Optimal Selection of PDC Bits for Coal Mining
Borehole construction is a widely used technical method for gas control, water damage prevention, and geological exploration in coal mines, which plays a crucial role in ensuring the safety and efficiency of coal production. In China, tens of millions of meters of drilling holes are created annually in coal mines. The construction process requires the support of drilling equipment such as drills, drill pipes, and Drill Bits. Among them, drills are essential tools for directly forming holes and breaking rocks.
The Poly crystalline diamond composite sheet (PDC) is extensively utilized in Mining Drill Bits due to its exceptional wear resistance and commendable impact toughness.PDC bitsexhibit prolonged drilling lifespan and heightened penetration efficiency when employed in medium-hard and lower rock formations, thereby yielding substantial economic benefits in coal mine drilling operations.In comparison to oil drilling, coal mining drilling possesses the technical attributes of being cost-effective, time-efficient, and densely designed.Consequently, the overall requirements for Pdc Bits are not excessively demanding.
The current challenges faced in mining PDC bits include: ①The entry threshold for mining PDC bits is relatively low, with a multitude of manufacturers offering varying levels of quality. ②The uses and requirements of underground drilling in coal mines exhibit evident disparities, while the classification of mining PDC bits is intricate and diverse. Furthermore, there is a lack of systematic and rational selection specifications in the subsequent utilization process.③With the increasing prevalence of complex hard strata in coal mines, conventional mining PDC bits fail to meet the construction requirements for efficient on-site drilling, resulting in instances of slow progress or even complete lack of footage.④Due to the challenging construction conditions in the coal mine, strict adherence to drilling procedures by construction personnel can sometimes be difficult.
In order to enhance the operational effectiveness of mining PDC bits, this paper provides a comprehensive analysis of the primary failure modes and investigates the underlying causes for each form of failure. Additionally, practical recommendations are proposed for the prudent selection and attentive utilization of these bits.
1.Analysis of the Form and Causes of PDC Bit Failure in Mining
Currently, the components of mining PDC bits primarily consist of the bit body, PDC cutters, gauge alloys, and other parts as depicted in Figure 1. The PDC cutters and gauge alloys are brazed onto the bit body to ensure their secure attachment. During regular drilling operations, the drill rod transmits torque and feed pressure to the bottom of the hole where it engages with the drill bit. The rock cutting process is accomplished through the utilization of PDC cutters on the drill bit. Additionally, gauge alloys safeguard against excessive wear by protecting the circumferential integrity of the bit body. However, it should be noted that various factors such as complex forces exerted on cutter at hole bottom during rock cutting with a PDC bit, changes in formation characteristics, selection of construction techniques and equipment, operational proficiency of personnel involved in construction activities along with quality control measures for bits themselves can significantly impact application effectiveness leading to diverse forms of failure for these bits. Through an investigation conducted at coal mine drilling sites, we have summarized and analyzed different failure modes and primary causes associated with PDC bits.
1.1 The PDC cutter failure occurs when the PDC is sintered under high temperature and high pressure conditions. The composite sheet consists of two main components: a diamond layer and a carbide matrix, as illustrated in Figure 2.
The primary failure modes of PDC cutters include normal wear, flaking off, blade collapse, and delamination.
(1) Regular wear and tear
The phenomenon of normal wear is a type of typical damage that occurs in PDC cutters during the rock cutting process, primarily characterized by significant macroscopic grinding loss in both the diamond layer and cemented carbide matrix of the cutters, without any apparent traces of cracking on the grinding surface.
(2) Chip drop
The phenomenon of chip drop occurs when PDC cutters detach from the bit body, resulting in bit failure. The most prominent characteristic is the complete separation of PDC cutters from the bit body, with no alloy residue found in the pit where welding between the bit body and PDC cutters takes place.
Primary factors contributing to the decline of falling film:
① The elevated temperature at the bottom of the borehole leads to challenges in dry drilling construction technology or water plug formation during the drilling process, while also impeding timely cooling of PDC cutters on the drill during high-speed rotary rock cutting. Consequently, there is a significant rise in temperature at the hole bottom, which can surpass the critical temperature of solder and result in solder melting and subsequent detachment of PDC cutters.
② The control of the drill welding process is not sufficiently stringent. In the welding procedure, there is a lack of proper cleaning prior to welding, faulty welds or inadequate exhaust gas management, and unreasonable heat preservation temperature or insufficient time after welding, all of which may result in bit detachment. Apart from demanding manufacturers to strictly regulate the production process (especially the PDC cutter welding process) to ensure complete and solid welds, it is also necessary to clarify the drilling construction procedure by opting for water drilling whenever possible and avoiding dry drilling construction. For deep hole drilling, it should be ensured that water returns before adding rod drilling. Additionally, each time before inserting the drill into the bottom of the hole, it is essential to check if there are any obstructions in order to prevent blockage.
(3) Blade breakage
The fracture of the diamond layer in PDC cutters represents a prevalent mode of failure for PDC bits. A characteristic feature of this failure mode is the localized rupture of the diamond layer, with some instances resulting in simultaneous breakage alongside the carbide matrix.
The main causes of PDC bit breakage:
① The cutter chosen based on the inherent characteristics of the PDC cutter exhibits low impact resistance. The combined strength of tungsten carbide (WC) and diamond micro-powder is insufficient, making it prone to failure under the stress of rock fragmentation;
② Factors related to the construction process indicate that the on-site construction pressure is excessively high. The pressure exerted by the PDC cutter on the bit significantly exceeds the cutter's pressure limit, leading to spalling of the diamond layer and subsequent collapse of the cutting edge.
③ Factors related to complex formations indicate that during the construction of hard, fractured formations, the impact load on PDC cutters exceeds their impact toughness, which may lead to the collapse of the cutting edge.
④ The design of the bit cutting angle is inadequate, as it does not adhere to the principle that a harder formation necessitates a larger cutting angle. In the construction of hard formations, a smaller cutting angle can result in excessive force on the PDC cutter, ultimately leading to the collapse of the cutting edge.
⑤ Other factors indicate that the arrangement of supporting bolts and anchor cables in underground roadways makes it highly susceptible for the drill bit to collapse upon encountering these elements. To prevent bit failure, it is essential not only to adhere strictly to the process parameters outlined in the drilling instruction manual but also to design and select the drill based on formation conditions. Specifically, when drilling into hard formations, it is advisable to increase the cutting angle of the bit appropriately, reduce its aggressiveness, and enhance protection for the bit. Additionally, selecting PDC cutters with higher impact toughness is recommended when drilling through hard fractured formations or modifying the external shape of the cutter's diamond layer to improve impact toughness; for instance, curved PDC cutters generally exhibit better impact resistance than flat types. Furthermore, a reasonable layout of holes during construction should be implemented to avoid drilling into bolts and anchor cables.
(4) Delamination
Delamination refers to the detachment of the diamond layer from the cemented carbide matrix in PDC composite sheets, as illustrated in Figure 3.
The primary cause of delamination in the diamond layer of PDC cutters is the substantial residual stress present between the diamond layer and the cemented carbide matrix, coupled with a significant difference in their coefficients of thermal expansion. During rock cutting, the diamond layer and cemented carbide matrix experience asynchronous contraction due to thermal stresses generated by frictional heat and cooling from washing fluids. The resultant impact load and residual stress lead to peeling of the diamond layer from the carbide matrix, ultimately resulting in drill bit failure. To mitigate delamination, it is essential to select appropriate bonding materials and processing technologies that address residual stress between these two components during manufacturing. Additionally, designing a new interface shape for the matrix can enhance bonding properties between the diamond layer and cemented carbide matrix.
1.2 Drill body failure is primarily characterized by blade fractures in the drill body, as illustrated in Figure 4.
The blade fracture of the bit body mostly occurs in the body bit and the steel body bit rarely occurs. The reasons for the fracture of the blade of the bit body are as follows: ① Striking the blade of the bit body when unscrewing the bit body because the crown of the bit body is sintered by powder metallurgy process at one time. compared with the ordinary steel body bit. the bit body bit has higher wear resistance. but the toughness of the body material is correspondingly reduced and it is easy to break the blade of the bit body when unscrewing the bit body; ② The control of the bit sintering process is not strict. and the sintered bit body is entrainment and the metal powder fails to form an alloy.In order to avoid the blade breaking. the user had better use tools such as dental pliers to assist the shackle when unscrewing the drill. so as to avoid knocking the blade; Secondly. manufacturers should strictly control the quality of the sintered body. not only to strictly control the sintering process. but also to monitor the metal powder from time to time to ensure that the powder meets the process requirements.
2. Optimized Design and Reasonable Selection of Mining PDC Bit
2.1 Optimization design of drill bit
(1) For the failure form of the bit, it is necessary to optimize its structure from the drill design. The main design parameters of PDC bit include cutting Angle α. side Angle β. azimuth Angle γ. etc. γ refers to the Angle between the orientation reference line of the PDC teeth and the center line of the bit. α refers to the Angle between the orientation reference line of the PDC tooth and the working plane of the cutter. as shown in Figure 5. β refers to the Angle between the plane through the center point and the center line of the bit and the working plane of the cutter. as shown in Figure 6. Preliminary research and comprehensive application analysis show that the cutting Angle and side Angle of PDC cutters can directly affect the life and drilling efficiency of the bit. and it is necessary to optimize the design of the bit according to different drilling formations. Cutting Angle design will greatly affect the drilling efficiency. or even no footage at all; If the cutting Angle is too small. the PDC cutting edge is prone to breakage and other phenomena. Therefore. the drill design should be considered comprehensively when selecting the parameter scheme choosing a moderate cutting Angle and side Angle rather than random design.
(2) In addition to optimizing the cutting angle of the bit, it is essential to also optimize PDC cutters. The quality of PDC cutters significantly influences mining PDC bits, as it directly determines their cutting performance and lifespan. A critical factor in enhancing the quality of PDC is the elimination of residual stress at the bonding interface between the diamond layer and cemented carbide matrix. Research indicates that maximum stress in PDC cutters with a planar interface structure is concentrated near both sides of this interface, with peak tensile and shear stresses occurring at the edge of the diamond layer. The presence of tensile stress can lead to abnormal fractures in PDC, while shear stress serves as a primary cause for delamination in PDC cutters.
(3) In addition to eliminating residual stress in PDC cutters, secondary treatments can further enhance their quality. Currently, the primary surface treatment methods include diamond surface grinding, edge chamfering, and shaped machining. The selection of PDC cutters can be optimized based on actual working conditions.
2.2 Reasonable selection of drill bits
Currently, PDC bits utilized in down hole drilling operations across most mining areas predominantly feature a three-wing concave design. However, the selection of these bits is often limited and lacks classification, resulting in a one-size-fits-all approach that does not adequately address the specific requirements of different strata. This is particularly problematic under complex formation conditions, where conventional concave bits struggle to deliver satisfactory performance, leading to shortened bit life and reduced drilling efficiency. Such inefficiencies not only escalate production costs but may also disrupt construction schedules and incur significant economic losses. Therefore, it is essential to select appropriate drill bits tailored to varying formation conditions.
Mining PDC bits can be categorized into two main types based on the bit body material: steel type and tire type. Steel-type bit bodies are primarily processed through mechanical methods, providing good toughness that makes them suitable for conventional formation drilling operations. In contrast, tire-type bit bodies are sintered using powder metallurgy processes, offering high wear resistance ideal for hard rock drilling or extended drilling tasks. Additionally, drill bits can be classified into various structures, including internal concave, arc corner, flat bottom, scraper, and others as illustrated in Figure 7. ①The concave structure design of the concave bit enhances straightening effects during conventional formation construction; ②The dispersed tooth arrangement of the arc bit results in a higher tooth density and improved drilling efficiency when working with medium-hard strata and above; ③The lip shape of the flat-bottom bit is nearly flat, which sharpens its side edges to facilitate skewing—this design is predominantly used for directional drilling applications; ④The scraper bit typically features a conical structure that provides greater aggressiveness and is mainly employed in soft strata or coal seam constructions.
The current mining PDC bits have been summarized and organized, along with the selection criteria for PDC bits under various working and formation conditions, as presented in Table 1 for reference.
3. The Use of Mining PDC Bit Precautions
In addition to carefully selecting drill bits, it is equally crucial to properly utilize these tools. Given the challenging construction conditions in coal mines, workers often struggle to strictly adhere to PDC bit operational guidelines, resulting in frequent instances of bit failure due to improper handling. In an effort to enhance drilling efficiency, some mining areas may indiscriminately increase bit weight during operations, leading to premature bit failure. Correctly utilizing PDC bits can optimize their performance and reduce production costs. Attention should be given to the following considerations when using PDC bits:
(1) Pay attention to the grading relationship between PDC bit and drill tool, ensuring that large diameter drill pipe is not matched with too small diameter drill pipe, and small diameter drill pipe is not matched with large diameter drill bit.
(2) The optimal approach is to utilize a specialized hole drill or an aged drill bit for nest construction, while ensuring that the drilling process avoids the vicinity of lane side bolts, anchor cables, and similar obstructions.
(3) When conducting drilling operations, it is essential to regulate the feed pressure in order to prevent accidents caused by excessive pressure within the borehole.
(4) Please utilize the clean water drilling technique, refraining from dry drilling or pressure air drilling. Prior to commencing pressure drilling operations, ensure proper water circulation within the hole to prevent potential combustion incidents at its base.
(5) When removing the bit, ensure to avoid clamping or striking the PDC cutter. Additionally, be cautious not to hit the body blade of the bit.
4. Conclusion
The safe and efficient green mining of coal has consistently been a central theme advocated by the coal industry. Effectively utilizing PDC bits for underground drilling operations in coal mines holds significant importance. This paper analyzes the failure modes of mining PDC bits, elucidating the primary causes behind various failure forms. It proposes optimal design and selection strategies for bits under different working conditions and geological formations, while summarizing precautions for their use, thereby providing a reference for improved selection and application of mining PDC bits. The judicious selection and proper utilization of mining PDC bits can maximize their effectiveness, minimize resource waste, and enhance safety, efficiency, and environmentally friendly practices in coal mining.