Calibration system for use in time lapse tomography

• US 5,946,271 A
• Filed: 03/21/1997
• Issued: 08/31/1999
• Est. Priority Date: 03/21/1997
• Status: Expired due to Fees

• Current Assignee: WesternGeco LLC
• Original Assignee: Western Atlas International Incorporated (Northrop Grumman Corporation)
• Inventors: Dragoset, William Henry Jr.
• Primary Examiner(s): Barlow, John
• Assistant Examiner(s): JOLLY, ANTHONY S
• Application Number:
  US08/822,299
• Time in Patent Office: 893 Days
• Field of Search: 367/48, 367/73, 367/13, 367/25, 367/26, 367/37, 367/71, 367/47
• US Class Current: 367/38
• CPC Class Codes: G01V 1/306 for determining physical properties of the subsurface, e.g. impedance, porosity or attenuation profiles
• CPC Class Codes: G01V 2210/612 Previously recorded data, e.g. time-lapse or 4D

Abstract

A discrete, minimally-equipped standard seismic calibration system is permanently installed over a subsurface reservoir structure that contains fluids of economic interest. Changes in the fluid content as a function of long-term time lapse may introduce changes in the acoustical characteristics of the reservoir rock layers, producing a corresponding change in the seismic signature recorded by the calibration system. The difference between two signatures over a selected time epoch defines a time-lapse calibration signature. The time lapse calibration signature can be applied to reduce the results of different conventional seismic data-acquisition systems of different vintages and technologies to a common standard.

Claims

1. A method for monitoring changes in the acoustical characteristics of a subsurface geologic reservoir as displayed within a preselected seismic reflection travel-time window, the changes being due to variations in the fluid distribution within said reservoir as a function of epochal time lapse, comprising:

a) installing at least one discrete localized acoustic calibration system, having a known impulse response, within a region associated with said reservoir;

b) using said localized acoustic calibration system, measuring a first acoustic signature characteristic of said reservoir at a first time epoch;

c) using said calibration system, measuring a second acoustic signature characteristic of said reservoir at a second time epoch;

d) defining a time-lapse calibration signature from the difference between said first and second acoustic signatures as measured within said preselected travel time window;

e) measuring a third acoustic signature characteristic of said reservoir at said first time epoch using a first distributed seismic data acquisition system, including a plurality of spatially distributed seismic data-sampling stations, said system having an unknown impulse response;

f) measuring a fourth acoustic signature characteristic of said reservoir at said second time epoch using a second distributed seismic data acquisition system including a plurality of spatially distributed seismic data-sampling stations, the system having an unknown impulse response;

g) defining a raw time-lapse signature from the difference between said measured third and fourth acoustic signatures;

h) substantially simulating said calibrated time-lapse signature by filtering the raw time lapse signature;

i) estimating changes in the acoustical characteristics of said reservoir, as a function of epochal time, from analysis of the waveform of the filtered time-lapse signature in comparison to the known impulse response of the calibration signature.

2. The method as defined by claim 1, comprising:

installing a plurality of discrete localized calibration systems within said region, each calibration system being ancillary to a nest of spatially-distributed seismic data-sampling stations;

executing steps b) through f);

defining a raw time-lapse signature, at each said spatially-distributed seismic data-sampling station, from the differences between the respective measured third and fourth acoustic signatures as derived from substantially co-located sampling stations;

substantially simulating the calibration signature defined by the corresponding ancillary calibration system by filtering the raw time-lapse signatures derived from said nest of sampling stations;

estimating local changes in the acoustical characteristics of said reservoir as a function of epochal time from analysis of the waveforms of the filtered time lapse signatures in comparison to the known impulse response the ancillary calibration system.

3. The method as defined by claim 2, wherein:

the raw time lapse signatures from any one nest of spatially-distributed sampling stations are averaged before the step of filtering.

4. The method as defined by claim 2, wherein:

the calibrated time lapse signatures from a plurality of ancillary calibration systems are averaged and the individual raw time lapse signatures from each spatially distributed seismic data-sampling point are filtered to simulate the averaged calibration time-lapse signature.

5. A method for monitoring changes in the acoustical characteristics of a subsurface geologic reservoir due to variations in the acoustical characteristics of said reservoir as a function of epochal time lapse, comprising:

a) installing at least one discrete localized acoustic calibration system, having a known impulse response, within a region associated with said reservoir;

b) using said localized acoustic calibration system, measuring a first acoustic signature characteristic of said reservoir at a first time epoch;

c) using said calibration system, measuring a second acoustic signature characteristic of said reservoir at a second time epoch;

d) defining a time-lapse calibration signature from the difference between said first and second acoustic signatures;

e) measuring a third acoustic signature characteristic of said reservoir at said first time epoch using a first distributed seismic data acquisition system, including a plurality of spatially distributed seismic data-sampling stations, said system having an unknown impulse response;

f) measuring a fourth acoustic signature characteristic of said reservoir at said second time epoch using a second distributed seismic data acquisition system including a plurality of spatially distributed seismic data-sampling stations, the system having an unknown impulse response;

g) defining a raw time-lapse signature from the difference between said measured third and fourth acoustic signatures;

h) substantially matching the raw time lapse signature to said calibrated time-lapse signature by filtering the raw time lapse signature;

i) estimating changes, over epochal time, in the acoustical characteristics of said reservoir from analysis of the waveform of the filtered time-lapse signature.

6. The method as defined by claim 5, comprising:

installing a plurality of discrete localized calibration systems within said region, each calibration system being ancillary to a nest of spatially-distributed seismic data-sampling stations;

executing steps b) through f);

defining a raw time-lapse signature, at each said spatially-distributed seismic data-sampling station, from the differences between the respective measured third and fourth acoustic signatures as derived from substantially co-located sampling stations;

substantially matching the calibration signature defined by the corresponding ancillary calibration system by filtering the raw time-lapse signatures derived from the corresponding nest of sampling stations;

estimating local changes in the acoustical characteristics of said reservoir as a function of epochal time from analysis of the waveforms of the filtered time lapse signatures.

7. A method for monitoring changes in the acoustical characteristics of a subsurface geologic reservoir as a function of epochal time lapse, comprising:

(a) using at least one discrete localized acoustic calibration system installed at a location responsive to acoustic signals reflected from said reservoir, said calibration system having a known impulse response, to measure a first acoustic signature characteristic of said reservoir at a first time epoch and a second acoustic signature characteristic of said reservoir at a second time epoch;

(b) defining a time-lapse calibration signature from the differences between said first and second acoustic signatures;

(c) using a first distributed seismic data system including a plurality of spatially distributed seismic data-sampling stations having an unknown impulse response for measuring a third acoustic signature characteristic of said reservoir at said first time epoch;

(d) using a second distributed seismic data system including a plurality of spatially distributed seismic data-sampling stations having an unknown impulse response for measuring a fourth acoustic signature characteristic of said reservoir at said second time epoch;

(e) defining a raw time-lapse signature from the difference between said measured third and fourth acoustic signatures;

(f) substantially matching the raw time lapse signature to said calibrated time-lapse signature by filtering the raw time lapse signature;

(g) estimating changes, over epochal time, in the acoustical characteristics of said reservoir from analysis of the waveform of the filtered time-lapse signature.