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SPE 104223 Lasers: The Next Bit Shahvir Pooniwala, SPE, Maharashtra Inst. of Technology
Copyright 2006, Society of Petroleum Engineers This paper was prepared for presentation at the 2006 SPE Eastern Regional Meeting held in Canton, Ohio, U.S.A., 11–13 October 2006. 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, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes 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 where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836 U.S.A., fax 01-972-952-9435.
Abstract Since the early twentieth century rotary drilling has revolutionized the procedure of extraction of crude oil replacing conventional methods like cable tool drilling. However with the advent of new technology it is time to look at future alternative, more efficient drilling methods. This paper acts as an eye opener to the feasibility of using laser drilling over modern currently used drilling techniques. The design and operation of a new laser-mechanical bit is put forth by the medium of this paper. This innovative bit shows probability of reducing rig time and increasing efficiency in drilling. The possible changes to be implemented in the present day drill string due to incorporation of this new bit is accounted for and an analysis of the possible advantages and disadvantages of this bit if implemented is also highlighted.
Introduction Rotary drilling has been widely used for extraction, in most of the oil fields in various parts of the world for more than a century. During this period many alternatives drilling techniques have been suggested, worked upon and tried so as to reduce the time and increase the efficiency of drilling. These techniques include the use of niche technology with tools commonly known as novel devices. This category of devices includes Water jets, Electron Beams, Cavitating Jets, Electric arcs, Plasmas and Lasers to name a few. In comparison with all the above devices, laser drilling if developed has shown the potential to be a futuristic advanced tool that will revamp the conventional rotary drilling system. LASER basically is an acronym for Light Amplification by Stimulated Emission of Radiation. It is basically a device which converts energy in one form to electromagnetic radiation beams (photons). These photons are basically produced due to the returning of atoms to their lower energy
state after their excitation to higher energy states. When this happens a photon is released. This high energy coherent light radiation can be focused to form intense high powered beams which can be used to fragment, melt or vaporize rocks depending on the input power, type of laser, adjusted focal length and interaction characteristics of the laser with the particular rock type. The other major laser parameters include discharge method (pulsed or continuous), wavelength, exposed time, pulse width, repetition rate, average power and peak power. These parameters determine the effective energy transfer to the rock. Lasers are currently being used as a potent tool with effective results, in various fields such as medical, metallurgical and for military applications. Currently lasers are widely used for precision cutting and welding of metals, ceramics and various other materials. Laser Drilling Majority of research in the field of laser drilling is focused on solely using a laser to vaporize the rock. These methods are proposed to have various advantages over currently used rotary drilling techniques which include: 1. Increasing Rate of Penetration (ROP)-Laser drilling shows the potential of having ROPs that is more than 100 times the presently ROPs 2. Provision of temporary casing 3. Reducing trip time and an increased bit life. 4. Lesser dependence on parameters such as weight on bit, mud circulation rate, rotary speed and bit design 5. Accurate and precise drilling since lasers travel in a straight line problems like dog legging are completely eliminated. 6. Providing enhanced well control, perforating and side-tracking capabilities 7. Single diameter bore hole 8. Achieving these breakthroughs with environmentally attractive, safe and cost effective technology1 Candiate Lasers Lasers of various types are available and are classified on the basis of their sources of production as Gas lasers, Dye lasers, Metal-vapour lasers, Solid-state lasers, semi-conductor lasers, Free-Electron lasers and Nuclear pumped lasers. However at present only a few lasers have been considered and short listed as suitable for drilling through rocks and tested with. These include:
1. Deuterium Fluoride (DF)/Hydrogen Fluoride (HF) 2. Free-Electron Laser (FEL) 3. Chemical Oxygen-Iodine Laser (COIL) 4. Carbon Dioxide Laser (CO2) 5. Carbon Monoxide Laser (CO) 6. Neodymium: Yttrium Aluminum Garnet (Nd:YAG) 7. Krypton Fluoride Excimer Laser (KrF) 8. Mid-Infrared Advanced Chemical Laser (MIRACL) 9. Direct Diode Laser1, 2 The main problem encountered in the present day scenario that prevents the commercial sole use of lasers for drilling is the size of extremely high powered lasers. Hence until compact lasers with enough power to vaporize the entire rock mass can be commercially manufactured, a LASER-MECHANICAL bit can be used. Theory Specific energy is useful for predicting the performance and power requirements when only lasers are used as the sole rock removal device. Specific energy, E, is defined as the amount of energy required to remove a unit volume of rock. It is calculated as;
P = P dV/dt dws
From specific energy which may be calculated from laboratory or field tests the drilling rate, R of a system can be calculated as;
However, when lasers are used in tandem with each other the term defined as Specific Kerfing Energy is used. When a combined laser-mechanical system is used, the rate equals;
1 ⎛ PL PM ⎞ ⎜ + ⎟ A ⎝ EL EM ⎠
Where, PL and Pm are the power inputs of laser and mechanical bit in watts and El and Em are the corresponding specific energies. In a laser-mechanical system the laser is used primarily to unsupport the rock so that it is easier to mechanically drill through the rock. It unsupports the rock prior to mechanical drilling reduceing the Em of the rock by more than half thus increasing the drilling rate. However it is found that since the calculated specific energy for a device may be same and cannot be used to differentiate for different kerfing situations say a deep narrow kerf and a broad shallow kerf. Specific energy is not an accurate measure of kerfing ability.Hence we use the term Specific Kerfing Energy (SKE). It is defined as the power per kerf depth multiplied by the speed the cutting mechanism is moving across a rock surface which is given by;
Power KerfDepth × TraverseSpeed
A comparison of specific energies using different drilling methods as well as using different types of lasers is provided (Table 1 and Table 2). As well as a range of the Specific Kerfing Energies required by various kerfing devices are given (Table 3). There are broadly four basic rock removal/disintegration mechanisms. They are: 1. Melting and vaporization which take place when the rock is subjected to temperatures above its melting point. 2. Thermal spalling which occurs due to heating of the rock surface rapidly producing high amount of stresses which exceed the strength of the rock. 3. Mechanical breakage which is brought about by mechanically drilling the rock. 4. Chemical reactions which occurs when chemicals that dissolve rocks are used.3 Spalling High powered lasers can weaken, spall, melt, and vaporize rocks, with thermal spallation being the most energy-efficient rock-removal mechanism. Laser rock spallation is a rockremoval process that uses laser-induced thermal stress to fracture the rock into small fragments before it melts. When high intensity laser energy is focused on a rock that has very low thermal conductivity, it causes the local rock temperature to increase instantaneously. This results in a local thermal stress that spalls the rock. Previous test data shows that laser rock spallation is the most energy efficient among all laser rock removal mechanisms and also has a higher rock removal rate when compared to conventional rotary drilling and flamejet spallation. Work has shown that application laser radiation to rock causes a significant decrease in rock mechanical strength due to an increase in the microcrack structure and resulting tensile stress field by the heat flow. Kilowatt level CO2 electric discharge convection lasers are successfully used for weakening rock by an unfocused beam or kerf-cutting rocks by a more intense energy beam. The bulk of the lased rock could be then removed by mechanical means. Investigation has shown that current lasers are more than sufficient to spall, melt or vaporize any lithology that may be encountered in the oil well drilling.1,2 Recently, kilowatt CO2 laser and pulsed Nd:YAG laser with better process parameter controls were used for testing. The specific energies, energy required to remove unit volume of rock, were measured for different lithologies and have shown a great agreement to theoretically calculated values. Different laser/rock interaction mechanisms from vaporization, melting, and spallation to weakening were shown on rock slab samples when the laser power density continuously reduced along the laser track. It was shown that rock spalling caused by laser-induced thermal stress is the most efficient mechanism.4
Laser Removal Rock Mechanic Basics When laser energy is applied to the rock, a temperature field in the rock is created. The temperature distribution during the initial period of lasing can be obtained by using the simplified heat conduction model by Carslaw and Jaeger6 which assumes portion of the rock beneath the constant laser beam to be part of a semi-infinite, homogeneous, elastic solid. The temperature distribution is given as:
T ( z, t ) =
2q Kt ⎡ z ⎤ ierfc ⎢ ⎥ k ⎣ 2 Kt ⎦
Where, T = Temperature at location z of the solid z = Normal distance into the rock from its surface (m) q = Constant laser energy flux (watts/cm2) K= Thermal diffusivity of the rock = k/ρc (m2/s) k = Thermal conductivity of the rock (W/m°C) p = Density of the rock (kg/m3) c = Specific heat of the rock (J/kg°C) t = Time from start of lasing (s) ierfc= Integral of the complement of the error function. The laser-induced stresses in the rock caused by the temperature distribution in the above equation are given as:
σx = σy =
EαT (1 − υ )
Where, E = Young's modulus (MN/m2) α = Coefficient of linear thermal expansion (0C-1) T = Temperature as calculated from equation (1) ν = Poisson's ratio The stress is proportional to the temperature and the value of the stress for any given temperature increases with increasing values of Young's modulus, the coefficient of expansion and Poisson's ratio. This stress is quite significant in the rocks that have low thermal conductivity and high thermal expansion. Spalling/cracking forms when the laser-induced stress from equation (2) just beneath the surface reaches the critical strength of the rock. If the mechanical and thermophysical properties of the rock exposed to the laser beam are available, one can use equation (2) and (1) to find the laser beam flux needed for generating rock spallation. Though the finite element technique ought to be used to determine the temperature and the corresponding thermal stresses for a rock that is not completely homogeneous and isotropic as assumed in the above simplified thermal stress analysis model, results from the analysis model provide only guidelines for selection of process parameters for laser spalling of rock.6 Similar to this model different models have been developed for each of the physical phenomena based on the finite difference method, then combining them into one numerical procedure using the Constrained Interpolation Profile Combined and Unified Procedure (CCUP) method is currently
being proposed for calculating the spalling parameters. CCUP based on FDM is developed to simulate large deformation of materials, fragmentation, multiphase problem and fluidstructure interaction problem. With this approach, the transient temperature and stress distributions in dry or watersaturated rocks exposed to a laser beam have been calculated. The spallation boundary and rock removal energy efficiency have been determined for different laser conditions. The modeling results provide a better understanding of laser rock spallation phenomenon and most importantly, guidelines for selecting processing parameters for fast rock removal.5, 10 Laser Mechanical Bit The laser mechanical bit works on the principal of first spalling the rock using a laser beam. It has been proved that temperatures induced by lasers weaken the rock. This is due to fracture development, mineral dehydration and vaporization that results in an increase in the void space. It is found that when compared to the unlased portion the various moduli such as Youngs modulus, shear modulus, bulk modulus and combined modulus of the rock were reduced (Table 4). This weakened rock is then drilled through using normal presently used mechanical bit techniques. This is achieved at a faster more efficient rate. The laser characteristics can be adjusted from surface depending on logging information to suit the formation characteristics. Beam Delivery Configurations Beam delivery configurations refer to transferring the produced high energy laser beam down hole. In various laser machines the most common method used is using mirrors. However due to the fragile nature of mirrors they cannot be used for rock drilling. The two feasible beam delivery configurations are: 1.
Assembly at surface: This configuration consists of the laser production apparatus at the surface. The produced laser beam is transferred down-hole using sturdy fiber optic cables widely used in various industrial laser machines. Though this assembly sounds relatively simple, the beam transfer via fiber optics could practically pose a problem as using fiber optics over such a large distance of the entire drilling depth for beam delivery has not been tried. However in applications like down-hole cameras optical cables have been a tried and tested method. Hence this system shows a great promise. After transfer of the beam from surface via the cable, the beam is fed into the laser head which is seated as an integral part into the normal currently used mechanical bit (Figure 1 and Figure 2).
Down-hole assembly: Some lasers such as Direct Diode Laser are compact enough to put the entire laser mechanism downhole. The basic requirement for functioning of a laser is electric supply to the laser tube. This electric supply can be produced using a down-hole motor. These motors which are widely
used in applications such as geo-steering and directional drilling are capable of giving an output of more than a kilowatt. In most laser types the tube is a place where various forms of energy are use to produce photons. It is in the tube that the lasing mediums are exited emitting photons. These photons are then transferred to the head from where it is focused on the rock surface (Figure 3). Laser Head The laser head is the final stage of the laser production system. It is basically an outlet for the laser beam to the rock. It consists of a converging or diverging lens to adjust the beam properties. This lens helps control the power, area of exposure (beam diameter) and distance of beam focusing from bit onto the formation. As an integral part of any drilling laser head a suction pipe is always present. This pipe clears the particles that are given out from the drilled area. These particles if not cleared may clog the laser lens reducing the laser efficiency. The design of the currently used mechanical bit must be modified such as the laser head perfectly seats into the bit. Care should be taken during designing that the laser head is perfectly aligned with the opening provided at the center of the bit, so as no obstruction to the beam is encountered. The laser head can be effectively incorporated in both rotary (tri-cone bits) as well as fixed cutter type of bits (PDC, Diamond impregnated, Hybrid and Natural Diamond bits). Hence, giving a wide range of combinations (Figure 4), depending on the drilling environments and formations that may be encountered. Additional Accessories A few additional equipments would have to be added for optimum performance of the laser-mechanical bit. These include: 1.
Focusing/De-focusing lens: this is an additional lens which can be used for finely adjusting the laser beam. The focusing lens converges the beam reducing the beam diameter, where as the de-focusing lens scatters the beam enabling an effectively larger beam diameter. Shutter: this is similar to the shutter found in cameras. It is it made of tough metal preferably the same composition as the bit and is located at the base of the bit acting as a gate. The main purpose of providing a shutter is to protect the laser when the laser is not in use during operations like tripping in and tripping out of the drill string. It prevents formation, debris and fluids from entering the laser head. Also protecting the head from back pressures and well kicks. Downhole cables: these are required for various miscellaneous operations such as to control the laser, supply power and get feed back information Micro processers/Computers: The laser functioning properties such as periodicity of laser shot, intensity of shot, etc. can be controlled by pre-programmed micro processors or via surface computers.
Advantages of Laser-Mechanical Bit 1. Faster ROP 2. Lesser wear of bit hence longer bit life 3. Reduction in number of trip times into and out of the hole. 4. Applicable successfully in all type of drilling operations. (Horizontal / Vertical / Directional). 5. The same laser production apparatus can be re-used for drilling a number of wells Conclusions Use of lasers for drilling was a realm not ventured into by many, this was because most people had dissenting and contradictory views towards laser drilling based on limited laboratory studies and experiments conducted 30 years ago, when lasers were at their infancy. The lasers used during that period had very low power, were difficult to focus, incapable of transmitting power over large distances, non-portable and largely unsafe. However with the advent of new developing technology in the field of lasers most of the above mentioned problems are put to rest. Recent tests have also proven laser spalling as the most efficient compared to other currently used rock removal techniques. Detailed models are also available that give accurate parameters with regard to laser spalling. Until high powered compact lasers which can fit downhole capable of vaporizing rocks can be commercially manufactured, a Laser-Mechanical bit can be an effective alternative technique of exploiting the merits of laser spalling to give an optimum drilling mechanism. However as with any new concept an initial feasibility study is very essential. Recommendations 1. An in-depth study on the technicalities of the delivery system should be undertaken. 2. Candidate lasers should be further short listed. 3. An initial cost assessment on the modifications to the drill string will have to be done. Since about 20% of rig time is spent in tripping operations and about 50% time is spent in reaching deeper depths it is logical that faster ROP and lesser bit wear will reduce the cost. However, will the higher initial cost be feasibly recovered by the rig time saved cost. 4. A study of the effects of vibrations on the fiber optic cables and a risk analysis in event of a failure to the optical cable will have to be made. 5. A problem may occur with drilling mud dehydrating and turning powdery due to the heat generated by the laser. As a remedial measure transparent drilling fluids may have to be used. Another solution could be using inert gases such as nitrogen as drilling fluids.
Nomenclature E=Specific Energy (J/cm3) P=Power Input (Watts) dV/dt= Volume Time Derivative (cm3/sec) d=Kerf Depth (cm) w=Kerf Width (cm) s=Traverse Speed (cm/sec) A=Hole Cross-Section Area (cm2) SKE= Specific Kerfing Energy (J/cm2) References 1. Graves, R.M. ,and. O'Brien, D.G.: "Star Wars Laser Technology Applied to Drilling and Completing Gas wells", SPE 49259, 1998 2. Graves, R.M., O'Brien, D.G. and O’Brien, E.A.: “Star Wars Laser Technology for Gas Drilling and Completions in the 21st Century” SPE 56625, 1999. 3. Maurer, W.C.: “Advanced drilling Techniques”, Petroleum Publishing Company, Tulsa (1980). 4. Graves, R.M., Gahan, B.C., Parker, R.A. and Araya, A.:“Comparision of Specific Energy Between Drilling With High Power Lasers and Other Drilling Methords” SPE 77627, 2002. 5. Bybee,K.: “Modeling Laser-Spallation Rock Drilling”, JPT(Feb 2006) 62 6. Xu, Z., Reed, C.B, Parker, R., and Graves,R.: “Laser spallation of rocks for oil well drilling”, Proceedings of 23rd International Congress on Applications of Laser & Electro-Optics, October 4-7, 2004, San Francisco, California. 7. Graves , R.M. , Gahan, B.C. , Parker , R.A. and Batarseh, S ,: “Temperature Induced by High Power Lasers: Effects on Reservoir Rock Strength and Mechanical Properties” SPE/ISRM 78154, 2002 8. Carstens, J.P., and Brown, C.A.: "Rock Cutting by Laser" SPE paper No. 3529, 46th Annual Meeting of the SPE, New Orleans, Louisiana (October 3-6, 1971). 9. Gahan, B.C. , Parker , R.A. , Batarseh, S., Figueroa, H., Reed ,C.B and Xu, Z.: “Laser Drilling: Determination of Energy Required to Remove Rock” SPE 71466, 2001. 10. Zhiyue Xu, Yuichiro Yamashita1, and Claude B. Reed,:“TWO-DIMENSIONAL MODELING OF LASER SPALLATION DRILLING OF ROCKS”,P532 11. Bensson, A.,Burr,B.,Dillard, S., Drake,E., Ivie, C. Ivie,B., Smith,R., and Watson,G.: “ On the Cutting Edge” ,Oilfield Review (Autumn 2000)36
Conn & Radtke, 1977
High Pressure Water Jet
High Pressure Water Jet
Summers & Henry, 1972
High Pressure Water Jet
Summers & Henry, 1972
Rotary Roller Drag
Graves, et al., 2002
Graves, et al., 1999
Graves & Batarseh, 2001
Figueroa, et al., 2002
Table 1: Specific Energy of Drilling Methods
Power Density (kW/cm2)
*Wright - Patterson Air Force Base, Ohio, USA. **Lebedev Radiophysics Institute, Moscow, Russia.
Table 2: Specific Energy of Different Lasers for Berea Sandstone4
SPECIFIC KERFING ENERGY (J/cm2)
250 – 500
1000 – 2000
3000 – 6000
20000 – 40000
50000 – 100000
Table 3: Specific Kerfing Energy of Drilling Methods3
Young's modulus (E) psi x106
Poisson's Ratio (ν)
Shear Bulk modulus Bulk compressibility Combined modulus modulus (G) (K) (Cb) (λ+2G) psi psi psi psi x106 x106 x10-6 x1012 LASED
Berea Yellow Sandstone
Berea Grey Sandstone
Mesaverde Shaly Sandstone
UNLASED Berea Yellow Sandstone
Berea Grey Sandstone
Mesaverde Shaly Sandstone
Table 4: Comparision of Elastic Moduli of Lased and Unlased Sample7
Figure 1: Laser-Mechanical Tri-cone Rotary Bit with fiber optic delivery system
Figure 2:Cross-section of Laser-Mechanical Tri-cone Rotary Bit (Modified after Reference11)
Figure 3: Laser-Mechanical Fixed Cutter Bit (Modified after Reference11)
Figure 4: Laser-Mechanical Tri-cone Rotary Bit with Down-hole configuration