Compound reconstruction method of directional hydraulic slotting and controllable acidification in coal seam with complex geological structure

Abstract

This invention relates to the field of coal seam modification engineering technology, and particularly to a method for combined modification of coal seams with directional hydraulic fracturing and controlled acidizing in complex geological structures. The method includes: obtaining the in-situ stress direction of the target coal seam segment and the spatial orientation of fault planes and folded flanks, thereby determining the arrangement orientation of the directional hydraulic fracturing; subsequently, directional hydraulic fracturing is implemented in the borehole to form multiple clusters of initial directional fracturing with different orientations along the borehole depth, used to define the initiation location and direction of fractures; based on this, a controlled acid system is injected into the coal seam segment, allowing the acid to enter the initial directional fracturing and natural fractures of the coal seam in a low-viscosity state, and during its advancement along the fractures, the system's pH value changes due to reaction with minerals in the coal, triggering the formation of in-situ temporary plugging zones in cementation units, guiding subsequent acid flow along different fracture channels; after continuous acid injection to complete the fracture acidizing treatment, the coal seam segment is subjected to backflow or replacement operations.

Description

Technical Field

[0001] This invention relates to the field of coal seam modification engineering technology, and in particular to a method for combined modification of coal seams with complex geological structures using directional hydraulic fracturing and controllable acidizing. Background Technology

[0002] In coalbed methane development and coal seam stimulation, hydraulic fracturing and hydraulic cutting technologies are widely used to improve the fracture structure and enhance the connectivity of coal seams, especially in medium- and low-permeability coal seams. These technologies create fractures by applying hydraulic pressure within the borehole to guide the movement of fluids within the coal seam. In practical engineering, to adapt to the heterogeneity of coal seams, directional hydraulic fracturing technology has been gradually developed. This involves controlling the direction of the water jet within the borehole to create initial fracturing with a specific orientation on the borehole wall, aiming to guide the direction of fracture initiation. Simultaneously, in some coal seam stimulation operations, acidizing measures are combined, injecting acidic fluids into the coal seam fractures to dissolve the mineral filling material, thereby improving the fracture space conditions. In coal seams with complex geological structures such as faults and folds, the stress field distribution is uneven and the direction of structural weak surfaces is diverse. In existing technologies, the arrangement of directional hydraulic fractures is often based solely on a single geostress direction or empirical judgment, making it difficult to simultaneously consider the in-situ geostress direction and the spatial orientation of structural surfaces. This results in a lack of effective constraints on the fracture initiation location and direction in complex structural areas, leading to significant uncertainty in the fracture initiation and propagation process, which affects the implementation of subsequent remediation operations. Summary of the Invention

[0003] To overcome the above shortcomings, this invention provides a method for combined directional hydraulic fracturing and controlled acidizing of coal seams with complex geological structures. This method aims to improve the problem that the location and direction of crack initiation are not effectively constrained in complex geological areas, and the uncertainty of crack initiation and propagation processes is large, which affects the implementation of subsequent modification operations.

[0004] This invention provides the following technical solution: a method for combined directional hydraulic fracturing and controllable acidizing of coal seams with complex geological structures, comprising the following steps: S1. Obtain the in-situ stress direction of the target coal seam section and the spatial orientation of the fault plane and folded wing. Determine the arrangement orientation of the directional hydraulic cut based on the in-situ stress direction and the spatial orientation of the fault plane and folded wing. S2. Directional hydraulic cutting is carried out along the borehole in the target coal seam section to form two sets of initial directional cutting slots with different orientations in the borehole. The initial directional cuttings are arranged in multiple clusters along the borehole depth to define the crack initiation position and crack initiation direction. S3. Inject a controllable acid system into the coal seam section after directional hydraulic fracturing. The controllable acid system includes a weak acid, a corrosion inhibitor, an iron ion stabilizer, and a pH-sensitive gelling unit. The controllable acid is in a low-viscosity state in the initial stage of injection and enters the initial directional fracturing and natural fractures of the coal seam. S4. During the process of controlled acid liquid advancing along the fracture, the controlled acid liquid reacts with the minerals in the coal and consumes hydrogen ions, causing the pH value of the system to rise. When the triggering condition of the pH value-sensitive cementing unit is reached, an in-situ temporary plugging zone is formed in the fracture, allowing the subsequently injected controlled acid liquid to flow along different fracture channels. S5. Continuously inject controlled acid solution to react with the mineral filling material in the fracture and acidify the fracture. S6. After the acidification treatment is completed, the coal seam section is subjected to backflow or replacement operation.

[0005] By adopting the above technical solution, the arrangement orientation of directional hydraulic cuts can be determined in coal seams with complex geological structures by comprehensively considering the in-situ stress direction and the spatial orientation of fault planes and folds. Multiple clusters of initial directional cuts with different orientations are formed in the borehole, thereby pre-defining the initiation location and direction of the cracks in the early stage of crack formation. This provides clear spatial constraints for the crack initiation and initial propagation process, reducing the uncertainty caused by the complexity of the stress field and the diversity of structural orientations during crack initiation and propagation.

[0006] Preferably, in step S1, the step of determining the arrangement orientation of the directional hydraulic cut based on the in-situ stress direction and the spatial orientation of the fault plane and the folded flank includes: Obtain the directions of maximum and minimum principal stress in the target coal seam section; Obtain the strike and dip of fault planes within the target coal seam segment, as well as the strike direction of the folded fins; Based on the directions of maximum and minimum principal stress, the orientation of the first directional hydraulic cut is determined to be conducive to crack initiation. Based on the strike and dip of the fault plane and the orientation of the folded wing, the orientation of the second directional hydraulic slot along the weak structural plane is determined. The orientations of the first and second directional hydraulic cuts are used as the orientations for the directional hydraulic cuts within the target coal seam section.

[0007] Preferably, in step S2, the step of performing directional hydraulic cutting along the borehole within the target coal seam section includes: A directional hydraulic slit cutting device is installed inside the borehole, and the orientation of the directional hydraulic slit cutting device is adjusted to be consistent with the orientation of the directional hydraulic slit cutting. High-pressure water jets are sprayed into the borehole using a directional hydraulic slitting device to directionally cut the borehole wall. A cutting groove with a predetermined orientation is formed on the borehole wall as an initial directional cut.

[0008] Preferably, in step S2, the step of arranging the initial directional cuts in multiple clusters along the borehole depth includes: The target coal seam section is divided into multiple slotted sections along the borehole axis; A cluster of initial directional cuts is formed within each cut segment; The initial directional cuts within adjacent cut sections are spaced apart in the borehole axial direction.

[0009] Preferably, in step S3, the step of ensuring the controllable acid solution is in a low-viscosity state during the initial injection stage includes: Before injecting the controlled acid system, the controlled acid system is prepared to a flow state in which no gelation reaction has occurred; When injecting the controlled acid system, the controlled acid system should be kept in a flowing state where no gelation reaction has occurred; In a controlled acid system in a flow state where no gelation reaction has occurred, it is introduced into the initial directional cuts and natural fractures of the coal seam.

[0010] Preferably, in step S4, the step of raising the pH value of the system to reach the trigger condition of the pH-sensitive gelling unit includes: This allows the controlled acid solution to advance along the initial directional cuts and natural fractures in the coal seam. During the process, the controlled acid reacts with the minerals in the coal and consumes hydrogen ions; As hydrogen ions are consumed, the pH value of the controllable acid solution system changes, reaching the triggering condition of the pH-sensitive gelation unit.

[0011] Preferably, in step S4, the step of forming an in-situ temporary plugging zone within the fissure includes: When the pH value of the controllable acid system reaches the triggering condition of the pH-sensitive gelling unit, the pH-sensitive gelling unit undergoes a state transition. A gel structure is formed within the fissure space due to a state transition; The gel structure is distributed inside the crack and forms an in-situ temporary plugging zone.

[0012] Preferably, in step S4, the step of flowing along different fracture channels includes: After forming an in-situ temporary plugging zone within the fracture, the controllable acid solution continues to flow in the fracture space not occupied by the in-situ temporary plugging zone. The controllable acid solution is advanced along the fracture channels formed by the initial directional cut and the fracture channels formed by the natural fractures in the coal seam.

[0013] Preferably, in step S5, the step of continuously injecting controllable acid to react with the mineral filling material in the fracture includes: A controllable acid solution with a continuous liquid phase is formed within the fracture space; This exposes the mineral filler in solid form to a controlled liquid acid solution. The reaction occurs at the interface between the liquid-phase controllable acid solution and the solid-phase mineral filler.

[0014] Preferably, in step S6, the step of performing backflow or replacement operation on the coal seam segment includes: Open the channel connecting the borehole to the ground, allowing the fluid in the coal seam section to be discharged along the borehole direction, forming a backflow operation; Alternatively, a displacement fluid can be injected into the borehole to displace and discharge the fluid in the coal seam along the borehole direction, thus forming a displacement operation.

[0015] The present invention has the following beneficial effects: 1. In this invention, by combining the in-situ stress direction with the spatial orientation of fault planes and fold flanks in coal seams with complex geological structures, the arrangement orientation of directional hydraulic cuts is determined, and multiple clusters of initial directional cuts with different orientations are formed in the borehole. This allows the initiation location and direction of the cracks to be predefined in space, thereby achieving effective control over the crack initiation process in areas with developed faults, folds, and other structures, and reducing the uncertainty in the crack initiation and propagation process.

[0016] 2. In this invention, based on directional hydraulic cutting, a controllable acid system with pH response characteristics is introduced. The acid enters the initial directional cutting and natural fractures of the coal seam in a low-viscosity state at the beginning of injection. During the process of advancing along the fractures, the pH value of the system changes due to the reaction with minerals in the coal, triggering the state transformation of the cementing unit and forming an in-situ temporary plugging zone in the fracture. This realizes the path adjustment of the acid in the fracture network, so that the acid can be continuously distributed and act in different fracture channels.

[0017] 3. In this invention, by forming a controllable acid environment with a continuous liquid phase in the fracture space, the acid and the mineral filling material in the fracture continuously react at the liquid-solid interface. After the acid treatment is completed, the flowback or replacement operation is carried out to achieve the orderly discharge or replacement of the working fluid. Thus, the entire transformation process forms a continuous and complete process flow in the fracture formation, acid reaction and process end stages, which is suitable for the transformation of coal seams with complex geological structures. Attached Figure Description

[0018] Figure 1 This is a flowchart of the method for combined directional hydraulic fracturing and controllable acidizing modification of coal seams with complex geological structures proposed in this invention. Detailed Implementation

[0019] 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.

[0020] In the first embodiment of the present invention, the present invention provides a method for combined directional hydraulic fracturing and controllable acidizing modification of coal seams with complex geological structures, such as... Figure 1 As shown, it includes the following steps: S1. Obtain the in-situ stress direction of the target coal seam section and the spatial orientation of the fault plane and folded wing. Determine the arrangement orientation of the directional hydraulic cut based on the in-situ stress direction and the spatial orientation of the fault plane and folded wing. Furthermore, in step S1, the step of determining the orientation of the directional hydraulic cut based on the in-situ stress direction and the spatial orientation of the fault plane and the folded flanks includes: Obtain the directions of maximum and minimum principal stress in the target coal seam section; Obtain the strike and dip of fault planes within the target coal seam segment, as well as the strike direction of the folded fins; Based on the directions of maximum and minimum principal stress, the orientation of the first directional hydraulic cut is determined to be conducive to crack initiation. Based on the strike and dip of the fault plane and the orientation of the folded wing, the orientation of the second directional hydraulic slot along the weak structural plane is determined. The orientations of the first and second directional hydraulic cuts are used as the orientations for the directional hydraulic cuts within the target coal seam section.

[0021] Specifically, given the complex geological structures of coal seams with faults, folds, and complex stress field distribution, a comprehensive analysis of the in-situ stress state and structural geometry of the target coal seam segment is first conducted to determine the orientation of the directional hydraulic cuts. In this process, the in-situ stress direction can be obtained through in-situ stress testing, well logging data interpretation, or geological inversion. By analyzing the stress state of the rock mass within the target coal seam, the directions of the maximum and minimum principal stresses are determined. The principal stress directions can be obtained by solving the stress tensor, which can be expressed as: ; in, These represent the normal stress components of the target coal seam in the three orthogonal directions; This represents the shear stress component in the corresponding direction; By performing eigenvalue decomposition on the stress tensor, the three principal stress values ​​are obtained. ,in For the maximum principal stress, For the minimum principal stress, the corresponding eigenvectors represent the direction vectors of the maximum principal stress. and the direction vector of minimum principal stress The direction vector is used to characterize the preferred orientation of fractures in the coal seam during the initiation stage; Simultaneously, the structural geometry within the target coal seam is analyzed. Geological interpretation determines the strike and dip angles of fault planes, as well as the strike direction of the folded limbs. The spatial orientation of the fault plane can be expressed through its normal vector. It can be expressed by the following formula: ; in, Indicates the strike angle of the fault plane; Indicates the dip angle of the fault plane; The orientation of the folded wing can be represented by a unit direction vector. This indicates that the direction vector is obtained by spatial fitting of the fold axis or the bedding extension direction, and is used to characterize the secondary weakening direction formed in the coal seam during the tectonic deformation process. Based on the combined in-situ stress direction and tectonic spatial orientation, the orientation of directional hydraulic cuts is determined in stages. The orientation of the first directional hydraulic cut is determined according to the direction of the maximum principal stress, and its direction vector... From the direction vector of maximum principal stress Sure: ; This arrangement is used to constrain the orientation of the cut during the crack initiation stage; Furthermore, the orientation of the second directional hydraulic slit arrangement is determined based on the direction of the structural weak surface, and its direction vector... It can be obtained through the normal vector of the fault plane with respect to the folded wing direction vector The combination of these factors determines the orientation of the second directional hydraulic cut, which, when both exist simultaneously, can be expressed as: ; Among them, symbols This represents the vector cross product operation. Indicates the magnitude of the vector; After completing the first directional hydraulic slot layout... Second directional hydraulic cut arrangement orientation Once determined, both are used together as the orientation of the directional hydraulic cut within the target coal seam section and input into the cut design parameters, so that the subsequent implementation of the directional hydraulic cut can be simultaneously controlled by the in-situ stress field characteristics and the spatial distribution of the structural weak surface. By using the above method, the orientation of the directional hydraulic slits can be quantitatively determined before the implementation of the directional hydraulic slits, so that the slit layout scheme has a clear physical basis and geometric constraints, providing directional input conditions for subsequent slit implementation steps.

[0022] S2. Directional hydraulic cutting is carried out along the borehole in the target coal seam section to form two sets of initial directional cutting slots with different orientations in the borehole. The initial directional cuttings are arranged in multiple clusters along the borehole depth to limit the crack initiation location and direction. Furthermore, in step S2, the step of performing directional hydraulic cutting along the borehole within the target coal seam section includes: A directional hydraulic slit cutting device is installed inside the borehole, and the orientation of the directional hydraulic slit cutting device is adjusted to be consistent with the orientation of the directional hydraulic slit cutting. High-pressure water jets are sprayed into the borehole using a directional hydraulic slitting device to directionally cut the borehole wall. A cutting groove with a predetermined orientation is formed on the borehole wall as an initial directional cut.

[0023] Furthermore, in step S2, the step of arranging the initial directional cuts in multiple clusters along the borehole depth includes: The target coal seam section is divided into multiple slotted sections along the borehole axis; A cluster of initial directional cuts is formed within each cut segment; The initial directional cuts within adjacent cut sections are spaced apart in the borehole axial direction.

[0024] Specifically, after determining the orientation of the directional hydraulic cuts, directional hydraulic cuts are carried out along the borehole in the target coal seam section to form an initial directional cut with clear spatial orientation on the borehole wall, thereby providing artificially controlled initiation conditions for subsequent fracture initiation and propagation. In this process, a directional hydraulic slitting device is first installed in the borehole. This directional hydraulic slitting device can be a hydraulic jet structure with directional jetting function. One possible way is to achieve spatial control of the water jet direction by setting a rotatable nozzle, an eccentric nozzle, or a directional jetting head. After the device is lowered into the borehole, the orientation of the directional hydraulic slitting device is adjusted by ground control or downhole adjustment mechanism so that the jetting direction is consistent with the aforementioned determined directional hydraulic slitting arrangement orientation, thereby ensuring that the subsequent cutting process has clear directional guidance. After the directional hydraulic slitting device completes its orientation adjustment, a high-pressure water jet is injected into the borehole through the device to directionally cut the borehole wall. During this process, the high-pressure water jet forms a concentrated action zone in a local area of ​​the borehole wall, causing erosion and damage to the borehole wall material within this area. This creates a slit with a predetermined orientation on the borehole wall, serving as the initial directional slit. The directionality of this cutting process can be described by the spatial relationship between the jet direction vector and the borehole axis, where the jet direction vector is denoted as . The direction vector of the borehole axis is denoted as The angle between the two It can be expressed by the following formula: ; By controlling the included angle and injection direction vector The azimuth angle in space enables directional control of the cutting groove direction; After completing the directional cutting at a single location, in order to form multiple clusters of initial directional cuts in the borehole depth direction, the target coal seam section is divided into multiple cut segments along the borehole axis. Each cut segment is arranged sequentially in the borehole axis direction. The above-mentioned directional hydraulic cutting operation is repeated in each cut segment to form a cluster of initial directional cuts, thereby forming a set of cutting slots with consistent orientation characteristics in the cut segment. Initial directional cuts within adjacent cut sections are spaced apart along the borehole axial direction. This spacing can be determined based on the coal seam thickness, borehole length, or construction parameters. One possible approach is to pre-set the axial interval length. The cut segments are distributed as follows: This indicates the axial distance between the centers of adjacent slot segments. By setting this interval, the initial directional slots can be distributed in multiple clusters along the drilling depth direction. By forming initial directional cut clusters with the same or different orientation characteristics in different cut segments and arranging them at intervals in the drilling depth direction, the initial directional cuts can simultaneously possess axial distribution characteristics and orientation control characteristics in space, thereby forming multiple preset crack initiation positions and directions in the coal seam, providing a clear initiation basis for the subsequent crack propagation process in the coal seam. Through the aforementioned directional hydraulic slotting process and multi-cluster arrangement, an initial directional slotting system with clear spatial orientation and axial distribution characteristics is formed on the borehole wall, enabling subsequent modification processes to be carried out under predetermined crack initiation positions and directions, thereby achieving spatial constraints on the crack formation process.

[0025] S3. Inject a controllable acid system into the coal seam section after directional hydraulic fracturing. The controllable acid system includes a weak acid, corrosion inhibitor, iron ion stabilizer and pH-sensitive gelling unit. The controllable acid is in a low viscosity state in the initial stage of injection and enters the initial directional fracturing and natural fractures of the coal seam. Furthermore, in step S3, the step of ensuring the controllable acid solution is in a low-viscosity state during the initial injection stage includes: Before injecting the controlled acid system, the controlled acid system is prepared to a flow state in which no gelation reaction has occurred; When injecting the controlled acid system, the controlled acid system should be kept in a flowing state where no gelation reaction has occurred; In a controlled acid system in a flow state where no gelation reaction has occurred, it is introduced into the initial directional cuts and natural fractures of the coal seam.

[0026] Specifically, after completing the initial directional hydraulic cutting to form the initial directional cutting slot, a controllable acid system is injected into the target coal seam section so that the controllable acid system enters the initial directional cutting and the natural fractures of the coal seam in a low-viscosity flow state in the initial stage of injection, thereby providing fluid entry conditions for subsequent advancement in the fracture network. In this process, the composition of the controllable acid system is determined according to the disclosure document as a compound system of weak acid, corrosion inhibitor, iron ion stabilizer and pH-sensitive gelling unit. The weak acid can be one or more of formic acid and acetic acid to form an acidic aqueous phase system. The corrosion inhibitor is used to form a chemical environment that inhibits corrosion when in contact with metal under acidic conditions. The iron ion stabilizer is used to control the change of iron ion form in the system. The pH-sensitive gelling unit is used to give the system a material basis for state change with pH value. The above components together constitute a flow system that has not undergone gelling reaction in the initial stage of injection. During the preparation stage, the controllable acid system is prepared to a flow state before injection, free from gelation. This flow state can be characterized by viscosity parameters. To avoid unnecessarily limiting viscosity values, online viscosity monitoring or laboratory rotational viscometer testing can be used to confirm the fluid state. Viscosity measurement can employ the Newtonian fluid approximation method, describing the flow state through the relationship between shear stress and shear rate, where shear stress is denoted as . The shear rate is denoted as Apparent viscosity is denoted as The following relation is satisfied: ; in This indicates the apparent viscosity of the controllable acid system at the initial stage of injection. This represents the shear stress experienced by the fluid under the viscosity measurement conditions. This represents the corresponding shear rate, obtained by... The monitoring ensures that the controllable acid system is in a pumpable, low-viscosity flow state, thereby meeting the fluid entry conditions in the initial stage of injection. During the injection phase, in order to keep the controllable acid system in a flowing state without gelation reaction, this can be achieved by controlling the time window from preparation to injection at the wellhead, the residence time in the injection pipeline, and the temperature conditions during the injection process. In particular, pH-sensitive gelation units remain in a non-crosslinked state under low pH conditions. During the injection phase, the injection fluid system can be kept in an acidic range to avoid premature state change in the surface pipeline or near-well section, thereby ensuring that the controllable acid system remains in a flowing state when injected into the wellbore and into the coal seam fracture space. In the process of fluid entering the fracture, the controllable acid system, driven by pressure difference, enters the initial directional slot and natural fractures of the coal seam through the borehole. The fluid's ability to enter the fracture can be approximated by laminar flow within the fracture. The fracture channel can be approximated as a parallel plate gap channel. The fracture aperture is denoted as b, the fracture width as w, the fracture length as L, and the fluid dynamic viscosity as . The pressure difference between the injection end and the distal end of the fracture is denoted as . Volumetric flow rate is denoted as The following relation is satisfied: ; in This represents the volumetric flow rate of a controllable acid system along the fissure channel. This indicates the pressure difference between the two ends of the fracture channel. denoted by b, b represents the initial dynamic viscosity of the injected fluid, w represents the fracture opening, and L represents the effective fracture length. By controlling the injection pressure and fluid viscosity, the fluid can enter the initial directional cut and the natural fracture network, and maintain a continuous flow state within different types of fracture channels. At the application level, by adjusting the controllable acid system to a flow state without gelation reaction before injection and maintaining this flow state during injection, the controllable acid system can enter the initial directional cut and natural fractures of the coal seam in the early stage of injection. This provides the preconditions for the subsequent pH value changes and material state transformations that occur in the fractures, thereby enabling subsequent process steps to continue in the fracture space according to the predetermined process.

[0027] S4. During the process of controlled acid liquid advancing along the fracture, the controlled acid liquid reacts with the minerals in the coal and consumes hydrogen ions, causing the pH value of the system to rise. When the triggering condition of the pH value-sensitive cementing unit is reached, an in-situ temporary plugging zone is formed in the fracture, allowing the subsequently injected controlled acid liquid to flow along different fracture channels. Furthermore, in step S4, the step of raising the pH value of the system to reach the trigger condition of the pH-sensitive gelation unit includes: This allows the controlled acid solution to advance along the initial directional cuts and natural fractures in the coal seam. During the process, the controlled acid reacts with the minerals in the coal and consumes hydrogen ions; As hydrogen ions are consumed, the pH value of the controllable acid solution system changes, reaching the triggering condition of the pH-sensitive gelation unit.

[0028] Furthermore, in step S4, the step of forming an in-situ temporary plugging zone within the crack includes: When the pH value of the controllable acid solution system reaches the triggering condition of the pH-sensitive gelation unit, the pH-sensitive gelation unit undergoes a state transition. A gel structure is formed within the fissure space due to a state transition; The gel structure is distributed inside the crack and forms an in-situ temporary plugging zone.

[0029] Furthermore, in step S4, the flow along different fracture channels includes: After forming an in-situ temporary plugging zone within the fracture, the controllable acid solution continues to flow in the fracture space not occupied by the in-situ temporary plugging zone. The controllable acid solution is advanced along the fracture channels formed by the initial directional cut and the fracture channels formed by the natural fractures in the coal seam.

[0030] Specifically, after the controllable acid system enters the initial directional cut and natural coal seam fractures in a flow state without gelation reaction, the controllable acid is continuously advanced along the fracture network. During the advancement, the reaction between the controllable acid and the minerals in the coal consumes hydrogen ions, causing the pH value of the system to change and reach the triggering condition of the pH-sensitive gelation unit. This forms an in-situ temporary plugging zone composed of gelation structure in the fracture space, and allows the subsequently injected controllable acid to continue flowing in the fracture space not occupied by the in-situ temporary plugging zone to enter different fracture channels. In this process, the channels through which the controllable acid propagates along the fractures include fracture channels formed by the initial directional cuts and fracture channels formed by the natural fractures of the coal seam. The spatial orientation of the initial directional cuts is determined by the orientation of the directional hydraulic cuts, and the spatial distribution of the natural fractures is formed by the primary fractures and tectonic fractures of the coal seam. The controllable acid enters the above-mentioned fracture channels under the drive of injection pressure and propagates along the channels. During the propagation process, it can be regarded as a controlled flow state within the fracture. During fracture propagation, the controllable acid solution reacts with minerals in the coal and consumes hydrogen ions. These minerals may include carbonate mineral infills distributed within the fracture or other mineral phases that can react with the acidic fluid. The reaction process leads to a decrease in the hydrogen ion activity in the solution, causing a change in the acid-base state of the system. The pH value of the system and the hydrogen ion activity satisfy the following relationship: ; in This indicates the pH parameter of a controllable acid solution system. Hydrogen ion activity is expressed as hydrogen ion concentration. In engineering applications, hydrogen ion activity can be approximated by hydrogen ion concentration, denoted as . As the reaction progresses, The pH value of the system gradually increases as the pH continues to decrease. In this implementation process, pH-sensitive gelling units are introduced into the controllable acid system. These pH-sensitive gelling units exhibit material properties that undergo state transitions with changes in the system's pH value. Their triggering conditions can be characterized by a trigger threshold parameter, denoted as [missing parameter]. When the pH of the system satisfies the following relationship, the gelation unit undergoes a state transition: ; in This indicates the trigger threshold of the pH-sensitive gelling unit. This threshold can be set or calibrated during the ground solution preparation stage based on the material composition and formulation design of the gelling unit. This ensures that the gelling unit remains in a flow state without state change in the initial stage of injection, and undergoes a state change after reaching the trigger condition during the fracture propagation process. When the system pH reaches the trigger condition, pH-sensitive gelation units undergo a state transition within the fracture space and form a gel structure. This gel structure is continuously distributed within the fracture space and occupies a portion of the fracture volume, thus constituting an in-situ temporary plugging zone. The formation state of this in-situ temporary plugging zone can be characterized by the volume fraction of gel within the fracture unit, denoted as [gel volume fraction denoted as ]. The volume of the fracture element is denoted as The gel volume is denoted as The following relation is satisfied: ; in This indicates the volume fraction of the gel structure within the fractured unit. This represents the volume occupied by the gel structure within the fractured unit. The spatial volume of the fracture unit is represented by the gel structure distributed inside the fracture and forming an in-situ temporary plugging zone, which changes the effective flow area within the fracture space. After the in-situ temporary plugging zone is formed, the subsequently injected controllable acid continues to flow in the fracture space not occupied by the in-situ temporary plugging zone, and advances along different fracture channels in the fracture network. The different fracture channels include fracture channels formed by the initial directional cutting and fracture channels formed by natural fractures in the coal seam. Through the distribution of the in-situ temporary plugging zone in the fracture space, the subsequently injected controllable acid forms a multi-path flow state in the fracture network. In the above manner, during the fracture propagation stage, the system's pH value is established by the hydrogen ion consumption caused by the reaction between controlled acid and minerals in the coal. Based on the triggering conditions of the pH-sensitive gelation unit, an in-situ temporary plugging zone composed of a gel structure is formed in the fracture. Then, the subsequently injected controlled acid continues to flow in the fracture space not occupied by the in-situ temporary plugging zone to enter different fracture channels, thereby making the operation process of step S4 a continuous and controlled implementation process in the fracture space.

[0031] S5. Continuously inject controlled acid solution to react with the mineral filling material in the fracture and acidify the fracture. Furthermore, in step S5, the step of continuously injecting controlled acid to react with the mineral filling material in the fracture includes: A controllable acid solution with a continuous liquid phase is formed within the fracture space; This exposes the mineral filler in solid form to a controlled liquid acid solution. The reaction occurs at the interface between the liquid-phase controllable acid solution and the solid-phase mineral filler.

[0032] Specifically, after forming an in-situ temporary plugging zone in the fracture space and guiding the subsequently injected controllable acid to continue flowing along the unplugged fracture channel, controllable acid is continuously injected into the coal seam section, so that the controllable acid reacts with the mineral filling material in the fracture space, thereby completing the acidizing treatment of the fracture. In this process, the continuously injected controllable acid enters the fracture space under the action of pressure and fracture network connectivity, and forms a continuous liquid phase fluid distribution state inside the fracture. The continuous liquid phase means that the controllable acid forms a continuous fluid phase in the fracture space, so that the fluid in the fracture does not exist in the form of isolated droplets or local stagnation, but covers the fracture wall and the internal space of the fracture with a continuous liquid phase. This state provides the basic conditions for subsequent liquid-solid contact reaction. In the fracture space, mineral filling materials are distributed in solid form on the fracture wall or inside the fracture. The mineral filling materials can be derived from primary mineral deposition in the coal seam, secondary minerals formed by tectonic activity, or filling materials formed during fracture development. Under the continuous and controllable action of liquid acid, the surface of the above-mentioned mineral filling materials is directly exposed to the liquid environment, thereby forming a stable phase interface between the liquid and solid phases. At this phase interface, the active components in the controllable acid solution react with the mineral filler. This reaction process is controlled by the combined effects of the mass transfer process from the reactants in the liquid phase to the solid surface and the reaction process on the solid surface. To describe the reaction behavior at the liquid-solid interface, the interfacial reaction rate can be used for characterization. The interfacial reaction rate is denoted as R and can be expressed as: ; Where R represents the amount of reaction between the liquid-phase controllable acid solution and the solid-phase mineral packing material per unit time, and k represents the interfacial reaction rate constant, used to characterize the kinetics of the liquid-solid reaction under given system conditions. This represents the effective reaction area of ​​the mineral filling material exposed to the liquid phase within the fracture space, and C represents the effective reactant concentration participating in the reaction in the controllable acid solution of the liquid phase. The above parameters together determine the extent of the reaction at the liquid-solid interface. Under continuous injection conditions, as the controllable acid continuously enters the fracture space, the continuous state of the liquid phase is maintained. The reactants consumed during the reaction are replenished by the subsequently injected controllable acid, so that the reaction at the interface between the liquid phase and the solid phase can continue in the fracture space. During the reaction, the morphology and distribution of the mineral filling material change as the reaction progresses, but the contact relationship between the liquid phase and the solid phase in the fracture space is always maintained. At the application level, by forming a continuous and controllable acid environment in the fracture space and exposing the mineral filling material in solid form to the liquid-phase controllable acid, a stable phase interface is formed between the liquid and solid phases in the fracture space. This allows the reaction process between the liquid-phase controllable acid and the solid mineral filling material to be completed under continuous injection conditions. This enables step S5 to be continuously implemented in the fracture space according to the predetermined process logic and provides a completed processing state for subsequent backflow or replacement operations.

[0033] S6. After the acidification treatment is completed, the coal seam section is subjected to backflow or replacement operation.

[0034] Furthermore, in step S6, the steps of performing backflow or replacement operations on the coal seam section include: Open the channel connecting the borehole to the ground, allowing the fluid in the coal seam section to be discharged along the borehole direction, forming a backflow operation; Alternatively, a displacement fluid can be injected into the borehole to displace and discharge the fluid in the coal seam along the borehole direction, thus forming a displacement operation.

[0035] Specifically, after the fracture acidizing treatment step is completed, a backflow or replacement operation is performed on the target coal seam section to discharge the working fluid in the coal seam section through the borehole and complete the process closure, so that the subsequent operation stage is in a controllable state of wellbore and coal seam fluid. In this process, the backflow operation is based on the premise of establishing a connection channel between the borehole and the surface. The connection channel can be achieved by opening the wellhead valve group, opening the surface manifold, or releasing the wellhead sealing state, so that a fluid passage is formed between the inside of the borehole and the surface recovery system. After the connection channel is opened, the fluid in the coal seam section is discharged along the borehole direction under the pressure difference between the bottom of the well and the surface. The discharged fluid can enter the surface separation and collection device. The fluid discharge process can be carried out by continuous discharge or intermittent discharge, and the specific method can be selected according to the wellhead control conditions and on-site operation organization. During the backflow process, in order to control the discharge process in a process-oriented manner, the discharge rate, pressure and properties of the recovered fluid discharged from the wellhead can be monitored. Discharge rate monitoring can be achieved through a wellhead flow meter, pressure monitoring can be achieved through a wellhead pressure sensor, and the properties of the recovered fluid can be achieved through on-site sampling and testing. The valve group opening or recovery manifold status can be adjusted based on the above monitoring information to ensure that the backflow process continues under the established wellhead operating conditions and avoids unexpected interruptions in the backflow process due to fluctuations in operating conditions. In another embodiment, a displacement operation can be used to replace or in conjunction with a backflow operation. The displacement operation is achieved by injecting displacement fluid into the borehole. The displacement fluid can be clean water, brine, or other liquid media compatible with the coal seam. The displacement fluid enters the borehole via a surface pumping device and enters the coal seam section along the borehole direction. During the displacement fluid injection process, the original fluid in the coal seam section is displaced and discharged along the borehole direction under the drive of the displacement fluid and enters the surface recovery system. The displacement process can be carried out continuously or in stages. The pressure and discharge rate of the displacement fluid injection can be adjusted according to the wellhead monitoring information to match the displacement process with the wellbore pressure-bearing capacity and the capacity of the field equipment. In the process control of the replacement operation, the cumulative injection volume of the replacement fluid, the cumulative recovery volume of the replacement fluid discharged, and the wellhead pressure change can be recorded. By analyzing the correspondence between the injection volume and the recovery volume of the replacement fluid, it can be determined whether the replacement process has reached the preset operation termination condition. The preset operation termination condition can be set based on indicators such as the content of working fluid components in the recovery fluid, the pH of the recovery fluid, or the content of solid particles in the recovery fluid. The timing of the end of the replacement stage is determined by monitoring the above indicators. In practical applications, backflow and replacement operations can be carried out independently or in combination in the same coal seam section. One possible approach is to first perform backflow after acidizing to remove the freely flowing fluid in the wellbore, and then perform replacement to drive the residual fluid in the fracture space to move towards the borehole and be discharged. Another possible approach is to first perform replacement to form a stable wellbore fluid environment, and then perform backflow after replacement to recover the replacement fluid in the wellbore and coal seam section. Through the above methods, a controllable fluid discharge process is formed in the coal seam section at the end of the process. Through the above-mentioned backflow or replacement operations, a clear fluid discharge path is formed between the borehole and the surface system in the acidized coal seam section, and the discharge or replacement of the working fluid is completed under monitoring and control conditions.

[0036] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for combined directional hydraulic fracturing and controlled acidizing stimulation of coal seams with complex geological structures, characterized in that, Includes the following steps: S1. Obtain the in-situ stress direction of the target coal seam section and the spatial orientation of the fault plane and folded wing. Determine the arrangement orientation of the directional hydraulic cut based on the in-situ stress direction and the spatial orientation of the fault plane and folded wing. In step S1, the step of determining the orientation of the directional hydraulic cut based on the in-situ stress direction and the spatial orientation of the fault plane and the folded flanks includes: Obtain the directions of maximum and minimum principal stress in the target coal seam section; Obtain the strike and dip of fault planes within the target coal seam segment, as well as the strike direction of the folded fins; Based on the directions of maximum and minimum principal stress, the orientation of the first directional hydraulic cut is determined to be conducive to crack initiation. Based on the strike and dip of the fault plane and the orientation of the folded wing, the orientation of the second directional hydraulic slot along the weak structural plane is determined. The orientation of the first and second directional hydraulic cuts is taken as the orientation of the directional hydraulic cuts within the target coal seam section. S2. Directional hydraulic cutting is carried out along the borehole in the target coal seam section to form two sets of initial directional cutting slots with different orientations in the borehole. The initial directional cuttings are arranged in multiple clusters along the borehole depth to define the crack initiation position and crack initiation direction. In step S2, the initial directional cuts are arranged in multiple clusters along the borehole depth, including: The target coal seam section is divided into multiple slotted sections along the borehole axis; A cluster of initial directional cuts is formed within each cut segment; The initial directional cuts within adjacent cut segments are spaced apart in the borehole axial direction; S3. Inject a controllable acid system into the coal seam section after directional hydraulic fracturing. The controllable acid system includes a weak acid, a corrosion inhibitor, an iron ion stabilizer, and a pH-sensitive gelling unit. The controllable acid is in a low-viscosity state in the initial stage of injection and enters the initial directional fracturing and natural fractures of the coal seam. In step S3, the step of ensuring that the controllable acid solution is in a low-viscosity state during the initial injection stage includes: Before injecting the controlled acid system, the controlled acid system is prepared to a flow state in which no gelation reaction has occurred; When injecting the controlled acid system, the controlled acid system should be kept in a flowing state where no gelation reaction has occurred; In a controlled acid system in a flow state where no gelation reaction has occurred, it is introduced into the initial directional cuts and natural fractures of the coal seam. S4. During the process of controlled acid liquid advancing along the fracture, the controlled acid liquid reacts with the minerals in the coal and consumes hydrogen ions, causing the pH value of the system to rise. When the triggering condition of the pH value-sensitive cementing unit is reached, an in-situ temporary plugging zone is formed in the fracture, allowing the subsequently injected controlled acid liquid to flow along different fracture channels. In step S4, the steps of raising the pH value of the system to reach the triggering condition of the pH-sensitive gelation unit include: This allows the controlled acid solution to advance along the initial directional cuts and natural fractures in the coal seam. During the process, the controlled acid reacts with the minerals in the coal and consumes hydrogen ions; As hydrogen ions are consumed, the pH value of the controllable acid solution system changes and reaches the triggering condition of the pH-sensitive gelation unit. S5. Continuously inject controlled acid solution to react with the mineral filling material in the fracture and acidify the fracture. S6. After the acidification treatment is completed, the coal seam section is subjected to backflow or replacement operation.

2. The method for combined directional hydraulic fracturing and controlled acidizing of coal seams with complex geological structures according to claim 1, characterized in that, In step S2, the step of performing directional hydraulic slotting along the borehole within the target coal seam section includes: A directional hydraulic slit cutting device is installed inside the borehole, and the orientation of the directional hydraulic slit cutting device is adjusted to be consistent with the orientation of the directional hydraulic slit cutting. High-pressure water jets are sprayed into the borehole using a directional hydraulic slitting device to directionally cut the borehole wall. A cutting groove with a predetermined orientation is formed on the borehole wall as an initial directional cut.

3. The method for combined directional hydraulic fracturing and controlled acidizing of coal seams with complex geological structures according to claim 1, characterized in that, In step S4, the step of forming an in-situ temporary plugging zone within the fissure includes: When the pH value of the controllable acid system reaches the triggering condition of the pH-sensitive gelling unit, the pH-sensitive gelling unit undergoes a state transition. A gel structure is formed within the fissure space due to a state transition; The gel structure is distributed inside the crack and forms an in-situ temporary plugging zone.

4. The method for combined directional hydraulic fracturing and controlled acidizing of coal seams with complex geological structures according to claim 1, characterized in that, In step S4, the step of flowing along different fracture channels includes: After forming an in-situ temporary plugging zone within the fracture, the controllable acid solution continues to flow in the fracture space not occupied by the in-situ temporary plugging zone. The controllable acid solution is advanced along the fracture channels formed by the initial directional cut and the fracture channels formed by the natural fractures in the coal seam.

5. The method for combined directional hydraulic fracturing and controlled acidizing of coal seams with complex geological structures according to claim 1, characterized in that, In step S5, the step of continuously injecting controlled acid to react with the mineral filling material in the fracture includes: A controllable acid solution with a continuous liquid phase is formed within the fracture space; This exposes the mineral filler in solid form to a controlled liquid acid solution. The reaction occurs at the interface between the liquid-phase controllable acid solution and the solid-phase mineral filler.

6. The method for combined directional hydraulic fracturing and controlled acidizing of coal seams with complex geological structures according to claim 1, characterized in that, In step S6, the step of performing backflow or replacement operation on the coal seam section includes: Open the channel connecting the borehole to the ground, allowing the fluid in the coal seam section to be discharged along the borehole direction, forming a backflow operation; Alternatively, a displacement fluid can be injected into the borehole to displace and discharge the fluid in the coal seam along the borehole direction, thus forming a displacement operation.

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