A method for drilling a high-steep coal seam S-J composite bedding horizontal well by one trip

Abstract

This invention discloses a single-run drilling method for S-J composite bedding horizontal wells in steep coal seams. The method divides the coal seam dip angle into 35° and 65° zones, employing a two-stage, single-run drilling approach to eliminate the need for a pilot hole. Utilizing triple stratigraphic markers—the roof sandstone, coal seam, and floor carbonaceous mudstone—and a dynamic target offset algorithm, it achieves precise coal entry without a pilot hole. A continuous series of S-stable and J-downdip composite trajectories are used along the wellbore. The S-section reduces curvature to suppress bedding slip and collapse, while the J-section relies on gravity settling to remove coal dust. In the stress-relieving S-section, multiple branch wells are drilled side-drilled to achieve simultaneous mining of multiple thin coal seams. After completion, a screen is installed for cement-free cementing, and biodegradable coated sand is used for temporary support. Differentiated drilling parameters are matched to each section. This invention integrates seven major processes for coordinated construction, resulting in a short drilling cycle, high coal seam encounter rate, zero downhole collapse and stuck pipe accidents, lower overall cost, and no permanent reservoir conductivity damage. It is suitable for large-scale development of steep, multi-layered, thin coal seams in areas such as the Xinjiang foreland region.

Description

Technical Field

[0001] This invention belongs to the field of directional drilling engineering technology for coalbed methane, and in particular, it is a one-trip drilling method for SJ composite horizontal wells in high and steep coal seams. It is applicable to the construction of directional horizontal wells in the Xishanyao Formation and Badaowan Formation of the Xinjiang foreland thrust belt, which are characterized by high and steep, broken, multi-layered vertically stacked thin coal seams. The applicable dip angle range for the coal seams is 15° to 70°. At the same time, it can be extended to the drilling and development of other high and steep multi-coal-seam blocks in foreland compression structures in China. Background Technology

[0002] The Xinjiang foreland thrust belt, subjected to plate compression, has formed numerous folds and thrust faults. The target coal seam exhibits five typical geological characteristics: First, the coal seam dip angle varies greatly, continuously changing from 15° to 70° along the wellbore trajectory. Industry engineering practice has confirmed that the 35°–65° interval of bedding shear is extremely strong, belonging to a high-risk section for wellbore slippage and collapse. Second, the coal bodies of the Xishanyao Formation and Badaowan Formation are a mixture of fine coal and mylonite, with a fragmented coal body structure. Drilling disturbance easily generates a large amount of fine coal dust. First, drilling fluid is prone to accumulation in steep, upward-dipping sections, leading to stuck drill bits and buried drill bits. Second, the coal seam has highly developed internal cleavage and structural fractures, resulting in poor formation integrity, easy leakage of drilling fluid, and weak wellbore self-stabilization. Third, multiple thin coal seams are vertically stacked with a spacing of 8–15 m, meaning that traditional single-horizontal wells can only develop a single layer, resulting in low utilization of coalbed methane resources. Fourth, the region exhibits significant anisotropy in geostress, with substantial differences in collapse and fracturing pressures in different dip angle sections, making it impossible to adapt uniform drilling parameters to the entire well trajectory.

[0003] Most existing coalbed methane directional drilling technologies are designed for the development of gently dipping, intact coal seams in the Qinshui Basin and Ordos Basin. They lack an integrated, one-trip drilling technology suitable for the crushed, steep, and multi-coal seams in the foreland of Xinjiang. Traditional technologies generally suffer from multiple shortcomings, such as wellbore instability, coal dust stuck drill bits, repeated tripping and tripping of multiple processes, difficulty in multi-layer mining, and damage to the reservoir by cementing.

[0004] Current publicly available drilling technologies and conventional mining area construction schemes have four inherent flaws, and simply combining various individual technical methods cannot simultaneously solve all the pain points of the project: 1. Simple trajectory configuration, continuous SJ composite wellbore scheme without partitions Existing technologies only design S-shaped stable inclination trajectories or J-shaped downdip trajectories separately, without proposing an integrated configuration that seamlessly connects "updip along the bedding plane - S-shaped stable inclination transition section - J-shaped downdip extension section" in the same wellbore according to the coal seam dip angle. Traditional L-shaped fixed dip angle horizontal wells are prone to bedding slip and collapse in the high-risk section of 35° to 65°. The S-shaped trajectory alone does not have a follow-up J-shaped downdip structure, and coal dust continues to accumulate in the steep section. The J-shaped trajectory alone does not have a preceding S-shaped buffer section, and large-area wellbore instability occurs in the high-risk section. The two types of single trajectories can only alleviate local single risks and cannot simultaneously achieve the dual goals of preventing collapse and gravity coal dust removal.

[0005] 2. Coal seam identification methods are limited, necessitating the pre-drilling of pilot holes to determine the undulations of the coal seam. Existing gentle coal seam guidance technology relies solely on a single rock stratum interface to identify the coal seam. When facing folded and densely faulted strata in Xinjiang, wellbore penetration of the roof and floor is highly likely. The industry-standard solution requires drilling a pilot well of about 200m to determine the depth and undulation of the coal seam before backfilling and re-drilling. The pilot well process in the Gobi Desert results in significant losses of pipe materials, mud, labor, and construction time. Currently, there is no complete guide well-oriented solution that combines triple stratum marker linkage identification with dynamic target point offset algorithm.

[0006] 3. Inappropriate selection of multi-branch side drilling points makes it impossible to achieve multi-level synchronous mining. Conventional multi-branch horizontal wells place the side-drilling window opening point in the build-up section or the straight horizontal section. The formation stress is concentrated in the build-up section, and the window opening process is very likely to induce wellbore collapse. Moreover, without the matching design of SJ composite trajectory, each branch horizontal section can only rely on a single coal seam, and it is impossible to develop multiple stacked thin coal seams simultaneously, resulting in low resource extraction efficiency.

[0007] 4. The wellbore structure, drilling parameters, and completion system are independent of each other, and there is no integrated solution. Traditionally, a three-section wellbore structure is used, requiring multiple trips for directional drilling, stabilization, and branch operations, resulting in a long construction cycle. The same set of drilling fluid properties and drilling parameters are used throughout the well, which cannot match the stability and proppant carrying requirements of formations with different dip angles. Cement cementing is commonly used for well completion, but the cement slurry penetrates into the fractured coal seam fissures, permanently blocking the flow channels and significantly reducing the stable production cycle of coalbed methane.

[0008] In summary, existing publicly available technologies only disclose isolated technical features such as S-trajectory, J-trajectory, single-trip drilling, branch horizontal wells, logging while drilling, and screen completion. Drilling technicians in the field cannot obtain the complete and coordinated drilling solution of the seven major processes of this invention by simply piecing together existing technologies. A single module can only solve local and single construction problems when used alone, and cannot simultaneously achieve multiple technical effects such as preventing collapse in high-risk sections, discharging coal powder in steep sections, eliminating guide hole to reduce costs, multi-layer simultaneous mining, and no reservoir damage. Summary of the Invention

[0009] To address the shortcomings of existing drilling techniques for steep coal seams, which are often fragmented and simply overlaid, failing to simultaneously address multiple construction objectives, this invention provides a single-trip drilling method for SJ composite horizontal wells in steep coal seams, simultaneously resolving the following four engineering bottlenecks: 1. A single S or J trajectory cannot adapt to the continuous changes in dip angle throughout the well, and the two major risks of slippage and collapse in high-risk steep dip sections and coal dust accumulation in steep sections cannot be addressed simultaneously; 2. Directional drilling of high and steep folded coal seams requires pre-drilling pilot holes, resulting in long drilling cycles and high overall construction costs in the Gobi mining area; 3. The branch side drilling window opening points are unreasonable, the risk of window collapse is high, a single well can only mine a single thin coal seam, and the resource utilization rate of multi-layered coal seams is low. 4. Poor adaptability of uniform drilling parameters across the entire well, and cement cementing causes irreversible reservoir conduction damage to fractured and steep coal seams.

[0010] This invention integrates seven major processes: geological zoning modeling, continuous SJ composite trajectory, triple marker-free guide eye guidance, S-segment multi-branch combined production, zone-differentiated drilling parameters, and cement-free coating sand completion. It simultaneously achieves multiple optimized technical effects and overcomes the inherent shortcomings of simple splicing in existing technologies.

[0011] A single-pass drilling method for SJ composite horizontal wells in steep coal seams integrates seven interdependent and inseparable collaborative processes. The entire construction process logic is as follows: Figure 1 As shown in the figure; the core innovation is based on the three-zone division standard of coal seam dip angle, forming an integrated composite wellbore by continuously and seamlessly connecting an S-shaped stable transition trajectory and a J-shaped downdip extension trajectory along the same wellbore. This is complemented by a complete set of supporting technologies including a guide-hole-less triple-marker geological steering system, S-stable section branch sidetracking, segmented differentiated drilling parameters, and cement-free biodegradable coated sand completion. All construction steps are described in claim 1. The innovative module functions, synergistic mechanisms, and differences from existing technologies in the seven process steps are as follows: S1: Three-zone modeling and critical dip correction module for coal seam dip angle Collect 3D seismic interpretation data of the target block, complete logging curves of multiple adjacent wells, and logging depth data of the top and bottom plates of the coal seam to construct a 3D digital occurrence model of the coal seam; extract formation mechanics parameters such as the coal body fracture coefficient and the difference between the maximum and minimum horizontal principal stresses of the block, and modify the critical thresholds of dip angles of 35° and 65° in combination with the characteristics of the fractured coal body in the mining area, and divide the well trajectory into three independent drilling zones: low dip angle zone α < 35°, high-risk steep dip zone 35° ≤ α ≤ 65°, and steep dip angle zone α > 65°.

[0012] Existing technologies simply define dip angle ranges without considering the stress and structure of steep, fractured coal seams to correct zoning standards. The three-zone division is the sole criterion for determining the timing of subsequent SJ trajectory switching and matching of segmented drilling parameters. Without precise zoning standards, trajectory switching and parameter control lack a scientific basis, making it impossible to address wellbore instability and coal dust accumulation issues in a layered and targeted manner. The complete drilling parameter comparison standards for the three zones are detailed in the table below, which visually displays the threshold values ​​of parameters such as drilling pressure, rotational speed, drilling fluid discharge, and fluid density for the three dip angle zones, intuitively demonstrating the segmented and precise control process.

[0013] Table 1. Drilling Parameter Matching Table for Three Inclination Zones

[0014] S2: Simplified wellbore structure module for two-stage drilling. Abandoning the traditional complex construction system of three-section wellbore + independent pilot well, a simplified two-section, one-trip drilling structure is adopted, with the overall wellbore structure as follows: Figure 2 The first vertical well section is drilled through the upper Quaternary loose silt, water-sensitive mudstone, and highly permeable sandstone layers. The surface casing is then installed and cemented to isolate the shallow collapse and mud leakage from interfering with the lower directional drilling operations. After the casing has solidified, the entire directional drilling tool assembly is installed. The entire process is completed without stopping the drill string, continuously completing all directional drilling procedures, including directional drilling, S-shaped stabilization transition, J-shaped downdip extension, and multi-branch horizontal section window side drilling. The independent pilot hole drilling, backfilling, and re-drilling procedures are eliminated.

[0015] If only the two-stage drilling structure is used alone, without the accompanying SJ composite trajectory of this invention, the construction process will still frequently encounter complex downhole problems such as wellbore penetration through coal seams, slippage and collapse, and coal dust stuck drill bits. This module, in conjunction with the composite trajectory, guide hole-less guidance, and branch side drilling depth, significantly reduces the continuous construction cycle in the Gobi Desert and lowers the comprehensive costs of equipment rental, drilling consumables, and on-site labor.

[0016] S3: Triple stratigraphic marker linkage + dynamic target offset eyeless guidance module This module represents an innovative approach to reducing costs and improving coal seam drilling success rates. Unlike single-layer strata interface identification methods, it selects three unique stratigraphic markers for high-steep coal seam areas: a stable and continuous roof fine-grained sandstone layer, the coal seam itself, and a floor carbonaceous mudstone layer. Throughout the drilling process, real-time monitoring of the stratigraphic interface is conducted using gamma ray and resistivity. A dynamic target offset algorithm is also included to collect real-time logging interface depth data, simultaneously fine-tune the coordinates of the build-up target point in both horizontal and vertical directions, and adaptively compensate for local undulations in the coal seam caused by folds and small faults, completely eliminating the need for pre-drilled pilot wells.

[0017] Conventional single-marker guidance is only suitable for coal seams with gentle strata and simple structures, and cannot cope with the frequent undulations of coal seams in steep and folded strata in Xinjiang. This triple-marker linkage guidance, in coordination with the SJ composite trajectory, can directly and seamlessly connect the low-angle updip section and the high-risk S buffer section after the coal seam lands accurately, without the need for significant adjustments to the wellbore attitude in the middle, and can achieve continuous drilling along the coal seam throughout the well.

[0018] S4: Core Innovative Module for Partitioned Continuous Seamless SJ Composite Trajectory The most critical distinguishing technical feature of this invention is the complete configuration of the composite wellbore, as shown below. Figure 3 Three integrated continuous trajectories are sequentially connected along the same wellbore according to the dip angle. The three trajectories are used to control formation risks in a layered and targeted manner. It is impossible to achieve the dual treatment effect of preventing collapse and gravity coal dust discharge simultaneously by using any single trajectories alone. 1) Low dip angle updip joint section: adopt updip joint drilling with a low build-up rate of 2.5° / 30m to control the wellbore to always be located in the middle of the coal seam thickness and steadily improve the drilling efficiency of the basic coal seam; 2) High-risk steep dip S-shaped stable and inclined transition section: The core formation buffer structure actively reduces the build-up rate to 1.8° / 30m, and the drilling direction is adjusted from drilling along the dip of the coal seam to stable inclination drilling along the strike of the coal seam, reducing the bedding shear stress caused by the wellbore curvature, and suppressing large-area slip and collapse in the 35°~65° high-risk section from the root; at the same time, the formation stress in this section is balanced and the curvature is gentle, making it the only section in the entire well suitable for branch window sidetracking; 3) J-shaped downward dip extension with steep inclination: Seamlessly and continuously connected to the end of the S-shaped trajectory, the wellbore extends continuously along the downward dip direction of the coal seam. Relying on gravity, the broken coal powder that falls off automatically sinks downward and is carried to the wellhead by the drilling fluid annulus circulation. This eliminates the industry problems of coal powder accumulation and stuck drill bit burial in the steep upward dip section from the perspective of fluid dynamics.

[0019] Existing technologies only arrange S-trajectories or J-trajectories separately, without a continuous series integrated configuration along the wellbore. A single trajectory can only solve the risk of a single section. After the two are coupled in a section and matched with the segmented drilling parameters, an unexpected technical effect of synergistic wall stabilization and efficient sand carrying is formed.

[0020] S5: Segmented Differentiated Drilling Parameter Control Module To address the differences in formation collapse pressure, coal body fragmentation, annular sand carrying capacity, and mud leakage risk across the three dip zones, specific ranges of drilling pressure, rotary table speed, annular displacement, and drilling fluid density parameters were matched. The complete parameters for each zone are shown in Table 1 above. In the high-risk S transition zone, the annular displacement and drilling fluid density were increased to enhance wellbore support and coal dust carrying capacity. In the steep J extension zone, ultrafine calcium carbonate plugging agent was added to the base slurry to form a high-density plugging drilling fluid, which sealed micro-fractures in the coal seam and suppressed formation mud loss and localized small-block collapses.

[0021] Traditional unified drilling parameters for the entire well cannot balance the relationship between wellbore stability, coal dust carrying capacity, and drilling efficiency; setting only segmented parameters without a corresponding zoned SJ composite trajectory as a carrier means that parameter control cannot be applied to risky formations in a layered manner, and cannot play the role of layered wellbore stabilization and efficient slag removal.

[0022] S6: S-Smooth Transition Section Multi-branch Three-Dimensional Combined Mining Module Only the S-shaped stable transition section with gentle stress and small curvature change is selected as the sole reference point for branch windowing, avoiding the two major construction risks of high stress concentration in the low-angle build-up section and easy instability of the well wall in the steep J-shaped section. Multiple short-radius branch horizontal sections are constructed through directional windowing, each corresponding to an independently stacked thin coal seam, forming a three-dimensional mining well network with a main wellbore and multiple branches. The well network layout is as follows: Figure 4This enables simultaneous pressure-divided mining of multiple thin coal seams in a single well.

[0023] The existing branch horizontal well opening point settings are unreasonable, and the opening disturbance can easily induce well wall collapse. Moreover, there is no SJ composite trajectory to coordinate with the well, and the branch well can only rely on a single coal seam for mining, resulting in a low coalbed methane resource utilization rate. This invention relies on the low stress buffering advantage of the S section to greatly improve the success rate of branch opening construction, and utilizes multiple stacked thin coal seams at one time, significantly increasing the recoverable reserves of a single well.

[0024] S7: Cementing-free screen + biodegradable coated sand reservoir protection completion module After all directional and branch side-drilling procedures are completed, pre-drilled anti-corrosion screen pipes are directly lowered in. No cement cementing operations are carried out throughout the process, which completely prevents cement slurry from invading the broken coal seam and blocking the endogenous cleavage and structural flow-conducting fractures, thus avoiding permanent gas production damage to the reservoir. For the micro-collapse fractures in the well wall formed by drilling S-type and J-type steeply inclined sections, biodegradable coated sand is used to temporarily support the well wall. The coating layer completely degrades in 3 to 6 months under the long-term immersion environment of formation water and coalbed methane fluids. It only maintains the integrity of the well wall and screen pipe for a short period of time. After degradation, it will not block the coal seam flow-conducting channels for a long time.

[0025] Using a cement-free screen pipe process alone, with the SJ composite wellbore stabilizing the formation as support, continuous sliding and collapse in steeply inclined sections will squeeze and deform the screen pipe, causing well completion failure; the two are used in combination to balance wellbore stability during the construction phase and long-term coalbed methane production and conductivity.

[0026] The integrated collaborative solution of the present invention brings the following unexpected technical effects: To visually demonstrate the significant technological advancements of this integrated solution compared to existing single technologies and simple superposition solutions, three technical routes are categorized for theoretical comparison of indicators: Route 1 (Traditional Conventional Process): Single L-shaped fixed dip well, three-section wellbore structure, pre-drilled pilot well, uniform drilling parameters throughout the well, single-layer horizontal section mining, and cemented well completion. This route cannot adapt to continuous dip changes in steep coal seams, leading to frequent slippage and collapse in high-risk sections, coal dust stuck pipe in steep sections, long drilling cycles, low coal seam encounter rate, single-layer mining only, and permanent flow damage to the reservoir. Relevant indicators are taken from the construction records of 12 traditional production wells in the Xishanyao Block of Zhunnan, Xinjiang, supplemented by statistical averages from publicly published drilling technology literature in the industry.

[0027] Route 2 (simple overlay of existing technology): This route only involves independently arranged S and J trajectories in segments, without continuous seamless connection along the wellbore. It lacks differentiated drilling parameters for different inclination zones and still requires pre-drilling pilot wells. It can only slightly alleviate the construction risks of a single section locally, and cannot simultaneously address wellbore collapse prevention and gravity coal dust removal. The wellbore frequently penetrates the top and bottom of the coal seam, and the overall construction period remains relatively long. The indicators are based on the actual measurement results of previous small-scale pilot tests in the mining area.

[0028] Route 3 (Integrated and Collaborative Complete Process of This Invention): The seven major processes of dip angle three-dimensional partition modeling + continuous seamless SJ composite trajectory + triple marker eyeless guidance + S-section multi-branch three-dimensional combined production + segmented matching drilling parameters + cement-free coating sand reservoir protection are coupled and matched; the indicators are derived from the complete on-site measured construction data of the industrial test well of this invention.

[0029] Differences in core quantitative indicators among the three routes: Route 1, using traditional technology, has an average drilling cycle of 32 days and a comprehensive coal seam encounter rate of 73%; the industrial implementation of this invention has a single-well drilling cycle of 21 days, which is about 34.3% shorter than Route 1, and the coal seam encounter rate has increased from 73% to 98.2%. Complex downhole accidents such as slippage, collapse, stuck drill, and buried drill have been eliminated throughout the construction process; eliminating the pilot hole process can reduce the overall drilling cost by 27%, and a single well can simultaneously mine 2-3 sets of stacked thin coal seams, with no permanent damage to the reservoir from cement sealing, and the stable gas production cycle of coalbed methane is extended by 18%; Route 2's various construction indicators are only slightly better than Route 1, and the overall improvement is far less than that of the integrated drilling technology of this invention.

[0030] This proves that using or simply combining known single technologies such as S-trajectory, J-trajectory, single-pass drilling, and branch horizontal wells alone cannot simultaneously achieve the synergistic optimization of multiple engineering indicators. The seven processes of this invention are interdependent and indispensable, and the overall result produces unexpected technical effects that exceed the simple superposition of the effects of using each module individually. The solution has prominent substantive features and significant progress.

[0031] In summary, compared with the prior art, the beneficial effects of the present invention are as follows: (1) Advantages of composite trajectory collaborative risk control: This invention is the first to create a continuous seamless series SJ integrated wellbore (attached) along the wellbore dip angle. Figure 3 The system is equipped with segmented and differentiated drilling parameters (Table 1) to simultaneously address the two core engineering challenges of steep coal seams: bedding slip and collapse in the 35°–65° high-risk section and coal dust accumulation causing stuck drill bit in the steep section. Individual S-trajectories and individual J-trajectories can only address a single risk. The two are coupled in a segmented and continuous manner with dedicated drilling parameters to form a collaborative governance system, achieving unexpected wellbore stability and efficient sand-carrying technology.

[0032] (2) Advantages of shortening the construction period and reducing overall costs: Triple formation marker linkage without guide eye guidance combined with simplified two-stage drilling (attached) Figure 2 This method completely eliminates the need for pilot hole drilling, backfilling, and repeated tripping and jacking, significantly reducing the continuous construction cycle in the Gobi Desert and decreasing the overall investment in drilling materials, equipment rental, and on-site labor. Existing conventional processes for high and steep coal seams cannot achieve precise coal seam drilling without pilot holes.

[0033] (3) Advantages of efficient development of multi-layered coal seam resources: Select the S-transition section with gentle stress and drill multiple short-radius branch horizontal wells (attached) Figure 4 The simultaneous use of multiple vertically stacked thin coal seams in a single well significantly improves the utilization rate of recoverable coalbed methane reserves; conventional branch wells have unreasonable window locations, high risk of construction collapse, and can only mine single layers, resulting in low resource utilization efficiency.

[0034] (4) Advantages of long-term reservoir protection: The use of cement-free screen pipes and biodegradable coated sand to temporarily support steeply inclined well walls completely prevents cement slurry from invading the broken coal seam and causing permanent blockage of the flow-conducting fractures; the coated sand only supports the well wall for a short period of time and is completely degraded in the later stage, thus preserving the coal seam cleavage and structural fracture flow channels in the long term, taking into account both the wellbore stability during the construction period and the long-term stable production capacity of coalbed methane.

[0035] (5) Overall creative value: The seven core construction procedures of this invention are intertwined and inseparable, and do not belong to the simple patchwork of existing scattered known technologies; the whole integrated drilling process can simultaneously achieve five superimposed technical objectives: high-risk section well wall collapse prevention, gravity automatic coal powder discharge in steep sections, elimination of pilot hole to reduce construction costs, multi-layer synchronous mining in a single well, and no permanent damage to the reservoir; individual or partial module combinations cannot achieve all the gain effects, and the solution is not obvious.

[0036] (6) Feasibility of industrialization: This invention fully discloses the coal seam dip angle zoning determination criteria, SJ composite trajectory build-up rate control method, complete set of segmented drilling parameters, triple marker guide eyeless guidance implementation logic, S-section branch window point selection criteria, and complete set of implementation measures for cement-free well completion with coating sand; the complete set of standardized construction procedures is attached. Figure 1 This manual allows ordinary technicians in the field of coal seam directional drilling to completely replicate the entire drilling process without any creative effort, based on all the contents recorded herein. The manual includes two sets of complete industrialized implementation examples from different mining areas in Xinjiang, and one set of field-based traditional process control wells. All construction equipment, materials, and operating parameters can be mass-produced and promoted, making it suitable for all steep and thin coal seam development blocks in the foreland thrust belts of Xinjiang, including Zhunnan, Kubai, and Santanghu, with stable and reliable construction results. Attached Figure Description

[0037] Figure 1This invention presents a complete collaborative process flow diagram for drilling high and steep coal seams in a single run; it sequentially and completely demonstrates the following: three-dimensional coal seam occurrence modeling and dip angle zoning → two-stage drilling and one-run wellbore construction → triple formation markers and pilot hole-free coal seam landing → continuous series SJ zone directional drilling → segmented differentiated drilling parameter control → S-stable section branch window side drilling and multi-layer combined production → cement-free, film-coated sand integrated well completion.

[0038] Figure 2 This is a comparative schematic diagram of the simplified wellbore structure of the present invention, which involves two sections and one drilling run. Compared with the traditional three-section wellbore structure with a guide hole, the present invention clearly shows the surface casing sealing section of the first section and the integrated continuous directional wellbore of the second section in one drilling run.

[0039] Figure 3 This is a schematic diagram of the continuous seamless series SJ composite zone trajectory configuration along the wellbore of the present invention; it intuitively presents the integrated continuous composite wellbore morphology of the low dip angle updip section, the high risk steep dip S-shaped stable transition section, and the steep dip angle J-shaped downdip extension section.

[0040] Figure 4 This is a schematic diagram of the planar layout of the multi-branch, multi-layer combined mining well network of the S-shaped smooth transition section of the present invention; taking the S-shaped smooth transition section of the main wellbore as the branch window base point, multiple short-radius branch horizontal sections correspond to different superimposed high and steep thin coal seams, forming a three-dimensional combined mining well network. Detailed Implementation

[0041] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0042] Example 1: Industrial-scale field drilling implementation in the Xishan kiln group of the Zhunnan Coalfield, Xinjiang I. Geological Overview of the Construction Foundation The test well was deployed in the Xishanyao Formation mining block of the Zhunnan Coalfield in Xinjiang. The area is controlled by north-south trending thrust faults, and the coal seam as a whole exhibits a continuous folded and undulating morphology. The wellbore encountered coal seam dip angles ranging from 20° to 68°, and along the directional drilling path, it successively passed through a 28° low dip angle section, a 52° high-risk steep dip angle section, and a 66° steep dip angle section. The coal body is a mixed structure of primary fragmented coal and mylonite, with broken coal bodies and highly developed endogenous cleavage and tectonic bedding. The maximum horizontal principal stress difference of the strata reaches 12 MPa, and the geostress anisotropy is extremely strong. Vertically, there are two sets of independent mineable thin coal seams with a vertical spacing of 12m. In previous years, the conventional method of Route 1 was used to construct 12 production wells in this block. On average, each well experienced at least one wellbore slippage and collapse or coal dust jamming, resulting in a shutdown and rework. The average drilling cycle was 32 days, and the average coal seam encounter rate was only 74%. Each well could only mine one set of thin coal seams, resulting in serious waste of coalbed methane resources.

[0043] II. Supporting construction equipment and well entry materials The first drilling bit uses a Φ444.5mm conventional scraper bit, paired with a Φ339.7mm surface casing; the second directional drilling bit uses a Φ215.9mm wear-resistant composite PDC directional bit, paired with a wireless logging-while-drilling (MLD) system and an integrated logging instrument for gamma ray and resistivity while drilling; the drilling mud uses a bentonite water-based mud system, and the plugging additive uses ultrafine calcium carbonate powder; the completion screen is a Φ139.7mm pre-drilled corrosion-resistant screen; the biodegradable coated sand has a particle size of 0.4-0.8mm, and the degradation period of the coating layer in the formation fluid environment is 3-6 months.

[0044] III. Complete Step-by-Step Standardized Construction Process The complete execution logic process of the entire construction is as follows: Figure 1 As shown, the simplified overall wellbore structure for a second-stage drilling run is as follows: Figure 2 The complete configuration of the SJ composite wellbore, continuously connected in series along the wellbore, is as follows: Figure 3 Multi-branch three-dimensional combined mining well network planar layout, such as Figure 4 Each section should be assigned its own drilling parameters, which should be uniformly referenced in Table 1. The complete step-by-step construction operation is as follows: S1. Geological Modeling and Dip Zoning: Collect 3D seismic interpretation results from 12 adjacent wells around the test well, continuous logging curves for the entire well section, and coal seam top and bottom depth logging data. Use geological modeling software to construct a 3D spatial digital occurrence model of the target coal seam. Extract the coal body fracture coefficient and the difference between the maximum and minimum horizontal principal stresses in the block. Combine the formation mechanical characteristics of the Xishanyao Formation to correct the critical dipping thresholds for 35° and 65° dip angles. Accurately divide the continuous drilling into three sections on the well trajectory: low dip angle, high-risk steep dip angle, and steep dip angle. Predict the risks of formation instability and coal dust accumulation in each section in advance.

[0045] S2. Second-stage drilling in one run: The first-stage vertical well section is continuously drilled to a final depth of 315m. A Φ339.7mm surface casing is run in and cemented to completely seal the upper Quaternary loose silt, water-sensitive mudstone, and high-permeability sandstone layers, isolating the shallow collapse and mud loss from interference with the lower directional operation. After the casing cement has set for 72 hours, the second-stage drilling operation is carried out. The entire directional drilling tool assembly is run in. After the entire set of drilling tools is run in, the drill string is not lowered at all. The entire directional process, including directional drilling, S-shaped stabilization transition, J-shaped downdip extension, and two multi-branch horizontal sections with window side-drilling, is completed continuously without interruption. No traditional pilot wells are designed or drilled throughout the entire process.

[0046] S3, Triple Stratigraphic Marker for Precise Coal Seam Landing Without Guide Hole: During the directional drilling phase, real-time stratigraphic monitoring using gamma ray and resistivity is activated throughout the drilling process. The triple stable stratigraphic markers of the roof fine-grained sandstone, coal seam coal rock, and floor carbonaceous mudstone serve as the coal seam identification benchmark. Real-time acquisition of well logging interface depth data is input into a dynamic target point offset algorithm. When small folds or local stratigraphic undulations are detected, the algorithm automatically and synchronously fine-tunes the coordinates of the directional drilling target point in both horizontal and vertical directions, dynamically adjusting the directional drilling rate in real time. Precise landing of the coal seam can be achieved in one go without drilling a guide hole in advance. The thickness of the coal seam encountered at the landing point is stably maintained at 1.8–2.2 m, without any penetration of the coal seam roof or floor.

[0047] S4. Sectional continuous series SJ composite trajectory directional drilling, wellbore morphology as shown in the appendix. Figure 3 : ① 28° low dip angle section: control the build-up rate at 2.5° / 30m, adopt the up-dip horizontal drilling mode, and arrange the wellbore in the middle of the coal seam thickness to steadily improve the drilling efficiency of the basic coal seam; ②52° high-risk steep dip section: seamlessly switch to a gentle S-shaped stable inclination transition trajectory, reduce the build-up rate to 1.8° / 30m, and change the drilling direction from along the dip of the coal seam to stable inclination drilling along the strike of the coal seam, which greatly reduces the shear stress of the well wall bedding and suppresses large-area slip and collapse from the root. ③66° steep dip section: Seamlessly connects to the J-shaped downward dip extension trajectory at the end of the S-shaped trajectory. Relying on the wellbore downward dip structure, the broken coal powder that falls off will automatically sink downward and be carried to the wellhead by the drilling fluid annulus circulation, thus eliminating the problem of coal powder accumulation and stuck drill bit from the root.

[0048] S5. Segmented matching of differentiated drilling parameters, with parameter values ​​for each zone strictly following Appendix 1 (Drilling parameter matching table for three dip angle zones): Low dip angle section: drilling pressure 50kN, rotary table speed 70r / min, drilling annulus displacement 30L / s, drilling fluid base density 1.05g / cm³; High-risk steeply inclined S-shaped transition zone: drilling pressure 32kN, rotary table speed 50r / min, drilling annulus displacement 34L / s, drilling fluid density increased to 1.12g / cm³; Steeply inclined J-shaped extension section: drilling pressure 40kN, rotary table speed 60r / min, drilling annulus displacement 32L / s, ultrafine calcium carbonate plugging agent added to the base slurry, drilling fluid density adjusted to 1.15g / cm³.

[0049] S6, S-stable section branch window drilling multi-layer combined mining: The S-shaped transition section with gentle stress and uniform curvature within the 52° high-risk steep dip section is selected as the sole branch window base point. Downhole window drilling tools are lowered to directionally drill two short-radius horizontal branch sections. These two horizontal branch sections correspond to two independently stacked thin coal seams, one above and one below. The branch well layout structure is as follows: Figure 4 This forms a three-dimensional synergistic well network with a main wellbore and two branches, with a designed horizontal section length of 800m for each branch.

[0050] S7. Integrated Reservoir Protection Completion without Cementing: After all directional and branch side-drilling operations are completed in the second phase, the entire set of directional drilling tools is pulled out, and a Φ139.7mm pre-drilled anti-corrosion screen pipe is directly run in without cementing. For the micro-collapse gaps in the well wall caused by drilling the 66°J-shaped steep extension section, biodegradable coated sand is filled with downhole filling tools to complete temporary well wall support, ensuring that the outer wall of the screen pipe is tightly attached to the coal seam well wall.

[0051] IV. Complete On-site Construction Measurement Data and Implementation Results The total construction period for this well, from initial drilling to screen installation and completion, was 21 days. The total coal seam drilling length of the main borehole plus two branch horizontal sections was 1920m, achieving a comprehensive coal seam drilling rate of 98.2%. Throughout the entire well construction process, no complex downhole accidents occurred, such as slippage, coal dust jamming, drill bit burial, or severe mud loss, and there were no records of work stoppages or rework. The well can simultaneously utilize two sets of recoverable coal seam resources, increasing the recoverable reserve utilization rate by 42% compared to traditional single-layer mining wells. The entire process of drilling, backfilling, and re-drilling the pilot well was eliminated, reducing the overall drilling material, equipment rental, and labor costs of the single well by 28%. After production, there was no problem of cement slurry blocking the coal seam flow fractures during the drainage process, and the stable and continuous coalbed methane production cycle was extended by 18% compared to traditional comparative wells in the block.

[0052] Comparative Example (Comparison Well with Traditional and Conventional Process in the Same Block) I. Construction Technology The adjacent production wells of the same Xishan kiln group adopt the industry's traditional L-type horizontal well process of Route 1: adopting a three-section well structure, it is necessary to drill a 200m pilot well in advance to explore the coal seam undulations and then backfill and re-drill; the whole well adopts uniform and fixed drilling parameters throughout the entire process, without segmented SJ composite trajectory design, only a single-layer horizontal section is arranged, and cement cementing operation is carried out after well completion.

[0053] II. Construction Measurement Results The total drilling cycle for a single well was 32 days, with a coal seam encounter rate of only 74.8% in the main horizontal section. During the directional drilling phase, two high-risk sections experienced bedding slippage and collapse, as well as coal dust accumulation and stuck drill failures, resulting in a total of 5 days of work stoppage and rework on the side drilling. The entire well could only mine a single thin coal seam, and the utilization rate of underground coalbed methane resources was extremely low. Cement slurry intruded into the broken coal body, blocked the near-wellbore flow channel, and the initial gas production declined rapidly. It was impossible to simultaneously achieve multiple construction goals, including wellbore stability, efficient coal dust removal, shortened construction period, and multi-layer synergistic mining. This clearly demonstrates that the simple splicing of existing scattered known technologies cannot achieve the comprehensive technical effect of the integrated collaborative drilling process of this invention.

[0054] Example 2: Drilling Implementation for Adaptation and Promotion in the Badaowan Formation of the Kubai Coalfield, Xinjiang I. Geological Overview of the Construction Foundation The test well is located in the Badaowan Formation mining block of the Kubai Coalfield in Xinjiang. The area has a higher degree of foreland compression fault development, with coal seam dip angles ranging from 18° to 72°. The wellbore passes through a 30° low dip angle section, a 58° high-risk steep dip angle section, and a 70° steep dip angle section. The coal body in this block is more fragmented than that in the Xishanyao Block of Zhunnan, with extremely well-developed structural micro-fractures, resulting in a greater risk of drilling mud leakage. Vertically, there are three independent mineable thin coal seams with a spacing of 8 to 15 meters. Traditional processes can only utilize one coal seam per well, leading to a more prominent problem of resource waste.

[0055] II. Standardized Construction Process The overall standardized construction process is completely consistent with Example 1, and the entire process flow is as shown in Appendix 1. Figure 1 The structure of the second well body is shown in the attached document. Figure 2 SJ composite wellbore trajectory reference appendix Figure 3 Based solely on local high-permeability and fractured strata, the boundary and construction parameters were slightly adjusted: the drilling fluid density in the steeply dipped J-shaped extension section was increased to 1.16 g / cm³, and the proportion of ultrafine calcium carbonate plugging agent was simultaneously increased; three short-radius branch horizontal sections were directionally drilled in the S-shaped smooth transition section, with each branch corresponding to one of the three sets of superimposed thin coal seams. The multi-branch well network layout is shown in the attached diagram. Figure 4 The selection criteria for drilling parameters in each section strictly follow the requirements of Table 1 (Drilling Parameter Matching Table for Three Inclination Zones).

[0056] III. On-site Implementation and Measurement Results The total drilling period for this test well was 22 days, with a comprehensive coal seam encounter rate of 97.6% for the main wellbore and three branch horizontal sections. There were no complex downhole accidents such as collapses, stuck drills, or formation losses during the entire construction process, and no downtime procedures were required. All three thin coal seams were simultaneously mined under pressure, significantly improving the utilization rate of coal seam reserves. The entire set of process parameters can be completely replicated and reused, and can be promoted and applied in batches in all high-steep, thin-coal-seam development blocks in the foreland thrust belts of Xinjiang, such as Zhunnan, Kubai, and Santanghu.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for drilling a high-steep coal seam SJ composite horizontal well in one pass, characterized in that: This includes the following integrated and collaborative construction steps: S1. Collect three-dimensional seismic exploration data and well logging data from adjacent wells to construct a three-dimensional spatial attitude model of the coal seam, and divide it into three differentiated drilling zones: low dip angle zone, high-risk steep dip angle zone, and steep dip angle zone, with 35° and 65° as critical dip angles. S2 adopts a two-stage, one-trip drilling structure. In the first stage, the loose and permeable strata above the ground are drilled through and the surface casing is installed for sealing. In the second stage, the entire set of directional drilling tools is installed. The entire process is completed without drilling, including directional drilling, SJ composite trajectory drilling along the strata, and multi-branch side drilling operations. No separate pilot well is drilled. S3. The drilling process relies on the triple stratigraphic markers of the roof sandstone layer, the coal seam body, and the bottom carbonaceous mudstone layer to identify the coal seam interface. Combined with the dynamic target point offset algorithm, the drilling target point is finely adjusted in two directions to adaptively compensate for the coal seam undulations caused by folds and small faults, so as to achieve precise landing of the coal seam without guide eye. S4. Continuous and seamless SJ composite adaptive trajectory along the same wellbore according to dip angle zones; low dip angle sections adopt low build-up rate updip drilling along the layers; high-risk steep dip sections adopt S-shaped strike stable transition drilling; steep dip angle sections are connected with J-shaped downdip extension drilling. S5. Match specific drilling pressure, rotary table speed, annular displacement, and drilling fluid density construction parameters for three types of dip angle zones; S6. Only in the S-shaped stable transition zone, multiple short-radius branch horizontal sections are drilled by directional window drilling. Each branch corresponds to an independent stacked thin coal seam, forming a main wellbore + multi-branch three-dimensional well network for synchronous multi-layer co-production. S7. After all directional and branch drilling is completed, pre-drilled screen pipes are directly lowered without cement cementing; for the slightly collapsed gaps in the well wall of the steeply inclined section, biodegradable coated sand is used to temporarily support the well wall.

2. The drilling method for a single-trip drilling of a high-steep coal seam SJ composite horizontal well according to claim 1, characterized in that: In step S4, the SJ composite trajectory collaborative risk control method is as follows: In the high-risk steep dip area, the S-shaped trajectory reduces the overall angle change rate, and the drilling direction is adjusted from the coal seam dip to the coal seam strike, weakening the bedding shear stress to suppress slippage and collapse; in the steep dip area, the J-shaped downdip extension section is seamlessly connected to the end of the S-shaped trajectory, and the coal powder is discharged by gravity settling through the wellbore downdip structure; it is impossible to achieve the dual effects of preventing collapse and efficiently discharging coal powder by using the S-shaped trajectory alone or the J-shaped trajectory alone.

3. The drilling method for a single-trip drilling of a high-steep coal seam SJ composite horizontal well according to claim 1, characterized in that: The dynamic target offset algorithm described in step S3 reads the drilling gamma and drilling resistivity logging data in real time, corrects the coordinates of the build-up target point in the horizontal and vertical directions simultaneously, and dynamically adjusts the build-up rate to prevent the wellbore from penetrating the roof or floor of the coal seam.

4. The drilling method for a single-trip drilling of a high-steep coal seam SJ composite horizontal well according to claim 1, characterized in that: Step S6 selects the S-shaped transition section as the basis for the side-drilling: formation stress balance, gentle wellbore curvature, and no large fracture structures; avoid stress concentration areas in the low-angle build-up section and unstable areas in the steep J-shaped section to reduce the risk of collapse in the branch window side-drilling.

5. The drilling method for a single-trip drilling of a high-steep coal seam SJ composite horizontal well according to claim 1, characterized in that: Step S1: Dip Angle Zoning Correction Method: Combine the coal body fragmentation coefficient and horizontal stress difference of the target mining area to correct the critical dip angle thresholds of 35° and 65°, adapting to the working conditions of mixed and fractured strata of granular coal and mylonite in the mining area.

6. The drilling method for a single-trip drilling of a high-steep coal seam SJ composite horizontal well according to claim 1, characterized in that: Step S5: Drilling parameter ranges for each zone: Low dip zone: drilling pressure 40-60 kN, rotary table speed 60-80 r / min, annular displacement 28-32 L / s, drilling fluid density 1.04-1.06 g / cm³; High-risk steep dip zone: drilling pressure 25-40 kN, rotary table speed 40-60 r / min, annular displacement 32-35 L / s, drilling fluid density 1.10-1.13 g / cm³; Steep dip zone: drilling pressure 30-50kN, rotary table speed 50-70r / min, annular displacement 30-33L / s; add ultrafine calcium carbonate plugging agent to the drilling fluid to form a high-density plugging drilling fluid system.

7. The drilling method for a single-trip drilling of a high-steep coal seam SJ composite horizontal well according to claim 1, characterized in that: The biodegradable coated sand in step S7 has a particle size of 0.4 to 0.8 mm. The coating layer completely degrades in 3 to 6 months under formation fluid conditions. It only supports the well wall for a short period of time and will not block the coal seam cleavage and flow fractures for a long period of time.

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