Pressure-maintaining coring method for fragile coal seam

Abstract

The invention discloses a pressure-maintaining coring method for a fragile coal seam, and relates to the technical field of pressure-maintaining coring, and the pressure-maintaining coring method comprises the following steps: S1, assembling a coring drill bit, and lowering the coring drill bit to a position L1 above a coring target layer; s2, a coring drill bit is lowered to the position L2, L1gt, above a coring target layer; l2, observing the hanging load change, judging the shaft bottom condition and cleaning the shaft bottom; s3, short-distance tripping is conducted, and the stability of the well wall is judged; s4, coring drilling is conducted according to preset drilling parameters, process parameters are recorded in the process, and drilling parameters are adjusted according to the process parameters; and S5, after coring is completed, core cutting is conducted, and the sample is pulled out to be recycled. The method achieves the purpose of avoiding low coring efficiency and poor sample quality caused by sand setting and chipping interference in the fragile coal seam in the coring process.

Description

Technical Field

[0001] The present invention relates to the technical field of pressure-maintained coring, and in particular to a pressure-maintained coring method for fragile coal seams. Background Art

[0002] In the exploration and development of coal resources, core drilling is an important technical means for obtaining geological samples from underground coal seams for detailed geological analysis and evaluation. Traditional coring processes often face many challenges when dealing with fragile coal seams. Due to the brittle and fragile nature of coal seams and coal-bearing strata, they are easily broken and crushed during drilling, resulting in large amounts of sand and debris deposited at the bottom of the well. These sand and debris not only interfere with coring operations, but may also cause the coring tool to misjudge the bottom of the well, thereby affecting the normal operation of the coring tool. In addition, when the coring tool drills over unstable sand and debris, it will disturb the coring target layer, further exacerbating the fracture of the coal seam, and even damage the coring tool, increasing the cost of coring.

[0003] In practice, the target coring layer is often located thousands of meters underground, making the bottomhole environment even more complex. Traditional coring techniques, unable to promptly remove sediment and debris, result in low sample yields, poor quality, and a high risk of tool damage. Therefore, how to efficiently and stably coring in fragile coal seams while protecting coring tools and reducing costs has become a pressing technical challenge.

[0004] In view of this, this application is hereby filed. Summary of the Invention

[0005] The purpose of the present invention is to provide a pressure-maintained coring method for fragile coal seams, so as to solve the problems of low coring efficiency and poor sample quality caused by interference from sand settling and falling blocks in fragile coal seams in existing coring processes.

[0006] The embodiment of the present invention is implemented through the following technical solution: The embodiment of the present invention provides a pressure-maintaining coring method for fragile coal seams, comprising the following steps: S1: Assemble the core drill bit and lower it to a position L1 above the target layer; S2: Lower the coring drill bit to a position L2 above the target coring layer, L1>L2, observe the change in the hanging weight to determine the bottom hole condition and clean the bottom hole; S3: Perform short trips to determine wellbore stability; S4: performing core drilling according to preset drilling parameters, recording the process parameters and adjusting the drilling parameters according to the process parameters; S5: After coring is completed, the core is cut and the drill is pulled out to recover the sample.

[0007] Specifically, during the lowering process, the changes in hanging weight are monitored in real time through the ground operating console. It should be noted that the hanging weight refers to the weight of the drill bit in the well, and under normal circumstances, the hanging weight should remain relatively stable. If the hanging weight drops abnormally, it indicates that there may be sand sedimentation or falling blocks at the bottom of the well. At this time, the bottom of the well is cleaned by circulating the drilling fluid until the hanging weight returns to normal, indicating that the bottom of the well has been cleaned. By observing the changes in hanging weight, the embodiment of the present invention can detect abnormal conditions at the bottom of the well in advance, avoid the coring tool from misjudging the bottom of the well position, reduce the risk of coring failure due to bottom instability, and improve the success rate of coring.

[0008] By performing short trips and drilling operations, the changes in the hanging weight are observed to determine the stability of the wellbore. An abnormal decrease in the hanging weight indicates wellbore instability, possibly indicating sand settling or falling blocks. In this case, a certain amount of thick slurry is injected from above the target coring layer to prevent the falling blocks and sand from reaching the wellbore bottom. This embodiment of the present invention can detect wellbore instability in advance, allowing timely measures to prevent wellbore collapse and ensure the safety and stability of the drilling process.

[0009] During the coring drilling process, process parameters such as drilling time, drilling pressure, and displacement are recorded in real time and adjusted based on changes in these parameters. For example, a significant increase in drilling time indicates that the formation has hardened or fractured, necessitating adjustments to the drilling pressure and displacement. By dynamically adjusting drilling parameters, embodiments of the present invention optimize the drilling process, improve drilling efficiency, reduce drill tool wear, and lower coring costs.

[0010] As an optional implementation, the short trip described in S3 includes raising the coring drill bit back to a position L1 above the target coring layer and then lowering it to a position L2 above the target coring layer, repeating this process several times. This short trip operation needs to be repeated several times to ensure the accuracy and reliability of the monitoring results. Repeating this operation allows for multiple verifications of wellbore stability, preventing misjudgments due to the chance of a single operation.

[0011] As an optional implementation, the determining of the wellbore stability problem in S3 includes observing the change of the suspended weight to determine the wellbore stability; If the hanging weight decreases abnormally, it indicates that the well wall is unstable. Inject thick slurry from above the core sampling target layer to prevent the falling of blocks and sand to the bottom of the well.

[0012] As an optional implementation, the lowering speed in S2 is 0.3-0.8 m / s; The method of observing the change of the hanging weight to judge the bottom condition of the well includes: observing the change of the hanging weight through the ground operating platform during the lowering process. If the hanging weight decreases abnormally, it indicates that there is sand or debris at the bottom of the well; The cleaning of the well bottom includes: circulating the drilling fluid until the suspended weight returns to normal, indicating that the well bottom has been cleaned.

[0013] As an optional implementation manner, recording the process parameters in S4 includes recording the process parameters every 0.5 m or 1 m of drilling, wherein the process parameters include at least one of drilling time, displacement, and bit pressure; The adjusting of the drilling parameters according to the process parameters includes: if the drilling time is greater than 1.5 times the drilling time before coring, the coring drill bit is lowered again to re-coring; If the drilling pressure increases by 10-50 kN, adjust the drilling pressure and continue drilling; If the recorded drilling time is restored to less than 1.5 times the drilling time before coring, it means that the crushing section has been passed and the original drilling pressure parameters can be restored to continue drilling. If the drilling pressure does not decrease or increases abnormally by more than 50 kN during drilling, or if drill slippage and / or drill holding occurs, it indicates that the core is blocked and the drilling should be stopped and the drill should be pulled out.

[0014] As an optional implementation manner, between S2 and S3 further includes: S6: If large pieces are found in the well that cannot be cleaned by circulating drilling fluid, reduce the displacement, increase the drilling pressure, grind and trim the pieces, and then clean them.

[0015] As an optional embodiment, the bottom end of the coring drill bit includes a coring cavity, and the upper end of the coring cavity is connected to the pressure-maintaining cavity; The outer wall profile of the coring cavity is a streamlined structure that gradually becomes larger from bottom to top, and the bottom of the flow channel is higher than the bottom of the coring cavity.

[0016] As an optional embodiment, the coring cavity includes an upper cavity and a lower cavity, the upper end of the upper cavity is connected to the pressure-maintaining cavity, and the bottom thereof is connected to the lower cavity; The inner diameter of the lower cavity is smaller than that of the upper cavity, the inner diameter of the upper cavity is smaller than that of the pressure-maintaining cavity, and the central axes of the upper cavity, the lower cavity and the pressure-maintaining cavity coincide with each other.

[0017] As an optional embodiment, at least three ridges are provided on the outside of the drill body, and there is a gap between two adjacent ridges; There are four rib plates, which are arranged in a circular array along the outer wall contour of the coring cavity.

[0018] As an optional embodiment, a plurality of PDC teeth are arranged on the outer surface of each rib plate; The PDC teeth are of an outwardly convex arc structure, and there is a gap between two adjacent PDC teeth.

[0019] The streamlined core drill bit and flow channel design of this embodiment allow debris, loose pieces, and sand generated during drilling to be smoothly discharged through the flow channel, preventing accumulation at the well bottom. A water barrier at the end of the drill bit prevents drilling fluid from eroding and contaminating the sample, ensuring sample integrity.

[0020] Compared with the prior art, the embodiments of the present invention have the following advantages and beneficial effects: 1. The embodiments of the present invention clean the well bottom to remove sand and debris at the well bottom to prevent sample contamination. By performing short trips and dynamically adjusting drilling parameters, the drilling process is optimized, drill tool wear is reduced, and drilling efficiency is improved. By detecting well wall instability in advance and taking measures, the risk of coring failure due to well wall instability is reduced.

[0021] 2. The core drill bit of this embodiment utilizes a streamlined body and flow channel configuration, allowing debris, loose pieces, and sand generated during drilling to be smoothly discharged through the flow channel, preventing accumulation at the well bottom. A water barrier at the end of the drill bit prevents drilling fluid from eroding and contaminating the sample, ensuring sample integrity.

[0022] 3. The coring process of the embodiment of the present invention prevents underground sand and debris from interfering with coring. The wellbore stability and whether the core is blocked are judged and measures are taken based on the data such as suspended weight, drilling time, and drilling pressure displayed on the ground operating console, effectively reducing the probability of breaking or even crushing the target layer of coring, improving the yield and quality of samples, protecting tools, reducing damage risks, and reducing costs.

[0023] In general, the embodiments of the present invention provide a pressure-maintained coring method for fragile coal seams, which achieves the purpose of avoiding low coring efficiency and poor sample quality caused by interference from sand settling and falling blocks in the coring process in fragile coal seams. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0025] Figure 1 This is a diagram showing the coring drill bit being lowered before cleaning the well bottom; Figure 2 This is a diagram showing the lowering of the core drill bit for cleaning the well bottom; Figure 3 This is the state diagram of injected thick slurry; Figure 4 This is the core drilling status diagram; Figure 5This is the state structure diagram of the core drill bit during the core drilling process, where Figure 5 aFor normal state, Figure 5 b corresponds to the sample crushing state, Figure 5 c corresponds to the state diagram through the crushing zone; Figure 6 This is a state diagram of sample recovery during drilling; Figure 7 Schematic diagram of the cross-sectional structure of a core drill bit; Figure 8 A perspective view of a core drill bit; Figure 9 This is a bottom view of the core drill bit; Figure 10 It is a state diagram of short trip drilling, where Figure 10 a corresponds to the descending state, Figure 10 b corresponds to the rising state.

[0026] Markings and corresponding parts names in the accompanying drawings: 1-Coring chamber, 2-Pressure-maintaining chamber, 3-Flow channel, 4-Upper chamber, 5-Lower chamber, 6-Rim plate, 7-Outer wall, 8-PDC tooth, 9-Drilling rig, 10-Drill string, 11-Control console, 12-Wellbore, 13-Coring tool, 14-Coring drill bit, 15-Sand and debris, 16-Surface, 17-Mud pool, 18-Coring target layer, 19-Sample, 20-Mud, 21-Thick slurry. DETAILED DESCRIPTION

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0028] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are intended to fall within the scope of protection of the present invention.

[0029] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0030] In the description of the present invention, it should be noted that the terms "first", "second", "third", etc. are only used to distinguish the description and cannot be understood as indicating or implying relative importance. Example

[0031] In the actual operation process, before coring, the drilling tool must be used to drill into the coring target layer. Due to the brittleness of coal seams and coal-bearing strata, they are easily broken and crushed during the drilling process. Due to the use of conventional processes, a large amount of sand and debris that cannot be removed in time will be deposited at the bottom of the well. In addition, the coring target layer is usually several thousand meters underground. The presence of sand and debris will cause the coring tool to misjudge the bottom of the well position, and then cause the coring tool to be unable to coring normally. Drilling on unstable sand and debris will also disturb the coring target layer, causing breakage and damage to the tool.

[0032] In order to solve the above problems, the embodiment of the present invention provides a pressure-maintaining coring method for fragile coal seams (please refer to Figures 1-6 As shown, Figures 1-6 This is a schematic diagram of different process states during pressure-maintained coring, including the drilling rig 9, drill string 10, control console 11, wellbore 12, coring tool 13, coring drill bit 14, sand and debris 15, ground surface 16, mud pool 17, coring target layer 18, sample 19, mud 20, and thick slurry 21. The following contents are included: 1. Select or prepare thick slurry Based on the geological parameters of the formation and the properties of the debris generated during drilling, a thick slurry with a viscosity range of 100-300 mPa·s is selected or prepared. The viscosity of the thick slurry can be controlled by adjusting the concentration of additives to ensure that it can effectively carry sediment and loose blocks in suspension without sinking. The selection and preparation methods of the thick slurry are not limited in the embodiments of the present invention and can be achieved using existing technologies as long as the purpose of effectively carrying sediment and loose blocks in suspension without sinking is achieved. For ease of understanding, in the embodiments of the present invention, the thick slurry is a high-viscosity mud prepared on-site, which is weighted and thickened by using ingredients such as barite and bentonite.

[0033] 2. Decentralize tools Assemble the coring tool and coring drill bit, ensuring all components are securely connected and not loose. Lower the tool to a depth of 300 to 800 meters above the target coring layer (L1). The specific depth can be determined based on the actual geological conditions and drilling design.

[0034] 3. Clean the bottom of the well Lower the tool at a speed of 0.3-0.8 m / s to a position L2 3-8 m above the target coring layer. During the lowering process, monitor the changes in the suspended weight from the surface control panel. If the suspended weight drops abnormally, it may indicate sand or debris at the bottom of the well. Circulate the drilling fluid until the suspended weight returns to normal, indicating that the bottom of the well has been cleared.

[0035] Reference Figure 1 As shown, the inner wall of the wellbore 12, i.e., the well wall, has a directly exposed stratum near the core sampling target layer 18, which may cause sand deposition and block falling.

[0036] Reference Figure 2 As shown, the coring tool 13 and the coring drill bit 14 are lowered, and data is recorded by the console 11. Sand and debris 15 are found. Mud is circulated through the drill string 10-coring tool 13-coring drill bit 14-wellbore 12-mud pool 17 to bring the sand and debris 15 at the bottom of the well to the mud pool 17 on the ground to clean the bottom of the well.

[0037] 4. Dealing with large falling blocks If there are large pieces of debris in the well that cannot be cleaned by circulating drilling fluid, adjust the displacement to 10~50 L / min, increase the drilling pressure to 50~200 kN, grind and trim the debris, and then clean it. The specific operation is as follows: Reduce the drilling fluid flow rate to 10-50 L / min to ensure that the drilling fluid can effectively carry the debris.

[0038] Increase the drilling pressure to 50~200 kN to allow the drill bit to grind and trim the large pieces.

[0039] The debris is carried out of the bottom of the well by circulating the drilling fluid until the bottom of the well is clear.

[0040] Continue to refer to Figure 2 As shown in the figure, if large pieces are found in the well and cannot be cleaned by circulating drilling fluid, reduce the displacement, increase the drilling pressure, grind and trim the pieces, and then clean them.

[0041] 5. Short trip drilling Reference Figure 10 As shown, the tool is raised to a depth of 300-800 meters and then lowered to a depth of 3-8 meters. Repeat this process several times. During each trip, observe the changes in the hanging weight to determine the wellbore stability. If the hanging weight remains stable over multiple trips, the wellbore is stable. If the hanging weight decreases abnormally, it indicates that the wellbore is unstable and requires further treatment.

[0042] 6. Dealing with wellbore instability If the wellbore is unstable, inject a thick slurry at least 100 meters above the target coring layer to prevent any debris and sand from falling to the bottom. After injecting the thick slurry, perform another short trip to confirm wellbore stability. If the wellbore remains unstable, repeat the above steps until the wellbore is stable.

[0043] Reference Figure 3 As shown, after cleaning the bottom of the well, the drill is tripped shortly. If sand settling and block falling 15 occur again, the bottom of the well is cleaned again, and a thick slurry 20 of more than 100 m is injected before coring is prepared. During the coring process, a specific drilling fluid (mud 20) is used to maintain the stability of the well wall, carry rock cuttings, protect the coring sample, and ensure the smooth progress of the coring process.

[0044] Reference Figure 4 As shown, the thick slurry 21 suspends the newly generated sand and debris 15 to prevent them from falling to the bottom of the well.

[0045] 7. Core drilling Carry out core drilling according to the designed parameters. During the drilling process, record the drilling time, displacement, drilling pressure and other data every 0.5 m or 1 m. The specific parameters are as follows: Drilling time: records the time required to drill each meter, in minutes / meter.

[0046] Displacement: Record the displacement of drilling fluid in liters per minute.

[0047] Weight on bit: records the pressure on the drill bit in kilonewtons.

[0048] If the drilling time is significantly higher than the pre-coring data (1.5-2.0 times the pre-coring data), lift the tool and lower it again to re-coring. If the bit weight or drilling time increases significantly compared to the previous range (bit weight increases by 10-50 kN, drilling time increases by 1.5-2.0 times), adjust the bit weight and continue drilling.

[0049] Combined with reference Figure 5 a and Figure 7 As shown, during core drilling, mud 20 and debris are isolated outside the tool by the core drill bit 14 and are quickly taken away through the streamlined watertight space. Figure 5 As shown in b, if the sample 19 or the core target layer 18 is broken, it can be discovered through the data of the console 11 or the phenomena of drilling stagnation and drilling slippage. Figure 5 As shown in c, increase the drilling pressure and continue drilling until you pass the crushed area before proceeding to the next step. If the crushed area cannot be passed by increasing the drilling pressure or grinding, you can directly pull out the drill, clean the tools and the bottom of the well, and then coring again or terminate coring.

[0050] 8. Determine the next step If the recorded drilling time returns to the normal range (close to 1.0~1.2 times the data before coring) during continued drilling, it means that the crushing section has been passed and the original drilling pressure parameters can be restored to continue drilling.

[0051] If the WOB does not decrease during drilling or increases abnormally (over 50 kN), or if the drill slips or is stuck, this indicates core blockage and the drilling process should be terminated and the drill pulled out. If coring is still required, clean the tool and the bottom of the well after returning to the surface, then drill again and repeat the above steps to coring.

[0052] 9. Coring completed After coring is completed, lift the tool to cut the core, and then pull the drill back to the surface (refer to Figure 6 During the drilling process, be sure to keep the drill tool stable to avoid damage to the sample.

[0053] As a preferred embodiment of the present invention, refer to Figure 7-9 As shown, the bottom end of the coring drill bit of the embodiment of the present invention includes a coring cavity 1, and the upper end of the coring cavity 1 is connected to the pressure-maintaining cavity 2; wherein, the outer wall 7 of the coring cavity 1 has a streamlined structure that gradually becomes larger from bottom to top, and the bottom of the flow channel 3 is higher than the bottom of the coring cavity 1.

[0054] Specifically, the coring cavity 1 is located at the bottom end of the drill bit body. It is the part that directly contacts the formation and obtains samples. The outline of its outer wall 7 is set to a streamlined structure that gradually becomes larger from bottom to top, which helps to reduce the accumulation of debris in the interior and avoid core blockage and core grinding problems; the pressure holding cavity 2 is located at the upper end of the coring cavity 1 and is connected to the coring cavity 1. Its main function is to maintain internal pressure and prevent external liquid from entering, thereby protecting the sample from contamination; the flow channel 3 is arranged inside the coring cavity 1, and its bottom is higher than the bottom of the coring cavity 1. This structure optimizes the arrangement of the flow channel 3, prevents external liquid from entering in reverse, improves the water-isolating effect, and prevents liquid from entering the tool and eroding and contaminating the sample.

[0055] During the drilling process, the coring cavity 1 is in direct contact with the formation to obtain samples. Due to the streamlined outer wall 7 profile of the coring cavity 1, the debris generated by drilling is automatically pushed outward and does not enter the interior of the tool, thereby reducing the problems of core blockage and core grinding. At the same time, the bottom of the flow channel 3 is higher than the bottom of the coring cavity 1, which improves the water-proof effect and prevents liquid from entering the interior of the tool to erode and contaminate the sample. After the sample is formed in the coring cavity 1, it is transferred to the pressure-maintaining cavity 2 through communication with the pressure-maintaining cavity 2. The pressure-maintaining cavity 2 maintains internal pressure to prevent external liquid from entering, thereby protecting the sample from contamination.

[0056] In addition, the coal seam is fragile, and the sample can be easily damaged by the vibration of the drill bit when entering the tool. In order to solve this problem, the coring cavity 1 of the embodiment of the present invention includes an upper cavity 4 and a lower cavity 5. The upper end of the upper cavity 4 is connected to the pressure-maintaining cavity 2, and its bottom is connected to the lower cavity 5; the inner diameter of the lower cavity 5 is smaller than the inner diameter of the upper cavity 4, and the inner diameter of the upper cavity 4 is smaller than the inner diameter of the pressure-maintaining cavity 2. The central axes of the upper cavity 4, the lower cavity 5 and the pressure-maintaining cavity 2 coincide.

[0057] In an embodiment of the present invention, the lower cavity 5 is the part that is in direct contact with the formation during the coring process and is responsible for obtaining samples. It allows samples to be formed and collected during the drilling process. The upper cavity 4 is located above the lower cavity 5 and is used to receive the sample transferred from the lower cavity 5 and provide a temporary storage space for the sample until the sample is transferred to the pressure-maintaining cavity 2; the pressure-maintaining cavity 2 is located above the coring cavity 1 and is used to maintain internal pressure and prevent external liquid from entering, thereby protecting the sample from contamination and ensuring stable transmission of the sample. In an embodiment of the present invention, by setting the inner diameter of the lower cavity 5 to be smaller than the inner diameter of the upper cavity 4, when the sample is transferred from the lower cavity 5 to the upper cavity 4, due to the larger inner diameter of the upper cavity 4, the spatial restriction on the sample is reduced, thereby reducing the crushing caused by friction and extrusion. At the same time, the size difference also helps to form a buffer area, so that the sample can be more stable during the transfer process, reducing the crushing caused by sudden pressure changes or impacts. Coal samples are more susceptible to breakage when disturbed, and this structural dimension difference reduces disturbance during sample transfer, ensuring that the sample remains as pristine as possible during coring. The alignment of the central axes of the upper and lower chambers 4 and 5, along with the pressure-maintaining chamber 2, ensures linearity and stability during sample transfer, minimizing deviation and vibration during transfer and improving sample integrity and coring accuracy.

[0058] Furthermore, at least three ribs 6 are provided on the outside of the drill bit body, with gaps between adjacent ribs 6. The gaps between the ribs 6 serve as fluid channels, guiding liquid and debris generated during drilling outward, thereby reducing contamination of the drill bit interior by debris and liquid. At the same time, the ribs 6 increase the structural strength of the drill bit, improve operational stability, and reduce the risk of sample breakage due to vibration.

[0059] It should be noted that the number of ribs 6 provided is not limited herein and can be set based on the dimensions of the outer side of the drill body. Three, four, or five ribs, for example, can be provided. The gaps between two adjacent ribs 6 can be set to be the same or different. For example, in an embodiment of the present invention, four ribs 6 are provided and arranged in an annular array along the contour of the outer wall 7 of the coring cavity 1. This structure can form four fluid channels.

[0060] Furthermore, a plurality of PDC teeth 8 are arranged on the outer side of each ridge plate 6. The PDC teeth 8 can be cutting elements made of polycrystalline diamond material with extremely high hardness and wear resistance. They are arranged on the outer side of each ridge plate 6 to form the cutting edge of the drill bit. The high hardness and wear resistance of the PDC teeth 8 enable it to effectively cut the formations, including hard rock and fragile coal seams, thereby improving drilling efficiency. The function of the PDC teeth 8 arranged on the first inclined plate 9 is to drill downward quickly, and the function of the PDC teeth 8 arranged on the outer side of the vertical plate 10 is to trim the well wall to maintain a constant inner diameter.

[0061] It should be noted that the number, shape, and size of the PDC teeth 8, as well as the gap size between adjacent PDC teeth 8, are not limited herein and can be set according to actual needs, as long as sufficient drilling strength can be achieved. As a preferred embodiment of the present invention, in order to improve processing efficiency, the gap size between two adjacent PDC teeth 8 with the same function can be set to be equal, and the shape and size of each PDC tooth 8 can be set to be the same.

[0062] The PDC teeth 8 are of an outwardly convex arc structure, with a gap between two adjacent PDC teeth 8. The outwardly convex arc structure helps to improve cutting efficiency, while reducing impact and vibration on the formation, and helps to extend the service life of the drill bit.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, improvements, and the like made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention. It should be noted that the structures or components illustrated in the accompanying drawings are not necessarily drawn to scale, and that descriptions of known components, processing techniques, and processes are omitted to avoid unnecessarily limiting the present invention.

Claims

1. A pressure-maintaining coring method for fragile coal seams, characterized in that: The following steps are involved: S1: Assemble the core drill bit and lower it to a position L1 above the target layer; S2: Lower the coring drill bit to a position L2 above the target coring layer, L1>L2, observe the change in the hanging weight to determine the bottom hole condition and clean the bottom hole; S3: Perform short trips to determine wellbore stability; S4: performing core drilling according to preset drilling parameters, recording the process parameters and adjusting the drilling parameters according to the process parameters; S5: After coring is completed, the core is cut and the drill is pulled out to recover the sample.

2. A pressure-maintaining coring method for fragile coal seams according to claim 1, characterized in that: The short tripping in S3 includes: lifting the coring drill bit back to a position L1 above the coring target layer, and then lowering it to a position L2 above the coring target layer, and repeating this process several times.

3. A pressure-maintaining coring method for fragile coal seams according to claim 2, characterized in that: The problem of judging the wellbore stability described in S3 includes observing the changes in the suspended weight to judge the wellbore stability; If the hanging weight decreases abnormally, it indicates that the well wall is unstable. Inject thick slurry from above the core sampling target layer to prevent the falling of blocks and sand to the bottom of the well.

4. The pressure-maintaining coring method for fragile coal seams according to claim 1, characterized in that: The lowering speed in S2 is 0.3~0.8 m / s; The method of observing the change of the hanging weight to judge the bottom condition of the well includes: observing the change of the hanging weight through the ground operating platform during the lowering process. If the hanging weight decreases abnormally, it indicates that there is sand or debris at the bottom of the well; The cleaning of the well bottom includes: circulating the drilling fluid until the suspended weight returns to normal, indicating that the well bottom has been cleaned.

5. The pressure-maintaining coring method for fragile coal seams according to claim 1, characterized in that: Recording the process parameters in S4 includes recording the process parameters every 0.5 m or 1 m of drilling, wherein the process parameters include at least one of drilling time, displacement, and bit pressure; The adjusting of the drilling parameters according to the process parameters includes: if the drilling time is greater than 1.5 times the drilling time before coring, the coring drill bit is lowered again to re-coring; If the drilling pressure increases by 10-50 kN, adjust the drilling pressure and continue drilling; If the recorded drilling time is restored to less than 1.5 times the drilling time before coring, it means that the crushing section has been passed and the original drilling pressure parameters can be restored to continue drilling. If the drilling pressure does not decrease or increases abnormally by more than 50 kN during drilling, or if drill slippage and / or drill holding occurs, it indicates that the core is blocked and the drilling should be stopped and the drill should be pulled out.

6. The pressure-maintaining coring method for fragile coal seams according to claim 1, characterized in that: Between S2 and S3 also include: S6: If large pieces are found in the well that cannot be cleaned by circulating drilling fluid, reduce the displacement, increase the drilling pressure, grind and trim the pieces, and then clean them.

7. The pressure-maintaining coring method for fragile coal seams according to claim 1, characterized in that: The bottom end of the coring drill bit comprises a coring cavity (1), and the upper end of the coring cavity (1) is connected to the pressure-maintaining cavity (2); The outer wall (7) of the coring cavity (1) has a streamlined structure with a contour that gradually increases in size from bottom to top, and the bottom of the flow channel (3) is higher than the bottom of the coring cavity (1).

8. The pressure-maintaining coring method for fragile coal seams according to claim 7, characterized in that: The coring cavity (1) comprises an upper cavity (4) and a lower cavity (5); the upper end of the upper cavity (4) is connected to the pressure-maintaining cavity (2), and the bottom thereof is connected to the lower cavity (5); The inner diameter of the lower cavity (5) is smaller than the inner diameter of the upper cavity (4), the inner diameter of the upper cavity (4) is smaller than the inner diameter of the pressure-maintaining cavity (2), and the central axes of the upper cavity (4), the lower cavity (5) and the pressure-maintaining cavity (2) coincide.

9. The pressure-maintaining coring method for fragile coal seams according to claim 7, characterized in that: At least three ridge plates (6) are provided on the outside of the drill body, and a gap exists between two adjacent ridge plates (6); Four rib plates (6) are provided, and the four rib plates (6) are arranged in a ring array along the contour of the outer wall (7) of the coring cavity (1).

10. The pressure-maintaining coring method for fragile coal seams according to claim 8, characterized in that: A plurality of PDC teeth (8) are arranged on the outer surface of each rib plate (6); The PDC teeth (8) are of an outwardly convex arc-shaped structure, and a gap exists between two adjacent PDC teeth (8).

Images