Experimental Investigation of Force Response, Efficiency, and Wear Behaviors of Polycrystalline Diamond Rock Cutters

Abstract

Polycrystalline diamond compact (PDC) cutters are the most extensively used tool for rock drilling in superdeep oil and gas exploration, in which the air drilling technology without drilling fluid is highly promoted. This study examined the performance of PDC cutters in air drilling, including their friction angle, cutting force, specific energy, and wear behaviors, using a home-made testing apparatus and a commercial tribometer. It also investigated the dependence of cutting force on cutting depth and back rake angle. Results obtained in both dry conditions and in drilling fluid media were compared, and a tentative explanation to the observed differences was brought about by these two environments.

1. Introduction

Polycrystalline diamond compact (PDC) drilling bits have gradually replaced conventional roller bits in oil and natural gas exploration worldwide since their invention [1,2]. The market share of PDC bits had reached more than 65% by 2010 and has since continued to grow [3]. PDC bits have a simple mechanical structure, usually consisting of a polycrystalline diamond layer and a hard metal substrate layer distributed on the main bit body and can be used with dozens of PDC cutters. PDC bits offer a superior performance to roller bits due to their higher penetration rate and longer service life [4].

The performance of the whole PDC bit during the drilling process is a synthetic result of many single cutters and, therefore, the study of PDC bits in a laboratory usually begins by investigating the behavior of a single cutter [5,6]. Specially designed apparatuses have been developed in various research institutes all around the world to carry out studies on single PDC cutters [7,8]. In order to continually improve the performance of PDC cutters and bits, great efforts have been made to understand the cutter–rock interaction and related phenomena surrounding their functionalities. In particular, considerable attention has been paid to the following: (1) the friction and heat transfer during the cutter–rock interaction, (2) optimization of the design of PDC cutter geometric parameters, including their spatial distribution, back rake angle, side rake angle, chamfer, and groove geometry [9,10], (3) the monitoring and control of the dynamic rock-cutting process, (4) cutting-force response in the cutting process and its influence factors, and (5) wear behaviors of cutters in the drilling process [11,12,13].

Among all the above-mentioned areas of research, cutting-force response is the most fundamental indicator and a prerequisite for understanding the behaviors of a single cutter. Experimental and simulation studies have been conducted to explore the forces acting on a PDC cutter in various cutting conditions. An independent experimental evaluation of friction and cutting force by Yahiaoui et al. revealed that impact friction increases cutting force and, therefore, reduces cutting performance [14]. When Huang et al. applied the discrete element modeling method to simulate the cutter–rock interaction, they found that the cutting force on a cutter is determined by a multi-directional flow mechanism and cannot be directly used to measure cutter–rock friction [15]. In a study exploring the formation of rock fragments, Menezes concluded that cutting force was significantly affected by the mechanical properties of the rock, as well as the factors of rake angle and velocity [16]. Che and Ehmann performed a series of studies on polycrystalline diamond cutting of rock, and obtained experimental results regarding a number of influencing factors on cutting force, such as cutting depth, feed rate, rake angle, and speed [4,8,17,18].

The industry’s need to drill superdeep wells for oil and gas exploration continues to facilitate the advancement of air drilling technology [19]. A major difference between air drilling technology and conventional drilling technology is that no drilling fluid is applied in air drilling. Increased recognition of the benefits of air drilling demands a better understanding of PDC cutter–rock interaction, especially the fundamental force response of the cutter, however, most previous studies on force response in rock cutting have focused on force measurement with drilling fluid and therefore, cannot be applied to force prediction for air drilling. In response to this need, this study aimed to investigate force response in dry conditions. Moreover, the results obtained in both dry conditions and the drilling fluid environment was compared. In our previous study, a polypropylene glycol (PPG) additive in drilling fluid was found to effectively reduce friction force between steel contacts [20]. Therefore, PPG was also added to drilling fluid in this study to investigate the resulting effects on cutting force. In addition, the possible wear mechanisms of the PDC cutter were examined. The overall objective of this paper was to tentatively explore the forces acting on a PDC cutter during the rock cutting process, in both dry and lubricated conditions, thus providing a possible understanding of the cutter–rock interaction for air drilling and other rock processing technology in which neither cooling nor lubricating liquids are applied.

2. Experimental Details

A testing apparatus was developed to measure the force acting on a PDC cutter during the rock cutting process. The structure of the apparatus is represented in Figure 1. The rock sample was fixed at the bottom of the apparatus while the PDC cutter was fixed to a holder. A force sensor was placed inside the holder to record the force acting on the PDC cutter during the cutting experiment. The rock sample was pre-treated with a hole at the center. The cutter was set to cut the inner surface of the rock in the experiments. The spatial position of the holder was adjustable, as was the PDC cutter, which was driven to rotate by a motor. Feeding was achieved by means of a linear actuator. In this study, the feeding rate was maintained at 0.04 mm/s, while the spindle speed was a constant 35 rpm.

The sharp PDC cutters with a diameter of 19 mm used in this study were commercial products purchased from a domestic market in China. Rock samples of Jining limestone were obtained from the field. The physical properties of limestone used in this study are listed in

Table 1. Physical properties of limestone used in this study.

Shear Strength	Compressive Strength	Tensile Strength
26.848 MPa	146.9 MPa	8.125 MPa

. The PDC cutter and rock samples used are shown in Figure 2. Different back rake angles and cutting depths were achieved by adjusting the position of the cutter to evaluate their impact on the force response. Five back rake angles, of between 45° to 65° with incremental differences of 5°, and four cutting depths, from 0.5 mm to 2 mm in increments of 0.5 mm, were applied. In order to compare the force response in both dry and drilling fluid conditions, force measurement was also performed when the cutter was immersed in drilling fluid, the formula for which is listed in Table 1. Furthermore, three types of PPG, namely PPG 200, PPG 400, and PPG 600, were added into the water-based drilling fluid in concentrations of 1 wt%, 2 wt%, 5 wt%, and 10 wt%. The numbers in the names indicate average molecular weight of the PPG polymers. The effects of the PPG additive on cutting force were investigated by the testing apparatus. Each experiment was repeated three times.

The force sensor in the apparatus was able to measure force normal to the cutter face. According to the force diagram, presented in Figure 3a, which is adjusted from Akbari and Miska’s model [7], the force directly measured in this study is the normal contact force. Normal contact force and tangent contact force are the two components of resultant force with respect to the cutter’s surface. In order to evaluate cutting force, it is necessary to know another important parameter, namely friction angle β, which is dependent on the back rake angle [7,15]. A commercial tribometer, being able to measure normal force and cutting force, was used to evaluate the friction angle at different back rake angles. A schematic expression of the experiments conducted by the tribometer is shown in Figure 3c and compared with the schematic testing apparatus in Figure 3b. In the experiments to determine F_n/F_c using the tribometer, spindle speed and normal force were set at constants of 40 rpm and 20 N, for the test’s duration of 30 min. The experiments were performed at ~22 °C with relative humidity of 45–55%. The friction angle was subsequently calculated using the following formula:

$$\mathsf{\beta }=\mathrm{arc}\tan (F_n/F_c)-\mathsf{\alpha }$$

(1)

where, F_n and F_c stands for normal force and cutting force respectively, and α is the back rake angle. Given the friction angle, back rake angle, and normal contact force, the cutting force in experiments conducted by the testing apparatus can be calculated as follows:

$$\frac{F_{nc}}{\cos \beta }=\frac{F_c}{\cos \left(\alpha +\beta \right)}$$

(2)

where, F_nc is the normal contact force, directly measured by the testing apparatus.

Wear behaviors of the PDC cutter were also evaluated using the tribometer. A conical cutter was used because no detectable wear was found on the cylindrical cutter. In the experiment, a load of 30 N was applied and the speed was maintained at 40 rpm. The duration of the experiment was 20 h. The tip of the conical cutter was subsequently examined using a three-dimensional laser profilometer.

3. Results and Discussion

3.1. Friction Angle

Normal force and cutting force were recorded using a tribometer and the results are presented in Figure 4. Values of F_n/F_c before and after data processing are compared and shown in Figure 4a, while the measurements of F_n/F_c at various back rake angles are presented in Figure 4b. Based on these results, it can be concluded that F_n/F_c is reduced as the back rake angle is increased.

The friction angles determined from the experimental measurements at different back rake angles are presented in Figure 5. In both dry conditions and the drilling fluid environment, the friction angle is determined to be negatively related to back rake angle in general. Variations of friction angles with back rake angles have also been reported by other researchers, although it is commonly regarded as a constant for rock [4,7,21]. In this paper, an explanation and a model is proposed by analyzing the rock flow regime ahead of the cutter [21], which is schematically shown in Figure 6. It can clearly be seen that rock material ahead of the cutter has two flow directions, namely forward to the cutter top and backward to the cutter bottom. Obviously, the portion of rock material moving forward depends on the inclination of the cutter, which, in this study, equates to the back rake angle. More rock material flows backward when the back rake angle increases. The ideal friction angle, which is a constant for a specific rock, is determined with the assumption that all rock material moves forward during the cutting process. As a result, the dependence of rock volume flowing forward on the back rake angle leads to changes in the friction angle.

An approximately linear decrease of the friction angle with an increasing back rake angle was observed in both dry conditions and in drilling fluid media. This result is in accordance with previous studies by Akbari et al. and Richard et al. [7,21]. Linear fitting was performed for both cases. The resulting lines are shown in Figure 5, where the slopes of the two fitted lines are closely aligned. The friction angle in the dry condition was approximately 8° lower than that in the drilling fluid media. One possible explanation is that the infiltration of drilling fluid into rock changes the properties of the rock. All natural rocks have a measure of porosity which allows for liquid to permeate the rock, leading to a change in the mechanical properties of the rock and friction angle.

3.2. Cutting Force and Specific Energy

Normal contact force was directly measured by the testing apparatus in this study. Measured normal contact force at a steady phase was used to calculate the cutting force using Equation (2). The impact of cutting depth on cutting force at a constant back rake angle of 65° is reported in Figure 7, in which the cutting force clearly increases with cutting depth. It is evident, therefore, that the contact area between cutter surface and rock increases at a greater cutting depth. This means that more rock materials need to be removed during the cutting process. As a consequence, a greater cutting force is required from an energetic point of view. However, a simple linear fitting for the measured results reveals that the cutting force is not zero at zero cutting depth, which may be attributed to the friction force between cutter and rock [8].

In addition to cutting depth, the back rake angle is another significant parameter affecting cutting force. The changes of cutting force with a back rake angle at a constant cutting depth of 1 mm were evaluated and the results are presented in Figure 8. In both dry conditions and drilling fluid immersion, the cutting force was found to increase quickly, in line with the back rake angle. At the same back rake angle, a greater cutting force is required to cut the rock under the dry condition. ‘Specific energy’, the ratio of cutting force to the active cutting area of the cutter, describes the energy required to remove a unit volume of rock under a specific condition, and it is, therefore, a commonly investigated factor in evaluating a cutter’s performance. Here, the active cutting areas of a PDC cutter at various back rake angles were determined, based on a simple geometrical calculation. The relationship between the resulting specific energy and the back rake angle is also shown in Figure 8, with hollow symbols as indicators. According to these results, specific energy is positively related to back rake angle in general. Similar to the results obtained for cutting force, specific energy is higher in dry conditions than in drilling fluid. A lower cutting force and specific energy are preferred when the cutting depth is the same, because less energy is then required to cut the same volume of rock. A higher cutting force and specific energy may, therefore, be possible reasons for the higher energy consumption observed in air drilling.

Additives are commonly used to improve drilling performance [22] and glycol is a widely used additive for water-based fluids [23]. In this study, three types of PPG were added into the water-based drilling fluid and their impacts on the cutting force is presented in Figure 9. Neither PPG 200 nor PPG 400 showed any statistical influence on the cutting force. Measured cutting forces were noted to fluctuate significantly along the reference line, with 1 wt% to 10 wt% concentration. However, PPG 600 slightly reduced the cutting force by approximately 5% with additive concentrations of between 2 wt% and 10 wt%.

3.3. Wear Behaviors

In order to evaluate the wear behaviors of conical PDC cutters, a lengthy, 20 h experiment was conducted in both dry conditions and drilling fluid media. The results are presented in Figure 10, with cutting tracks on the limestone shown in Figure 10a,b. It is clear that the cutting track formed in the dry condition is narrower than that obtained in the drilling fluid, thus indicating that more rock is cut and removed with the existence of drilling fluid, when all other parameters are the same. Profiles of the cutter tip were characterized using the profilometer and shown in Figure 10c–f. Remarkable differences were observed between the two-dimensional (2D) profiles of the two cutters. A layer of tribofilm was found on the cutter in the dry conditions, whereas scratches formed on the cutter in drilling fluid. The formation of the tribofilm layer is demonstrated by the bump shown in the three-dimensional (3D) image in Figure 10e. During the cutting process, hard PDC easily breaks the rock surface producing small chips of rock, some of which remain in the cutting area and are ground during the cutter–rock interaction until tiny rock particles are produced. In addition, a massive amount of heat is generated during the cutting in dry conditions. Localized temperatures on the cutter tip become notably high and the rock particles are thus sintered together to form a new phase. Similar tribofilm formation has been reported in other brittle materials, such as alumina and shale rock [24,25,26]. The newly formed tribofilm attaches to the cutter and forms the structure observed in Figure 10c,e. The hard PDC is covered by the new phase, which forms a structure similar to balling, thus leading to reduced cutting efficiency, as seen in Figure 10a. In real air drilling, however, the aggregation and grinding of rock chip can be avoided because of air circulation.

This scenario is distinguished for the cutter in drilling fluid media, in which the rock chips removed from the rock flow away with the fluid, so no smaller particles are produced. In addition, the heat generated at the cutter tip dissipates quickly in water, so the temperature remains low. As a result, PDC can continually cut and remove rock during the cutting process, leading to fewer scratches and higher cutting efficiency.

4. Conclusions

The force response and wear behaviors of PDC cutters were investigated in both dry conditions and in drilling fluid. Friction angles decreased linearly with an increasing back rake angle, while at the same back rake angle, the friction angles measured in dry conditions were smaller than those in the drilling fluid environment.

Cutting force positively depended on cutting depth and back rake angle. Generally, specific energy increased with back rake angles, except a slight decrease at 60°. All other parameters being constant, the cutting force and specific energy in dry conditions are higher than those in drilling fluid environments, leading to greater energy consumption in air drilling.

PPG 600 exhibited a very limited influence on reducing cutting force, while PPG 200 and PPG 400 showed no obvious effect. In terms of wear behaviors, in dry conditions, a tribofilm layer was found on the cutter surface due to particle sintering, causing a lower cutting efficiency than drilling fluid environment.