Mapping subsurface fractures using nonlinearity measurements

• US 6,597,632 B2
• Filed: 03/01/2001
• Issued: 07/22/2003
• Est. Priority Date: 03/01/2001
• Status: Active Grant

• Current Assignee: Nonlinear Seismic Imaging, Inc.
• Original Assignee: Nonlinear Seismic Imaging, Inc.
• Inventors: Khan, Tawassul Ali
• Primary Examiner(s): Toatley, Jr., Gregory J.
• Application Number:
  US09/795,137
• Publication Number:
  US 20020122352A1
• Time in Patent Office: 873 Days
• Field of Search: 367/30, 367/57, 367/32, 166/250.1
• US Class Current: 367/32
• CPC Class Codes: G01V 1/288 Event detection in seismic signals, e.g. microseismics G01V1/36 takes precedence
• CPC Class Codes: G01V 2210/65 Source localisation, e.g. faults, hypocenters or reservoirs

Abstract

This Patent describes a method for determining the location and orientation of the open natural fractures in an earth formation by analyzing the interaction of the two seismic signals. One is a low frequency signal transmitted from the earth'"'"'s surface and the other a high frequency signal transmitted from a wellbore. The compressional and the rarefaction cycles of the lower frequency signal are used to modulate the width of the open fractures, which changes their transmission characteristics. As a result, the amplitude of the high frequency signal gets modulated as it propagates through the open fractures. The result of the interaction of the high and low frequency seismic signals is recorded in another wellbore. The spectral analysis of the modulated signal that is recorded during compression and rarefaction cycles of the lower frequency surface generated signal is used to determine the location and the orientation of the open natural fractures.

Claims

1. A method for mapping the open fractures in an earth formation from two seismic signals transmitted into said formation:

one high-frequency seismic signal transmitted from within one wellbore and received in another wellbore, and the other low-frequency seismic signal transmitted from the surface;

both seismic signals synchronized and transmitted at preselected frequencies, the interaction of the two transmitted signals received and recorded by the down hole receivers;

the method comprising;

spectrally analyzing said received signals;

determining the interaction of the low- and high-frequency signals as they propagate through the open fractures; and

determining the relative spectral amplitudes of the high-frequency signal that is transmitted from one wellbore and received in the other;

specifically measuring the amplitude difference of the fundamental of the higher frequency and its harmonics during rarefaction and compression cycles of the lower-frequency signal.

2. The method as defined in claim 1 further comprising:

determining the spectral difference of the high-frequency ‘

carrier’

signal between the rarefaction and compression cycle time windows of the lower-frequency ‘

modulation’

signal for each specific surface source location which is part of a geometric pattern; and

moving the surface source to the next location of the surface geometric pattern until the maximum and minimum spectral differences of the high-frequency ‘

carrier’

wave between the ‘

compression’ and

rarefaction cycle time windows of the lower-frequency ‘

modulation’

wave have been established.

3. The method in claim 1 further comprising:

determining of the location of the surface source that provides maximum relative spectral difference of the wellbore generated high-frequency ‘

carrier’

signal, the spectral measurements of the high-frequency signal made during the compression and rarefaction cycle time windows of the lower-frequency surface generated ‘

modulation’

wave, the location of the surface source identified to provide maximum spectral difference will provide the information related to the orientation of the fractures.

4. The method in claim 1 further comprising:

determining the location of the surface source that provides minimum spectral difference of the wellbore generated high-frequency ‘

carrier’

signal during the compression and rarefaction cycle time windows of the low-frequency surface generated ‘

modulation’

wave, the minimum difference will provide the information related to the orientation and the location of the fractures.

5. The method in claim 1 further comprising:

determining the relative amplitudes of the fundamental frequency of the wellbore generated high-frequency ‘

carrier’

wave during the compression and rarefaction cycle time windows of the surface generated lower-frequency ‘

modulation’

wave, these measurements made for every surface source location in the surface geometric pattern.

6. The method in claim 1 further comprising:

determining the relative amplitudes of the harmonic content of the wellbore generated high-frequency ‘

carrier’

wave during the compression and rarefaction cycle time windows of the surface generated lower-frequency ‘

modulation’

wave, these measurements made for every surface source location.

7. The method in claim 1 further comprising:

determining the number of compression and rarefaction cycles of the surface generated ‘

modulation’

wave in the recorded wellbore signal, summing the corresponding compression cycle and rarefaction cycle data to improve the signal to noise ratio.

8. The method in claim 1 further comprising:

using the identified surface source locations that provide maximum and minimum spectral variation of the ‘

carrier’

wave during compression and rarefaction cycles of the ‘

modulation’

wave, also the locations that provide maximum differences in fundamental frequency amplitude and the relative amplitude of the harmonics, and using this information to map the orientation and location of the open fractures between the wellbores.

9. The method in claim 1 further comprising:

using multiple receiver wells for simultaneous recording while transmitting from a single source well, mapping the orientation and location between each source/receiver well pair, then using this information to image the open fractures over a specific area in an oilfield.