Method and system for identifying structural surfaces and flow paths induced by fluid injection

Abstract

The present invention relates to a method and system for identifying structural surfaces and flow paths induced by fluid injection. The method comprises selecting rocks in a reservoir to be injected with fluid for a rock injection experiment, obtaining corresponding acoustic emission point cloud data, acoustic emission source mechanism, and pore fluid pressure; determining identification features of structural surfaces and flow paths according to the acoustic emission point cloud data, acoustic emission source mechanism, and pore fluid pressure; and performing microseismic monitoring on the reservoir to be injected with fluid to obtain monitoring results, and identifying structural surfaces and flow paths induced by fluid injection according to the identification features of structural surfaces and flow paths based on the monitoring results. The present invention can break through the limitations of traditional methods in describing and characterizing reservoir structural surfaces, thereby more comprehensively characterizing and characterizing underground geological structures.

Description

Technical Field

[0001] The present invention relates to the technical field of geological structure identification, and more particularly to a method and system for identifying structural surfaces and flow paths induced by fluid injection. Background Art

[0002] As the process of carbon sequestration and low-carbon industrial structure continues to accelerate, injection projects (or fracturing projects) are being carried out in various regions. However, the implementation of injection and fracturing projects has an unignorable impact on the rock structure of underground reservoirs. The implementation of injection projects may cause the reactivation of existing faults and other structural surfaces. Therefore, it is necessary to make timely adjustments to the induced seismic events monitored during the implementation of the project. Therefore, it is crucial to describe and characterize accurate reservoir structural surface information.

[0003] The reservoir rock structure surface refers to the various structures produced in the rock mass under the action of tectonic stress, including faults, joints, bedding and fracture zones. This rock structure surface has a certain direction, large extension and small thickness.

[0004] At present, the characterization and description of underground geological structures (volumes) are mainly achieved by processing and interpreting 3D seismic data. However, 3D seismic data generally occurs on faults, and during the formation of faults, not only does the rock break, but there is also a relative displacement between the two broken rock bodies. Therefore, not all geological structures can obtain corresponding 3D seismic data.

[0005] Some researchers have also tried to describe the reservoir structure by constructing a fracture network based on the event point positioning results in microseismic monitoring. However, the fracture network construction only connects all the event points and can only represent the fluid impact range, that is, the volume of the formation stimulated by the fluid, but cannot represent the fluid movement path and cannot be studied and analyzed to accurately obtain structural surface information.

[0006] Therefore, how to overcome the above limitations and develop new geological structure identification methods is an urgent problem that technicians in this field need to solve. Summary of the invention

[0007] In view of this, the present invention provides a method and system for identifying structural surfaces and flow paths induced by fluid injection, aiming to solve the limitations of traditional geological structure identification methods in describing and depicting accurate reservoir structural surface information. In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0008] A method for identifying a structural surface and a flow path induced by fluid injection, comprising:

[0009] Select the rocks in the reservoir to be injected with fluid to conduct rock injection experiments, and obtain the corresponding acoustic emission point cloud data, acoustic emission source mechanism and pore fluid pressure;

[0010] Determine the identification features of structural surfaces and flow paths based on acoustic emission point cloud data, acoustic emission focal mechanisms, and pore fluid pressure;

[0011] Microseismic monitoring is performed on the reservoir to be injected with fluid to obtain monitoring results, and based on the monitoring results, the structural surface and flow path induced by the fluid injection are identified according to the identification characteristics of the structural surface and the flow path.

[0012] Furthermore, surface fitting is performed on the acoustic emission point cloud data, and identification features of the structural surface and the flow path are determined based on the tangent vector and normal vector of the surface.

[0013] Furthermore, the identification feature of the structural surface is expressed as:

[0014] The product of the tangent vector and the normal vector of the surface is 0, the focal mechanism is shear type, and the pore fluid pressure is less than the ground stress;

[0015] The identification characteristics of the flow path are expressed as:

[0016] The product of the tangent vector and the normal vector of the surface is not 0, the focal mechanism is tensional, and the pore fluid pressure is greater than the ground stress.

[0017] Furthermore, the geostress is the confining pressure surrounding the rock, which can be directly obtained through rock injection experiments or logging data.

[0018] Furthermore, a reservoir core to be tested is selected and made into the size required for the true triaxial experiment, so as to simulate fluid injection through the true triaxial experiment.

[0019] Furthermore, the monitoring results obtained from microseismic monitoring are preferentially classified in time and space to identify the structural surfaces and flow paths induced by fluid injection based on the simultaneous and spatial isomorphic data.

[0020] A fluid injection induced structural surface and flow path identification system, which internally stores identification features of structural surfaces and flow paths determined by the fluid injection induced structural surface and flow path identification method as described above,

[0021] Used to receive and identify the structural surface and flow path induced by fluid injection based on the identification characteristics of the structural surface and flow path according to the microseismic monitoring results of the reservoir to be tested.

[0022] It can be seen from the above technical solutions that the method and system for identifying structural surfaces and flow paths induced by fluid injection provided by the present invention comprehensively consider multi-dimensional information such as acoustic emission point cloud data, acoustic emission source mechanism, and pore fluid pressure when determining the identification characteristics of structural surfaces and flow paths. Compared with traditional three-dimensional seismic data processing and interpretation methods, the present invention can more comprehensively characterize and depict underground geological structural surfaces (bodies), breaking through the limitations of traditional methods in data acquisition.

[0023] Furthermore, through the precise prediction and description of the structural surface and the fluid dominant path, the injection project is effectively monitored. When a large number of events are detected on the structural surface during the monitoring and prediction process, the project can be adjusted in time, thus providing a highly reliable guarantee for the implementation of subsequent injection projects. In addition, the fluid dominant path of the current project is explored, which has a certain guiding role in the implementation of subsequent projects.

[0024] This scheme performs well in rigid-brittle rock mass, but is not limited to reservoirs of this type. Rigid-brittle rock mass responds significantly to stress. Rigid-brittle strata reservoirs contain more structural information such as joints and faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0026] Figure 1 A flow chart of the method for identifying structural surfaces and flow paths induced by fluid injection provided by the present invention;

[0027] Figure 2 A schematic diagram of a true triaxial experiment provided by the present invention;

[0028] Figure 3 This is a schematic diagram of the recognition results provided by the present invention. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] The structural planes in the rock mass have a larger spatial distribution relative to the rock. Among them, the larger scale representative ones are faults. The regular polygons in the middle represent rocks. Rocks are units smaller than the rock mass. The rock mass may be composed of multiple rocks. Rock structures such as joints and bedding may be formed inside the rock. These structures are areas that are prone to microseismic events during the implementation of the fluid injection project. The events occur independently or in clusters. Timely and accurate identification of the structural planes and paths triggered during the fluid injection process can achieve effective monitoring of the injection project.

[0031] In order to overcome the defects in the prior art, the present application provides a method for identifying a structural surface and a flow path induced by fluid injection, such as Figure 1 As shown, the method is implemented by the following steps:

[0032] Select the rocks in the reservoir to be tested for rock injection experiments to obtain the corresponding acoustic emission point cloud data, acoustic emission source mechanism and pore fluid pressure;

[0033] Determine the identification features of structural surfaces and flow paths based on acoustic emission point cloud data, acoustic emission focal mechanisms, and pore fluid pressure;

[0034] Microseismic monitoring is performed on the reservoir to be tested to obtain monitoring results, and based on the monitoring results, the structural surface and flow path induced by fluid injection are identified according to the identification characteristics of the structural surface and the flow path.

[0035] In this embodiment, the above method mainly includes two parts:

[0036] The first part is to conduct rock injection experiments on the rocks in the reservoir to be tested;

[0037] Since it is necessary to monitor reservoir structures of different lithologies in practical applications, it is necessary to first conduct rock experiments on the rocks in the reservoir to be tested. In this embodiment, a rock injection experiment is conducted based on true triaxial injection, and the steps include:

[0038] 1) Select the reservoir rock in the area to be tested, give priority to cores and select rocks with structural surfaces inside the rock, and make the sample size suitable for the triaxial experimental instrument (schematic diagram as shown in the figure) Figure 2 , the arrow in the figure shows the stress loading direction, and the vertical pipe in the cube is the fluid injection channel);

[0039] 2) Conduct triaxial fluid injection experiments, that is, inject fluid into the rock sample. The experimental process is equipped with an acoustic emission monitoring system to monitor the acoustic emission events occurring during the injection process in real time; at the same time, the acoustic emission focal mechanism P and the pore fluid pressure Pp are obtained;

[0040] In this embodiment, there is waveform data at the acoustic emission event point, and the focal mechanism solution is obtained by inverting the waveform data, including tension, compression, shear and other types;

[0041] The pore pressure can be monitored and obtained through the pore pressure monitoring device equipped during the experiment;

[0042] 3) Based on the acoustic emission events obtained in the above steps, i.e., the acoustic emission point cloud data, as well as the acoustic emission source mechanism and pore fluid pressure, the identification features of the structural surface and the flow path are determined, i.e., the corresponding relationship between the above data and the structural surface / flow path.

[0043] This process mainly uses experimental means to obtain the mechanism and spatiotemporal laws of events occurring in rocks with structural surfaces during fluid injection, and summarizes and classifies the relationship between them and the existing structural surfaces, laying the foundation for subsequent reservoir microseismic monitoring and analysis.

[0044] In this embodiment, the identification features of the structural surface and the flow path are different. The structural surface is characterized by being relatively straight and nearly straight on a large scale. The time and space intervals may not be very dense, that is, there will be no large-scale curves with various curved shapes. However, after the subsequent fluid injection into the fluid path, microseismic events will continue to occur on its path. Compared with the structural surface, its spatial structural morphology is more random, that is, the morphology from the injection point to another position is diverse, as is the distribution of the event point cloud, and its occurrence frequency is also more dense.

[0045] During the rock injection experiment, the spatial and temporal distribution of the acoustic emission event point (x), the focal mechanism of the acoustic event (P), and the pore fluid pressure (pp) can be finally obtained.

[0046] In this embodiment, the acoustic emission point cloud data is firstly fitted with a surface (including a plane or a curved surface) to obtain the tangent vector and normal vector of the surface, and the tangent vector t and normal vector n of the fitted plane are further used to characterize the characteristics of the structural surface and the fluid path; at the same time, by statistically analyzing the focal mechanism components of the event cluster and the pore fluid pressure, the following conclusions are finally obtained:

[0047] t*n=0 is the structural surface feature, t*n≠0 is the fluid path feature;

[0048] P is the focal mechanism information calculated from the event point set, and the focal extension mechanism of the structural surface is biased towards the shear type, while the focal extension mechanism of the fluid path is biased towards the tension type;

[0049] Regarding the pore fluid pressure PP, compared with the structural surface, the fluid path has a special PP feature, that is, at the time of the event, there is a fluid pressure pp greater than the ground stress σ, where the ground stress is the confining pressure wrapped in the rock, which is directly obtained through rock injection experiments or logging data.

[0050] When conducting rock experiments, the confining pressure information must be set first. The ground stress is simulated by the confining pressure. The basis for this is the horizontal stress and vertical stress (obtained through well logging) of the actual reservoir to be tested. The vertical stress is generated by the influence of gravity. In addition to well logging, there is also the hydraulic fracturing method for measuring horizontal stress.

[0051] In an exemplary embodiment, the identification feature of the structural surface is expressed as:

[0052] The product of the tangent vector and the normal vector of the surface is 0, the focal mechanism is shear type, and the pore fluid pressure is less than the ground stress; that is, F1 = (t*n = 0, P = JQ, pp < σ);

[0053] The identification characteristics of the flow path are expressed as:

[0054] The product of the tangent vector and the normal vector of the surface is not 0, the focal mechanism is tensional, and the pore fluid pressure is greater than the ground stress; that is, F2 = (t*n≠0, P = LZ, pp>σ).

[0055] In the second part, microseismic monitoring is performed on the reservoir to be tested to obtain monitoring results, and based on the monitoring results, the structural surfaces and flow paths induced by fluid injection are identified according to the identification characteristics of the structural surfaces and flow paths.

[0056] The monitoring result is the event point cloud obtained by microseismic monitoring.

[0057] In this embodiment, before identifying the structural surface and flow path induced by fluid injection based on the event point cloud, the event point cloud is first classified in time and space; that is, the principle of simultaneous and spatial isomorphism is followed. Simultaneous time and space refer to event points that occur close in time and space, and isomorphism refers to events with similar occurrence mechanisms.

[0058] In a preferred embodiment, the microseismic event point clouds can be divided according to Δx and Δt (distance and occurrence time difference); that is, the distance (Δx) less than 20m and the occurrence time interval (Δt) less than 1h are divided into one category.

[0059] Then, the structural surface and flow path identification features determined in the first part are introduced into the spatiotemporal distribution of microseismic events to identify the structural surface and flow path.

[0060] In actual monitoring, the focal mechanism of microseismic events can be obtained by inversion calculation using the monitored waveform, but the pore pressure cannot be directly monitored. In this embodiment, the pore pressure is equal to the ground stress minus the effective stress.

[0061] Effective stress is the root cause of rupture events, which is equal to the unit mass of rock multiplied by the rupture distance (the rupture distance can be obtained at the same time as the focal mechanism of the microseismic event is inverted), that is,

[0062] pp=σ-σ' σ'=m*d

[0063] Where σ' represents the effective stress, m represents the unit mass, and d represents the rupture distance.

[0064] It should be noted that, in general, the in-situ formation stress is greater than the pore pressure (but not absolutely). When the pore pressure is greater than the ground stress, it can cause rock damage. The destructive stress is caused by the fluid and is called effective stress, that is, the magnitude of the stress that causes rock damage. The positive and negative values ​​represent the type and direction of damage.

[0065] The present invention combines laboratory rock physics experiments with field monitoring, and analyzes the classification results of events caused by different influences in advance through rock experiments, so as to obtain the relationship between the relevant event development and the rock structure surface, so as to determine the identification characteristics of the identification surface and the flow path. In one embodiment, the identification results are as follows: Figure 3 As shown, the red arrow is the fluid path, the yellow arrow is the structural surface, and the colored background is the event point distribution.

[0066] The present invention can be used to guide the interpretation results of events collected by microseismic engineering, thereby making the interpretation results more reliable and accurate.

[0067] Through the prediction and description of the structural surface and the fluid dominant path by the identification method disclosed in this application, effective monitoring of the injection project can be achieved. If a large number of events are found on the structural surface during the monitoring and prediction process, the project needs to be adjusted in time, which can provide a high degree of credibility for the implementation of subsequent injection projects. The fluid dominant path of the current project is explored, which plays a certain guiding role in the implementation of subsequent projects.

[0068] On the other hand, the present application provides a fluid injection-induced structural surface and flow path identification system, the system internally stores identification features of structural surfaces and flow paths determined by the fluid injection-induced structural surface and flow path identification method as described above,

[0069] Used to receive and identify the structural surface and flow path induced by fluid injection based on the identification characteristics of the structural surface and flow path according to the microseismic monitoring results of the reservoir to be tested.

[0070] The present invention not only breaks through the limitations of traditional three-dimensional seismic data processing and interpretation methods in data acquisition, but also improves the accuracy of depicting reservoir structural surface information. Furthermore, the establishment of a corresponding system provides a more convenient and efficient way for geological structure identification, which helps to timely adjust the induced seismic events monitored during the implementation of the project and ensure the stability and safety of the rock structure surface of the underground reservoir.

[0071] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0072] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for identifying structural surfaces and flow paths induced by fluid injection, characterized in that: Select rocks in the reservoir to be injected with fluid to conduct rock injection experiments, and obtain corresponding acoustic emission point cloud data, acoustic emission source mechanism and pore fluid pressure; Determine the identification features of structural surfaces and flow paths based on acoustic emission point cloud data, acoustic emission focal mechanisms, and pore fluid pressure; The identification features of the structural surface are expressed as: The product of the tangent vector and the normal vector of the surface is 0, the focal mechanism is shear type, and the pore fluid pressure is less than the ground stress; The identification characteristics of the flow path are expressed as: The product of the tangent vector and the normal vector of the surface is not 0, the focal mechanism is tensional, and the pore fluid pressure is greater than the ground stress; Microseismic monitoring is performed on the reservoir to be injected with fluid to obtain monitoring results, and based on the monitoring results, the structural surface and flow path induced by the fluid injection are identified according to the identification characteristics of the structural surface and the flow path.

2. A method for identifying structural surfaces and flow paths induced by fluid injection according to claim 1, characterized in that: The acoustic emission point cloud data is fitted with a surface, and the identification features of the structural surface and flow path are determined based on the tangent vector and normal vector of the surface.

3. A method for identifying structural surfaces and flow paths induced by fluid injection according to claim 1, characterized in that: The geostress is the confining pressure surrounding the rock and can be directly obtained through rock injection experiments or well logging data.

4. A method for identifying structural surfaces and flow paths induced by fluid injection according to claim 1, characterized in that: The reservoir core to be tested is selected and made into the required size for the true triaxial experiment to simulate fluid injection through the true triaxial experiment.

5. A method for identifying structural surfaces and flow paths induced by fluid injection according to claim 1, characterized in that: Priority is given to spatiotemporal classification of the monitoring results obtained from microseismic monitoring in order to identify the structural surfaces and flow paths induced by fluid injection based on spatiotemporal isomorphic data.

6. A system for identifying structural surfaces and flow paths induced by fluid injection, characterized in that: The identification features of the structural surface and the flow path determined by the method for identifying the structural surface and the flow path induced by fluid injection according to any one of claims 1 to 5 are stored internally. Used to receive and, based on the microseismic monitoring results of the reservoir to be tested and based on the identification features of the structural surfaces and flow paths, identify the structural surfaces and flow paths induced by fluid injection.

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