Fracturing fluid viscosity dynamic regulation and control device and method for true triaxial hydraulic fracturing experiment

Abstract

The invention discloses a fracturing fluid viscosity dynamic regulation and control device and method for a true triaxial hydraulic fracturing experiment, and the method comprises the following steps: (1) starting and detecting whether a true triaxial hydraulic fracturing experiment system operates normally, verifying the accuracy and rationality of a Smith time delay compensation algorithm and adaptive PID control, and applying in-situ stress to a fracturing sample; (2) setting an initial viscosity value, injecting fracturing fluid into the fracturing sample at a constant flow, and synchronously starting an acoustic emission and resistivity signal receiver; (3) the fracturing effect is evaluated according to implementation feedback information such as acoustic emission / resistivity signals and fracturing pressure in the fracturing process, and the fluid mixing proportion is updated to track a new viscosity set value containing time delay compensation; (4) repeating the step (3) until a relatively complex hydraulic fracture structure is formed or the fracture thoroughly penetrates through the fracturing sample; and (5) based on different viscosity change paths set in the step (3), quantitatively analyzing the inhibition or promotion effect of viscosity change on fracture expansion, turning and branches, and realizing the maximization of the fracture network volume of hydraulic fracturing. A double-cylinder injection system capable of adjusting the fluid mixing proportion in real time is introduced, the limitation of traditional static mixing of the fracturing fluid is broken through in combination with a Smith time delay compensation algorithm and self-adaptive PID control, and the complex influence mechanism of dynamic viscosity change of the fracturing fluid on crack propagation and propping agent migration is explored; and the simulation capability and the research depth of the experiment are improved.

Description

Technical Field

[0001] This invention belongs to the field of coal-bearing reservoir fracturing and permeability enhancement and coalbed methane development technology, specifically relating to a device and method for dynamic control of fracturing fluid viscosity in true triaxial hydraulic fracturing experiments. Background Technology

[0002] Currently, underground extraction still dominates the development and utilization of coalbed methane in China. However, driven by factors such as industrial restructuring, economic development needs, and national strategic planning, the energy supply and demand pattern continues to evolve, and coalbed methane development is gradually shifting towards deeper reservoirs. The hydrogeological conditions of deep reservoirs and the synergistic effect of the rebalancing of matter and energy between Earth's spheres make it difficult for underground mining operations to maintain a dynamic balance between extraction and drilling. Therefore, there is an urgent need to develop systematic and efficient pressure relief and permeability enhancement technologies to improve the comprehensive utilization level of deep coalbed methane resources.

[0003] Hydraulic fracturing activates the migration pathways of coalbed methane within a reservoir by artificially applying external loads, offering advantages such as a large stimulation range and significant effectiveness. Among these, fracturing fluid viscosity, a core parameter directly affecting fluid flow resistance and filtration characteristics within the fracture, is currently typically used by pre-mixing in a tank to establish a fixed value or by varying it in stages. However, actual reservoir geological conditions are complex and variable, and the pre-set viscosity may not match the formation's dynamic response during injection, thus limiting fracture propagation and proppant placement effectiveness, ultimately restricting production capacity or increasing environmental risks.

[0004] Furthermore, physical simulation of fracturing effects using large-scale true triaxial hydraulic fracturing experimental equipment in the laboratory can effectively guide the implementation of fracturing projects. However, the viscosity of fracturing fluid in current equipment is usually a fixed value or manually adjusted, which cannot simulate the dynamic viscosity changes in the formation caused by temperature, shear, and filtration loss; it also has limitations such as large viscosity control lag and lack of closed-loop control mechanism; and it is difficult to study the impact of instantaneous viscosity changes on fracture propagation morphology, resulting in large deviations between experimental results and field conditions.

[0005] In summary, the purpose of this invention is to provide a device and method for dynamically controlling the viscosity of fracturing fluid. By introducing a dual-cylinder injection system that allows real-time adjustment of the fluid mixing ratio, and combining the Smith time-delay compensation algorithm with adaptive PID control, the limitations of traditional static mixing of fracturing fluid are overcome. This provides new insights for in-depth experimental research into the complex influence mechanism of dynamic changes in fracturing fluid viscosity on fracture propagation and proppant migration, optimizing fracturing design and evaluating new formulations, and more effectively utilizing fracturing fluid energy while reducing filtration loss. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a device and method for dynamic control of fracturing fluid viscosity in true triaxial hydraulic fracturing experiments, filling the technical gap in real-time control and dynamic feedback of fracturing fluid viscosity in large-scale physical simulations.

[0007] According to the present invention, a device and method for dynamic control of fracturing fluid viscosity for true triaxial hydraulic fracturing experiments are proposed. The device includes a triaxial stress hydraulic loading pump (1), a fracturing sample (2), a fracturing fluid injection pipe (3), a metal conductive plate (4), an acoustic emission probe (5), an acoustic emission signal receiver (6), a resistivity signal receiver (7), a terminal control module (8), a fracturing fluid mixer (9), a low-viscosity fracturing fluid storage tank (10), a high-viscosity fracturing fluid storage tank (11), a dual-cylinder constant speed and constant pressure pump (12), an air compressor (13), a fluid viscometer (14), a high-viscosity fracturing fluid injection line control valve (15), and a low-viscosity fracturing fluid injection line control valve (16).

[0008] The triaxial stress hydraulic loading pump (1) can apply stress values ​​in three different directions to the fracturing sample (2) to truly simulate the in-situ stress environment of the deep reservoir.

[0009] The fracturing sample (2) has a drill hole at its center connected to the fracturing fluid injection pipe (3), and metal conductive plates (4) and acoustic emission probes (5) are attached to both sides of the fracturing sample (2).

[0010] The fracturing fluid injection pipe (3) is connected in sequence to the fluid viscometer (14), the fracturing fluid mixer (9), the low / high viscosity fracturing fluid storage tank (10) / (11), the dual-cylinder constant speed and constant pressure pump (12), and the air compressor (13) to jointly complete the fluid injection behavior of the fracturing sample (2).

[0011] The metal conductive plate (4) is connected in sequence to the resistivity signal receiver (7) and the terminal control module (8). By monitoring the resistivity change of the fracturing sample (2) during the hydraulic fracturing process, non-contact detection of the crack and inversion of the crack width change trend can be achieved.

[0012] The acoustic emission probe (5) is connected in sequence to the acoustic emission signal receiver (6) and the terminal control module (8). By monitoring the acoustic signal of the damage of the fracturing sample (2) during the hydraulic fracturing process, the three-dimensional positioning of the dynamic expansion of the crack is realized.

[0013] The low-viscosity fracturing fluid storage tank (10) and the high-viscosity fracturing fluid storage tank (11) are arranged side by side, and the low-viscosity fracturing fluid injection pipeline control valve (16) and the high-viscosity fracturing fluid injection pipeline control valve (15) are respectively installed in the connecting pipelines of the two to the fracturing fluid mixing agitator (9).

[0014] The high-viscosity fracturing fluid injection pipeline control valve (15) and the low-viscosity fracturing fluid injection pipeline control valve (16) are both connected to the terminal control module (8). The terminal control module (8) can evaluate the propagation behavior of hydraulic fractures based on the feedback information from the acoustic emission signal receiver (6) and the resistivity signal receiver (7), and realize the real-time adjustment of the fluid mixing ratio.

[0015] The terminal control module (8) is highly integrated with the fluid viscometer (14), the high-viscosity fracturing fluid injection pipeline control valve (15), the low-viscosity fracturing fluid injection pipeline control valve (16), the acoustic emission signal receiver (6), and the resistivity signal receiver (7). The real-time measured viscosity, acoustic emission, and resistivity information are used as feedback signals. Combined with the Smith time delay compensation algorithm and adaptive PID control, the output viscosity can quickly and accurately track the dynamic set value, thus achieving high-precision dynamic viscosity regulation.

[0016] The method includes the following five steps:

[0017] S1: Pre-fracturing commissioning;

[0018] Start the true triaxial hydraulic fracturing test system, check whether each hydraulic pump, control valve and instrument is normal, and verify the accuracy and rationality of Smith time delay compensation algorithm and adaptive PID control. After all are correct, load the fracturing sample into the chamber and connect it to the fracturing fluid injection pipe. Start the triaxial stress hydraulic loading pump to apply in-situ stress to the fracturing sample in three directions and maintain it for 15-20 minutes.

[0019] S2: Constant viscosity hydraulic fracturing;

[0020] Start the dual-cylinder constant speed and pressure pump and fracturing fluid mixing agitator, set the initial viscosity value and inject the fracturing fluid into the fracturing sample at a constant flow rate, and simultaneously turn on the acoustic emission and resistivity signal receiver.

[0021] S3: Dynamic control of fracturing fluid viscosity;

[0022] The fracturing effect is evaluated based on parameters such as acoustic emission signals, resistivity signals, and fracturing pressure collected in real time during the hydraulic fracturing process. The fluid transport time delay is calculated based on the Smith time delay compensation algorithm, and the fluid mixing ratio is updated to track the new viscosity setpoint.

[0023] S4: Hydraulic crack control;

[0024] Repeat step S3 until a relatively complex hydraulic fracture structure is formed or the fracture completely penetrates the fracturing sample; then stop the dual-cylinder constant speed and pressure pump and the fracturing fluid mixer in sequence, unload the in-situ triaxial stress of the fracturing sample, record the dynamic evolution of parameters such as acoustic emission, resistivity, fracturing pressure and fluid viscosity throughout the process, and shut down the true triaxial hydraulic fracturing experimental system.

[0025] S5: Study on the influence of dynamic viscosity changes on crack propagation;

[0026] In step S3, different viscosity change paths are set, and the effects of viscosity on fracture propagation, turning and branching are quantitatively analyzed based on the evolution curves of acoustic emission event location, fracture width and fracturing pressure. The variable viscosity control strategy of fracturing fluid is optimized to maximize the fracture network volume of hydraulic fracturing.

[0027] Furthermore, in step S1, the fracturing sample specifications are 300(±5)mm×300(±5)mm×300(±5)mm, the fracturing fluid injection pipe diameter is Φ10mm, the maximum working pressure of the triaxial stress hydraulic loading pump is 70MPa, the flow rate range is 1-65mL / min, and the repeatability is no more than 0.1% (1mL / min). When applying in-situ stress to the fracturing sample, a step-by-step pressure increase method is adopted to prevent sample damage caused by uneven stress. After the stress is loaded to the predetermined value, wait 15-20 minutes to complete the stress balance construction around the wellbore.

[0028] Furthermore, the Smith time-delay compensation algorithm and adaptive PID control in step S1 serve as the core of fracturing fluid viscosity feedback regulation, and the control relationship satisfies the following calculation formula:

[0029] The transmission delay model is shown in equation (1):

[0030]

[0031] In the formula: T d The transmission time delay is s; L is the tube length from the mixer outlet to the viscometer, taken as 0.5m; A is the pipe cross-sectional area, taken as 7.85×10⁻⁶. -5 m 2 Q total The injection flow rate is set to 20 mL / min.

[0032] The estimated viscosity is calculated using equation (2):

[0033]

[0034] In the formula: V p (t) represents the predicted viscosity value from the Smith prediction output, in mPa·s; V meansτ is the viscosity value measured by the viscometer, in mPa·s; G(τ) is the time-domain form of the transfer function, and the corresponding transfer function is shown in equation (3); This represents the proportion of high-viscosity fracturing fluid, ranging from 0 to 1.

[0035]

[0036] In the formula: G(s) is the system transfer function, representing the first-order lag element and pure delay; K is the steady-state gain coefficient, dimensionless, taken as 0.95; θ is the pure lag time including the mixed delay and the measurement delay, taken as 0.2s; τ is the time constant reflecting the system inertia, experimentally measured as 0.6s.

[0037] The error calculation for adaptive PID control is shown in equation (4):

[0038] e(t) = V set (t)-V p (t) (4)

[0039] In the formula: e(t) is the control error; V set (t) represents the target viscosity set for the experiment, in mPa·s.

[0040] Since the actual control system is discrete, the incremental PID output is shown in equation (5):

[0041]

[0042] In the formula: K represents the ratio adjustment amount output by the PID controller, which is dimensionless. p ,K i ,K d These are the coefficients of the discrete incremental PID controller, which are adaptively adjusted according to the magnitude of the error.

[0043] Ultimately, the flow distribution of high / low viscosity fracturing fluid is based on the execution equation of the feedback control system:

[0044]

[0045] In the formula: Q h For the distribution flow rate of high-viscosity fracturing fluid, m 3 / s;Q l For the distribution flow rate of low-viscosity fracturing fluid, m 3 / s.

[0046] Furthermore, constraints are set for the flow distribution of the high / low viscosity fracturing fluid as shown in Equation (7) to ensure that the high viscosity fracturing fluid fully participates in the mixing and to prevent insufficient low viscosity fracturing fluid from causing the pump to run dry. If the range is exceeded, the amplitude will be limited.

[0047]

[0048] Furthermore, the flow rate range of the dual-cylinder constant speed and constant pressure pump in step S2 is 0-100 mL / min, the maximum working pressure is 70 MPa, and it can perform three different injection modes: constant flow, constant pressure, and tracking; the initial viscosity value should be controlled as much as possible in the high viscosity range (≥40 mPa·s).

[0049] Furthermore, in step S3, the width of the hydraulic fracture can be inverted based on the change in resistivity signal; assuming that the resistivity of the fracture and water is constant and the circuit structure of different parts is in a "series-parallel" relationship, the relationship between the resistance and fracture width corresponding to the fracture during hydraulic fracturing is shown in equation (8):

[0050]

[0051] In the formula: R1 is the resistivity of the hydraulic fracture, Ω; ρ w ρ is the resistivity of the fracturing fluid, in Ω; d d1 is the resistivity of the fracturing specimen, in Ω; l is the length of the fracturing specimen (the distance between the two metal conductive plates), in m; d1 is the width of the hydraulic fracture, in m.

[0052] Furthermore, in step S4, the hydraulic fracture regulation achieved by dynamically controlling the viscosity of the fracturing fluid can refer to the design concept of alternating complex fracture networks with high (≥40 mPa·s) / low viscosity (<10 mPa·s) fracturing fluid and long-range support for the sand-carrying fracture network with medium viscosity (10-40 mPa·s) fracturing fluid.

[0053] Furthermore, in step S5, when studying the influence of viscosity dynamics on fracture propagation, the viscosity change path can be set as a step increase from low viscosity (<10 mPa·s) to high viscosity (≥40 mPa·s), a linear decrease from high viscosity (≥40 mPa·s) to low viscosity (<10 mPa·s), or an oscillating fluctuation of medium viscosity (10-40 mPa·s); respectively simulating geological structural zones encountering high temperatures or local high shear stress, continuous dilution of fracturing fluid, and near-wellbore turbulence or fracture branching interference.

[0054] Compared with the prior art, the device and method for dynamic control of fracturing fluid viscosity for true triaxial hydraulic fracturing experiments disclosed in this invention have the following beneficial effects:

[0055] This invention can simulate the dynamic viscosity changes of fracturing fluid caused by temperature, shear, and filtration loss during hydraulic fracturing. It introduces time delay compensation and adaptive control to overcome the limitations of traditional static mixing of fracturing fluid, greatly reduce hysteresis error, and enhance the real-time performance of dynamic viscosity control.

[0056] This invention helps to explore the complex influence mechanism of dynamic changes in fracturing fluid viscosity on fracture propagation and proppant migration, and provides new ideas for optimizing fracturing design, evaluating new formulations, utilizing fracturing fluid energy more effectively, and reducing filtration loss.

[0057] This invention combines viscosity control with real-time monitoring methods such as acoustic emission, resistivity, and pressure. Based on a series of feedback information such as crack propagation morphology, viscosity is dynamically adjusted to optimize or study specific effects (such as inhibiting branching or promoting deflection), thereby enhancing the simulation capability and research depth of the experiment. Attached Figure Description

[0058] To make the objectives, methods, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the drawings described below are merely some practical examples of this invention. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort. In particular:

[0059] Figure 1 This is an overall structural diagram of a device and method for dynamic control of fracturing fluid viscosity in true triaxial hydraulic fracturing experiments, as disclosed in an embodiment of the present invention.

[0060] Figure 2 This is a flowchart illustrating the dynamic control of fracturing fluid viscosity in a device and method for true triaxial hydraulic fracturing experiments, as disclosed in an embodiment of the present invention.

[0061] 1: Triaxial stress hydraulic loading pump; 2: Fracturing sample; 3: Fracturing fluid injection pipe; 4: Metal conductive plate; 5: Acoustic emission probe; 6: Acoustic emission signal receiver; 7: Resistivity signal receiver; 8: Terminal control module; 9: Fracturing fluid mixer; 10: Low viscosity fracturing fluid storage tank; 11: High viscosity fracturing fluid storage tank; 12: Dual-cylinder constant speed and constant pressure pump; 13: Air compressor; 14: Fluid viscometer; 15: High viscosity fracturing fluid injection line control valve; 16: Low viscosity fracturing fluid injection line control valve. Detailed Implementation

[0062] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. These preferred embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0063] Figure 1 This is an overall structural diagram of the device provided in an embodiment of the present invention; Figure 2This is a flowchart illustrating the dynamic control of fracturing fluid viscosity according to an embodiment of the present invention. The present invention discloses a device and method for dynamic control of fracturing fluid viscosity in true triaxial hydraulic fracturing experiments, specifically comprising the following five steps.

[0064] S1: Pre-fracturing commissioning;

[0065] Start the true triaxial hydraulic fracturing test system and check whether the triaxial stress hydraulic loading pump (1), the dual-cylinder constant speed and constant pressure pump (12), the fluid viscometer (14), the fracturing fluid mixing agitator (9), the high viscosity fracturing fluid injection pipeline control valve (15), and the low viscosity fracturing fluid injection pipeline control valve (16) are operating normally and verify the accuracy and rationality of the Smith time delay compensation algorithm and adaptive PID control. After all are correct, the fracturing sample (2) with a size of 300mm×300mm×300mm is loaded into the chamber and connected to the fracturing fluid injection pipe (3) with a diameter of Φ10mm. Start the triaxial stress hydraulic loading pump (1) to apply in-situ stress of no more than 70MPa to the three directions of the fracturing sample (2) in a step-by-step manner according to the engineering conditions or experimental plan. Maintain for 15 minutes to complete the stress balance construction around the wellbore and prevent non-fracturing damage caused by uneven stress.

[0066] Among them, the Smith time-delay compensation algorithm and adaptive PID control are the core of fracturing fluid viscosity feedback regulation, and the control relationship satisfies the following calculation formula:

[0067]

[0068] In the formula: T d The transmission time delay is s; L is the tube length from the mixer outlet to the viscometer, taken as 0.5m; A is the pipe cross-sectional area, taken as 7.85×10⁻⁶. -5 m 2 Q total The injection flow rate is set to 20 mL / min.

[0069] The estimated viscosity is calculated using equation (2):

[0070]

[0071] In the formula: V p (t) represents the predicted viscosity value from the Smith prediction output, in mPa·s; V means τ is the viscosity value measured by the viscometer, in mPa·s; G(τ) is the time-domain form of the transfer function, and the corresponding transfer function is shown in equation (3); This represents the proportion of high-viscosity fracturing fluid, ranging from 0 to 1.

[0072]

[0073] In the formula: G(s) is the system transfer function, representing the first-order lag element and pure delay; K is the steady-state gain coefficient, dimensionless, taken as 0.95; θ is the pure lag time including the mixed delay and the measurement delay, taken as 0.2s; τ is the time constant reflecting the system inertia, experimentally measured as 0.6s.

[0074] The error calculation for adaptive PID control is shown in equation (4):

[0075] e(t) = V set (t)-V p (t) (4)

[0076] In the formula: e(t) is the control error; V set (t) represents the target viscosity set for the experiment, in mPa·s.

[0077] Since the actual control system is discrete, the incremental PID output is shown in equation (5):

[0078]

[0079] In the formula: K represents the ratio adjustment amount output by the PID controller, which is dimensionless. p ,K i ,K d These are the coefficients of the discrete incremental PID controller, which are adaptively adjusted according to the magnitude of the error.

[0080] Ultimately, the flow distribution of high / low viscosity fracturing fluid is based on the execution equation of the feedback control system:

[0081]

[0082] In the formula: Q h For the distribution flow rate of high-viscosity fracturing fluid, m 3 / s;Q l For the distribution flow rate of low-viscosity fracturing fluid, m 3 / s.

[0083] The flow distribution constraints for the high / low viscosity fracturing fluids mentioned above are shown in Equation (7), to ensure that the high viscosity fracturing fluid fully participates in the mixing and to prevent insufficient low viscosity fracturing fluid from causing the pump to run dry. If the range is exceeded, the amplitude will be limited.

[0084]

[0085] S2: Constant viscosity hydraulic fracturing;

[0086] Start the dual-cylinder constant speed and constant pressure pump (12) and fracturing fluid mixing agitator (9), set the initial viscosity value, and inject the fracturing fluid into the fracturing sample (2) at a constant flow rate of 20 mL / min. Simultaneously turn on the acoustic emission signal receiver (6) and resistivity signal receiver (7). In order to better connect the coal and rock cleavage and create a wide and long main hydraulic fracture, the initial constant flow constant viscosity hydraulic fracturing uses a high viscosity fracturing fluid of 45 mPa·s. set (t) Feedback is sent to the terminal control module (8) and commands are output to the high-viscosity fracturing fluid injection pipeline control valve (15) and the low-viscosity fracturing fluid injection pipeline control valve (16) to control the distribution flow rate Q of the fracturing fluid. h and Q l .

[0087] S3: Dynamic control of fracturing fluid viscosity;

[0088] The fracturing effect is evaluated based on parameters such as acoustic emission three-dimensional positioning signals, resistivity fracture width change signals, and fracturing pressure acquired in real time during the hydraulic fracturing process. The fluid transport time delay T is calculated based on the Smith time delay compensation algorithm. d And update the fluid mixing ratio To track the new viscosity setpoint V set (t); The width of the hydraulic fracture can be inverted using the resistivity signal. It can be assumed that the resistivity of the fracture and water is constant and that the circuit structure of different parts is in a "series-parallel" relationship. Then the relationship between the resistance and the fracture width during hydraulic fracturing is shown in equation (8):

[0089]

[0090] In the formula: R1 is the resistivity of the hydraulic fracture, Ω; ρ w ρ is the resistivity of the fracturing fluid, in Ω; d d1 is the resistivity of the fracturing specimen, in Ω; l is the length of the fracturing specimen (the distance between the two metal conductive plates), in m; d1 is the width of the hydraulic fracture, in m.

[0091] S4: Hydraulic crack control;

[0092] Repeat step S3 until a relatively complex hydraulic fracture structure is formed or the fracture completely penetrates the fracturing sample (2); stop the dual-cylinder constant speed and pressure pump (12) and fracturing fluid mixing agitator (9) in sequence, unload the in-situ stress of the triaxial fracturing sample (2), record the dynamic evolution of parameters such as acoustic emission, resistivity, fracturing pressure and fluid viscosity throughout the process, and shut down the true triaxial hydraulic fracturing experimental system.

[0093] The dynamic control of fracturing fluid viscosity to regulate hydraulic fractures can refer to the design concept of alternating high (≥40 mPa·s) / low viscosity (<10 mPa·s) fracturing fluids to create complex fracture networks and medium viscosity (10-40 mPa·s) fracturing fluids to carry sand and provide long-range support for the fracture network. High viscosity fracturing fluid is used to connect coal and rock cleavages to create wide and long-extending main hydraulic fractures, low viscosity fracturing fluid is used to create small and complex micro fractures to construct complex hydraulic fracture structures, and medium viscosity fracturing fluid has good sand-carrying capacity, taking into account the characteristics and advantages of both main fractures and complex micro fractures. Taking the initial high-viscosity fracturing fluid of 45 mPa·s as an example, based on sufficient fracture creation and connection, when the resistivity inversion shows a fracture width > 2 mm, the fracturing fluid is changed to a low-viscosity fracturing fluid of 5 mPa·s to develop fine and complex micro-fractures, making the fractures longitudinally finely layered and laterally extending to distant ends; when the number of events detected by acoustic emission monitoring is dense, it is adjusted to a high-viscosity fracturing fluid of 45 mPa·s to suppress branch fractures and further strengthen the main fracture to form a three-dimensional fracture network structure; finally, based on the high elasticity characteristics of the medium-viscosity fracturing fluid of 20 mPa·s, the complex fracture network is effectively supported, further improving the fracture conductivity.

[0094] S5: Study on the influence of dynamic viscosity changes on crack propagation;

[0095] In step S3, different viscosity change paths are set, and the effects of viscosity on fracture propagation, turning and branching are quantitatively analyzed based on the evolution curves of acoustic emission event location, fracture width and fracturing pressure. The variable viscosity control strategy of fracturing fluid is optimized to maximize the fracture network volume of hydraulic fracturing.

[0096] When studying the impact of viscosity dynamics on fracture propagation, the viscosity change path can be set as a step increase from low viscosity (<10 mPa·s) to high viscosity (≥40 mPa·s), a linear decrease from high viscosity (≥40 mPa·s) to low viscosity (<10 mPa·s), or an oscillating fluctuation of medium viscosity (10-40 mPa·s); respectively simulating geological structural zones with high temperature or local high shear stress, continuous dilution of fracturing fluid and near-wellbore turbulence or fracture branching interference.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the method of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the method described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding method to deviate from the scope defined by the claims of the present invention.

Claims

1. A device and method for dynamic control of fracturing fluid viscosity in true triaxial hydraulic fracturing experiments, the device comprising a triaxial stress hydraulic loading pump, a fracturing sample, a fracturing fluid injection pipe, a metal conductive plate, an acoustic emission probe, an acoustic emission signal receiver, a resistivity signal receiver, a terminal control module, a fracturing fluid mixer, a low-viscosity fracturing fluid storage tank, a high-viscosity fracturing fluid storage tank, a dual-cylinder constant-speed constant-pressure pump, an air compressor, a fluid viscometer, control valves for the high-viscosity fracturing fluid injection pipeline, and control valves for the low-viscosity fracturing fluid injection pipeline; The method includes the following five steps: S1: Pre-fracturing commissioning; Start the true triaxial hydraulic fracturing test system, check whether each hydraulic pump, control valve and instrument is normal, and verify the accuracy and rationality of Smith time delay compensation algorithm and adaptive PID control. After all are correct, load the fracturing sample into the chamber and connect it to the fracturing fluid injection pipe. Start the triaxial stress hydraulic loading pump to apply in-situ stress to the fracturing sample in three directions and maintain it for 15-20 minutes. S2: Constant viscosity hydraulic fracturing; Start the dual-cylinder constant speed and pressure pump and fracturing fluid mixing agitator, set the initial viscosity value and inject the fracturing fluid into the fracturing sample at a constant flow rate, and simultaneously turn on the acoustic emission and resistivity signal receiver. S3: Dynamic control of fracturing fluid viscosity; The fracturing effect is evaluated based on parameters such as acoustic emission signals, resistivity signals, and fracturing pressure collected in real time during the hydraulic fracturing process. The fluid transport time delay is calculated based on the Smith time delay compensation algorithm, and the fluid mixing ratio is updated to track the new viscosity setpoint. S4: Hydraulic crack control; Repeat step S3 until a relatively complex hydraulic fracture structure is formed or the fracture completely penetrates the fracturing sample; then stop the dual-cylinder constant speed and pressure pump and the fracturing fluid mixer in sequence, unload the in-situ triaxial stress of the fracturing sample, record the dynamic evolution of parameters such as acoustic emission, resistivity, fracturing pressure and fluid viscosity throughout the process, and shut down the true triaxial hydraulic fracturing experimental system. S5: Study on the influence of dynamic viscosity changes on crack propagation; In step S3, different viscosity change paths are set, and the effects of viscosity on fracture propagation, turning and branching are quantitatively analyzed based on the evolution curves of acoustic emission event location, fracture width and fracturing pressure. The variable viscosity control strategy of fracturing fluid is optimized to maximize the fracture network volume of hydraulic fracturing.

2. The device for dynamic viscosity control of fracturing fluid in true triaxial hydraulic fracturing experiments as described in claim 1, characterized in that, The triaxial stress hydraulic loading pump can apply stress values ​​in three different directions to the fracturing sample, realistically simulating the in-situ stress environment of deep reservoirs. The fracturing specimen has a drilled hole at its center connected to a fracturing fluid injection pipe, and metal conductive plates and acoustic emission probes are attached to both sides of the fracturing specimen. The fracturing fluid injection pipe is sequentially connected to a fluid viscometer, a fracturing fluid mixer, a low / high viscosity fracturing fluid storage tank, a dual-cylinder constant speed and constant pressure pump, and an air compressor to jointly complete the fluid injection behavior of the fracturing sample. The metal conductive plate is sequentially connected to the resistivity signal receiver and the terminal control module. By monitoring the change in resistivity of the fracturing sample during hydraulic fracturing, non-contact detection of the fracture and inversion of the fracture width variation trend can be achieved. The acoustic emission probe is sequentially connected to the acoustic emission signal receiver and the terminal control module. By monitoring the acoustic signal of damage to the fracturing sample during the hydraulic fracturing process, the three-dimensional positioning of the dynamic propagation of the fracture can be achieved. The low-viscosity fracturing fluid storage tank and the high-viscosity fracturing fluid storage tank are arranged side by side, and the low-viscosity fracturing fluid injection pipeline control valve and the high-viscosity fracturing fluid injection pipeline control valve are respectively installed in the connecting pipelines of the two tanks and the fracturing fluid mixing agitator. Both the high-viscosity fracturing fluid injection pipeline control valve and the low-viscosity fracturing fluid injection pipeline control valve are connected to the terminal control module. The terminal control module can evaluate the propagation behavior of hydraulic fractures based on feedback information from acoustic emission signal receivers and resistivity signal receivers, and realize real-time adjustment of fluid mixing ratio. The terminal control module is highly integrated with the fluid viscometer, the high-viscosity fracturing fluid injection pipeline control valve, the low-viscosity fracturing fluid injection pipeline control valve, the acoustic emission signal receiver, and the resistivity signal receiver. It uses the real-time measured viscosity, acoustic emission, and resistivity information as feedback signals, and combines the Smith time delay compensation algorithm and adaptive PID control to enable the output viscosity to quickly and accurately track the dynamic set value, thus achieving high-precision dynamic viscosity control.

3. The method for dynamic control of fracturing fluid viscosity for true triaxial hydraulic fracturing experiments as described in claim 1, characterized in that, The Smith time-delay compensation algorithm and adaptive PID control in step S1 are the core of fracturing fluid viscosity feedback regulation, and the control relationship satisfies the following calculation formula: The transmission delay model is shown in equation (1): The estimated viscosity is calculated using equation (2): The error calculation for adaptive PID control is shown in equation (4): e(t)=V set (t)-V p (t) (4) Since the actual control system is discrete, the incremental PID output is shown in equation (5): The flow distribution of high / low viscosity fracturing fluids is based on the execution equation of the feedback control system: The constraints for the flow distribution of the high / low viscosity fracturing fluids mentioned above are shown in Equation (7): In the formula: T d The transmission delay is s; L is the tube length from the mixer outlet to the viscometer, taken as 0.5m; A is the pipe cross-sectional area, taken as 7.85×10⁻⁶. -5 m 2 Q total The injection flow rate is 20 mL / min; V p (t) represents the predicted viscosity value from the Smith prediction output, in mPa·s; V means τ is the viscosity value measured by the viscometer, in mPa·s; G(τ) is the time-domain form of the transfer function, and the corresponding transfer function is shown in equation (3); The proportion of high-viscosity fracturing fluid is taken as 0-1; G(s) is the system transfer function, representing the first-order hysteresis element and pure delay; K is the steady-state gain coefficient, dimensionless, taken as 0.95; θ is the pure hysteresis time including mixing delay and measurement delay, taken as 0.2s; τ is the time constant reflecting the system inertia, experimentally measured as 0.6s; e(t) is the control error; V set (t) represents the target viscosity set for the experiment, in mPa·s; K represents the ratio adjustment amount output by the PID controller, which is dimensionless. p ,K i ,K d These are the coefficients of a discrete incremental PID controller, which are adaptively adjusted according to the error magnitude; Q h For the distribution flow rate of high-viscosity fracturing fluid, m 3 / s;Q l For the distribution flow rate of low-viscosity fracturing fluid, m 3 / s.

4. The method for dynamic control of fracturing fluid viscosity for true triaxial hydraulic fracturing experiments as described in claim 1, characterized in that, In step S2, the flow rate range of the dual-cylinder constant speed and constant pressure pump is 0-100 mL / min, the maximum working pressure is 70 MPa, and it can perform three different injection modes: constant flow, constant pressure, and tracking. The initial viscosity value should be controlled as much as possible in the high viscosity range (≥40 mPa·s).

5. The method for dynamic control of fracturing fluid viscosity for true triaxial hydraulic fracturing experiments as described in claim 1, characterized in that, In step S3, the width of the hydraulic fracture can be inverted based on the change in resistivity signal; assuming that the resistivity of the fracture and water is constant and the circuit structure of different parts is in a "series-parallel" relationship, the relationship between the resistance and fracture width corresponding to the fracture during hydraulic fracturing is shown in equation (8): In the formula: R1 is the resistivity of the hydraulic fracture, Ω; ρ w ρ is the resistivity of the fracturing fluid, in Ω; d d1 is the resistivity of the fracturing specimen, in Ω; l is the length of the fracturing specimen (the distance between the two metal conductive plates), in m; d1 is the width of the hydraulic fracture, in m.

6. The method for dynamic control of fracturing fluid viscosity for true triaxial hydraulic fracturing experiments as described in claim 1, characterized in that, In step S4, the hydraulic fracture regulation is achieved by dynamically controlling the viscosity of the fracturing fluid. This can be achieved by referring to the design concept of alternating high (≥40 mPa·s) / low viscosity (<10 mPa·s) fracturing fluid to create complex fracture networks and medium viscosity (10-40 mPa·s) fracturing fluid to carry sand and provide long-range support for the fracture network.

7. The method for dynamic control of fracturing fluid viscosity for true triaxial hydraulic fracturing experiments as described in claim 1, characterized in that, In step S5, when studying the influence of viscosity dynamics on fracture propagation, the viscosity change path can be set as a step increase from low viscosity (<10 mPa·s) to high viscosity (≥40 mPa·s), a linear decrease from high viscosity (≥40 mPa·s) to low viscosity (<10 mPa·s), or an oscillating fluctuation of medium viscosity (10-40 mPa·s); respectively simulating geological structural zones with high temperature or local high shear stress, continuous dilution of fracturing fluid and near-wellbore turbulence or fracture branching interference.

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