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The characterization of wear in roller cone drill bit by rock material – Sandstone
Accepted Manuscript The characterization of wear in roller cone drill bit by rock material – Sandstone Jurij Šporin, Primož Mrvar, Mitja Petrič, Goran Vižintin, Željko Vukelić PII:
S0920-4105(18)30958-6
DOI:
https://doi.org/10.1016/j.petrol.2018.10.090
Reference:
PETROL 5453
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 16 May 2018 Revised Date:
24 October 2018
Accepted Date: 25 October 2018
Please cite this article as: Šporin, J., Mrvar, Primož., Petrič, M., Vižintin, G., Vukelić, Ž., The characterization of wear in roller cone drill bit by rock material – Sandstone, Journal of Petroleum Science and Engineering (2018), doi: https://doi.org/10.1016/j.petrol.2018.10.090. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The characterization of wear in roller cone drill bit by rock material – sandstone ACCEPTED MANUSCRIPT Jurij Šporin a*, Primož Mrvar b, Mitja Petrič b, Goran Vižintin a, Željko Vukelić a a
University of Ljubljana, Faculty of Natural Science and Engineering, Department of Geotechnology, mining and environment, Aškerčeva 12, Ljubljana, Slovenia b University of Ljubljana, Faculty of Natural Science and Engineering, Department of Materials and Metallurgy, Aškerčeva 12, Ljubljana, Slovenia
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[email protected] [email protected] [email protected] [email protected] [email protected] *
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Corresponding author: University of Ljubljana, Faculty of Natural Science and Engineering, Aškerčeva 12, 1000 Ljubljana, Slovenia E-mail address: [email protected]
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Key words: roller cone bit, mechanism of drill bit wear, carbide coating, erosion channel, micro crack, aggressive drill mud Abstract
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Roller cone drill bit efficiency time during drilling depends, to a large extent, on the properties of the materials from which the components of the roller cone bit are made. The faster the roller cone bit is worn, due to the effects of the rock material through which we drill and the drilling regimes used during the drilling, the worse is the efficiency of drilling and the higher is the number of drill bits needed to drill the well. The aim of our research was to determine the mechanisms of wear on roller cone bit materials according to the characteristics of the steel material of the roller cone bit and the characteristics of the rock material, in this case, sandstone on the drilling regime. The results of the discovered wear mechanisms help us to anticipate the possible improvement of the resistance of the unprotected roller cone bit steel material to excessive wear and, consequently, the longevity of the roller cone bit. For this purpose, we examined the steel materials of the roller cone bit, which we used during well drilling and the rock material, sandstone, through which we drilled with the roller cone bit under consideration. The results of our analysis present the mechanisms that result in the wear of the roller cone bit material under the given conditions of the rock material and the drilling regime. The results of the discovered wear mechanism can be used to improve the material of the roller cone bits in order to achieve a longer operating life and decrease the related costs of drilling. Introduction
The efficiency of the roller cone bit is proportional to the quality of the material from which it is produced and depends on the mechanical characteristics of the rock material through which it is drilled as well as on the drilling regime. By drilling regime, what is meant is the load on the drill bit, the number of drill bit rotations and the type, quantity and pressure of the drilling mud used. Among the factors that reduce the time of the effective operation of the roller cone bit, we can include the material from which the roller and the teeth are made, the drilling regime, and the geomechanical properties of the rock material through which we drill. With the wear of drill bits in general and, consequently, the reduced rate of penetration, several authors were studied Karasawa (2016), Timonin (2017), Njobuenwu (2007), Naganawa (2012) among many others, but only a few research is being done to discover the mechanism of roller cone bit wear.
Fig. 1: Drill bit 215.9 mm (8 ½") ST 117
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In our study, we analysed the mechanical properties of the steel material of the Smith ST117 roller cone bit, ACCEPTED MANUSCRIPT diameter 215.9 mm (8 ½"), which was drilled through 610.70 m of sandstone. The rock material through which it was drilled is basically weak bound sandstone, which, due to its high content of silicates, has a very aggressive level of abrasion on the steel part of the roller cone bit. The Smith ST117 roller cone drill bit is a drill bit for drilling into soft rock materials. The teeth on the rollers are made from the same steel material as the rollers. The teeth on the rollers are covered with a carbide coating to prevent wear resulting from rock penetration and shear movement during roller rotation.
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The roller cone drill bit’s temperature increases during drilling and is cooled down by drilling mud, which is usually water based, with other materials added (bentonite, polymer …), which improves the rheological properties of drilling mud. In Timonin’s (2017) paper, the wear of PDC bits was inspected, which differs in some ways from the operation of the roller cone bit in the mode of its effect on rock. However, in the paper, the method of wear that can be applied to the operation and wear of the conventional roller cone bits is shown taking into account their mode of operation on the rock and the flow of drilling mud around the rollers. In the case of a high content of silicates in the drilling mud, the flow of the dispersed medium is reflected in the wear of the steel material on the roller cone bit. The result of this is the loss of steel material in the area of the tooth. The consequence of this loss is the reduction and change in the geometry of the teeth, which no longer have a functional value in terms of effective breaking of the rock massif. Similarly, Zhao (2018). examined the loss of steel material due to the erosion of solid particles in the housing of the drill bit, where they found that the rate of the erosion on steel material increases by increasing the flow of the dispersed material containing erosion-acting particles. In the investigation of the properties of the steel material of the roller cone drill bit, we inspected the materials of the bit body, rollers and teeth. We examined the method of wear and determined the critical points of erosion effect on the steel material of roller cone bits. For this purpose, we analysed the rock material that the examined roller cone bit drilled through. At that point, we evaluated the mechanism of wear of roller cone drill bit in the field of material microstructure. To reach that goal, we use the theoretical and methodical basics of previously published articles by Rashidi (2015, 2008) and Abbas (2018). Rashidi (2008), specifies that the variables that influence the rate of penetration (ROP) of roller cone bits, among other influential parameters, also refers to the wear on the roller's teeth. The influential parameters on ROP are shown in Equation 1. ROP = f ∙ f ∙ f ∙ f ∙ f ∙ f ∙ f ∙ f
(1)
where, ROP is the rate of penetration, f1 is the effect of formation strength or rock durability, f2 is the effect of formation depth, f3 is the effect of formation compaction or pore pressure, f4 is the effect of differential
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pressure, f5 is the effect of bit diameter and bit weight, f6 is the effect of rotary speed, f7 is the effect of tooth ACCEPTED MANUSCRIPT wear and f8 is the effect of bit hydraulics. In the Abbas (2018) article, a literature survey is reported dealing with methods of wear quantification in terms of drilling parameters and mechanical properties of steel materials of drill bits. Also shown are the empirical wear models as well as wear prediction techniques. In our research, we focused on a problem that shows the dependence of the resistance of the roller cone bit steel material on the rock material. In the subject that we dealt with, the guideline discussed how to improve the basic material (steel) of the rollers and teeth of the rolling cone bit, so that it is more resistant to the abrasion caused by the flow of the dispersed medium with the content of the rock material debris through which it is drilled. Materials and Methods
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In the course of the investigations to characterize the wear the roller cone bit, detailed investigations were carried out on the materials through which it was drilled, drilling parameters and the materials from which the roller cone bit was made. To this end, in the first phase, we carried out a review of the roller cone bit, which included an overview of the condition of the bit after drilling through the rock formation, according to IADC (International Association of Drilling Contractors) methodology. Here, we examined the state of the teeth and bit rollers after drilling through the rock material. After a visual inspection of the bit, we cut it apart to perform tests to discover the metallurgical properties of the steel from which the roller cone drill bit was made and to produce test samples for the investigation of material damage that occurred during the drilling work. From the obtained sample of rock material, we carried out laboratory analyses to determine the mechanical and mineralogical properties of the rock through which it was drilled with the considered drill bit. The following investigations were carried out on a sample of rock material: - a visual overview of rock material, - an overview of the geochemical and mineralogical composition of the sample of the rock material through which it was drilled using the XRF (X-ray fluorescence) method, using a Thermo NITON XL3t XRF analyser - an overview of the strength-deformation properties of the rock material was carried out according to ASTM standard D7012-10. While drilling through the known rock material, drilling parameters were also monitored as follows: - the load on the drill bit, - the number of rotations of the drill string, - the quantity of the pumped drill mud, - the pressure of the pumped drill mud, - the penetration rate, - the length of the drilling interval. In the following paper, we carried out a complete analysis of the steel materials of the roller cone bit, which covered the following examinations: - the chemical composition of roller body steel with a ARL MA-310 optical emission spectrometer, - the composition of the carbide coating of bit teeth with the XRF (X-ray fluorescence) method using a Thermo NITON XL3t XRF analyser, - a cross-sectional view of the bit teeth with an Jeol JSM 5610 electron microscope using the EDS/SEM analysis (energy dispersive spectroscopy / scanning electron microscope), - DSC (differential scanning calorimetry) of roller steel and tooth carbide coating with the Nech Jupiter STA449C - a dilatometric analysis of the roller steel and the tooth carbide coating with a Bähr DIL 801 low temperature dilatometer, - Vickers hardness tests on the roller steel with a 100 g load with a Shimadzu type M microhardness tester. 2.1
Drilling Work
Drilling work was carried out using a N-1000 drill rig made in 1992. The area where we observed the ACCEPTED MANUSCRIPT performance of the 215,9 mm (8 ½") roller cone drill bit included a drilling interval from 871.80 m to 1482.50 m in depth, which amounts to 610.70 m in length. The sample of rock material was extracted from a depth of 1482.50 m and is also the subject of our research. The extracted rock sample represents a characteristic sample of sandstone, which was in the observed area – drilling interval. The average penetration rate during the observed interval was 4.68 m/h. The rest of the drilling parameters can be seen in Table 1. Table 1: Drilling parameters 871.04 – 1150.00 30 50 0.92 4.5
1150.00 – 1320.00 40 55 0.92 4.5
1320.00 – 1482.50 50 60 0.92 5
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Depth (m) Load on drill bit (kN) Rotary speed (rpm) Pump capacity (m3/min) Pump pressure (MPa)
Table 2: Mud characteristics
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Value 1080 35 5.8 - 6,0 8-9 9.6 – 9.7
Rock Material
Unit kg/m3 s ml/30 min mPas pH
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Parameter Density Viscosity (Fann funnel) Filtration Plastic viscosity (Fann viscometer) pH
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During the drilling, 14 mass. % bentonite drilling mud was used. The mud characteristics are shown in Table 2.
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It was at a depth of 1482.80 m that we extracted the sample of rock material to determine its mechanical and mineralogical properties. The core sampling was performed using a double core barrel 158.75 mm/101.6 mm.
Fig. 2: Sample of rock material A geological investigation was performed on the rock material. Geologically, the rock material is finegrained sandstone with a carbonate binder, poorly bonded with grains of mica, hornblende and quartz. We carried out investigations on the rock sample to determine the hardness deformation properties according to the ASTM D7012-10 standard (Standard test method for compressive strength and elastic moduli of intact rock core specimens under varying stress and temperatures). The density of the rock sample was determined according to the ISO/TS 17892-2:2004 standard (Geotechnical investigation and testing Laboratory testing of soil - Part 2: Determination of density of fine-grained soil) according to a non-
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accredited procedure and using the linear measurement method, i.e. measuring both the dimensions of the ACCEPTED MANUSCRIPT sample and its weight. In the Hoek cell, the rock sample was loaded axially symmetrically at a different lateral pressure from about 0.5 to about 1.7 MPa. Considering that only one sample was available, we performed a multistage triaxial test, which means that the sample was loaded slowly and in a controlled manner until the collapse began. Then the lateral pressure was increased and the procedure was repeated. In addition, the deformation properties of the material were determined in the triaxial state. The modulus of elasticity E was determined on the central (linear) part of the curve. Since only one test specimen was available, the compressive strength was evaluated indirectly by means of a spot strength index test. The part of the sample that had previously been used in a triaxial cell was applied to the point load until its collapse in the diametric direction. Since the uniaxial compressive strength of sedimentary and metamorphic rocks depends on the degree and type of cementation, the correlation value for the poorly bonded sedimentary rock was taken into consideration in order to estimate the uniaxial compressive strength ac = Is50 * 15 (Romana (1999)). The test was performed in accordance with ASTM D5731 and ISRM (International Society for Rock Mechanics) recommendations. We also carried out an investigation on mineralogical composition using the XRF (X-Ray florescence spectrometry) method, where we determined the proportion of each mineral’s representation. The size of grains in the rock sample was determined on the basis of a sample scan with a scanning electronic microscope (SEM) and then with the measurements using the image Analyses 5.0 system. Results
Through the analyses described in the Materials and Methods chapter, we obtained the following results, which will be described below. 3.1
Rock material
Density
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Table 3: Results of the geomechanical properties of rock material Cohesion Angle of internal friction
ρ
c
ϕ
[Mg/m3] 2.007
[MPa] 0.361
[°]
Compressive strength from spot strength index test
Spot strength index
E
σc
Is50
[MPa] 3098
[MPa] 1.219
[kPa] 81.3
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Elastic module
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The shear strength at the primary stress state (1500 m × 0.02 MN/m3 = 30 MPa) was estimated using HoekBrown's criterion and by also taking into account the actual results of the laboratory triaxial test. The results shown in Figure 3 and 4 present the established shear parameters for a state of normal stress a = 30 MPa, c = 2.648 MPa and ϕ = 13.8° (τ = 9.83 MPa).
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Fig. 3: Determining rock strength parameters using Hoek-Brown criterion
Fig. 4: Normal stress vs. Shear stress of rock sample The results of the mineralogical composition of the rock material determined by the XRF method are shown in Tables 4, 5 and 6.
Table 4: Mineralogical composition of the sample rock material - geochemical SiO2 % 47.49
Al2O3 % 10.57
Fe2O3 % 3.30
MgO % 4.63
CaO ACCEPTED K2O TiO2 MANUSCRIPT % % % 9.82 2.24 0.54
Table 5: Elemental composition of the sample of rock material Si
Al
Mg
% % 22.20 5.59
Fe
Ca
K
S
Mn
Nb
Pb
Rb
Sr
Zn
Zr
Cr
Ti
Ba
% % % % % ppm 2.79 2.31 7.02 1.86 0.05 214
ppm 9
ppm 9
ppm 72
ppm 126
ppm 38
ppm 124
ppm 39
ppm 3263
ppm 476
Quartz
Dolomite
% 47.30
% 23.80
Calcite Plagioclases Muscovite/ Illite % 3.40
% 12.10
% 8.30
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Table 6: Mineral composition of the sample of rock material - mineral (geological) Chlorite
Kaolinite
Sum
% 3.10
% 2.00
% 100.00
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The size of the rock particles was estimated using the microscope picture. Based on Figure 5, we estimated that the particle size varies from a few µm to 3 mm.
Fig. 5: Microphotography of the sample of rock material 3.2
Roller Cone Bit
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After drilling, the drill bit was macroscopically examined using IADC methodology, where we evaluated the condition of the rolls and the teeth of the drill bit. We found that the teeth of the bit were evenly worn and the formation of erosion channels at the top of the teeth was quite evident. The teeth of the external line were eroded on the side that was not protected by the carbide coating, in contrast to its initial state. On all of the drill bit’s teeth, erosion channels were clearly visible. The results of dull inspection of the drill bit can be seen in Table 7. Table 7: Results of dull inspection of drill bit according to IADC dull grading system Cutting structure Bearings / Gauge Other seals dull char. Inner rows Outer rows Dull char. Location T1 T2 SS A E I ER
Reason pulled TD
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Fig. 6: Drill bit condition after the drilled interval of 610.70 m
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Fig. 7: Macroscopic cross sectional view through the teeth of the drill bit, with the reconstruction of the original geometry Roller Steel Material Analysis
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We examined the roller material for both the steel body with teeth and the carbide coating of the tooth. An examination of the chemical composition of the roller steel was carried out on an optical emission spectrometer. The results of the chemical composition of the investigated steel can be seen in Table 8. Table 8: Chemical analysis of the investigated roller steel Element Unit Result C mass.% 0.145 Si mass.% 0.25 Mn mass.% 0.59 P mass.% 0.007 S mass.% 0.002 Cr mass.% 0.11 Ni mass.% 3.50 Cu mass.% 0.18 Mo mass.% 0.201 V mass.% 0.01 Ti mass.% 0.005 Nb mass.% 0.005
Al N
mass.% mass.%
0.053 < 0.003
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Based on the results of the chemical analysis (Table 8), we have found that, in this case, the steel material of the tooth base is the so-called cold work tool steel with increased toughness. In the execution of the metallographic investigations by scanning electron microscopy, we analysed the tooth section where we examined the basic tooth material – cold work tool steel, the joint with the carbide coating and the carbide coating itself. EDS analyses were also performed, which showed the chemical composition of the involved phases. Figure 8 shows the SEM image of the sample in the area of the carbide coating, the mixed zone and the steel of the tooth. Figures 9 -13 show the spectra of EDS analyses and chemical compositions of the analysed phases, where it can be seen that the tooth steel composition corresponds to the chemical analysis of the roller steel shown in Table 8. (Analysed) Site 2 in Figure 8 shows that the steel had elevated Mn, Co, and W proportions, which came from the carbide coating and the carbide matrices based on the cobalt alloy in which the particles of tungsten carbide (WC) are allocated. The analysis at points 3 and 5 in Figure 8 confirm this. The presence of other elements is a consequence of a larger analytical volume, which also analyses material from the background.
Fig. 8: SEM image of bit tooth, from carbide coating, mixing zone till steel tooth base (from left to right): 1 – tooth body, 2 – carbide coating matrices, 3 – carbide material, 4 – carbide material, 5 – carbide material
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The elemental composition of the tooth base (1 - Figure 8) determined according to the SEM method is shown in Figure 9.
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Fig. 9: EDS spectrum of tooth base; location 1 in Figure 8.
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The elemental composition of carbide coating matrices (2 - Figure 8) determined according to the SEM method is shown in Figure 10.
Fig. 10: EDS spectrum of carbide coating matrices; location 2 in Figure 8. The elemental composition of carbide coating (3 - Figure 8) determined according to the SEM method is shown in Figure 11.
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Fig. 11: EDS spectrum of carbide coating; location 3 in Figure 8.
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The elemental composition of carbide coating (4 - Figure 8) determined according to the SEM method is shown in Figure 12.
Fig. 12: EDS spectrum of carbide coating; location 4 in Figure 8. The elemental composition of carbide coating (5 - Figure 8) determined according to the SEM method is shown in Figure 13.
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Fig. 13: EDS spectrum of carbide coating; location 5 in Figure 8.
Simultaneous Thermal Analysis of Tooth Steel and Dilatometric Analysis
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The results of simultaneous thermal analysis (STA) (Figure 14) of the steel sample from the body of the roller show that an eutectoid transformation in solid state starts at a temperature of 695.2 °C, which is characterized by an endothermic peak. When this was completed (738 °C), the metal base was transformed into an austenite structure. At a temperature of 1352.1 °C, the low temperature eutectics began to melt into the investigated steel and then, at 1483.5 °C, into the primary austenitic crystals. The melting process was completed at a temperature of 1524.5 °C.
Fig. 14: DSC heating curve of the steel of the roller and body of the bit tooth. Figure 15 shows the heating curve of the investigated carbide coating sample, which, on the surface, still contained a mixing zone between the steel of the tooth body and the carbide coating. At a temperature of 725.7 °C, an eutectoid transformation began, which is undoubtedly related to the part of the demisting zone between the steel and the carbide coating. Otherwise, this is not typical for carbide coatings. At 1002.6 °C, the dissolution of the elements from the mixing zone began in the cobalt matrices of the carbide coating. The first melting was registered at 1289.6 °C and was terminated at a temperature of more than 1600 °C.
Fig. 15: DSC heating curve of carbide coating of the bit tooth.
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The dilatometric heating curve of the roller steel and the dilatometric heating curve of the carbide coating is shown in Figure 16. From the measurements, it can be seen that the steel extends linearly to the temperatures of eutectoid transformation during heating. The inclination of the curve corresponds to the linear thermal expansion coefficient of the steel for cold work tool steel. On the contrary, the curve of the investigated sample of carbide coating, which contains a thin layer of the mixing zone, shows a relatively flat curve. Therefore, the temperature expansion coefficient of the carbide coating is considerably lower. If we look at the difference in the zone of working temperatures that can reach a local temperature even at 500 °C, despite intense cooling, the absolute difference is 0.15 %. This is important because it results in a strong increase in internal stresses, particularly in the mixing zone, which is the preferred site for the initiation and propagation of cracks. The diagram shows the temperature of the beginning of the eutectoid transformation, which starts at 725.7 °C and ends with the austenitization of the steel at a temperature of 820 °C. The dilatometric curve of the carbide coating shows a deviation in the field of eutectoid transformation, which is related to the eutectoid transformation of the thin layer of the mixing zone on the carbide coating except that the reference temperatures are slightly higher. The cause of higher temperatures is the chemical composition of the mixing zone.
Fig. 16: Dilatometric analysis of the steel of the roller and carbide coatings 1 – carbide coating, 2 – tooth steel material We also performed micro hardness tests according to Vickers (Figure 17). In Table 9, it can be seen that the roller has a slightly higher level of hardness than the teeth, where the hardnesses are, on average, 380.3775
HV and 327.5854 HV respectively. The hardness in the carbide coatings were expected to be much higher ACCEPTED MANUSCRIPT and reach up to 2200 HV.
23.74 23.74 23.93 23.8 24
23.42 23.95 23.62 23.87 23.87
333.5333 Tooth core 326.1611 328.0846 326.4349 323.7129 327.5854 Average
17.65 9.46 9.11 13.24 16.55 12.51
17.94 9.41 9.23 12.56 14.98 11.72
585.6402 Carbide coating 2083.258 2205.404 1114.416 746.1719 1263.514 1482.553 Average
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HV0,1 369.2692 Roller 386.3155 379.3581 375.1048 391.8399 380.3775 Average
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d2 22.85 21.89 22.3 22.21 21.59
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d1 21.97 21.93 21.92 22.26 21.92
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Table 9: Vickers hardness test results
Fig. 17: Displaying the Vickers hardness test on the carbide coating of the tooth 4.
Discussion
In the implementation of the drilling regime, it can be determined that the drill bit should be loaded in accordance with the recommendations of the manufacturer of the bit. The load recommendation for this type of drill bit is between 40 to 200 kN. The load on the drill bit did not exceed 50 kN. At the beginning of
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Fig. 18: Characteristic wear of roller drill bit
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drilling, the load on the bit was a little too low (30 kN), but the load increased after 278.96 m of drilling, to ACCEPTED the minimum recommended load. A lower load at theMANUSCRIPT starting phase was used because the rock material was very weakly bonded and the operator did not want to push too much of the load on the bit, because of the danger that the well would deviate. With depth, the sandstone became more hardly bonded and the operator increased the load on the roller cone bit. The rotary speed was compliant with manufacturer recommendations and was between 50 and 60 rpm. When the penetration rate started to rapidly decrease, the drill bit was extracted from the well. A visual inspection of the fatigue of the bit, which led to the poorer operation of the bit, was carried out. It was found that the wear on the bit occurred in the form of the loss of tooth materials in places where the teeth were not protected by a carbide coating. Such characteristic wear of the bit is shown in Figure 18.
Fig. 19: Simplified sketch of mud flow through the roller of drill bit Figure 18 illustrates the loss of steel material due to the erosion action of the mud, which contained a large proportion of silicate particles (Table 4). The wear was reflected in the reduction of the dimensions of the body of the tooth and the formation of erosion channels at the tip of the tooth. The formation of erosion channels at the top of the tooth can be attributed to the decay of the carbide coating during the drilling process, which allowed the flushing of mud with silicate particles (Table 4) to erode into the newly opened surfaces, as shown in Figure 19. Due to the considerable differences in the strength and thermal expansion properties between the carbide materials that represents the coating (protection) of the tooth body and the steel material of the tooth body and also due to various expansions or elastic material characteristics, cracks formed between the materials as
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shown at the Figure 20. A consequence of this is the split of the carbide coating from the tooth body. The ACCEPTED MANUSCRIPT result of this split between materials was the formation of a new steel surface through which mud containing a large amount of silicate abrasive components flowed. The abrasive components, which eroded the newly opened surfaces at the tips of the teeth, were caused by the formation of erosion channels and consequently the loss of the steel tooth material. The display of the formation of micro cracks resulting from the different elastic properties of the materials is shown in Figure 20. The picture shows the edge of the erosion channel at the top of the tooth. On the left side of the image, we can see a lining of carbide coating and, on the right side, the steel material of the tooth and the formation of a micro crack can be seen. At the top of the tooth, micro erosion channels are visible. The carbide coating at the top of the tooth, which is more resistant to erosion, but fragile, gradually disintegrates due to the effects of pressure (WOB) and shear conditions combined with heating and cooling during the drilling process. This process can be seen in the top left side of Figure 20. In our case, we did not manage to determine the temperature of the steel during the drilling beyond doubt. We can only be sure that the temperature on the top of the teeth, during drilling, was lower than the temperature of the recrystallization of drill bit steel. That recrystallization could be measured by the Vickers hardness test along with an observation of the crystalline structure of the steel with electronic microscope. We did not find any signs of recrystallization of steel material. The temperature of the recrystallization of steel material of our drill bit is 695.2°C. This was determined by simultaneous thermal analysis. Because of this, we could determine the temperature influence by dilatometric test. During this test, we find out that the difference between the material properties of the tooth steel and carbide coating are expressed at a temperature higher than 100°C as shown in Figure 16. These temperatures are present during the phase when the teeth are in contact with the rock material. During the journey of tooth around the cone axis, the temperature of tooth steel decreases because of the influence of the mud. If this warming and cooling time is short enough, there is no immediate deviation in the carbide coating, but only in the formation of micro cracks at the contact of the carbide coating and tooth steel. These micro cracks are extended (during this time period) along the contact between the materials, which leads to a deviation of the carbide coating. This phenomenon was only observed at the top of the tooth, while on the side of tooth, the formation of micro cracks in the carbide coating was not detected on the investigated drill bit.
Fig. 20: The formation of micro cracks as a result of various elastic properties of materials Similar phenomena can also be observed on the tooth body itself, which is not in the area of the carbide coating. Figure 21 shows the formation of micro cracks and erosion channels along the edge of the tooth.
Conclusion
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Fig. 21: Micro cracks and erosion channels on the tooth body
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The results of our research, which included investigations of rock and steel material, represent the wear investigated in the roller cone drill bit in sandstone. With the described investigation methods, we discovered the mechanism of wear of the teeth steel basis as well as in the carbide coating according to influence of sandstone which contains a large content of abrasive particles. The characterization of the wear of the roller drill bit, which is described in our work, is a complex analysis of several factors. On the one hand, the interaction between the rock material and the drill bit is closely connected to the drilling regime. In this case, we dealt with the action of the steel material of the drill bit on the rock material - sandstone. During the drilling process, the drill bit was heated while in contact with the rock material and then cooled due to the influence of the drilling mud. At this stage, the differences between the deformability properties of the steel material of the rollers and the teeth, and, on the other hand, the deformability properties of the carbide coating become evident. In this case, due to the different temperature expansion coefficients, the formation of micro cracks on the steel material of the tooth causes the penetration of a dispersed medium containing erosion microparticles in the newly created cracks. The carbide coating of the tooth is more resistant to erosion, but fragile, which causes it to gradually disintegrate during the drilling process due to the effects of pressure and shear conditions combined with the heating and cooling. During our research, we discovered that the wear on the tooth steel is more progressive than the wear on the carbide coating. The surfaces of the teeth, which are not covered with a carbide coating, are more exposed to the influence of erosion. The erosion effect is represented by the formation of erosion channels into which the mud, which contains a large percentage of aggressive particles, flows in. The disintegration of the carbide coating, which is a slower process than the steel wear, is a consequence of the bit load during drilling and the temperature differences, which occur during the roller rotation. Because of the different rate of wear of both materials, the geometry of the teeth changes and, thus, the effectiveness of the drill bit. Our roller cone drill bit, which was the subject of our investigations, belongs to the class of the so-called self-sharpening drill bits, for which the manufacturers predict that the geometry of tooth changes during the drilling process. With the discovered rate and development of wear of the steel materials on the surfaces of teeth on the drill bit, we determined the way that the self-sharpening effect actually occurs, so the effective operation of this type of drill bit is prolonged. We had drilled through 610.70 m of sandstone when the ROP started to rapidly decrease. The improvement of the resistance of tooth steel, which is rather tough, is offered in the surface treatment of this. The surface treatment of tooth steel, in terms of increased erosion resistance, can be carried out by means of metallurgical processes or the subsequent treatment of the surface of the entire tooth prior to the application of the carbide coating.
As such, the protection against erosion resistance begins with experiments in nano materials, which already ACCEPTED MANUSCRIPT show a potential new trend in the development of erosion resistance in the rollers and tooth material of roller cone drill bits. References Abbas, R.K., 2018, A review on the wear of oil drill bits (conventional and state of the art approaches for wear reduction and quantification), Engineering Failure Analysis, Volume 90, pp. 554 – 584, https://doi.org/10.1016/j.engfailanal.2018.03.026
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Al-Sudani, J. A., 2017, Real-time monitoring of mechanical specific energy and bit wear using control engineering systems, Journal of Petroleum Science and Engineering, Issue 149, pp. 171 – 182, https://doi.org/10.1016/j.petrol.2016.10.038 Besson, A. & others, 2000, On the cutting edge, Oilfield Review, Volume 12, Issue 3, pp. 36 – 57, https://www.slb.com/~/media/Files/resources/oilfield_review/ors00/aut00/p36_57.pdf
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Cotler, R.A., Goodman, E.C., Hendrickson, R.R., Leslie, W.C., 1981, Elevated temperature fracture toughness and fatigue testing of steels for geothermal applications, Final report, University of Michigan, p. 140, https://digital.library.unt.edu/ark:/67531/metadc1114108/
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Geoffroy H., Nguyen Minh D., 1997, Study on interaction between rock and worn PDC's cutter, International Journal of Rock Mechanics & Mining Sciences, Volume 34, Paper No. 095, https://doi.org/10.1016/S1365-1609(97)00036-1 Günen, A., 2016, Micro-Abrasion Wear Behavior of Thermal-Spray-Coated Steel Tooth Drill Bits, Acta physica Polonica A, Volume 130, p. 217 – 222, http://przyrbwn.icm.edu.pl/APP/PDF/130/a130z1p056.pdf
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He, W., Chen, Y., He, J., Xiong, W., Tang, T., Yang, H.O., 2016, Spherical contact mechanical analysis of roller cone drill bits journal bearing, Petroleum, Volume 2, Issue 2, pp. 208 – 214, https://doi.org/10.1016/j.petlm.2016.03.002
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Jones, H.G., Norgren, S.M., Kritikos, M., Mingard, K.P., Gee, M.G., 2017, Examination of wear damage to rock-mining hardmetal drill bits, International Journal of Refractory Metals and Hard Materials, Volume 66, p. 1–10, https://doi.org/10.1016/j.ijrmhm.2017.01.013
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Karasawa, H., Ohno, T., Miyazaki, K., Eko, A., 2016, Experimental results on the effect of Bit wear on torque response, International Journal of Rock Mechanics & Mining Sciences, Volume 84, p. 1–9, https://doi.org/10.1016/j.ijrmms.2016.01.013 Mancini, P., 1996, New mechanisms of rock – bit wear in geothermal wells, 21 workshop on geothermal reservoir engineering, Standford University, California USA, January 22 – 24, p. 8, https://digital.library.unt.edu/ark:/67531/metadc884890/ Momeni, S., Moseley, S., Ante, M., Allaart, J., 2017, The wear of WC-Co drill bits during rotary-percussive drilling of reinforced concrete, International Journal of Refractory Metals and Hard Materials, Volume 62, pp. 202 – 209, https://doi.org/10.1016/j.ijrmhm.2016.08.006 Naganawa, S., 2012, Feasibility study on roller-cone bit wear detection from axial bit vibration, Journal of Petroleum Science and Engineering, Volume 82-83, p. 140–150, https://doi.org/10.1016/j.petrol.2012.01.014 Neville, A., & Hodgkiess, T., 1999, Characterisation of high-grade alloy behaviour in severe erosioncorrosion conditions, Wear, Volume 233–235, p 596–607, https://doi.org/10.1016/S0043-1648(99)00220-3
Njobuenwu, D.O., Wobo, C.A., 2007, Effect of drilled solids on drilling rate and performance, Journal of ACCEPTED MANUSCRIPT Petroleum Science and Engineering, Volume 55, p. 271–276, DOI: 10.1016/j.petrol.2006.08.012 Olsson, M., Yvell, K., Heinrichs, J., Bengtsson, M., Jacobson, S., 2017, Surface degradation mechanisms of cemented carbide drill buttons in iron ore rock drilling, Wear, Volume 388 - 389, pp. 81 – 92, https://doi.org/10.1016/j.wear.2017.03.004 Pang, B., Wang, S., Wang, Q., Yang, K., Lu, H., Hassan, M., Jiang, X., 2018, Numerical prediction of cuttings transport behaviour in well drilling using kinetic theory of granular flow, Journal of Petroleum Science and Engineering, Volume 161, p. 190 – 203, https://doi.org/10.1016/j.petrol.2017.11.028
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Peter, W., 2010, Wear – resistant nanocomposite stainless steel coating and bits for geothermal drilling, Oak ridge national laboratory, p. 12, https://www.energy.gov/sites/prod/files/2014/02/f7/high_peter_nanocomposite_bits_0.pdf
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Rashidi, B., Hareland, G., Nygaard, R., 2008, Real-Time Drill Bit Wear Prediction by Combining Rock Energy and Drilling Strength Concepts, paper for presentation at the Abu Dhabi International Petroleum Conference and Exhibition, Abu Dhabi, UAE [3rd – 6th November 2008], https://www.onepetro.org/conference-paper/SPE-117109-MS
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Rashidi, B., Hareland, G., Wu, Z., 2015, Performance, simulation and field application modelling of rollercone bits, Journal of Petroleum Science and Engineering, Volume 133, p. 507 – 517, https://doi.org/10.1016/j.petrol.2015.06.003 Romana, M., 1999, Correlation between uniaxial compressive and point-load (Franklin test) strengths for different rock classes, International Society for Rock Mechanics and Rock Engineering, 9th ISRM Congress, 25-28 August, Paris, France, https://www.onepetro.org/conference-paper/ISRM-9CONGRESS-1999-136
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Timonin, VV. et al, 2017, Causes of wear of PDC bits and ways of improving their wear resistance, Conference Series: Earth and Environmental Science 53, p. 5, http://iopscience.iop.org/article/10.1088/1755-1315/53/1/012027
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Zhao, J., Zhang, G., Xu, Y., Wang, R., Zhou, W., Yang, D., 2018, Experimental and theoretical evaluation of solid particle erosion in an internal flow passage within a drilling bit, Journal of Petroleum Science and Engineering, Volume 160, p. 582–596, https://doi.org/10.1016/j.petrol.2017.10.068
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ACCEPTED MANUSCRIPT The characterization of the wear of the drill bit, is a complex analysis of several factors. The differences between the deformability properties of the steel material on drill bit are evident. The carbide coating gradually disintegrate during the drilling process. The abrasive components in drilling mud are forming the erosion channels.
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• • • •
S0920-4105(18)30958-6
DOI:
https://doi.org/10.1016/j.petrol.2018.10.090
Reference:
PETROL 5453
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 16 May 2018 Revised Date:
24 October 2018
Accepted Date: 25 October 2018
Please cite this article as: Šporin, J., Mrvar, Primož., Petrič, M., Vižintin, G., Vukelić, Ž., The characterization of wear in roller cone drill bit by rock material – Sandstone, Journal of Petroleum Science and Engineering (2018), doi: https://doi.org/10.1016/j.petrol.2018.10.090. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The characterization of wear in roller cone drill bit by rock material – sandstone ACCEPTED MANUSCRIPT Jurij Šporin a*, Primož Mrvar b, Mitja Petrič b, Goran Vižintin a, Željko Vukelić a a
University of Ljubljana, Faculty of Natural Science and Engineering, Department of Geotechnology, mining and environment, Aškerčeva 12, Ljubljana, Slovenia b University of Ljubljana, Faculty of Natural Science and Engineering, Department of Materials and Metallurgy, Aškerčeva 12, Ljubljana, Slovenia
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[email protected] [email protected] [email protected] [email protected] [email protected] *
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Corresponding author: University of Ljubljana, Faculty of Natural Science and Engineering, Aškerčeva 12, 1000 Ljubljana, Slovenia E-mail address: [email protected]
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Key words: roller cone bit, mechanism of drill bit wear, carbide coating, erosion channel, micro crack, aggressive drill mud Abstract
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Roller cone drill bit efficiency time during drilling depends, to a large extent, on the properties of the materials from which the components of the roller cone bit are made. The faster the roller cone bit is worn, due to the effects of the rock material through which we drill and the drilling regimes used during the drilling, the worse is the efficiency of drilling and the higher is the number of drill bits needed to drill the well. The aim of our research was to determine the mechanisms of wear on roller cone bit materials according to the characteristics of the steel material of the roller cone bit and the characteristics of the rock material, in this case, sandstone on the drilling regime. The results of the discovered wear mechanisms help us to anticipate the possible improvement of the resistance of the unprotected roller cone bit steel material to excessive wear and, consequently, the longevity of the roller cone bit. For this purpose, we examined the steel materials of the roller cone bit, which we used during well drilling and the rock material, sandstone, through which we drilled with the roller cone bit under consideration. The results of our analysis present the mechanisms that result in the wear of the roller cone bit material under the given conditions of the rock material and the drilling regime. The results of the discovered wear mechanism can be used to improve the material of the roller cone bits in order to achieve a longer operating life and decrease the related costs of drilling. Introduction
The efficiency of the roller cone bit is proportional to the quality of the material from which it is produced and depends on the mechanical characteristics of the rock material through which it is drilled as well as on the drilling regime. By drilling regime, what is meant is the load on the drill bit, the number of drill bit rotations and the type, quantity and pressure of the drilling mud used. Among the factors that reduce the time of the effective operation of the roller cone bit, we can include the material from which the roller and the teeth are made, the drilling regime, and the geomechanical properties of the rock material through which we drill. With the wear of drill bits in general and, consequently, the reduced rate of penetration, several authors were studied Karasawa (2016), Timonin (2017), Njobuenwu (2007), Naganawa (2012) among many others, but only a few research is being done to discover the mechanism of roller cone bit wear.
Fig. 1: Drill bit 215.9 mm (8 ½") ST 117
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In our study, we analysed the mechanical properties of the steel material of the Smith ST117 roller cone bit, ACCEPTED MANUSCRIPT diameter 215.9 mm (8 ½"), which was drilled through 610.70 m of sandstone. The rock material through which it was drilled is basically weak bound sandstone, which, due to its high content of silicates, has a very aggressive level of abrasion on the steel part of the roller cone bit. The Smith ST117 roller cone drill bit is a drill bit for drilling into soft rock materials. The teeth on the rollers are made from the same steel material as the rollers. The teeth on the rollers are covered with a carbide coating to prevent wear resulting from rock penetration and shear movement during roller rotation.
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The roller cone drill bit’s temperature increases during drilling and is cooled down by drilling mud, which is usually water based, with other materials added (bentonite, polymer …), which improves the rheological properties of drilling mud. In Timonin’s (2017) paper, the wear of PDC bits was inspected, which differs in some ways from the operation of the roller cone bit in the mode of its effect on rock. However, in the paper, the method of wear that can be applied to the operation and wear of the conventional roller cone bits is shown taking into account their mode of operation on the rock and the flow of drilling mud around the rollers. In the case of a high content of silicates in the drilling mud, the flow of the dispersed medium is reflected in the wear of the steel material on the roller cone bit. The result of this is the loss of steel material in the area of the tooth. The consequence of this loss is the reduction and change in the geometry of the teeth, which no longer have a functional value in terms of effective breaking of the rock massif. Similarly, Zhao (2018). examined the loss of steel material due to the erosion of solid particles in the housing of the drill bit, where they found that the rate of the erosion on steel material increases by increasing the flow of the dispersed material containing erosion-acting particles. In the investigation of the properties of the steel material of the roller cone drill bit, we inspected the materials of the bit body, rollers and teeth. We examined the method of wear and determined the critical points of erosion effect on the steel material of roller cone bits. For this purpose, we analysed the rock material that the examined roller cone bit drilled through. At that point, we evaluated the mechanism of wear of roller cone drill bit in the field of material microstructure. To reach that goal, we use the theoretical and methodical basics of previously published articles by Rashidi (2015, 2008) and Abbas (2018). Rashidi (2008), specifies that the variables that influence the rate of penetration (ROP) of roller cone bits, among other influential parameters, also refers to the wear on the roller's teeth. The influential parameters on ROP are shown in Equation 1. ROP = f ∙ f ∙ f ∙ f ∙ f ∙ f ∙ f ∙ f
(1)
where, ROP is the rate of penetration, f1 is the effect of formation strength or rock durability, f2 is the effect of formation depth, f3 is the effect of formation compaction or pore pressure, f4 is the effect of differential
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pressure, f5 is the effect of bit diameter and bit weight, f6 is the effect of rotary speed, f7 is the effect of tooth ACCEPTED MANUSCRIPT wear and f8 is the effect of bit hydraulics. In the Abbas (2018) article, a literature survey is reported dealing with methods of wear quantification in terms of drilling parameters and mechanical properties of steel materials of drill bits. Also shown are the empirical wear models as well as wear prediction techniques. In our research, we focused on a problem that shows the dependence of the resistance of the roller cone bit steel material on the rock material. In the subject that we dealt with, the guideline discussed how to improve the basic material (steel) of the rollers and teeth of the rolling cone bit, so that it is more resistant to the abrasion caused by the flow of the dispersed medium with the content of the rock material debris through which it is drilled. Materials and Methods
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In the course of the investigations to characterize the wear the roller cone bit, detailed investigations were carried out on the materials through which it was drilled, drilling parameters and the materials from which the roller cone bit was made. To this end, in the first phase, we carried out a review of the roller cone bit, which included an overview of the condition of the bit after drilling through the rock formation, according to IADC (International Association of Drilling Contractors) methodology. Here, we examined the state of the teeth and bit rollers after drilling through the rock material. After a visual inspection of the bit, we cut it apart to perform tests to discover the metallurgical properties of the steel from which the roller cone drill bit was made and to produce test samples for the investigation of material damage that occurred during the drilling work. From the obtained sample of rock material, we carried out laboratory analyses to determine the mechanical and mineralogical properties of the rock through which it was drilled with the considered drill bit. The following investigations were carried out on a sample of rock material: - a visual overview of rock material, - an overview of the geochemical and mineralogical composition of the sample of the rock material through which it was drilled using the XRF (X-ray fluorescence) method, using a Thermo NITON XL3t XRF analyser - an overview of the strength-deformation properties of the rock material was carried out according to ASTM standard D7012-10. While drilling through the known rock material, drilling parameters were also monitored as follows: - the load on the drill bit, - the number of rotations of the drill string, - the quantity of the pumped drill mud, - the pressure of the pumped drill mud, - the penetration rate, - the length of the drilling interval. In the following paper, we carried out a complete analysis of the steel materials of the roller cone bit, which covered the following examinations: - the chemical composition of roller body steel with a ARL MA-310 optical emission spectrometer, - the composition of the carbide coating of bit teeth with the XRF (X-ray fluorescence) method using a Thermo NITON XL3t XRF analyser, - a cross-sectional view of the bit teeth with an Jeol JSM 5610 electron microscope using the EDS/SEM analysis (energy dispersive spectroscopy / scanning electron microscope), - DSC (differential scanning calorimetry) of roller steel and tooth carbide coating with the Nech Jupiter STA449C - a dilatometric analysis of the roller steel and the tooth carbide coating with a Bähr DIL 801 low temperature dilatometer, - Vickers hardness tests on the roller steel with a 100 g load with a Shimadzu type M microhardness tester. 2.1
Drilling Work
Drilling work was carried out using a N-1000 drill rig made in 1992. The area where we observed the ACCEPTED MANUSCRIPT performance of the 215,9 mm (8 ½") roller cone drill bit included a drilling interval from 871.80 m to 1482.50 m in depth, which amounts to 610.70 m in length. The sample of rock material was extracted from a depth of 1482.50 m and is also the subject of our research. The extracted rock sample represents a characteristic sample of sandstone, which was in the observed area – drilling interval. The average penetration rate during the observed interval was 4.68 m/h. The rest of the drilling parameters can be seen in Table 1. Table 1: Drilling parameters 871.04 – 1150.00 30 50 0.92 4.5
1150.00 – 1320.00 40 55 0.92 4.5
1320.00 – 1482.50 50 60 0.92 5
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Depth (m) Load on drill bit (kN) Rotary speed (rpm) Pump capacity (m3/min) Pump pressure (MPa)
Table 2: Mud characteristics
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Value 1080 35 5.8 - 6,0 8-9 9.6 – 9.7
Rock Material
Unit kg/m3 s ml/30 min mPas pH
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Parameter Density Viscosity (Fann funnel) Filtration Plastic viscosity (Fann viscometer) pH
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During the drilling, 14 mass. % bentonite drilling mud was used. The mud characteristics are shown in Table 2.
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It was at a depth of 1482.80 m that we extracted the sample of rock material to determine its mechanical and mineralogical properties. The core sampling was performed using a double core barrel 158.75 mm/101.6 mm.
Fig. 2: Sample of rock material A geological investigation was performed on the rock material. Geologically, the rock material is finegrained sandstone with a carbonate binder, poorly bonded with grains of mica, hornblende and quartz. We carried out investigations on the rock sample to determine the hardness deformation properties according to the ASTM D7012-10 standard (Standard test method for compressive strength and elastic moduli of intact rock core specimens under varying stress and temperatures). The density of the rock sample was determined according to the ISO/TS 17892-2:2004 standard (Geotechnical investigation and testing Laboratory testing of soil - Part 2: Determination of density of fine-grained soil) according to a non-
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accredited procedure and using the linear measurement method, i.e. measuring both the dimensions of the ACCEPTED MANUSCRIPT sample and its weight. In the Hoek cell, the rock sample was loaded axially symmetrically at a different lateral pressure from about 0.5 to about 1.7 MPa. Considering that only one sample was available, we performed a multistage triaxial test, which means that the sample was loaded slowly and in a controlled manner until the collapse began. Then the lateral pressure was increased and the procedure was repeated. In addition, the deformation properties of the material were determined in the triaxial state. The modulus of elasticity E was determined on the central (linear) part of the curve. Since only one test specimen was available, the compressive strength was evaluated indirectly by means of a spot strength index test. The part of the sample that had previously been used in a triaxial cell was applied to the point load until its collapse in the diametric direction. Since the uniaxial compressive strength of sedimentary and metamorphic rocks depends on the degree and type of cementation, the correlation value for the poorly bonded sedimentary rock was taken into consideration in order to estimate the uniaxial compressive strength ac = Is50 * 15 (Romana (1999)). The test was performed in accordance with ASTM D5731 and ISRM (International Society for Rock Mechanics) recommendations. We also carried out an investigation on mineralogical composition using the XRF (X-Ray florescence spectrometry) method, where we determined the proportion of each mineral’s representation. The size of grains in the rock sample was determined on the basis of a sample scan with a scanning electronic microscope (SEM) and then with the measurements using the image Analyses 5.0 system. Results
Through the analyses described in the Materials and Methods chapter, we obtained the following results, which will be described below. 3.1
Rock material
Density
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Table 3: Results of the geomechanical properties of rock material Cohesion Angle of internal friction
ρ
c
ϕ
[Mg/m3] 2.007
[MPa] 0.361
[°]
Compressive strength from spot strength index test
Spot strength index
E
σc
Is50
[MPa] 3098
[MPa] 1.219
[kPa] 81.3
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Elastic module
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The shear strength at the primary stress state (1500 m × 0.02 MN/m3 = 30 MPa) was estimated using HoekBrown's criterion and by also taking into account the actual results of the laboratory triaxial test. The results shown in Figure 3 and 4 present the established shear parameters for a state of normal stress a = 30 MPa, c = 2.648 MPa and ϕ = 13.8° (τ = 9.83 MPa).
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Fig. 3: Determining rock strength parameters using Hoek-Brown criterion
Fig. 4: Normal stress vs. Shear stress of rock sample The results of the mineralogical composition of the rock material determined by the XRF method are shown in Tables 4, 5 and 6.
Table 4: Mineralogical composition of the sample rock material - geochemical SiO2 % 47.49
Al2O3 % 10.57
Fe2O3 % 3.30
MgO % 4.63
CaO ACCEPTED K2O TiO2 MANUSCRIPT % % % 9.82 2.24 0.54
Table 5: Elemental composition of the sample of rock material Si
Al
Mg
% % 22.20 5.59
Fe
Ca
K
S
Mn
Nb
Pb
Rb
Sr
Zn
Zr
Cr
Ti
Ba
% % % % % ppm 2.79 2.31 7.02 1.86 0.05 214
ppm 9
ppm 9
ppm 72
ppm 126
ppm 38
ppm 124
ppm 39
ppm 3263
ppm 476
Quartz
Dolomite
% 47.30
% 23.80
Calcite Plagioclases Muscovite/ Illite % 3.40
% 12.10
% 8.30
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Table 6: Mineral composition of the sample of rock material - mineral (geological) Chlorite
Kaolinite
Sum
% 3.10
% 2.00
% 100.00
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The size of the rock particles was estimated using the microscope picture. Based on Figure 5, we estimated that the particle size varies from a few µm to 3 mm.
Fig. 5: Microphotography of the sample of rock material 3.2
Roller Cone Bit
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After drilling, the drill bit was macroscopically examined using IADC methodology, where we evaluated the condition of the rolls and the teeth of the drill bit. We found that the teeth of the bit were evenly worn and the formation of erosion channels at the top of the teeth was quite evident. The teeth of the external line were eroded on the side that was not protected by the carbide coating, in contrast to its initial state. On all of the drill bit’s teeth, erosion channels were clearly visible. The results of dull inspection of the drill bit can be seen in Table 7. Table 7: Results of dull inspection of drill bit according to IADC dull grading system Cutting structure Bearings / Gauge Other seals dull char. Inner rows Outer rows Dull char. Location T1 T2 SS A E I ER
Reason pulled TD
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Fig. 6: Drill bit condition after the drilled interval of 610.70 m
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Fig. 7: Macroscopic cross sectional view through the teeth of the drill bit, with the reconstruction of the original geometry Roller Steel Material Analysis
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We examined the roller material for both the steel body with teeth and the carbide coating of the tooth. An examination of the chemical composition of the roller steel was carried out on an optical emission spectrometer. The results of the chemical composition of the investigated steel can be seen in Table 8. Table 8: Chemical analysis of the investigated roller steel Element Unit Result C mass.% 0.145 Si mass.% 0.25 Mn mass.% 0.59 P mass.% 0.007 S mass.% 0.002 Cr mass.% 0.11 Ni mass.% 3.50 Cu mass.% 0.18 Mo mass.% 0.201 V mass.% 0.01 Ti mass.% 0.005 Nb mass.% 0.005
Al N
mass.% mass.%
0.053 < 0.003
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Based on the results of the chemical analysis (Table 8), we have found that, in this case, the steel material of the tooth base is the so-called cold work tool steel with increased toughness. In the execution of the metallographic investigations by scanning electron microscopy, we analysed the tooth section where we examined the basic tooth material – cold work tool steel, the joint with the carbide coating and the carbide coating itself. EDS analyses were also performed, which showed the chemical composition of the involved phases. Figure 8 shows the SEM image of the sample in the area of the carbide coating, the mixed zone and the steel of the tooth. Figures 9 -13 show the spectra of EDS analyses and chemical compositions of the analysed phases, where it can be seen that the tooth steel composition corresponds to the chemical analysis of the roller steel shown in Table 8. (Analysed) Site 2 in Figure 8 shows that the steel had elevated Mn, Co, and W proportions, which came from the carbide coating and the carbide matrices based on the cobalt alloy in which the particles of tungsten carbide (WC) are allocated. The analysis at points 3 and 5 in Figure 8 confirm this. The presence of other elements is a consequence of a larger analytical volume, which also analyses material from the background.
Fig. 8: SEM image of bit tooth, from carbide coating, mixing zone till steel tooth base (from left to right): 1 – tooth body, 2 – carbide coating matrices, 3 – carbide material, 4 – carbide material, 5 – carbide material
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The elemental composition of the tooth base (1 - Figure 8) determined according to the SEM method is shown in Figure 9.
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Fig. 9: EDS spectrum of tooth base; location 1 in Figure 8.
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The elemental composition of carbide coating matrices (2 - Figure 8) determined according to the SEM method is shown in Figure 10.
Fig. 10: EDS spectrum of carbide coating matrices; location 2 in Figure 8. The elemental composition of carbide coating (3 - Figure 8) determined according to the SEM method is shown in Figure 11.
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Fig. 11: EDS spectrum of carbide coating; location 3 in Figure 8.
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The elemental composition of carbide coating (4 - Figure 8) determined according to the SEM method is shown in Figure 12.
Fig. 12: EDS spectrum of carbide coating; location 4 in Figure 8. The elemental composition of carbide coating (5 - Figure 8) determined according to the SEM method is shown in Figure 13.
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Fig. 13: EDS spectrum of carbide coating; location 5 in Figure 8.
Simultaneous Thermal Analysis of Tooth Steel and Dilatometric Analysis
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The results of simultaneous thermal analysis (STA) (Figure 14) of the steel sample from the body of the roller show that an eutectoid transformation in solid state starts at a temperature of 695.2 °C, which is characterized by an endothermic peak. When this was completed (738 °C), the metal base was transformed into an austenite structure. At a temperature of 1352.1 °C, the low temperature eutectics began to melt into the investigated steel and then, at 1483.5 °C, into the primary austenitic crystals. The melting process was completed at a temperature of 1524.5 °C.
Fig. 14: DSC heating curve of the steel of the roller and body of the bit tooth. Figure 15 shows the heating curve of the investigated carbide coating sample, which, on the surface, still contained a mixing zone between the steel of the tooth body and the carbide coating. At a temperature of 725.7 °C, an eutectoid transformation began, which is undoubtedly related to the part of the demisting zone between the steel and the carbide coating. Otherwise, this is not typical for carbide coatings. At 1002.6 °C, the dissolution of the elements from the mixing zone began in the cobalt matrices of the carbide coating. The first melting was registered at 1289.6 °C and was terminated at a temperature of more than 1600 °C.
Fig. 15: DSC heating curve of carbide coating of the bit tooth.
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The dilatometric heating curve of the roller steel and the dilatometric heating curve of the carbide coating is shown in Figure 16. From the measurements, it can be seen that the steel extends linearly to the temperatures of eutectoid transformation during heating. The inclination of the curve corresponds to the linear thermal expansion coefficient of the steel for cold work tool steel. On the contrary, the curve of the investigated sample of carbide coating, which contains a thin layer of the mixing zone, shows a relatively flat curve. Therefore, the temperature expansion coefficient of the carbide coating is considerably lower. If we look at the difference in the zone of working temperatures that can reach a local temperature even at 500 °C, despite intense cooling, the absolute difference is 0.15 %. This is important because it results in a strong increase in internal stresses, particularly in the mixing zone, which is the preferred site for the initiation and propagation of cracks. The diagram shows the temperature of the beginning of the eutectoid transformation, which starts at 725.7 °C and ends with the austenitization of the steel at a temperature of 820 °C. The dilatometric curve of the carbide coating shows a deviation in the field of eutectoid transformation, which is related to the eutectoid transformation of the thin layer of the mixing zone on the carbide coating except that the reference temperatures are slightly higher. The cause of higher temperatures is the chemical composition of the mixing zone.
Fig. 16: Dilatometric analysis of the steel of the roller and carbide coatings 1 – carbide coating, 2 – tooth steel material We also performed micro hardness tests according to Vickers (Figure 17). In Table 9, it can be seen that the roller has a slightly higher level of hardness than the teeth, where the hardnesses are, on average, 380.3775
HV and 327.5854 HV respectively. The hardness in the carbide coatings were expected to be much higher ACCEPTED MANUSCRIPT and reach up to 2200 HV.
23.74 23.74 23.93 23.8 24
23.42 23.95 23.62 23.87 23.87
333.5333 Tooth core 326.1611 328.0846 326.4349 323.7129 327.5854 Average
17.65 9.46 9.11 13.24 16.55 12.51
17.94 9.41 9.23 12.56 14.98 11.72
585.6402 Carbide coating 2083.258 2205.404 1114.416 746.1719 1263.514 1482.553 Average
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HV0,1 369.2692 Roller 386.3155 379.3581 375.1048 391.8399 380.3775 Average
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d2 22.85 21.89 22.3 22.21 21.59
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d1 21.97 21.93 21.92 22.26 21.92
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Table 9: Vickers hardness test results
Fig. 17: Displaying the Vickers hardness test on the carbide coating of the tooth 4.
Discussion
In the implementation of the drilling regime, it can be determined that the drill bit should be loaded in accordance with the recommendations of the manufacturer of the bit. The load recommendation for this type of drill bit is between 40 to 200 kN. The load on the drill bit did not exceed 50 kN. At the beginning of
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Fig. 18: Characteristic wear of roller drill bit
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drilling, the load on the bit was a little too low (30 kN), but the load increased after 278.96 m of drilling, to ACCEPTED the minimum recommended load. A lower load at theMANUSCRIPT starting phase was used because the rock material was very weakly bonded and the operator did not want to push too much of the load on the bit, because of the danger that the well would deviate. With depth, the sandstone became more hardly bonded and the operator increased the load on the roller cone bit. The rotary speed was compliant with manufacturer recommendations and was between 50 and 60 rpm. When the penetration rate started to rapidly decrease, the drill bit was extracted from the well. A visual inspection of the fatigue of the bit, which led to the poorer operation of the bit, was carried out. It was found that the wear on the bit occurred in the form of the loss of tooth materials in places where the teeth were not protected by a carbide coating. Such characteristic wear of the bit is shown in Figure 18.
Fig. 19: Simplified sketch of mud flow through the roller of drill bit Figure 18 illustrates the loss of steel material due to the erosion action of the mud, which contained a large proportion of silicate particles (Table 4). The wear was reflected in the reduction of the dimensions of the body of the tooth and the formation of erosion channels at the tip of the tooth. The formation of erosion channels at the top of the tooth can be attributed to the decay of the carbide coating during the drilling process, which allowed the flushing of mud with silicate particles (Table 4) to erode into the newly opened surfaces, as shown in Figure 19. Due to the considerable differences in the strength and thermal expansion properties between the carbide materials that represents the coating (protection) of the tooth body and the steel material of the tooth body and also due to various expansions or elastic material characteristics, cracks formed between the materials as
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shown at the Figure 20. A consequence of this is the split of the carbide coating from the tooth body. The ACCEPTED MANUSCRIPT result of this split between materials was the formation of a new steel surface through which mud containing a large amount of silicate abrasive components flowed. The abrasive components, which eroded the newly opened surfaces at the tips of the teeth, were caused by the formation of erosion channels and consequently the loss of the steel tooth material. The display of the formation of micro cracks resulting from the different elastic properties of the materials is shown in Figure 20. The picture shows the edge of the erosion channel at the top of the tooth. On the left side of the image, we can see a lining of carbide coating and, on the right side, the steel material of the tooth and the formation of a micro crack can be seen. At the top of the tooth, micro erosion channels are visible. The carbide coating at the top of the tooth, which is more resistant to erosion, but fragile, gradually disintegrates due to the effects of pressure (WOB) and shear conditions combined with heating and cooling during the drilling process. This process can be seen in the top left side of Figure 20. In our case, we did not manage to determine the temperature of the steel during the drilling beyond doubt. We can only be sure that the temperature on the top of the teeth, during drilling, was lower than the temperature of the recrystallization of drill bit steel. That recrystallization could be measured by the Vickers hardness test along with an observation of the crystalline structure of the steel with electronic microscope. We did not find any signs of recrystallization of steel material. The temperature of the recrystallization of steel material of our drill bit is 695.2°C. This was determined by simultaneous thermal analysis. Because of this, we could determine the temperature influence by dilatometric test. During this test, we find out that the difference between the material properties of the tooth steel and carbide coating are expressed at a temperature higher than 100°C as shown in Figure 16. These temperatures are present during the phase when the teeth are in contact with the rock material. During the journey of tooth around the cone axis, the temperature of tooth steel decreases because of the influence of the mud. If this warming and cooling time is short enough, there is no immediate deviation in the carbide coating, but only in the formation of micro cracks at the contact of the carbide coating and tooth steel. These micro cracks are extended (during this time period) along the contact between the materials, which leads to a deviation of the carbide coating. This phenomenon was only observed at the top of the tooth, while on the side of tooth, the formation of micro cracks in the carbide coating was not detected on the investigated drill bit.
Fig. 20: The formation of micro cracks as a result of various elastic properties of materials Similar phenomena can also be observed on the tooth body itself, which is not in the area of the carbide coating. Figure 21 shows the formation of micro cracks and erosion channels along the edge of the tooth.
Conclusion
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Fig. 21: Micro cracks and erosion channels on the tooth body
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The results of our research, which included investigations of rock and steel material, represent the wear investigated in the roller cone drill bit in sandstone. With the described investigation methods, we discovered the mechanism of wear of the teeth steel basis as well as in the carbide coating according to influence of sandstone which contains a large content of abrasive particles. The characterization of the wear of the roller drill bit, which is described in our work, is a complex analysis of several factors. On the one hand, the interaction between the rock material and the drill bit is closely connected to the drilling regime. In this case, we dealt with the action of the steel material of the drill bit on the rock material - sandstone. During the drilling process, the drill bit was heated while in contact with the rock material and then cooled due to the influence of the drilling mud. At this stage, the differences between the deformability properties of the steel material of the rollers and the teeth, and, on the other hand, the deformability properties of the carbide coating become evident. In this case, due to the different temperature expansion coefficients, the formation of micro cracks on the steel material of the tooth causes the penetration of a dispersed medium containing erosion microparticles in the newly created cracks. The carbide coating of the tooth is more resistant to erosion, but fragile, which causes it to gradually disintegrate during the drilling process due to the effects of pressure and shear conditions combined with the heating and cooling. During our research, we discovered that the wear on the tooth steel is more progressive than the wear on the carbide coating. The surfaces of the teeth, which are not covered with a carbide coating, are more exposed to the influence of erosion. The erosion effect is represented by the formation of erosion channels into which the mud, which contains a large percentage of aggressive particles, flows in. The disintegration of the carbide coating, which is a slower process than the steel wear, is a consequence of the bit load during drilling and the temperature differences, which occur during the roller rotation. Because of the different rate of wear of both materials, the geometry of the teeth changes and, thus, the effectiveness of the drill bit. Our roller cone drill bit, which was the subject of our investigations, belongs to the class of the so-called self-sharpening drill bits, for which the manufacturers predict that the geometry of tooth changes during the drilling process. With the discovered rate and development of wear of the steel materials on the surfaces of teeth on the drill bit, we determined the way that the self-sharpening effect actually occurs, so the effective operation of this type of drill bit is prolonged. We had drilled through 610.70 m of sandstone when the ROP started to rapidly decrease. The improvement of the resistance of tooth steel, which is rather tough, is offered in the surface treatment of this. The surface treatment of tooth steel, in terms of increased erosion resistance, can be carried out by means of metallurgical processes or the subsequent treatment of the surface of the entire tooth prior to the application of the carbide coating.
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ACCEPTED MANUSCRIPT The characterization of the wear of the drill bit, is a complex analysis of several factors. The differences between the deformability properties of the steel material on drill bit are evident. The carbide coating gradually disintegrate during the drilling process. The abrasive components in drilling mud are forming the erosion channels.
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