SPE 113177

Underwater Laser Drilling: Drilling Underwater Granite by CO2 Laser

Toshio Kobayashi, SPE, and Masahiro Nakamura, Japan Drilling Co., LTD; Komei Okatsu, SPE, Japan Oil, Gas and Metals National Corporation ; Kiyonobu Ohtani, Institute of Fluid Science, Tohoku Univ; and Kazuyoshi Takayama, TUBERO, Tohoku Univ.

Copyright 2008, Society of Petroleum Engineers This paper was prepared for presentation at the 2008 Indian Oil and Gas Technical Conference and Exhibition held in Mumbai, India, 46 March 2008. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract The purpose of this research is to develop Underwater Laser Drilling Technology in drilling and completing oil and natural gas wells. Recent challenges in oil fields are caused by: the geometry of oil or gas wells is becoming more complicated in profile; drilling in hard formations like granite requires more tripping times for changing rotary bits made of steel; as the water depth at which floater rigs can operate increases, tripping has to take longer than ever. Underwater Laser Drilling is a key to solution of these challenges since the laser may allow for non-contact / non-rotating drilling method. Granite is one of the most difficult rocks to be drilled by lasers. Consequently, no successful report has been found on underwater granites drilled by lasers. This paper describes experimental tests of irradiating carbon dioxide (CO2) laser to granite submerged in water/ bentonite solutions. Sample granite was submerged at 50mm from the water-surface and then a CO2 laser with a wavelength of 10.6m which has a high absorption coefficient in water was intermittently irradiated into the 4wt% bentonite solutions. A laser induced underwater shock waves, cavitation bubble formations, and micro-water jet formations upon bubble collapse. The high power laser beam locally melted the granite into molten glass, which successively turned to a molten glass bead. The generated glass beads were small enough to be removed out of the created hole. A high-speed video camera was employed to record the laser beam in the water. We observed the generation of initial cone shaped water bubbles through which the laser beams reached bottom rock surface with a minimum absorption energy loss. The laser beam indeed drilled the granite specimen submerged in the solution. The research will continue to collect data for designing a prototype. Introduction

Melting Rocks. Relatively low-power continuous wave CO2 lasers with output powers1 kW had been used for most of the experimental results on laser-rock interaction before 2000. It was reported by Mukhamedgalieva et al (1975, 1976, 1978, 1981. 1982) that CW CO2 laser radiation on various rocks forms a laser plume. They concluded that the laser-plume absorbs laser radiation. This absorption leads both to a temperature rise of the plume and a further increase in absorption of laser radiation by the plume, causing the rock destruction process to completely cease (Mukhamedgalieva et al (1976)). [1] Spallation of Rocks. To avoid melting rocks, some researches have been conducted for identifying a transition zone between spallation and melt zone during a laser radiation. From 1997, Gas Technology Institute has been carrying the experiments to identify the specific energy requirements required to remove rock from test samples of sandstone, shale and limestone. According to their report, to break rock by mechanically or thermally induce stresses, sufficient power must be applied to the rock such that the induce stresses exceed the rock's strength [2]. Thermal Spallation Drilling using Flame-Jet was researched by R.M.Rauenzahn et al in 1989. Exposing certain polycrystalline rocks to rapid surface heating will fracture into thin, disk-like fragments. Several hard-rock drilling methods using flame-jets make use of this behaviour for efficient granite quarrying or blast-hole formation. For prediction of chip size distributions and rock surface temperatures at spallation under any intense heat source, weibull's theory of rock failure has been applied. The estimated spallation temperatures induced by flame-jet heating were to be below 520 centigrade for Barre and Westerly granites. They also described that laser-induce spallation

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experiments were less successful, partially because extremely localized heating delivered by the beam greatly complicated the subsequent thermal stress analysis [3]. Previous Work. Reviewing the above-described previous work, the reason why laser technology has not been applied to the drilling of rocks are presumably as follows: (1) a laser plume formed under the laser radiation on various rocks absorbs a laser radiation, causing a temperature rise of the plume and hampering the rock destruction process; (2) Laser-induced spallation is difficult to continuously happen, partially because extremely localized heating by the beam greatly complicates the subsequent thermal stress analysis and partially because localized heating by the laser beam causes an uniform increase in the temperature of the whole rock as time passes, thereby failing to induce mechanical or thermal stresses enough to break the rock. Feature of our research. Taking into consideration that the above-mentioned methods are less successful in drilling rocks, we have reached the idea that laser-induced mechanical forces could be utilized for generating a cavity in a rock by irradiating lasers on rocks in water. A laser beam can travel in some distance in a transparent fluid. However, scattered and absorbed by particles contained in muddy water, the laser beam cannot reach a rock submerged in such circumstances. That is why the laser is considered to be incapable of processing materials in the muddy waters. The laser-induced mechanical forces includes: shock-waves, bubble formations, and water jets that are induced by laser-irradiation into water. Such phenomena are essentially elucidated by bubble-dynamics and shock wave dynamics. Irradiating a laser beam on a rock forms molten glass that spreads on the surface of the rock and hampers the rock from being further irradiated. Utilizing mechanical forces induced by laser-radiation in water may form molten glasses into shapes that can be easily handled, thereby allowing a continuously laser-drilling of rocks submerged in the water. It is an objective of our research to establish a laser-drilling method that can effectively work even in opaque water contaminated with solid particulates. The experiments of laser-drilling rocks were conducted, employing a 10.6m CO2 laser having a high water-absorption rate. The CO2 laser was intermittently irradiated perpendicularly from a muddy water surface. Irradiated laser beam was absorbed in the water to form cone-shaped bubbles that had an irregular interface. Since the induced bubbles kept its shape until a subsequent laser beam reaches the rock, the laser beam could effectively drill the rock. A high speed video camera was used for recording a formation of the cone-shaped bubbles induced by the laser-radiation in the water. The Inada granite placed at 50mm in depth from the opaque water surface was successfully drilled to have a hole with a 47mm maximum depth.

Experimental setup

A schematic diagram of the experimental setup is shown in Fig. 1. A 300mm x 300mm x 300mm acrylics chamber was used. A bentonite/water 4wt% solution was filled in the chamber. A CO2 laser with maximum output of 5kW and 10.6m wavelength was used. The CO2 laser has a ring mode. A high speed video-camera was placed at the side of the chamber to photograph a dynamic behavior of the formation of bubbles induced by a laser-irradiation. A rock specimen was placed in the chamber so that a distance from the water surface to the top surface of the granite makes 50mm. (Fig.1) The rock specimens: Inada Granite. The rock specimens used in this experiment were Inada granite occurring in the vicinity of Inada, Kasama City, Ibaraki, Japan; its K-Ar chronology is 60 Ma. Inada granite is a granite representative in Japan, and it is often used as construction materials and experiment materials. The Inada granite contains 33.7% quartz, 32.1% potash feldspar, 30.3% plagioclase, 3.8% biotite, etc.[4] Physical characteristics of Inada granite are as follows: uniaxial compressive strength, 180MPa; splitting tensile strength 0.75MPa; youngs modulus 65GPa; density 2.63g/cm3; porosity 0.75%; specific heat 0.0167~0.0377J/g/K; thermal conductivity 54.4~83.7 W/m/K [5].

Results and Discussion A bubble formation induced by irradiating CO2 laser in water. Bubbles were induced by CO2 laser that was irradiated from above the water surface. The length and width of bubbles vary with elapsed times as shown in Fig. 2. The length and width of the induced bubbles were obtained from sequential photographs captured by the high speed camera. The framing speed of the high-speed camera was 1000 frame/s with exposure time 60s and resolution 800pixel x 600pixel. On the bottom row of each photograph in Fig. 2, the elapsed time from the arrival of the laser beam at the water surface was shown. Successive photographs are taken at 3ms intervals. The laser had a peak output power of 5kW, a pulse width of 50ms, a repetition rate of 10Hz, a pulsed energy of 250J, and a duty ratio of 50%. The experiments using the high speed camera were conducted both with 10mm diameter laser spot positioned on the water surface and with 10mm diameter laser spot positioned at 50mm in depth from the water surface. The depth of the focused spot will be denoted Lf, so Lf=0mm indicates an experiment conducted with the 10mm diameter laser spot positioned on the water surface and Lf=50mm indicates an experiment with the 10mm diameter laser spot positioned at a 50mm depth below the surface. An experiment conducted with a 10mm diameter laser spot positioned on the water surface is denoted by Lf=0mm below while other experiment with a

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10mm diameter laser spot positioned at 50mm in depth from the water surface is denoted by Lf=50mm. The laser irradiation on the water induced a cone-shaped bubble that evolved downwardly (Fig.3 (a) - (j)). The maximum length of the bubble is 55.20mm at t=25ms. The maximum length of the bubble for Lf=50mm is about 27% longer than that of the bubble for Lf=0mm, showing a travelling distance of a laser beam can be proportional to a focused position of the laser-beam in the water. Evolving to the maximum length, the bubble starts to shrink, dissociate, collapse, and disappear (Fig. 3). For Lf=0mm, the length of the bubble increases linearly with the eelapsed time. The maximum bubble length was 43.5mm. The bubble starts to shrink when the bubble evolves to the maximum length. The bubble is relatively shortened at point a (t=31ms). The length of the bubble nearly levels out in the range from a to b. During a-b period, the bubble is constricted at a longitudinally mid position and then dissociates. The bubble is accelerated to shrink in the length, and completely disappear in a short time. For the width, the bubble gradually evolves to reach the maximum width of 17.70mm at t=30ms. The width of the bubble nearly levels out until the bubble dissociates at point b (t=41ms). For Lf=50mm, the length of the bubble increases linearly up to point c. A gradient in the length for Lf=50mm is larger than that for Lf=0mm. The length of the bubble reaches the maximum at point c, starts to shrink, and evolve again up to point d. The bubble shrinks rapidly both in length and in width after point d. A shrinking rate for Lf=50mm is larger than that for Lf=0mm. The bubble gradually evolves and reaches the maximum width of 28.12mm at t=58ms. The width of the bubble nearly levels out until the bubble dissociates at t=81ms (Fig.2).

Measured temperatures of the induced bubbles generated by irradiating laser in water. The temperature of the bubble reaching a thermocouple was measured for this purpose. It was predicted that the measurement maximum temperature exceeded the maximum measured temperature of the thermocouple. In order to protect the thermocouple, silicone grease was applied at the tip of the thermocouple. The thermocouple was placed at 50mm in depth from the water surface in a same manner as the rock specimen was located in the water. Fig 4 (a) is a picture of the laser induced bubble extended from the water surface to the thermocouple tip. The thermocouple can indicate 90% of the measured value in 25ms. Laser irradiating conditions were identical to the conditions of irradiating the submerged rock in the water with a laser of 250 J/pulse, and of the repetition rate of 10Hz. Fig 4 (b) shows a graphic chart of elapsed time and measured temperature when the laser was irradiated for ten seconds. The results were 642 degree C in elapsed time 3 seconds, 2371.79 degree C in 5.44 seconds, and 2459.94 degree C in 9.84 seconds.

Observation of behaviour of the plumes generated by irradiating CO2 laser in air and water

Behaviour of the plumes generated by irradiating granite with CO2 laser in air. Fig. 5 is a sequential photograph of the plumes generated by irradiating granite with a CO2 laser in air. Photographs shown were taken by the high speed camera horizontally placed close to the rock specimen. Sequential photographs exhibit behaviours of the plumes in a hole generated in the granite. The laser was irradiated under the condition of a peak power of 5kWrepetition rate 10Hz, and duty ratio, 50%. Elapsed times shown at the bottom of each photograph were taken from when the plume could be first seen through a filter placed in front of the high speed camera. A time interval of adjoining photographs is 5.0ms. The granite significantly melts to generate a plume at the elapsed time of t=5ms. For t=550ms, the region where brightness is high becomes larger, and rock vapor generated by melting the granite is observed to be rising. The rock vapor indicates that the rock sample is being irradiated with the laser. For this period, the length of the rising rock vapor becomes longer as the time elapsed. In contrast, no plume coming out of a generated hole is observed (see photographs ( t=550ms)). This is because the plume is pushed down in the generated hole by the rock vapor pressure. The region with high brightness is becoming smaller at t =55ms. When the laser irradiation ceases, the rock vapor pressure is lost, causing the plume to come out of the generated hole. The photographs (t=8590ms) show the plume coming out of the generated hole. From the observation described above, the plume comes out of the hole in a 35 to 40mss time after the laser irradiation ceases. Consequently, it is concluded that the plume coming out of the generated hole was hampered by rock vapor pressure during the laser irradiation.

Behaviour of the underwater plumes generated by irradiating granite with CO2 laser in the water. The behaviour of the plumes generated by irradiating the granite with CO2 laser in the water is shown in Fig.6. Photographs shown in Fig.6 were taken by the high speed camera horizontally placed close to the chamber. Sequential photographs exhibits behaviours of the plumes that exploded from the hole generated in the specimen granite. The laser was irradiated under the condition of a peak power of 5kW, repetition rate 10Hz, and duty ratio 50%. Elapsed times shown at the bottom of each photograph were taken from when the plume could be first seen through a filter placed in front of the high speed camera. A time interval of adjoining photographs is 1.6ms. The granite significantly melts to generate a plume at the elapsed time of t=1.6ms. For t=3.2ms, particulates (beads) are observed to disperse. For t=4.8ms, the region where brightness is high becomes larger. For t=6.4ms, the region with high brightness is separated into small portions. The plume then grows gradually for t=8, 9.6ms. For t=14.4ms, the region with high brightness rapidly expands toward the water surface. Subsequently, the expansion of the plume nearly levels out up to t=19.2ms. Next, the enhanced expansion is observed from t=20.8ms to t=28.8ms. The explosive behaviour of the plume that upwardly ejects is observed in this duration (t=20.8ms to 28.8ms). The region with high brightness is becoming smaller at t=30.4ms. From this time to t=36.8ms, the ejection of the plume gradually disappears.

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From the observation described above, the explosive ejection of the plume occurred in a 20mss time after the plume was generated at t=1.6ms, persisting for 8ms. Consequently, it is understood that the ejection of the plume occurred explosively in underwater laser irradiation (Fig.6).

Comparison of dynamic behaviour of molten granites in air and water In this section, we compare dynamic

behaviour of molten granite in the water with those in the air. It is an objective of the comparison to clarify the mechanism which allows underwater laser irradiation to eject the plume, generate a cavity that is not closed by a glassy object. Fig. 8 shows dynamic behaviour of the plumes which were generated by the 10th shot of the laser irradiation. For comparing dynamic behaviour of the molten granites, the photographs for in water are laid on the upper row with those for in air arranged on the lower row. The elapsed times are exhibited on the bottom of the photographs for in air. An optical visible filter was used for taking the photographs. The photographs showing the behaviour of the plumes in water are denoted by a1 to a10, while the photographs showing those in air are denoted by b1 to b15. A starting point of the elapsed time is set at 1096ms after the trigger signal from the laser oscillator. The elapsed times are denoted by t1 to t15, with the subscripts corresponding to those for in-water and in-air photographs. To compare the dynamic behaviour of the plume in water with those in air, the starting time t1 in Fig.8 is set at 18ms that elapses from 1096ms when 10th shot irradiated from the laser oscillator reaches the rock in water. The heights of the plumes obtained from the photographs shown in Fig. 8 are plotted against the elapsed time in Fig.10. The solid line shows the height of the plumes during the laser irradiation, while the dotted line shows those after the completion of the laser irradiation. In Fig.10, the heights of the plume in air are plotted against the elapsed time starting from 0ms while those in water are plotted from 18ms when the evolving bubble reaches the rock in water. The difference in the starting time of each line in Fig.10 are caused by the follows: in air, the laser beam directly reaches the rock to process; in water the laser beam induces the bubble formation that reaches the rock, allowing the laser beam to interact with the rock. The laser beam in water is thus delayed in reaching the rock in comparison to that in air. For the laser irradiation in air, processing time by the laser irradiation ranges from 0.6ms to 54.4ms. By contrast, for the laser irradiation in water, processing time by the laser irradiation ranges from 17.8ms to 51.6ms. In water, the behaviour of the plume becomes invisible through the filter on the completion of the laser irradiation. Meanwhile, the behaviour of the plume in air is still visible through the filter even after the completion of the laser irradiation, so that the height of the plume can be measured for b11 to b15 (Fig.10). In the photographs denoted by a3 to a8, the explosive ejection of the plume with the bubbles collapsed was observed. The ejection of the plume in the water appears to be occurring while the bubbles collapse. In the photographs denoted by b1 to b9, the rock evaporation other than the ejection of the plume was observed. In air, the dynamic behaviour of the plume appears to be stabilized by the reaction force of the rock evaporation during the laser irradiation. The heights of the plume obtained from the photographs denoted by b1 to b10 are plotted against the elapsed time in Fig. 10. In the range denoted by "Irradiating laser" in Fig.10, the heights of the plume in air nearly level out until the laser irradiation ceases. Subsequently, the plume in water appears to be two or more times as high as that in air (Fig.10).

In Fig.10, the remarkable ejection of the plume in air is observed for b11 to b15 that were taken after the laser irradiation was completed. From the observation of the dynamic behaviour shown in Fig.9 and Fig.10, the rock evaporation induced in air prevented the plume from being ejected from a cavity generated in the rock during the laser irradiation. For the laser irradiation in air, it is understood that the ejection of the plume occurred after the completion of the laser radiation, causing the cavity in the rock to be closed. Meanwhile, for the laser irradiation in water, it can be concluded that the ejection of the plume occurred during the laser irradiation.

Drilling granite in transparent and opaque water

Generating a through hole in granite submerged in transparent water. As rock processes on each laser shot with the

laser irradiation nozzle fixed in terms of elevation, the diameter of a laser spot on the rock specimen surface changes, and the laser intensity on the rock surface becomes weak. The laser irradiation intensity has to be kept constant to process the rock specimen in high efficiency. It is therefore necessary to lower the laser irradiation nozzle during irradiating the rock specimen. Taking the irradiation efficiency into consideration, we obtained the optimal lowering velocity of the nozzle. On rock penetration experiments, a lowering speed of the nozzle was changed to the Inada granite submerged at 50mm in depth from the water surface. Fig.11 shows high-speed photographs of the rock specimen which is processed for creating a through-hole. The penetration time of the rock specimen at each nozzle lowering speed is calculated using the high-speed photographs. Comparing the results, the optimal nozzle lowering speed for each rock sample could be obtained. Two samples used were Inada granites of 20mm and 30 mm in thickness. A laser was focused so that a spot diameter of 10mm was made on the surface of the rock sample submerged at 50mm depth in transparent water. With a lowering speed of the nozzle constant, the laser was irradiated and high speed photographs were taken to calculate the time required for penetrating the rock specimen. Experiments were repeated in order that penetration-times for each rock specimen were studied as a function of nozzle-lowering-speed parameters. Penetration times were obtained by the analysis of photographs captured by the high speed camera. For the specimen of 20mm in thickness, the shortest penetration time was achieved at the nozzle lowering speed of 0.5 mm/s. For the specimen of 30mm in thickness, the rock could be penetrated not at the nozzle lowering speed of 0.5 mm/s, but 0.4 mm/s at which the shortest penetration time was obtained.

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Observation of lased hole- section Fig.13, 14 show specimens obtained in the experiments conducted in the transparent water. Comparative observations were made of two rock specimens obtained from the experiments in the transparent water, one 20mm thick, the other 35mm thick. Both specimens were placed at a 50mm depth from the water surface. The completely through hole was successfully created in the 20mm rock specimen, while the hole in the 35mm rock specimen failed to penetrate all the way through. Both specimens were sectioned for closer observation. The portion enclosed by the line in Fig.14 (a) shows the rock that was sectioned. The walls of the lased holes were coated with thin layers of glass. The through-hole specimen had a hole about 3mm in diameter with a comparatively linear wall. The glass coating the wall was thin and light blue. Fig. 14 (b) shows the hole-plugged specimen that had a hole about 1mm in diameter and about 5mm at maximum. Both specimens were partly covered with glass in the plasticity domain that was easily removable with a nail. The plasticity was greater in the hole-plugged specimen than in the through-hole specimen. For both specimens, a white powdery material was observed in the plasticity domain, showing that a scratched quartz/feldspar was induced by the different thermal expansion rates of the mineral particles contained in granite.

Generating a hole in granite submerged in the 4wt% bentonite solution. The experiment of irradiating Inada granite

submerged in a 4wt% bentonite solution with CO2 laser was conducted (Fig 12 (a)). The demension of the specimen is 50mm in thickness by 70mm in width and 70mm in length. Since CO2 laser has the high absorption rate in water, irradiating CO2 laser can induce bubbles. We found that the granite could be drilled even in the opaque bentonite solution by using the bubbles generated by CO2 laser irradiation (Fig 12 (b)). A cavity in the granite could be created. The dimension of the cavity is 47mm in maximum depth, 15mm in maximum diameter (Fig 15 (a)). The irradiation of CO2 laser melted quartz glass and SiO2, which were contained in the granite. The molten quartz glass and SiO2 were made into small particles that could be easily removed from the cavity. Fig 15 (b) shows glass beads collected from the chamber after the experiment. The diameters of the glass beads range approximately from 0.2 to 3.0 mm. This is because the collapsing pressure of the induced bubbles and a shock wave made the molten quartz glass and SiO2 dispersed. The laser-induced bubble removes molten rock from the cavity, preventing the molten rock from solidifying in the hole. A complete cavity could be thus created in the granite submerged in the 4wt% bentonite solution. Conclusions 1) A high speed video camera was employed to observe the dynamic behaviour of bubbles induced by a CO2 laser beam having a high water absorption rate. The bubble length depends greatly on the focused position of the laser beam. The deeper the laser beam is focused into the water, the farther the laser beam can propagate through the induced bubble. 2) CO2 laser irradiation at a wavelength with a high liquid absorption rate induces bubbles in opaque water. A very well defined hole in granite can be created by employing the bubbles formed in the opaque water. 3) For the plumes generated by irradiating granite with CO2 laser in the air, the plume was hampered from ejecting from the generated hole by rock vapor pressure during the laser irradiation. The ejection of the plume occurs after the completion of laser irradiation and closes the hole created in the rock. For the plumes generated by irradiating granite with CO2 laser in the water, in contrast, the ejection of the plume occurs during laser irradiation. The pressure of collapse of the induced bubbles causes the ejection of plumes from holes created in granite. 4) The granite could be drilled not only in the transparent water but also in the opaque bentonite solution by CO2 laser irradiation that can effectively induce bubbles. The induced bubble removes molten rock from a hole, preventing the molten rock from solidifying in the hole. This results in a complete cavity in the granite. Acknowledgement This paper was prepared based on a joint research conducted by Japan Drilling Co., Ltd and Tohoku University, which has been carried out as a part of JOGMEC TRC-led (Japan Oil, Gas and Metals National Corporation,Technology Research Center) project titled Development of Laser Drilling System. The authors would like to extend their thanks to JOGMEC TRC for providing a chance to present our result of the joint research and would like to express our thanks to the Geoscience Research Laboratory for their cooperation. References [1] Ramona M. Graves. (1998) Targeted Literature Review, GRI-98/0163 [2] B.C. Gahan. (2001) Laser Drilling, Determination of Energy Required to Remove Rock, the 2001 SPE annual technical conference and exhibition, New Orleans, SPE 71466. [3] R.M. Rauenzahn, J.W. Tester. (1989) Rock Failure Mechanism of Flame-Jet Thermal Spallation Drilling-Theory and Experimental Testing, Int. J. Rock Mech., Min. Sci. & Geomech. Vol. 26, No. 5, 381-399. [4] M. Sasada: (1991) Inada Granite, Chishitsu News no. 441, 34-40. [5] K. Ikeda, K. Nishimura, T. Satoh, M. Sato. (2000) Development of laser associated cutting method for dangerous rock slope, Proceedings of the 30th Symposium on Rock Mechanics, 203-207.

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Fig. 1 Experimental Set-up for irradiation of CO2 laser.

Fig.2 Time variation of bubble length and width (: bubble length (Lf=0mm), : bubble length (Lf=50mm), : bubble width (Lf=0mm),

: bubble width (Lf=50mm)).

Elapsed time (ms)Elapsed time (ms)

Bub

ble

wid

th (m

m)

Bub

ble

leng

th (m

m)

Water chamber

Transparent water or opaque water

Trigger signal cable

High speed camera

CO2 Laser Oscillator

50mm

Mirror

Granite

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Fig. 3 Sequential photographs of CO2 laser-induced bubble phenomena (laser focal position Lf=50mm, laser energy: 250J/pulse, laser frequency 10Hz).

Fig. 4 Measured temperatures of the induced bubble by the thermocouple at the time of bubble collapse.

(a) t=1ms (b) t=4ms (c) t=7ms (d) t=10ms (e) t=13ms (f) t=16ms (g) t=19ms (h) t=22ms

(i) t=25ms (j) t=28ms (k) t=31ms (l) t=34ms (m) t=37ms (n) t=40ms (o) t=43ms (p) t=46ms

(a) Measured temperature of the induced bubble by the thermo couple

(b) Induced bubble reaching the tip of the thermocouple

Indu

ced

bubb

le le

ngth

0 5 10 150

500

1000

1500

2000

2500

Temperature (C)

Elapsed time (sec)

2371.79 C 2459.94C

50m

m

Thermocouple

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t=0ms t=5ms t=10ms t=15ms t=20ms t=25ms t=30ms

t=35ms t=40ms t=45ms t=50ms t=55ms t=60ms t=65ms

t=70ms t=75ms t=80ms t=85ms t=90ms t=95ms t=99.8ms

Fig. 5 Dynamic behaviour of plume generated by irradiating granite with CO2 laser in the air, taken by high-speed camera at 5000f/s (P=5.0kw, Pulse width 50ms, 250J). Sequential photographs exhibit behaviours of the plumes in a hole generated in the granite in the air. The granite significantly melts to generate a plume at the elapsed time of t=5ms. For t=550ms, the region where brightness is high becomes larger, and rock vapour generated by melting the granite is observed to be rising. The rock vapour indicates that the rock sample is being irradiated with the laser. For this period, the length of the rising rock vapour becomes longer as the time elapsed. In contrast, no plume coming out of a generated hole is observed (see photographs (t=550ms)). This is because the plume is pushed down in the generated hole by the rock vapour pressure. The region with high brightness is becoming smaller at t =55ms. When the laser irradiation ceases, the rock vapour pressure is lost, causing the plume to come out of the generated hole. The photographs (t=8590ms) show the plume coming out of the generated hole. The plume comes out of the hole in a 35 to 40mss time after the laser irradiation ceases. Consequently, it is concluded that the plume coming out of the generated hole was hampered by rock vapour pressure during the laser irradiation.

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Fig. 6 Dynamic behaviour of plume generated by irradiating CO2 laser in the water, taken by high-speed camera at 5000f/s (P=5.0kw, Pulse width 50ms, 250J). The granite in the water significantly melts to generate a plume at the elapsed time of t=1.6ms. For t=3.2ms, particulates (beads) are observed to disperse. For t=4.8ms, the region where brightness is high becomes larger. For t=6.4ms, the region with high brightness is separated into small portions. The plume then grows gradually for t=8, 9.6ms. For t=14.4ms, the region with high brightness rapidly expands toward the water surface. Subsequently, the expansion of the plume nearly levels out up to t=19.2ms. Next, the enhanced expansion is observed from t=20.8ms to t=28.8ms. The explosive behaviour of the plume that upwardly ejects is observed in this duration (t=20.8ms to 28.8ms). The region with high brightness is becoming smaller at t=30.4ms. From this time to t=36.8ms, the ejection of the plume gradually disappears. The explosive ejection of the plume occurred in a 20mss time after the plume was generated at t=1.6ms, persisting for 8ms. Consequently, it is understood that the ejection of the plume occurred explosively in underwater laser irradiation.

t=0ms t=1.6ms t=3.2ms t=4.8ms t=6.4ms

t=8ms t=9.6ms t=11.2ms t=12.8ms t=14.4ms

t=16ms t=17.6ms t=19.2ms t=20.8ms t=22.4ms

t=24ms t=25.6ms t=27.2ms t=28.8ms t=30.4ms

t=32ms t=33.6ms t=35.2ms t=36.8ms t=38.4ms

10mm

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Fig. 8 Comparison of dynamic behaviour of molten granite taken by high-speed camera at 5000f/s, P=5.0kw, Pulse width 50ms, pulse energy, 250J).

Fig. 9 Dynamic behaviour of plume coming out of the hole after the completion of the laser-irradiation in the air

(taken by high-speed camera at 5000f/s, P=5.0kw, Pulse width 50ms, pulse energy, 250J).

in water

in air

(a1) (a2) (a3) (a4) (a5)

(b1) (b2) (b3) (b4) (b5)

in water

in air

(a6) (a7) (a8) (a9) (a10)

(b6) (b7) (b8) (b9) (b10)

t 1 =0ms

t 2=2.6ms

t3=5ms

t4=8.4ms

t5=16ms

t 6=22.8ms t 7 =26.4ms t8=28.6ms t9=30.6ms t10=33ms

10mm

(b11) (b12) (b13) (b14) (b15)

t 11 =66ms t 12=69.4ms t13=73.6ms t14=74.6ms t15=76.8ms

in air 10mm

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Fig. 10 Comparison of the height of plumes generated in the air and in the water (:in the air, :in the water).

t=0s t=81.236s t=81.240s t=81.244s t=81.248s t=81.252s

Fig.11 Photographs of the granite submerged in transparent water during irradiating laser (Specimen thickness: 30.10mm).

Fig 12 Drilling granite submerged in 4wt% bentonite solution. Irradiating Inada granite submerged in the 4 wt% bentonite solution with CO2 laser was conducted (a). The dimension of the specimen is 50mm in thickness by 70mm in width and 70mm in length. The granite could be drilled even in the opaque bentonite solution by CO2 laser irradiation. A cavity in the granite could be generated. The dimension of the cavity is 47mm in maximum depth, 15mm in maximum diameter (b).

(a) Irradiating the granite submerged in opaque water (4% bentonite solution) with CO2 Laser.

(b) The processed granite after displacing the bentonite solution from the chamber

12 SPE-113177

Fig. 13 Top view of rock specimens 30mm long, 30mm wide, and 20mm high.

Fig.14 Hole sections of rock specimens lased in transparent water.

((1): lased hole, (2): molten glass, (3): plasticity domain).

Fig. 15 Granite lased by CO2 laser in opaque water ( output power 5kW, repetition rate 10Hz, duty ratio 50%, pulse energy 250J/pulse, laser spot diameter 10mm at granite surface).

(1)

20mm

(3)

(2)

(1)

35mm

(3)

(2)

(a) Through-hole specimen (b) Hole-plugged specimen

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remove

glass beads

(a) Through-hole specimen (b) Hole-plugged specimen

(a) Lased hole: 47mm in maximum depth, 15mm in maximum diameter in granite specimen 70mm long, 70mm wide, and 50mm high

(b) Glass beads collected in water chamber after the experiments: minimum diameter 0.2mm, maximum diameter 3mm

3mm

70mm