ARMA 11-266 Wear Analysis and Optimization on Impregnated Diamond Bits in Vibration Assisted Rotary Drilling (VARD) Abtahi A., Butt S., and Molgaard J.., Arvani F., Memorial University of Newfoundland, St. John s, NL, Canada Copyright 2011 ARMA, American Rock Mechanics Association This paper was prepared for presentation at the 45 th US Rock Mechanics / Geomechanics Symposium held in San Francisco, CA, June 26 29, 2011. This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented. ABSTRACT: This is an investigation to find, understand, and optimize bit wear using vibration assisted rotary drilling. The wear of embedded diamond bits is being studied with and without vibration, to better understand the mechanisms of wear, the effect of vibration on them, and to study relationships between drilling parameters including rotational velocity, vibration, bit pressure, and profile, focusing separately on the wear mechanisms for the bit matrix and the embedded diamonds. Tests reported here, analyze the effect of different drilling conditions on bit matrix wear, diamond wear, and power consumption, working mainly with short runs in which small amounts of wear occurred. Concrete was selected as a rock analog as it can provide a range of properties, such as uniaxial compressive strength and relative abrasion resistance, by varying the proportions and curing of the included materials. Some preliminary results and observations are reported. Wear caused changes in bit profile. Effect of bit profile is found to be critical for rate of penetration (ROP) and bit life. For a given profile, the ranges of the optimum WOB, the maximum ROP, and minimum weight loss may overlap, but conditions for maximum ROP and minimum wear rate are not always identical. 1. INTRODUCTION It is known that vibration combined with rotation increases the rate of penetration in drilling relative to conventional drilling, e.g. Wiercigroch M. studied the effect of resonance enhanced drilling [1]; also Li H. et al showed that vibration can increase the rate of penetration in rotary drilling [2]. However, from our investigation it decreases the bit life. The advanced drilling group in Memorial University of Newfoundland has investigated different aspects of bit performance in vibration drilling. One of the important issues is bit wear and relating it to bit performance, not just for the bit life, but also to rate of penetration. Miller and Ball classified bit wear as five different types, recently exposed or unworn diamonds, wear flat, micofracture, hackly macrofracture, and pull- out hole [3]. They did their experiments with different types of rocks and found that for stable drilling in any given rock type a characteristic threshold pressure existed above which desirable microfracture of the exposed diamonds was promoted over undesirable wear flat generation. At lower load, flats are produced by sliding wear, with the silicate minerals ploughing plastic grooves in the heated surfaces of the diamonds. D.N. Wright, et al reported some results for drilling on two different rock types. In drilling sandstone erosion of the matrix predominated, in particular around diamond particles [4]. In both sandstone and granite were eventually pull out of the diamond particles. In the case of granite, there were particles pulled out after they fractured; with sandstone pull out occurring before fracture. With both rocks breakdown occurred when the supporting matrix had been eroded. They also found that the rate of diamond protrusion can be directly related to its position on the end face of bit and the rate of exposure of diamonds related to the abrasion of the matrix. Xuefeng Tian, and Shifeng Tian did some experimental studies on wear mechanisms in hard rock drilling [5]. A single-diamond was used for rock cutting to understand the coefficient of friction and wear process at contact surface. The coefficient of friction depends on rock fracture characteristics at the contact surface. It was found that the penetration per revolution was the predominant parameter influencing wear behavior. They also found that wear depends on drilling conditions and diamond temperature. Excessive penetration rate increases temperature and it results in micro-burn and adhesion of diamond and drilling detritus together.
2- EXPERIMENTAL MACHINE AND MATERIALS The laboratory drilling machine of VARD project is an electrical powered drill rig with two rotary speeds of 300 and 600 RPM. The drill can easily move up and down on a rail guide. The sample is mounted on an electromagnetic shaker attached to the base, to vibrate the sample at different frequencies and amplitudes instead of vibrating the bit. A constant weight on bit (WOB) is applied by hanging a weight from a wheel (Fig. 1). All data is saved on a computer including current, vibration amplitude, and drilling depth, using a group of accelerometers and LVDT. Water pressure and flow rate is read during experiments from gauges. The LVDT is attached to the table to register the amplitude of the vibration and a frequency inverter is used to set the frequency. Also a rotary encoder is attached to the drill bit and travels with the bit along the rig frame to measure the displacement or drilling depth for each drilling. A tachometer is used for measuring the exact rotary speed. Concrete samples were used instead of rocks to have identical samples in weight on the shaking table, uniaxial compressive strength (UCS) values, and abrasivity. The concrete mixture is shown below: 0.3 (1) 0.5 (2) Concrete samples were cured in 23 degree Celsius and 100% humidity to reach the maximum UCS value. UCS values tested in correspondence to ASTM C873after 7 and 28 days were respectively 35 and 38 MPa. Another UCS test was done on core samples during the drilling runs and the value was 46MPa. 3- PROCEDURES FOR WEAR MEASURMENT Methods have been investigated and designed for measuring the wear of the matrix and diamonds on impregnated coring bits. A typical way of calculating the volume of wear is through measuring the mass or length of the bit. In this project, wear is being studied by placing indentations on the surfaces and using replicas of the surfaces. Replication is the best way to save all of the bit information permanently after each test. Two types of replica are being produced, using silicon rubber and epoxy materials. They keep the full shape of the bit. First a negative replica of the face of the bit is molded using silicone mold-making resins then a positive epoxy replica is cast in the negative replica (Fig. 2). The positive replica is gold coated for use under optical microscopes (Fig. 3). Fig. 1. Drill rig The replication method is very precise and it saves all of the data such as profile shape, length, and size of the head of the bit permanently. This makes it possible to follow changes at any location on a bit throughout an experiment without any concern regarding missing any information or measurement after each experiment. Fig. 2. Negative replica Fig. 3. Positive replica
Indentations are used as reference marks to aid accurate measurements of changes in length and profile. Using a center punch or electro discharge machining (EDM), it is possible to make indentations on different locations on the bit. Usually, indentations are placed on the water way surfaces, bit end and side faces. Water ways can show the profile changes and change in the length of teeth (Fig. 4). Side face and end face can show depth change in addition to change in position of the indentation relative to bit face. Fig. 5. unused bit profile Fig. 6. flat end profile Fig. 7. 2flat end with groove Fig. 8. rounded edge Fig. 4. indentation on waterway face The bit surfaces before and after experiments were compared, using pictures from the actual bit, taken from end or side face with an optical microscope. In this case, the best magnification is chosen first and then pictures of the best places are captured. 4- EXPERIMENTAL PARAMETERS Four new coring bits were used with dimension of 410mm length, 26.8 mm outside diameter, and 19mm inside diameter. The mass of the new bit is 0.56 kg. The end face area of the bit was 150. For all the experiments the constant parameters were, 60 Hz vibration frequency, 600 RPM, and water flow rate of 3100 with the supply pressure 4800 Pa. For each experiment, other parameters of drilling such as WOB, vibration amplitude and drilling depths were varied. In addition to the parameters just described, the bit profile changes during drilling. Unused bits have a V profile for the bit matrix with two sharp ridges (Fig. 5). The ridges wear down quite quickly to two flats separated by a groove (Fig. 7). With continuing wear the profile changes to a complete flat (Fig. 6). Finally, continuing wear changes the bit profile to rounded edges (Fig. 8). The changes in profile may affect the performance of the bit, but bits were used in all these states. 5- RESULTS AND DISCUSSION Preliminary tests were performed with several WOB values, with vibration amplitude of 0.48mm and without vibration (Fig. 9). A maximum ROP for each set of conditions was found. The drill depths were 40mm for each test, conducted in sequence in the same hole up to total depth of 240mm. On the basis of these, subsequent tests were performed with a drill depth of 100mm, each in a separate hole and a WOB range from 60 to 111 kg, within which a maximum ROP was expected. ROP (mm/sec) 3.00 2.50 2.00 1.50 1.00 0.50 bit 1, no vibration bit 2, vibration amplitude 0.48mm Fig. 9. Effect of vibration on ROP on unused bit Figure 10 and 11 shows effect of conventional (no vibration) and vibration drilling. Figure 3 and 4 shows the effect of different vibration amplitude levels on ROP and weight loss. Each bit has a different profile shape. For the first run of experiment (Fig. 10 and 11), bit 1 and 2 had flat end surface with edges a little rounded. Bit 3 and 4 had grooved end face, but bit 4 was in transition from a grooved shape to a flat end; as shown in pictures from waterways can display it very well (Fig. 12).
ROP (mm/sec) 3.50 3.30 3.10 2.90 2.70 2.50 2.30 2.10 1.90 1.70 1.50 bit 1, no vibration bit 3 vibration amplitude 0.48mm Fig.10. Effect of vibration on ROP vs. WOB. weight loss (g) 0.3 0.25 0.2 0.15 0.1 0.05 0 Fig.11. Effect of vibration on Weight loss vs. WOB. Fig. 12. bit shape profile before experiment bit 2 vibration amplitude 0.48mm bit 4, no vibration Bit 1, no vibration Bit 3 vibration amplitude 0.48mm Bit 2 Vibration amplitude 0.48mm Bit 4, no vibration Figure 13 and 14 show the effect of two vibration amplitudes with different profile shapes. Bit 1 and 2 had the flat end faces with rounded edges, and bit 3 was in transition to a flat end face; bit 4 had a flat end face. ROP (mm/sec) 3.60 3.40 3.20 3.00 2.80 2.60 2.40 2.20 2.00 Bit 1 Vibration amplitude 0.38mm Bit 3 Vibration amplitude 0.38mm Bit 2 Vibration amplitude 0.58mm Bit 4 Vibration amplitude 0.58mm Fig.13. Effect of different vibration level on ROP vs. WOB. Weight loss (g) 0.25 0.2 0.15 0.1 0.05 0 Bit 1 Vibration amplitude 0.38mm Bit 3 Vibration amplitude 0.38mm Bit 2 vibration amplitude 0.58mm Bit 4 vibration amplitude 0.58mm Fig.14. Effect of different vibration on Weight loss vs. WOB. Examination of the bits shows 3 major diamond situations; unworn, fractured, and diamond pull out. For the bits exposed to vibration pull out of diamonds was a common wear pattern. Generally, two types of wear occurred, matrix wear and diamond wear. In this study, matrix wear was very significant and it was associated with changes in the ROP. Diamond wear just shows two main classes of wear; diamond pull out and fracture. Matrix wear caused the profile change. Profiles can be classified as grooved (having two narrow flat end faces with a groove in the center), flat end, and rounded edge. At each state, the ROP was different, and the profile changed between states, the ROP also changed. Figure 9 shows the ROP for different WOB with two unused bits, bits 1 and 2. Vibration drilling has almost 1.5 times more ROP than conventional drilling. Fig.10 shows ROP for all four bits with the same experimental conditions but with different bit profiles. Bit 3 with the grooved profile had the highest ROP (Fig. 12). Bit 2 with a flat end-face, had intermediate range of ROP. Bit 4 was changing from grooved shape to flat end face, and it had higher ROP without vibration than bit 2 with vibration; this shows
that with the V profile the bit achieves a higher ROP. Bit 1 was transitioning to a rounded edge (Fig. 12). This had the lowest ROP compared with other shapes. Figure 11 shows the weight loss for the same experiments of Figure 10; in which there is the same trend for all of the bits; a decline from lower WOBs to higher WOBs. Figure 13 and 14 shows the effect of different vibration amplitude on bit performance; in this case bit 4 reached the highest ROP with the flat end face. Bits 1 and 2 had almost same profile, but different vibration amplitudes. Bit 2 had a higher ROP than bit 1. Bit 3 was in transition from a grooved shape to a flat end face, and it had higher ROP than bit 1 with same WOB and vibration amplitude but just difference in profile shape; it shows that the transition situation also produced higher ROP. Figure 14 shows the weight losses for the same experiments as in Figure 13. In this case weight loss increased a little up to 80kg WOB, but after that, it started to decrease with adding more WOB. Drilling productivity is affected by two critical factors in conventional drilling, ROP and bit wear. Optimizing one of them can affect the other one; Higher ROP usually causes more wear. Another important issue is vibration; it can assist drilling, increasing ROP, but it can affect bit wear to an undesired rate. All of these factors should be compared to obtain the best drilling conditions. For example, in Figure 13, bit 4 has a highest ROP for the range of WOB from 80 to 100 kg in comparison with the other bits. It also has the vibration amplitude 0.58mm. As seen in Figure 14, it has very low wear rate (weight loss) for the range of WOB from 95 to 105. With these plots, it is possible to choose the best condition; first, compare profiles, then choose optimum range of WOB for ROP, and lastly locate conditions for minimum wear rate. This method can combine all of the factors that contribute to an optimum. The diamonds are, of course, the cutters, but the role of wear of diamonds has yet to be studied in detail. The main reason of matrix wear is diamond pull-out when a bit loses diamonds, there has to be matrix wear to expose new diamonds for cutting. 7- FUTURE WORK Future work will include study of the effect of fluid flow rate, as there in some evidence that this affects the flushing of cuttings and matrix wear. We expect also to check the effect of bit rotational speed and verify the effects of varying vibration frequency and amplitude. 8- ACKNOWLEDGEMENT This investigation has been funded by the Atlantic Innovation Fund (contract no. 781-2636-192044), Industrial Research and Innovation Fund, Husky Energy and Suncor Energy. REFERENCES 1. Wiercigrokh, M. 2007. Resonance enhanced drilling: method and apparatus. World Organization patent no. WO/2007/141550, filed June 06, 2007, and published December 13, 2007. 2. Li, H., S. Butt, K. Munaswamy, and F. Arvani. 2010. Experimental Investigation of Bit Vibration on Rotary Drilling Penetration Rate. In the 44th US Rock Mechanics Symposium and 5th U.S.-Canada Rock Mechanics Symposium, held in Salt Lake City, UT June 27-30, 2010. 3. Miller, D. and A. Ball, 1990. The wear of diamonds in impregnated diamond bit drilling. J. Wear. Res. 0043-1648 4. Wright, D. N., S. M. Wilson, W. F. Brown, and U. Ovens. 1990. Segment wear on diamond impregnated mining bits. J. IDR industrial diamond review, ISSN 0019-8145. 5. Tian, X. and S. Tian. 1994. The wear mechanisms of impregnated diamond bits. J. Wear. Res.0043-1648. 6- CONCLUSIONS Optimum drilling productivity can be achieved by optimizing both bit weight loss and ROP. A decline in wear rate with increasing WOB was observed when vibration was combined with rotation Vibration is not the only variable that may cause increased bit wear. Matrix wear leads to changes in profile which, in turn, has a significant effect on ROP; this study shoes profile change can be a more important factor than vibration.