Method for monitoring point of interest in subterranean layer

Abstract

The invention relates to a method for monitoring a region of interest of a subterranean formation, the region of interest comprising a point of interest, the method comprising: extracting a first subset (1) of raw geophysical data associated with said point of interest from a set (0) of raw geophysical data previously acquired to generate a first image (Im1) of the subterranean formation; constructing a projection model (MOD1) on the basis of the first subset of data (1); reducing the first subset of data (1) to a second subset (2); creating a reference set (B) by applying the projection model (MOD1); performing the acquisition forming a third subset (3); creating a monitoring set (M) by applying the projection model (MOD1); comparing the first reference set (B) with the first monitoring set (M) in order to identify the evolution of the subsurface layer to be observed.

Description

Technical Field

[0001] The present invention relates to the field of acquiring and processing geophysical data.

[0002] More particularly, the present application relates to methods of monitoring a subsurface region of interest based on geophysical data. Background Art

[0003] According to prior art, it is known that the collection of geophysical data can be used to generate geophysical structural images of terrestrial or marine subsurface layers. Such images can help those skilled in the art determine the presence or absence of areas of interest, such as the presence or absence of natural resources, with a high degree of certainty.

[0004] Devices used to generate images of subsurface formations typically include multiple sources capable of generating seismic waves in different frequency ranges. These waves propagate through the subsurface and the region of interest, with some of them reflected back to receivers. By studying how the reflected waves propagate through the subsurface, an image of the subsurface can be generated.

[0005] This standard existing technique can be used repeatedly over time to generate a series of images of the subsurface, thereby determining and monitoring the evolution of the geophysical characteristics of the subsurface. Such images are called 4D images, that is, volumetric images of the subsurface evolving over time.

[0006] Current 4D research solutions are designed to completely update the 3D acquisition of the subsurface in a given area. In other words, a vast amount of seismic data must be acquired to generate a new, complete 3D image. Consequently, existing technologies require extensive computer processing time, ranging from weeks to months, and requiring massive amounts of data, ranging from gigabits to terabits.

[0007] 4D studies remain costly in terms of resources, especially considering the software resources needed to process the huge influx of data, which takes a long time because 4D images acquired at different calendar periods have to be generated and the evolution of the geophysical structure of the area of ​​interest has to be determined.

[0008] The applicant has previously proposed a number of improvements to the prior art, particularly with a view to reducing the costs of such equipment in terms of financial, material and software resources, for example by proposing a method for identifying, among a set of sources and a set of receivers, ideally positioned receivers for measuring the evolution of the geophysical structure in an area of ​​interest.

[0009] Despite these acquisition improvements, the raw records of acquired geophysical data may still require a specific processing stage to extract geophysical data that can be used for monitoring.

[0010] The present invention aims to overcome the shortcomings of the prior art by providing a method for extracting useful geophysical data for monitoring the operation of a subterranean formation to be monitored. Summary of the Invention

[0011] More specifically, the present invention relates to a method for monitoring at least one area of ​​interest of a subsurface to be observed, wherein the area of ​​interest includes a point of interest and an area adjacent to the point of interest, the method comprising:

[0012] Extracting a first subset of raw geophysical data associated with a neighborhood of the point of interest from a dataset of raw geophysical data previously acquired and preprocessed to generate a first previous image of the subsurface to be monitored;

[0013] constructing a first projection model based on the first data subset, wherein the first projection model is obtained by geophysical data separation and is used to extract effective geophysical data used in generating a first image in the vicinity of the point of interest;

[0014] reducing the first data subset to a second subset containing a predetermined minimum amount of raw geophysical data;

[0015] creating a first effective geophysical data reference set by applying a first projection operator to the second subset, the first projection operator being configured such that a first correlation coefficient between the obtained first effective geophysical data reference set and the first projection model is greater than 0.7;

[0016] Acquiring raw geophysical data to form a third subset of raw geophysical data, the third subset of raw geophysical data containing the same minimum number of raw geophysical data as the second subset, and each datum in the third subset corresponding to one datum in the second subset;

[0017] creating a first valid monitoring geophysical dataset by applying the same first projection operator from the first projection model to the third subset;

[0018] • Comparing the first reference set with the first monitoring set to identify the evolution of the subsurface layer to be observed.

[0019] The "neighborhood" of the point of interest is defined as the area corresponding to the resolution of the image generated by the geophysical method employed at the point of interest. For example, if the geophysical method is seismic, the resolution is typically assessed in terms of a quarter wavelength, where the wavelength is a function of the wave propagation velocity in the geological structure and the dominant frequency of the recorded signal.

[0020] Due to the combination of these features, this monitoring method can be used to observe the evolution of the geophysical response of a region of interest using limited data acquisition—data that, while insufficient to generate a 4D image, is sufficient to detect any potential changes in geophysical properties. Consequently, the software resources and time required to process the available geophysical data are significantly reduced. Furthermore, and particularly so, the constructed projection model is particularly immutable over time, meaning that once established, it can be reused.

[0021] Advantageously, the monitoring method further comprises, after creating the first monitoring geophysical dataset:

[0022] Extracting a second image from the first image, the second image corresponding to a neighborhood of the point of interest;

[0023] constructing a second projection model based on the first effective geophysical data reference set and the second image, where the second projection model is obtained by geophysical data separation and is used to extract effective geophysical data used in generating a second image in the vicinity of the point of interest;

[0024] creating a second effective geophysical data reference set (Bi) by applying a second projection operator to the first effective geophysical data reference set, wherein the second projection operator is configured such that a second correlation coefficient between the obtained second effective geophysical data reference set and a second projection model is greater than 0.7;

[0025] creating a second valid monitoring geophysical dataset by applying the same projection operator from a second projection model to the first valid monitoring geophysical dataset;

[0026] • comparing the second reference set of valid geophysical data with the second set of valid monitoring geophysical data to identify the evolution of the subsurface layer to be monitored.

[0027] In this configuration, the quality of the analysis produced by applying such projection models is significantly improved.

[0028] Advantageously, at least one new raw geophysical data acquisition iteration forms the third subset, and subsequent steps are performed over time, thereby allowing the evolution of the geophysical structure to be observed over regular or irregular calendar time intervals, for example, from weeks to months.

[0029] Advantageously, the minimum amount of valid geophysical data is equal to one record. In this configuration, the amount of data to be processed is significantly reduced, thereby reducing the resources required to monitor the subsurface region of interest. To this end, the records are selected, for example, by an inverse ray tracing method starting from the point of interest to identify the source location that generated the first subset of records and the receiver representing the point of interest.

[0030] Advantageously, the raw geophysical data are seismic wave records, and the effective geophysical data are reflected seismic waves, surface seismic waves or refracted seismic waves.

[0031] Advantageously, the acquisition of the previous seismic wave data is a two-dimensional, three-dimensional or four-dimensional acquisition, and the acquisition of the original geophysical data used to form the third subset is another four-dimensional acquisition.

[0032] Advantageously, the projection on the projection model comprises at least:

[0033] Deconvolution and / or frequency filtering of geophysical data;

[0034] Separation of geophysical data by applying slope filters defined in the time-distance plane and / or the frequency-wavenumber plane;

[0035] Static and dynamic correction of geophysical data.

[0036] In such a configuration, any predefined set of standard physical processes, whether single-channel or multi-channel, can be part of the projection model. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The invention will be better understood on reading the following description, given purely by way of example, and with reference to the accompanying drawings, provided as non-limiting examples, in which like references refer to similar objects and in which:

[0038] Figure 1 A flowchart describing the steps of a method for monitoring an underground layer to be observed according to a first embodiment of the present invention;

[0039] Figure 2 A flowchart illustrating steps of a method for monitoring an underground layer to be observed according to another embodiment of the present invention;

[0040] Figure 3 Schematic diagram illustrating a subset of the original geophysical data;

[0041] Figure 4 To illustrate the use Figure 3 Schematic diagram of the projection model constructed from the original geophysical dataset shown;

[0042] Figure 5 To illustrate the reduction Figure 3 A schematic diagram illustrating a record obtained by subsetting data and applying signal processing operations successively;

[0043] Figure 6 is a set of graphs comparing Figure 4 A single record of the projection model and Figure 5 Post-processing records;

[0044] Figure 7 A schematic diagram illustrating a subset of raw geophysical data and multiple acquisitions, each acquisition forming a subset of the raw geophysical data and each acquisition being conducted in a different calendar period;

[0045] Figure 8 To illustrate the effective reference geophysical dataset and to Figure 7 Schematic diagram of multiple effective Earth monitoring datasets obtained from data;

[0046] Figure 9 To illustrate and Figure 8 Schematic diagram of the difference in arrivals detected at the point of interest compared to the data;

[0047] Figure 10 To illustrate the effect of applying the second projection model to Figure 8 Schematic diagram of the third effective geophysical dataset obtained using the data.

[0048] It should be noted that the accompanying drawings illustrate embodiments of the present invention in detail; although these drawings are not intended to be limiting, their main function is to more clearly define the present invention when necessary. DETAILED DESCRIPTION

[0049] The present invention relates to a method for monitoring at least one area of ​​interest in an underground layer to be observed by a geophysical method, wherein the at least one area of ​​interest includes a point of interest and an area adjacent to the point of interest. Figure 1 In the illustrative embodiment shown, the monitoring method is specifically implemented by means of a seismic method, but any other geophysical method is also applicable.

[0050] The "neighborhood" of the point of interest is defined as an area corresponding to the resolution of the image generated by the geophysical method employed at the point of interest. For example, if the geophysical method is seismic, the resolution is typically one-quarter wavelength, where the wavelength is a function of the wave propagation velocity in the geological structure and the dominant frequency of the recorded signal.

[0051] The following description will gradually explain the Figure 1 The monitoring method of the first embodiment comprises the first step of extracting a first original geophysical data subset 1 related to the neighborhood of the point of interest from an original geophysical data set 0 previously acquired and preprocessed to generate a first previous image Im1 of the subsurface to be monitored.

[0052] The term "previously" is defined as a raw geophysical dataset 0 that has already been used in an imaging process, such as two-dimensional or three-dimensional imaging, to generate a first image Im1 of a subsurface layer. "Two-dimensional or three-dimensional" is defined as imaging performed in two or three spatial dimensions. Therefore, while a processed version of the raw geophysical dataset 0 exists, having undergone signal processing operations, the present method utilizes an unprocessed version of the dataset 0, such as a version stored on a storage medium.

[0053] Figure 3 The illustrated raw geophysical data subset 1 comprises at least raw geophysical data that, when previously processed, contributed to a portion of a first image Im1 encompassing a point of interest and a portion of its vicinity. In the depicted embodiment, the subset 1 is a set of raw two-dimensional seismic recordings. The term "two-dimensional" is defined as recordings based on two spatial coordinates.

[0054] The method further comprises the step of constructing a first projection model MOD1 based on the first data subset 1, the first projection model MOD1 being obtained by geophysical data separation and being used to extract valid geophysical data used in generating a first image Im1 in the vicinity of the point of interest. The projection model is a geophysical model, in this case a seismic model, that improves the quality of the original geophysical data subset, enabling monitoring of the area of ​​interest using valid geophysical data.

[0055] To create the first projection model MOD1, a single-channel processing is first performed, such as direct access suppression filtering and / or deconvolution filtering and / or frequency filtering. Optionally, a multi-channel processing is performed after the single-channel processing. The multi-channel processing may include, for example, a correction known to those skilled in the art as "normal moveout correction", frequency plane filtering, frequency-wavenumber plane filtering, frequency-distance plane filtering, singular value decomposition or any multi-channel signal processing method known in the art. In terms of seismic methods, the constructed first projection model contains effective geophysical data, i.e., data useful for generating the first image Im1, such as Figure 3 Alternatively, the geophysical data may be refracted seismic waves or surface seismic waves. The first projection model MOD1 is stored, for example, in a storage medium for subsequent use in implementing the method.

[0056] The method further comprises the step of reducing the first data subset 1 to a second subset 2, wherein the second subset 2 comprises a predetermined minimum amount of original geophysical data. In the above embodiment, the minimum amount of geophysical data is a single record, such as Figure 5 Here, the records are selected, for example, by an inverse ray tracing method starting from a point of interest to identify the source group positions generating the first subset 1 records and the receivers representing the point of interest.

[0057] The method further includes a step of creating a first effective geophysical data reference set B by applying the first projection model MOD1 to the second subset 2. This operation is referred to as "projecting" the second subset 2 onto the first projection model MOD1. The projection can, for example, be performed by a first projection operator to improve the quality of the records obtained in the previous inverse ray tracing step. The first projection operator processes the records to improve their quality and relevance, such that the first effective geophysical data set B approaches the first ideal projection model MOD1. The term "approximate" is defined as follows: the first projection operator is constructed by combining conventional signal processing techniques to obtain a target correlation coefficient between the first effective geophysical data reference set B and the first projection model MOD1. In other words, the correlation coefficient between the first effective geophysical data reference set B and the first projection model MOD1 is greater than a predetermined threshold, specifically 0.7. Therefore, the projected second subset 2 "approximates" the first projection model.

[0058] Figure 6 From left to right, the following diagrams illustrate: a first projection model MOD1, a first effective geophysical data reference set B, and a comparison between the first projection model MOD1 and the geophysical data reference set B. A correlation coefficient between the first projection model MOD1 and the geophysical data reference set B is calculated and is considered to be very high, i.e., greater than 0.99. Advantageously, the first correlation coefficient is greater than or equal to 0.7.

[0059] The method further includes the step of acquiring raw geophysical data to form a third raw geophysical data subset 3, wherein the third raw geophysical data subset 3 includes the same minimum number of raw geophysical data as the second raw geophysical data subset 2, and each data in the third raw geophysical data subset 3 is similar to a data in the second raw geophysical data subset 2. The term "similar" should be understood as meaning that the third raw geophysical data subset 3 is obtained through a single acquisition that is capable of monitoring the same point of interest as the second raw geophysical data subset 2 or an adjacent area thereof.

[0060] Advantageously, at least one new iteration of the acquisition of raw geophysical data forming the third subset 3 is performed over time. Figure 7 The same figure shows a second subset 2 at position zero, designated as a calendar reference date, and five subsets 3 collected at different calendar dates 31, 32, 33, 34, 35, a process known in English as "Monitoring spotgathers" or "MSG". The collections can be made at regular or irregular time intervals.

[0061] The method further comprises creating a first effective monitoring geophysical data set M by applying a third subset 3 of the same first projection operator obtained by the first projection model MOD1. When the acquisition of the third subsets 3 is iterated over time, each third subset 3 is projected onto the first projection model MOD1 using the first projection operator in the same way as the original geophysical data subset 2 was previously projected onto the first projection model MOD1. Figure 8 The figure shows the MSG of the geophysical data reference set B at the zero position and five sets of valid monitoring geophysical data M acquired on different calendar dates M1, M2, M3, M4 and M5. The acquisition can be performed at regular or irregular intervals in calendar time.

[0062] The method also includes comparing a first reference set B with a first monitoring set M to identify the evolution of the subsurface to be monitored. When multiple valid monitoring geophysical datasets M are present, each monitoring set is individually compared to the baseline reference set. In this configuration, the monitoring method can be used to observe the evolution of the geophysical response in the area of ​​interest using limited data acquisition—data that, while insufficient to generate a 4D image, is sufficient to detect any potential changes in geophysical properties. Consequently, the software resources and time required to process the valid geophysical data are significantly reduced.

[0063] Preferably, multiple comparisons can be performed using different theoretical models. Figure 9 The temporal drift measurements between a reference geophysical data set B and multiple valid monitoring geophysical data sets M acquired on different dates are shown. For each date, the arrival time difference dDT of the seismic echoes at the point of interest between the valid geophysical data set B and each valid monitoring geophysical data set M is calculated, along with the associated uncertainty, which depends on the signal-to-noise ratio and bandwidth known to those skilled in the art.

[0064] The following description will be based on Figure 2 Another embodiment is shown to illustrate the present invention. In this embodiment, the monitoring method further comprises, after creating the first valid monitoring geophysical dataset M:

[0065] Extracting a second image Im2 from the first image Im1, where the second image Im2 corresponds to the area adjacent to the point of interest;

[0066] constructing a second projection model MOD2 based on the first effective geophysical data reference set B and the second image Im2, wherein the second projection model MOD2 is obtained by geophysical data separation and is used to extract effective geophysical data used in generating the second image Im2 in the vicinity of the point of interest;

[0067] Creating a second reference geophysical data reference set Bi by applying a second projection operator to the first effective geophysical data reference set B, wherein the second projection operator is configured such that a second correlation coefficient between the obtained second effective geophysical data reference set Bi and the second projection model MOD2 is greater than 0.7;

[0068] Creating a second valid monitoring geophysical dataset Mi by applying the same projection operator from the second projection model MOD2 to the first valid monitoring geophysical dataset M;

[0069] Comparing said second reference set of valid geophysical data Bi with the second set of valid monitoring geophysical data Mi to identify the evolution of the subsurface layer to be monitored.

[0070] The second projection operator processes the records to further improve their quality and relevance, causing the second effective geophysical data set Bi to approach the second ideal projection model MOD2. The term "approach" is defined as: the second projection operator is constructed by combining conventional signal processing techniques to obtain a second target correlation coefficient between the second effective geophysical data reference set Bi and the second projection model MOD2. The second operator is configured to obtain the obtained second target correlation coefficient greater than 0.7.

[0071] Figure 10 The MSG of the second effective geophysical data reference set Bi at the calendar reference position and five second effective monitoring geophysical data sets Mi collected on different dates Mi1, Mi2, Mi3, Mi4 and Mi5 are shown. Based on this MSG, it is possible to perform analyses to draw conclusions about the temporal evolution of the geophysical structure of the area of ​​interest, especially to perform Figure 9 The illustrated embodiment is similar to the time drift measurement.

[0072] It should also be noted that the present invention is not limited to the embodiments described above. In fact, it will be apparent to those skilled in the art based on the information provided that various modifications can be made to the embodiments described.

[0073] In the above detailed description of the present invention, the terms used should not be construed to limit the present invention to the embodiments described herein, but should be construed to include any equivalent solutions that can be foreseen by those skilled in the art using their common knowledge and newly available information.

Claims

1. A method for monitoring at least one area of ​​interest in a subsurface to be observed, wherein the at least one area of ​​interest includes a point of interest and an area adjacent to the point of interest, the method comprising: Extracting a first subset of raw geophysical data (1) associated with the vicinity of the point of interest from a raw geophysical data set (0) previously acquired and pre-processed to generate a first previous image (Im1) of the subsurface to be monitored; constructing a first projection model (MOD1) based on the first data subset (1), wherein the first projection model (MOD1) is obtained by geophysical data separation and is used to extract effective geophysical data used when generating a first image (Im1) in the vicinity of the point of interest; reducing said first data subset (1) to a second subset (2) containing a predetermined minimum amount of raw geophysical data; creating a first effective geophysical data reference set (B) by applying a first projection operator to said second subset (2), said first projection operator being configured such that a first correlation coefficient between the obtained first effective geophysical data reference set (B) and a first projection model (MOD1) is greater than 0.7; collecting raw geophysical data to form a third subset of raw geophysical data (3), the third subset of raw geophysical data (3) containing the same minimum number of raw geophysical data as the second subset (2), and each data in the third subset (3) corresponding to one data in the second subset (2); creating a first valid monitoring geophysical dataset (M) by applying the same first projection operator from the first projection model (MOD1) to a third subset (3); • Comparing said first reference set (B) with the first monitoring set (M) to identify the evolution of the subsurface layer to be observed.

2. The monitoring method according to claim 1, characterized in that: After the creation of the first monitoring geophysical dataset (M), it also includes: Extracting a second image (Im2) from the first image (Im1), the second image (Im2) corresponding to a neighborhood of the point of interest; constructing a second projection model (MOD2) based on the first effective geophysical data reference set (B) and the second image (Im2), wherein the second projection model (MOD2) is obtained by geophysical data separation and is used to extract effective geophysical data used when generating the second image (Im2) in the vicinity of the point of interest; creating a second effective geophysical data reference set (Bi) by applying a second projection operator to the first effective geophysical data reference set (B), the second projection operator being configured such that a second correlation coefficient between the obtained second effective geophysical data reference set (Bi) and a second projection model (MOD2) is greater than 0.7; creating a second valid monitoring geophysical dataset (Mi) by applying the same projection operator from a second projection model (MOD2) to said first valid monitoring geophysical dataset (M); • Comparing said second reference set of valid geophysical data (Bi) with a second set of valid monitoring geophysical data (Mi) to identify the evolution of the subsurface layer to be monitored.

3. The monitoring method according to any one of the preceding claims, characterized in that At least one new iteration of raw geophysical data acquisition is performed to form a third subset (3), and subsequent steps are completed over time.

4. The monitoring method according to any one of the preceding claims, characterized in that The minimum amount of effective geophysical data is equal to one record.

5. The monitoring method according to any one of the preceding claims, characterized in that The original geophysical data is seismic wave records, and the effective geophysical data is reflected seismic waves, surface seismic waves, or refracted seismic waves.

6. The monitoring method according to claim 5, characterized in that: The acquisition of the previous seismic wave data is a two-dimensional, three-dimensional or four-dimensional acquisition, and the acquisition of the original geophysical data used to form the third subset (3) is another four-dimensional acquisition.

7. The monitoring method according to any one of the preceding claims, characterized in that The application of the first projection model (MOD1) comprises at least: Deconvolution or frequency filtering of geophysical data; Separation of geophysical data by applying slope filters defined in the time-distance plane and / or the frequency-wavenumber plane; Static and dynamic correction of geophysical data.

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