Mapping subsurface fractures using nonlinearity measurements

• US 20020122352A1
• Filed: 03/01/2001
• Published: 09/05/2002
• Est. Priority Date: 03/01/2001
• Status: Active Grant

• Current Assignee: Nonlinear Seismic Imaging, Inc.
• Original Assignee: Nonlinear Seismic Imaging, Inc.
• Inventors: Khan, Tawassul A.
• Granted Patent: US 6,597,632 B2
• Time in Patent Office: Days
• US Class Current: 367/57
• 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

A method for determining the location and the orientation of the open natural fractures in an earth formation from the interaction of the two seismic signals, one signal transmitted into the formation from one wellbore and the second signal transmitted from the surface of the earth; the interaction of the two seismic waves as they are transmitted through the fractured space is recorded in the second wellbore. The two seismic waves have two selected discrete frequencies. The seismic signal or the seismic wave transmitted from the surface is a low frequency of large amplitude identified as ‘modulation’ wave and the signal transmitted from the wellbore is a higher frequency by an order of one hundred times, identified as ‘carrier’ wave. The interaction of the ‘carrier’ wave during the compression and rarefaction cycles of the ‘modulation’ wave is spectrally analyzed. The relative amplitudes of the fundamentals frequency of the ‘carrier’ wave and its harmonics changes during compression and rarefaction cycles of the ‘modulation’ wave, since the fractures are squeezed during compression and open during rarefaction cycles of the large amplitude low-frequency signal. This method includes the determination of the spectral changes in the ‘carrier’ wave during the alternate half cycles of rarefaction and compression of the low-frequency signal.

When the low-frequency wave arrives at or near right angles to the fractures the difference is maximized and when arrives parallel or near parallel to the fractures the difference is minimized. By using the surface source at various locations on the surface in a predetermined geometric pattern and analyzing the results of the interaction of the two waves from each location, the orientation and the location of the fractures can be determined.

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.