WO2022237177A1 - 碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法 - Google Patents

碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法 Download PDF

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WO2022237177A1
WO2022237177A1 PCT/CN2021/140941 CN2021140941W WO2022237177A1 WO 2022237177 A1 WO2022237177 A1 WO 2022237177A1 CN 2021140941 W CN2021140941 W CN 2021140941W WO 2022237177 A1 WO2022237177 A1 WO 2022237177A1
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horizontal
coal seam
well
drilling
horizontal well
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PCT/CN2021/140941
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English (en)
French (fr)
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张群
姜在炳
李彬刚
许耀波
王成
周加佳
李浩哲
陈刚
降文萍
杜新锋
张培河
刘柏根
舒建生
范耀
刘嘉
赵龙
庞涛
刘磊
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中煤科工集团西安研究院有限公司
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Priority to AU2021445202A priority Critical patent/AU2021445202A1/en
Publication of WO2022237177A1 publication Critical patent/WO2022237177A1/zh

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    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B47/00—Survey of boreholes or wells
    • E21B47/02—Determining slope or direction
    • E21B47/022—Determining slope or direction of the borehole, e.g. using geomagnetism
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A90/00—Technologies having an indirect contribution to adaptation to climate change
    • Y02A90/30—Assessment of water resources

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  • the invention belongs to the technical field of coal bed gas extraction, and in particular relates to a method for extracting coal bed gas from a horizontal well with staged fracturing on the top or bottom plate of a broken soft low-permeability coal seam.
  • CBM is mainly stored in coal seams in the form of adsorption.
  • the gas adsorbed in the coal seam is desorbed and converted into free gas, and then migrates to the coalbed methane well through micropore diffusion and fracture seepage in the coal seam, thereby realizing the extraction of coalbed methane.
  • Fragile soft and low permeability coal seams have the characteristics of low porosity, low permeability, and crushed and soft coal seams. They are widely distributed in Carboniferous-Permian coal seams in my country, but the extraction efficiency of coalbed methane is low.
  • coalbed methane gas
  • coalbed methane gas
  • coalbed gas fracturing technology is gradually developing from vertical well fracturing to horizontal well staged fracturing technology.
  • staged fracturing of horizontal wells can greatly increase the drainage area of a single well and effectively increase the gas production of a single coalbed methane well. Therefore, there is an urgent need for a coalbed methane extraction method for soft and low-permeability coal seams to achieve efficient extraction of coalbed methane.
  • the method of enhanced drainage of coalbed methane staged fracturing horizontal wells used in the prior art aims at the problem of coalbed methane development in structurally soft coal, and proposes to control the wellbore trajectory of the horizontal well between 0.5m and 1.5m above the top boundary of the coal seam.
  • the purpose of strengthening the drainage of structural soft coalbed methane is achieved through directional perforation and fracturing in stages in the horizontal well section. This method is relatively strict and difficult to control the wellbore trajectory of horizontal wells, and the cost of boundary detection technology required is high.
  • the present invention provides a coalbed methane extraction method in a fractured, soft and low-permeability coal seam roof or floor segmented fracturing horizontal well, so as to solve the problem of coalbed methane in the fragmented, soft and low-permeability coal seam in the prior art.
  • Technical problems of low drainage efficiency are a coalbed methane extraction method in a fractured, soft and low-permeability coal seam roof or floor segmented fracturing horizontal well.
  • the present invention takes the following technical solutions:
  • a method for extracting coalbed methane from a horizontal well with staged fracturing on the roof or floor of a broken, soft and low-permeability coal seam comprising the following steps:
  • Step 1 Collect the exploration data of the target mining area, and determine the horizontal well type according to the collected exploration data and mine data.
  • the horizontal well includes a vertical section and a horizontal section of the horizontal well.
  • the horizontal well type includes L type, U type type and W type;
  • Step 2 determining the location of the horizontal well according to the collected exploration data and mine data
  • Step 3 Determine the target layer of the horizontal section of the horizontal well in the coal seam roof or floor according to the collected exploration data and mine data, and the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam;
  • Step 4 determine the layout azimuth of the horizontal section of the horizontal well according to the minimum horizontal principal stress direction of the target mining area obtained by previous exploration or testing;
  • Step 5 Complete the drilling of the vertical well, and complete the first and second drilling and cementing of the horizontal well;
  • Step 6 Use the azimuth gamma geosteering method to carry out geosteering drilling in the horizontal section of the horizontal well, adjust the drilling trajectory of the horizontal section of the horizontal well in real time, so that the drilling trajectory of the drill bit is located in the target layer described in step 3, and make the Keep the vertical distance determined in step 3 between the horizontal section of the horizontal well and the top/bottom of the coal seam;
  • Step 7 Carry out segmental isolation on the horizontal section of the horizontal well, implement directional perforation or multi-cluster directional perforation toward the coal seam, and then perform multi-cluster temporary plugging and turn to segmental fracturing construction;
  • Step 8 After completing multi-cluster temporary plugging and turning to staged fracturing, perform flowback, well cleaning operations, and then conduct coalbed methane drainage and gas recovery.
  • the vertical distance between the horizontal section of the horizontal well described in step 3 and the top/bottom of the coal seam is determined by the following steps:
  • Step 3.1 According to the collected exploration data and mine data, determine the fracture extension data of the horizontal section of the horizontal well in the target horizon through numerical simulation.
  • the fracture extension data includes the total height of the fracture, the half-length of the fracture, The height of the upper part of the fracture and the height of the lower part of the fracture;
  • Step 3.2 Determine the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam according to the obtained fracture extension data.
  • the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam is 0.5-8 meters.
  • the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam is 1.5 meters.
  • the minimum horizontal principal stress direction of the target mining area obtained from the test is measured through small-scale fracturing tests, dipole sonic logging or stress relief methods.
  • the azimuth angle of the layout of the horizontal section of the horizontal well described in step 4 is -15° to 15°.
  • the azimuth angle of the layout of the horizontal section of the horizontal well described in step 4 is 0°.
  • step 5 specifically includes:
  • the well type is an L-shaped well
  • the vertical well drilling is completed, and then the first and second horizontal wells are drilled to the landing point, and then the casing is injected and cemented;
  • casing cementing is performed after the vertical well drilling is completed, and then the first and second drilling of the two horizontal wells are respectively completed, and the casing is cemented after drilling to the landing point.
  • the geosteering drilling in the horizontal section of the horizontal well as described in step 6 specifically includes:
  • Step 6.1 establishing the relationship between the formation absorption coefficient and the gamma ray intensity in the target mining area
  • Step 6.2 establishing the relationship between the thickness of the coal seam roof or floor and the gamma ray intensity in the target mining area;
  • Step 6.3 according to the relationship between the formation absorption coefficient and the gamma ray intensity established in step 6.1 and the relationship between the layer roof or floor thickness and the gamma ray intensity established in step 6.2, determine the gamma curve amplitudes of different data acquisition sectors in the target mining area,
  • the gamma curve amplitude includes an upper gamma curve amplitude and a lower gamma curve amplitude;
  • Step 6.4 determining the drilling distance and the vertical distance from the drill bit to the interface between the coal seam and the coal seam roof/coal seam floor;
  • Step 6.5 according to the drilling distance and vertical distance between the coal seam and the coal seam roof/coal seam floor interface obtained in step 6.4 and the upper gamma curve amplitude and lower gamma curve amplitude obtained in step 6.3, determine the geosteering drilling in the horizontal section of the horizontal well track, and conduct geosteering drilling in the horizontal section of the horizontal well.
  • J is the gamma ray intensity in MeV
  • ac is the formation absorption coefficient
  • ⁇ is the position parameter
  • ⁇ is the scale parameter
  • the relationship between the thickness of the coal seam roof or floor and the gamma ray intensity in the target mining area of step 6.2 is:
  • J is the gamma ray intensity in MeV
  • H is the thickness of the coal seam roof or floor in m
  • ⁇ is the position parameter
  • ⁇ is the scale parameter
  • step 6.3 calculates the gamma curve amplitudes of different data acquisition sectors as:
  • f is the amplitude of the gamma curve
  • the unit is API
  • Si is the gamma count value of the i-th data collection sector
  • i is an integer ranging from 1 to 8
  • N j is the data collected in Si gamma ray intensity count value at detection depth j in the sector;
  • G 1j , G 2j , G 3j , and G 4j are the contributions of the gamma-ray intensity count values of the azimuth logging tool while drilling at depth j and tool face angles of 0°, 90°, 180°, and 270°, respectively
  • ⁇ is the tool face angle of the azimuth logging tool while drilling
  • V is the drilling speed of the azimuth logging tool while drilling, in m/s
  • R is the rotation speed of the azimuth logging tool in r/s s.
  • step 6.4 determines the drilling distance and the vertical distance from the drill bit to the interface between the coal seam and the coal seam roof/coal seam floor as follows:
  • L is the drilling distance from the drill bit to the coal-rock interface, in m;
  • L c is the vertical distance from the drill bit to the coal-rock interface, in m;
  • L ac is the distance from the azimuth logging tool while drilling to the drill bit, in is m;
  • D is the distance from the detector at the opening point to the coal-rock interface, in m;
  • ⁇ is the angle between the coal-rock interface and the instrument, in °;
  • dep is the measurement distance, in m;
  • n is the number of measurements, n is a positive integer ⁇ 1, dev n is the dip angle measured by the azimuth logging tool for the nth time, the unit is °, and the value range is 0 to 360°;
  • a formation is the formation dip angle, the unit is °, take The value range is 0-360°;
  • D n is the distance from the detector to the coal bed interface in the nth measurement, in m, and D n+1 is the distance
  • step 6.5 specifically includes:
  • the amplitudes of the upper and lower gamma curves gradually increase and the amplitude of the upper gamma curve increases first, and the drilling distance gradually increases.
  • the drilling distance is less than 5-8m or the vertical distance
  • the amplitude of the upper and lower gamma curves exceeds 70 API, adjust the drill bit to drill along the original drilling direction with an instrument tool face angle of 180°;
  • the amplitudes of the upper and lower gamma curves gradually increase and the amplitude of the lower gamma curve increases first, and the drilling distance gradually increases.
  • the drilling distance is less than 5-8m or the vertical distance
  • the amplitude of the upper and lower gamma curves exceeds 70 API, adjust the drill bit to drill along the original drilling direction with an instrument tool face angle of 180°;
  • step 7 specifically includes the following sub-steps:
  • Step 7.1 determining the number of fracturing stages and clustered perforation positions in the horizontal section of the horizontal well;
  • Step 7.2 implement multi-cluster directional perforation vertically downward/upward to the coal seam direction in the first section of the horizontal section of the horizontal well;
  • multi-cluster directional perforation is carried out in the first section of the horizontal section of the horizontal well by means of tubing or coiled tubing;
  • Step 7.3 using the bridge plug staged fracturing method to perform multi-cluster temporary plugging and diverting staged fracturing construction on the first section of the horizontal section of the horizontal well;
  • Step 7.4 repeating steps 7.2 and 7.3, performing perforation and fracturing construction on the remaining well sections of the horizontal section of the horizontal well until the fracturing construction of the entire well section is completed;
  • Step 7.5 After completing the fracturing construction of the whole well section, shut down the well and wait for the pressure to spread below the fracture closure pressure, then perform blowout and flowback operations.
  • the operating parameters for implementing multi-cluster directional perforation vertically downward/upward in the direction of the coal seam described in step 7.2 include: the perforating gun is type 95 or 89, the perforating charge is type 102, and the perforation density is 10-16 holes/m, and the perforation phase angle is vertical downward/upward.
  • the operating parameters for multi-cluster temporary plugging and diversion to staged fracturing in the first section of the horizontal section of the horizontal well described in step 7.3 include: the fracturing fluid is active water fracturing fluid, the proppant is quartz sand, and the fracturing fluid is activated water.
  • the sanding intensity of the split section is 8-15m 3 /m
  • the injection displacement is 8-15m 3 /min
  • the average sand ratio is 10-15%
  • the pre-fluid ratio is greater than or equal to 40%.
  • the elevation of the landing point drilled in the horizontal section of the horizontal well is 3-20m higher than the elevation at the junction of the horizontal section and the vertical section of the horizontal well;
  • the elevation of the landing point drilled in the horizontal section is 3-20m lower than the elevation at the end of the horizontal section of the horizontal well.
  • step 8 specifically includes:
  • Step 8.1 After the discharge and flowback operation is completed, run the milling pipe string to drill and mill the bridge plugs in a unified manner. After milling and milling all the bridge plugs in the horizontal section of the horizontal well, continue to run the milling pipe string to the artificial well of the horizontal well At the bottom, positively circulate clean water 1.5 times the volume of the wellbore of the horizontal well, and then raise the milling string;
  • Step 8.2 Carry out circular well flushing operation on the horizontal wellbore, observe the situation of the outlet liquid return until the water quality of the inlet and outlet is consistent, and the outlet liquid is clean and free of impurities and dirt, stop the well flushing operation;
  • Step 8.3. According to the requirements of drainage design specifications, install horizontal well drainage equipment, and perform drainage operations on horizontal wells.
  • the present invention has beneficial technical effects as follows:
  • the present invention selects different horizontal well types according to different geological conditions and coalbed methane extraction requirements, which is conducive to adapting measures to local conditions, saving costs, and maximizing benefits.
  • the method of the present invention sets the elevation difference between the landing point elevation of the horizontal well and the end of the horizontal well, which is conducive to the accumulation of water in the horizontal section flowing to the bottom of the vertical well under the action of gravity, thereby facilitating rapid drainage and gas recovery.
  • the method of the present invention can ensure that the transverse fracture perpendicular to the wellbore of the horizontal section is formed during hydraulic fracturing by setting the azimuth angle of the horizontal section of the horizontal well. Because the broken soft coal seam is rich in gas and low in permeability as a whole, the transverse fracture can ensure that the horizontal well has Higher production capacity realizes efficient extraction of coalbed methane in coal mines.
  • the method of the present invention utilizes the azimuth gamma geosteering method to carry out geosteering drilling in the horizontal section of a horizontal well, and by adjusting the drilling trajectory in real time during the drilling process, it effectively solves the problem that the actual drilling trajectory cannot be monitored in real time, and the drilling trajectory
  • the adjustment process has the problems of cumbersomeness, invisibility and insufficient effectiveness.
  • the drilling rate of the horizontal section of the horizontal well in the target layer can reach more than 95%.
  • the method of the present invention realizes the reformation of the coalbed methane reservoir through the multi-cluster temporary plugging and turning to the staged fracturing process, which is beneficial to realize the densely cut multi-cluster staged fracturing volume reformation of the broken soft coal seam, thereby forming complex network fractures, Obtain larger reconstruction volume and increase single well production.
  • Fig. 1 is the flowchart of the inventive method
  • Fig. 2 is a schematic diagram of the relationship between horizontal well drilling direction and fracture extension
  • Fig. 3 is a schematic diagram of the extension form of a single fracture arranged at different layout azimuths in the horizontal section of the horizontal well;
  • a is the layout azimuth of the horizontal section of the horizontal well at 0°
  • b is the layout azimuth of the horizontal section of the horizontal well at 15°
  • c is the layout azimuth of the horizontal section of the horizontal well at 45°
  • d is the horizontal section of the horizontal well
  • the layout azimuth angle is 60°
  • Fig. 4 is a schematic diagram of the extended form of double fractures arranged at different layout azimuths in the horizontal section of the horizontal well;
  • Fig. 5 is a schematic diagram of the relationship between formation absorption coefficient and gamma ray intensity of the present invention.
  • Fig. 6 is a schematic diagram of the distance from the drill bit to the interface in the azimuth gamma geosteering of the present invention.
  • Fig. 7 is a schematic diagram of geosteering drilling trajectory of the present invention.
  • Fig. 8 is the upper and lower gamma curve amplitudes and the drilling trajectory obtained when the distance between the azimuth logging tool while drilling and the drill bit is 1 meter in embodiment 1;
  • Fig. 9 is the effect diagram of carrying out multi-cluster general fracturing to the horizontal section of the horizontal well in embodiment 1;
  • Fig. 10 is an effect diagram of multi-cluster temporary plugging diversion staged fracturing on the horizontal section of the horizontal well in Example 1.
  • the method for extracting coalbed methane by staged fracturing horizontal wells on the top or bottom plate of broken soft low permeability coal seams designed by the present invention uses staged fracturing horizontal wells to extract coalbed methane on the top or bottom plate of broken soft low permeability coal seams.
  • the primary task is to Identify the coal seam roof and floor interface, coal seam structure and location in the target mining area, and then obtain the water content and gas content of the coal seam, as well as the lithology, thickness, coal seam rock mechanics parameters and physical parameters of the coal seam roof or floor; optimize the orientation of the horizontal well Arranged parallel to the direction of the minimum horizontal principal stress; in the process of measuring with the azimuth logging tool while drilling, the azimuth measurement is realized by using the probe set on the azimuth logging tool while drilling, and the measured azimuth data is recorded in 8 sectors, and then Synthesize the natural gamma ray intensity measurements in the four azimuths of up, down, left, and right and upload them to the ground in real time, adjust the drilling trajectory of the horizontal section of the horizontal well in real time according to the obtained data, and complete the drilling; finally implement directional perforation or multi-cluster directional Cluster temporary plugging turned to staged fracturing construction, coalbed methane drainage and gas recovery.
  • U-shaped horizontal well It consists of a vertical well and a horizontal butt well.
  • L-shaped horizontal well A horizontal well consisting of a single horizontal well.
  • W-type horizontal well It consists of a vertical well connected with two horizontal wells.
  • Fracture upper height refers to the fracture height below the wellbore of the horizontal section of the horizontal well.
  • Fracture lower height refers to the fracture height above the wellbore in the horizontal section of a horizontal well.
  • Drilling distance the distance from the drill bit to the coal-rock interface along the drilling direction.
  • Vertical distance the vertical distance from the drill bit to the coal-rock interface.
  • the layout azimuth of the horizontal section of the horizontal well the acute angle formed between the layout direction of the horizontal section of the horizontal well and the direction of the minimum horizontal principal stress.
  • Cable pumping bridge plug perforation combined operation suitable for casing cementing and completion methods, combining cable pumping perforation technology with bridge plug staged fracturing technology, which can realize the completion of bridge plug setting and bridge plug fracturing in one trip by cable transmission
  • Perforation operation can realize multi-stage continuous fracturing construction under pressure.
  • the bridge plug and perforating gun joint tool connected together are sent to the bridge plug setting position by cable pumping, and then ignited and set, and then the perforating gun is lifted to the perforation position to carry out perforation operation. After the perforation operation Pull out the cable for bare casing fracturing, and then proceed to the next stage of pumping, bridge plug, perforation and fracturing operations. Repeat the above steps to complete the perforation and fracturing operations for the entire well section.
  • This embodiment provides a coalbed methane extraction method in a broken-soft low-permeability coal seam roof or floor staged fracturing horizontal well, including the following steps:
  • Step 1 Collect the exploration data and mine data of the target mining area, and determine the horizontal well type according to the collected exploration data and mine data.
  • the horizontal well includes a vertical section and a horizontal section of the horizontal well.
  • the horizontal well type includes L Type, U type and W type;
  • the exploration data and mine data of the target mining area specifically include: the topographical conditions and geological conditions of the target mining area, coal mining planning in the mining area, and the layout of underground roadways, etc.
  • Collecting geological condition data can understand the structure of coal seam and guide the selection of horizontal well type.
  • Collecting coal mining planning and underground roadway layout in the mining area can provide a preliminary understanding of the underground mining situation of the coal mine, determine the distance between the coal mining area and the target extraction area, and determine whether it is necessary to achieve full coverage of the horizontal section of the horizontal well in the length direction of the working face.
  • U-shaped horizontal wells are preferred if the degree of previous research in the target mining area is low or the geological structure is complex.
  • the advantage of the U-shaped horizontal well is that the position of the coal seam can be detected through the vertical well; the well cleaning operation is convenient after the staged fracturing operation and before the drainage operation; the drainage operation in the vertical well greatly reduces the displacement of the drainage pipe. The impact of grinding and pulverized coal output on the drainage process.
  • L-shaped horizontal wells are preferred.
  • the advantage of L-shaped horizontal wells is that it can save drilling costs, reduce road repair costs, and reduce land occupation.
  • W-shaped horizontal wells are preferred.
  • the advantage of the W-shaped horizontal well is that one vertical well can be used to drain two horizontal wells at the same time.
  • the W-type horizontal well after the vertical well drilling is completed, two horizontal wells are drilled from both ends of the working face to the vertical well, and the two horizontal wells are connected to the vertical well, and the drainage operation is carried out in the vertical well.
  • Step 2 determining the location of the horizontal well according to the collected exploration data and mine data
  • the coal reservoir data of the target mining area includes coal seam thickness, coal bed gas content, and coal seam permeability .
  • Maps comprehensively evaluate the geological conditions of coalbed methane extraction on the surface of the mine field, and determine the target area and well location of the horizontal well: arrange the horizontal well in the area where the thickness of the coal seam, the gas content of the coal seam, and the permeability of the coal seam are all high.
  • Step 3 Determine the target layer of the horizontal section of the horizontal well in the coal seam roof or floor according to the collected exploration data and mine data, and the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam;
  • Step 3.1 According to the collected exploration data and mine data, determine the fracture extension data of the horizontal section of the horizontal well in the target horizon through numerical simulation.
  • the fracture extension data includes the total height of the fracture, the half-length of the fracture, The height of the upper part of the fracture and the height of the lower part of the fracture;
  • the height H coal of the fracture above the bottom surface of the coal seam and the height H floor of the fracture below the bottom surface of the coal seam are calculated.
  • the calculation method is:
  • Step 3.2 Determine the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam according to the obtained fracture extension data.
  • the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam is 0.5-8 meters.
  • the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam is 1.5 meters.
  • Step 4 determine the layout azimuth of the horizontal section of the horizontal well according to the minimum horizontal principal stress direction of the target mining area obtained by previous exploration or testing;
  • the minimum horizontal principal stress direction of the target mining area obtained from the test is measured through small-scale fracturing tests, dipole sonic logging or stress relief methods.
  • the azimuth angle of the horizontal section of the horizontal well is -15° ⁇ 15°
  • the horizontal section layout azimuth of the horizontal well is 0°.
  • Fig. 3 The extension of the single fracture generated after the horizontal section of the horizontal well is laid out at different azimuth angles is shown in Fig. 3.
  • the angle between the horizontal well and the direction of the minimum horizontal principal stress is 0°-15°
  • the fracturing formation is perpendicular to the wellbore.
  • the transverse fractures have a larger contact area with the formation; as the angle increases (the angle reaches 45°-60°), the fracture turns after initiation and finally extends along the direction of the maximum horizontal principal stress. Steering occurs, and the larger the included angle is, the larger the crack turning distance is.
  • the small fracture width at the fracture turning point is not conducive to the migration of proppant in the fracture, and the near-wellbore bending friction is large when the fracture initiates, which leads to an increase in the fracture initiation pressure during surface construction, which is not conducive to fracturing operation.
  • the angle between the azimuth of the horizontal section of the horizontal well and the direction of the minimum horizontal principal stress ranges from -15° to 15°, and the preferred angle is 0°.
  • the main purpose of setting the azimuth of the horizontal section of the horizontal well is to obtain the best fracture extension effect and gas effect.
  • Step 5 Complete the drilling of vertical wells, and the first and second drilling of horizontal wells;
  • the vertical well When the well type of the horizontal well is U-shaped or W-shaped, the vertical well is first drilled on the ground to the coal seam, and the coal seam is sampled, tested, logged, and tested, and then continues to drill to the preset position under the coal seam to carry out in-situ stress measurement. Obtain the in-situ stress profile of the vertical well, run the casing and cement the well; then drill the vertical section of the horizontal well, then construct the pilot hole, log while drilling, determine the pilot layer, fill the pilot well, and sidetrack to the landing point Back casing cementing;
  • the horizontal well is firstly drilled in the vertical section, and then the pilot hole is constructed, logging while drilling, the pilot layer is determined, the pilot well is buried, and the casing is run after sidetracking to the landing point Cementing.
  • the elevation of the landing point drilled in the horizontal section of the horizontal well is 3-20m higher than the elevation at the junction of the horizontal section of the horizontal well and the vertical well section;
  • the elevation of the landing point is 3-20m lower than the elevation of the end of the horizontal section of the horizontal well.
  • Step 6 Use the azimuth gamma geosteering method to carry out geosteering drilling in the horizontal section of the horizontal well, adjust the drilling trajectory of the horizontal section of the horizontal well in real time, so that the drilling trajectory of the drill bit is located in the target layer described in step 3, and make the Keep the vertical distance determined in step 3 between the horizontal section of the horizontal well and the top/bottom of the coal seam;
  • Step 6.1 establishing the relationship between the formation absorption coefficient and the gamma ray intensity in the target mining area
  • the measurement of the azimuth while drilling is also affected by the properties of the surrounding rock of the coal seam roof or coal seam floor, the thickness of the coal seam roof/bottom and the absorption coefficient of the formation.
  • the coal seam roof or The intensities of gamma rays are different when the properties of the surrounding rock of the coal seam floor, the thickness of the coal seam roof/floor and the absorption coefficient of the formation are different.
  • the absorption coefficient of the formation is 0.08, 0.085, 0.09, and 0.1
  • the gamma ray intensity increases with the decrease of the absorption coefficient.
  • J is the gamma ray intensity in MeV
  • H is the thickness of the coal seam roof or floor in m
  • ⁇ is the position parameter
  • ⁇ is the scale parameter
  • Step 6.2 establishing the relationship between the thickness of the coal seam roof or floor and the gamma ray intensity in the target mining area;
  • the absorption coefficient of the medium in the borehole is the same, the absorption coefficients of the coal seam, coal seam roof/floor and mud are all 0.15, the absorption coefficient of the coal seam roof/floor surrounding rock is 0.08, and the thickness of the coal seam roof/floor surrounding rock is 1m, 3m and 6m, after obtaining the data through the simulation test, the relationship between the thickness of the coal seam roof or floor and the gamma ray intensity in the target mining area is obtained by fitting:
  • f is the amplitude of the gamma curve
  • the unit is API
  • Si is the gamma count value of the i-th data collection sector
  • i is an integer ranging from 1 to 8
  • N j is the data collected in Si gamma ray intensity count value at detection depth j in the sector;
  • Step 6.3 according to the relationship between the formation absorption coefficient and the gamma ray intensity established in step 6.1 and the relationship between the layer roof or floor thickness and the gamma ray intensity established in step 6.2, determine the gamma curve amplitudes of different data acquisition sectors in the target mining area,
  • the gamma curve amplitude includes an upper gamma curve amplitude and a lower gamma curve amplitude;
  • f is the amplitude of the gamma curve
  • the unit is API
  • Si is the gamma count value of the i-th data collection sector
  • i is an integer ranging from 1 to 8
  • N j is the data collected in Si gamma ray intensity count value at detection depth j in the sector;
  • the azimuth logging tool while drilling is ⁇ 0 at the initial time t 0 , and the stuck time during the rotation of the probe of the azimuth logging tool while drilling (that is, the time when the drill collar stops rotating) is t s , then the azimuth logging tool while drilling
  • the tool face angle ⁇ of the well instrument probe at time t is:
  • the rotation speed R of the azimuth logging tool while drilling is:
  • the gamma ray intensity count value of the detection depth j of the azimuth logging tool probe in the data acquisition sector can be obtained:
  • G 1j , G 2j , G 3j , and G 4j are the contributions of the gamma-ray intensity count values of the azimuth logging tool while drilling at depth j and tool face angles of 0°, 90°, 180°, and 270°, respectively
  • ⁇ is the tool face angle of the azimuth logging tool while drilling, in °
  • V is the drilling speed of the azimuth logging tool while drilling, in m/s
  • R is the rotation speed of the azimuth logging tool while drilling, The unit is r/s.
  • Step 6.4 determine the drilling distance and the vertical distance from the drill bit to the interface between the coal seam and the coal seam roof/coal seam floor as follows:
  • L is the drilling distance from the drill bit to the coal-rock interface, in m;
  • L c is the vertical distance from the drill bit to the coal-rock interface, in m;
  • L ac is the distance from the azimuth logging tool while drilling to the drill bit, in is m;
  • D is the distance from the detector at the opening point to the coal-rock interface, in m;
  • ⁇ is the angle between the coal-rock interface and the instrument, in °;
  • dep is the measurement distance, in m;
  • n is the number of measurements, n is a positive integer ⁇ 1, dev n is the measured inclination angle of the nth measurement of the azimuth logging tool while drilling, the unit is °, and the value range is 0-360°;
  • a formation is the formation dip angle, the unit is °, which is taken The value range is 0-360°;
  • D n is the distance from the detector to the coal bed interface in the nth measurement, in m, and D
  • the distance between the azimuth logging tool while drilling and the coal-rock interface is determined according to the rising or falling edge of the drilling curve, and then converted to calculate the distance from the drill bit to the interface along the drilling direction.
  • Step 6.5 according to the drilling distance and vertical distance between the coal seam and the coal seam roof/coal seam floor interface obtained in step 6.4 and the upper gamma curve amplitude and lower gamma curve amplitude obtained in step 6.3, determine the geosteering drilling in the horizontal section of the horizontal well track, and conduct geosteering drilling in the horizontal section of the horizontal well.
  • the amplitudes of the upper and lower gamma curves gradually increase and the amplitude of the upper gamma curve increases first, and the drilling distance gradually increases.
  • the drilling distance is less than 5-8m or the vertical distance
  • the amplitude of the upper and lower gamma curves exceeds 70 API, adjust the drill bit to drill along the original drilling direction with an instrument tool face angle of 180°;
  • the amplitudes of the upper and lower gamma curves gradually increase and the amplitude of the lower gamma curve increases first, and the drilling distance gradually increases.
  • the drilling distance is less than 5-8m or the vertical distance
  • the amplitude of the upper and lower gamma curves exceeds 70 API, adjust the drill bit to drill along the original drilling direction with an instrument tool face angle of 180°;
  • Step 7 Carry out segmental isolation of the horizontal section, implement directional perforation or multi-cluster directional perforation in the direction of the coal seam, and then perform segmental fracturing construction, specifically including:
  • Step 7.1 determining the number of fracturing stages and clustered perforation positions in the horizontal section of the horizontal well;
  • Step 7.2 implement multi-cluster directional perforation vertically downward/upward to the coal seam direction in the first section of the horizontal section of the horizontal well;
  • multi-cluster directional perforation is carried out in the first section of the horizontal section of the horizontal well by means of tubing or coiled tubing;
  • the operating parameters for implementing multi-cluster directional perforation vertically downward/upward to the coal seam include: perforating gun type 95 or 89, perforating charge 102, perforation density 10-16 holes/m, perforation The phase angle is vertical down/up.
  • Step 7.3 using the bridge plug staged fracturing method to perform multi-cluster temporary plugging and diverting staged fracturing construction on the first section of the horizontal section of the horizontal well;
  • the operating parameters for the multi-cluster temporary plugging diversion to staged fracturing in the first section of the horizontal section of the horizontal well include: the fracturing fluid is active water fracturing fluid, the proppant is quartz sand, and the sanding intensity of the fracturing section is 8 ⁇ 15m 3 /m, the injection displacement is 8-15m 3 /min, the average sand ratio is 10-15%, and the pre-fluid ratio is greater than or equal to 40%.
  • Step 7.4 repeating steps 7.2 and 7.3, performing perforation and fracturing construction on the remaining well sections of the horizontal section of the horizontal well until the fracturing construction of the entire well section is completed;
  • Step 7.5 After completing the fracturing construction of the whole well section, shut down the well and wait for the pressure to spread below the fracture closure pressure, then perform blowout and flowback operations.
  • Step 8 After completing multi-cluster temporary plugging and turning to staged fracturing, perform flowback, well flushing operations, and then conduct coalbed methane drainage and gas recovery.
  • Step 8.1 After the discharge and flowback operation is completed, run the milling pipe string to drill and mill the bridge plugs in a unified manner. After milling and milling all the bridge plugs in the horizontal section of the horizontal well, continue to run the milling pipe string to the artificial well of the horizontal well At the bottom, positively circulate clean water 1.5 times the volume of the wellbore of the horizontal well, and then raise the milling string;
  • Step 8.2 Carry out circular well flushing operation on the horizontal wellbore, observe the condition of the outlet fluid return until the water quality of the inlet and outlet is consistent, and the outlet liquid is clean and free of impurities, stop the well flushing operation;
  • Step 8.3. According to the requirements of drainage design specifications, install horizontal well drainage equipment, and perform drainage of horizontal wells
  • the target mining area is a certain mining area in Huaibei, which is a typical high-gas outburst mining area in my country.
  • Huaibei which is a typical high-gas outburst mining area in my country.
  • the geological structure is complex, the gas disaster is serious, and there have been many gas outbursts and explosion accidents. Therefore, It is extremely necessary to carry out gas pre-extraction before coal mining.
  • Underground rock tunnels are usually drilled through layers or horizontally drilled in coal tunnels for pre-extraction of gas. Due to the soft and low permeability of coal seams, the gas extraction volume of a single hole is low, and it takes a long time for the drainage to reach the standard, and the contradiction between mining and replacement is very prominent.
  • vertical fracturing vertical wells were used for surface coalbed methane extraction, which achieved certain results.
  • this method has disadvantages such as long extraction time, small single well control area, and large peripheral engineering volume.
  • Step 1 Due to the lack of previous geological exploration data in the target mining area, the average well spacing of exploration holes is 400-500m, and there is no requirement for full coverage of the working face. Therefore, for the target mining area, the horizontal well type is determined to be U-shaped.
  • Step 2 determining the well location of the horizontal well according to the collected exploration data
  • coal seam roof contour line where the horizontal section of the horizontal well is located should be 5m higher than the coal seam roof contour line where the vertical well is located, so as to facilitate drainage and gas recovery in the later stage.
  • Step 3 According to the exploration data of the target mining area collected in the early stage, it is determined that the Permian coal-bearing layers in the Huaibei Mine field are 19-58 layers, and there are 8 recoverable and partially recoverable coal layers.
  • the upper 3, 4, Coal seams 5, 6, and 7 are thin coal seams with high ash content and poor coal seam stability. They are only partially mineable, and the average total mineable thickness is 31.75m.
  • Coal seams 8, 9 and 10 are the main coal seams in this mining area. Coal seam 8 and coal seam 9 are relatively close in the vertical direction, so coal seam 8 and coal seam 9 are taken as the target coal seams for this CBM development, and horizontal wells are arranged in the roof rock above the top of coal seam 8.
  • the fracturing simulation test was carried out using the three-dimensional fracturing numerical simulation software MFrac Suite, and the horizontal section of the horizontal well was arranged in the roof of the coal seam to obtain the results when the distance between the horizontal section of the horizontal well and the top surface of the coal seam was 0.5m to 8m.
  • the extension data of the fracture the results are as follows:
  • the half-length of the fracture is 86.109 meters
  • the total height of the fracture is 58.964 meters
  • the height of the fracture above the top of the coal seam is 31.915 meters
  • the fracture is pressed below the top of the coal seam Height: 27.049 meters;
  • the half-length of the fracture is 80.155m
  • the total height of the fracture is 66.728m
  • the height of the fracture above the top of the coal seam is 36.361m
  • the fracture is below the top of the coal seam Height: 30.367 meters
  • the half-length of the fracture is 74.356m
  • the total height of the fracture is 74.621m
  • the height of the fracture above the top of the coal seam is 41.161m
  • the fracture is below the top of the coal seam Height: 33.46 meters.
  • the cracks can cross the interface between the coal seam and the coal seam roof to achieve penetration and expansion, thereby communicating with the lower coal seam and providing a channel for the coalbed methane to enter the wellbore.
  • the horizontal section of the horizontal well is easy to drill into the target broken soft coal seam. coal seam.
  • the height H coal of the fracture below the top surface of the coal seam is preset to be 15m. This is because the total thickness from the top surface of the 8th coal seam to the bottom surface of the 9th coal seam in the target area is about 15m on average.
  • the crack height below the top surface of the coal seam is ⁇ 15m
  • the preset fracture half-length L is 80-100m. This is due to the combination of previous construction experience and numerical simulation of production capacity in this mining area. When the half-length of the fracture is within this range, better drainage effects can be obtained.
  • the distance between the horizontal section of the horizontal well and the top of the coal seam should be less than 4.0 m, and to promote the extension of the fracture from the top of the coal seam to the lower coal seam .
  • the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam is set at 0.5-2.0 m.
  • the vertical distance between the horizontal section of the horizontal well and the top/bottom of the coal seam is 1.5 meters.
  • Step 4 Combined with the distribution map of in-situ stress in China, and according to the in-situ stress direction in the mining area obtained from cross-dipole acoustic logging interpretation, set the azimuth of the horizontal section of the horizontal well to 0°.
  • Step 5 Complete the vertical well drilling, and complete the first and second drilling of horizontal wells;
  • the vertical well belongs to the CBM parameter well and the production well, and is also the drainage well of the later horizontal well.
  • the designed well depth of the vertical well is 816.00m, and the actual drilling depth is 806.00m. In the process of drilling, it is necessary to obtain the coal seam's buried depth, thickness, structure and other qualitative parameters, and to complete the coalbed gas content and injection/pressure drop well test.
  • the vertical well adopts a two-hole structure.
  • the first spud was drilled to a depth of 256.50m, and the second spud was completed to a depth of 806.00m. After drilling, the ⁇ 177.8mm production casing was run in and cemented.
  • fiberglass casing is used to complete the well at 724.9m ⁇ 733.15m, and a total of 4.50m of holes are reamed at 726.65 ⁇ 731.15m, and the diameter of the hole is 0.5m.
  • the pilot well perform logging while drilling, determine the pilot horizon, fill the pilot well, and run casing cementing after sidetracking to the landing point;
  • the vertical distance is about 1.5m.
  • the inclined pilot hole is drilled first to detect the position of the coal seam, so as to realize the precise control of the landing point when drilling the horizontal section of the horizontal well.
  • Step 6 Use the azimuth gamma geosteering method to carry out geosteering drilling in the horizontal section of the horizontal well, adjust the drilling trajectory of the horizontal section of the horizontal well in real time, so that the drilling trajectory of the drill bit is located in the target layer described in step 3, and make the Keep the vertical distance determined in step 3 between the horizontal section of the horizontal well and the top/bottom of the coal seam;
  • Step 6.1 establishing the relationship between the formation absorption coefficient and the gamma ray intensity in the target mining area
  • Step 6.2 establishing the relationship between the thickness of the coal seam roof or floor and the gamma ray intensity in the target mining area;
  • Step 6.3 according to the relationship between the formation absorption coefficient and the gamma ray intensity established in step 6.1 and the relationship between the layer roof or floor thickness and the gamma ray intensity established in step 6.2, determine the gamma curve amplitudes of different data acquisition sectors in the target mining area,
  • the gamma curve amplitude includes an upper gamma curve amplitude and a lower gamma curve amplitude;
  • Step 6.4 determining the drilling distance and the vertical distance from the drill bit to the interface between the coal seam and the coal seam roof/coal seam floor;
  • Step 6.5 according to the drilling distance and vertical distance between the coal seam and the coal seam roof/coal seam floor interface obtained in step 6.4 and the upper gamma curve amplitude and lower gamma curve amplitude obtained in step 6.3, determine the geosteering drilling in the horizontal section of the horizontal well track, and conduct geosteering drilling in the horizontal section of the horizontal well.
  • the azimuth logging tool when used for geosteering drilling in the horizontal section of a horizontal well, the azimuth logging tool is set on the drill collar to obtain the real-time gamma curve amplitude, the gamma
  • the curve amplitude includes the upper gamma curve amplitude and the lower gamma curve amplitude. See Figure 8 for specific results. It should be noted that the depth unit in Figure 8 is 100 meters.
  • the drill bit first drilled along the coal seam.
  • the amplitudes of the upper and lower gamma curves gradually increased, and the amplitude of the upper gamma curve was earlier than that of the lower gamma curve.
  • the drilling distance first decreased and then increased, indicating that the drill bit encountered the coal-rock interface at this time.
  • the amplitude of the upper and lower gamma curves reached about 78API and the drilling depth reached about 800 meters, the The amplitude of the gamma curve no longer changes, indicating that the drill bit has penetrated the coal seam and drilled in the coal seam roof stratum.
  • the drilling trajectory of the drill bit needs to be adjusted, and the tool face angle is 180° along the original drilling direction. drilled into.
  • the amplitude of the upper and lower gamma curves does not change, indicating that the drill bit is still drilling in the surrounding rock of the coal seam roof at this time.
  • the amplitude of the upper and lower gamma curves gradually changes from about 78API to less than 20API, and the amplitude of the lower gamma curve decreases before the amplitude of the upper gamma curve.
  • the encounter distance decreased first and then increased, and the amplitude of the upper/lower gamma curves decreased significantly, indicating that the drill bit entered the coal seam with low radioactivity at this time.
  • the amplitude of the upper and lower gamma curves gradually increases again, and the amplitude of the lower gamma curve becomes larger than that of the upper gamma curve, indicating that the drill bit is drilling into the coal seam floor at this time, and the amplitude of the upper and lower gamma curves increases gradually.
  • the amplitude of the horse curve reaches about 120 API, and the amplitude of the upper and lower gamma curves does not change, indicating that the drill bit is drilling in the surrounding rock of the coal seam floor at this time, and the drilling trajectory of the drill bit needs to be adjusted.
  • Step 7 Carry out segmental isolation on the horizontal section of the horizontal well, implement directional perforation or multi-cluster directional perforation toward the coal seam, and then perform multi-cluster temporary plugging and turn to segmental fracturing construction;
  • the length of the horizontal section of the horizontal well to be laid is about 800m
  • the spacing of the fracturing sections is 80m
  • the temporary plugging perforation clusters in each section are 3 clusters
  • the cluster spacing is 20m.
  • the running position is 20m away from the shaft of the vertical well.
  • the first section of the horizontal section of the horizontal well is perforated by the cable pumping bridge plug perforation joint method.
  • the pressure test is carried out for 5 minutes.
  • the setting of a section of perforation cluster is completed; then lift up the perforating gun to ignite and perforate; continue to lift up the perforating gun to the perforation position of the second cluster of the first section, and ignite and perforate; continue to lift up the perforating gun to the first
  • the perforation position of the third cluster in the section, ignition perforation, a total of 3 perforation clusters, each section of perforation 1.5m, the perforation density is 14 holes/m.
  • the pre-fluid, sand-carrying fluid and displacement fluid are pumped to carry out the first stage of fracturing construction.
  • the proportion of pre-fluid is 40%, and the formula of fracturing fluid is: clean water + 1% KCl + 0.05% sterilization
  • the proppant is Lanzhou quartz sand, the fracturing fluid injection displacement is 10m 3 /min, and the average sand ratio is 10-15%.
  • Fig. 9 shows the fracture morphology when injected at a constant displacement of 30mL/min
  • Fig. 10 The fracture shape is given when the initial displacement is 30ml/min and the coal seam is immediately switched to 20ml/min 30s after the fracture.
  • the injection displacement of the pre-stage fluid is 11-15 m 3 /min at the initial stage, and then decreases to 8-10m 3 /min, so as to achieve the purpose of "high displacement to promote fracture penetration, medium and low displacement to promote uniform crack expansion".
  • the fractures can penetrate and spread across the coal-rock interface, connecting the wellbore and the lower coal seam, but in the case of injection at a constant displacement of 30mL/min, the fracture area in the roof of the coal seam is small , and when the initial discharge rate is 30ml/min, and the coal seam is immediately switched to 20ml/min 30s after the coal seam breaks, the cracks on the roof of the coal seam and in the coal seam are more fully developed, and the fracture area is larger, which is more conducive to later drainage.
  • Step 8 After completing multi-cluster temporary plugging and turning to staged fracturing, perform flowback, well cleaning operations, and then conduct coalbed methane drainage and gas recovery.
  • Step 8.1 After the discharge and flowback operation is completed, run the milling pipe string to drill and mill the bridge plugs in a unified manner. After milling and milling all the bridge plugs in the horizontal section of the horizontal well, continue to run the milling pipe string to the artificial well of the horizontal well At the bottom, positively circulate clean water 1.5 times the volume of the wellbore of the horizontal well, and then raise the milling string;
  • Step 8.2 Carry out circular well flushing operation on the horizontal wellbore, observe the situation of the outlet liquid return until the water quality of the inlet and outlet is consistent, and the outlet liquid is clean and free of impurities and dirt, stop the well flushing operation;
  • Step 8.3. According to the requirements of drainage design specifications, install horizontal well drainage equipment, and perform drainage operations on horizontal wells.

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Abstract

本发明公开一种碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，包括：根据勘探数据和矿井数据确定水平井井型；根据勘探数据和矿井数据确定水平井井位；根据勘探数据和矿井数据确定水平井水平段在煤层顶板或底板的目标层位，以及水平井水平段与煤层顶/底面的垂直距离；根据目标矿区最小水平主应力方向确定水平井水平段的布设方位角；完成直井段钻井；利用方位伽玛地质导向方法进行水平井水平段地质导向钻进，实时调整水平井水平段的钻进轨迹；对水平井水平段进行分段封隔、向煤层方向实施定向射孔或多簇定向射孔，进行多簇暂堵转向分段压裂施工；完成多簇暂堵转向分段压裂施工后进行返排、通洗井、煤层气排水采气。

Description

碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法 技术领域
本发明属于煤层气抽采技术领域，具体涉及一种碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法。
背景技术
煤层气主要以吸附的形式储集于煤层中。在煤层气的抽采过程中，煤层中吸附的气体经过解吸转换为游离态气体，之后通过煤层中微孔扩散、裂缝渗流运移至煤层气井中，从而实现煤层气的抽采。碎软低渗煤层具有孔隙度低、渗透率低、煤层碎软等特征，在我国石炭-二叠纪煤层中分布广泛，但是煤层气抽采效率低。目前常用的压裂直井产气量低，无法满足煤层气高效开发的要求，也无法达到地面预抽煤层气快速降低煤矿煤层气(瓦斯)含量和压力的目的。随着我国煤层气勘探开发技术的迅速发展以及对煤层气快速、高效抽采的技术需求，煤层气压裂技术也逐渐由直井压裂向水平井分段压裂技术方向发展，相比于直井压裂改造，水平井分段压裂可以大大提高单井泄流面积，有效提高煤层气单井的产气量。因此，亟需一种针对碎软低渗煤层的煤层气抽采方法，实现煤层气的高效抽采。
现有技术中所采用的煤层气分段压裂水平井强化抽采方法针对构造软煤煤层气开发问题，提出将水平井的井眼轨迹控制在煤层顶界以上0.5m～1.5m之间的泥岩或砂岩顶板中，通过在水平井段分段向下定向射孔和分段压裂，达到强化抽采构造软煤煤层气的目的。该方法对水平井井眼轨迹控制较为苛刻，不易控制，所需的边界探测技术成本高。此外，将水平井布置在泥岩层中，由于泥岩较强的塑性、起裂压力高、裂缝延伸困难，不利于裂缝跨界面穿层扩展沟通井筒与煤层，而且现有技术中采用的向下定向射孔分段压裂，为降低成本，往往设计的压裂段间距较大，从而导致压裂缝少、抽采效率低、不利于发挥长水平段优势。为提高压裂段数，目前也有采用段内多簇射孔压裂的手段，但是由于多簇射孔孔眼对压裂液的分流作用以及缝间应力干扰，致使部分射孔簇裂缝不能实现穿层扩展，形成无效裂缝。
发明内容
针对现有技术中的缺陷和不足，本发明提供了一种碎软低渗煤层顶板或底板 分段压裂水平井煤层气抽采方法，以解决现有技术中碎软低渗煤层中煤层气抽采效率低的技术问题。
为达到上述目的，本发明采取如下的技术方案：
碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，该方法包括以下步骤：
步骤1、收集目标矿区的勘探数据，并根据收集到的勘探数据和矿井数据确定水平井井型，所述水平井包括直井段和水平井水平段，所述水平井井型包括L型、U型和W型；
步骤2、根据收集到的勘探数据和矿井数据确定水平井井位；
步骤3、根据收集到的勘探数据和矿井数据确定水平井水平段在煤层顶板或底板中的目标层位，以及水平井水平段与煤层顶/底面的垂直距离；
步骤4、根据前期勘探或测试得到的目标矿区最小水平主应力方向确定水平井水平段的布设方位角；
步骤5、完成直井钻井，完成水平井一开、二开钻井和固井；
步骤6、利用方位伽玛地质导向方法进行水平井水平段地质导向钻进，实时调整水平井水平段的钻进轨迹，使钻头的钻进轨迹位于步骤3所述的目标层位中，并使水平井水平段与煤层顶/底面之间保持步骤3所确定的垂直距离；
当井型为L型井时，钻进至水平井水平段达到预设长度后下套管固井；
当井型为U型或W型时，钻进至水平井水平段与直井段对接后下套管固井；
步骤7、对水平井水平段进行分段封隔、向煤层方向实施定向射孔或多簇定向射孔，然后进行多簇暂堵转向分段压裂施工；
步骤8、完成多簇暂堵转向分段压裂施工后进行返排、通洗井作业，然后进行煤层气排水采气。
本发明还具有以下技术特征：
具体的，步骤3所述的水平井水平段与煤层顶/底面的垂直距离通过以下步骤确定：
步骤3.1、根据收集到的勘探数据和矿井数据，通过数值模拟确定水平井水平段在目标层位内的压裂缝延伸数据，所述压裂缝延伸数据包括压裂缝总高度、压裂缝半长、压裂缝上部裂缝高度、压裂缝下部裂缝高度；
步骤3.2、根据得到的压裂缝延伸数据，确定水平井水平段与煤层顶/底面的垂直距离。
更进一步的，所述的水平井水平段与煤层顶/底面的垂直距离为0.5～8米。
更进一步的，所述的水平井水平段与煤层顶/底面的垂直距离为1.5米。
更进一步的，所述测试得到的目标矿区最小水平主应力方向通过小型压裂测试、偶极子声波测井或应力解除法测得。
更进一步的，步骤4所述的水平井水平段布设方位角为-15°～15°。
更进一步的，步骤4所述的水平井水平段布设方位角为0°。
更进一步的，所述步骤5具体包括：
当井型为L型井时，完成直井钻井，然后水平井一开、二开钻井钻进至着陆点后下套管注水泥固井；
当井型为U型时，完成直井钻井后下套管注水泥固井，然后进行水平井一开、二开钻井，钻进至着陆点后再下套管注水泥固井；
当井型为W型时，完成直井钻井后下套管注水泥固井，然后分别完成两口水平井的一开、二开钻井，钻进至着陆点后下套管注水泥固井。
更进一步的，步骤6所述的进行水平井水平段地质导向钻进具体包括：
步骤6.1、建立目标矿区内地层吸收系数与伽马射线强度关系；
步骤6.2、建立目标矿区内煤层顶板或底板厚度与伽马射线强度关系；
步骤6.3、根据步骤6.1建立的地层吸收系数与伽马射线强度关系和步骤6.2建立的层顶板或底板厚度与伽马射线强度关系，确定目标矿区内不同数据采集扇区的伽马曲线幅值，所述伽马曲线幅值包括上伽马曲线幅值和下伽马曲线幅值；
步骤6.4、确定钻头到煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离；
步骤6.5、根据步骤6.4得到的煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离及步骤6.3得到的上伽马曲线幅值和下伽马曲线幅值，确定水平井水平段地质导向钻进轨迹，并进行水平井水平段地质导向钻进。
更进一步的，所述的步骤6.1中目标矿区内地层吸收系数与伽马射线强度关系为：
Figure PCTCN2021140941-appb-000001
式中，J为伽马射线强度，单位为MeV，ac为地层吸收系数，η为位置参数，σ为尺度参数。
更进一步的，所述的步骤6.2的目标矿区内煤层顶板或底板厚度与伽马射线强度关系为：
Figure PCTCN2021140941-appb-000002
式中，J为伽马射线强度，单位为MeV，H为煤层顶板或底板厚度，单位为m，η为位置参数，σ为尺度参数。
更进一步的，所述的步骤6.3计算不同数据采集扇区的伽马曲线幅值为：
f(Si)＝J(H)*J(ac)*N j
式中，f为伽马曲线幅值，单位为API，Si为第i个数据采集扇区的伽马计数值，且i为取值范围为1～8的整数，N j为在Si数据采集扇区内的探测深度j的伽马射线强度计数值；
Figure PCTCN2021140941-appb-000003
其中，G 1j、G 2j、G 3j、G 4j分别为随钻方位测井仪在深度j且工具面向角为0°、90°、180°、和270°的伽马射线强度计数值的贡献值，θ为随钻方位测井仪的工具面向角，V为随钻方位测井仪的钻进速度，单位为m/s，R为随钻方位测井仪的转数，单位为r/s。。
更进一步的，所述的步骤6.4确定钻头到煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离如下：
L＝D/sin(α)-L ac
L c＝L·sin(α)
D n+1＝D n-dep·sin(α)
其中，α＝α 地层-dev n
式中，L为钻头到煤岩界面的钻遇距离，单位为m；L c为钻头到煤岩界面的垂直距离，单位为m；L ac为随钻方位测井仪到钻头的距离，单位为m；D为开孔点处探测器到煤岩层界面距离，单位为m；α为煤岩层界面与仪器的夹角，单位为°；dep为测量间距，单位为m；n为测量次数，n为≥1的正整数，dev n为随钻方位测井仪第n次测量的测量倾角，单位为°，取值范围为0～360°；a 地 层为地层倾角，单位为°，取值范围0～360°；D n为第n次测量时探测器到煤岩层界面距离，单位为m，D n+1为第n+1次测量时探测器到煤岩层界面距离，单位为m。
更进一步的，所述的步骤6.5具体包括：
当从煤层钻进到顶板岩层时，上下伽马曲线幅值均逐渐变大且上伽马曲线幅值先增大，并且钻遇距离逐渐增大，当钻遇距离小于5～8m或垂直距离为0.1～0.5m、上下伽马曲线幅值均超过70API时，调整钻头沿原钻进方向仪器工具面向角180°钻进；
当从煤层钻进到底板岩层时，上下伽马曲线幅值均逐渐变大且下伽马曲线幅值先增大，并且钻遇距离逐渐增大，当钻遇距离小于5～8m或垂直距离为0.1～0.5m、上下伽马曲线幅值均超过70API时，调整钻头沿原钻进方向仪器工具面向角180°钻进；
当顺煤层钻进时，若上下伽马曲线幅值均超过70API，则说明钻头进入煤层顶板/底板；若上下伽马曲线幅值均小于50API时，则说明钻头顺煤层钻进。
更进一步的，步骤7具体包括以下子步骤：
步骤7.1、确定水平井水平段的压裂段数及分簇射孔位置；
步骤7.2、对水平井水平段的第一段向煤层方向垂直向下/向上实施多簇定向射孔；
当井型为L型井时，对水平井水平段的第一段采用油管或连续油管输送方式实施多簇定向射孔；
当井型为U型或W型时，对水平井水平段的第一段采用电缆泵送桥塞射孔联作方式实施多簇定向射孔；
步骤7.3、采用桥塞分段压裂方式，对水平井水平段的第一段进行多簇暂堵转向分段压裂施工；
步骤7.4、重复步骤7.2和步骤7.3，对水平井水平段的剩余井段进行射孔压裂施工，直至完成全井段压裂施工；
步骤7.5、完成全井段压裂施工后，关井等待压力扩散至裂缝闭合压力以下时，进行放喷返排作业。
更进一步的，步骤7.2所述的向煤层方向垂直向下/向上实施多簇定向射孔的操作参数包括：射孔枪为95型或89型、射孔弹为102型、射孔孔密为10～16孔/米、射孔相位角为垂直向下/向上。
更进一步的，步骤7.3所述的对水平井水平段的第一段进行多簇暂堵转向分段压裂施工操作参数包括：压裂液为活性水压裂液，支撑剂为石英砂，压裂段的加砂强度为8～15m 3/m，注入排量为8～15m 3/min，平均砂比为10～15％，前置液比大于等于40％。
更进一步的，对于U型水平井或W型水平井，水平井水平段钻进的着陆点标高比水平井水平段与直井段对接处的标高高3～20m；对于L型水平井，水平井水平段钻进的着陆点标高比水平井水平段末端的标高低3～20m。
更进一步的，所述步骤8具体包括：
步骤8.1、放喷返排作业结束后，下入磨铣管柱将桥塞统一钻铣，将水平井水平段的所有桥塞全部磨铣结束后，继续下磨铣管柱至水平井人工井底，正循环1.5倍水平井井筒容积的清水，然后提出磨铣管柱；
步骤8.2、对水平井筒进行循环洗井作业，观察出口返出液情况至进出口水质一致，且出口液体干净无杂质污物，停止洗井作业；
步骤8.3、按照排采设计规范要求，安装水平井排采设备，对水平井进行排采作业。
本发明与现有技术相比，有益的技术效果是：
(1)本发明根据不同的地质条件以及煤层气抽采要求，选择不同的水平井井型，有利于因地制宜、节约成本、实现效益最大化。
(2)本发明方法设置了水平井着陆点标高与水平井末端的标高差值，有利于水平段内的积水在重力作用下流向直井井底，从而有利于快速排水采气。
(3)本发明方法通过水平井水平段布设方位角的设置可保证水力压裂时形成与水平段井筒垂直的横向缝，由于碎软煤层整体富气低渗，而横向缝能够保证 水平井具有较高的产能，实现煤矿煤层气的高效抽采。
(4)本发明方法利用方位伽玛地质导向方法进行水平井水平段地质导向钻进，通过在钻进过程中实时调整钻进轨迹，有效解决了实际钻进轨迹不能实时监测，且钻进轨迹调整过程存在繁琐、不直观和有效性不足的问题，采用本发明方法，水平井水平段在目标层位的钻遇率达到95％以上。
(5)本发明方法通过多簇暂堵转向分段压裂工艺实现煤层气储层的改造，有利于实现对碎软煤层的密切割多簇分段压裂体积改造，从而形成复杂网络裂缝，获得更大的改造体积，提高单井产量。
附图说明
图1为本发明方法的流程图；
图2为水平井钻进方向与裂缝延伸的关系示意图；
图3为水平井水平段以不同布设方位角设置的单裂缝延伸形态示意图；
其中，a为水平井水平段的布设方位角为0°，b为水平井水平段的布设方位角为15°，c为水平井水平段的布设方位角为45°，d为水平井水平段的布设方位角为60°；
图4为水平井水平段以不同布设方位角设置的双裂缝延伸形态示意图；
图5为本发明的地层吸收系数与伽马射线强度关系示意图；
图6为本发明的方位伽玛地质导向中钻头到界面的距离示意图；
图7为本发明的地质导向钻进轨迹示意图；
图8为实施例1中随钻方位测井仪到钻头的距离为1米时得到的上下伽马曲线幅值及钻进轨迹；
图9为实施例1中对水平井水平段进行多簇笼统压裂的效果图；
图10为实施例1中对水平井水平段进行多簇暂堵转向分段压裂的效果图。
以下结合说明书附图和具体实施方式对本发明做具体说明。
具体实施方式
本发明所设计的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，在碎软低渗煤层顶板或底板使用分段压裂水平井抽采煤层气，首要任务是明确目标矿区煤层顶底板界面、煤层构造及位置，其次是要获取煤层的含水量和含气量，以及煤层顶板或底板的岩性、厚度、煤层岩石力学参数和物性参数等数据；优选 水平井方位与最小水平主应力方向平行布设；在采用随钻方位测井仪测量的过程中，利用设置在随钻方位测井仪上的探头实现方位测量，测量的方位数据分8个扇区记录，然后合成上下左右四个方位自然伽马强度测量值实时上传到地面，根据得到的数据实时调整水平井水平段的钻进轨迹，完成钻进；最后实施定向射孔或多簇定向射孔，进行多簇暂堵转向分段压裂施工、煤层气排水采气。
对本发明所涉及的技术术语解释如下：
U型水平井：由一口直井和一口水平对接井组成。
L型水平井：由单一水平井组成的水平井。
W型水平井：由一口直井和两口水平井对接组成。
压裂缝上部高度：指水平井水平段井筒下方的裂缝高度。
压裂缝下部高度：指水平井水平段井筒上方的裂缝高度。
钻遇距离：钻头沿钻进方向到煤岩界面的距离。
垂直距离：钻头到煤岩界面的垂线距离。
水平井水平段的布设方位角：水平井水平段的布设方向与最小水平主应力方向之间构成的锐角。
电缆泵送桥塞射孔联作：适用于套管固井完井方式，将电缆泵送射孔工艺与桥塞分段压裂工艺相结合，可实现电缆传输一趟完成桥塞坐封与射孔作业，可在带压情况下实现多段连续压裂施工。通过电缆泵送方式将连载一起的桥塞和射孔枪联作工具送至桥塞坐封位置后点火坐封，再上提射孔枪至射孔位置后进行射孔作业，射孔作业后起出电缆进行光套管压裂，压后再进行下一级泵送桥塞射孔联作以及压裂施工作业，重复以上步骤完成全井段的射孔压裂施工作业。
遵从上述技术方案，以下给出本发明的具体实施例，需要说明的是本发明并不局限于以下具体实施例，凡在本申请技术方案基础上做的等同变换均落入本发明的保护范围。下面结合实施例对本发明做进一步详细说明。
本实施例给出了一种碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，包括以下步骤：
步骤1、收集目标矿区的勘探数据和矿井数据，并根据收集到的勘探数据和矿井数据确定水平井井型，所述水平井包括直井段和水平井水平段，所述水平井井型包括L型、U型和W型；
其中，目标矿区的勘探数据和矿井数据具体包括：目标矿区的地形条件、地质条件、矿区煤炭采掘规划、井下巷道布置情况等。
收集地质条件数据可以了解煤层构造情况，指导水平井井型的选择。
收集矿区煤炭采掘规划、井下巷道布置情况能够初步了解煤矿井下开采情况，判断煤矿采掘区与目标抽采区域的距离，判断是否需要实现水平井水平段在工作面长度方向的全覆盖。
对于地处平原的矿区，如果目标矿区的前期研究程度低或地质构造复杂，优选U型水平井。U型水平井的优势在于，可通过直井探测煤层位置；在分段压裂作业后、排采作业前的通洗井作业方便；在直井中进行排采作业，大大降低了排采杆管偏磨以及煤粉产出对排采工序的影响。
如果目标矿区的前期研究程度高或地质构造简单，优选L型水平井。L型水平井的优势在于，可节约钻井成本、减少修路费用、减少占地面积。
为实现水平井水平段在工作面长度方向的全覆盖，优选W型水平井。W型水平井的优势在于：能够利用一口直井同时对两口水平井进行排采。对于W型水平井，在直井钻井结束后，分别从工作面的两端向直井方向钻进两口水平井，并将两口水平井与直井对接连通，在直井中进行排采作业。
步骤2、根据收集到的勘探数据和矿井数据确定水平井井位；
具体包括：根据收集到的目标矿区的地质与煤储层数据、目标矿区的煤炭采掘规划来确定水平井井位，所述目标矿区的煤储层数据包括煤层厚度、煤层气含量、煤层渗透率。首先，根据收集到的目标矿区的地质条件及煤层气勘探开发数据，编绘主采煤层底板等高线、煤层厚度等值线、煤层含气量等值线、煤层渗透率等值线等基础图件，综合评价井田地面煤层气抽采的地质条件，确定水平井的靶区及井位：将水平井布置在煤层厚度、煤层含气量、煤层渗透率均较高的区域。
步骤3、根据收集到的勘探数据和矿井数据确定水平井水平段在煤层顶板或底板中的目标层位，以及水平井水平段与煤层顶/底面的垂直距离；
步骤3.1、根据收集到的勘探数据和矿井数据，通过数值模拟确定水平井水平段在目标层位内的压裂缝延伸数据，所述压裂缝延伸数据包括压裂缝总高度、压裂缝半长、压裂缝上部裂缝高度、压裂缝下部裂缝高度；
具体包括：对于设置在煤层顶板中的水平井，根据水平井水平段井筒距离煤 层顶面的距离d，计算得到煤层顶面以上裂缝高度H roof，以及煤层顶面以下裂缝高度H coal，计算方法为：
H roof＝H up+d
H coal＝H down-d
对于设置在煤层底板中的水平井，根据水平井水平段井筒距离煤层底面的距离d，计算得到煤层底面以上裂缝高度H coal，以及煤层底面以下裂缝高度H floor，计算方法为：
H coal＝H up-d
H floor＝H down+d
步骤3.2、根据得到的压裂缝延伸数据，确定水平井水平段与煤层顶/底面的垂直距离。
作为一种优选方案，水平井水平段与煤层顶/底面的垂直距离为0.5～8米。
作为一种优选方案，所述的水平井水平段与煤层顶/底面的垂直距离为1.5米。
步骤4、根据前期勘探或测试得到的目标矿区最小水平主应力方向确定水平井水平段的布设方位角；
具体的，所述测试得到的目标矿区最小水平主应力方向通过小型压裂测试、偶极子声波测井或应力解除法测得。
更进一步的，所述水平井水平段布设方位角为-15°～15°，
作为本实施例的优选方案，水平井水平段布设方位角为0°。
如图2所示，在水平井水平段布设方位角为0°时，水力压裂后将形成与水平井水平段垂直的横向裂缝。当水平井水平段的走向与煤层最小水平主应力方向垂直，即水平井水平段布设方位角为90°时，水力压裂将形成与水平段井筒平行的纵向裂缝。
水平井水平段以不同布设方位角布设后生成的单裂缝的延伸情况如图3所示，当水平井与最小水平主应力方向夹角为0°～15°时，压裂时形成与井筒垂直的横向裂缝与地层具有较大的接触面积；随着夹角的增大(夹角达到45°～60°)，裂缝起裂后发生转向，最终仍旧沿着最大水平主应力方向延伸，但裂缝发生转向，夹角越大，裂缝转向距离越大。裂缝转向处裂缝宽度小，不利于支撑剂在裂缝内 的运移，并且裂缝起裂时近井弯曲摩阻较大，导致地面施工时的起裂压力升高，不利于压裂施工。
水平井水平段以不同布设方位角布设后生成的双裂缝的延伸情况如图4所示，当水平井水平段布设方位角为0°时，先压裂缝对后压裂缝的延伸并无明显影响，两条裂缝均能获得充分的延伸，从而有利于煤层气井产能的提高。而当水平井水平段布设方位角为45°时，后压裂缝由于受到先压裂缝的影响而在两翼出现不对称的情况，裂缝上翼延伸受限，压裂效果变差。
因此，水平井水平段方位与最小水平主应力方向夹角的范围为-15°～15°，优选的夹角为0°，设置水平井水平段布设方位角的主要目的是获得最佳裂缝延伸效果和产气效果。
步骤5、完成直井钻井，完成水平井一开、二开钻井；
当水平井井型为U型或W型时，首先在地面钻设直井至煤层，并对煤层取样测试、测井、试井，然后继续钻进至煤层下的预设位置，开展地应力测井获取直井地应力剖面，下套管固井；然后进行水平井直井段钻井，再施工导眼井、随钻测井、确定导向层位、填埋导眼井，并在侧钻至着陆点后下套管固井；
当井型为L型时，首先进行水平井直井段钻井，然后依次施工导眼井、随钻测井、确定导向层位、填埋导眼井，并在侧钻至着陆点后下套管固井。
对于U型水平井或W型水平井，水平井水平段钻进的着陆点标高比水平井水平段与直井段对接处的标高高3～20m；对于L型水平井，水平井水平段钻进的着陆点标高比水平井水平段末端的标高低3～20m。
步骤6、利用方位伽玛地质导向方法进行水平井水平段地质导向钻进，实时调整水平井水平段的钻进轨迹，使钻头的钻进轨迹位于步骤3所述的目标层位中，并使水平井水平段与煤层顶/底面之间保持步骤3所确定的垂直距离；
当井型为L型井时，钻进至水平井水平段达到预设长度后下套管固井；
当井型为U型或W型时，钻进至水平井水平段与直井段对接后下套管固井；
在煤层气的勘探开发中，随钻方位仪器随钻进设备顺着煤层钻进，煤层顶板/底板的天然放射性与煤层差异较大，当随钻方位仪钻遇到界面时，能够通过上下伽马的幅度变换顺序及测斜数据判别界面，并及时调整钻头方向，维持钻进设备顺煤层钻进。
具体包括：
步骤6.1、建立目标矿区内地层吸收系数与伽马射线强度关系；
随钻方位仪器在钻孔中测量，除受钻进层的伽马射线正常影响外，还受到煤层顶板或煤层底板围岩性质、煤层顶板/底板厚度和地层吸收系数的影响，当煤层顶板或煤层底板围岩性质、煤层顶板/底板厚度和地层吸收系数不同时，伽马射线的强度也不相同。如图5所示，当地层吸收系数分别为0.08、0.085、0.09、0.1时，伽马射线强度随吸收系数的减小而增大。
设钻孔中煤层顶板或底板厚度相同，吸收系数分别为0.08、0.085、0.09、0.1，通过模拟试验得到伽马射线强度的变化曲线，然后通过拟合得到目标矿区内地层吸收系数与伽马射线强度的关系：
Figure PCTCN2021140941-appb-000004
式中，J为伽马射线强度，单位为MeV，H为煤层顶板或底板厚度，单位为m，η为位置参数，σ为尺度参数。
步骤6.2、建立目标矿区内煤层顶板或底板厚度与伽马射线强度关系；
设钻孔中介质的吸收系数相同，煤层、煤层顶板/底板和泥浆的吸收系数都为0.15，煤层顶板/底板围岩的吸收系数为0.08，煤层顶板/底板围岩厚度分别为1m、3m和6m，通过模拟试验获取数据后，拟合得到目标矿区内煤层顶板或底板厚度与伽马射线强度关系为：
Figure PCTCN2021140941-appb-000005
式中，f为伽马曲线幅值，单位为API，Si为第i个数据采集扇区的伽马计数值，且i为取值范围为1～8的整数，N j为在Si数据采集扇区内的探测深度j的伽马射线强度计数值；
步骤6.3、根据步骤6.1建立的地层吸收系数与伽马射线强度关系和步骤6.2建立的层顶板或底板厚度与伽马射线强度关系，确定目标矿区内不同数据采集扇区的伽马曲线幅值，所述伽马曲线幅值包括上伽马曲线幅值和下伽马曲线幅值；
f(Si)＝J(H)*J(ac)*N j
式中，f为伽马曲线幅值，单位为API，Si为第i个数据采集扇区的伽马计数值，且i为取值范围为1～8的整数，N j为在Si数据采集扇区内的探测深度j的伽马射线强度计数值；
若随钻方位测井仪的探头在初始时刻t 0角度为θ 0，随钻方位测井仪探头转动过程中的遇卡时间(即钻铤停止转动时间)为t s，则随钻方位测井仪探头在t时刻的工具面向角θ为：
Figure PCTCN2021140941-appb-000006
若θ大于2π，则将θ对2π取余，得到测井仪探头此时所处Si扇区的伽马计数值。
若随钻方位测井仪的采样间隔为L 0，钻头的钻进速度为V，则在一个采样间隔内，随钻方位测井仪探头转动的转数R为：
Figure PCTCN2021140941-appb-000007
将R和θ代入下式，即可求得随钻方位测井仪探头在数据采集扇区内的探测深度j的伽马射线强度计数值：
Figure PCTCN2021140941-appb-000008
其中，G 1j、G 2j、G 3j、G 4j分别为随钻方位测井仪在深度j且工具面向角为0°、90°、180°、和270°的伽马射线强度计数值的贡献值，θ为随钻方位测井仪的工具面向角，单位为°，V为随钻方位测井仪的钻进速度，单位为m/s，R为随钻方位测井仪的转数，单位为r/s。
步骤6.4、确定钻头到煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离如下：
L＝D/sin(α)-L ac
L c＝L·sin(α)
D n+1＝D n-dep·sin(α)
其中，α＝α 地层-dev n
式中，L为钻头到煤岩界面的钻遇距离，单位为m；L c为钻头到煤岩界面的垂直距离，单位为m；L ac为随钻方位测井仪到钻头的距离，单位为m；D为开孔点处探测器到煤岩层界面距离，单位为m；α为煤岩层界面与仪器的夹角，单位为°；dep为测量间距，单位为m；n为测量次数，n为≥1的正整数，dev n为随钻方位测井仪第n次测量的测量倾角，单位为°，取值范围为0～360°；a 地 层为地层倾角，单位为°，取值范围0～360°；D n为第n次测量时探测器到煤岩层界面距离，单位为m，D n+1为第n+1次测量时探测器到煤岩层界面距离，单位为m。
由于随钻方位测井仪的直径与随钻方位测井仪的探测深度大小相近，所以在计算时不能忽略。根据钻井曲线的上升或下降沿来确定随钻方位测井仪与煤岩界面的距离，进而转换为计算钻头到界面沿钻进方向上的距离。
步骤6.5、根据步骤6.4得到的煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离及步骤6.3得到的上伽马曲线幅值和下伽马曲线幅值，确定水平井水平段地质导向钻进轨迹，并进行水平井水平段地质导向钻进。
当从煤层钻进到顶板岩层时，上下伽马曲线幅值均逐渐变大且上伽马曲线幅值先增大，并且钻遇距离逐渐增大，当钻遇距离小于5～8m或垂直距离为0.1～0.5m、上下伽马曲线幅值均超过70API时，调整钻头沿原钻进方向仪器工具面向角180°钻进；
当从煤层钻进到底板岩层时，上下伽马曲线幅值均逐渐变大且下伽马曲线幅值先增大，并且钻遇距离逐渐增大，当钻遇距离小于5～8m或垂直距离为0.1～0.5m、上下伽马曲线幅值均超过70API时，调整钻头沿原钻进方向仪器工具面向角180°钻进；
当顺煤层钻进时，若上下伽马曲线幅值均超过70API，则说明钻头进入煤层顶板/底板；若上下伽马曲线幅值均小于50API时，则说明钻头顺煤层钻进。
步骤7、对水平段进行分段封隔、向煤层方向实施定向射孔或多簇定向射孔，然后进行分段压裂施工，具体包括：
步骤7.1、确定水平井水平段的压裂段数及分簇射孔位置；
步骤7.2、对水平井水平段的第一段向煤层方向垂直向下/向上实施多簇定向射孔；
当井型为L型井时，对水平井水平段的第一段采用油管或连续油管输送方式实施多簇定向射孔；
当井型为U型或W型时，对水平井水平段的第一段采用电缆泵送桥塞射孔联作方式实施多簇定向射孔；
向煤层方向垂直向下/向上实施多簇定向射孔的操作参数包括：射孔枪为95型或89型、射孔弹为102型、射孔孔密为10～16孔/米、射孔相位角为垂直向下/向上。
步骤7.3、采用桥塞分段压裂方式，对水平井水平段的第一段进行多簇暂堵转向分段压裂施工；
对水平井水平段的第一段进行多簇暂堵转向分段压裂施工操作参数包括：压裂液为活性水压裂液，支撑剂为石英砂，压裂段的加砂强度为8～15m 3/m，注入排量为8～15m 3/min，平均砂比为10～15％，前置液比大于等于40％。
步骤7.4、重复步骤7.2和步骤7.3，对水平井水平段的剩余井段进行射孔压裂施工，直至完成全井段压裂施工；
步骤7.5、完成全井段压裂施工后，关井等待压力扩散至裂缝闭合压力以下时，进行放喷返排作业。
步骤8：完成多簇暂堵转向分段压裂施工后进行返排、通洗井作业，然后进行煤层气排水采气。
具体包括：
步骤8.1、放喷返排作业结束后，下入磨铣管柱将桥塞统一钻铣，将水平井水平段的所有桥塞全部磨铣结束后，继续下磨铣管柱至水平井人工井底，正循环1.5倍水平井井筒容积的清水，然后提出磨铣管柱；
步骤8.2、对水平井筒进行循环洗井作业，观察出口返出液情况至进出口水 质一致，且出口液体干净无杂质污物，停止洗井作业；
步骤8.3、按照排采设计规范要求，安装水平井排采设备，对水平井进行排
采作业。实施例1
遵从上述技术方案，在本实施例中，目标矿区为淮北某矿区，该矿区是我国典型的高瓦斯突出矿区，地质构造复杂，瓦斯灾害严重，曾发生过多起瓦斯突出和爆炸事故，因此，采煤前开展瓦斯预抽极为必要。通常井下常规采用底板岩巷穿层钻孔或煤巷顺层水平钻孔预抽瓦斯，由于煤层碎软低渗，单孔瓦斯抽采量低，抽采达标时间长，采掘接替矛盾十分突出。初期采用垂直压裂直井进行地面煤层气抽采，取得了一定的效果，但是该方式存在抽采时间长、单井控制面积小、外围工程量大等缺陷。
因此，对目标矿区前期的勘探井、煤层气开发井的数据进行了收集，选择8年后的采煤区域施工顶板水平井。
按照以下步骤进行碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采：
步骤1、由于目标矿区前期地质勘探数据较少，勘探孔平均井距为400～500m，并且无工作面全覆盖要求，因此，对于目标矿区，确定水平井井型为U型。
步骤2、根据收集到的勘探数据确定水平井井位；
保证水平井水平段所处的煤层顶板等高线应比直井井位所处的煤层顶板等高线高出5m，以利于后期排水采气。
步骤3、根据前期收集到的目标矿区的勘探数据，确定淮北矿的井田内二叠系含煤19～58层，可采及局部可采煤层共8层，其中，上部的3、4、5、6、7煤层为薄煤层，灰分高，煤层稳定性较差，仅可局部可采，平均可采总厚度为31.75m。8、9、10煤层为该矿区的主采煤层。8煤层与9煤层在纵向上距离较近，所以将8煤层和9煤层作为本次煤层气开发的目标煤层，水平井布置在8煤顶面以上的顶板岩层中。
在本实施例中，采用三维压裂数值模拟软件MFrac Suite进行了压裂模拟试验，将水平井水平段布置在煤层顶板中，以获取水平井水平段与煤层顶面距离为0.5m～8m时压裂缝的延伸数据，结果如下：
在水平井水平段与煤层顶面距离为0.5m时，压裂缝半长为：91.23米，压裂缝总高度为：55.297米，煤层顶面以上裂缝高度为：29.702米，煤层顶面以下压 裂缝高度为：25.595米；
在水平井水平段与煤层顶面距离为1.0m时，压裂缝半长为：86.109米，压裂缝总高度为：58.964米，煤层顶面以上裂缝高度为：31.915米，煤层顶面以下压裂缝高度为：27.049米；
在水平井水平段与煤层顶面距离为2.0m时，压裂缝半长为：85.132米，压裂缝总高度为：61.068米，煤层顶面以上裂缝高度为：33.084米，煤层顶面以下压裂缝高度为：27.984米；
在水平井水平段与煤层顶面距离为4.0m时，压裂缝半长为：80.155米，压裂缝总高度为：66.728米，煤层顶面以上裂缝高度为：36.361米，煤层顶面以下压裂缝高度为：30.367米；
在水平井水平段与煤层顶面距离为8.0m时，压裂缝半长为：74.356米，压裂缝总高度为：74.621米，煤层顶面以上裂缝高度为：41.161米，煤层顶面以下压裂缝高度为：33.46米。
由上述数据可以看出，将水平井水平段布置在煤层顶板中，裂缝能够跨过煤层与煤层顶板的界面实现穿层扩展，从而沟通下部的煤层，为煤层气进入井筒提供通道。
当水平井水平段与煤层顶面的距离为0.5m向8m增加时，裂缝高度小幅度增加，并且煤层顶面以上的裂缝高度也逐渐增大，其对于煤层气渗流进入水平井水平段的井筒帮助不大。
当水平井水平段与煤层顶面的距离小于0.5m时，水平井水平段钻进时易钻入目标碎软煤层，一方面易发生埋钻、卡钻等井下事故，另一方面也易污染煤层。
当水平井水平段与煤层顶面的距离大于8m时，压裂时煤层顶面以上的无效裂缝长度将大幅度增加，会使压裂施工时的无效投入增加，另一方面距离越大，水平井水平段井筒与煤层间的地层情况也会更加复杂，地层间接触界面增多，增大水力压裂裂缝沟通煤层的难度。
此外，水平井水平段与煤层顶面的距离越小，裂缝从顶板起裂后越能快速穿层扩展进入下部煤层，裂缝穿层扩展时间越短，并且煤层顶面以上裂缝高度越小，即产生的无效裂缝的长度越小。
本实施例中预设煤层顶面以下裂缝高度H coal为15m，这是因为在目标区域 内8煤层顶面至9层煤底面的总厚度平均约为15m，当煤层顶面以下裂缝高度≥15m时，能够为8煤层和9煤层的煤层气流动提供通道。预设压裂缝半长L为80～100m，这是由于结合该矿区以往施工经验和产能数值模拟，当裂缝半长在该区间内时能够获得较好的抽采效果。
根据表1中的结果，为使裂缝长度和煤层顶面以上裂缝高度达到预设值，水平井水平段与煤层顶面的距离应小于4.0m，并且为促进裂缝从煤层顶板向下部煤层中延伸。
因此，在本实施例中，水平井水平段与煤层顶/底面的垂直距离设定为0.5～2.0m。作为优选，水平井水平段与煤层顶/底面的垂直距离为1.5米。
步骤4、结合中国地应力分布图，根据交叉偶极子声波测井解释得到的矿区地应力方向，将水平井水平段布设方位角设置为0°。
步骤5、完成直井钻井，完成水平井一开、二开钻井；；
在本实施例中，直井属于煤层气参数井和生产井，也是后期水平井的排采井，直井的设计井深为816.00m，实际完钻井深806.00m。在钻井的过程中，需要获取煤层的埋深、厚度、结构等质参数，并完成煤层气含量和注入/压降试井测试。
直井采用二开井身结构。一开井深256.50m，二开钻至井深806.00m完钻。完钻后下入Ф177.8mm生产套管并固井。
为便于后期水平井对接和扩孔造穴作业，在724.9m～733.15m处采用玻璃钢套管完井，并且对726.65～731.15m处共计4.50m进行扩孔造穴，穴的直径为0.5m。
然后依次施工导眼井、进行随钻测井、确定导向层位、填埋导眼井，并在侧钻至着陆点后下套管固井；为保证水平井水平段与煤层顶/底面的垂直距离为1.5m左右，在施工过程中，先钻斜导眼井探测煤层的位置，以实现水平井水平段钻进时着陆点的精确控制。
步骤6、利用方位伽玛地质导向方法进行水平井水平段地质导向钻进，实时调整水平井水平段的钻进轨迹，使钻头的钻进轨迹位于步骤3所述的目标层位中，并使水平井水平段与煤层顶/底面之间保持步骤3所确定的垂直距离；
包括以下子步骤：
步骤6.1、建立目标矿区内地层吸收系数与伽马射线强度关系；
步骤6.2、建立目标矿区内煤层顶板或底板厚度与伽马射线强度关系；
步骤6.3、根据步骤6.1建立的地层吸收系数与伽马射线强度关系和步骤6.2建立的层顶板或底板厚度与伽马射线强度关系，确定目标矿区内不同数据采集扇区的伽马曲线幅值，所述伽马曲线幅值包括上伽马曲线幅值和下伽马曲线幅值；
步骤6.4、确定钻头到煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离；
步骤6.5、根据步骤6.4得到的煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离及步骤6.3得到的上伽马曲线幅值和下伽马曲线幅值，确定水平井水平段地质导向钻进轨迹，并进行水平井水平段地质导向钻进。
在本实施例中，在使用随钻方位测井仪进行水平井水平段地质导向钻进时，将随钻方位测井仪设置在钻铤上，获得实时伽马曲线幅值，所述伽马曲线幅值包括上伽马曲线幅值和下伽马曲线幅值，具体结果参见图8，需要说明的是，在图8中深度单位为百米，
如图8所示，钻头首先顺煤层钻进，当钻进深度达到700米左右时，上下伽马曲线幅值均出现逐渐变大，且上伽马曲线幅值先于下伽马曲线幅值变大，在此过程中，钻遇距离先减小后增大，指示此时钻头钻遇煤岩界面，在上下伽马曲线幅值均达到约78API、钻进深度达到800米左右时，上下伽马曲线幅值不再发生变化，说明此时钻头已经穿出煤层并在煤层顶板岩层中钻进，此时需要对钻头的钻进轨迹进行调整，沿原钻进方向仪器工具面向角180°钻进。
当钻进深度为800～2850米时，上下伽马曲线幅值并未发生变化，指示此时钻头仍在煤层顶板围岩中钻进。
当钻进深度达到2850～2950米时，上下伽马曲线幅值从约78API逐渐变为小于20API，且下伽马曲线幅值先于上伽马曲线幅值变小，在此过程中，钻遇距离先减小后增大，上/下伽马曲线幅值出现明显降低，说明此时钻头进入了具有低放射性的煤层。
当钻进深度达到2950～3300米时，上下伽马曲线幅值保持不变，指示此时钻头仍在煤层中钻进。
当钻进深度超过3300米时，上下伽马曲线幅值再次出现逐渐变大且下伽马曲线幅值先于上伽马曲线幅值变大，指示此时钻头钻遇煤层底板，在上下伽马曲线幅值均达到120API左右，上下伽马曲线幅值不再发生变化，指示此时钻头在煤层底板围岩中钻进，需要对钻头的钻进轨迹进行调整。
实际施工中，根据经验，当顺煤层钻进时，若上下伽马曲线幅值均超过70API，则说明钻头进入煤层顶板/底板；若上下伽马曲线幅值均小于50API时，则说明钻头顺煤层钻进，或者当钻遇距离小于5～8m或垂直距离为0.1～0.5m时，需要调整钻头的钻进方向。
步骤7、对水平井水平段进行分段封隔、向煤层方向实施定向射孔或多簇定向射孔，然后进行多簇暂堵转向分段压裂施工；
在本实施例中，要布设的水平井水平段的长度约为800m，设置的压裂段间距为80m，每段内暂堵射孔簇为3簇，簇间距为20m，第一个桥塞下入位置距离直井井筒20m。
在分段压裂施工过程中，水平井水平段的第一段采用电缆泵送桥塞射孔联作方式进行射孔，桥塞到达设计井深后，打压测试5min，若压力未降低，则第一段射孔簇坐封完成；然后上提射孔枪点火射孔；继续上提射孔枪至第一段第二簇的射孔位置，点火射孔；继续上提射孔枪至第一段第三簇的射孔位置，点火射孔，共计射孔3簇，每段射孔1.5m，射孔密度为14孔/米。射孔成功后，泵注前置液、携砂液和顶替液，进行第一段的压裂施工，前置液比例为40％，压裂液配方为：清水+1％KCl+0.05％杀菌剂，支撑剂为兰州石英砂，压裂液注入排量为10m 3/min，平均砂比为为10～15％。
完成所有压裂段的压裂施工后，水平井水平段共计压裂9段27簇。
为验证注入排量变化对压裂缝的影响，进行了对比实验，结果如图9和图10所示，其中，图9给出了以30mL/min的恒定排量注入时的裂缝形态，图10给出了初始排量为30ml/min，煤层破裂后30s立即切换到20ml/min时的裂缝形态。
从对比结果可以看出，相对于常规的多簇笼统压裂施工，在多簇暂堵转向分段压裂施工中，前置的液初始阶段注入排量11～15m 3/min，随后降至8～10m 3/min，从而可以实现“大排量促进裂缝穿层、中低排量促进裂缝均匀扩展”的目的。在两种注入模式下，压裂缝均实现了跨煤岩界面的穿层扩展，沟通了井筒与下部煤层，但是在以30mL/min的恒定排量注入的情况下，煤层顶板内的裂缝面积小，而当初始排量为30ml/min，煤层破裂后30s立即切换到20ml/min时，煤层顶板和煤层内的裂缝发育更加充分，裂缝面积更大，更有利于后期排采。
步骤8、完成多簇暂堵转向分段压裂施工后进行返排、通洗井作业，然后进行煤层气排水采气。
步骤8.1、放喷返排作业结束后，下入磨铣管柱将桥塞统一钻铣，将水平井水平段的所有桥塞全部磨铣结束后，继续下磨铣管柱至水平井人工井底，正循环1.5倍水平井井筒容积的清水，然后提出磨铣管柱；
步骤8.2、对水平井筒进行循环洗井作业，观察出口返出液情况至进出口水质一致，且出口液体干净无杂质污物，停止洗井作业；
步骤8.3、按照排采设计规范要求，安装水平井排采设备，对水平井进行排采作业。

Claims (19)

  1. 碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，该方法包括以下步骤：
    步骤1、收集目标矿区的勘探数据和矿井数据，并根据收集到的勘探数据和矿井数据确定水平井井型，所述水平井包括直井段和水平井水平段，所述水平井井型包括L型、U型和W型；
    步骤2、根据收集到的勘探数据和矿井数据确定水平井井位；
    步骤3、根据收集到的勘探数据和矿井数据确定水平井水平段在煤层顶板或底板中的目标层位，以及水平井水平段与煤层顶/底面的垂直距离；
    步骤4、根据前期勘探或测试得到的目标矿区最小水平主应力方向确定水平井水平段的布设方位角；
    步骤5、完成直井钻井，完成水平井一开、二开钻井和固井；
    步骤6、利用方位伽玛地质导向方法进行水平井水平段地质导向钻进，实时调整水平井水平段的钻进轨迹，使钻头的钻进轨迹位于步骤3所述的目标层位中，并使水平井水平段与煤层顶/底面之间保持步骤3所确定的垂直距离；
    当井型为L型井时，钻进至水平井水平段达到预设长度后下套管固井；
    当井型为U型或W型时，钻进至水平井水平段与直井段对接后下套管固井；
    步骤7、对水平井水平段进行分段封隔、向煤层方向实施定向射孔或多簇定向射孔，然后进行多簇暂堵转向分段压裂施工；
    步骤8、完成多簇暂堵转向分段压裂施工后进行返排、通洗井作业，然后进行煤层气排水采气。
  2. 如权利要求1所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，步骤3所述的水平井水平段与煤层顶/底面的垂直距离通过以下步骤确定：
    步骤3.1、根据收集到的勘探数据和矿井数据，通过数值模拟确定水平井水平段在目标层位内的压裂缝延伸数据，所述压裂缝延伸数据包括压裂缝总高度、压裂缝半长、压裂缝上部裂缝高度、压裂缝下部裂缝高度；
    步骤3.2、根据得到的压裂缝延伸数据，确定水平井水平段与煤层顶/底面的垂直距离。
  3. 如权利要求1所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述的水平井水平段与煤层顶/底面的垂直距离为0.5～8米。
  4. 如权利要求1所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述的水平井水平段与煤层顶/底面的垂直距离为1.5米。
  5. 如权利要求1所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述测试得到的目标矿区最小水平主应力方向通过小型压裂测试、偶极子声波测井或应力解除法测得。
  6. 如权利要求1所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，步骤4所述的水平井水平段布设方位角为-15°～15°。
  7. 如权利要求6所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，步骤4所述的水平井水平段布设方位角为0°。
  8. 如权利要求1所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述步骤5具体包括：
    当井型为L型井时，完成直井钻井，然后水平井一开、二开钻井钻进至着陆点后下套管注水泥固井；
    当井型为U型时，完成直井钻井后下套管注水泥固井，然后进行水平井一开、二开钻井，钻进至着陆点后再下套管注水泥固井；
    当井型为W型时，完成直井钻井后下套管注水泥固井，然后分别完成两口水平井的一开、二开钻井，钻进至着陆点后下套管注水泥固井。
  9. 如权利要求1所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，步骤6所述的利用方位伽玛地质导向方法进行水平井水平段地质导向钻进具体包括：
    步骤6.1、建立目标矿区内地层吸收系数与伽马射线强度关系；
    步骤6.2、建立目标矿区内煤层顶板或底板厚度与伽马射线强度关系；
    步骤6.3、根据步骤6.1建立的地层吸收系数与伽马射线强度关系和步骤6.2建立的层顶板或底板厚度与伽马射线强度关系，确定目标矿区内不同数据采集扇区的伽马曲线幅值，所述伽马曲线幅值包括上伽马曲线幅值和下伽马曲线幅值；
    步骤6.4、确定钻头到煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离；
    步骤6.5、根据步骤6.4得到的煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离及步骤6.3得到的上伽马曲线幅值和下伽马曲线幅值，确定水平井水平段地质导向钻进轨迹，并进行水平井水平段地质导向钻进。
  10. 如权利要求9所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述的步骤6.1中目标矿区内地层吸收系数与伽马射线强度关系为：
    Figure PCTCN2021140941-appb-100001
    式中，J为伽马射线强度，单位为MeV，ac为地层吸收系数，η为位置参数，σ为尺度参数。
  11. 如权利要求9所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述的步骤6.2的目标矿区内煤层顶板或底板厚度与伽马射线强度关系为：
    Figure PCTCN2021140941-appb-100002
    式中，J为伽马射线强度，单位为MeV，H为煤层顶板或底板厚度，单位为m，η
    为位置参数，σ为尺度参数。
  12. 如权利要求9所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述的步骤6.3计算不同数据采集扇区的伽马曲线幅值为：
    f(Si)＝J(H)*J(ac)*N j
    式中，f为伽马曲线幅值，单位为API，Si为第i个数据采集扇区的伽马计数值，且i为取值范围为1～8的整数，N j为在Si数据采集扇区内的探测深度j的伽马射线强度计数值；
    Figure PCTCN2021140941-appb-100003
    其中，G 1j、G 2j、G 3j、G 4j分别为随钻方位测井仪在深度j且工具面向角为0°、 90°、180°、和270°的伽马射线强度计数值的贡献值，θ为随钻方位测井仪的工具面向角，单位为°，V为随钻方位测井仪的钻进速度，单位为m/s，R为随钻方位测井仪的转数，单位为r/s。
  13. 如权利要求9所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述的步骤6.4确定钻头到煤层与煤层顶板/煤层底板界面的钻遇距离和垂直距离如下：
    L＝D/sin(α)-L ac
    L c＝L·sin(α)
    D n+1＝D n-dep·sin(α)
    其中，α＝α 地层-dev n
    式中，L为钻头到煤岩界面的钻遇距离，单位为m；L c为钻头到煤岩界面的垂直距离，单位为m；L ac为随钻方位测井仪到钻头的距离，单位为m；D为开孔点处探测器到煤岩层界面距离，单位为m；α为煤岩层界面与仪器的夹角，单位为°；dep为测量间距，单位为m；n为测量次数，n为≥1的正整数，dev n为随钻方位测井仪第n次测量的测量倾角，单位为°，取值范围为0～360°；a 地 层为地层倾角，单位为°，取值范围0～360°；D n为第n次测量时探测器到煤岩层界面距离，单位为m，D n+1为第n+1次测量时探测器到煤岩层界面距离，单位为m。
  14. 如权利要求9所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述的步骤6.5具体包括：
    当从煤层钻进到顶板岩层时，上下伽马曲线幅值均逐渐变大且上伽马曲线幅值先增大，并且钻遇距离逐渐增大，当钻遇距离小于5～8m或垂直距离为0.1～0.5m、上下伽马曲线幅值均超过70API时，调整钻头沿原钻进方向仪器工具面向角180°钻进；
    当从煤层钻进到底板岩层时，上下伽马曲线幅值均逐渐变大且下伽马曲线幅值先增大，并且钻遇距离逐渐增大，当钻遇距离小于5～8m或垂直距离为0.1～0.5m、上下伽马曲线幅值均超过70API时，调整钻头沿原钻进方向仪器工具面向角 180°钻进；
    当顺煤层钻进时，若上下伽马曲线幅值均超过70API，则说明钻头进入煤层顶板/底板；若上下伽马曲线幅值均小于50API时，则说明钻头顺煤层钻进。
  15. 如权利要求1所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，步骤7具体包括以下子步骤：
    步骤7.1、确定水平井水平段的压裂段数及分簇射孔位置；
    步骤7.2、对水平井水平段的第一段向煤层方向垂直向下/向上实施多簇定向射孔；
    当井型为L型井时，对水平井水平段的第一段采用油管或连续油管输送方式实施多簇定向射孔；
    当井型为U型或W型时，对水平井水平段的第一段采用电缆泵送桥塞射孔联作方式实施多簇定向射孔；
    步骤7.3、采用桥塞分段压裂方式，对水平井水平段的第一段进行多簇暂堵转向分段压裂施工；
    步骤7.4、重复步骤7.2和步骤7.3，对水平井水平段的剩余井段进行射孔压裂施工，直至完成全井段压裂施工；
    步骤7.5、完成全井段压裂施工后，关井等待压力扩散至裂缝闭合压力以下时，进行放喷返排作业。
  16. 如权利要求15所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，步骤7.2所述的向煤层方向垂直向下/向上实施多簇定向射孔的操作参数包括：射孔枪为95型或89型、射孔弹为102型、射孔孔密为10～16孔/米、射孔相位角为垂直向下/向上。
  17. 如权利要求15所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，步骤7.3所述的对水平井水平段的第一段进行多簇暂堵转向分段压裂施工操作参数包括：压裂液为活性水压裂液，支撑剂为石英砂，压裂段的加砂强度为8～15m 3/m，注入排量为8～15m 3/min，平均砂比为10～15％，前置液比大于等于40％。
  18. 如权利要求1所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，对于U型水平井或W型水平井，水平井水平段钻进的着 陆点标高比水平井水平段与直井段对接处的标高高3～20m；对于L型水平井，水平井水平段钻进的着陆点标高比水平井水平段末端的标高低3～20m。
  19. 如权利要求17所述的碎软低渗煤层顶板或底板分段压裂水平井煤层气抽采方法，其特征在于，所述步骤8具体包括：
    步骤8.1、放喷返排作业结束后，下入磨铣管柱将桥塞统一钻铣，将水平井水平段的所有桥塞全部磨铣结束后，继续下磨铣管柱至水平井人工井底，正循环1.5倍水平井井筒容积的清水，然后提出磨铣管柱；
    步骤8.2、对水平井筒进行循环洗井作业，观察出口返出液情况至进出口水质一致，且出口液体干净无杂质污物，停止洗井作业；
    步骤8.3、按照排采设计规范要求，安装水平井排采设备，对水平井进行排采作业。
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