Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx
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Original article
Investigation of effect of CO2 laser parameters on drilling characteristics of rocks encountered during mining A. Bharatish ⇑, B. Kishore Kumar, R. Rajath, H.N. Narasimha Murthy Department of Mechanical Engineering, R V College of Engineering, Bangalore 560059, India
a r t i c l e
i n f o
Article history: Received 11 May 2017 Accepted 28 December 2017 Available online xxxx Keywords: Laser drilling Specific energy Rate of penetration Rock minerals Response surface methodology
a b s t r a c t Application of laser technology for drilling rock samples reduces mining costs because of its higher transmission capabilities providing an alternative to conventional drill bits and blasting techniques. This paper investigates the effect of laser drilling parameters such as laser power, frequency, assist gas pressure and piercing time on drilling characteristics of rock mineral samples such as limestone, shale and sandstone using 12 kW CO2 Laser. For limestone, minimum specific energy of 46.14 kJ/mm3 and maximum rate of penetration 15.14 mm/s was achieved at 1000 W laser power, 1 kHz frequency, 6 bar assist gas pressure and 0.1 s piercing time. For sandstone, minimum specific energy of 14.33 kJ/mm3 and maximum ROP of 57.46 mm/s was achieved at 1000 W, 1 kHz, 2 bar and 0.1 s. For shale, minimum specific energy of 8.13 kJ/mm3 and ROP of 45.05 mm/s was achieved at 300 W, 5 kHz, 2 bar and 0.1 s, based on Response Surface Methodology. Morphological studies were reported on the drilled rock samples. Ó 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction and literature review Application of advanced high power laser technology to oil and gas well drilling has attracted the attention of many research institutes, petroleum industries, and universities (Dong-Gyu and Gwang-Won, 2009). The conventional methods and equipment used for drilling oil, gas and water wells have been developed to crush various earth and rock formations (Williams, 1986). However, they do not perform well in the hardest rock formations because of slower drilling rate and higher tool wear (Bazargan et al., 2013). Laser technology applied to drilling and completion operations can reduce drilling time by eliminating the rock contact and need of mechanical bit replacement. With the adoption of laser technology for perforation, the rock is cleaned and fluid flow paths for oil and gas production are retained without destruction (Xu et al., 2003a,b). Other advantages of laser include the creation of a melted rock wellbore lining which can eliminate the need for steel casing and improved flow performance, if adopted as a perfo⇑ Corresponding author. E-mail address:
[email protected] (A. Bharatish). Peer review under responsibility of King Saud University.
Production and hosting by Elsevier
rator (Batarseh et al., 2003). The rocks encountered in oil and gas drilling include shale, a rock with fine grain size, sandstone, made up of grains predominantly 62 microns and larger; and limestone. The three different methods by which rock can be removed by lasers include thermal spalling, melting and vaporization. The most efficient rock removal mechanism would be to minimize energy to remove unit volume of rock (specific energy) while maximizing the rate of penetration (ROP) (Xu et al. (2002)). Modern high power lasers have enough power to spall melt and vaporize all types of rocks. Some of the authors have reported laser – rock mineral interaction studies using Nd:YAG and CO2 lasers. Gas Institute of Technology (GTI) investigated the interaction of high power lasers with different rock types to determine the extent of application in drilling for mining purposes. GTI identified the specific energy (SE) to remove rock samples such as sandstone, shale and limestone using 1.6 kW pulsed Nd: YAG laser beam (Parker et al., 2003a). Ezzedine et al. (2015) investigated the influence of repetition rate, pulse width, specific energy on hole diameter of sandstone and lime stone using 1400 W CO2 laser. The spallation temperatures induced by flame jet heating was below 5200 °C and no solid phase transitions were reported. Parker et al. (2003b) investigated the rock removal efficiency of the multiple laser beams by drilling large diameter holes in rocks using 1.6 MW Mid-infrared Advanced Chemical Laser (MIRACL). It was found that combination of several small illuminated spots created a larger hole. Gahan et al. (2001) investigated the effect of laser exposure time on specific energy
https://doi.org/10.1016/j.jksues.2017.12.003 1018-3639/Ó 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Bharatish, A., et al. Investigation of effect of CO2 laser parameters on drilling characteristics of rocks encountered during mining. Journal of King Saud University – Engineering Sciences (2018), https://doi.org/10.1016/j.jksues.2017.12.003
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A. Bharatish et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx
Nomenclature Lp F Pt Agp
Laser Power (W) Frequency of laser (kHz) Piercing time of laser (s) Assist gas pressure (Bar)
using 1.6 kW pulsed Nd:YAG laser and found that the specific energy increased with increase in exposure time for shale mineral when compared to limestone and sandstone. Rad et al. (2014) investigated the thermal influence of gas laser drilling in different composition of limestone samples. CO2 laser achieved greater rate of penetration (ROP) compared to conventional operations and were more effective than the solid state lasers. Hafez et al. (2015) carried out high specific power laser drilling and a computer control ROP laser system. It was concluded that the rock removal mechanism may be shifted from melting to spallation to achieve efficient laser drilling. Salehi et al. (2007) developed a test plan to measure the amount of energy required to remove the material from the rock samples by minimising the secondary effect that tends to absorb laser power. Increase in beam repetition rate increased the material removal rate and increase in pulse width and repetitive pulse rate decreased the specific energy. Williams (1986) investigated the thermal spallation during the drilling process. It was reported that rapid heating during drilling produced a thin surface layer with high temperature resulting in formation of spall. Xu et al. (2003a, b) investigated the interaction of 1.6 kW Nd:YAG laser with rock samples such as shale, limestone and sandstone. The effect of laser parameters on specific energy and spallation was evaluated. The lowest SE values were obtained in the spalling zone just prior to the onset of mineral melt. The study found that increasing beam repetition rate within the same material removal mechanism increased the material removal rate. Damian et al. (2016) reported the coupled thermal-mechanical physics inherent to laser rock interaction. COMSOL Multiphysics was used to study the heat transfer with phase change in laser-drilling of rocks. The numerical results showed that it was possible to use an anisotropic thermal conductivity, in the melting and vaporization phases, to account for removing and evaporating materials. Leonenko et al. (2013) investigated the laser treatment of carbonate rocks using continuous 600 W fiber-optic ytterbium laser radiation. It was found that the depth of cut grew with increasing laser radiation power and the dolomite specimen had smaller cut depth than that in the limestone specimen. The review of literature indicated that the physical phenomenon behind the laser drilling of rock minerals is not fully explored because of its strong dependence on material composition and grain size. Simultaneous optimisation of laser rock drilling responses in order to reduce the operation time is not yet reported. Hence the present research focuses on studying the effect of laser rock drilling parameters such as laser power, frequency, assist gas pressure and piercing time on specific energy and rate of penetration in rock minerals such as limestone, sand stone and shale, based on Taguchi’s orthogonal array technique. ANOVA was used to identify the significant effects of laser parameters on measured rock drilling responses. Multiple regression models were obtained to correlate laser parameters with drilling responses. Simultaneous optimisation of drilling responses was achieved using Response Surface Methodology. Morphological features of laser drilled holes in rock samples were investigated using scanning electron microscope and Xray diffraction methods.
SE ROP RSM TSM
Specific Energy Rate of penetration Response Surface Methodology Trichomonas Stereo Measuring Microscope
2. Experimental studies Laser drilling technique was employed by using 12 kW Trumph Laser Cell 1005. CO2 laser in continuous mode was used and smaller diameter holes were obtained. The specifications of the laser machine are as indicated in Table 1.The rock minerals such as limestone, sandstone and shale were used in the experimental studies and trepanning drilling technique was used for drilling holes in samples. Properties of the rock samples are presented in Table 2. Experiments were conducted based on L9 Taguchi’s orthogonal array as shown in Table 3. Parameters like laser power (Lp), frequency (F), piercing time (Pt) and assist gas pressure (Agp) were chosen as factors. The specific energy and rate of penetration were selected as responses. When laser drilling was attempted on rock minerals samples with laser power lesser than 300 W, through holes were not achieved and cracks were formed on the top surface of the sample, thus resulting in failure.
2.1. Scheme of measuring the drilling responses Diameter and area of the drilled holes in rock minerals were measured using Trichomonas Stereo Measuring Microscope (TSM). The images were captured from TSM and were imported to Image J Software and Diameter of the hole and thickness was measured from which Drilled area was obtained. The drilled rock samples are as shown in Fig. 1(a)–(c).
2.2. Specific energy (SE) The amount of energy required to remove a unit volume of rock is known as specific energy. It is the absorbed energy that causes the rock heating and destruction. SE was evaluated using Eq. (1)
Specific EnergyðJ=mm3 Þ ¼
Energy input Volume removed
ð1Þ
The volume of the material removed is calculated using Eq. (2)
Volume removedðmm3 Þ ¼
p
ðhole depthÞ ðR2 þ r2 3 þ ðR rÞÞ
ð2Þ
where R and r are entrance and exit radius of hole respectively.
Table 1 Specifications of Laser Machine. Machine Model
Trumpf Laser Cell 1005 CO2 laser
Wavelength Frequency Power Working distance Maximum field size Beam diameter Mode of operation
10.6 m 10 Hz–10 kHz 1800 W–12,000 W 500 mm 2000 mm 1500 mm 0.25 mm Continuous type
Please cite this article in press as: Bharatish, A., et al. Investigation of effect of CO2 laser parameters on drilling characteristics of rocks encountered during mining. Journal of King Saud University – Engineering Sciences (2018), https://doi.org/10.1016/j.jksues.2017.12.003
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A. Bharatish et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx Table 2 Properties of Rock Minerals.
4. Response surface methodology models (RSM)
Properties
Limestone
Sandstone
Shale
Porosity (%) Density (kg/cm3) Specific heat (J/gK) Thermal Conductivity (W/mK) Young’s Modulus (GPa) Compressive Strength (N/mm2) Hardness (Mosh’s Scale)
0–20 2.5 to 2.7 0.908 0.0048 20.70 60–170
5–30 2.3 to 2.4 0.92 0.0125 18.45 90 to 140
0–10 2.3 0.391 0.0042 8.46 70–120
3 to 4
6.5 to 7
3
2.3. Rate of penetration (ROP) The amount of material removed in unit time is known as rate of penetration and defined as ratio of specific power to specific energy (Jaeger and Cook, 1976) given in Eq. (3)
ROPðmm=sÞ ¼
Specific power Specific energy
ð3Þ
The specific power of a laser is defined as the laser power per cross sectional unit area of the beam, given by Eq. (4)
Specific PowerðW=mm2 Þ ¼
Laser Power Area
ð4Þ
The relationship between the laser drilling parameters and the responses was modelled using RSM. The general first order RSM model used to predict the influence of laser parameters on the response factor is given by Eq. (5).
Y i ¼ b0 þ ðb1 X i1 Þ þ ðb2 X i2 Þ þ :::: þ ðbq X iq Þ þ i ði ¼ 1; 2; 3 . . . . . . ::NÞ
ð5Þ
where yi is the response factor and xij are the values of ith observation and jth level of the drilling parameters .The terms bi are the regression coefficients. For modelling purpose, the higher order linear effects were considered and the interactive effects were neglected. The residual Єi is a measure of the experimental error. First order RSM model was developed to predict the mathematical relationship between measured responses such as specific energy and rate of penetration and the process parameters such as laser power, frequency, piercing time and assist gas pressure. The regression equations and corresponding R-Squared values for drilling responses of rock minerals obtained from Minitab-17 are presented in Table 5. In general, the response surface representing the SE as a function of laser drilling parameters such as laser power (Lp), frequency (Fr) and assist gas pressure (Agp) and piercing time(Pt) were represented by Eq. (6)
SE ¼ b0 þ ðb1 Lp Þ þ ðb2 Fr Þ þ ðb3 Agp Þ þ ðb4 Pt Þ 3. Analysis of variance (ANOVA) of drilling responses ANOVA was performed in order to determine the influence of parameters such as laser power, pulse frequency, piercing time and assist gas pressure on SE and ROP. The assessment was made based on F and p distributions as shown in Table 4.
ð6Þ
Based on the experimental results of laser drilling, the regression models established for correlating SE and ROP with the laser parameters for limestone, sandstone and shale are presented in the Table 5. The adequacy of the model was further analysed by R-squared values. These values represented the confidence level of regression
Table 3 L9 Orthogonal array experimental layout for Limestone and Sandstone. Sl No
1 2 3 4 5 6 7 8 9
P (W)
1000 1000 1000 1200 1200 1200 1500 1500 1500
F (kHz)
1 2 5 1 2 5 1 2 5
Agp (bar)
2 4 6 4 6 2 6 2 4
Pt (s)
0.1 0.2 0.3 0.3 0.1 0.2 0.2 0.3 0.1
Limestone
Sandstone
Shale
SE (kJ/mm3)
ROP (mm/s)
SE (kJ/mm3)
ROP (mm/s)
SE (kJ/mm3)
ROP (mm/s)
79.53 133.92 196.01 299.83 75.61 193.16 216.08 322.70 366.48
15.86 8.01 5.29 5.27 16.10 8.03 7.85 5.15 6.58
16.65 34.07 55.50 63.99 21.34 39.82 47.52 66.03 81.95
59.97 28.16 19.46 19.67 56.62 29.32 26.98 17.33 15.38
8.13 16.72 20.75 33.94 9.96 19.31 39.68 63.35 19.90
47.59 22.07 15.51 16.08 46.58 24.39 22.90 15.58 47.24
Fig. 1. Laser Drilled holes in a) Limestone b) sandstone c) shale.
Please cite this article in press as: Bharatish, A., et al. Investigation of effect of CO2 laser parameters on drilling characteristics of rocks encountered during mining. Journal of King Saud University – Engineering Sciences (2018), https://doi.org/10.1016/j.jksues.2017.12.003
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Table 4 ANOVA Results of Laser Drilling on Rock Materials. Limestone
Shale ROP (mm/s)
SE (kJ/mm3)
Sandstone
SE (kJ/mm3)
ROP (mm/s)
Ftab
SE (kJ/mm3)
ROP (mm/s)
Fcal
P
Fcal
P
Fcal
P
Fcal
P
Fcal
P
Fcal
P
6.10 0.84 1.15 29.54
16.20 2.25 3.05 78.48
6.42 1.05 0.94 14048.6
0.0457 0.0075 0.0067 99.940
8.66 1.27 0.72 8.44
45.30 6.65 3.79 44.23
0.65 1.70 1.34 1723.91
0.03 0.09 0.07 99.78
2.21 0.93 1.06 1.40
39.43 16.55 19.02 24.98
1.305 0.967 1.03 2.58
0.045 0.0075 0.0067 99.94
2 2 2 2
Fcal – Fisher’s calculated value, Ftab – Fisher’s tabulated value.
Table 5 Regression models for Rock minerals. Rock minerals
Responses
Regression Equations
R-sq value
Limestone
SE (kJ/mm3) ROP (mm/s)
147 + 1.183Lp + 3.3 Fr 43.6 Agp – 30Pt
86.10%
20.71–0.00034 Lp 0.003 Fr + 0.017 Agp 53.26 Pt
92.62%
SE (kJ/mm3) ROP (mm/s)
62.5 + 0.0610 Lp + 4.58 Fr + 0.15 Agp + 109.3 Pt 107.8–0.0334 Lp 3.70 Fr 0.30 Agp 125.8 Pt
71.54%
SE (kJ/mm3) ROP (mm/s)
10.78 + 0.03740 Power 2.169 Fr 1.701 Agp + 133.4 Pt 60.61 + 0.00004 Lp + 0.11 Fr 0.21 Agp 157.1 Pt
97.25%
Sandstone
Shale
78.35%
91.36%
and indicated that experimental and predicted values were in good agreement with each other.
5. Results and discussion 5.1. Influence of laser parameters on specific energy and ROP of limestone From ANOVA results of SE & ROP, Fcal value was greater than Ftab value for laser power and piercing time, thus both were found to be influencing factors for SE and ROP. From RSM plot as shown in Fig. 2, SE increased with increase in laser power and piercing time. Increase in piercing time lead to heating of interaction region and secondary effects such as material melting which consumes addi-
tional laser energy. This melt acts as a barrier which prevents the laser beam from fully interacting with limestone. Also, extra heat dissipation will result in thermal expansion, fracture formation and mineral decomposition tending to cause increase in specific energy values. ROP decreased with increase in piercing time at constant laser power. This was mainly because increase in piercing time causes increase in duration of laser exposure leading to higher interaction with melt rather than substrate. This causes increase in width of melt zone in radial direction leading to lower ROP. This is in agreement with Hafez et al. (2015). The authors eventhough reported that the dominant rock removal mechanism was melting, they could not confirm the correlation between ROP and exposure time, which was established from present study. The desirability function approach is used for the optimization of multiple responses. It finds the experimental factors which provide the most desirable response values. For each response, a desirability function assigns numbers between 0 and 1 to the possible values of the response. The response value equal to zero represents a completely undesirable value and the response value equal to one represents a completely desirable or ideal response value. The individual desirabilities are combined using the geometric mean, which gives the overall or composite desirability. Using Minitab, Minimum Specific energy (46.142 kJ/mm3) and maximum ROP (15.141 mm/ s) were obtained at 1000 W laser power, 1 kHz pulse frequency, 6 bar assist gas pressure and 0.1 s piercing time. 5.2. Influence of laser parameters on SE and ROP of sandstone From ANOVA results (Table 5), Fcal value was greater than Ftab value for laser power with respect to SE, thus only laser power
Fig. 2. Response surface methodology plot for limestone.
Please cite this article in press as: Bharatish, A., et al. Investigation of effect of CO2 laser parameters on drilling characteristics of rocks encountered during mining. Journal of King Saud University – Engineering Sciences (2018), https://doi.org/10.1016/j.jksues.2017.12.003
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was found to be the influencing factor for SE. For ROP, since Fcal value was greater than Ftab value for piercing time, same was influential. RSM plot (Fig. 3) depicts the effect of laser power and piercing time on SE. It was observed that, specific energy increased with increase in laser power. Sandstone mainly contains clay particles entrapped with water molecules. Increase in laser power causes increase in surface temperature which in turn leads to vaporization of water molecules. This increases the volume and pressure in the pore and causes fractures. These fractures represent the loss of energy, which results in higher values of specific energy of sandstone Xu et al. (2003a,b). ROP decreased with increase in piercing time whereas power increased with increase in ROP throughout the process. As the piercing time increases energy accumulates in the form of heat, thus increasing local temperature of minerals of sandstone to their melting point and forms a glassy melt. Since sandstone has higher thermal conductivity compared to limestone and shale, higher amount of heat energy is transferred due to proximity of grains, resulting in higher melt. This melt acts as a barrier
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for laser material interaction and hence ROP decreases (Gahan et al., 2001). Minimum Specific energy (14.33 kJ/mm3) and maximum ROP (57.46 mm/s) were obtained at 1000 W laser power, 1 kHz laser frequency, 2 bar assist gas pressure and 0.1 s piercing time. 5.3. Influence of laser parameters on specific energy and ROP of shale ANOVA results (Table 5) indicated that SE was influenced by laser power and piercing time and ROP was influenced by only piercing time since corresponding Fcal value was found to be greater than Ftab value. The effect of laser power and piercing time on specific energy was indicated by RSM plot as shown in Fig. 4. It was observed that, specific energy increased with increase in power and piercing time. The shale sample was easily drilled than limestone or the sandstone. Increase in laser power causes higher material removal rate as well as induces secondary effects, such as ex-solving gases which absorbs the beam energy
Fig. 3. Response surface methodology plot for sandstone.
Fig. 4. Response surface methodology plot for shale.
Please cite this article in press as: Bharatish, A., et al. Investigation of effect of CO2 laser parameters on drilling characteristics of rocks encountered during mining. Journal of King Saud University – Engineering Sciences (2018), https://doi.org/10.1016/j.jksues.2017.12.003
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A. Bharatish et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx
or particles released from the sample (Parker et al., 2003c, 18). ROP decreased with increase in piercing time whereas power remained constant throughout the process. Increase in piercing time causes non overlapping of laser spots and hence there is a possibility of spikes and ridges of remnant materials forming between the spots. This caused decrease in ROP (Parker et al., 2003b, 9). Specific energy and ROP obtained as 8.13 kJ/mm3 and 45.05 mm/s respectively.
5.4. Morphological studies 5.4.1. Limestone The laser drilled holes were examined under scanning electron microscope (Model Hitachi SU-1500 SEM 15.0 kV) in order to characterize the geometrical and metallurgical features and ascertain the influence of laser parameters on drilling characteristics. The laser drilled rock minerals were gold sputtered to study the region
Fig. 5. a) SEM Image of laser drilled hole in limestone b) XRD plot for limestone –. 1, 2, 3 -Ca37.6Mo12N56O3Sr13.4 – Calcium Strontium Tetranitridomolybdate Nitride Oxide. 4, 12 – Ca1O3Si1 – Calcium Catena-silicate. 5, 6 – C2Ca1Mg1O6 – Dolomite. 7 – Ca0.1Ce0.5Mg0.05O1.85 – Cerium Calcium Magnesium Oxide. 8, 9, 10, 11 – Al0.5Ca2Mg0.75O7Si1.75 – Akermanite – Gehlenite.
Fig. 6. a) SEM image of laser drilled hole in sandstone b) XRD plot of sandstone –. 1 – H36 Ca2 Cd3 Cl10 O18 – Octaaquacalcium Catena-decachlorotricadmate Dihydrate. 2, 4 – Ca1 O3 Si1 – Calcium Catena-silicate. 3 – Ca1 Cu3 O12 V4 – Calcium Tricopper(II) Tetrakis(trioxovanadate(IV)).
Fig. 7. a) SEM image of laser drilled hole in shale b) XRD plot of shale. 1 – Ca0.75 Hf1 O3 Sr0.25 – Calcium Strontium Hafnate. 2, 4 – H16 As1 Ca1 K1 O12 – Calcium Potassium Arsenate Octahydrate. 3 – H4 Ca3 Cl2 O6 – Tricalcium Bis(chlorate(I)) Hydroxide. 5 – Ca0.3 Mn1 O3 Pr0.7 – Praseodumium Calcium Manganese Oxide (0.7/0.3/1/3).
Please cite this article in press as: Bharatish, A., et al. Investigation of effect of CO2 laser parameters on drilling characteristics of rocks encountered during mining. Journal of King Saud University – Engineering Sciences (2018), https://doi.org/10.1016/j.jksues.2017.12.003
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around the hole for grain structure and micro cracks. Fig. 5(a) shows the hole drilled in limestone at 1000 W laser power, 1 kHz pulse frequency, 6 bar assist gas pressure and 0.1 s piercing time and Fig. 5(b) shows the XRD plot of the same. Laser drilled limestone indicated a large melted zone on the top surface along with micro cracks and pores mainly due to higher laser power intensity and lower piercing time 5.4.2. Sandstone Fig. 6(a) shows the hole drilled in sandstone at 1000 W laser power, 2 kHz pulse frequency, 4 bar assist gas pressure and 0.2 s piercing time. During the laser sandstone interaction, the greater percentage of quartz in the rock samples causes the higher laser energy consumption in secondary mechanism, including melting and vaporization. The clay particles get dehydrated when exposed to high temperature which enhances the permeability and creates micro fractures as evidenced in the Fig. 6 (a). A sharp peak with high intensity was detected at the 2h value of 15.34° which confirmed the presence of quartz after laser drilling process as shown in the Fig. 6(b). 5.4.3. Shale Fig. 7(a) shows the hole drilled in shale at 300 W laser power, 5 kHz pulse frequency, 6 bar assist gas pressure and 0.3 s piercing time. During the laser shale interaction, shale requires least specific energy to remove a unit volume of rock since it do not contain any moisture content in inter layers. A sharp peak with high intensity was detected at the 2h value of 74.35° which confirmed the presence of calcium strontium hafnate after laser drilling process as shown in the Fig. 7(b). 6. Conclusions CO2 laser drilling studies on rock mineral samples such as limestone, sandstone and shale was carried out to investigate the effect of laser parameters on specific energy and rate of penetration characteristics. The following conclusions were arrived at: 1. SE and ROP of limestone were influenced by laser power and piercing time. SE and ROP of sandstone were influenced by laser power and piercing time respectively. SE of shale was influenced by laser power and piercing time and ROP of shale was influenced by piercing time. 2. For limestone, minimum specific energy of 46.14 kJ/mm3 and maximum ROP of 15.14 mm/s could be achieved at 1000 W Laser power, 1 kHz Frequency, 6 bar Assist gas pressure and 0.1 s Piercing time. For sandstone, minimum specific energy of 14.33 kJ/mm3 and maximum ROP of 57.46 mm/s could be achieved at 1000 W, 1 kHz, 2 bar and 0.1 s. For shale, minimum specific energy of 8.13 kJ/mm3 and ROP of 45.05 mm/s could be achieved at 300 W, 5 kHz, 2 bar and 0.1 s, based on Response surface methodology results. 3. The morphological studies of laser drilled limestone indicated a large melted zone on the top surface along with micro cracks and pores mainly due to higher laser power intensity and lower
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piercing time. During the laser sandstone interaction, the greater percentage of quartz in the rock samples caused the higher laser energy consumption in secondary mechanism, including melting and vaporization. The clay particles were dehydrated when encountered to high temperature enhancing the permeability and formation of micro fractures. Shale required least specific energy (8.13 kJ/mm3) to remove a unit volume of rock because of the absence of moisture content in inter layers.
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Please cite this article in press as: Bharatish, A., et al. Investigation of effect of CO2 laser parameters on drilling characteristics of rocks encountered during mining. Journal of King Saud University – Engineering Sciences (2018), https://doi.org/10.1016/j.jksues.2017.12.003
