is:

1. A method for determining alteration of a region of an earth formation surrounding an earth borehole, comprising the steps of: providing a logging device that is moveablethrough the borehole; transmitting sonic energy into the formation and receiving, at a plurality of transmitter-to-receiver spacings on said logging device, sonic energy that has traveled through the formation, and producing signals representative ofthe received sonic energy for said plurality of transmitter-to-receiver spacings; determining, from said signals, sonic transit times and differential transit times for the respective transmitter-to-receiver spacings; deriving a test statistic fromsaid differential transit times; and determining the presence of alteration of a region of the formations from said test statistic.

2. The method as defined by claim 1, wherein said determined sonic transit times and differential transit times are sonic compressional transit times and sonic compressional differential transit times.

3. The method as defined by claim 1, wherein said step of determining the presence of alteration of a region of the formations from said test statistic comprises comparing said test statistic to a threshold.

4. The method as defined by claim 2, wherein said step of determining the presence of alteration of a region of the formations from said test statistic comprises comparing said test statistic to a threshold.

5. The method as defined by claim 1, wherein said test statistic includes a component that depends on the degree to which the differential transit times decrease monotonically as a function of transmitter-to-receiver spacing.

6. The method as defined by claim 2, wherein said test statistic includes a component that depends on the degree to which the differential transit times decrease monotonically as a function of transmitter-to-receiver spacing.

7. The method as defined by claim 2, wherein said test statistic includes a component that depends on the degree to which the differential transit times, as a function of transmitter-to-receiver spacing, corresponds to a line of negative slope.

8. The method as defined by claim 2, wherein said test statistic includes components that depend on the degree to which the differential transit times, as a function of transmitter-to-receiver spacing, corresponds to a line of negative slopefollowed by a line of substantially zero slope.

9. The method as defined by claim 2, further comprising determining the compressional slowness of the virgin earth formation from said test statistic.

10. The method as defined by claim 8, further comprising determining the compressional slowness of the virgin earth formation from said test statistic.

11. The method as defined by claim 1, wherein said test statistic T.sub.1 is of the form ##EQU8## where DTT.sub.i are the individual differential transit times, DTT is the average of the differential transit times, TR.sub.i are the individualtransmitter-to-receiver spacings, and m and c are constants.

12. The method as defined by claim 2, wherein said test statistic T.sub.1 is of the form ##EQU9## where DTT.sub.i are the individual differential transit times, DTT is the average of the differential transit times, TR.sub.i are the individualtransmitter-to-receiver spacings, and m and c are constants.

13. The method as defined by claim 2, wherein said test statistic T.sub.1 is of the form ##EQU10## where DTT.sub.i are the individual differential transit times, DTT is the average of the differential transit times, TR.sub.i are the individualtransmitter-to-receiver spacings, m and c are constants, and W.sub.ii are weighing coefficients on the diagonal of the inverse of the covariance matrix.

14. The method as defined by claim 2, wherein said test statistic T.sub.1 is of the form ##EQU11## where ##EQU12## where DTT.sub.i are the individual differential transit times, {overscore (DTT)} is the average of the differential transit times,TR.sub.i are the individual transmitter-to-receiver spacings, and m, c.sub.0, c.sub.1, and R.sub.c are constants.

15. Apparatus for determining alteration of a region of an earth formation surrounding an earth borehole, comprising: a logging device that is moveable through the borehole; means on said logging device for transmitting sonic energy into theformation and receiving, at a plurality of transmitter-to-receiver spacings on said logging device, sonic energy that has traveled through the formation, and for producing signals representative of the received sonic energy for said plurality oftransmitter-to-receiver spacings; means for determining, from said signals, sonic transit times and differential transit times for the respective transmitter-to-receiver spacings; means for deriving a test statistic from said differential transittimes; and means for determining the presence of alteration of a region of the formations from said test statistic.

16. Apparatus as defined by claim 15, wherein said means for determining the presence of alteration of a region of the formations from said test statistic comprises means for comparing said test statistic to a threshold.

17. A method for determining alteration of a region of the earth formation, for use in conjunction with a technique for sonic logging of an earth formation that includes: providing a logging device that is moveable through the borehole; transmitting sonic energy into the formation and receiving, at a plurality of transmitter-to-receiver spacings, sonic energy that has traveled through the formation, and producing signals representative of the received sonic energy for said plurality oftransmitter-to-receiver spacings; comprising the steps of: determining, from said signals, sonic transit times and differential transit times for the respective transmitter-to-receiver spacings; deriving a test statistic from said differential transittimes; and determining the presence of alteration of a region of the formations from said test statistic.

18. The method as defined by claim 17, wherein said determined sonic transit times and differential transit times are sonic compressional transit times and sonic compressional differential transit times.

19. The method as defined by claim 17, wherein said step of determining the presence of alteration of a region of the formations from said test statistic comprises comparing said test statistic to a threshold.

20. The method as defined by claim 17, wherein said test statistic includes a component that depends on the degree to which the differential transit times decrease monotonically as a function of transmitter-to-receiver spacing.

21. The method as defined by claim 17, wherein said test statistic includes a component that depends on the degree to which the differential transit times, as a function of transmitter-to-receiver spacing, corresponds to a line of negativeslope.

22. The method as defined by claim 17, wherein said test statistic includes components that depend on the degree to which the differential transit times, as a function of transmitter-to-receiver spacing, corresponds to a line of negative slopefollowed by a line of substantially zero slope.

23. The method as defined by claim 18, further comprising determining the compressional slowness of the virgin earth formation from said test statistic.

24. The method as defined by claim 17, wherein said test statistic T.sub.1 is of the form ##EQU13## where DTT.sub.i are the individual differential transit times, DTT is the average of the differential transit times, TR.sub.i are the individualtransmitter-to-receiver spacings, and m and c are constants.

25. The method as defined by claim 17, wherein said test statistic T.sub.1 is of the form ##EQU14## where DTT.sub.i are the individual differential transit times, DTT is the average of the differential transit times, TR.sub.i are the individualtransmitter-to-receiver spacings, m and c are constants, and W.sub.ii are weighing coefficients on the diagonal of the inverse of the covariance matrix.

26. The method as defined by claim 17, wherein said test statistic T.sub.1 is of the form ##EQU15## where ##EQU16## where DTT.sub.i are the individual differential transit times, DTT is the average of the differential transit times, TR.sub.i arethe individual transmitter-to-receiver spacings, and m, c.sub.0, c.sub.1, and R.sub.c are constants.

27. A method for determining whether a region of an earth formation surrounding an earth borehole is homogeneous, comprising the steps of: providing a logging device that is moveable through the borehole; transmitting sonic energy into theformation and receiving, at a plurality of transmitter-to-receiver spacings on said logging device, sonic energy that has traveled through the formation, and producing signals representative of the received sonic energy for said plurality oftransmitter-to-receiver spacings; determining, from said signals, sonic transit times and differential transit times for the respective transmitter-to-receiver spacings; deriving a test statistic from said differential transit times; and determining,from said test statistic, whether said region of the formation is homogeneous.

28. The method as defined by claim 27, wherein said determined sonic transit times and differential transit times are sonic compressional transit times and sonic compressional differential transit times.

29. The method as defined by claim 27, wherein said step of determining, from said test statistic, whether said region of the formation is homogeneous, comprises comparing said test statistic to a threshold.

30. The method as defined by claim 27, wherein said test statistic includes a component that depends on the degree to which the differential transit times decrease monotonically as a function of transmitter-to-receiver spacing.

31. The method as defined by claim 27, wherein said test statistic includes a component that depends on the degree to which the differential transit times, as a function of transmitter-to-receiver spacing, corresponds to a line of negativeslope.

32. The method as defined by claim 27, wherein said test statistic includes components that depend on the degree to which the differential transit times, as a function of transmitter-to-receiver spacing, corresponds to a line of negative slopefollowed by a line of substantially zero slope.

33. The method as defined by claim 27, wherein said test statistic T.sub.1 is of the form ##EQU17## where DTT.sub.i are the individual differential transit times, DTT is the average of the differential transit times, TR.sub.i are the individualtransmitter-to-receiver spacings, and m and c are constants.

34. The method as defined by claim 27, wherein said test statistic T.sub.1 is of the form ##EQU18## where DTT.sub.i are the individual differential transit times, DTT is the average of the differential transit times, TR.sub.i are the individualtransmitter-to-receiver spacings, m and c are constants, and W.sub.ii are weighing coefficients on the diagonal of the inverse of the covariance matrix.

35. The method as defined by claim 27, wherein said test statistic T.sub.1 is of the form ##EQU19## where ##EQU20## where DTT.sub.i are the individual differential transit times, {overscore (DTT)} is the average of the differential transittimes, TR.sub.i are the individual transmitter-to-receiver spacings, and m, c.sub.0, c.sub.1, and R.sub.c are constants.