CA-3038118-C - METHOD OF BOREHOLE TIME-LAPSE MONITORING USING SEISMIC WAVES

Abstract

Receiver-consistent scalars of seismic receiver channels are used for time-lapse monitoring of a sub-surface earth formation. Signals are induced by seismic waves propagating through the earth formation adjacent to each respective seismic receiver channel. Each seismic receiver channel is acoustically coupled to the earth formation as present directly adjacent to the location of the seismic receiver channel in question. The base receiver-consistent scalars and the monitor receiver-consistent scalars of seismic receiver channels can be outputted to reveal changes in these receiver-consistent scalars. These changes can be used to delineate information about physical changes in the subsurface earth formation. The changes in the based receiver-consistent scalars and the monitor receiver-consistent scalars may be displayed visually.

Inventors

• Albena Alexandrova Mateeva
• Paul Maarten ZWARTJES

Assignees

• SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.

Dates

Publication Date

20260505

Application Date

20171004

Priority Date

20161006

Claims (1)

1. 85102948 CLAIMS: 1. A method of time-lapse monitoring of a subsurface earth formation adjacent to a seismic receiver spread, comprising: 5 - selecting a seismic receiver spread disposed in an earth formation, wherein said seismic receiver spread comprises a plurality of seismic receiver channels each seismic receiver channel having a seismic receiver channel location, whereby each seismic receiver channel is acoustically coupled to the earth formation that is present adjacent to the seismic receiver channel location; - collecting base survey data, comprising measuring base survey signals in the seismic receiver 10 spread induced by seismic waves propagating through the earth formation; - determining base receiver-consistent scalars for the seismic receiver channels from the base survey data, wherein each said receiver-consistent scalar is a measure of signal amplitude induced by seismic waves propagating through the earth formation adjacent to each respective seismic receiver channel; 15 - allowing a time lapse between said collecting of base survey data and collecting of monitor survey data; - after said time lapse, collecting the monitoring survey data, comprising measuring monitor survey signals in the seismic receiver spread induced by seismic waves propagating through the earth formation; 20 - determining monitor receiver-consistent scalars for the seismic receiver channels from the monitor survey data; - outputting base and monitor receiver-consistent scalars to reveal changes in the receiver-consistent scalars before and after the time lapse. 25 2. The method of claim 1, wherein the values of signal amplitude are determined using a channel-consistent scalar technique. 3. The method of claim 1 or 2, wherein signal amplitude is quantified by an inverse root-meansquare average amplitude of a plurality of coherent seismic events. 14 Date Re9ue/Date Received 2024-05-09 85102948 4. The method of any one of claims 1 to 3, further comprising the step of: - using the changes in the receiver-consistent scalars to delineate information about physical changes in the subsurface comprising changes in the earth formation that is present adjacent to the seismic receiver channel locations. 5. The method of any one of claims 1 to 4, further comprising a step of depth-matching of monitor receiver-consistent scalars of one or more of the seismic receiver channels with base receiver-consistent scalars of corresponding seismic receiver channels. 10 6. The method of claim 5, further comprising extracting a set of depth shifts for said one or more of the seismic receiver channels between base and monitor surveys. 7. The method of claim 6, comprising outputting said set of depth shifts and delineating information about relative movement between rock layers in the subsurface earth formation and the 15 seismic receiver channels that has occurred during the time lapse. 8. The method of any one of claims 1 to 7, further comprising depth aligning the base receiverconsistent scalars and monitor receiver-consistent scalars, and outputting differences between base receiver-consistent scalars and monitor receiver-consistent scalars for one or more of the seismic 20 receiver channels. 9. The method of any one of claims 1 to 8, further comprising delineating a change in an elastic property of the earth formation that is present adjacent to the seismic receiver channel locations. 25 10. The method of any one of claims 1 to 9, wherein the monitor receiver-consistent scalars for the seismic receiver channels are determined from the monitor survey data by following the same procedure that is used to determine the base receiver-consistent scalars from the base survey data. 11. The method of any one of g claims 1 to 10, wherein the time lapse is at least one full day. Date Re9ue/Date Received 2024-05-09 85102948 12. The method of any one of claims 1 to 11, wherein the seismic receiver channels in the seismic receiver spread form a string of seismic receiver channels. 13. The method of any one of claims 1 to 12, wherein the seismic receiver spread is formed by a 5 Distributed Acoustic Sensing (DAS) system which subdivides an optical fiber in a plurality of DAS receiver channels whereby the seismic receiver channels are said DAS receiver channels. 14. The method of claim 13, wherein said optical fiber is packaged in a cable and operated as a DAS optical fiber, preferably wherein a plurality of optical fibers is packaged in said cable and 10 operated as DAS optical fibers. 15. The method of claim 14, where said optical fiber is sensitive to broadside seismic waves relative to said cable. 15 16. The method of any one of claims 1 to 15, wherein said seismic spread is disposed in a borehole in the earth formation. 17. The method of any one of claims 1 to 16, wherein signal amplitude is determined using an average amplitude of a plurality of coherent seismic events. 18. The method of any one of claims 1 to 17, wherein the receiver-consistent scalars are a measure of elastic properties of the formation adjacent to the respective seismic-receiver channel to which the seismic-receiver channel is coupled. 25 19. The method of any one of claims 1 to 18, wherein said changes in receiver-consistent scalars reflect changes of elastic properties of the formation adjacent to the respective seismic receiver channel to which the seismic-receiver channel is coupled. 16 Date Re9ue/Date Received 2024-05-09 85102948 20. The method of any one of claims 1 to 19, wherein each receiver-consistent scalar is a single numerical value which characterizes the response of a specific receiver as configured within its local environment coupled to the earth formation, by removing contributions or effects of other circumstances that affect a factual response. 17 Date Re9ue/Date Received 2024-05-09

Description

85102948 METHOD OF BOREHOLE TIME-LAPSE MONITORING USING SEISMIC WAVES Field of the Invention In accordance with a first aspect of the present invention, there is provided a method of timelapse monitoring of a subsurface earth formation. Background of the Invention 5 Various seismic techniques have been developed which employ a seismic receiver spread disposed in a borehole in an earth formation. Examples include tomographic techniques (such as include cross-borehole seismic tomography), and Vertical Seismic Profiling (VSP). Distributed Acoustic Sensing (DAS) is a useful novel technology to provide such a seismic receiver spread in a borehole in an earth formation for seismic data acquisition purposes. A 10 description of this technology is provided in an article "Distributed acoustic sensing for reservoir monitoring with vertical seismic profiling" by Alhena Mateeva et al., which appeared in Geophysical Prospecting, Vol. 62, pp. 679-692 (2014). Conceptually, DAS measurements are simple. A DAS interrogator unit sends laser pulses along an optical fiber disposed in a wellbore, and measures signals of back-scattered light. The optical fiber can be subdivided into DAS receiver 15 channels ( corresponding, for instance, to VSP receiver levels) based on the time of flight of a laser pulse along it. DAS also has been proposed for time-lapse monitoring, particularly time-lapse VSP. In the context ofVSP, a time-lapse surveying is also known as 4D surveying. However, 4D VSP is far from easy. Not only are high level of repeatability and high signal quality (particularly high signal- 20 to-noise ratio) required, which makes 4D acquisition technically challenging, but also the positioning of detected 4D changes in the formation requires significant effort in both acquisition and processing, especially away from the VSP well(s). These difficulties exist irrespective of whether the VSP is based on DAS or conventional receivers such as hydrophones/geophones. 1 Date Re9ue/Date Received 2024-05-09 85102948 Summary of the invention In a first aspect there is provided a method of time-lapse monitoring of a subsurface earth formation, comprising: - selecting a seismic receiver spread disposed in a borehole in an earth formation, wherein said 5 seismic receiver spread comprises a plurality of seismic receiver channels each seismic receiver channel having a seismic receiver channel location, whereby each seismic receiver channel is acoustically coupled to the earth formation that is present adjacent to the seismic receiver channel location; - collecting base survey data, comprising measuring base survey signals in the seismic receiver 10 spread induced by seismic waves propagating through the earth formation; - determining base receiver-consistent scalars for the seismic receiver channels from the base survey data; - allowing a time lapse between said collecting of base survey data and collecting of monitor survey data; 15 - after said time lapse, collecting the monitoring survey data, comprising measuring monitor survey signals in the seismic receiver spread induced by seismic waves propagating through the earth formation; - determining monitor receiver-consistent scalars for the seismic receiver channels from the monitor survey data; 20 - outputting base and monitor receiver-consistent scalars to reveal changes in the receiver-consistent scalars before and after the time lapse. Each said receiver-consistent scalar is a measure of signal strength (amplitude) induced by seismic waves propagating through the earth formation adjacent to each respective seismic receiver channel. The base and monitor receiver-consistent scalars suitably quantify signal amplitude. 25 The changes in the receiver-consistent scalars may be used to delineate information about changes in the subsurface. Amongst changes in the subsurface that can be delineated are changes in the earth formation that is present adjacent to the seismic receiver channel locations (for instance, changes in elastic properties of the formation), and changes in relative positions between rock layers in the subsurface earth formation and the seismic receiver channels that have occurred during the 30 time lapse, for example due to well completion deformations or due to subsidence. 2 Date Re9ue/Date Received 2024-05-09 85102948 Alternative, or in addition, to using the changes in the receiver-consistent scalars to delineate information about changes in the subsurface, the changes in receiver-consistent scalars at one or more of the seismic receiver channels may be used for depth-matching of the base and monitor survey data. Subsequent to such depth-matching, information about changes in the subsurface may 5 be delineated from the changes in the receiver-consistent scalars. In addition, or instead thereof, more generally the base and monitor survey data (depth-matched or not) may be used in any suitable 4D analysis method. Brief description of the drawing The appended drawing, which i