A method for improving the recovery rate of caving coal based on regionalization of up-down well combined operation

Abstract

The application discloses a regionalized method for improving top coal caving rate based on well up and down combined operation, which comprises the following steps: step one, constructing an accurate three-dimensional geological model of the target coal seam roof; step two, constructing a horizontal directional long borehole along the minimum horizontal principal stress direction of the coal seam roof; step three, performing directional sand blasting and tight cutting in the horizontal directional long borehole; step four, performing fracturing construction on the horizontal directional long borehole by well up and down combination; step five, performing directional sand blasting and tight cutting and fracturing construction on all the horizontal directional long boreholes; and step six, mining the bottom coal, and the reformed top coal naturally falls under the overburden pressure, thereby improving the top coal caving rate. The application can inject high-pressure water into the target top coal through the horizontal directional long borehole of the coal seam roof, when the water pressure reaches the coal rock mass cracking pressure, cracks are generated in the coal seam roof and the top coal and are extended, the integrity of the top coal is damaged, and the purposes of weakening the coal rock mass and improving the top coal caving rate are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of top coal caving, in particular to a regionalized method for improving top coal caving recovery rate based on up-down well combined operation. BACKGROUND

[0002] China is rich in coal resources, and thick and super-thick coal seams are widely distributed. At present, most thick and super-thick coal seams are mined by top coal caving, that is, after the bottom coal is mined, the top coal falls naturally under the confining pressure of overburden rock. However, in some areas, due to the large thickness and high hardness of the top coal, the upper top coal is not fully broken, the broken size is large and not easy to be released, the recovery rate is low, and resources are wasted.

[0003] In the prior art, in view of the problem that the top coal is difficult to be released, methods such as short borehole water injection softening or underground blasting are used. However, the construction engineering quantity of borehole water injection is huge, the cost is high, and the effect is not ideal; underground blasting is easy to produce dust and toxic and harmful gases, which constitutes an unfavorable factor for mine safety production, and it is difficult to realize regionalized low-cost improvement of top coal caving recovery rate. SUMMARY

[0004] In view of the deficiencies in the prior art, the purpose of the present application is to provide a regionalized method for improving top coal caving recovery rate based on up-down well combined operation and a control method, to solve the problems in the prior art that the engineering quantity is huge, the cost is high, and the effect is not ideal, and underground blasting is easy to produce dust and toxic and harmful gases, which constitutes an unfavorable factor for mine safety production.

[0005] In order to solve the above technical problems, the present application adopts the following technical scheme: a regionalized method for improving top coal caving recovery rate based on up-down well combined operation, comprising the following steps:

[0006] Step 1: constructing an accurate three-dimensional geological model of the coal seam roof of the target coal seam;

[0007] Step 2: determining the minimum horizontal principal stress direction of the coal seam roof, and constructing a horizontal directional long borehole along the minimum horizontal principal stress direction of the coal seam roof according to the accurate three-dimensional geological model of the coal seam roof;

[0008] The horizontal directional long borehole is at least one and is arranged at intervals and can cover the working face;

[0009] Step 3: performing directional sand blasting and tight cutting in the horizontal directional long borehole, and the direction of the directional sand blasting and tight cutting is towards the target coal seam direction;

[0010] Step 4: performing fracturing construction on the horizontal directional long borehole by up-down well joint operation;

[0011] Step five: repeat step three and step four, and directional sand blasting and fracturing construction are carried out on all horizontal directional long boreholes, and the transformation of top coal is completed;

[0012] Step six: mining the bottom coal, the transformed top coal is naturally caved under the pressure of overburden rock, and the recovery rate of top coal caving is improved.

[0013] The application also has the following technical features:

[0014] The precise three-dimensional geological model of the target coal seam roof comprises:

[0015] The roadway drilling data, the cut eye geological sampling data, the three-dimensional seismic analysis data, the coal seam roof contour data set in the working face range are used as sample data, the known data is interpolated using the Kriging method, and the accurate coal seam roof geological form is obtained.

[0016] The minimum horizontal principal stress direction determination method is measured through the downhole small-scale hydraulic fracturing test.

[0017] When the horizontal directional long borehole is constructed, the position of the horizontal directional long borehole is determined according to the while-drilling measurement data in the construction process, and the position of the horizontal directional long borehole is controlled in the coal seam roof and in the range of 0.5-1.0 m above the top boundary of the target coal seam.

[0018] The directional sand blasting and close cutting in the horizontal directional long borehole comprises: using the downhole bottom sealing drag segmented fracturing tool string to carry out directional sand blasting and close cutting in the horizontal directional long borehole towards the target coal seam direction.

[0019] The downhole bottom sealing drag segmented fracturing tool string comprises a sealing joint, a mechanical hydraulic release, a centralizer, a directional spray gun, a balance valve, a packer, a coupling positioner and a leading shoe connected in sequence.

[0020] The sealing joint is used for connecting the coiled tubing.

[0021] The mechanical hydraulic release is used for preventing the downhole bottom sealing drag segmented fracturing tool string from being sand stuck to cause the coiled tubing to be unable to be retrieved, and when the coiled tubing is pulled back, the pin is cut off by the pulling force of the coiled tubing, so that the coiled tubing and the lower tool string are separated.

[0022] The centralizer is used for ensuring that the downhole bottom sealing drag segmented fracturing tool string is located at the middle part of the horizontal directional long borehole.

[0023] The directional spray gun is used for vertical downward directional perforation.

[0024] The balance valve is used for cooperating with the bottom packer to realize unblocking, and when the bottom packer is unblocked, the differential pressure of the bottom packer is balanced, and the unblocking load is reduced.

[0025] The packer is used for sealing the fracturing section and the non-fracturing section.

[0026] The coupling positioner is used for measuring the position of the steel casing coupling in the hole.

[0027] The leading shoe is used for preventing the downhole bottom seal trailing staged fracturing tool string from colliding with the well wall.

[0028] The directional lance comprises a first rotary sealing nipple, a directional nozzle, a director and a second rotary sealing nipple connected in sequence, the first rotary sealing nipple is communicated with the centralizer, and the second sealing nipple is connected with the balance valve.

[0029] The directional sandblasting dense cutting in the horizontal directional long borehole is performed by using the downhole bottom seal trailing staged fracturing tool string to perform directional sandblasting dense cutting in the horizontal directional long borehole towards the target coal seam.

[0030] The method comprises the following steps:

[0031] After the downhole bottom seal trailing staged fracturing tool string is assembled, the coiled tubing is connected to the sealing joint, the other end of the coiled tubing is connected to the ground fracturing vehicle, the downhole bottom seal trailing staged fracturing tool string is pushed to the designed position in the horizontal directional long borehole, the directional lance is started to perform directional work, after the directional lance is oriented, the coiled tubing is pushed back to set the packer, the packer seals the fracturing section and the non-fracturing section, then the ground fracturing equipment is started to inject the sand fracturing fluid into the coiled tubing, the directional lance is used to perform directional sandblasting dense cutting on the horizontal directional long borehole, the coal seam roof near the horizontal directional long borehole and the target coal seam.

[0032] The uphole and downhole combined fracturing of the horizontal directional long borehole refers to that after the directional sandblasting dense cutting is completed, the downhole fracturing pump is started to inject the preflush, the sand fluid and the displacement fluid into the annulus between the coiled tubing and the casing in sequence, and the fracturing of the horizontal directional long borehole is performed.

[0033] After the first section is completed, the coiled tubing is pulled back to release the packer, the downhole bottom seal trailing staged fracturing tool string is moved to the next perforation position, and the above operation is repeated until the directional sandblasting dense cutting and the fracturing of the entire horizontal directional long borehole are completed.

[0034] The interval distance of the horizontal directional long borehole is calculated according to the following formula:

[0035]

[0036] The number of the horizontal directional long borehole is calculated according to the following formula:

[0037]

[0038] In the formula:

[0039]

[0040]

[0041] L The interval distance of horizontally oriented long boreholes, in meters (m). N The number of horizontally oriented long boreholes; M The length of the main roadway in the mining area is in meters (m). It is the ratio of coal seam fracture length to roof fracture length, generally between 0.7 and 0.9, and is dimensionless; C The filtration coefficient of fracturing fluid is given in m / s. 0.5 ; Q For construction displacement, m 3 / s; H The crack height is in meters (m). S p For initial filtrate loss of fracturing fluid, m 3 / m 2 ; μ The viscosity of a Newtonian fluid is given in Pa·s. G Here, denoted as rock shear modulus, Pa; v Poisson's ratio of the rock; w (0, t) represents the maximum width of the elliptical cross-section at time t, in meters; t is the duration of the formal start of hydraulic fracturing, in seconds. x Let be the half-length of the rectangle in the KGD hydraulic fracture model, in meters. yes x The error compensation function.

[0042] Before performing directional sandblasting and dense cutting on the horizontally oriented long borehole, a steel casing is inserted into the horizontally oriented long borehole and cemented.

[0043] The process of running a steel casing into a horizontally oriented long borehole and cementing it includes:

[0044] After the horizontal directional long borehole is completed, the borehole is flushed until no more sand or debris is returned.

[0045] Select a steel casing with a diameter one grade smaller than that of the horizontal directional borehole. Install a tapered guide shoe at the front end of the steel casing, tighten the casing clamp in one go, and push the steel casing into the designed depth. High-pressure injection of cement slurry containing a quick-setting agent into the steel casing fills the annulus of the horizontal directional borehole wall and the steel casing with cement slurry until it returns to the borehole opening. Allow it to solidify for 48 hours to complete the cementing operation, so that the steel casing and the horizontal directional borehole solidify into a whole.

[0046] Compared with the prior art, the present invention has the following technical effects:

[0047] (I) The present application can inject high-pressure water into target top coal through horizontal directional long borehole of coal seam roof, when water pressure reaches the coal rock mass cracking pressure, cracks are generated in coal seam roof and top coal and are extended, the integrity of top coal is destroyed, and the mechanical properties are changed, so that the purpose of weakening coal rock mass and improving top coal recovery rate is achieved.

[0048] (II) The present application can construct a precise three-dimensional geological model of the roof of a super-thick top-coal caving seam by using a multi-parameter fusion technique, and the horizontal directional long borehole is arranged at a reasonable position in the underground.

[0049] (III) The present application is operated in a well-up and well-down mode, compared with the conventional ground fracturing method, the working face (mining area) fracturing area is fully covered, the engineering quantity of the ground directional drilling is reduced, the engineering cost is saved, and the economic practicability is strong; compared with the conventional underground fracturing method, the ground fracturing equipment is used for construction, the disadvantages of small displacement of underground fracturing pump, large difficulty of sand adding, difficult equipment transportation, and poor coal crushing effect are solved, low-cost, large-displacement sand adding staged fracturing modification is realized, and three-dimensional top coal crushing modification is realized.

[0050] (IV) The present application adopts the lower steel casing cementing and completion mode, can be matched with the optimized directional coiled tubing hydraulic sandblasting perforation annular staged fracturing process technology, and is greatly improved in the aspects of directional sandblasting perforation and staged fracturing scale without moving the pipe string. Compared with underground blasting, gas emission and coal dust are reduced. Compared with the conventional water injection weakening, the injection displacement and pressure are improved, the proppant is increased, the top coal is better crushed, the horizontal directional long borehole working face is fully covered, and the regional top coal caving recovery rate is improved. BRIEF DESCRIPTION OF DRAWINGS

[0051] Figure 1 It is a schematic diagram of the principle of the present application;

[0052] Figure 2 It is a schematic diagram of the horizontal directional long borehole after cementing of the present application;

[0053] Figure 3 It is a schematic diagram of the structure of the downhole bottom sealing and dragging staged fracturing tool string of the present application;

[0054] Figure 4 It is a schematic diagram of the structure of the directional spray gun of the present application;

[0055] Figure 5 It is a schematic diagram of the close cutting direction of the directional sandblasting of the present application;

[0056] Figure 6 It is a schematic diagram of the well-up and well-down combined operation of the present application;

[0057] Meaning of each reference numeral in the drawings:

[0058] 1 - surface wellhead; 2 - surface-to-roadway through well; 3 - well-to-roadway connecting device; 4 - high pressure pipeline in roadway; 5 - main roadway in mining area; 6 - coiled tubing truck; 7 - injection head truck; 8 - downhole fracturing pump; 9 - wellhead device of horizontal directional well; 10 - coal seam floor; 11 - target coal seam; 12 - horizontal directional long borehole; 13 - fracturing fracture; 14 - coal seam roof; 15 - downhole bottom sealing drag segmented fracturing tool string; 16 - casing; 17 - cement sheath; 18 - long borehole wall; 19 - coiled tubing; 20 - casing tubing annulus;

[0059] 11-1 top coal, 11-2 bottom coal;

[0060] 15-1 sealing joint, 15-2 mechanical hydraulic release, 15-3 centralizer, 15-4 directional spray gun, 15-5 balance valve, 15-6 packer, 15-7 coupling positioner, 15-8 leading shoe;

[0061] 15-4-1 first rotary sealing nipple, 15-4-2 directional spray head, 15-4-3 orienter, 15-4-4 second rotary sealing nipple.

[0062] The specific content of the present application is further explained in detail in combination with the following examples. DETAILED DESCRIPTION

[0063] The following specific examples of the present application are given, it should be noted that the present application is not limited to the following specific examples, any equivalent transformation made on the basis of the technical scheme of the present application falls within the protection scope of the present application.

[0064] The terms "upper", "lower", "front", "back", "top", "bottom" and the like used in the present application indicate the orientation or positional relationship only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, "inner", "outer" refers to the inner and outer of the corresponding part contour, the above terms should not be understood as a limitation on the present application.

[0065] In the present application, the terms "mounting", "connection", "connection", "fixing" and the like should be understood broadly without the opposite description, for example, it can be fixed connection, or detachable connection or integrated; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium; it can be the internal communication of two elements or the interaction relationship of two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0066] All components in the present application, such as no special description, all use the components known in the prior art.

[0067] Example 1:

[0068] Following the above technical solutions, such as Figures 1-6 As shown, taking the 401 mining area of ​​a coal mine in Binchang, Shaanxi Province as an example and referring to the accompanying drawings, the present invention will be further described. The mine is designed to have an annual production capacity of 600 Mt. The No. 4 coal seam is the main coal seam, the average coal thickness in the 401 mining area is 17.50 m, and the apparent density of the coal seam is 1.36 t / m³. 3 The Protodyakonov hardness coefficient is 1.48. The mining technology is fully mechanized top-coal caving with complete roof caving management. The mining depth is 3.5m, and the caving depth is 10.5m, with a mining-to-caving ratio of 1:3. The coal seam roof is fine-grained sandstone, and the coal seam floor is mudstone. The strata from the surface to the target roadway are, in sequence: Quaternary (Q), Luohe Formation (K1...). l Yijun Group (K1) y ), Stability Group (J2) a ), J2 group z ) and Yan'an Group (J2) y ).

[0069] A method for improving the top coal caving recovery rate based on combined underground and surface operations includes the following steps:

[0070] Step 1: Construct an accurate three-dimensional geological model of the roof of the target coal seam;

[0071] Specifically, firstly, borehole data, cut geological data, 3D seismic exploration data, and coal seam roof contour data within the mining area are collected as sample data, and multi-parameter data fusion is performed. The Kriging method is then used to interpolate the data volume to obtain the accurate coal seam roof morphology.

[0072] (1) Obtain VSP logging data, borehole coal seam data and three-dimensional seismic exploration data within the mining area;

[0073] (2) Based on the geological characteristics and lithological variations of the mine, the strata are divided into 6 major velocity layers from top to bottom, namely:

[0074] ① Quaternary (Q) ~ Luohe Formation (K1) l The soil is mainly loess, with a layer velocity of V. Q ;

[0075] ②Luohe Formation (K1) l ~ Yijun Group (K1) y The rock is mainly composed of medium-grained sandstone with a layer velocity of V. K1l ;

[0076] ③ Yijun Group (K1) y ) ~ Stability Group (J2) a The rock is mainly conglomerate with a layer velocity of V. K1y ;

[0077] ④J2 J2 a ) ~ J2 z , fine-grained sandstone and mudstone interbedded, interval velocity V J2a ;

[0078] ⑤J2 z ) ~ J2 y , fine-grained sandstone, mudstone second, interval velocity V J2z ;

[0079] ⑥J2 y ) ~ 4 coal seam, mainly sandy mudstone, interval velocity V J 2 y ;

[0080] (3) using the VSP single well logging data and three-dimensional seismic stack velocity body in the mine range, by dix formula respectively calculated each well site at the interval velocity V int .

[0081] (4) according to the formula V int = V f + λZ middle (1)

[0082] with the midpoint depth of the layer Z middle as the horizontal coordinate, the layer velocity V int as the vertical coordinate, using the interval velocity data of all well points, linear fitting, the slope value of the line segment is the constant λ value, and the intersection of the vertical coordinate is the V f value. Sublayer fitting, get each stratum's λ value and v f value. V f gridding to get each layer's v f plane.

[0083] In the formula: V int - interval velocity, V f - velocity constant, λ- slope, constant, Z middle - midpoint depth of the target layer

[0084] (5) according to the formula (2)

[0085] In the formula: Z 上 - the bottom boundary depth of the overlying stratum, △t - one-way travel time of the underlying stratum

[0086] Based on the v f plane, using (2) formula to calculate the Quaternary interval velocity V Q , since the Quaternary is the surface rock layer, therefore Z 上 is 0; get the Quaternary interval velocity VQ Subsequently, the Quaternary (Q) and Luohe Formation (K1) were interpreted using Landmark software. l K1 can be obtained by performing multiplication on the T0 mesh of the interface. l Depth data of the top interface.

[0087] (6) Use the depth of the top surface of the Luohe Formation as Z 上 Substituting into formula (2), calculate the layer velocity V of the Luohe Formation. K1l Multiplying this by the time T0 at the interface of the layer yields the thickness of the Luohe Formation. Adding this thickness to the depth of the top surface of the Luohe Formation gives the depth of the top surface of the Yijun Formation. Similarly, the layer velocity V of the Yijun Formation is calculated. K1y Formation thickness and top interface depth; layer velocity V of the Anding Formation. J2a Stratification thickness and top interface depth; layer velocity V of the Zhiluo Formation J2z Stratigraphic thickness and top interface depth; layer velocity V in the section from Yan'an Formation to coal seam 4. J2y The thickness of the strata and the depth of the top interface of coal seam 4 were determined. The seismic data volume was discretized to form a 1m×1m×1m three-dimensional interface grid.

[0088] (7) The discretized seismic data volume forms a set of three-dimensional data points. Combined with the measured three-dimensional data points of borehole point data and tunnel cut-in data, the three different data volumes are converted into a set of spatial three-dimensional point data volumes that can be used for geological modeling, and the seismic-borehole-tunnel data are fused.

[0089] (8) The above data fusion is interpolated using the Kriging method, and a large amount of borehole data and roadway data are used to constrain the seismic depth value to obtain accurate coal seam roof data shape.

[0090] Because the Huanglong Jurassic Coalfield in the mining area has a simple geological structure, but the coal seams are undulating and have a certain trend, interpolation can be performed using the Kriging method with external drift.

[0091] (9) Based on the coal seam roof morphology data, use commercial 3D modeling software to perform precise geological modeling and obtain an accurate 3D geological model of the coal seam roof.

[0092] Step 2: Determine the direction of the minimum horizontal principal stress of the coal seam roof. Based on the accurate three-dimensional geological model of the coal seam roof, construct a horizontal directional long borehole along the direction of the minimum horizontal principal stress of the coal seam roof. Insert a steel casing into the horizontal directional long borehole and cement it.

[0093] There shall be at least one horizontally oriented long borehole, which shall be spaced apart and able to cover the working face;

[0094] The direction of the minimum horizontal principal stress was determined by a small-scale downhole hydraulic fracturing test.

[0095] This involves drilling a short borehole into the roof of the coal seam within the coal seam roadway, sealing the front and back of the borehole, using a fracturing pump to fracture the rock, inserting an endoscope into the borehole to observe the azimuth angle of the borehole wall fracture, and setting this as the direction of the maximum principal stress at the working face; the direction perpendicular to the direction of the maximum principal stress is taken as the direction of the minimum horizontal principal stress.

[0096] Specifically, applying geological guidance technology to construct horizontally oriented long boreholes along the direction of minimum horizontal principal stress in the roof of coal seams where top coal is difficult to fall refers to setting up a set of horizontally oriented long boreholes within the accurate three-dimensional geological model constructed in step one, with a borehole diameter greater than or equal to Φ150mm, the direction of the horizontally oriented long boreholes along the minimum horizontal principal stress of the working face, and the bottom of the horizontally oriented long boreholes 0.5 to 1.0m away from the roof of the coal seam;

[0097] The location of the horizontally oriented long borehole trajectory is determined based on the measurement while drilling data, and the horizontally oriented long borehole is controlled to be within 0.5 to 1.0 m above the top boundary of the target coal seam.

[0098] Specifically, the horizontally oriented long boreholes can fully cover the entire area from the cutting hole to the main roadway of the mining area. The horizontally oriented long boreholes are arranged in parallel and have a length of more than 750m.

[0099] Specifically, because the cracks are horizontally arranged within the coal seam roof, the propagation rate of the cracks within the roof mudstone is greater than that within the coal seam. This causes the cracks in the coal seam to extend rapidly laterally, resulting in a longer crack length within the roof than within the coal seam. Extensive numerical simulation experiments have shown that the ratio of the length of the cracks within the coal seam to the length of the cracks in the roof is generally between 0.7 and 0.9.

[0100] The interval between horizontally oriented long boreholes is calculated according to equation (5), and the number of horizontally oriented long boreholes is calculated according to equation (6):

[0101]

[0102] The number of horizontally oriented long boreholes is calculated according to the following formula:

[0103]

[0104] In the formula:

[0105]

[0106]

[0107] L The interval distance of horizontally oriented long boreholes, in meters (m). N The number of horizontally oriented long boreholes;M The length of the main roadway in the mining area is in meters (m). It is the ratio of coal seam fracture length to roof fracture length, generally between 0.7 and 0.9, and is dimensionless; C The filtration coefficient of fracturing fluid is given in m / s. 0.5 ; Q For construction displacement, m 3 / s; H The crack height is in meters (m). S p For initial filtrate loss of fracturing fluid, m 3 / m 2 ; μ The viscosity of a Newtonian fluid is given in Pa·s. G Here, denoted as rock shear modulus, Pa; v Poisson's ratio of the rock; w (0, t) represents the maximum width of the elliptical cross-section at time t, in meters; t is the duration of the formal start of hydraulic fracturing, in seconds. x Let be the half-length of the rectangle in the KGD hydraulic fracture model, in meters. yes x The error compensation function.

[0108] The length M of the main roadway in the mining area is 1500m. The final calculation shows that the interval L of the horizontal directional long boreholes is 300m, and the number N of the horizontal directional long boreholes is equal to 5.

[0109] Specifically, after construction, the entire horizontal directional long borehole is flushed until no more sand or cuttings are returned. A steel casing with an outer diameter of Φ110mm is selected for casing 16. A tapered guide shoe is installed at the front end of the steel casing to reduce friction. The casing clamps are tightened sequentially using downhole drilling equipment, and the steel casing is pushed to the designed depth. Cement slurry containing a quick-setting agent is injected under high pressure into the steel casing. When the pressure reaches 2MPa, the piston at the front end of the tapered guide shoe is compressed, and cement enters the annulus between the steel casing and the borehole wall from the opening at the front end of the guide shoe. High-pressure injection of cement slurry continues, filling the annulus between the borehole wall and the steel casing until it returns to the borehole opening. After 48 hours of setting, a cement sheath 17 is formed, completing the cementing operation. This solidifies the steel casing and the horizontal directional long borehole into a single unit. The structure of the horizontal directional long borehole is referenced. Figure 2 .

[0110] In the specific construction process: In order to transport high-pressure fluid from the ground to the mine, based on the geological conditions of the work area and the model established in step one, a breakthrough well was constructed on the ground to form a dedicated underground roadway. The breakthrough well adopted the three-stage cementing technology.

[0111] A precise surface-to-tunnel connection shaft 2 is constructed from the surface into the target tunnel to ensure that the tubing of the surface fracturing equipment is led to the main tunnel 5 of the underground mining area. A shaft-to-tunnel connection device 3 is designed at the bottom of the shaft. The initial end of the shaft-to-tunnel connection device 3 is sealed to the surface fracturing equipment 1, and the end is sealed to the high-pressure pipeline 4 in the tunnel to ensure the safety of the construction process.

[0112] This step specifically includes: using a drilling rig to construct a surface-to-tunnel connection well 2 from the surface to the target layer (mining area main roadway). The borehole depth structure adopts a three-stage design to ensure that the tubing of the surface fracturing equipment is brought to the downhole working face. The first stage drills from the surface to 10m below the bedrock, with a borehole diameter greater than or equal to Φ300mm, and runs a Φ255mm steel casing, cemented; the second stage drills to 10m above the mining area main roadway, with a borehole diameter greater than or equal to Φ210mm, and runs a Φ195mm steel casing, cemented; the third stage drills to the target layer (mining area main roadway or bottom roadway), with a borehole diameter greater than or equal to Φ150mm, and runs a Φ137mm steel casing; anti-vibration washers are installed at the top of the roadway, and a well-to-tunnel connection device 3 is designed at the bottom of the well. The surface fracturing equipment 1 is sealed to the upper end of the well-to-tunnel connection device 3, and the high-pressure pipeline 4 is sealed to the lower end of the well-to-tunnel connection device 3 to ensure the safety of the construction process. Full-hole coring was performed for geological logging. Rock mechanics samples were collected from the coal seam roof and coal seam to measure rock physical parameters such as tensile strength, shear modulus, Young's modulus, Poisson's ratio, and filtration coefficient.

[0113] The fracturing wellhead is connected, and the bottom end of the well on the surface is connected to the long borehole downhole via a high-pressure resistant pipeline.

[0114] Specifically, the outlet of the shaft-tunnel connection device is connected to the head of the horizontal directional long borehole (including the shaft-tunnel connection device 3, the high-pressure pipeline 4 in the tunnel, and the horizontal directional wellhead device 9, etc.) through a high-pressure pipeline, forming a channel for transporting high-pressure fluid from the surface to the long borehole in the coal mine. During the extension of the underground tunnel, straight sections are connected using steel oil pipes, and high-pressure hoses are used for connection at the points where the tunnel direction changes. Specifically, each section of the steel pipe is 3m long, connected using augers and laid on the side wall of the tunnel. The inner diameter of the connecting pipe is φ100mm to meet the requirements of large-volume fracturing.

[0115] Step 3: Perform directional sandblasting and dense cutting within the horizontal directional long borehole, wherein the direction of the directional sandblasting and dense cutting is toward the target coal seam;

[0116] The downhole bottom seal-driven segmented fracturing tool string is used to perform directional sandblasting and dense cutting in a horizontal directional long borehole. The downhole bottom seal-driven segmented fracturing tool string includes a sealing joint 15-1, a mechanical hydraulic release 15-2, a centralizer 15-3, a directional spray gun 15-4, a balance valve 15-5, a packer 15-6, a coupling positioner 15-7, and a guide shoe 15-8 connected in sequence.

[0117] The directional spray gun comprises a first rotary sealing section 15-4-1, a directional nozzle 15-4-2, a directional device 15-4-3, and a second rotary sealing section 15-4-4 connected in sequence. The first rotary sealing section 15-4-1 is connected to the centralizer 15-3, and the second rotary sealing section 15-4-4 is connected to the balance valve 15-5.

[0118] The directional spray gun comprises a rotary sealing section 15-4-1, a directional nozzle 15-4-2, a directional device 15-4-3, and a rotary sealing section 15-4-1 connected in sequence.

[0119] Sealing joint 15-1: Used to connect the continuous tubing 19, preventing high-pressure water leakage at the joint during high-pressure water injection.

[0120] Mechanical hydraulic release mechanism 15-2: During the pullback of coiled tubing 19, to prevent sand from getting stuck in the downhole bottom seal dragging segmented fracturing tool string 15, which would prevent the coiled tubing 19 from being retracted. The purpose of setting up mechanical hydraulic release mechanism 15-2 is that, under the above circumstances, when the pulling force is greater than 5 tons, mechanical hydraulic release mechanism 15-2 will break, the coiled tubing 19 and the downhole bottom seal dragging segmented fracturing tool string 15 will be disconnected, and the coiled tubing 19 can continue to be pulled back, leaving only the downhole bottom seal dragging segmented fracturing tool string in the hole.

[0121] Centralizer 15-3: A device that ensures that the downhole bottom seal drag segmented fracturing tool string 15 is located in the middle of the borehole.

[0122] Directional spray gun 15-4: A special tool for vertical downward directional perforation.

[0123] Balance valve 15-5: By using liquid pressure to create a pressure differential in the tubing, it helps packer 15-6 to be safely set, allowing the balance valve to be repeatedly opened and closed downhole for reuse.

[0124] The balancing valve 15-5 is used to cooperate with the bottom packer to achieve unsealing. When the bottom packer is unsealed, it balances the pressure difference of the bottom packer and reduces the unsealing load.

[0125] Packer 15-6: Packer 15-6 is an in-hole tool used to seal the fractured section and the non-fractured section during the fracturing process.

[0126] Coupling positioner 15-7: An instrument based on the principle of electromagnetic induction for measuring the position of steel sleeve couplings in a hole.

[0127] Guide shoe 15-8: A cone-shaped body that is fitted onto the front end of the downhole bottom seal drag segmented fracturing tool string 15 to prevent the downhole bottom seal drag segmented fracturing tool string 15 from hitting the well wall and to guide the downhole bottom seal drag segmented fracturing tool string 15 smoothly down to the bottom of the well.

[0128] The aforementioned method of performing directional sandblasting and dense cutting in a horizontally directional long borehole utilizes the downhole bottom seal to drag a segmented fracturing tool string to perform directional sandblasting and dense cutting toward the target coal seam within the horizontally directional long borehole.

[0129] Includes the following steps:

[0130] After assembling the downhole bottom seal drag segmented fracturing tool string, connect the coiled tubing 19 to the sealing joint. Connect the other end of the coiled tubing to the well-to-tunnel connection device 3. Push the downhole bottom seal drag segmented fracturing tool string 15 into the designed position inside the horizontal directional long borehole and begin directional spraying. After the directional spraying gun 15-4 completes the directional work, push back the coiled tubing 19 to set the packer 15-6. Then, turn on the surface fracturing equipment 1 to inject sand-mixed fracturing fluid into the coiled tubing. Use the directional spraying gun 15-4 to perform directional sandblasting and dense cutting on the horizontal directional long borehole casing 16, the coal seam roof 14 near the horizontal directional long borehole, and the target coal seam 11.

[0131] First, a pipeline pressure test is conducted. The surface fracturing wellhead 1, the well-to-tunnel connection device 3, and the pressure-resistant valve inside the tunnel controlling the planned horizontal directional long borehole are opened. The long borehole fracturing wellhead 9 is closed, and the surface fracturing equipment is started to conduct the pipeline test. The pressure is gradually increased to 70MPa. The surface high-pressure manifold and the underground high-pressure pipe and connection device show no significant pressure drop or leakage for 15 minutes. The pressure test is qualified.

[0132] The downhole bottom seal is installed at the front end of the coiled tubing 19, and the segmented fracturing tool string 15 is dragged along it. (Refer to...) Figure 3 The downhole bottom seal drag-type fracturing tool string 15 has a directional spray gun 15-4 capable of directional perforation, see reference. Figure 4 Start the coiled tubing truck. Push the downhole bottom seal drag segmented fracturing tool string 15 into the designed position and begin nozzle orientation. After the downhole bottom seal drag segmented fracturing tool string 15 enters the horizontal directional long borehole, the directional steel balls of the directional nozzle 15-4 roll down to the bottom of the annulus under the action of gravity, realizing the downward directional perforation of the nozzle of the directional nozzle 15-4.

[0133] Push the pullback coiled tubing 19 to set the packer 15-6. Use the surface fracturing equipment 1 to inject sand-mixed fracturing fluid into the coiled tubing. Use the directional spray gun 15-4 to perform directional sandblasting and dense cutting on the horizontal directional long borehole casing 16, cement sheath 17, the coal seam roof 14 near the horizontal directional long borehole, and the target coal seam 11. (Refer to...) Figure 5 .

[0134] Specifically, the perforation stage displacement of the surface fracturing equipment should be greater than or equal to 3.0 m³ / s. 3 / min, pressure greater than or equal to 70MPa, sand content of mixed sand fluid greater than or equal to 2%, abrasive selected as 50-70 mesh quartz sand; interval between two adjacent fracturing sections less than or equal to 30m;

[0135] Step 4: Joint fracturing operations are carried out on the horizontally directional long borehole, both above and below ground.

[0136] The surface fracturing equipment 1 and downhole equipment (including coiled tubing truck 6, injection head truck 7, and downhole fracturing pump 8) are used in a joint operation to carry out segmented fracturing operations on the horizontal directional long borehole 12.

[0137] After directional sandblasting and dense cutting are completed, the surface fracturing equipment 1 is continuously activated to continuously inject high-pressure water into the coiled tubing 19; the downhole fracturing pump 8 is activated to continuously inject sand-mixed fluid to replenish fracturing fluid in the annulus 20 between the casing 16 and the coiled tubing 19. Utilizing Bernoulli's principle, the throttling effect of the hydraulic jet nozzle is used to convert the high-pressure perforation fluid into high-speed perforation fluid, which continues to erode and cut the steel casing 16, cement sheath 17, and formation (including the coal seam roof 14 and the target coal seam 11). The surface-downhole combined operation is used for fracturing. The pressurization of the jet fluid and the superposition of the annulus pressure greatly exceed the fracturing pressure of the coal seam roof and coal seam, and the top coal 11-1 is fully fractured and broken.

[0138] Specifically, the displacement of the downhole fracturing equipment is greater than or equal to 2.0 m³ / h. 3 / min, pressure greater than or equal to 50MPa; ground fracturing truck displacement greater than or equal to 3.0m 3 / min, pressure greater than or equal to 70MPa, sand content of sand-mixing liquid greater than or equal to 10%, and abrasive selected as 50-70 mesh quartz sand;

[0139] Specifically, steps three and four are continuous operations. The combined wellbore and surface fracturing technology is a new technology that integrates large-volume, infinitely segmented, and economically reasonable directional perforation fracturing. It can achieve simultaneous perforation and fracturing without moving the tubing string, and densely cut the target section, reducing operation time and increasing work efficiency.

[0140] After observing the fracturing of the coal (rock) seam, shut down the surface fracturing equipment 1 and the downhole fracturing pump 8, and pull back the coiled tubing 19 under pressure to release the packer 15-6; after the packer 15-6 is released, continue to pull back the coiled tubing 19 (without venting) to move the directional nozzle 15-4 to the next perforation target section until the entire target section of the hole is cut and fracturing.

[0141] Step 5: Repeat steps 3 and 4 to carry out directional sandblasting and fracturing operations on all horizontal directional long boreholes to complete the modification of the top coal seam.

[0142] To minimize inter-fracturing interference, an alternating fracturing method was used for horizontally oriented long boreholes. By changing the valve direction of the high-pressure connection device between the surface well and the underground long borehole, pre-installed in the main roadway or working roadway of the mining area, alternating segmented fracturing operations were performed on each horizontally oriented long borehole. Specifically, horizontally oriented long borehole I12-1 was fracturing first, followed by horizontally oriented long borehole III12-3, then horizontally oriented long borehole II12-2, and subsequently horizontally oriented long boreholes V12-5 and IV12-4, sequentially completing continuous segmented fracturing of all horizontally oriented long boreholes in the mining area.

[0143] Step Six: Mining the bottom coal. The modified top coal naturally collapses under the pressure of the overlying strata, which improves the recovery rate of top coal caving.

[0144] refer to Figure 1 Because the top coal was pre-processed by crushing, its integrity was greatly reduced. With the repeated up-and-down movement of the support, the top coal was crushed more thoroughly, and it could collapse in time as the bottom coal was mined, thus improving the recovery rate.

[0145] This patent proposes a regionalized method to improve the top coal recovery rate based on combined underground and surface operations. This technology injects high-pressure water into the target top coal through a horizontally directional long borehole in the coal seam roof. When the water pressure reaches the fracturing pressure of the coal and rock mass, cracks are generated in the coal seam roof and top coal and extend, destroying the integrity of the top coal and changing its mechanical properties, thereby weakening the coal and rock mass and improving the top coal recovery rate.

[0146] This invention can create a dense network of numerous cracks with relatively even orientation and length inside the top coal that is difficult to cavitate using conventional top caving methods. This artificially pre-cracks the top coal that is difficult to cavitate, reduces the overall strength of the top coal, and thus enables the top coal to cavitate fully, thereby improving the top coal caving recovery rate.

[0147] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions conceived by those skilled in the art within the scope of the technology disclosed in the present invention without creative effort are covered within the scope of protection of the present invention.

Claims

1. A method for improving the top coal caving recovery rate based on combined underground and surface operations, characterized in that, Includes the following steps: Step 1: Construct an accurate three-dimensional geological model of the roof of the target coal seam; Step 2: Determine the direction of the minimum horizontal principal stress in the coal seam roof, and construct a horizontal directional long borehole along the direction of the minimum horizontal principal stress in the coal seam roof according to the accurate three-dimensional geological model of the coal seam roof. There shall be at least one horizontally oriented long borehole, which shall be spaced apart and able to cover the working face; Step 3: Perform directional sandblasting and dense cutting within the horizontal directional long borehole, wherein the direction of the directional sandblasting and dense cutting is toward the target coal seam; Step 4: Joint fracturing operations are carried out on the horizontally directional long borehole, both above and below ground. Step 5: Repeat steps 3 and 4 to carry out directional sandblasting and fracturing operations on all horizontal directional long boreholes to complete the modification of the top coal seam; Step Six: Mining the bottom coal, the modified top coal naturally collapses under the pressure of the overburden, which improves the recovery rate of top coal caving; The interval distance of the horizontally oriented long boreholes is calculated according to the following formula: The number of horizontally oriented long boreholes is calculated according to the following formula: In the formula: The interval distance between horizontally oriented long boreholes, in meters (m). N The number of horizontally oriented long boreholes; M The length of the main roadway in the mining area is in meters (m). This is the ratio of coal seam fracture length to roof fracture length, dimensionless; C The filtration coefficient of fracturing fluid is given in m / s. 0.5 ; Q For construction displacement, m 3 / s; H The crack height is in meters (m). w Let the width of the crack at any point be m; S p For initial filtrate loss of fracturing fluid, m 3 / m 2 ; μ The viscosity of a Newtonian fluid is given in Pa·s. G Here, denoted as rock shear modulus, Pa; v Poisson's ratio of the rock; w (0, t) represents the maximum width of the crack at time t on the elliptical cross-section at the crack opening, in meters; t is the duration of the formal start of fracturing, in seconds; x is the half-length of the rectangle in the KGD hydraulic fracture model, in meters. It is the error compensation function for x.

2. The method for improving the top coal caving recovery rate based on combined underground and surface operations as described in claim 1, characterized in that, The construction of an accurate three-dimensional geological model of the target coal seam roof includes: The data were collected from roadway borehole data, cut geological sampling data, 3D seismic analysis data, and coal seam roof contour data within the working face area. The Kriging method was used to interpolate the known data to obtain the accurate geological morphology of the coal seam roof.

3. The method for improving the top coal caving recovery rate based on combined underground and surface operations as described in claim 1, characterized in that, The method for determining the direction of the minimum horizontal principal stress is as follows: it is obtained through a small-scale downhole hydraulic fracturing test.

4. The method for improving the top coal caving recovery rate based on combined underground and surface operations as described in claim 3, characterized in that, When constructing horizontal directional long boreholes, the location of the horizontal directional long boreholes is determined based on the drilling measurement data during the construction process, ensuring that the location of the horizontal directional long boreholes is in the coal seam roof and within 0.5 to 1.0 m above the top boundary of the target coal seam.

5. The method for improving the top coal caving recovery rate based on combined underground and surface operations as described in claim 1, characterized in that, In step three, the downhole bottom seal is used to drag the segmented fracturing tool string to perform directional sandblasting and dense cutting in the horizontal directional long borehole. The downhole bottom seal dragged segmented fracturing tool string includes a sealing joint, a mechanical hydraulic release, a centralizer, a directional spray gun, a balance valve, a packer, a coupling positioner, and a guide shoe connected in sequence. The sealing joint is used to connect a continuous tubing; The aforementioned mechanical hydraulic release mechanism is used to prevent sand from getting stuck in the downhole bottom seal dragging the segmented fracturing tool string, thus preventing the coiled tubing from being retracted. The centralizer is used to ensure that the downhole bottom seal drag segmented fracturing tool string is located in the middle of the horizontally oriented long borehole; The directional spray gun is used for vertically downward directional injection. The aforementioned balancing valve is used to cooperate with the bottom packer to achieve unsealing. When the bottom packer is unsealed, it balances the pressure difference of the bottom packer and reduces the unsealing load. The packer is used to isolate the fracturing section and the non-fracturing section; The coupling locator is used to measure the position of the steel sleeve coupling in the hole; The aforementioned guide shoe is used to prevent the downhole bottom seal from dragging the segmented fracturing tool string and hitting the well wall; The directional spray gun comprises a first rotary sealing section, a directional nozzle, a directional device, and a second rotary sealing section connected in sequence. The first rotary sealing section is connected to a centralizer, and the second rotary sealing section is connected to a balance valve.

6. The method for improving the top coal caving recovery rate based on combined underground and surface operations as described in claim 5, characterized in that, Step three specifically includes the following steps: The assembled downhole bottom seal is dragged and the segmented fracturing tool string is pushed into the designed position inside the horizontal directional long borehole to complete the orientation of the directional spray gun. The directional spray gun is used to perform directional sandblasting and dense cutting on the horizontal directional long borehole, the coal seam roof near the horizontal directional long borehole, and the target coal seam. After the directional sandblasting and dense cutting is completed, the downhole bottom seal is moved and the segmented fracturing tool string is dragged to the designed position inside the next horizontal directional long borehole. The above operation is repeated until the entire horizontal directional long borehole has been directionally sandblasted and densely cut.

7. The method for improving the top coal caving recovery rate based on combined underground and surface operations as described in claim 1, characterized in that, Before performing directional sandblasting and dense cutting on the horizontally oriented long borehole, a steel casing is inserted into the horizontally oriented long borehole and cemented.

8. The method for improving the top coal caving recovery rate based on combined underground and surface operations as described in claim 7, characterized in that, The aforementioned horizontal directional long borehole drilling, including the insertion of steel casing and cementing, comprises: After the horizontal directional long borehole is completed, the borehole is flushed until no more sand or debris is returned. Select a steel casing with a diameter one grade smaller than that of the horizontal directional borehole. Install a tapered guide shoe at the front end of the steel casing, tighten the casing clamp in one go, and push the steel casing into the designed depth. High-pressure injection of cement slurry containing a quick-setting agent into the steel casing fills the annulus of the horizontal directional borehole wall and the steel casing with cement slurry until it returns to the borehole opening. Allow it to solidify for 48 hours to complete the cementing operation, so that the steel casing and the horizontal directional borehole solidify into a whole.

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