Semblance processing for an acoustic measurement-while-drilling system for imaging of formation boundaries

• US 6,581,010 B2
• Filed: 07/08/2002
• Issued: 06/17/2003
• Est. Priority Date: 01/24/1997
• Status: Expired due to Term

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: Bolshakov, Alexi, Dubinsky, Vladimir, Leggett, James V. III
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Taylor, Victor J.
• Application Number:
  US10/190,955
• Publication Number:
  US 20030014190A1
• Time in Patent Office: 344 Days
• Field of Search: 702/9, 367/76, 367/25, 367/57, 367/46, 367/45, 367/50, 367/24, 367/41, 367/35, 175/45, 175/40, 703/175
• US Class Current: 702/9
• CPC Class Codes: E21B 43/08 Screens or liners expandable screens or liners E21B43/108
• CPC Class Codes: E21B 43/12 Methods or apparatus for controlling the flow of the obtained fluid to or in wells E21B43/25 takes precedence; valve arrangements E21B34/00
• CPC Class Codes: E21B 43/14 Obtaining from a multiple-zone well
• CPC Class Codes: G01V 1/48 Processing data

Abstract

The present invention provides a system for drilling boreholes having a downhole subassembly which contains an acoustic measurement-while-drilling system. It uses the acoustic velocity through the formations surrounding the borehole and an acoustic transmitter and a set of receivers for determining the bed boundaries surrounding the borehole formation. An acoustic isolator on the tool may be used to attenuate body waves traveling between the transmitter and the receivers. Additional attenuation of the body waves is provided by a threshold filter based upon the absolute maximum of the received signals. Semblance of the data is determined in a slowness/intercept-time domain. Coherence and semblance filtering methods are used to differentiate between reflection signals and noise. The position and orientation of the bed boundary relative to the tool are determined. A further processing step uses the relative position and orientation determined for a number of tool positions to further discriminate against noise and obtain an absolute position and depth of the bed boundaries.

Claims

1. A downhole tool for determination of the position and orientation of a bed boundary in a formation surrounding a wellbore during drilling of the wellbore, the downhole tool comprising:

(a) a transmitter for transmitting acoustic signals at at least one position in the wellbore during drilling of the wellbore, said acoustic signals having an acoustic velocity;

(b) a plurality of spaced apart receivers for detecting signals including reflected signals from the bed boundary; and

(c) a processor for prefiltering the detected signals using a prefilter based upon a threshold determination for selectively attenuating unwanted signal components traveling between the transmitter and the plurality of receivers and providing prefiltered signals, the processor further using said acoustic velocity and the prefiltered signals for determining a distance and orientation of the bed boundary relative to the tool.

2. The downhole tool of claim 1 further comprising an acoustic isolator between the transmitter and the plurality of spaced apart receivers, said isolator attenuating body waves present in said unwanted signal components.

3. The downhole tool of claim 1 wherein the at least one position comprises a plurality of positions and the processor is further adapted to process the data from said plurality of positions to determine therefrom the absolute depth and position of the bed boundary corresponding to said plurality of positions.

4. The downhole tool of claim 1 wherein the processor determines a plurality of semblances of the prefiltered signals in a slowness domain to determine the distance and orientation of the bed boundary relative to the tool.

5. The downhole tool of claim 2 wherein the processor determines a plurality of semblances of the prefiltered signals in a slowness domain to determine the absolute depth and position of the bed boundary corresponding to said plurality of positions.

6. The downhole tool of claim 4 wherein the processor determines the absolute position of the bed boundary using said semblances and a predetermined window.

7. The downhole tool according to claim 4, wherein the plurality of receivers include a near receiver which is utilized for determining a near receiver travel time.

8. The downhole tool of claim 1 wherein arrival times of reflected signals at the plurality of receivers are one of (i) a hyperbolic function of a transmitter to receiver distance, and (ii) a linear function of a transmitter to receiver distance.

9. The downhole tool of claim 1 wherein the reflected signals comprise at least one of (i) compressional waves, and (ii) shear waves.

10. A method of determining the position of a bed boundary during the drilling of a wellbore, comprising:

(a) using a transmitter on a downhole tool to transmit acoustic signals at at least one downhole location;

(b) using a plurality of spaced apart receivers on the downhole tool to receive acoustic signals, said received signals including reflected signals from the bed boundary and unwanted signals traveling from the transmitter to the plurality of receivers;

(c) using a processor for prefiltering the received signals based on a threshold determination and selectively attenuating the unwanted signals, giving a plurality of prefiltered signals; and

(d) using the processor for determining a relative position and orientation of the bed boundary with respect to the downhole tool from the plurality of prefiltered signals.

11. The method of claim 10 wherein the prefiltering further comprises applying a threshold filter based upon an absolute maximum value of the received signals following a semblance processing.

12. The method of claim 10 wherein the at least one downhole location comprises a plurality of downhole locations, the method further comprising:

(i) providing the processor with supplemental and survey information relating to a position and orientation of the downhole tool at the plurality of downhole locations; and

(ii) using the processor to determine an absolute position and orientation of the bed boundary from the determined relative position and orientation of the bed boundary, the survey information relating to the position and orientation of the tool, and the supplemental information.

13. The method of claim 10 wherein the determination of the relative position further comprises using the processor to determine a plurality of semblances of the received signals in a slowness domain.

14. The method of claim 13 wherein the determination of the absolute position further comprises using the processor to analyze said plurality of semblances to obtain a set of maxima of the semblances.

15. The method of claim 10 wherein arrival times of the reflected signals at the plurality of receivers are at least one of (i) a linear function of a transmitter to receiver distance, and (ii) a hyperbolic function of a transmitter to receiver distance.

16. The method of claim 10 wherein the reflected signals are at least one of (i) compressional waves, and (ii) shear waves.

17. The method of claim 10 further comprising using an attenuator on the downhole tool for attenuating the body waves.

18. A method of determining the position of a bed boundary during the drilling of a wellbore, comprising:

(a) using a transmitter on a downhole tool to transmit acoustic signals at a plurality of downhole locations of the tool;

(b) using a plurality of spaced apart receivers on the downhole tool to receive acoustic signals reflected by the bed boundary at each of said plurality of spaced apart receivers at each of said plurality of downhole locations; and

(c) using a processor at each of said plurality of downhole locations to;

(i) prefiltering the received signals to attenuate an unwanted portion of the acoustic signals traveling from the transmitter to the receiver;

(ii) determining semblances of the signals received by the plurality of receivers in a slowness-minimum offset time domain,

(ii) filtering the determined semblances to give a set of filtered semblances,

(iii) determining from said set of filtered semblances and a velocity of the acoustic signals, a relative position and orientation of the bed boundary with respect to the tool, and

(d) determining an absolute position and depth of the boundary from said relative positions and orientations and from supplemental information.

19. The method of claim 18 wherein the step of using the processor to filter the determined semblances comprises at least one of (i) a coherence filtering, and, (ii) a semblance filtering.

20. The method of claim 18 wherein the prefiltering further comprises applying a threshold filter based upon an absolute maximum value of the received signals.