An experimental method for fluid injection-induced earthquakes under true triaxial stress conditions

Abstract

The present invention discloses a method for fluid injection-induced earthquake experiments under true triaxial stress conditions. The method first creates a cubic rock model containing a fault, obliquely cuts it to form a fault plane, lays out water injection and water pressure detection holes, and injects water to saturate the fault plane. A true triaxial device is then used to apply three-dimensional unequal stresses and maintain stability. Displacement is then used to drive the fault to spontaneous instability, and the inter-seismic instability period is calculated. Finally, a cyclic or constant-rate water injection scheme is set to simulate industrial fluid activity and explore the mechanism of water injection-induced earthquakes. This method for fluid injection-induced earthquake experiments under true triaxial stress conditions can accurately simulate actual stratum stress and water injection conditions, providing a reliable means for studying water injection-induced earthquakes and having significant implications for earthquake prediction, geological disaster prevention and control, and related industrial safety.

Description

Technical Field

[0001] The invention belongs to the technical field of earth science experiments, and in particular relates to a fluid injection-induced earthquake experimental method under true triaxial stress conditions. Background Art

[0002] With the acceleration of global industrialization and the continued rise in energy demand, industrial activities such as hydraulic fracturing and wastewater injection are expanding. These activities involve the forced injection of large amounts of high-pressure water into the ground. Faults are widespread within the complex underground geological structure. High-pressure water injection can disrupt the inherent mechanical equilibrium of these faults, causing them to activate. Once activated, faults can experience unstable slip, triggering seismic activity. In severe cases, these can even lead to destructive earthquakes, posing significant threats to human life, property, and the ecological environment.

[0003] Laboratory-induced seismic experiments have emerged to deepen our understanding of earthquake development and its mechanisms. Due to their relatively low cost, highly controllable experimental conditions, and comprehensive scope, these experiments have become a crucial tool for earthquake research in the Earth sciences, attracting widespread attention and in-depth research from both academia and industry. Based on the loading method, these experiments currently include direct shear injection, biaxial injection, and triaxial injection.

[0004] Among the many experimental methods, triaxial injection experiments are the most widely used due to their advantages in simulating underground stress environments. Most existing triaxial injection experimental devices are improved upon traditional confining pressure triaxial devices. However, the occurrence environment of faults in actual strata exhibits a true triaxial stress state, that is, the principal stresses in the three directions are unequal. Clearly, existing injection devices and methods based on confining pressure triaxial cannot accurately simulate the stress state of faults in real strata and have obvious limitations. In contrast, true triaxial injection devices can more closely mimic the occurrence of actual induced earthquakes, demonstrating better results and greater innovation in induced earthquake research, and are more conducive to revealing the inherent mechanisms of water injection-induced earthquakes. However, existing true triaxial water injection devices mainly focus on rock seepage or fault instability experiments, and lack systematic and targeted simulation of the complex process of water injection-induced earthquakes. In particular, in the design of the water injection steps, they fail to fully consider the various water injection methods in actual industrial activities and the complex physical processes of earthquake occurrence. Summary of the Invention

[0005] The purpose of the present invention is to provide an experimental method for fluid injection-induced earthquakes under true triaxial stress conditions, so as to solve the problem raised in the above background technology of how to accurately simulate in a laboratory environment the complete process of water injection operation causing fault instability and ultimately inducing earthquakes under complex conditions where the three-dimensional stresses in the real formation are unequal.

[0006] To achieve the above object, the present invention provides the following technical solution: a fluid injection-induced earthquake experimental method under true triaxial stress conditions, comprising the following steps:

[0007] S1. Use rock material to make a cube model with a side length of a cm × a cm × a cm. Cut it diagonally to form a fault plane. Drill a water injection hole from the bottom to the fault plane in the center of the fault footwall, and evenly set water pressure detection holes along the fault plane.

[0008] S2. Connect a high-precision high-pressure water pump to the water injection hole and inject water to infiltrate the fault surface until it is saturated;

[0009] S3. Use a true triaxial device to apply force to the fault model to achieve a true triaxial stress state in which the major, medium, and minor principal stresses are unequal, and maintain this state for d min;

[0010] S4, switch the true triaxial device from force loading to displacement loading, drive the fault model to spontaneously destabilize, and calculate the interseismic instability period T;

[0011] S5. By setting the number of cycles N, with a period of T / N, and alternating between oscillating water injection at a constant injection rate of e ml / min and no water injection, or injecting water at a constant rate of f ml / min with an interseismic instability period T as an interval, the mechanism of water injection-induced earthquakes can be explored.

[0012] In a further embodiment, the manufacturing accuracy of the side length of the cube model is controlled within ±m cm, and the cutting angle error of the fault plane is controlled within ±n° to ensure the accuracy of the model.

[0013] In a further embodiment, during the process of water injection to infiltrate the fault surface, the water injection rate is controlled between g ml / min and h ml / min to ensure the stability of the infiltration process.

[0014] In a further embodiment, the accuracy of the loading force of the true triaxial device is controlled within ±xN, and the stress loading rate is controlled within y MPa / min to ensure the accuracy and stability of the loading process.

[0015] In a further embodiment, during the displacement-driven spontaneous fault destabilization process, the displacement rate is controlled between z cm / min and w cm / min to achieve stable spontaneous fault destabilization.

[0016] In a further embodiment, during the water injection-induced earthquake process, parameters such as water pressure changes in the water pressure detection holes, fault displacement, and stress changes are monitored synchronously and jointly analyzed.

[0017] Technical effects and advantages of the present invention:

[0018] This fluid injection-induced earthquake experiment under true triaxial stress conditions can realistically simulate, in a laboratory environment, the complex conditions of unequal triaxial stress in real formations, as well as the entire process of water injection triggering fault instability and inducing earthquakes. Through carefully designed fault model processing, true triaxial stress loading, and a variety of water injection schemes, it overcomes the difficulty of existing experimental methods in simulating real formation stress states, making the experimental results more realistic and providing a reliable data foundation for in-depth research on the mechanism of water injection-induced earthquakes.

[0019] By systematically varying parameters such as the injection rate and number of cycles, and incorporating the process of displacement-driven spontaneous fault instability, we can comprehensively and deeply explore the source physics of water-injection-induced earthquakes. This method not only observes the macroscopic phenomenon of fault instability but also, through joint analysis of multiple sets of data such as simultaneously monitored water pressure changes, fault displacement, and stress changes, deeply explores the inherent physical processes of earthquake occurrence, filling the current gap in our understanding of the source mechanism of water-injection-induced earthquakes.

[0020] Based on the experimental results of this invention, researchers and relevant departments can more accurately assess the risk of earthquakes caused by underground fluid injection activities and formulate more scientific and effective earthquake prevention and response strategies, thereby reducing the threat of earthquake disasters to human life, property safety and the ecological environment;

[0021] This experimental method can provide a scientific basis for optimizing water injection schemes. By simulating the impact of different water injection methods on fault stability, industrial practitioners can choose safer and more reasonable water injection parameters to avoid earthquake activities caused by improper water injection operations, ensure the safe development of industrial activities, and promote the sustainable development of the energy industry. The fluid injection-induced earthquake experimental method under true triaxial stress conditions can accurately simulate real formation stress and water injection conditions, providing a reliable means for studying water injection-induced earthquakes, which is of great significance to earthquake prediction, geological disaster prevention and control, and related industrial safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0023] Figure 1 This is a true triaxial loading schematic diagram of the present invention;

[0024] Figure 2 It is a water injection model diagram of the present invention;

[0025] Figure 3 is a flow chart of the experimental method of the present invention;

[0026] Figure 4 It is a schematic diagram of the loading result of the present invention.

[0027] In the figure: 1. Fault plane; 2. True triaxial loading end; 3. High-precision high-pressure water pump; 4. Water injection hole; 5. Water pressure detection hole. DETAILED DESCRIPTION

[0028] In the following description, numerous specific details are provided to provide a more thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without one or more of these details. In other instances, certain technical features well known in the art are not described to avoid confusion with the present invention.

[0029] Unless otherwise defined, the directions of up, down, left, right, front, back, inside and outside involved in this document are based on the directions of up, down, left, right, front, back, inside and outside shown in the figures of the present invention, and are explained here together.

[0030] The present invention provides Figure 1-4 The method for fluid injection-induced earthquake experiment under true triaxial stress conditions includes the following steps:

[0031] 1. Fault model processing and drilling layout:

[0032] High-quality rock materials were used, and high-precision cutting equipment was used to produce a cube model with a side length of a cm × a cm × a cm. For rock samples containing faults, the entire cube was precisely cut along the diagonal to obtain a fault surface of a cm × a cm. During the cutting process, the cutting accuracy was strictly controlled to ensure that the flatness and angular errors of the fault surface were within a very small range to ensure the accuracy of the experimental results.

[0033] At the center of the fault footwall, a water injection hole 4 with a diameter of b cm was drilled from the bottom toward the fault plane. Professional drilling and cooling techniques were used during the drilling process to ensure hole quality and avoid thermal effects or mechanical damage during drilling that could affect the mechanical properties of the rock. Simultaneously, water pressure monitoring holes 5 with a diameter of c cm were drilled at locations evenly spaced along the fault plane. This ensured a uniform distribution of the monitoring holes and enabled comprehensive monitoring of water pressure changes. The selection of drilling locations was based on numerical simulations and previous experimental experience to ensure that the water pressure monitoring holes could accurately capture key information about water pressure changes on the fault plane.

[0034] 2. Fault surface infiltration treatment: Reliably connect the high-precision high-pressure water pump 3 to the water injection hole 4, turn on the high-precision high-pressure water pump 3, and inject water at a stable low speed. During the water injection process, closely observe the bottom of the fault footwall. When water slowly seeps out from the bottom, it is determined that the fault surface is saturated and the water injection is stopped. During the infiltration process, the water injection flow rate and water pressure changes are monitored in real time through high-precision flow sensors and pressure sensors to ensure the stability and controllability of the infiltration process;

[0035] 3. True triaxial servo loading: Using the true triaxial loading terminal 2, force loading is used to load the fault model to a true triaxial stress state with varying principal stresses, including large, medium, and small. During the loading process, stress changes are monitored in real time. The loading rate and amount are adjusted to ensure a smooth and accurate loading process. After loading is completed, the stress state is maintained for d min to ensure a stable stress. During the stress loading process, a large amount of actual formation stress data and geomechanical models are referenced to reasonably set the loaded stress value to realistically simulate the background stress state of the fault in the actual geotechnical environment.

[0036] 4. Displacement-driven spontaneous fault instability: Switch the true triaxial device to displacement loading mode, adjust the displacement rate appropriately, and drive the fault model to spontaneous instability. After three stick-slip events occur in the fault model, accurately calculate the interseismic instability period T. During the displacement driving process, high-precision displacement sensors and force sensors are used to monitor the changes in fault displacement and applied force in real time, providing reliable data for accurate calculation of the interseismic instability period.

[0037] 5. Water injection to induce laboratory earthquake: Turn on the high-precision high-pressure water injection pump, set the number of cycles to N, and use T / N as the period. Use a constant injection rate of e ml / min and alternate with no water injection to perform oscillating water injection. Observe the relationship between the effective stress of the displacement driving axis and the fault displacement (such as Figure 4 As shown in the figure, the mechanism and influencing factors of water injection-induced earthquakes are deeply explored. During the water injection process, multiple sets of data such as water pressure changes, fault displacement, and stress changes on the fault surface are monitored simultaneously. The multi-parameter joint analysis method is used to comprehensively and in-depth study the physical process of water injection-induced earthquakes.

[0038] In the process of implementing the present invention, the understanding and use of relevant terms should follow the following principles:

[0039] Directional terms: The directions or positional relationships indicated by the terms "up", "down", "left", "right", "front", "back", "top", "bottom", "inside", "outside", "vertical", "horizontal", "lateral", "longitudinal", etc. are determined based on the directions or positional relationships shown in the accompanying drawings. These terms are mainly used to describe the present invention and its embodiments, and do not limit the indicated devices, elements or components to have a specific direction, or to be constructed and operated in a specific direction. At the same time, these terms may have other meanings in some cases. For example, "up" may indicate a certain dependency or connection relationship in certain circumstances. Ordinary technicians in this field should accurately understand their meanings according to the specific circumstances. In actual experimental operations, these direction terms should be used accurately according to the layout of the experimental equipment and the operating procedures to ensure the accuracy and consistency of the experimental operations.

[0040] Connection terms: The terms "installed", "set", "provided with", "connected", and "connected" should be understood in a broad sense. They can be fixed connections, detachable connections, or integral structures; they can be mechanical connections, electrical connections; they can be directly connected, or indirectly connected through an intermediate medium, or they can be internal connections between two devices, elements, or components. Those skilled in the art should understand the specific meanings of the above terms in the present invention based on the specific circumstances. During the construction and debugging of the experimental device, the connection method should be reasonably selected according to the experimental requirements and equipment characteristics to ensure the stability and reliability of the experimental device.

[0041] Distinguishing Terms: Terms such as "first" and "second" are primarily used to distinguish different devices, elements, or components (which may or may not be of the same type and construction). They are not intended to indicate or imply the relative importance or quantity of the devices, elements, or components indicated. Unless otherwise specified, "plurality" means two or more. When recording and analyzing experimental data, these distinguishing terms should be used accurately to facilitate the classification, management, and comparative analysis of different experimental subjects and data, thereby improving the accuracy and reproducibility of experimental data.

[0042] In describing the embodiments of the present invention, the technical solutions of the present invention should be clearly and completely understood and applied. The embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A fluid injection-induced earthquake experimental method under true triaxial stress conditions, characterized by: The following steps are involved: S1. A cube model with a side length of a cm × a cm × a cm is made of rock material, and a fault plane (1) is formed by diagonally cutting the cube model. A water injection hole (4) is drilled from the bottom to the fault plane (1) at the center of the fault footwall, and water pressure detection holes (5) are evenly set along the fault plane (1). S2, connecting the high-precision high-pressure water pump (3) to the water injection hole (4), and injecting water to infiltrate the fault surface (1) until it is saturated; S3. Use a true triaxial device to apply force to the fault model to achieve a true triaxial stress state in which the major, medium, and minor principal stresses are unequal, and maintain this state for d min; S4, switch the true triaxial device from force loading to displacement loading, drive the fault model to spontaneously destabilize, and calculate the interseismic instability period T; S5. By setting the number of cycles N, with a period of T / N, and alternating between oscillating water injection at a constant injection rate of e ml / min and no water injection, or injecting water at a constant rate of f ml / min with an interseismic instability period T as an interval, the mechanism of water injection-induced earthquakes can be explored.

2. The fluid injection-induced earthquake experimental method under true triaxial stress conditions according to claim 1, characterized in that: The manufacturing accuracy of the side length of the cube model is controlled within ±m cm, and the cutting angle error of the fault plane (1) is controlled within ±n° to ensure the accuracy of the model.

3. The fluid injection-induced earthquake experimental method under true triaxial stress conditions according to claim 1, characterized in that: During the process of water injection into the fault surface (1), the water injection rate is controlled between g ml / min and h ml / min to ensure the stability of the infiltration process.

4. The fluid injection-induced earthquake experimental method under true triaxial stress conditions according to claim 1, characterized in that: The loading force accuracy of the true triaxial device is controlled within ±xN, and the stress loading rate is controlled within y MPa / min to ensure the accuracy and stability of the loading process.

5. The fluid injection-induced earthquake experimental method under true triaxial stress conditions according to claim 1, characterized in that: During the displacement-driven spontaneous fault instability process, the displacement rate is controlled between z cm / min and w cm / min to achieve stable spontaneous fault instability.

6. The fluid injection-induced earthquake experimental method under true triaxial stress conditions according to claim 1, characterized in that: During the water injection-induced earthquake process, the water pressure change, fault displacement, and stress change parameters of the water pressure detection hole (5) are monitored synchronously and jointly analyzed.

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