Drilling and completion construction method of L-shaped horizontal well for multi-layer co-production of coal bed gas in thin coal seam

Abstract

The invention discloses a coal bed gas L-shaped horizontal well drilling and completion construction method, which can realize a specific construction process for simultaneously mining multi-layer thin coal seam resources, has the characteristics of capability of mining multi-layer coal seams and close connection of construction links, and effectively promotes the coal bed gas multi-layer co-mining method to be applied from theory to engineering practice. Through construction of the 40-degree guide hole and the 70-degree guide hole, the vertical depth, the vertical thickness and the dip angle of the target coal seam can be accurately detected, accurate landing is completed, coal entering is conducted in a proper well inclination mode, the drilling rate of horizontal section construction of the target coal seam is guaranteed, the well completion rate is greatly improved, and meanwhile the development and utilization rate of coal seam gas resources is also guaranteed. The process of later fracturing construction is effectively omitted, the construction difficulty is reduced, the engineering cost can be greatly reduced, the requirement for efficient development of thin coal seam resources is met, large-scale popularization of the thin coal seam multi-layer coal seam gas L-shaped horizontal well can be achieved, the coal seam gas development technical level is improved, and economic benefits are maximized; the method is suitable for large-scale industrial use and popularization.

Description

Technical Field

[0001] The invention belongs to the technical field of surface drilling and production of coalbed methane, and particularly relates to a construction method for drilling and completion of an L-shaped horizontal well for multi-layer co-production of coalbed methane in thin coal seams. Background Art

[0002] Coalbed methane is a hydrocarbon gas existing in coal seams and coal-bearing strata, with methane as the main component, and it is an associated mineral resource of coal. The main exploitation methods of coalbed methane are surface drilling and underground drainage. Among them, surface drilling first extracts gas and then mines coal, which not only develops clean energy but also improves the safety production conditions of coal mines. Therefore, the surface drilling exploitation method has been widely promoted.

[0003] At present, surface drilling for coalbed methane in China mainly exploits the No. 3 coal seam and No. 15 coal seam with relatively large vertical thickness. However, with the continuous exploitation of coalbed methane resources in the No. 3 coal seam and No. 15 coal seam with relatively large vertical thickness, the gas content is continuously decreasing, and thin coal seam resources can be further exploited to achieve the sustainable development and utilization of coalbed methane resources.

[0004] Due to the relatively thin coal seams of thin coal seam resources, it is difficult to ensure the drilling encounter rate of the target coal seam during construction, the drilling workload per unit gas production is large, and the development and utilization rate of thin coal seam resources is low. Currently, surface drilling either has a small exploitation range or can only exploit a single target coal seam. For coalbed methane resources with multi-layer coexistence in thin coal seams, the drilling cost of single-layer exploitation one by one is high, and the construction period is long, which cannot meet the requirements of efficient development of thin coal seam resources. Therefore, multi-layer co-production of thin coal seams is the future development direction of efficient exploitation of coalbed methane resources, and the existing coal mining methods need to fracture the target coal seam, with relatively large construction difficulty and high cost. Summary of the Invention

[0005] The purpose of the invention is to provide a construction method for drilling and completion of an L-shaped horizontal well for multi-layer co-production of coalbed methane in thin coal seams, so as to solve the problems of relatively large construction difficulty and high cost in the existing coal mining methods.

[0006] In order to solve the above technical problems, the invention is implemented by adopting the following technical solutions:

[0007] A construction method for drilling and completion of an L-shaped horizontal well for multi-layer co-production of coalbed methane in thin coal seams includes the following steps:

[0008] Step 1, determine the first target coal seam and the second target coal seam, as well as the corresponding first L-shaped horizontal well kick-off point and the second L-shaped horizontal well kick-off point according to the coal seam spatial distribution of the multi-layer co-production of coalbed methane in thin coal seams to be constructed and the requirements of later drainage construction;

[0009] Step 2: Use a roller cone bit to vertically drill until reaching below the bedrock surface and then complete the drilling; Lower a surface casing into the formed borehole, and grout the annulus between the surface casing and the borehole until the grout returns to the ground and then wait for setting;

[0010] Step 3: Use a PDC bit to continue drilling until reaching the kick-off point of the first L-shaped horizontal well and then conduct a 40° pilot hole operation. When the well inclination reaches 40°, conduct steady drilling, successively penetrate the second target coal seam and the first target coal seam and then complete the drilling to form a 40° pilot hole; Backfill the 40° pilot hole and wait for setting;

[0011] Step 4: Use a PDC bit to sidetrack from the kick-off point of the first L-shaped horizontal well to conduct a 70° pilot hole operation. When the inclination reaches 70°, conduct steady drilling, successively penetrate the second target coal seam and the first target coal seam and then complete the drilling to form a 70° pilot hole; Backfill the 70° pilot hole and wait for setting;

[0012] Determine the vertical depth, vertical thickness, and dip angle of the first target coal seam and the second target coal seam based on the 40° pilot hole and the 70° pilot hole. Then, determine the wellbore trajectory and horizontal section length of the first L-shaped horizontal well and the wellbore trajectory and horizontal section length of the second L-shaped horizontal well according to the vertical depth, vertical thickness, and dip angle of the first target coal seam and the second target coal seam;

[0013] Step 5: Use a PDC bit to drill along the wellbore trajectory of the first L-shaped horizontal well from the kick-off point until reaching the first target coal seam and then complete the drilling; Lower the first technical casing, and grout the annulus between the first technical casing and the borehole until the grout returns to the ground and then wait for setting;

[0014] Step 6: Use a PDC bit to horizontally drill along the first target coal seam until reaching the horizontal section length of the first target coal seam and then complete the drilling to form the first L-shaped horizontal well. Lower the first production casing into the first L-shaped horizontal well, and inject the cleaning agent into the annulus between the first production casing and the first technical casing using the reverse circulation method and discharge it from the first production casing for reverse circulation cleaning;

[0015] Step 7: Open the stage collar of the first production casing, inject cement slurry from the inside of the first production casing until the return height reaches the kick-off point of the second L-shaped horizontal well for cementing. After cementing, close the stage collar and conduct large-displacement and long-time circulation until there is no cement inside the first production casing in the first L-shaped horizontal well, completing the drilling and completion construction of the first L-shaped horizontal well;

[0016] Step 8: Lower a casing cutter into the first L-shaped horizontal well, cut the first production casing below the kick-off point of the second L-shaped horizontal well to divide it into upper and lower parts, and recover the upper part of the first production casing;

[0017] Step 9: Run a whipstock into the kick-off point of the second L-shaped horizontal well for plugging. After that, use a milling cone to open a window in the first production casing from the kick-off point of the second L-shaped horizontal well and perform sidetracking. After the window opening is completed, pull out the drill string, replace the PDC bit, and repeat the reaming operation for entering and exiting the window multiple times.

[0018] Step 10: Use a PDC bit to drill along the wellbore trajectory of the second L-shaped horizontal well to the second target coal seam, and continue horizontal drilling until reaching the horizontal section length of the second L-shaped horizontal well, then complete the drilling to form the second L-shaped horizontal well.

[0019] Run a second production casing into the second L-shaped horizontal well, and grout the annulus between the second production casing and the borehole until the slurry returns to the ground and then wait for it to set.

[0020] Step 11: Run a second production casing into the second L-shaped horizontal well, and inject the cleaning agent into the annulus between the second production casing and the second production casing using the reverse circulation method, and discharge it from the second production casing for reverse circulation cleaning.

[0021] Step 12: Open the stage collar of the second production casing, inject cement slurry from the inside of the second production casing until it returns to the kick-off point of the second L-shaped horizontal well. After cementing, close the stage collar and perform large-displacement and long-time circulation until there is no cement inside the second production casing of the second L-shaped horizontal well, and complete the drilling and completion construction of the second L-shaped horizontal well.

[0022] Step 13: Sweep the plug in the second L-shaped horizontal well; run a casing cutter into the second L-shaped horizontal well, cut the second production casing at the kick-off point of the second L-shaped horizontal well into upper and lower parts. After recovering the upper part of the second production casing, sweep the plug in the first L-shaped horizontal well.

[0023] Step 14: Connect the surface casing and the lower part of the first production casing to complete the drilling and completion construction of the L-shaped horizontal well for multi-layer co-production of coalbed methane in thin coal seams.

[0024] The present invention also has the following features:

[0025] Further, in Step 3, during the process of using a PDC bit to drill to the kick-off point of the first L-shaped horizontal well, control the full angle change rate within 6° / 30m.

[0026] Further, the first production casing and the second production casing have the same specifications;

[0027] The first production casing includes a Φ139.7mm steel pipe of grade N80, a stage collar string, and a fiberglass screen pipe string.

[0028] Further, the first production casing and the second production casing have the same specifications;

[0029] The first technical casing pipe mentioned above is a Φ193.7mm technical casing pipe with steel grade J55.

[0030] Further, in step 9, when running a whipstock to block at the kick-off point of the second L-shaped horizontal well, a combined method of using an isolation packer and a slip setting is adopted to achieve the blockage below the whipstock.

[0031] Further, in step 14, the surface casing pipe has a specification of Φ273.1mm;

[0032] The wellhead of the surface casing pipe is 70 cm below the ground surface, and the wellhead of the first technical casing pipe is 30 cm below the ground surface;

[0033] The surface casing pipe and the first technical casing pipe are welded with an annular steel plate, and a casing head is fixedly sleeved at the connection of the surface casing pipe and the first technical casing pipe.

[0034] Further, in step 2, a roller cone bit with a diameter of Φ346.1mm is used to drill to 10 m below the bedrock surface;

[0035] In step 3, a PDC bit with a diameter of Φ241.3mm is used to drill to the kick-off point of the first L-shaped horizontal well;

[0036] After the well inclination reaches 40°, the drill string is pulled out of the hole, and a PDC bit with a diameter of Φ215.9mm is used in combination with a single bend screw drill with a diameter of Φ165mm / 1.5° to continue the steady drilling;

[0037] The setting time is 48 hours;

[0038] In step 5, a PDC bit with a diameter of Φ241.3mm is used to drill along the wellbore trajectory of the first L-shaped horizontal well from the kick-off point of the first L-shaped horizontal well until reaching the first target coal seam and penetrating 1 - 2 m into the coal seam, and then the drilling is completed;

[0039] In step 6, a PDC bit with a diameter of Φ171.5mm is used to drill along the first target coal seam;

[0040] In step 9, a mill bit with a diameter of Φ218mm is used to open a window and sidetrack the first technical casing pipe from the kick-off point of the second L-shaped horizontal well. After the window opening is completed, the drill string is pulled out of the hole and replaced with a PDC bit with a diameter of Φ215.9mm to drill 15 m, and the operation of reaming in and out of the window is repeated multiple times;

[0041] In step 10, a PDC bit with a diameter of Φ215.9mm is used to drill along the wellbore trajectory of the second L-shaped horizontal well until reaching the second target coal seam and penetrating 1 - 2 m into the coal seam.

[0042] Further, the cleaning agent used is a foam cleaning agent.

[0043] Compared with the prior art, the present invention has the following technical effects:

[0044] (Ⅰ) The construction method of the L-shaped horizontal well for coalbed methane drilling and completion in the present invention can realize the specific construction process for simultaneous exploitation of multi-layer thin coal seam resources. It has the characteristics of being able to exploit multi-layer coal seams and having closely connected construction links at each construction stage, effectively promoting the transformation of the method of multi-layer co-production of coalbed methane from theory to engineering practice application.

[0045] Among them, through the construction of the 40° pilot hole and the 70° pilot hole, the vertical depth, vertical thickness and dip angle of the target coal seam can be accurately explored, precise landing can be completed, and the coal seam can be entered at an appropriate well inclination to ensure the drilling encounter rate of the horizontal section of the target coal seam. While greatly improving the well completion rate, the development and utilization rate of coalbed methane resources is also guaranteed.

[0046] At the same time, the subsequent fracturing construction process is effectively omitted, the construction difficulty is reduced, and it has the characteristics of excellent well completion engineering quality, short drilling cycle and high construction efficiency. It can greatly reduce the engineering cost, meet the requirements of efficient development of thin coal seam resources, realize large-scale popularization of L-shaped horizontal wells for multi-layer coalbed methane in thin coal seams, improve the technical level of coalbed methane development and maximize economic benefits, and is suitable for large-scale industrial use and popularization. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is a schematic diagram of exploring coal by the 40° pilot hole and the 70° pilot hole in the present invention;

[0048] Figure 2 It is a schematic diagram of drilling and completion of the L-shaped horizontal well in the first target coal seam of the present invention;

[0049] Figure 3 It is a schematic diagram of window cutting of the technical casing at the kick-off point of the second L-shaped horizontal well by the whipstock and the packer in the present invention;

[0050] Figure 4 It is a schematic diagram of drilling and completion of the L-shaped horizontal well in the second target coal seam of the present invention;

[0051] Figure 5 It is a schematic diagram of the well completion effect of the multi-layer L-shaped horizontal well in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0052] It should be noted that all components in the present invention, unless otherwise specified, are all components known in the prior art. For example, the roller cone bit uses a commonly known roller cone bit.

[0053] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent transformations made on the basis of the technical solutions of this application fall within the protection scope of the present invention.

[0054] A construction method for drilling and completing an L-shaped horizontal well for multi-layer co-production of coalbed methane in thin coal seams includes the following steps:

[0055] Step 1: Determine the kick-off point of the second L-shaped horizontal well, the kick-off point of the first L-shaped horizontal well, the second target coal seam, and the first target coal seam according to the spatial distribution of the coal seams for multi-layer co-mining of coalbed methane in thin coal seams to be constructed and the requirements for later drainage construction;

[0056] It should be noted that the spatial distribution of the coal seams for multi-layer co-mining of coalbed methane in thin coal seams to be constructed and the requirements for later drainage construction can be directly obtained by those skilled in the art;

[0057] The method for determining the second target coal seam, the first target coal seam, the kick-off point of the second L-shaped horizontal well, and the kick-off point of the first L-shaped horizontal well based on the above content is also well-known content. Those skilled in the art can determine the above content based on relevant knowledge in the art, which does not belong to the content discussed in this embodiment and will not be elaborated further.

[0058] Step 2: Use a roller bit to drill below the bedrock surface. After forming a borehole, retract the roller bit; Lower a surface casing into the borehole, and grout the annulus between the surface casing and the borehole until the slurry returns to the ground and then wait for it to set.

[0059] Step 3: Use a PDC bit to continue drilling until the kick-off point of the first L-shaped horizontal well is reached, and then conduct a 40° pilot hole construction. When the well inclination reaches 40°, change to steady drilling and complete the drilling after passing through the second target coal seam and the first target coal seam in sequence.

[0060] Step 4: Conduct a 70° pilot hole construction by sidetracking from the kick-off point of the first L-shaped horizontal well. When the inclination reaches 70°, continue steady drilling, pass through the second target coal seam and the first target coal seam in sequence until below the first target coal seam, and then end the drilling to form a 70° pilot hole; Backfill the 70° pilot hole and wait for it to set;

[0061] The schematic diagram after the completion of the final 40° pilot hole and 70° pilot hole is as Figure 1 shown. The purpose of Steps 3 and 4 is to determine the vertical depth, vertical thickness, and dip angle of the first target coal seam and the second target coal seam according to the results of the 40° and 70° pilot holes, and determine the wellbore trajectory and horizontal section length of the first L-shaped horizontal well and the second L-shaped horizontal well based on the above data.

[0062] It should be noted that determining the vertical depth, vertical thickness, and dip angle of the first target coal seam and the second target coal seam according to the results of the 40° and 70° pilot holes and determining the wellbore trajectory and horizontal section length of the first L-shaped horizontal well and the second L-shaped horizontal well based on the above data are content that those skilled in the art can directly obtain based on relevant knowledge in the art and do not belong to the scope of discussion in this embodiment and will not be elaborated further.

[0063] Step 5: After starting from the kick-off point of the first L-shaped horizontal well and drilling along the wellbore trajectory of the first L-shaped horizontal well to the first target coal seam using a PDC bit, complete the drilling, run in the first technical casing, and grout the annulus between the first technical casing and the borehole until the slurry returns to the ground and then wait for it to set;

[0064] Step 6: Use a PDC bit to drill horizontally along the first target coal seam until reaching the horizontal section length of the first target coal seam and then complete the drilling to form the first L-shaped horizontal well. Run in the first production casing into the first L-shaped horizontal well, and use the reverse circulation method to let the cleaning agent enter from the annulus between the first production casing and the first technical casing and be discharged from the first production casing for reverse circulation cleaning;

[0065] According to the conventional selection, the cementing string of the stage collar of the first production casing needs to be set above the first target coal seam.

[0066] Step 7: Open the stage collar of the cementing string of the stage collar, inject cement slurry from the inside of the first production casing until the return height is below the kick-off point of the second L-shaped horizontal well. After cementing, close the stage collar and conduct large-displacement and long-time circulation until there is no cement in the inside of the first production casing of the first L-shaped horizontal well, and complete the drilling and completion construction of the first L-shaped horizontal well;

[0067] Among them, the schematic diagram after the drilling and completion construction of the first L-shaped horizontal well is as Figure 2 shown.

[0068] Step 8: Run in a casing cutter into the first L-shaped horizontal well, cut the first production casing below the kick-off point of the second L-shaped horizontal well to divide it into upper and lower parts, and recover the upper part of the first production casing;

[0069] Step 9: Run in a whipstock to block at the kick-off point of the second L-shaped horizontal well. After that, use a milling cone to open a window and sidetrack the first technical casing from the kick-off point of the second L-shaped horizontal well. After the window opening is completed, pull out the drill string and replace it with a PDC bit to repeat the window in and out reaming operation multiple times;

[0070] Among them, the schematic diagram of running in the whipstock to block is as Figure 3 shown.

[0071] Step 10: Use a PDC bit to drill along the wellbore trajectory of the second L-shaped horizontal well to the second target coal seam, and continue to drill horizontally until reaching the horizontal section length of the second L-shaped horizontal well and then complete the drilling to form the second L-shaped horizontal well;

[0072] Run in the second technical casing into the second L-shaped horizontal well, and grout the annulus between the second technical casing and the borehole until the slurry returns to the ground and then wait for it to set;

[0073] Step 11, lowering a second production casing into the second L-shaped horizontal well, and using a reverse circulation method to inject a cleaning agent from the annular space between the second production casing and the second technical casing, and discharge the cleaning agent from the second production casing for reverse circulation cleaning;

[0074] Step 12, setting the graded collar cementing string of the second production casing at the upper part of the second target coal seam, opening the graded collar, injecting cement slurry from the inside of the first production casing back to below the inclination point of the second L-shaped horizontal well, closing the graded collar after cementing to perform large displacement and long-term circulation until there is no cement inside the second production casing of the second L-shaped horizontal well, and completing the drilling and completion construction of the second L-shaped horizontal well;

[0075] The drilling and completion diagram of the second L-type horizontal well is shown in the figure below: Figure 4 shown.

[0076] Step 13, sweeping and plugging the second L-shaped horizontal well; lowering a casing cutter into the second L-shaped horizontal well, cutting the second production casing at the inclination point of the second L-shaped horizontal well to divide it into an upper part and a lower part, and recovering the upper first production casing; after recovering the upper second production casing, sweeping and plugging the first L-shaped horizontal well;

[0077] Step 14, connecting the surface casing and the first technical casing to complete the drilling and completion construction of the L-shaped horizontal well for multi-layer co-mining of coalbed methane in thin coal seams.

[0078] As a preferred solution, in step 3, during the process of drilling to the first L-shaped horizontal well inclination point using a PDC drill bit, the full angle change rate is controlled within 6° / 30m.

[0079] As a preferred solution, the first production casing and the second production casing have the same specifications;

[0080] The first production casing comprises a Φ139.7 mm steel pipe of steel grade N80, a graded hoop pipe string and a glass fiber reinforced plastic screen pipe string.

[0081] According to conventional selection in the art, the graded hoop strings of the first production casing and the second production casing are respectively arranged 3-5 m above the first target coal seam and the second target coal seam.

[0082] Specifically, both the first production casing and the second production casing need to be lowered to within 5m of the well bottom, wherein the graded hoop string is arranged 3-5m above the first target coal seam, a fiberglass screen is lowered in the horizontal section, and steel pipes are lowered in the rest.

[0083] The cleaning agent is a foam cleaning agent.

[0084] As a preferred solution, the first technical casing and the second technical casing have the same specifications;

[0085] The first technical casing described above is a Φ193.7mm technical casing with steel grade J55.

[0086] As a preferred solution, in step 9, as Figure 3 shown, when a whipstock is run into the kick-off point of the second L-shaped horizontal well for plugging, a combined method of using an isolation packer and a slip setting is adopted to achieve plugging at the lower part of the whipstock. Among them, the plugging method of combining an isolation packer and a slip setting is known in the art and can improve the plugging efficiency.

[0087] As a preferred solution, in step 14, the surface casing has a specification of Φ273.1mm;

[0088] The wellhead of the surface casing is 70 cm below the ground surface, and the wellhead of the first technical casing is 30 cm below the ground surface;

[0089] The surface casing and the first technical casing are welded with a ring-shaped steel plate, and a casing head is fixedly sleeved at the connection between the surface casing and the first technical casing.

[0090] The following gives the specific numerical selections in this embodiment:

[0091] In step 2, a roller cone bit with a diameter of Φ346.1mm is used to drill to 10 m below the bedrock surface;

[0092] In step 3, a PDC bit with a diameter of Φ241.3mm is used to drill to the kick-off point of the first L-shaped horizontal well;

[0093] After the well inclination reaches 40°, the drill string is pulled out of the hole, and a PDC bit with a diameter of Φ215.9mm is used in combination with a single bend screw with a size of Φ165mm / 1.5° to continue drilling steadily;

[0094] The waiting-on-cement time is 48 hours;

[0095] In step 5, a PDC bit with a diameter of Φ241.3mm is used to drill along the wellbore trajectory of the first L-shaped horizontal well from the kick-off point of the first L-shaped horizontal well until it enters the first target coal seam and drills 1 - 2 m into the coal seam before completion;

[0096] In step 6, a PDC bit with a diameter of Φ171.5mm is used to drill along the first target coal seam;

[0097] In step 9, a mill bit with a diameter of Φ218mm is used to open a window and sidetrack the first technical casing from the kick-off point of the second L-shaped horizontal well. After the window opening is completed, the drill string is pulled out of the hole and replaced with a PDC bit with a diameter of Φ215.9mm to drill 15 m, and the operations of drilling in and out of the window are repeated multiple times;

[0098] In step 10, a PDC bit with a diameter of Φ215.9mm is used to drill along the wellbore trajectory of the second L-shaped horizontal well until it enters the second target coal seam and drills 1 - 2 m into the coal seam.

Claims

1. A method for drilling and completing a L-shaped horizontal well for multi-layered coalbed methane mining in thin coal seams, characterized in that: The following steps are involved: Step 1: Determine the first target coal seam and the second target coal seam, and the corresponding first L-shaped horizontal well inclination point and second L-shaped horizontal well inclination point according to the spatial distribution of the coal seams for multi-layer coal seam gas mining in the thin coal seams to be constructed and the requirements for subsequent drainage and mining construction; Step 2, use a roller drill bit to vertically drill to below the bedrock surface and then complete the drilling; insert a surface casing into the formed borehole, and inject grout into the annulus between the surface casing and the borehole until the slurry returns to the ground and solidifies; Step 3, use the PDC drill bit to continue drilling to the first L-shaped horizontal well inclination point and then conduct 40° pilot hole construction. When the well inclination reaches 40°, perform steady drilling, and drill through the second target coal seam and the first target coal seam in sequence to complete the drilling, forming a 40° pilot hole; backfill the 40° pilot hole and wait for setting; Step 4: Use a PDC drill bit to perform side drilling of a 70° pilot hole from the inclination point of the first L-shaped horizontal well. When the inclination reaches 70°, perform steady drilling, and drill through the second target coal seam and the first target coal seam in sequence to complete the drilling, forming a 70° pilot hole; backfill the 70° pilot hole and wait for setting; Determine the vertical depth, vertical thickness and inclination of the first target coal seam and the second target coal seam according to the 40° pilot eye and the 70° pilot eye, and then determine the wellbore trajectory and horizontal section length of the first L-shaped horizontal well and the wellbore trajectory and horizontal section length of the second L-shaped horizontal well according to the vertical depth, vertical thickness and inclination of the first target coal seam and the second target coal seam; Step 5: Drill from the first L-shaped horizontal well inclination point along the wellbore trajectory of the first L-shaped horizontal well using a PDC drill bit to the first target coal seam and complete the drilling; run the first technical casing, and inject grout into the annulus between the first technical casing and the borehole until the slurry returns to the ground and solidifies; Step 6, use a PDC drill bit to drill horizontally along the first target coal seam until the horizontal section length of the first target coal seam is reached and the drilling is completed to form a first L-shaped horizontal well, and a first production casing is lowered into the first L-shaped horizontal well, and a cleaning agent is injected from the annular area between the first production casing and the first technical casing by a reverse circulation method, and discharged from the first production casing for reverse circulation cleaning; Step 7, open the staging collar of the first production casing, inject cement slurry from the inside of the first production casing back to the second L-shaped horizontal well inclination point for cementing, close the staging collar after cementing, and perform large-volume and long-term circulation until there is no cement inside the first production casing in the first L-shaped horizontal well, thereby completing the drilling and completion construction of the first L-shaped horizontal well; Step 8, inserting a casing cutter into the first L-shaped horizontal well, cutting the first production casing below the inclination point of the second L-shaped horizontal well to separate it into an upper part and a lower part, and recovering the upper first production casing; Step 9: Run a whipstock into the second L-shaped horizontal well inclination point for plugging, then use a milling cone to perform window side drilling on the first technical casing from the second L-shaped horizontal well inclination point, and after the window drilling is completed, start drilling, replace the PDC drill bit, and repeat the window in and out eye-marking operation for multiple times; Step 10, using a PDC drill bit to drill along the wellbore trajectory of the second L-shaped horizontal well to the second target coal seam, and continuing to drill horizontally until the horizontal section length of the second L-shaped horizontal well is reached, and then completing the drilling to form the second L-shaped horizontal well; A second technical casing is run into the second L-shaped horizontal well, and grouting is performed into the annulus between the second technical casing and the borehole until the slurry returns to the surface and solidifies; Step 11, lowering a second production casing into the second L-shaped horizontal well, and using a reverse circulation method to inject a cleaning agent from the annular space between the second production casing and the second technical casing, and discharge the cleaning agent from the second production casing for reverse circulation cleaning; Step 12, opening the staging collar of the second production casing, injecting cement slurry from the inside of the second production casing back to the inclination point of the second L-shaped horizontal well, closing the staging collar after cementing to perform large-volume and long-term circulation until there is no cement inside the second production casing of the second L-shaped horizontal well, and completing the drilling and completion construction of the second L-shaped horizontal well; Step 13, sweeping and plugging the second L-shaped horizontal well; inserting a casing cutter into the second L-shaped horizontal well, cutting the second production casing at the inclination point of the second L-shaped horizontal well to divide it into an upper part and a lower part, and after recovering the upper second production casing, sweeping and plugging the first L-shaped horizontal well; Step 14, connecting the surface casing and the first technical casing at the bottom to complete the drilling and completion construction of the L-shaped horizontal well for multi-layer co-mining of coalbed methane in thin coal seams.

2. The L-shaped horizontal well drilling and completion construction method for multi-layered coalbed methane mining in thin coal seams as claimed in claim 1, characterized in that: In step 3, during the process of drilling to the first L-shaped horizontal well inclination point using the PDC drill bit, the full angle change rate is controlled within 6° / 30m.

3. The L-shaped horizontal well drilling and completion construction method for multi-layered coalbed methane mining in thin coal seams as claimed in claim 1, characterized in that: The first production casing and the second production casing have the same specifications; The first production casing comprises a Φ139.7 mm steel pipe of steel grade N80, a graded hoop pipe string and a glass fiber reinforced plastic screen pipe string.

4. The L-shaped horizontal well drilling and completion construction method for multi-layered coalbed methane mining in thin coal seams as claimed in claim 1, characterized in that: The first technical casing and the second technical casing have the same specifications; The first technical casing adopts a Φ193.7 mm technical casing of steel grade J55.

5. The L-shaped horizontal well drilling and completion construction method for multi-layered coalbed methane mining in thin coal seams as claimed in claim 1, characterized in that: In step 9, when a whipstock is lowered into the inclination point of the second L-shaped horizontal well for plugging, a combination of an isolation packer and a slip setting seal is used at the lower part of the whipstock to achieve plugging.

6. The L-shaped horizontal well drilling and completion construction method for multi-layered coalbed methane mining in thin coal seams as claimed in claim 1, characterized in that: In step 14, the surface casing has a specification of Φ273.1 mm; The head of the surface casing is 70 cm below the ground, and the head of the first technical casing is 30 cm below the ground; The surface casing and the first technical casing are welded by an annular steel plate, and a casing head is fixedly sleeved at the connection between the surface casing and the first technical casing.

7. The L-shaped horizontal well drilling and completion construction method for multi-layered coalbed methane mining in thin coal seams as claimed in claim 1, characterized in that: In step 2, a Φ346.1 mm roller drill bit is used to drill to 10 m below the bedrock surface; In step 3, a Φ241.3 mm PDC drill bit is used to drill to the first L-shaped horizontal well inclination point; When the well inclination reaches 40°, the drill is started and a Φ215.9mm PDC drill bit is used in combination with a Φ165mm / 1.5° single-bend screw to continue steady drilling. The waiting time for setting is 48 hours; In step 5, a Φ241.3 mm PDC drill bit is used to drill from the inclination point of the first L-shaped horizontal well along the wellbore trajectory of the first L-shaped horizontal well to the first target coal seam and then 1-2 m of coal is incorporated to complete the drilling; In step 6, a Φ171.5 mm PDC drill bit is used to drill along the first target coal seam; In step 9, a Φ218 mm milling cone is used to perform window side drilling on the first technical casing from the inclination point of the second L-shaped horizontal well. After the window is completed, the drill is replaced with a Φ215.9 mm PDC drill bit to drill 15 m, and the window in and out eye-marking operation is repeated multiple times; In step 10, a Φ215.9 mm PDC drill bit is used to drill along the wellbore trajectory of the second L-shaped horizontal well to the second target coal seam and incorporate 1-2 m of coal.

8. The L-shaped horizontal well drilling and completion construction method for multi-layered coalbed methane mining in thin coal seams according to any one of claims 1 to 7, characterized in that: The cleaning agent is a foam cleaning agent.

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