Study on the limestone fracturing characteristics of PDC cutters in high-temperature environments

Abstract

Addressing the problems of low footage and premature failure of existing PDC bits in high-temperature geothermal wells, this study investigates the limestone fragmentation characteristics of PDC cutters under high-temperature conditions. Adopting a combined approach of experimental testing, numerical simulation, and full-scale drilling verification, the effects of key parameters—temperature, rake angle, and cutting depth—on rock-breaking characteristics. The findings reveal that: (1)The mechanical parameters of limestone, such as compressive strength and shear strength, first increase and then decrease with rising temperature, reaching a peak at 250°C, where rock-breaking difficulty is the highest; (2)Single-cutter cutting tests indicate that 350°C is the optimal rock-breaking temperature, and the specific energy of fragmentation is the lowest with the highest rock-breaking efficiency when the rake angle is 15° and the cutting depth is 2mm; (3) Simulations demonstrated that both the temperature and contact stress on the cutter face increase with penetration depth but decrease with rake angle; (4)A new Φ118mm drill bit is designed based on the research. Experimental results show that the bit operates well in high-temperature environments, with a torque increase of 5.8% and a rate of penetration improvement of 31.9% compared to conventional PDC bits. This study clarifies the optimal rock-breaking parameter combination of PDC cutters under high-temperature conditions, providing key technical support for the cutter profile optimization, structural design, and drilling parameter regulation of PDC bits for high-temperature geothermal wells.

Introduction

Geothermal energy, as a renewable and clean energy source, is gaining increasing global attention as non-renewable resources like petroleum and coal become increasingly scarce.^1,2 Geothermal energy, as a renewable resource, has emerged as an effective alternative to non-renewable resources. The utilization efficiency of geothermal resources is projected to reach 30%-80% of global medium-to-high-temperature energy, with increasingly widespread applications.^3–6 While drilling challenges are relatively minor in low-to-medium temperature formations, drill bit performance significantly degrades in high-temperature environments due to altered bottomhole conditions. High-temperature formations typically range from 150-650°C and consist of highly abrasive, hard metamorphic or igneous rocks (e.g., granite, gneiss). PDC bits exhibit markedly reduced performance in high-temperature geothermal drilling.^7,8 Extensive research on high-temperature geothermal drilling has been conducted globally. Regarding high-temperature-resistant drill bits, Smith’s Kadera series roller cone bits achieve enhanced high-temperature performance through synergistic optimization of sealing materials and lubrication systems. Abandoning traditional sealants, they employ fiber-reinforced fluorocarbon compounds as core components for roller cone seals, substantially improving the seal structure’s resistance to high temperatures and wear. Simultaneously, specialized synthetic lubricants and functional additives are injected into the seal chamber, forming an efficient lubrication protection system that reduces high-temperature friction losses in bearings. Field data indicates this series achieves approximately 33% higher average penetration rates than conventional products. Upon well exit, bearings and sealing components remain intact and fully functional, ⁹ substantially reducing operational costs and downtime risks in high-temperature formations. Baker Hughes focused on enhancing bearing system high-temperature adaptability, launching the Vanguard™ series geothermal-specific drill bits. These drill bits replace traditional elastic seals with metal sealing structures, leveraging the high-temperature stability of metal materials to prevent seal failure. Combined with high-viscosity specialized grease, they maintain stable form and lubrication performance within high-temperature ranges, providing continuous bearing protection, ¹⁰ enabling stable operation under extreme high-temperature geothermal drilling conditions and providing reliable equipment support for geothermal development. Yang Yingxin’s team at Southwest Petroleum University addressed severe tooth loss and breakage in roller cone bits during high-temperature geothermal drilling by researching tooth setting strength under elevated temperatures. ¹¹ By analyzing the impact of high temperatures on the bonding strength between teeth and the roller body, they optimized the setting structure and process parameters, developing two improved roller bit types: “8½″HF637HDY” and “8½″HF637Y”. In geothermal drilling technology, Li Guangqiao et al. developed a novel flat rubber seal ring for high-temperature roller bit ¹² features adjustable contact area, delivering extended service life under high-temperature, high-speed conditions with no severe field damage, offering new insights for high-temperature seal design. The HT-1 high-temperature protective agent developed by CNPC Great Wall Drilling’s Drilling Engineering Technology Research Institute enhances the activity of foam fluids in high-temperature drilling. ¹³ Additionally, Jianghan Drill Bits extensively deployed its “HJT Series” tricone bits in Kenyan geothermal wells. This series incorporates sliding bearing structures with metal seals, paired with specialized diameter-maintaining features and reinforced palm-back structures, tailored to meet local geothermal drilling requirements. PDC bits have achieved over 92% of total drilling footage in oil and gas operations. However, existing geothermal PDC bits perform poorly under high-temperature conditions, exhibiting low penetration rates and rapid failure.^14,15 Consequently, research on rock fragmentation characteristics in high-temperature environments is urgently needed to provide foundational data for developing PDC bits suited for such conditions.

Experimental study on single-tooth fracturing of limestone in high-temperature environments

Testing of rock mechanical properties under high-temperature conditions

Rock mechanical properties significantly influence the rock-cutting performance of PDC bits during actual operations. In geothermal wells, rocks undergo substantial mechanical property changes due to high temperatures compared to ambient conditions. To provide effective mechanical data support for setting rock parameters in subsequent numerical simulations, rock samples were heated to specified temperatures and then subjected to uniaxial confining pressure tests to determine relevant mechanical properties. For convenience in mechanical testing machines, typical test specimens are cylindrical cores with a diameter of approximately 50mm and a height-to-diameter ratio of about 2. The tests employ a high-temperature, high-pressure triaxial rock mechanical testing machine capable of reaching temperatures up to 400°C. To ensure the accuracy of rock mechanical parameters throughout the testing process, each core undergoes three identical test sets under identical conditions. The axial loading rate of the rock mechanics testing machine is controlled between 0.5 and 90MPa/s, with displacement controlled at 0.1mm/min (Figure 1).

Figure 1.

Determination of rock mechanical properties. (a) High-temperature, high-pressure rock mechanics testing machine, (b)limestone sample, (c) failure morphology.

The specific rock parameters, including Poisson’s ratio, elastic modulus, shear strength, and compressive strength measured at different temperatures, are shown in Table 1.

Table 1.

Rock mechanical properties.

Temperature (°C)	Compressive strength (MPa)	Modulus of elasticity (GPa)	Shear strength (MPa)	Poisson’s ratio	Internal friction angle (°)
25	112.63	30.798	15.88	0.177	22.35
150	124.32	32.539	16.92	0.168	23.18
200	135.47	33.492	17.95	0.161	24.55
250	150.11	33.69	19.42	0.154	26.92
300	79.56	24.167	14.61	0.145	18.79
350	68.82	23.027	14.22	0.141	18.62

The mechanical relationship between limestone compressive strength and shear strength with temperature variation is shown in Figure 2. The figure indicates that both compressive strength and shear strength of limestone exhibit a trend of initially increasing and then decreasing with rising temperature. Maximum values occur at 250°C, while minimum values are observed at 350°C.

Figure 2.

Curves showing the relationship between mechanical properties of limestone at different temperatures. (a) Compressive strength variation graph, (b)shear strength variation graph.

PDC cutter rock cutting experiment under high-temperature conditions

The rock-cutting efficiency of PDC drill bits in geothermal wells is influenced by multiple parameters during actual high-temperature operations. This experiment employs a controlled-variable methodology to investigate how environmental temperature, penetration depth, rake angle, and tooth geometry affect rock-cutting efficiency. The diameter of the PDC cutters used in this single-tooth cutting test was 15.875mm, consistent with the numerical simulation model, and the planer cutting feed rate was also set to 0.3m/s, consistent with the numerical simulation. Table 2 details the experimental content.

Table 2.

Experimental details.

Serial number	Variable	Rock type	Cutting tooth specifications
1	Cutting Depth (0.5mm,1mm, 1.5mm, 2mm, 2.5mm, 3mm)	Limestone	φ15.875mm
2	Rake angle (5°, 10°, 15°,20°,25°,30°)	Limestone	φ15.875mm
3	Temperature Room temperature, 150°C, 200°C, 250°C, 300°C, 350°C	Limestone	φ15.875mm

The single-tooth rock-breaking test was conducted on a modified bullhead planer used as a single-tooth cutting rock-breaking tester. This tester offers advantages such as adjustable cutting speed, low impact force, smooth cutting motion, and constant cutting force. Its experimental principle and equipment are illustrated in Figure 3. The equipment used includes the single-tooth cutting rock-breaking tester, data acquisition system, triaxial force sensor, and cutting tooth.

Figure 3.

Schematic diagram of single-tooth cutting test.

Prior to testing, rocks undergo a preheating process to reach a preset temperature. Each temperature condition requires at least 4 hours of heating to ensure uniform internal and external temperatures. After the heating period, rocks are removed and wrapped in insulating material to maintain temperature. The heated rocks are placed on the testing machine. Before cutting, the rock temperature was reconfirmed using a thermometer to reconfirm it, followed by the cutting test. Simultaneously, a thermal imaging system monitors temperature rise during the experiment (Figure 4).

Figure 4.

Rock heating schematic.

Before the single-tooth cutting test, place the rock sample in the rock clamping tool and secure it so that the scraped upper surface of the sample is parallel to the workbench surface. Adjust the cutter holder to set the cutting depth to the required level. Place the insulated rock sample on the testing machine, measure the temperature with a high-temperature gun, and then perform the cutting, as shown in Figure 5. After cutting, collect the broken rock chips and record the cutting process with high-speed photography. Figure 5(b) shows rock scratches under different temperature conditions. The figure indicates that at temperatures not exceeding 200°C, the degree of brittle fracture in the cut groove is essentially consistent. As temperature increases, rock strength decreases and brittleness increases, leading to a pronounced brittle fracture phenomenon in the cut groove. This enhances rock breaking efficiency. Figure 5 displays thermal imaging monitoring during cutting of rock at ambient temperature and at 250°C, respectively.

Figure 5.

Temperature measurement during rock cutting process with pdc cutters. (a) Cutting process graph, (b) scratch trace graph, (c) thermal imaging of room-temperature graph, (d) thermal imaging of high-temperature graph.

During single-tooth cutting at a fixed depth, rock fragmentation primarily relies on tangential force. The magnitude of the tangential force reflects the difficulty of rock cutting: the smaller the tangential force, the easier the rock breaks and the lower the cutting power consumption. Therefore, the ratio of work done by tangential force to the volume of fragmented rock sample serves as an efficiency metric for rock fragmentation at different temperatures.^16,17 The specific work required for rock fragmentation during single-tooth cutting is:

$A\ast =\frac{W^{*}}{V^{*}}=\frac{{\displaystyle \sum }_{i=1}^n{f}_{ti}·v·t}{V^{*}}$

(1)

where $W^{*}$ is the work done by the cutting tooth, in joules (J); $f_{ti}$ is the number of discrete load data points collected, in newtons (N); $v$ is the cutting speed of the cutting tooth, in meters per second (m/s); $t$ is the cutting time, in seconds (s); $A*$ is the specific crushing work, in joules per millimeter cubed (J/mm³); $V^{*}$ is the crushed rock volume, in cubic millimeters (mm³).

Analysis of single-tooth cutting test results

Figure 6(a) shows the force characteristics of PDC circular cutters after cutting limestone. It can be observed that as temperature increases, the tangential force of PDC teeth first rises and then decreases, reaching a peak around 250°C. After approximately 300°C, the tangential force remains relatively stable. Compared to cutting at ambient temperature, the tangential force decreases by approximately 38.5%–39.6% between 250°C and 350°C.When cutting limestone, once the temperature exceeds 250°C, thermal stress arises due to the rock’s inherent heterogeneity. The rock contains various particulate materials with differing coefficients of thermal expansion. Under varying heating temperatures, these internal materials expand, causing mismatched thermal deformation among the rock particles and leading to thermal cracking. When temperature reaches a certain threshold, internal expansion stabilizes. As temperature continues to rise, sample expansion plateaus, resulting in minimal fluctuations in cutting load. Thus, the optimal fracturing temperature for limestone is 350°C.Figure 6(b) illustrates the relationship between temperature and rock-breaking specific energy during limestone cutting operations. The graph shows that rock-breaking specific energy first increases then decreases with rising temperature, peaking around 150°C. At 350°C, rock-breaking specific energy is lowest, indicating maximum crushing efficiency.

Figure 6.

Variation of tangential force and rock-breaking specific energy during limestone cutting with temperature. (a) Tangential force variation graph, (b) variation of rock-breaking specific energy.

Figure 7 shows the rock cuttings after cutting under high-temperature conditions. It can be observed that higher temperatures result in more scraped rock particles with larger cutting sizes, indicating that increased temperature facilitates limestone crushing.

Figure 7.

Rock cuttings at different temperatures. (a)25°C, (b)200°C, (c)350°C.

Figure 8 shows the tangential force and rock-breaking specific energy when a single-tooth scraper with different rake angles cuts limestone at a depth of 2mm. Observing vertically, as the temperature rises from ambient to 350°C, both tangential force and rock-breaking specific energy first increase then decrease. They peak at 250°C, where rock-breaking efficiency is lowest. Beyond 250°C, both values drop sharply, reaching their lowest at 350°C, where rock-breaking efficiency is highest. Horizontally, as the rake angle increases, both the tangential force and rock-breaking specific energy first increase and then decrease. The tangential force and rock-breaking specific energy are highest at a rake angle of 30°,lowest at 15°.

Figure 8.

Tangential force and specific energy during limestone cutting at different rake angles. (a) Tangential force variation graph, (b) variation of specific energy.

Figure 9 shows the tangential force and rock-breaking specific energy of PDC cutting teeth during limestone cutting at different temperatures and penetration depths. Longitudinally, as temperature increases from ambient to 250°C, both tangential force and rock-breaking specific energy rise, peaking at 250°C where rock-breaking efficiency is lowest. Beyond 250°C, both values drop sharply, reaching their lowest at 350°C. Horizontally, increasing penetration depth enhances efficiency by increasing tangential force and decreasing rock-breaking specific energy. Efficiency is lowest at 0.5 mm penetration and highest at 3 mm penetration.

Figure 9.

Tangential force and rock-breaking specific energy for limestone cutting at different penetration depths. (a) Tangential force variation diagram, (b) variation of specific crushing power.

Numerical simulation of single-tooth crushing of limestone under high-temperature environment

Model establishment

Numerical simulation of single-tooth cutting was conducted using the finite element deletion method. The rock strength criterion is crucial for numerical simulation, as it reflects the rock failure mechanism. To accurately characterize the rock failure mechanism, the plastic behavior of rock was modeled using the Drucker-Prager criterion, with shear damage adopted as the damage initiation criterion for rock failure. The evolution process of the damage stage was described by equivalent plastic displacement ( ${\overline{\mathit{u}}}^{\mathit{p}\mathit{l}}$ ) or fracture energy ( ${\mathit{G}}_{\mathit{f}}$ ), as shown in Equations (2) and (3).^18,19

$\{\begin{array}{l}{\dot{\overline{u}}}^{pl}=0\hspace{0.17em}Before\hspace{0.17em}damage\\ {\dot{\overline{u}}}^{pl}=L{\dot{\overline{\epsilon }}}^{pl}\hspace{0.17em}After\hspace{0.17em}damage\hspace{0.17em}initiation\end{array}$

(2)

$G_f={\int }_{{\overline{\epsilon }}_0^{pl}}^{{\overline{\epsilon }}_f^{pl}}L{\sigma }_{y}d{\overline{\epsilon }}^{pl}={\int }_0^{{\overline{u}}_f^{pl}}{\sigma }_{y}d{\overline{u}}^{pl}$

(3)

where: ${\dot{\overline{\epsilon }}}^{pl}$ is the plastic strain rate; ${\overline{\epsilon }}_0^{pl}$ is the equivalent plastic strain at damage initiation; ${\overline{\epsilon }}_f^{pl}$ is the equivalent plastic strain at failure; ${\sigma }_y$ is the yield stress at damage initiation; ${\overline{u}}^{pl}$ is the equivalent plastic displacement; $G_f$ is the fracture energy.

When the equivalent plastic displacement ( ${\overline{u}}^{pl}$ ) or fracture energy ( $G_f$ ) reaches a critical value, the element is removed to characterize the failure process of the cut rock. The stress-strain curve of the rock constitutive model is shown in Figure 10. ²⁰

Figure 10.

Stress-strain curve of the rock constitutive model.

The cutting model is shown in Figure 11. The simulation employed a rock specimen with dimensions of 100mm (length)×80 mm (width)×40mm (height) and a cutting tooth with a diameter of 15.875mm, thickness of 8mm, composite plate thickness of 2mm, rake angle of 15°, and flank angle of 0°.The general contact algorithm is adopted for the contact type in the PDC cutter–rock model. According to the guidelines of numerical simulation software, the first contact surface should be the harder and coarser-meshed surface, so the surface of the PDC cutter is selected as the first contact surface, and the rock region to be cut is selected as the second contact surface. A fully fixed constraint is applied to the bottom of the rock model. The PDC cutter scratches the rock at a velocity of 0.3 m/s, and a temperature condition is imposed on the rock model. The mesh employed hexahedral eight-node linear reduced integral elements (C3D8RT), with denser meshing applied to the rock cutting section. The rock remained stationary while the cutting tooth performed linear cutting at 0.3m/s. A predefined temperature field identical to the experimental conditions in Chapter 2 was applied to the rock. For computational convenience, the following assumptions were made for the model ²¹ : (1) The rock is treated as a homogeneous, continuous, isotropic material; (2) Damaged rock elements are immediately removed without secondary damage; (3) The effects of confining pressure and cutting tooth wear are neglected; (4) Only thermal conduction between rock and teeth is considered.

Figure 11.

Cutting model. (a) Model boundary conditions, (b) model mesh generation.

Analysis of simulation results

Figure 12 shows the temperature contour maps of PDC cutters during limestone cutting at different temperatures. The figure indicates that the maximum tooth surface temperature increases significantly with rising ambient temperature: At room temperature, the temperature rise is small, originating solely from frictional heat generation. In high-temperature environments, the superimposed effect of rock heat transfer causes a more pronounced temperature increase. The figure shows multi-colored temperature bands radiating outward from the cutting edge, with temperatures increasing closer to the edge. This occurs because during cutting, temperatures continuously accumulate on the tooth surface, while the rock generates heat under compression and transfers it to the tooth surface through contact. Therefore, under high-temperature conditions, higher ambient temperatures facilitate greater heat accumulation at the cutting edge, accelerating tooth wear.

Figure 12.

Temperature contour maps of cutter surfaces at different temperatures.

To effectively analyze cutting edge stresses during tooth-rock contact, the Cpress stress values at element nodes within the -90° to 90° range of the cutting edge were extracted. The lowest point corresponds to an azimuth angle of 0°, as shown in Figure 13. The figure reveals the contact stress distribution zone on the PDC cutting tooth surface—the area where cutting contact occurs with the rock. As ambient temperature increases, this contact stress zone gradually expands, and the stress distribution becomes more uniform.

Figure 13.

Stress contour maps of cutter surfaces at different temperatures.

Figure 14 shows the variation curve of contact stress on the tooth surface during PDC tooth cutting of rock at different ambient temperatures. The figure reveals a clear upward trend in contact stress from room temperature to 250°C. Beyond 250°C, contact stress decreases significantly, with the largest fluctuation amplitude occurring at 250°C.This pattern directly correlates with the temperature-dependent mechanical properties of rock: when rock temperature reaches 250°C, its compressive strength and other mechanical properties peak, causing synchronous increases in contact stress on the cutting teeth. At this point, the drill bit experiences more uneven loading, increased drilling resistance, and significantly heightened risk of cutting tooth wear. However, when temperatures exceed 250°C, the mechanical properties of the rock, such as compressive strength, gradually decrease with rising temperature. The rock becomes more brittle and easier to cut, leading to a continuous decrease in the contact stress on the cutting tooth surfaces.

Figure 14.

Variation of tooth surface contact stress at different temperatures.

Figure 15 shows the tooth surface temperature variation during rock cutting at different rake angles for limestone at 250°C.The data indicates that the maximum tooth surface temperatures at rake angles of 5°, 10°, 15°, 20°, 25°, and 30° are 323.2°C, 331.9°C, 341.9°C, 353.9°C, 378.2°C, and 385.4°C respectively. While the rake angle has a relatively minor effect on the maximum temperature, the temperature gradient range decreases as the rake angle increases. This occurs because as the rake approaches 0°, the tooth surface becomes more parallel to the rock, resulting in a larger area for heat transfer between the tooth and the rock.

Figure 15.

Temperature field on the tooth surface at different rake angles.

Figure 16 shows the variation in tooth surface contact stress when cutting teeth scrape rock in 250°C limestone under different forward rake angles. As the forward rake angle increases, both the cutting edge stress and stress fluctuations of PDC teeth rise. This is because a larger rake angle expands the contact area in the tooth edge’s extreme angle region, intensifying stress fluctuations and tooth-rock squeezing effects. This can lead to non-uniform wear of the cutting teeth, adversely affecting their service life.

Figure 16.

Variation of tooth surface contact stress at different rake angles.

Figure 17 shows the temperature variation on the cutting edge surface when cutting rock at different penetration depths in 250°C limestone. The figure shows that at 250°C, the maximum tooth surface temperatures for penetration depths of 0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, and 3mm are 276°C, 282°C, 329°C, 345°C, 362.3°C, and 385.4°C respectively. Both the temperature and the temperature gradient range near the tooth edge increase with cutting depth. This occurs because greater cutting depth expands the contact area between the tooth and rock, intensifying friction-induced heat generation and thermal conduction. Ultimately, deeper penetration depths make cutting teeth more susceptible to thermal wear failure.

Figure 17.

Temperature distribution on the tooth surface at different penetration depths.

Figure 18 shows the variation in tooth surface contact stress during rock cutting by cutting teeth at different penetration depths in 250°C limestone. The figure indicates that increased penetration depth elevates PDC tooth contact stress and intensifies stress curve fluctuations. The enlarged contact area increases friction and cutting forces, leading to more uneven loading on the cutting teeth. Therefore, greater penetration depth results in more fluctuating stress curves and increased susceptibility to tooth wear.

Figure 18.

Variation of tooth surface contact stress at different penetration depths.

Full-drilling test under high-temperature conditions

Unlike single-tooth cutting, actual drilling involves a composite process of cutting -extrusion-shearing-fracturing rather than pure cutting. To comprehensively study PDC bit performance in fracturing limestone, full-drilling tests were conducted.

Test principle

A newly designed Φ118mm drill bit was used for the full-drilling test. The experiment was conducted on the “YD100” drilling simulator at Southwest Petroleum University, with the principle illustrated in Figure 19. The parameters of the new drill bit and the conventional drill bit are compared in Table 3.

Figure 19.

Principle of full-drilling test.

Table 3.

Parameter comparison of two drill bits.

PDC bit	New-type PDC drill bit	Conventional drill bit
Blade shape	Spiral blades	Straight blades
Rake angle	All 15°	5°, 10°, 15°
Cutter size	φ13.44 mm	φ13.44 mm
Number of cutters	17	17
Rear row cutter design	Equipped with ball-tooth cutters	No rear row cutters
No rear row cutters	6	4
Nozzle diameter	12mm	12mm
Test temperature	400°C	400°C

The experimental rock sample consisted of limestone, measuring 350mm in length and width, and 200mm in height. The drill bit was heated prior to drilling (Figure 20).

Figure 20.

Heating process of PDC drill bit.

Test results

Laboratory testing yielded the results shown in Figure 21. Observation of Figure 21(b) reveals no significant wear on the drill bit after completing the drilling. From Figure 21(c) and (d), it is found that the bottom-hole morphology formed by the new-type drill bit is favorable, and its contour is clearer than that formed by the conventional drill bit. This indicates that the cutting effect of the new drill bit on the rock remains basically stable during each rotation cycle, and the cutting trajectory presents regular repetition. This characteristic enables the drill bit to bear relatively uniform loads during the drilling process, so its stability is better than that of the conventional drill bit.

Figure 21.

Experimental results diagram. (a) Bit pullout temperature plot, (b) bit after pullout, (c)bottom hole with new-type drill bit, (d)bottom hole with conventional drill bit.

Comparing the drill bit with a conventional drill bit, the results are shown in Figure 22. Under identical conditions of 400°C and 15kN drilling pressure during the experiment, the torque and mechanical drilling speed of both drill bits were compared. Observations revealed that the torque of the conventional drill bit was 998.83 N·m, while the torque of the new PDC drill bit reached 1062.58N·m, 5.8% higher than the conventional drill bit. The increased torque indicates that the new PDC drill bit exerts greater cutting force on rock during drilling, resulting in superior rock-cutting capability. Furthermore, the ample torque effectively suppresses load fluctuations during drilling, helping maintain stable drilling conditions. Experimental data also indicates that the mechanical drilling rate of the conventional bit is 5.11m/h, while the new PDC bit achieves 6.74m/h—a 31.9% improvement over the conventional bit. This result fully demonstrates that the new PDC bit possesses higher rock-cutting efficiency and better adapts to the complex working environment of geothermal wells. Therefore, for the design of high-temperature geothermal drill bits, the optimal cutting angle of the cutters is 15°, the cutting depth should not be excessively large, with 2 mm being optimal, and the adoption of spiral blades is more appropriate.

Figure 22.

Experimental results diagram. (a) Torque comparison chart, (b) mechanical drilling rate comparison chart.

Conclusions

This study investigates the limestone rock-cutting characteristics of PDC cutters in high-temperature environments through experiments and simulations. Single- -tooth cutting tests were conducted to record the mechanical relationship during limestone cutting with individual PDC cutters. Additionally, digital simulations of single-tooth cutting were performed to analyze the variation patterns of cutting tooth stress fields and temperature fields under different parameter conditions. Subsequently, full-drilling experiments were conducted using the designed PDC bit and compared with conventional bits. The aim is to provide a theoretical basis for geothermal well bit design.

1. Research on rock mechanical properties under high temperatures was conducted. Results indicate that under elevated temperatures, the compressive strength, shear strength, and internal friction angle of limestone first increase and then decrease, with maximum values occurring around 250°C.

2. Single-tooth cutting rock-breaking experiments were conducted under varying temperature conditions. Results indicate that elevated temperatures alter limestone’s internal structure, enhancing fracturability. Compared to ambient conditions, tangential force decreases by approximately 38.5%-39.6% at 250°C-350°C. At 350°C, limestone cutting efficiency peaks with maximum tooth penetration capability. Simultaneously, high-speed cameras and thermal imaging instruments were employed to observe cuttings generation and temperature variations.

3. Single-tooth rock-cutting experiments were conducted under varying parameters. Results indicated that a 15° rake angle produced the lowest tangential force, the lowest specific rock-cutting power, and the highest rock-cutting efficiency. Conversely, increasing cutting depth led to higher tangential forces, lower specific rock-cutting power, greater rock chip generation, and larger cutting sizes.

4. Digital simulations of single-tooth cutting rock breaking under varying parameters were conducted. Results indicated that as the cutting tooth’s ambient temperature increased, the contact stress zone gradually expanded and stress distribution became more uniform. Both the cutting tooth surface temperature and contact stress increased with penetration depth but decreased with increasing rake angle.

5. A new Φ118mm drill bit was designed and subjected to full-drilling rock-breaking tests. Both the drill bit and rock were heated during testing. Results showed no significant wear on the drill bit after completion, with excellent wellbore bottom morphology. Compared to conventional PDC drill bits, torque was increased by 5.8% and mechanical drilling speed increased by 31.9%.