Research on the Fixation Strength of High-Temperature Geothermal Drilling Cone Bit Teeth

Abstract

During the drilling process of high-temperature geothermal wells, the high temperature at the bottom of the well and the complex lithology of the formation lead to poor tooth loss prevention in cone drill bits. This issue seriously affects the life and efficiency of geothermal drilling. The stability of the wellbore is one of the key issues in the drilling process of high-temperature geothermal wells, and the fixed-tooth strength of the roller drill bit directly affects the stability of the wellbore and drilling efficiency. The heat transfer effect of the wellbore will exacerbate the thermal expansion and performance degradation of the drill bit material in high-temperature environments, leading to a decrease in the strength of the fixed teeth. To address this, this study used a high-temperature experimental apparatus to systematically test the fixed-tooth strength of roller drill bits. By using five types of tooth spacing: 4, 6, 8, 10, and 12 mm, three types of tooth diameters: 12, 14, and 16 mm, and three types of interference fit: 0.075, 0.095, and 0.115 mm, the maximum fastening force of fixed teeth was measured under different conditions, and its variation pattern was analyzed. The experimental results show that the higher the temperature, the weaker the tooth-fixing strength. Under the same perforation distance, the maximum fastening force decreases with increasing temperature. Compared with normal temperature, the maximum fastening force decreases by about 49.6–64.5%. At the same temperature, the maximum fastening force is the largest when the perforation distance is 10 mm. When the temperature increases, the maximum fastening force increases with the tooth diameter; that is, the larger the tooth diameter, the better the tooth-fixing effect. At the same temperature, the maximum fastening force first increases and then decreases with increasing interference. The maximum fastening force is the largest when the interference is 0.095 mm. At 120 °C, 180 °C, and 240 °C, the maximum fastening force is reduced by 21.9%, 29.4%, and 56.6%, respectively, compared to normal temperature. The study reveals the variation law of tooth-fixing strength under high-temperature conditions and proposes tooth-fixing methods and suggestions suitable for high-temperature geothermal wells. This provides a scientific basis for solving the problem of tooth loss of roller bits in high-temperature geothermal drilling and has important theoretical and practical application value.

1. Introduction

The formation lithology of high-temperature geothermal drilling is complex, the bottom hole temperature is high, and there are many uncertain factors. In recent years, significant progress has been made in the geothermal energy extraction technology of high-temperature geothermal wells, especially in high-temperature drilling technology and efficient utilization of geothermal resources. The development of high-temperature geothermal wells faces challenges from high temperature, high pressure, and complex geological conditions, especially the wear and failure of drill bits during the drilling process. Research has shown that the performance of drill bit materials significantly decreases under high-temperature environments, leading to a decrease in drilling efficiency [1,2,3,4,5]. Related studies have investigated the crack propagation behavior of CO₂ fracturing fluid in unconventional low-permeability reservoirs, and found that the physical and chemical properties of the fracturing fluid and reservoir characteristics have a significant impact on crack propagation [6]. In addition, the wellhead stability during the development of combustible ice reservoirs in the northern South China Sea was analyzed, and it was pointed out that changes in formation pressure and wellhead structure design were key factors. Optimization suggestions were also proposed. These studies provide important references for the development of related fields [7]. In high-temperature geothermal drilling, the cone bit is mainly used. However, tooth loss has become the most serious failure form of the cone bit (as shown in Figure 1), which greatly shortens the bit’s service life and increases drilling costs [8,9,10,11,12]. The Kadera bit, developed by Smith Company of the United States, was used in a geothermal well in Italy. The drilling time was 3–37% lower than that of the traditional cone bit, and the bit exhibited good tooth-fixing ability [13,14]. To improve the fixed-tooth strength of the cone bit, experimental research on the assembly stress of the cone bit was conducted. It was concluded that the stress on the tooth causes deformation of the tooth hole, the interference increases gradually, and the surface circumferential and radial stress of the tooth hole is transformed into tensile stress. This is an important factor leading to tooth-hole cracking [15,16]. Experimental analysis and numerical simulation were conducted to study the fixed-tooth strength of three-cone drill bits, and a personalized new product of three-cone drill bits was developed. The results showed that the fixed-tooth strength significantly decreased under high-temperature conditions [17,18,19,20]. Generally, chamfering the root of the teeth with different interference fits is beneficial for reducing stress concentration at the root of the tooth hole [21,22]. Numerical simulation of tooth interference fit assembly revealed that the gradient of compressive stress distribution on the hole wall increases during the assembly process, and the deformation around the hole increases. A shorter assembly distance can achieve a more reasonable stress distribution [23,24]. A shock-resistant drill bit has been developed for hard rock formations and high-temperature environments, effectively improving mechanical drilling speed and reducing drilling costs [25,26]. In addition, researchers have analyzed and calculated the contact stiffness and damping of the interference fit interface under different interference amounts through numerical simulation methods. They also calculated the frequency response function of the interference fit and verified the correctness of the contact stiffness and damping [27]. A new experimental testing method for tooth fixation ability has been proposed for the unique drilling technology of high-temperature geothermal wells [28,29,30,31].

Although scholars both domestically and internationally have conducted extensive simulation research on tooth fixation, various simulation models have oversimplified the tooth fixation process. Additionally, while there are experimental studies on tooth fixation, these experiments have not been designed to account for factors such as temperature and tooth stress. Therefore, this paper presents an experimental study on the tooth-fixing technology of cone bits in high-temperature environments, aiming to propose methods and suggestions for mitigating tooth loss in cone bits during high-temperature drilling operations.

2. Materials and Equipment

During the drilling process of high-temperature geothermal wells, the phenomenon of tooth loss in roller bits seriously affects drilling efficiency and equipment life. In order to reduce the failure rate and extend the service life of the equipment, this study proposes a new method for testing the strength of tooth fastening and conducts relevant experimental research. Through experiments, the influence of factors such as temperature, tooth diameter, and tooth-hole spacing on the tooth fixation strength of roller drill bits is analyzed in depth. The goal is to grasp their inherent laws and provide a scientific basis for optimizing the design of roller drill bits. The roller base material used in the experiment is 20CrMo, and the tooth material is YG16C. Both materials are provided by Henan Hard Alloy Company in China (Zhengzhou Hard Alloy Company, Henan Province, China), and the material specifications and performance indicators are determined according to experimental requirements. A procurement contract is signed with the supplier to clarify the quality standards and delivery time. Strict quality inspection shall be conducted before delivery to ensure that the materials meet the experimental requirements. The processed base and teeth, prepared according to the experimental requirements, are shown in Figure 2. The properties of the base and cutting tooth materials are listed in

Table 1. The material properties of cone matrix and cutting teeth.

	Elastic Modulus (GPa)	Poisson’s Ratio	Expansion Coefficient (K−1)	Yield Limit (MPa)	Strength Limit (MPa)
20CrMo	210	0.28	13.2 × 10−6	626	885
YG16C	640	0.23	5.3 × 10−6	1120	5460

.

In this experiment, a hydraulic universal testing machine (as shown in Figure 3) is used to complete the loading process. The hydraulic universal testing machine model AG-IS 100 kN, with a maximum test pressure of 100 kN and an accuracy level of 0.5, is suitable for testing the mechanical properties of various materials such as tension, compression, and bending. It is manufactured in Taizhou City, Jiangsu Province, China. A pressure sensor is threaded onto the indenter of the press, and the displacement of the indenter is monitored by a displacement sensor. Both sensors are connected to a computer with data acquisition capabilities, enabling real-time acquisition and storage of pressure and displacement data. The tooth and the experimental specimen (tooth wheel matrix) are placed on the workbench plane, with the indenter and tooth axis always aligned in a straight line. The indenter can press into and out of the tooth through the upward movement of the hydraulic cylinder.

3. Testing Process

This experiment was meticulously designed and executed in strict compliance with the relevant standards of the International Organization for Standardization (ISO), particularly in material testing, mechanical property evaluation, and high-temperature environment simulation. Specific reference was made to ISO 6892-1:2019 [32], “Tensile testing of metallic materials—Part 1: Room temperature test method”. The tooth hole type employed in the fastening force test experiment is a through-hole. The experiment investigates the influence of different interference fits on the tooth-holding force at various temperatures. Specifically, the fastening force of the tooth hole is measured by applying axial loading to press the tooth out of the hole. In the experiment, tooth diameters of 16 mm, 14 mm, and 12 mm were used, with tooth hole distances of 4 mm, 6 mm, 8 mm, 10 mm, and 12 mm, respectively. Three interference fit ranges were tested: 0.065–0.085 mm, 0.085–0.105 mm, and 0.105–0.125 mm. Axial loading experiments were conducted at room temperature, 120 °C, 180 °C, and 240 °C. To eliminate experimental variability, each group of experiments was repeated five times. The temperature of the heated test piece was measured using an HT-8830 infrared thermometer, as illustrated in Figure 4.

In the fastening force experiment, the tooth-pressing process was carried out at different temperatures. During the pressing process, the press must be controlled to pressurize slowly. The tooth axis should always remain perpendicular to the working surface to avoid damaging the tooth hole surface by pressing the tooth askew. After the tooth reaches the specified depth in the tooth hole, it should be unloaded slowly to zero. When pressing out the teeth to test the fastening force, the press must be controlled to load slowly. Once the pressure curve stabilizes, it can be considered that the teeth have begun to move. The test is concluded when the tooth displacement reaches 1 mm. The experimental process is shown in Figure 5.

In the high-temperature fastening force test, heat insulation is essential between the test piece and the pressure machine surface, as well as between the test piece and the indenter. This insulation prevents rapid temperature drop of the test piece due to heat conduction. The entire extrusion and disassembly process is shown in Figure 6.

In the experiment, to account for the influence of adjacent teeth on the tooth-tightening force, it is necessary to press all five teeth on the test piece into their respective holes before beginning the process of pressing out the teeth. The experimental process is shown in Figure 7.

4. Experimental Result and Numerical Simulation

4.1. Curve Analysis of Tooth Ballast Process

The displacement and drilling pressure curves of the teeth during pressing and extruding were obtained by the experimental acquisition system, as shown in Figure 8. During the tooth-pressing process, the maximum tooth insertion force occurs at point A, corresponding to the continuous increase in the load. In the extrusion process, the teeth initially move slightly to overcome the friction on the joint surface as the load continues to increase. Significant displacement movement begins once the maximum static friction, caused by the interference fit, is overcome. Since the contact area between the teeth and the perforations remains constant, the pressure curve stabilizes after the teeth start moving, and the load reaches the maximum fastening force at point B.

4.2. The Influence of Perforation Distance on Fastening Force

Establish a finite element three-dimensional model to analyze the effect of different tooth spacings on the fixed-tooth force. After the three-dimensional model is established, import it into Abaqus for calculation. The three-dimensional model is shown in Figure 9. The tooth spacings are set at 4 mm, 6 mm, 8 mm, 10 mm, and 12 mm, and their respective influences are analyzed. The stress cloud map is shown in Figure 10.

The relationship between the maximum fastening force during simulation and experimental testing, i.e., the pressure value during tooth pressing, and the perforation distance at different temperatures is shown in Figure 11. From the simulation data, it can be seen that the larger the distance between the teeth, the greater the maximum fastening force and the better the fastening effect. When the tooth pitch increases to 10 mm, the increase in surface contact pressure is most significant. This is because when the distance between adjacent teeth is too close, stress superposition during the tooth insertion process causes plastic deformation of the tooth hole material. The hole wall cannot maintain the fastening effect on the teeth, resulting in a decrease in fastening force. This is consistent with the experimental results.

From the experimental data, it can be seen that at the same temperature, the maximum fastening force first increases and then decreases as the perforation distance increases. The maximum fastening force is highest when the perforation distance is 10 mm, with values of 2080.8 kg, 1652.1 kg, 1284 kg, and 856.9 kg at different temperatures, respectively. Conversely, the fastening force is lowest when the perforation distance is 4 mm, decreasing by approximately 28.5–41.6% compared to that at 10 mm. Therefore, under an interference of 0.115 mm, the tooth-fixing strength is optimal when the tooth perforation spacing is 10 mm. Under the same tooth spacing, the maximum fastening force decreases with increasing temperature, being highest at normal temperature. Compared to normal temperature, the maximum fastening force decreases by approximately 49.6–64.5% at elevated temperatures, indicating that higher temperatures significantly weaken the tooth-fixing strength of the cone bit. The main reason for this phenomenon is the influence of the thermal expansion effect. Under a high-temperature environment, the matrix material (20CrMo) and tooth material (YG16C) of the roller drill bit will undergo thermal expansion. When the distance between the perforations is 10 mm, the thermal expansion of the material may lead to a reduction in the interference fit between the perforations and the teeth. Due to the fact that interference fit is a key factor in the strength of fixed teeth, a decrease in interference fit directly leads to a decrease in fastening force. In addition, thermal expansion may also cause plastic deformation of the tooth hole wall, further weakening the fastening effect between the teeth and the tooth hole. The second is the change in material properties, where the strength and hardness of the material decrease at high temperatures, resulting in a weakened support capacity of the tooth hole wall. When the tooth spacing is 10 mm, the decrease in the performance of this material may be more significant, making the tooth hole wall unable to effectively resist the pressure of the teeth, resulting in a rapid decrease in fastening force. The third is caused by stress concentration. When the spacing between teeth is small, the stress between adjacent teeth may overlap, which may lead to stress concentration. At high temperatures, this stress concentration will further intensify the deformation of the tooth hole wall, reducing its fastening ability to the teeth. At 120 °C, the main reason for the continued rebound after a slight decrease in tooth spacing is due to the nonlinear characteristics of thermal expansion. The thermal expansion of the material is not completely linear, especially near the yield point of the material. At 120 °C, the thermal expansion of the material may cause a slight decrease in the tooth spacing, but as the temperature further increases, the plastic deformation of the material begins to take effect, causing the tooth spacing to increase again. This nonlinear thermal expansion characteristic may lead to a brief decrease in tooth spacing and subsequent rebound. The second is the elastic recovery of the material. At 120 °C, although the elastic modulus of the material decreases, it still has a certain degree of elastic recovery ability. When the distance between teeth decreases slightly due to thermal expansion, the elastic recovery of the material may cause the distance between teeth to increase again, leading to a rebound in fastening force.

4.3. Effect of Tooth Diameter on Fastening Force

The fixation strength of different tooth diameters was analyzed under an interference fit of 0.115 mm. Figure 12 illustrates the relationship between tooth diameter and maximum fastening force at various temperatures. The fastening force consistently decreases as the temperature rises. When the tooth diameter is held constant, the tooth fixation effect is poorest at 240 °C, with the maximum fastening force decreasing by approximately 58.7–73.9% compared to that at room temperature. At room temperature, the maximum fastening force initially decreases and then increases with increasing tooth diameter, reaching a peak of 1392.8 kg when the tooth diameter is 16 mm. As the temperature increases, the maximum fastening force also increases with larger tooth diameters, peaking at 16 mm. In other words, at higher temperatures, larger tooth diameters result in better tooth fixation effects. For a tooth diameter of 16 mm, the maximum fastening forces at elevated temperatures are 1053.2 kg at 120 °C, 895 kg at 180 °C, and 549.2 kg at 240 °C, respectively.

Establish different tooth diameter models for finite element analysis, and their models and stress cloud maps are shown in Figure 13. It can be seen that the larger the diameter of the teeth, the better the fastening effect. This is because appropriately increasing the diameter of the teeth can significantly increase the contact area between the teeth and the surface of the tooth cavity, thereby effectively improving the clamping force of the tooth cavity on the teeth. This design optimization can effectively prevent the teeth from falling off during use and improve the stability and reliability of the structure.

Extracting the maximum fastening force from the stress cloud map, as shown in Figure 14, the simulation results are highly consistent with the experimental data. This not only verifies the accuracy and reliability of the finite element analysis but also provides strong theoretical support for subsequent structural optimization design.

4.4. The Effect of Interference on the Fastening Force

As shown in Figure 15, which illustrates the relationship between the interference amount and the maximum fastening force of 16 mm teeth at different temperatures, it can be seen that the maximum tooth-fixing force decreases with increasing temperature under the same interference amount. Specifically, when the interference amount is 0.075 mm, the maximum fastening force at 120 °C, 180 °C, and 240 °C decreases by approximately 25.9–64.4% compared to normal temperature. When the interference is 0.095 mm and 0.115 mm, the maximum fastening force is 56.6% and 60.6% lower than that at normal temperature, respectively. The maximum fastening force decreases with increasing temperature mainly due to the degradation of material properties. As the temperature increases, the mechanical properties of the base material (20CrMo) and tooth material (YG16C) of the roller drill bit significantly decrease. The second is the thermal expansion effect. At high temperatures, the thermal expansion coefficients of the matrix material and the tooth material are different, resulting in a decrease in the interference fit between the tooth hole and the tooth. Thirdly, stress relaxation occurs in materials at high temperatures, where the stress gradually decreases over time under constant strain. This phenomenon will gradually weaken the fastening force between the tooth hole wall and the teeth.

Simulation analysis was conducted on different interference amounts, and the stress cloud maps and extracted maximum stresses are shown in Figure 16 and Figure 17. It can be seen that the tooth fastening force decreases with increasing temperature, and the larger the interference amount, the smaller the maximum fastening force. This is because when the interference amount is too large, the tooth hole wall undergoes plastic deformation. The hole wall cannot maintain the fastening effect on the teeth, resulting in a decrease in the strength of the fixed teeth. This is consistent with the experimental results. The main reason for the maximum fastening force, when the interference fit is 0.095 mm, is that the contact pressure between the tooth hole wall and the teeth is moderate, providing sufficient friction and support to ensure maximum fastening force. The second is the optimization of stress distribution. An appropriate interference fit can optimize the stress distribution between the tooth hole wall and the teeth, avoiding stress concentration. A 0.095 mm interference fit can ensure more uniform contact between the tooth hole wall and the teeth, thereby improving the fastening force. The third is the matching of material properties. In high-temperature environments, an appropriate interference fit can better match the thermal expansion coefficients of the matrix material and the tooth material, thereby reducing the decrease in fastening force caused by differences in thermal expansion.

5. Conclusions and Discuss

5.1. Conclusions

This study proposes an experimental testing method for the tooth fixation ability of roller drill bits in high-temperature geothermal wells, and conducts systematic tooth fixation experiments in high-temperature environments. Through finite element analysis and indoor experiments, the influence of tooth diameter, tooth pitch, interference fit, and temperature on the strength of fixed teeth was studied, aiming to provide a theoretical basis for the design optimization of roller drill bits. The experimental results indicate that:

(1)

The higher the temperature, the weaker the strength of the fixed teeth. At the same tooth spacing, the maximum fastening force decreases continuously with increasing temperature, and compared to room temperature, the maximum fastening force decreases by about 49.6–64.5%. This indicates that high temperatures significantly weaken the tooth fixation strength of the roller drill bit.

(2)

At the same temperature, the maximum fastening force is highest when the tooth spacing is 10 mm, with maximum values of 2080.8 kg (room temperature), 1652.1 kg (120 °C), 1284 kg (180 °C), and 856.9 kg (240 °C), respectively. When the interference fit is 0.115 mm and the tooth spacing is 10 mm, the best tooth-fixing effect is achieved.

(3)

When the tooth diameter is 16 mm, the maximum fastening force is the highest, with a maximum value of 1392.8 kg. As the temperature increases, the maximum fastening force increases with the increase in tooth diameter. When the interference fit is 0.115 mm, choosing a tooth diameter of 16 mm is more conducive to improving the strength of the fixed teeth.

(4)

At the same temperature, as the interference fit increases, the maximum fixed-tooth force first increases and then decreases, and the maximum fixed-tooth force is highest when the interference fit is 0.095 mm. At 120 °C, 180 °C, and 240 °C, the maximum fastening force decreased by 21.9%, 29.4%, and 56.6%, respectively, compared to room temperature.

5.2. Discuss

This study systematically analyzed the variation law of fixed-tooth strength of roller drill bits under a high-temperature environment through a combination of experiments and numerical simulations. The research results provide an important theoretical basis for optimizing the design of roller drill bits. The following is a further discussion of the research results:

(1)

The impact of high temperature on material properties. Under high-temperature conditions, the mechanical properties of the matrix material (20CrMo) and tooth material (YG16C) of the roller drill bit significantly decrease. The strength limit and hardness of the material decrease, and the difference in thermal expansion coefficient leads to a reduction in interference fit, thereby weakening the fastening force between the teeth and the tooth hole.

(2)

Optimization of tooth spacing. The experimental results show that the fixed-tooth strength is the highest when the tooth spacing is 10 mm. This may be because appropriate tooth spacing can better balance the stress on teeth and the thermal expansion of materials. Too small a tooth spacing can lead to stress concentration, while too large a tooth spacing can reduce the support effect of the teeth. This discovery provides an important reference for the design of roller drill bits.

(3)

Selection of tooth diameter. In high-temperature environments, larger tooth diameters (such as 16 mm) can provide stronger resistance to thermal expansion and structural stability. Therefore, when drilling high-temperature geothermal wells, it is recommended to prioritize selecting teeth with larger diameters to improve the strength of the fixed teeth.

(4)

Optimization of interference fit. The experimental results show that the fixed-tooth strength is the highest when the interference fit is 0.095 mm. The optimization of interference fit is crucial for improving the strength of fixed teeth. Excessive interference fit may lead to material fatigue and stress concentration, and instead reduce the strength of fixed teeth.

This study provides a scientific basis for the design and optimization of roller bits for drilling high-temperature geothermal wells. In practical applications, it is recommended to choose the appropriate tooth spacing, tooth diameter, and interference fit based on the specific temperature and geological conditions of the geothermal well. In addition, future research can further explore new materials and fixed-tooth technologies to better address the challenges in high-temperature geothermal drilling.