A downhole gas-liquid-solid three-phase synergistic fracturing method and system

Abstract

The application relates to the technical field of coal mining and gas control, and discloses an underground gas-liquid-solid three-phase cooperative fracturing method and system, wherein the method comprises the following steps: starting a water sand pump group to output a water sand mixed liquid; exciting a high-energy gas generator to generate a high-pressure gas pulse, so as to force a water way one-way valve to be closed; driving a water sand mixed liquid column in a fracturing pipe column to accelerate into a coal seam crack, and transporting a proppant to a deep part of the coal seam crack; when the pressure at the end of the fracturing pipe column is less than the fluid pressure in the coal seam crack, an orifice one-way valve is closed under the action of a pressure difference; the pressure in a three-way current collector continues to decrease to be less than the output pressure of the water sand pump group, and the water way one-way valve is opened under the action of a pressure difference. Through the gas-driven liquid piston effect and fluid diode control, the proppant is transported to the deep part of the crack, and automatic pressure maintaining and locking after operation are realized, backflow and sand spitting are effectively prevented, and the long-term permeability improvement effect of coal seam fracturing is improved.

Description

Technical Field

[0001] This invention relates to the field of coal mining and gas control technology, specifically to an underground gas-liquid-solid three-phase synergistic fracturing method and system. Background Technology

[0002] In the process of underground gas disaster control and coalbed methane development in coal mines, artificial fracturing to enhance permeability in low-permeability coal seams is a key technical means to improve gas extraction efficiency. While the widely used hydraulic fracturing technology is mature, it has inherent limitations in geological adaptability. When external water-based fracturing fluid enters the pores of the coal seam, it easily triggers the hydration and expansion of clay minerals in the coal matrix, creating a capillary retention effect at the micropore throat, resulting in severe water-locking damage. This objectively increases the resistance to gas desorption and flow. Furthermore, conventional hydraulic fracturing mainly relies on the viscosity of the fluid to carry the proppant. However, when using low-viscosity fracturing fluids, proppant particles settle rapidly under gravity, tending to accumulate near the wellbore and making it difficult to transport to the deeper fractures. This leads to the fracture ends closing again under in-situ stress due to a lack of effective support, limiting the extension range of the effective flow channel.

[0003] An existing supplementary approach involves high-energy gas fracturing technology, which utilizes the combustion of solid propellants to generate high-pressure gas, for coal seam permeability enhancement. This technology leverages the shock wave effect of gas pulses to create a complex radial fracture network without causing water-sensitive damage to the formation. However, the extremely low dynamic viscosity of the gas medium objectively limits its ability to suspend and transport solid proppant over long distances. After the gas pulse energy dissipates, the newly formed fracture surfaces, lacking mechanical support from the solid medium, rapidly rebound and close under formation confining pressure. This leads to a rapid decline in the permeability enhancement effect over time, making it difficult to maintain a long-term gas extraction channel.

[0004] To comprehensively utilize the proppant-carrying capacity of liquids and the fracture-creating capacity of gases, existing research has attempted to employ a combined gas-liquid fracturing process. However, in practical engineering, this process faces severe problems of fluid backflow and proppant regurgitation. During the pressure decay phase after the pulse work, the pressure drop rate within the injection pipeline is much faster than the pressure dissipation rate within the coal seam fractures, resulting in a reverse high-pressure differential between the pipeline and the fractures, pointing towards the wellbore. Due to the lack of an effective downhole fluid control mechanism, the high-energy fluid within the fractures carries the still-stabilized proppant back into the pipeline at high speed. This reverse flow not only leads to a significant loss of proppant within the fractures, causing them to lose support and fail, but also causes high concentrations of proppant-laden fluid to backflow into the wellhead equipment, leading to pipeline blockage and equipment wear, severely restricting the safety and effectiveness of fracturing operations. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a downhole gas-liquid-solid three-phase synergistic anti-backflow fracturing method and system, which solves the problems in existing coal seam fracturing technologies where single hydraulic fracturing is unable to transport proppant to the deep part of the fracture, and simple gas fracturing cannot effectively lay proppant, leading to fracture closure in a short period of time.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] The first aspect of this invention provides a downhole gas-liquid-solid three-phase synergistic anti-backflow fracturing method, comprising the following steps:

[0008] Connect the water-sand pump set to the first inlet of the three-way manifold via a water circuit check valve, connect the high-energy gas generator to the second inlet of the three-way manifold via a gas circuit check valve, connect the outlet of the three-way manifold to the fracturing tubing, and install an orifice check valve at the end of the fracturing tubing.

[0009] The water-sand pump unit is started to output water-sand mixture. Under pressure, the water-sand mixture opens the water circuit check valve and the orifice check valve, enters the coal seam borehole to form initial fractures and fills proppant.

[0010] The water-sand pump set continues to operate, stimulating the high-energy gas generator to produce high-pressure gas pulses. The high-pressure gas pulses cause the pressure inside the three-way manifold to rise to a level greater than the output pressure of the water-sand pump set, forcing the water circuit check valve to close.

[0011] High-pressure gas pulses enter the three-way manifold through the gas circuit check valve, driving the water-sand mixture column in the fracturing string to accelerate into the coal seam fracture, transporting the proppant to the deep part of the coal seam fracture;

[0012] The gas pulse ends, causing the pressure inside the fracturing string to decrease. When the pressure at the end of the fracturing string is less than the fluid pressure inside the coal seam fracture, the orifice check valve closes under the action of the pressure difference.

[0013] As the pressure inside the three-way manifold continues to decrease until it is lower than the output pressure of the water-sand pump unit, the water circuit check valve opens under the action of pressure difference, and the water-sand pump unit replenishes the water-sand mixture into the fracturing string and restores the pipeline pressure.

[0014] Preferably, in step S2, the initial crack filling with proppant specifically includes:

[0015] The water-sand pump set pressurizes the water-sand mixture to a set pumping pressure, which is greater than the sum of the minimum principal stress of the coal seam and the tensile strength of the coal seam. The pressurized water-sand mixture overcomes the frictional resistance along the pipeline and flows through the three-way manifold.

[0016] The water circuit check valve remains open under the positive pressure of the fluid, while the air circuit check valve remains closed under the lateral pressure of the fluid.

[0017] The water-sand mixture enters the coal seam borehole through the one-way valve at the orifice. Under the action of quasi-static hydraulic pressure, it causes tensile fracturing of the coal seam wall, forming a fracture channel extending outward from the near-wellbore zone. The proppant in the water-sand mixture remains in the near-end region of the fracture channel along with the fluid.

[0018] Preferably, in step S3, the step of activating the high-energy gas generator to produce a high-pressure gas pulse specifically includes:

[0019] The control system sends an ignition signal to the gas-generating propellant column in the high-energy gas generator to ignite the propellant column and carry out a chemical reaction.

[0020] The gas-generating propellant column releases a large amount of high-temperature and high-pressure gas within milliseconds. The gas rapidly accumulates in the confined space, forming a pressure peak with a steep rising edge.

[0021] The pressure peak establishes a transient gas pressure value higher than the current hydrostatic pressure inside the fracturing string before reaching the gas circuit check valve.

[0022] Preferably, in step S3, the pressure inside the three-way manifold increases to a level greater than the output pressure of the water-sand pump set, specifically including:

[0023] The gas circuit check valve opens under the thrust of the high-pressure gas pulse, and the high-pressure gas instantly fills the internal cavity of the three-way manifold;

[0024] The fluid pressure inside the three-way manifold jumps to a peak value the instant gas is injected, and the peak pressure is 2 to 4 times the output pressure of the water sand pump set;

[0025] The outlet side of the water circuit check valve bears the peak pressure, and the inlet side bears the output pressure of the water sand pump group. The reverse pressure difference formed between the outlet side and the inlet side drives the valve core of the water circuit check valve to return to its seat and press the sealing surface tightly, cutting off the fluid passage between the water sand pump group and the three-way manifold.

[0026] Preferably, in step S4, the accelerated entry of the water-sand mixture column into the coal seam fracture within the driving fracturing string includes:

[0027] The high-pressure gas pulse entering the three-way manifold forms a gas-liquid interface with the water-sand mixture column retained in the fracturing tubing.

[0028] The high-pressure gas pulse acts as a gas-phase piston, transferring momentum to the incompressible water-sand mixture column through expansion and work.

[0029] The water-sand mixture column gains acceleration under the drive of the gas pressure at the rear end, which is transformed into a secondary hydraulic impact on the tip of the coal seam fracture. At the same time, the high-speed flow of the fluid carries the proppant across the near end region of the fracture channel and transports it to the deep closed region of the fracture channel.

[0030] Preferably, in step S5, the pressure attenuation within the fracturing string includes:

[0031] As the high-pressure gas pulse propels the water-sand mixture column into the coal seam fracture, the gas volume expands within the fracturing tubing and fracture passage.

[0032] The gas-generating propellant in the high-energy gas generator has finished reacting, and the gas supply has stopped.

[0033] Some gases and fluids are lost to the coal seam matrix through the pores in the fracture walls of the coal seam.

[0034] Preferably, in step S5, the pressure at the end of the fracturing string being less than the fluid pressure within the coal seam fracture specifically includes:

[0035] After hydraulic fracturing, high-pressure fluid and elastic potential energy accumulate in the coal seam fractures, which maintains the first pressure value inside the fractures.

[0036] Due to gas expansion and cooling, the pressure inside the fracturing string decreases to the second pressure value;

[0037] When the second pressure value is lower than the first pressure value, a reverse pressure differential is formed on both sides of the valve core of the orifice check valve, pointing from the coal seam to the fracturing string. The reverse pressure differential, in conjunction with the elasticity of the internal reset mechanism of the orifice check valve, drives the valve core to close, isolating the fracturing string from the coal seam borehole, so that an independent pressure-holding cavity is formed inside the coal seam fracture.

[0038] Preferably, in step S6, the water-sand pump group replenishes the fracturing tubing with a water-sand mixture and restores the pipeline pressure, specifically including:

[0039] After the water circuit check valve is opened, the water-sand mixture output by the water-sand pump unit flows into the three-way manifold and the fracturing tubing string;

[0040] The water-sand mixture entering the fracturing string fills the volume space left after the high-pressure gas pulse expansion and filtration, and replaces the residual gas in the pipeline;

[0041] The water-sand pump unit continuously injects water-sand mixture until the fluid pressure in the three-way manifold and fracturing string rises and stabilizes to the output pressure of the water-sand pump unit, thus completing the continuous reconstruction of the liquid phase medium after a single pulse fracturing.

[0042] Preferably, after the continuous reconstruction of the liquid medium following a single pulse fracturing, the next independent gas-generating propellant column in the high-energy gas generator is activated in a preset order, and steps S3 to S6 are repeated based on the high-pressure gas pulse generated by the gas-generating propellant column until all the pre-set gas-generating propellant columns in the high-energy gas generator are activated.

[0043] A second aspect of this invention provides a downhole gas-liquid-solid three-phase synergistic anti-backflow fracturing system, which is applied to the method described in the first aspect above. The system includes:

[0044] Water-sand pump set, used to provide a continuous quasi-static high-pressure water-sand mixture;

[0045] High-energy gas generator, used to generate transient high-pressure gas pulses;

[0046] A three-way manifold has a first inlet, a second inlet, and an outlet, used to combine liquid and gaseous fluids.

[0047] A water circuit check valve is installed between the first inlet of the water-sand pump unit and the three-way manifold.

[0048] A one-way valve for the gas path is installed between the high-energy gas generator and the second inlet of the three-way manifold.

[0049] The fracturing string is connected at the outlet of the three-way manifold and extends into the coal seam borehole.

[0050] An orifice check valve is installed at the end of the fracturing string;

[0051] The water circuit check valve, the gas circuit check valve, and the orifice check valve together constitute a fluid logic control loop, which automatically switches between open and closed states according to the changes in fluid pressure in the pipeline, realizing unidirectional coordinated transport and energy locking of gas, liquid, and solid three-phase fluids.

[0052] This invention provides a downhole gas-liquid-solid three-phase synergistic anti-backflow fracturing method and system. It has the following beneficial effects:

[0053] 1. This invention utilizes high-pressure gas pulses as a power source to drive a water-sand mixture column within the fracturing string, establishing a power transmission mechanism of gas-driven fluid and liquid-carrying proppant. By pre-storing an incompressible water-sand mixture within the fracturing string, the high-pressure expansion work instantaneously released by the gas pulse imparts extremely high acceleration to the liquid column, enabling the liquid medium to carry proppant and conduct a secondary impact on the fracture tip. This method overcomes the shortcomings of traditional gas fracturing, which cannot effectively carry proppant due to the low viscosity of the gas. Simultaneously, it utilizes the high-energy explosive characteristics of gas to solve the problems of difficult fracturing initiation in deep coal seams and limited proppant transport distance in traditional hydraulic fracturing, achieving effective proppant placement deep within the fracture.

[0054] 2. This invention achieves one-way locking and backflow prevention of fracturing energy by installing a one-way valve at the end of the fracturing string. During the depressurization phase after the gas pulse ends, once the pressure inside the string is lower than the pressure inside the fracture, the one-way valve automatically closes due to the pressure difference, sealing the high-pressure fluid and proppant inside the coal seam fracture. This mechanism effectively prevents the "sand spraying" phenomenon caused by a sudden drop in pipeline pressure, which results in the proppant being ejected with the high-pressure gas-liquid backflow. It ensures that the proppant filling the fracture is not lost, thereby maintaining the effective support width and conductivity of the fracture.

[0055] 3. This invention employs a fluid logic control loop composed of a three-way manifold and one-way valves in the water and gas circuits, achieving automatic switching between continuous pumping and pulse impact, as well as equipment protection. At the instant a gas pulse several times the pumping pressure is generated, the system automatically closes the water circuit one-way valve using the reverse pressure difference. This physically blocks the impact of high-pressure gas on the low-pressure water-sand pump unit without the need for complex electronic actuators, preventing pump damage. Simultaneously, after the pulse energy is released, the system automatically resets and quickly replenishes the pipeline volume using the water-sand pump unit, achieving continuous cyclic operation of static pressure fracture creation, dynamic pressure fracture expansion, and automatic fluid replenishment, simplifying downhole operation procedures. Attached Figure Description

[0056] Figure 1 This is a schematic diagram of the method flow of the present invention;

[0057] Figure 2 This is a system architecture diagram of the present invention;

[0058] Figure 3 This is a schematic diagram of the anti-backflow gas-liquid-solid three-phase synergistic fracturing and permeability enhancement method for underground coal mines according to the present invention;

[0059] Figure 4 This is a diagram of the interconnected fracturing pump unit, borehole, and interconnected high-energy gas generator of the present invention;

[0060] Figure 5 This is a schematic diagram of hydraulic fracturing and solid medium pre-positioning according to the present invention;

[0061] Figure 6 This is a schematic diagram of the gas-driven liquid-solid pulse impact and deep slot expansion method of the present invention;

[0062] Figure 7 This is a schematic diagram of the unidirectional energy locking and cyclic fracturing of the present invention.

[0063] Among them, 1. Water-sand pump set; 2. High-pressure resistant pipeline; 3. Water circuit check valve; 4. Three-way manifold; 5. Gas circuit check valve; 6. High-energy gas generator; 7. Orifice check valve; 8. Drilling high-pressure resistant pipeline; 9. Coal seam. Detailed Implementation

[0064] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0065] Figure 1 This is a flowchart of a downhole gas-liquid-solid three-phase synergistic anti-backflow fracturing method according to an embodiment of the present invention.

[0066] Figure 2 This is a structural diagram of a downhole gas-liquid-solid three-phase synergistic anti-backflow fracturing system according to an embodiment of the present invention.

[0067] Figure 3 This is a schematic diagram of the anti-backflow gas-liquid-solid three-phase synergistic fracturing and permeability enhancement method for underground coal mines according to the present invention.

[0068] Figure 4 This is a diagram of the interconnected fracturing pump unit, borehole, and interconnected high-energy gas generator of the present invention.

[0069] Figure 4 This is a schematic diagram of the working state of the three-way manifold 4, which connects the fracturing pump unit to the borehole (liquid-solid); and connects the gas-producing agent storage tank to the borehole (gas phase), with continuous pressure supplied to the pump unit end.

[0070] Figure 5 This is a schematic diagram of hydraulic fracturing and solid medium pre-positioning according to the present invention.

[0071] Figure 5 When the medium-water sand pump 1 starts, the high-pressure water-sand mixture it outputs pushes open the water circuit check valve 3 and the orifice check valve 7, and is continuously injected into the coal seam 9 through the borehole 8, creating initial cracks in the coal body and pre-positioning proppant. At this time, the right-side gas circuit check valve 5 is in the closed state.

[0072] Figure 6 This is a schematic diagram of the gas-driven liquid-solid pulse impact and deep groove expansion method of the present invention.

[0073] Figure 6 The high-energy gas generator 6 on the right side generates high-pressure gas, which pushes open the gas circuit check valve 5 and enters the three-way valve; the huge pressure difference forces the water circuit check valve 3 on the left side to close, protecting the water pump from being backflush by the high pressure; the high-pressure gas drives the water and sand fluid in the pipeline and the initial fracture of the coal seam 9 to violently impact the coal seam 9 through the orifice check valve 7, realizing the secondary expansion of the fracture and deep sand carrying.

[0074] Figure 7 This is a schematic diagram of the unidirectional energy locking and cyclic fracturing of the present invention.

[0075] Figure 7The high-pressure fluid in the middle coal seam 9 attempted to flow back, but was effectively blocked by the one-way valve 7 at the orifice, thus "locking in energy" to prevent sand backflow. At the same time, due to the pressure reduction in the three-way valve, the water-sand pump 1 on the left automatically pushed open the one-way valve 3 of the water circuit to replenish the pipeline with new water-sand medium, preparing materials for the next pulse impact.

[0076] Reference Figures 1-7 This invention provides a downhole gas-liquid-solid three-phase synergistic anti-backflow fracturing method, comprising the following steps:

[0077] S100. The water-sand pump unit 1 is used as a continuous liquid phase power source and connected to the first inlet of the three-way manifold 4 through the water circuit check valve 3; the high-energy gas generator 6 is used as a transient gas phase power source and connected to the second inlet of the three-way manifold 4 through the gas circuit check valve 5. The outlet end of the three-way manifold 4 is connected to the fracturing tubing extending to the borehole of the underground coal seam 9, and a borehole check valve 7 is installed at the very end of the fracturing tubing to construct a closed fluid transport circuit with unidirectional conduction characteristics.

[0078] S200. Start the water-sand pump unit 1, outputting a water-sand mixture containing proppant. After the fluid pressure overcomes the friction resistance along the pipeline, it pushes open the water circuit check valve 3 and the orifice check valve 7. At this time, the gas circuit check valve 5 is in the closed state. The water-sand mixture enters the borehole of coal seam 9, using the quasi-static pressure of the liquid to generate initial fractures in coal seam 9, and relying on the fluid carrying effect to lay the pre-proppant in the near-wellbore zone of the fractures.

[0079] S300. While the water-sand pump unit 1 is running, the high-energy gas generator 6 is triggered. The propellant inside the generator reacts to produce a high-pressure gas pulse. This pulse pressure rapidly builds up in the three-way manifold 4. When the pressure exceeds the output pressure of the water-sand pump unit 1, a reverse pressure difference is formed between the three-way manifold 4 and the water-sand pump unit 1, forcing the water circuit check valve 3 to close instantaneously. At this time, the system physically isolates the high-pressure gas from the low-pressure water-sand pump unit 1, preventing fluid backflow.

[0080] S400 and the gas-line check valve 5 open under the pressure of high-pressure gas, and the gas pulse enters the three-way manifold 4 and the fracturing string. Since the pipeline is pre-filled with an incompressible water-sand mixture, the high-pressure gas acts as a gas-phase piston, propelling the water-sand mixture column in front to accelerate. The high-energy liquid column impacts the coal seam 9 fracture, exerting a secondary fracturing effect on the fracture tip, and carrying the suspended proppant deeper into the fracture.

[0081] S500. As the gas expands due to work and the reaction of the proppant ends, the pressure inside the fracturing string begins to decrease. When the pressure at the end of the fracturing string drops below the fluid pressure inside the fracture in coal seam 9, the one-way valve 7 at the orifice automatically closes under the action of the pressure difference. This action blocks the backflow channel of fluid from the fracture to the pipeline, locking the injected high-energy fluid and proppant within the fracture space and maintaining the fracture in an open state.

[0082] S600. As the pressure in the three-way manifold 4 and the pipeline continues to decrease, when the system pressure drops below the output pressure of the water-sand pump set 1, the water circuit check valve 3 reopens under the action of positive pressure differential. The fluid output from the water-sand pump set 1 automatically flows into the three-way manifold 4 and the fracturing string, filling the void in the pipeline, displacing residual gas, and restoring the system pressure to the initial injection level, preparing the medium for the next round of pulse ignition.

[0083] Reference Figures 1-7 In this embodiment, the system deployment and connection process described in step S100 constructs a high-pressure fluid dynamics transport network, which is physically connected by a power source unit, a fluid control unit, and an execution unit.

[0084] Specifically, the water-sand pump unit 1, acting as a continuous liquid-phase power source, is equipped with a sand mixing device and a high-pressure plunger pump for preparing and outputting a water-sand mixture with a specific sand ratio. The current output pressure of the water-sand pump unit 1... The parameters were set based on the geomechanical parameters of coal seam 9. The basis for these settings is as follows:

[0085] ;

[0086] In the formula, This represents the minimum principal stress in coal seam 9, used to characterize the rock mass closure pressure; This indicates the current output pressure of water-sand pump unit 1; This represents the tensile strength of the rock in coal seam 9, used to characterize the resistance to crack initiation; This represents the total frictional loss of the fluid flowing within the fracturing tubing. The outlet of the water-sand pump unit 1 is connected to the input of the water circuit check valve 3 via a high-pressure pipeline. The output of the water circuit check valve 3 is rigidly connected to the first inlet of the three-way manifold 4. The water circuit check valve 3 is a spring-reset high-pressure check valve, with its opening pressure set between 0.5 MPa and 1.0 MPa to ensure that the valve only opens when the pump unit actively pressurizes, preventing natural convection of the fluid under low pressure.

[0087] The high-energy gas generator 6 serves as a gas-phase pulse power source, containing one or more independent charging chambers pre-loaded with solid propellant or high-energy gas generating propellant. The gas release port of the high-energy gas generator 6 is connected to the input of the gas path check valve 5, and the output of the gas path check valve 5 is connected to the second inlet of the three-way manifold 4. The gas path check valve 5 employs a hard-seal structure, with its reverse pressure resistance designed to be no less than 1.5 times the rated output pressure of the water-sand pump set 1, to prevent the water-sand mixture from backflowing into the generator chamber during the pumping stage, causing the propellant to become damp and fail.

[0088] The three-way manifold 4 is the system's fluid exchange hub, made of high-strength alloy steel in T- or Y-type high-pressure resistant pipe fittings. Its structural strength design must meet the maximum burst pressure requirement, i.e., withstand the transient peak gas pressure generated by the high-energy gas generator 6. Without plastic deformation, the outlet of the three-way manifold 4 connects to the beginning of the fracturing string, which extends to the predetermined borehole depth of coal seam 9. At the end of the fracturing string (i.e., the end entering coal seam 9), a one-way valve 7 is threadedly connected. The one-way valve 7 is the end-effector of the system; its valve core and seat are made of wear-resistant hard alloy material to resist the scouring of fluids containing high concentrations of proppant. The one-way valve 7 is configured to be normally closed, opening only when the pressure inside the string exceeds a certain threshold above the ambient pressure outside. When the pressure differential reverses, it closes rapidly using spring force and reverse fluid pressure, forming the end-effector of the system. The entire string system is sealed within the borehole by a sealing device to ensure isolation between the fracturing zone and the wellbore annulus.

[0089] Reference Figures 1-7 In this embodiment, the static pressure fracturing and pre-support process described in step S200 is the basic stage of the entire fracturing operation. Its main purpose is to establish an initial fracture network in the coal seam 9 and pre-place a certain amount of proppant.

[0090] In specific operation, water-sand pump unit 1 is started to pump the prepared water-sand mixture at a constant discharge rate. Injection system. The water-sand mixture consists of a base fluid (usually water or low-viscosity fracturing fluid) and solid proppant particles suspended within it. The density of the mixture... Determined by the volume ratio of the solid and liquid phases, it follows the mixing density formula:

[0091] ;

[0092] In the formula, The volume fraction of the proppant should be controlled between 5% and 10%. The density of the proppant (such as coated ceramic particles); This represents the density of the base liquid.

[0093] As the water-sand pump unit 1 continues to operate, the fluid pressure within the system gradually increases. When the pressure at the output end of the water-sand pump unit 1 overcomes the opening resistance of the water circuit check valve 3 and the friction resistance along the pipeline, the valve core of the water circuit check valve 3 opens, and the fluid enters the fracturing string through the first inlet of the three-way manifold 4. At this time, the high-energy gas generator 6 is in standby mode, and the pressure on the inlet side of the gas circuit check valve 5 is atmospheric pressure or slightly positive pressure, which is much lower than the high-pressure liquid flow in the three-way manifold 4. Therefore, the gas circuit check valve 5 remains tightly closed under the action of a huge lateral pressure difference, preventing high-pressure water-sand liquid from entering the gas generator.

[0094] The high-pressure water-sand mixture flows along the fracturing string to the end, pushing open the one-way valve 7 at the orifice and entering the open hole or perforated section of the coal seam 9 borehole. As the injected volume accumulates, the fluid pressure at the wellbore rock interface continuously increases. When the fluid pressure inside the borehole... When the fracture mechanics initiation criterion is met—that is, when the stress exceeds the sum of the tangential stress and tensile strength of the rock—tensile failure occurs in the coal and rock, forming an initial crack. At this stage, crack propagation is primarily driven by quasi-static hydraulic pressure, and the crack volume... With injection time It exhibits a roughly linear growth relationship:

[0095] ;

[0096] In the formula, This indicates the volume of fracturing fluid lost to the coal seam matrix. Indicates the injection time; This indicates the injection displacement or flow rate of the high-pressure water-sand mixture; This represents the time differential element. During this process, the proppant carried by the water-sand mixture enters the newly formed fracture channel along with the fluid. Limited by the carrying capacity of the fluid and the velocity distribution within the fracture, the proppant mainly settles and accumulates in the near-wellbore zone or near the fracture entrance, forming a so-called "sand embankment" or high-concentration proppant zone, providing the material basis for subsequent deep propulsion via gas-phase pulses. This stage continues until the injected volume of the mixture reaches the preset pre-fill volume standard.

[0097] Reference Figures 1-7 In this embodiment, the process described in step S300 realizes the switching of the system from quasi-static hydraulic mode to transient pneumatic impact mode. Its core lies in utilizing the explosive characteristics of high-pressure gas and the physical logic control of fluid diodes.

[0098] With the water-sand pump unit 1 maintaining a constant output pressure, the surface or downhole control system sends an electrical pulse signal to the ignition head of the high-energy gas generator 6. Upon receiving the signal, the selected individual gas-producing propellant column is ignited, resulting in a violent redox reaction. This chemical reaction releases a massive amount of gaseous products (typically including nitrogen, carbon dioxide, and water vapor) within millisecond timescales, causing the pressure within the confined space inside the generator to rise exponentially.

[0099] Define the pressure change function of the high-pressure gas pulse as follows: Within a very short time after ignition (typically 10ms to 15ms), the gas pressure rapidly reaches its peak. To achieve effective impact and shut-off functions, this peak pressure is designed to be the current output pressure of the water-sand pump unit 1. Multiple gain:

[0100] ;

[0101] In the formula, Indicates the peak value of the gas pressure; This indicates the current output pressure of water-sand pump unit 1; This represents the gain coefficient; it means that the generated high-pressure gas mass has an energy density far exceeding the hydrostatic pressure of the system.

[0102] When the generated transient gas pressure builds up to a value higher than the fluid pressure inside the three-way manifold 4 before reaching the one-way valve 5, the valve core of the one-way valve 5 is pushed open, and high-pressure gas instantly rushes into the cavity of the three-way manifold 4. At this time, the pressure at the node of the three-way manifold 4... It rises rapidly in sync with the gas pressure. Due to... The pressure on the outlet side (connected to the manifold) of the water circuit check valve 3 sharply exceeds the pressure on the inlet side (connected to the water pump). This huge reverse pressure difference acts directly on the valve core of the water circuit check valve 3, generating a strong mechanical closing force. :

[0103] ;

[0104] In the formula, This refers to the effective force-bearing area of ​​the valve core. To assist the spring force; Indicates the mechanical closing force; This indicates the pressure at the 4th node of the three-way junction box; This indicates the current output pressure of the water-sand pump unit 1. This closing force compels the water circuit check valve 3 to instantly reseat and tighten the sealing surface, physically cutting off the liquid phase input channel. This process is similar to the diode cutoff effect in electronic circuits; without the need for external electronic sensors or actuators, it automatically isolates and protects the low-pressure pump equipment using the fluid's own pressure fluctuations, preventing high-energy gas backflow from damaging the pump body. At this point, the system enters a pure gas phase drive state, with the gas path unobstructed and the water path cut off.

[0105] Reference Figures 1-7 In this embodiment, the gas-liquid-solid coupling drive and deep transport process described in step S400 utilizes the expansion work of high-pressure gas pulses to transform a stationary or low-speed moving liquid medium into a high-speed jet, thereby realizing dynamic fracture creation and long-distance transport of proppant in coal seam 9.

[0106] When a high-pressure gas pulse enters the three-way manifold 4 and the fracturing tubing via the gas circuit check valve 5, a clear gas-liquid interface is formed between the gas leading edge and the existing water-sand mixture column within the tubing. Due to the high compressibility and expansibility of the gas, while the water-sand mixture can be considered an incompressible fluid, the high-pressure gas mass acts as a massless "gas-phase piston" at the interface. The gas performs work on the surroundings through an adiabatic or polytropic expansion process, converting its internal energy into the kinetic energy of the preceding liquid column.

[0107] According to the law of conservation of momentum and the unsteady form of Bernoulli's equation, the accelerated water-sand mixture column acquires extremely high acceleration within the fracturing string. This acceleration is directly proportional to the pressure difference across the gas-liquid interface and inversely proportional to the mass of the liquid column.

[0108] ;

[0109] In the formula, The cross-sectional area of ​​the tubular column; This refers to the pressure at the crack inlet. The total mass of the driven liquid column; This is the component of gravitational acceleration; The coefficient of friction; Pipe diameter; For flow rate; The instantaneous acceleration of the liquid column; This indicates the gas pulse pressure.

[0110] Driven by high-pressure gas, a high-energy column of water-sand mixture was propelled into the No. 9 coal seam fracture in the form of a pulsed jet. This high-speed flow produced two significant physical effects:

[0111] Secondary impact widening effect. The high-speed liquid column generates extremely high "water hammer" pressure at the crack tip. This dynamic pressure is superimposed on the original hydrostatic pressure, causing the stress intensity factor at the crack tip to instantly exceed the fracture toughness of the rock. This leads to the formation of complex secondary branches and microcracks, greatly increasing the complexity and volume of the crack network.

[0112] Proppant deep transport effect. The dramatic increase in fluid velocity enhances the proppant-carrying capacity of the liquid. According to the particle transport theory in fluid mechanics, the suspension capacity of particles is related to a high power of the fluid velocity. High-speed turbulence effectively suppresses the gravitational settling of proppant particles, causing the proppant "sand embankment" that was originally trapped in the near-wellbore area to be picked up again and transported at high speed with the fluid, crossing the near-end region of the fracture and penetrating into the far-end closure zone. This solves the problem of proppant's difficulty in penetrating deep fractures in conventional hydraulic fracturing, ensuring that deep fractures still have high conductivity after closure. The entire process achieves efficient energy transfer from chemical energy to gas internal energy, then to liquid column kinetic energy, and finally to rock fracturing work and particle displacement work.

[0113] Reference Figures 1-7 In this embodiment, the energy locking and anti-backflow mechanism described in step S500 is to automatically protect the fracturing results by utilizing the dynamic reversal characteristics of the pressure field at the end of the pulse energy release.

[0114] As the high-pressure gas propels the water-sand mixture column into the fracture and continues to expand, and the propellant in the high-energy gas generator 6 is exhausted and stops supplying gas, the fluid pressure inside the fracturing string decreases. It began to exhibit exponential decay characteristics. Meanwhile, the fracture in coal seam 9, acting as an elastic energy storage body, was in a high-pressure open state after receiving the high-energy fluid injection. Furthermore, due to the elastic recoil force of the fracture wall and the fluid filtration resistance, a relatively stable high pressure value was maintained inside the fracture. .

[0115] Transient pressure within the fracturing string Decay to below the residual pressure within the crack When, that is, the condition is met:

[0116] ;

[0117] In the formula, This indicates the transient fluid pressure within the fracturing tubing; This indicates the remaining pressure within the crack.

[0118] At this point, a reverse pressure differential is formed on both sides of the one-way valve 7 at the orifice end of the fracturing string, pointing from the coal seam 9 into the interior of the string. This reverse pressure difference acts on the back of the valve core of the orifice check valve 7, generating a reverse thrust. Combined with the preload of the built-in return spring in the orifice check valve 7... The total closing force on the valve core for:

[0119] ;

[0120] In the formula, This refers to the valve seat sealing area; Indicates reverse pressure difference; This indicates the preload of the return spring; This indicates the total closing force acting on the valve core. Driven by this, the one-way valve 7 at the orifice quickly closed, cutting off the fluid connection between the fracturing string and the borehole of the coal seam 9.

[0121] This action achieves a physical "energy lock-in." At this point, an independent high-pressure holding chamber is formed inside the fractures of coal seam 9, forcibly sealing the injected high-pressure gas, fracturing fluid, and proppant within the fracture network. This mechanism effectively overcomes the "sand ejection" and backflow problems commonly encountered in traditional gas fracturing or pulse fracturing. Without this one-way locking mechanism, a rapid drop in tubing pressure would cause the high-pressure fluid within the fractures to carry the proppant back towards the wellbore at high speed, resulting in proppant loss and wellbore blockage. By promptly closing the orifice check valve 7, the system forces the high-pressure gas and fracturing fluid to be filtered out only through microfractures into the deeper matrix of coal seam 9. This not only prevents proppant backflow but also prolongs the interaction time of the high-pressure fluid with the coal body, further improving gas extraction efficiency through gas desorption and displacement effects.

[0122] Reference Figures 1-7 In this embodiment, the process described in step S600 marks the end of a single pulse operation cycle and the preparation for the next cycle, reflecting the system's self-recovery and continuous operation capabilities.

[0123] After the orifice check valve 7 closes to maintain pressure in the fracture, the pressure in the three-way manifold 4 and the fracturing string continues to decrease. At this time, the system is in a "cavity" state, with the pipeline mainly filled with low-pressure residual gas from the previous pulse work, and the volume is reduced due to gas expansion and partial filtration. In contrast, the water-sand pump unit 1 maintains its rated operating condition, and the fluid pressure at its output end... Maintain at the set level.

[0124] When the residual pressure inside the three-way combiner 4 Reduce to less than the output pressure of water sand pump set 1 When, that is, the following condition is met:

[0125] ;

[0126] In the formula, This is the opening differential pressure threshold for the water circuit check valve 3. At this time, the positive differential pressure overcomes the valve core back pressure and the return spring force, pushing the water circuit check valve 3 to reopen. The fresh water-sand mixture pumped by the water-sand pump unit 1 then rushes into the three-way manifold 4 and quickly fills the space of the fracturing tubing.

[0127] The injected water-sand mixture serves a dual purpose: first, volume compensation, filling the physical space left by gas expansion and eliminating air pockets in the pipeline; second, medium reconstruction, compressing or displacing the low-density residual gas through an exhaust valve (if the system is equipped with one), allowing the pipeline to be refilled with incompressible liquid medium. As the liquid replenishment proceeds, the pressure in the pipeline gradually rises and eventually stabilizes at the output pressure level of the water-sand pump set, and the system returns to the initial static pressure injection state, completing the continuous liquid phase reconstruction after a single pulse.

[0128] Subsequently, the control system, according to the preset fracturing program, sends an excitation signal to the next-stage independent gas-producing charge within the high-energy gas generator 6. The system repeats steps S300 to S600, namely, generating high-pressure gas pulses again, closing the water circuit check valve 3, using gas-driven liquid columns to impact the fractures, maintaining pressure at the orifice check valve 7, and reopening the water circuit for replenishment. Through this iterative cycle of "static pressure—dynamic pressure—pressure maintenance—replenishment," the relay bursts of multi-stage charges deliver multiple consecutive, powerful impacts to coal seam 9. This multi-wave pulse load can overcome the fatigue strength of the rock, continuously expanding the fracture network until all pre-set charges are consumed, achieving full modification of the target coal seam 9 area.

[0129] Reference Figures 1-7 This embodiment provides a downhole gas-liquid-solid three-phase synergistic anti-backflow fracturing system. The system consists of a surface power and control unit, a downhole fluid transmission unit, and a terminal execution unit. The components are physically connected through a high-pressure resistant pipeline 2 to form a closed gas-liquid-solid multiphase fluid operation loop.

[0130] Specifically, the system mainly includes a water-sand pump set 1, a high-energy gas generator 6, a three-way manifold 4, a water circuit check valve 3, a gas circuit check valve 5, a fracturing tubing string, and an orifice check valve 7.

[0131] Water-sand pump unit 1 is the liquid-phase power source of the system, used to provide a continuous quasi-static high-pressure water-sand mixture. The output end of water-sand pump unit 1 is connected to the input end of water circuit check valve 3 via a high-pressure pipeline. Water circuit check valve 3 is installed between water-sand pump unit 1 and three-way manifold 4, and its output end is airtightly connected to the first inlet of three-way manifold 4. Water circuit check valve 3 is equipped with a one-way conduction mechanism, configured to allow fluid to flow only from water-sand pump unit 1 to three-way manifold 4, and automatically shuts off when the outlet pressure is higher than the inlet pressure to physically block the reverse fluid path.

[0132] The high-energy gas generator 6 is the system's gas-phase pulse power source, used to generate transient high-pressure gas pulses. It internally houses a propellant combustion chamber and an electric ignition control module, with its output connected to the input of the gas path check valve 5. The gas path check valve 5 is installed between the high-energy gas generator 6 and the three-way manifold 4, with its output airtightly connected to the second inlet of the three-way manifold 4. The gas path check valve 5 also possesses unidirectional conduction characteristics, allowing high-pressure gas to flow from the generator into the manifold while simultaneously preventing the mixture from flowing back into the generator.

[0133] The three-way manifold 4, serving as the fluid junction node of the system, has three ports: a first inlet, a second inlet, and an outlet. The first inlet is connected to the water circuit check valve 3 to receive liquid fluid; the second inlet is connected to the gas circuit check valve 5 to receive gaseous fluid. The internal cavity structure of the three-way manifold 4 is designed to withstand the mixing impact of the gas and liquid phases and transient peak pressures. The outlet end of the three-way manifold 4 is rigidly connected to the beginning of the fracturing string.

[0134] The fracturing string is the transmission channel connecting the surface / near-well equipment with the deep working area of ​​coal seam 9. Its starting end is connected to the outlet of the three-way manifold 4, and the tubing extends along the borehole to the predetermined fracturing position in coal seam 9. The fracturing string uses high-pressure resistant seamless steel tubing or coiled tubing, and has the mechanical strength to withstand the expansion of internal high-pressure gas and external formation stress.

[0135] The orifice check valve 7 is installed at the end of the fracturing tubing, specifically at the very front of the borehole extending into coal seam 9. The inlet side of the orifice check valve 7 communicates with the interior of the fracturing tubing, while the outlet side faces directly into the borehole space of coal seam 9. The orifice check valve 7 contains a return spring and a valve core assembly, configured to open only when the fluid pressure within the tubing overcomes the external environmental pressure and the spring preload, allowing the water-sand mixture and gas to enter coal seam 9. Conversely, when a pressure drop within the tubing causes a reverse pressure differential, the valve core automatically closes, isolating the fracturing tubing from the borehole in coal seam 9.

[0136] The water circuit check valve 3, the gas circuit check valve 5, and the orifice check valve 7 are physically connected to form a fluid logic control loop. This loop does not require external electronic sensors or active control signals. It can automatically switch the opening or closing state of each valve by relying solely on the real-time changes in fluid pressure and the direction of pressure difference between the nodes in the pipeline. This allows for automatic timing coordination of all process actions, including static pressure injection, pneumatic shut-off protection, gas-driven liquid pulse transport, and crack pressure holding and backflow prevention. This achieves unidirectional coordinated transport and energy locking of the gas-liquid-solid three-phase fluid.

Claims

1. A downhole gas-liquid-solid three-phase synergistic fracturing method, characterized in that, Includes the following steps: Connect the water-sand pump set to the first inlet of the three-way manifold via a water circuit check valve, connect the high-energy gas generator to the second inlet of the three-way manifold via a gas circuit check valve, connect the outlet of the three-way manifold to the fracturing tubing, and install an orifice check valve at the end of the fracturing tubing. The water-sand pump unit is started to output water-sand mixture. Under pressure, the water-sand mixture opens the water circuit check valve and the orifice check valve, enters the coal seam borehole to form initial fractures and fills proppant. The water-sand pump set continues to operate, stimulating the high-energy gas generator to produce high-pressure gas pulses. The high-pressure gas pulses cause the pressure inside the three-way manifold to rise to a level greater than the output pressure of the water-sand pump set, forcing the water circuit check valve to close. High-pressure gas pulses enter the three-way manifold through the gas circuit check valve, driving the water-sand mixture column in the fracturing string to accelerate into the coal seam fractures, transporting the proppant to the depth of the coal seam fractures; The gas pulse ends, causing the pressure inside the fracturing string to decrease. When the pressure at the end of the fracturing string is less than the fluid pressure inside the coal seam fracture, the orifice check valve automatically closes under the reverse pressure differential from the coal seam to the fracturing string, isolating the fracturing string from the coal seam borehole and forming an independent pressure-holding cavity inside the coal seam fracture to lock the proppant inside the fracture. As the pressure inside the three-way manifold continues to decrease to below the output pressure of the water-sand pump set, the water circuit check valve automatically opens under the action of positive pressure differential, and the water-sand pump set replenishes the water-sand mixture into the fracturing string and restores the pipeline pressure. The water circuit check valve, air circuit check valve, and orifice check valve together constitute the fluid logic control loop, which automatically switches between open and closed states based on changes in fluid pressure within the pipeline. The fluid logic control loop is configured as follows: When a high-pressure gas pulse is injected into the three-way manifold, the peak pressure generated by the gas pulse causes the pressure inside the three-way manifold to jump to 2 to 4 times the output pressure of the water sand pump group, thereby creating a reverse pressure difference on both sides of the water circuit check valve to drive it to close automatically, so as to physically isolate the water sand pump group from the high-pressure gas. When the pressure inside the fracturing string decreases to below the fluid pressure inside the coal seam fracture after the gas pulse ends, the one-way valve at the orifice is automatically closed by using the reverse pressure differential to isolate the fracturing string from the coal seam borehole and form an independent pressure-holding cavity. When the pressure inside the three-way manifold further decreases to less than the output pressure of the water-sand pump set, the positive pressure difference drives the water circuit check valve to open automatically, so as to replenish the water-sand mixture into the fracturing string and restore the pipeline pressure, thereby completing the continuous reconstruction of the liquid phase medium after a single pulse.

2. The downhole gas-liquid-solid three-phase synergistic fracturing method according to claim 1, characterized in that, in, The process of drilling into the coal seam to form initial fractures and filling them with proppant specifically includes: The water-sand pump set pressurizes the water-sand mixture to a set pumping pressure, which is greater than the sum of the minimum principal stress of the coal seam and the tensile strength of the coal seam. The pressurized water-sand mixture overcomes the frictional resistance along the pipeline and flows through the three-way manifold. The water circuit check valve remains open under the positive pressure of the fluid, while the air circuit check valve remains closed under the lateral pressure of the fluid. The water-sand mixture enters the coal seam borehole through the one-way valve at the orifice. Under the action of quasi-static hydraulic pressure, it causes tensile fracturing of the coal seam wall, forming a fracture channel extending outward from the near-wellbore zone. The proppant in the water-sand mixture remains in the near-end region of the fracture channel along with the fluid.

3. The downhole gas-liquid-solid three-phase synergistic fracturing method according to claim 1, characterized in that, in, The specific steps involved in generating a high-pressure gas pulse using the high-energy gas generator are as follows: The control system sends an ignition signal to the gas-generating propellant column in the high-energy gas generator to ignite the propellant column and carry out a chemical reaction. The gas-generating propellant column releases a large amount of high-temperature and high-pressure gas within milliseconds. The gas rapidly accumulates in the confined space, forming a pressure peak with a steep rising edge. The pressure peak establishes a transient gas pressure value higher than the current hydrostatic pressure inside the fracturing string before reaching the gas circuit check valve.

4. The downhole gas-liquid-solid three-phase synergistic fracturing method according to claim 1, characterized in that, in, The pressure inside the three-way manifold increases to a level greater than the output pressure of the water-sand pump set, specifically including: The gas circuit check valve opens under the thrust of the high-pressure gas pulse, and the high-pressure gas instantly fills the internal cavity of the three-way manifold; The fluid pressure inside the three-way manifold jumps to its peak value the instant gas is injected. The outlet side of the water circuit check valve bears the peak pressure, and the inlet side bears the output pressure of the water sand pump group. The reverse pressure difference formed between the outlet side and the inlet side drives the valve core of the water circuit check valve to return to its seat and press the sealing surface tightly, cutting off the fluid passage between the water sand pump group and the three-way manifold.

5. The downhole gas-liquid-solid three-phase synergistic fracturing method according to claim 1, characterized in that, in, The accelerated entry of the water-sand mixture column within the fracturing string into the coal seam fractures includes: The high-pressure gas pulse entering the three-way manifold forms a gas-liquid interface with the water-sand mixture column retained in the fracturing tubing. The high-pressure gas pulse acts as a gas-phase piston, transferring momentum to the incompressible water-sand mixture column through expansion and work. The water-sand mixture column gains acceleration under the drive of the gas pressure at the rear end, which is transformed into a secondary hydraulic impact on the tip of the coal seam fracture. At the same time, the high-speed flow of the fluid carries the proppant across the near end region of the fracture channel and transports it to the deep closed region of the fracture channel.

6. The downhole gas-liquid-solid three-phase synergistic fracturing method according to claim 1, characterized in that, in, The pressure attenuation within the fracturing string includes: As the high-pressure gas pulse propels the water-sand mixture column into the coal seam fracture, the gas volume expands within the fracturing tubing and fracture passage. The gas-generating propellant in the high-energy gas generator has finished reacting, and the gas supply has stopped. Some gases and fluids are lost to the coal seam matrix through the pores in the fracture walls of the coal seam.

7. The downhole gas-liquid-solid three-phase synergistic fracturing method according to claim 1, characterized in that, in, Specifically, the pressure at the end of the fracturing string being less than the fluid pressure within the coal seam fracture includes: After hydraulic fracturing, high-pressure fluid and elastic potential energy accumulate in the coal seam fractures, which maintains the first pressure value inside the fractures. Due to gas expansion and cooling, the pressure inside the fracturing string decreases to the second pressure value; When the second pressure value is lower than the first pressure value, a reverse pressure differential is formed on both sides of the valve core of the orifice check valve, pointing from the coal seam to the fracturing string. The reverse pressure differential, in conjunction with the elasticity of the internal reset mechanism of the orifice check valve, drives the valve core to close, isolating the fracturing string from the coal seam borehole, so that an independent pressure-holding cavity is formed inside the coal seam fracture.

8. The downhole gas-liquid-solid three-phase synergistic fracturing method according to claim 1, characterized in that, in, The water-sand pump unit replenishes the water-sand mixture into the fracturing tubing and restores the pipeline pressure, specifically including: After the water circuit check valve is opened, the water-sand mixture output by the water-sand pump unit flows into the three-way manifold and the fracturing tubing string; The water-sand mixture entering the fracturing string fills the volume space left after the high-pressure gas pulse expansion and filtration, and replaces the residual gas in the pipeline; The water-sand pump unit continuously injects water-sand mixture until the fluid pressure in the three-way manifold and fracturing string rises and stabilizes to the output pressure of the water-sand pump unit, thus completing the continuous reconstruction of the liquid phase medium after a single pulse fracturing.

9. A downhole gas-liquid-solid three-phase synergistic fracturing method according to claim 8, characterized in that, After the continuous reconstruction of the liquid medium following a single pulse fracturing, the next independent gas-generating propellant column in the high-energy gas generator is activated in a preset order. Based on the high-pressure gas pulse generated by the gas-generating propellant column, the steps in claim 1, where the self-activated high-energy gas generator generates a high-pressure gas pulse to replenish the water-sand mixture in the fracturing tubing and restore the pipeline pressure, are repeatedly executed until all the pre-set gas-generating propellant columns in the high-energy gas generator are activated.

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