Laser Drilling Using Nd:YAG on Limestone, Sandstone and Shale Samples: ROP Estimation and the Development of a Constant ROP Drilling System

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1 Cairo University From the SelectedWorks of Seif-Eddeen K Fateen 2015 Laser Drilling Using Nd:YAG on Limestone, Sandstone and Shale Samples: ROP Estimation and the Development of a Constant ROP Drilling System Seif-Eddeen K Fateen Available at:

2 SPE MS Laser Drilling Using Nd:YAG on Limestone, Sandstone and Shale Samples: ROP Estimation and the Development of a Constant ROP Drilling System Ahmed Hafez, Ibrahim El-Sayed, Omar El-Said, Seif-Eddeen Fateen, and Karam Beshay, The American University in Cairo, Egypt Copyright 2015, Society of Petroleum Engineers This paper was prepared for presentation at the SPE North Africa Technical Conference and Exhibition held in Cairo, Egypt, September 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 Background/Scope: Mechanical drilling, the gold standard to reach oil and gas reservoirs, is associated with drilling risks and increased time and costs. Laser drilling has emerged as a potential new drilling tool. However, studies about its practical applications are limited. This study investigates the feasibility and performance of laser technology in drilling oil wells. Methods: Core samples from the three lithologies were irradiated with Nd:YAG laser at different power intensities and exposure times while holding other parameters constant. Some core samples were saturated with brine to investigate the effect of fluid saturation. We developed a novel laser drilling system that enables users to input the lithology of encountered formations and output the power intensity needed to be drawn from the electric source to maintain the constant ROP. Results: The Nd:YAG laser beam fully penetrated the cores in all the study trials conducted in shale. In limestone, the laser beam fully penetrated the cores in 5/13 trials and partially penetrated them in 7 trials. The experiments could not be performed in sandstone due to rough surfaces and high porosity. The ROP values ranged between 58 and 66 ft/hr (mean ft/hr) in shale and between and ft/hr (mean ft/hr ft/hr) in limestone. The specific energy values ranged between 650 and 5433 KJ/cm 3 (mean KJ/cm 3 ) for the limestone samples. A statistically significant inverse correlation was observed between laser energy and ROP (r , P 0.001). The developed software was tested and validated. Added value: The study provides further evidence that laser drilling technology is a promising new technology for oil well drilling. The current study utilized high specific power laser which has not been previously used and developed a novel computer based constant ROP laser system. The rock removal mechanism should be shifted away from melting to spallation to achieve an efficient laser drilling mechanism through the optimization of the laser specific power and decreasing the specific energy during rock removal. Laser beam reflection, sample thickness and fluid saturation were identified as potential factors that influence the efficiency and performance of the laser drilling operation.

3 2 SPE MS Introduction To date, oil and gas reservoirs remain solely accessible via conventional drilling using mechanical rotating drill bits. Despite the evolution of this technology and its well proven reliability and performance, mechanical-bit drilling techniques pose various risks and challenges the most important of which is the rate of penetration (ROP). A small ROP extends the time needed to drill oil or gas well resulting in increased costs as drilling rigs are rented by the day. Thus, several attempts were made to achieve ROP values higher than those currently obtained with mechanical bits. Achieving a higher ROP will accelerate well drilling and reduce rig rental costs. In the long term, more wells can be drilled in the same time frame, and field development plans can proceed in a timely manner or even ahead of schedule, thus maximizing oil and gas production and profits. Maintaining constant ROPs using mechanical bits is difficult due to its dependency on several factors such as operating conditions, drilling fluid rheological properties, and formation & bit characteristics. The non-uniform ROP increases the risk of deviating from the well trajectory or drilling program and makes it difficult for the driller to respond to the erratic and unpredictable down-hole conditions in a timely manner resulting in prematurely or unknowingly entering an over-pressurized zone. Thus, maintaining a constant ROP will ensure that the drilling program will be followed and field development will proceed as scheduled resulting in saving extra costs that would otherwise have been allocated for extended rentals and service crew standby, and will accelerate field production. Several drilling techniques have been suggested to replace mechanical bits and improve ROP. Laser drilling has recently emerged as an attractive alternative. Several laser types such as CO2, COIL and Nd:YAG have been suggested. ROP obtained by laser is a function of the type of laser, the laser power intensity and the type of formation. Other parameters influencing ROP by laser may include laser beam diameter, the distance between the laser source and the penetrated sample and the beam s angle of incidence. Thus, the current study investigated the efficacy and performance of laser drilling as a potential alternative drilling technique. Specifically, the current study aimed to Investigate the effectiveness, feasibility, and performance of drilling an oil well using Nd:YAG laser Determine factors that affect laser-rock penetration Estimate potential ROP values across the most predominantly encountered formations in oilfield drilling using Nd:YAG laser Develop a laser drilling system capable of delivering a constant ROP at minimum electric power input. Statement of Theory and Definitions There are three fundamental ways in which incident electromagnetic (E incident ) waves can interact with a solid body, namely reflection (E reflected ), scattering (E scattered ), and absorption (E absorbed )(Gahan et al. 2001): (1) Logically, minimizing the percentage of waves reflected and scattered will maximize the effectiveness of the desired laser rock interactions: absorption. When energy is absorbed by the rock, it can spall, melt, or vaporize (Elahifar et al. 2011). As radiation is absorbed, the grains in contact with the beam quickly increase in temperature and expand by a small yet significant amount. This expansion across the surface of the rock causes grains to collide, crack, and eventually fracture off in small kerfs. If the energy being absorbed is greater than the energy dissipated by the rock, the local temperature will quickly rise to the melting point (Gahan et al. 2001). The ultimate

4 SPE MS 3 product is a smooth glass-like melt that coats the walls of the hole (Elahifar et al. 2011). If temperature continues to increase rapidly, the rock will begin to vaporize. Spallation is the most desirable method of interaction as it maximizes the energy directed towards breaking off cuttings. To maximize efficiency of a laser system, it is necessary to increase the amount of absorbed rays that spall the rock. However, there are several factors that affect the degree at which spallation occurs such as formation type, laser type, beam continuity, beam specific power, duration of exposure, fluid saturation, and the purging gas used. Formation Type Different mineral and chemical compositions react differently to the incident radiation affecting the range of laser power intensities at which spallation occurs. Elahifar et al. (2011) has demonstrated that granite can withstand higher laser power intensities while remaining in the spallation zone than sandstone which began to exhibit lower ROP, indicating a shift from spallation to melting, after increasing laser power intensity past 4.8 kw. Granite, on the other hand, continued to have a higher ROP as laser power increased past 4.8 kw, indicating that it remained in the spallation zone. Bjorndalen et al. (2003) observed that the melting region in limestone is larger than that in sandstone. Shale by its nature contains significant amounts of bound water that absorbs amounts of radiation that would otherwise be used to spall the rock, thus reducing the effectiveness of laser radiation on shale (Sinha and Gour 2006). Laser Type High power laser beams are needed to drill through rocks. Sinha and Gour (2006) and O Brien et al. (1999) list seven laser candidates, namely; Deuterium Flouride Laser (DF), Free-Electron Laser (FEL), Chemical-Oxygen Iodine Laser (COIL), Carbon Dioxide Laser (CO 2 ), Carbon Monoxide Laser (CO), Neodymium: Yttrium Aluminum Garnet (Nd:YAG)and Krypton Fluoride (KrF). The major factor in selecting a laser type is the produced wavelength, which ranges from 0.1 mto100 m. This factor, along with the formation type, is the most significant determinant for deciding the quantity of absorbed radiation and in turn the degree of spallation. Beam Continuity Incident laser beams can either be continuous or pulsed. Elahifar et al. (2011) showed that a continuous beam yields higher ROP than a pulsed beam, keeping all other parameters constant. However, it was noticed that a continuous laser beam caused a shift to the melting zone much sooner than a pulsed beam. When a pulsed laser was used, the duration in which spallation remained dominant increased. Beam Specific Power The important factor for ROP is not the overall power of the laser but the specific power (SP) which is defined as the laser power per cross sectional unit area of the beam (Gahan et al. 2001) expressed as (2) As the specific power increases, more energy is transferred to the rock and ROP increases up to the point where the rock removal mechanism shifts from spallation to melting. Duration of Exposure As the laser beam exposure duration increases, the probability of increasing the temperature past spallation increases. The effect is more pronounced in a continuous beam than a pulsed beam (Graves et al. 2002). Fluid Saturations The fluids within the rock pores absorb energy that would otherwise be utilized in rock spallation. Thus, a dry formation will exhibit higher ROP than a fully saturated formation (O brien et al. 1999).

5 4 SPE MS Purging Gas Unlike conventional drilling, drilling muds cannot be used with laser technology due to the high temperatures involved. Instead, a gas purging system is used to remove the cuttings. Elahifar et al. (2011) has demonstrated that the purging gas used to lift cuttings to the surface had an effect on the ROP, even though this effect was small. He also observed that COIL performs better within a nitrogen environment than an argon environment. Specific Energy Specific energy (SE) is used to compare different laser setups. It is defined as the power needed to remove a unit volume of rock (Sinha and Gour 2006). (3) The equation can then be rearranged to give Maurer equation: (4) Specific energy can be calculated from the specific power and the ROP of the setup. The specific energy calculated applies only for the parameters in which it was tested. Thus, SE will change if the specific power, the laser type, the formation, or any other factor is altered. In order to maximize ROP, it is thus required to minimize specific energy (Graves et al. 2002). Experiments involving laser drilling were performed with a variety of laser types, formations and parameters to identify the specific energy of the system. Thus far, such experiments have been conducted solely in labs on core samples and to date no field testing has been done. In this study, we tested the performance of Nd:YAG in penetrating different rock samples. Methods Sample Preparation Several core samples of different lithologies were used in this experiment. The rock samples included: Limestone Three limestone core sections, cm ( in) in thickness and 2.52 cm (0.992 in) in diameter, were used and labeled C1, C2 and C5 in addition to other smaller core sections, C9 and C10, with thickness 0.9 cm (0.354 in) and 1.3 cm (0.511 in). The rock mechanical properties were tested using a longer core section, CL2, with a thickness of cm (2.427 in). The lengths of these core sections were adjusted using the trimming machine. Shale Two shale samples were obtained and labeled F1 and F2. Sandstone A sandstone core section, 2 cm (0.787 in) thick and 2.52 cm (0.992 in) in diameter, was used in the experiment and labeled A4. Another core section, AS1, which was 6.7 cm (2.637 in) thick and 2.52 cm (0.992 in) in diameter, was used to measure the rock properties. Equipment The samples were prepared at the geo-mechanics lab of the department of petroleum engineering, the American University in Cairo (PENG-AUC). The core samples were subdivided into smaller sections and smoothened using the trimming machine (VINCI Technologies, Nanterre, France).Before measuring the rock properties, the core samples were cleaned by a Dean Stark Extractor (VINCI Technologies, Nanterre. France). The porosity of the samples was measured with a Helium Porosimeter AP121 (VINCI Technologies, Nanterre, France) (Fig. 1) and the permeability was measured using a steady state gas permeameter GasPerm Production 2Q1TT (VINCI Technologies, Nanterre, France (Fig. 2). The limestone core samples were saturated with the manual saturator (VINCI Technologies, Nanterre, France).

6 SPE MS 5 Figure 1 Helium porosimeter Figure 2 Gas permeameter Laser penetration was conducted at the laser facilities of the Central Metallurgical Research and Development Institute (CMRDI) in Tebin, Cairo, Egypt. We utilized the laser device TruLaser Cell 3010 manufactured by TRUMPF Laser Technology, Ditzingen, Germany. The type of laser used was Neodymium-doped yttrium aluminum garnet (Nd:YAG) with a wavelength of 1062 nm and maximum power intensity of 2 KW. The device was used in accordance with the manufacturer s guidelines and standards of procedures (SOP)(Fig. 3). Figure 3 Tru-laser cell 3010 laser device

7 6 SPE MS Rock Properties Measurements Sample Cleaning Before measuring the rock mechanical properties, the rock samples were cleaned to remove any impurities using the Dean Stark Extractor according to the manufacturer s guidelines and Standard of Procedure (SOP). Each sample was left in the device for more than 10 hours. After removal from the Dean Stark, the samples were placed in the drying oven to remove the solvent previously introduced to the sample while in the Dean Stark. Permeability We measured the permeability of core samples using a steady state gas permeameter according to the manufacturer s guidelines and Standard of Procedure (SOP). Porosity The porosity of core samples was determined using a Helium Porosimeter according to the manufacturer s guidelines and Standard of Procedure (SOP). Laser Irradiation We exposed rock samples of different lithologies to Nd: YAG laser irradiation at different power intensities and exposure times in 13 different trials. In all trials, the angle of laser beam incidence on the rock sample was 85, the beam diameter was 0.1 mm ( in) and the distance between laser source and sample was 2 mm ( in). Shale We performed trials 1 and 2 on shale sample F1, which was chosen due to its smooth surface and dimensions that were appropriate for the operation of the laser device while sample F2 was excluded from the analysis due to its rough surfaces and inappropriate size. In trials 1 and 2, the laser device was programmed to create a circular hole of 0.5 cm diameter by rotating the nozzle (laser source) about a fixed axis at a speed of 1m/min. This is equivalent to an exposure time of second. In trials 1 and 2, the sample F1 was dry and not saturated with any fluids. Dry Limestone In all trials performed thereafter, the laser nozzle (source) remained static. For safety reason and to prevent any possible damage to the laser device or nozzle, we had to start with a limestone sample of small thickness (0.9 cm) and then increase the thickness gradually to 2 cm. Trials 3 through 6 were performed on limestone samples C9, C10 and C5, which were dry and not saturated with any fluids.trials 11 through 13 were performed on limestone sample C5 to verify the results obtained in the first round of trials. Dry Sandstone In trial 10, we irradiated consolidated sandstone sample A4 with an 800W laser beam for 0.7 seconds. Sample A4 was dry and not saturated with any fluids. However, the sensor of the laser system could not detect the sample due to its rough surface and abundance of pores. As a result, we were unable to conduct our experiment on the sandstone sample. Fluid Saturation To better simulate actual conditions, we saturated limestone core samples C1 and C2 with brine with salinity 300,000 ppm using the manual saturator according to manufacturer s guidelines and SOP. Due to the low permeability of limestone, the two samples were left in the saturator for two days. Laser Irradiation-Phase 2 The second round of trials was also performed on saturated limestone using the same laser device at CMRDI. Trials 7 through 9 were performed on the limestone samples C1 and C2 which were saturated with brine. The conditions of trials 1 through 13 are summarized in Table 1.

8 SPE MS 7 Table 1 Experimental conditions for trials 1 through 13 Trial number Sample Lithology Saturation Power intensity (W) Duration of exposure (s) Angle of incidence 1 F1 Shale dry F1 Shale dry C9 Limestone dry C10 Limestone dry C5 Limestone dry C5 Limestone dry C1 Limestone Saturated-brine C1 Limestone Saturated-brine C2 Limestone Saturated-brine A4 Consolidated dry sandstone 11 C5 Limestone dry C5 Limestone dry C5 Limestone dry Depth of Penetration Estimation After the cores have been irradiated with laser, we measured the depth of laser penetration to estimate the ROP. In case of full laser penetration (the laser beam caused a hole that is visible at both the top and bottom surfaces of the core), the length of the core, measured using a vernier caliper, was recorded as the depth of penetration. In case of partial laser penetration (the laser caused a hole that is visible at the top surface but not on the bottom surface), we used sandpaper to file the bottom surface of the core sample until the bottom end of the hole became visible. We then measured the length of the core after sanding using the caliper and recorded that value as the depth of penetration. Program Development We developed a computer program algorithm that provides an estimate for the required power to be withdrawn for a certain laser unit to achieve a constant ROP. This program algorithm maybe utilized by drilling engineers to maintain a constant ROP. The program was designed to fully operate on the basis of Maurer s equation, which relates specific power to specific energy. In this sequence, the user provides three inputs for the program to begin its calculations: the desired rate of penetration, formation lithology and percentage of tolerance. Extensive experimental data can be used to provide mathematical correlations between specific power and specific energy for each rock type. Accordingly, the program starts by assuming initial power intensity and then identifies the corresponding specific energy from the mathematical correlations obtained from the experiments. These two values are then used in Maurer s equation to estimate ROP. If the resulting rate of penetration value isn t equal to the desired input, the program will continue to reiterate, assuming a different power intensity in each iteration, until the ROP value is within the specified tolerance. Once achieved, the program will output the required specific power to be withdrawn from the laser system for such an operational rate of penetration to be feasible under the input conditions. Fig. 4 provides a schematic for the program s flowchart.

9 8 SPE MS Figure 4 Program flowchart Results Rock Properties Before the experiment was performed, the porosity, permeability, and grain density of the core samples included in our study were measured. The results of these measurements are summarized in table 2.Inthe sandstone samples, the permeability was very high ( md) and the porosity was also high (29.88%) falling towards the upper limit of the range of typical values for sandstone porosity (5% to 30%). The grain density for the sandstone sample (2.64 g/cc) was very close to the accepted value for the grain density of quartz, 2.65 g/cc, the major constituent mineral of sandstone.

10 SPE MS 9 Table 2 Measured rock properties Lithology Porosity Grain Density (g/cc) Permeability (md) Consolidated 29.88% , Sandstone (A) Limestone (C) 23.45% sshale (F) Size Restrictions Size Restrictions Size Restrictions In contrast, the permeability of limestone was very low (0.575 g/cc) while its porosity was moderate (23.45%) and falls towards the upper limit of typical values of limestone porosity (0 23%). The grain density of the limestone sample was 2.69 g/cc which was very close to the grain density of calcite (2.71 g/cc), the major mineral forming limestone. We were unable to measure the porosity and permeability of the shale samples due to size and shape restrictions. However, typical porosity values for shale range between 0 and 10%. We also expect the permeability of the shale samples to be low, given that shale formations typically serve as source or cap rocks. Qualitative Analysis Full penetration occurs when the laser beam creates a hole that extends through the entire thickness (length) of the sample and becomes visible on both the top and bottom surfaces. Partial penetration occurs when the laser beam creates a hole whose depth is less than the thickness (length) of the core sample and is visible on only the top surface and not the bottom surface. Shale In shale sample F1, the laser device was programmed to create a hole 0.5 cm (0.196 in) in diameter by rotating the laser source (nozzle) about a fixed axis. Fig. 5 illustrates the top and bottom surfaces of the sample F1 after irradiation in which two circular pink regions ( 0.5 cm in diameter) can be observed. The first circular region was created from trail 1 (500W, 0.942s) and the second circular region was created from trial 2 (700W, 0.942s).The pink color circular region resulted from melting of the rock material due to the excessive heat from the irradiation process. A circular ring indentation that does not fully penetrate the sample was also created by the laser beam along the circumference of the circular pink region. In the center of each region, the laser beam created a hole that fully penetrates the shale sample. Figure 5 Top and bottom surfaces of shale sample F1 after laser irradiation Sandstone The sensor of the laser nozzle did not detect sandstone sample A4 due to its rough surface and abundance of pore spaces. Dry Limestone (Phase 1) In all the trials involving limestone, the nozzle of the laser beam remained static. The laser beam fully penetrated the core sample creating a hole that is visible on both the top and

11 10 SPE MS bottom surfaces of the core. Some black burn marks were observed on the top surface. This penetration pattern was achieved in trial 3 (500W, 0.5s) on C9 (Fig. 6) and C10 (Fig. 7) in trial 4 (500W, 0.5s). The results of trial 5 (500W, 0.7s) and trial 6 (700W, 0.7s) on sample C5 are shown in Fig. 8. It is observed that the laser beam partially penetrated the core sample in trial 5 as the hole is visible only at the top surface and not on the bottom surface. In trial 6, on the other hand, the laser beam fully penetrated the core sample. Burn marks are also visible around both holes on the top surface. Figure 6 Top and bottom surfaces of limestone sample C9 after laser irradiation Figure 7 Top and bottom surfaces of sample C10 after laser irradiation Figure 8 Top and bottom surfaces of sample C5 after laser irradiation with trials 5 and 6 Saturated Limestone In both trial 7 (500W, 0.7s) and trial 8 (200W, 0.5s), the laser only partially penetrated the core sample C1 (Fig. 9). However, a portion of the incident laser beam was reflected off the surface of the core sample during laser irradiation. The reason of this reflection is not well understood. The burn marks around both holes are very faint when compared to the previous trials. The laser irradiation had a similar impact on sample C2 in trial 9 (500W, 0.7s). (Fig 10)

12 SPE MS 11 Figure 9 Top and bottom surfaces of sample C1 after laser irradiation Figure 10 Top and bottom surfaces of sample C2 after laser irradiation Dry Limestone (Phase 2) Fig. 11 illustrates sample C5 after trial 11 (700W, 0.7s), trial 12 (800W, 0.7s) and trial 13 (800W, 1s). In these three trials, the laser beam partially penetrated the core sample. Reflection also occurred in these three trials. In addition, the burn marks around the holes are very faint. A summary of the qualitative analysis of trials 1 through 13 is presented in table 3. Figure 11 Top surface of sample C5 after trials 11, 12 and 13

13 12 SPE MS Table 3 Summary of qualitative analysis Trial number Sample Penetration 1 F1 Full 2 F1 Full 3 C9 Full 4 C10 Full 5 C5 Partial 6 C5 Full 7 C1 Partial 8 C1 Partial 9 C2 Partial 10 A4 Not Applicable 11 C5 Partial 12 C5 Partial 13 C5 Partial Quantitative Analysis Rate of Penetration Using the data for depth of penetration and duration of laser exposure, the ROP was calculated using the equation ROP depth of penetration/duration of laser exposure A distinction should be made between minimum ROP and actual ROP. The minimum ROP is that achieved in trials where the laser fully penetrated the core sample of a specific thickness. The laser at that power intensity and duration could have caused a deeper penetration if the core sample were thicker (longer). The actual ROP is the ROP recorded in trials where the laser partially penetrated the core sample. In trials 1 through 13, the ROP values ranged between 58.6 and 66.2 ft/hr (mean ft/hr) for shale and and ft/hr (mean ft/hr ft/hr) for limestone (Table 4). In shale, the ROP values would have been higher than the achieved rates if the laser nozzle remained static in trials 1 and 2. No ROP value could be reported for the sandstone sample A4 because the sensor of the laser nozzle could not detect the sample owing to its rough surface. A portion of the incident laser beam was reflected off the surface of the core in trials 7 through 13. Table 4 Summary of quantitative analysis Trial number Sample Power intensity (W) Duration of exposure (s) Duration of exposure (hr) Penetration depth (mm) Penetration depth (ft) ROP (ft/hr) Actual / minimum 1 F Minimum 2 F Minimum 3 C Minimum 4 C Minimum 5 C Actual 6 C Not recorded 7 C Actual 8 C Actual 9 C Actual 10 A None 11 C Actual 12 C Actual 13 C Actual

14 SPE MS 13 Specific Energy and Power The specific energy for limestone was calculated using Maurer equation from samples C1, C2 and C5 in trials 5, 7 through 9 and 11 through 13 because actual ROP values were estimated for them. The specific energy for the other trials could not be estimated because the ROP values measured were minimum ROP. The specific energy values for these trials ranged between KJ/cm 2 and KJ/cm 2 (mean KJ/cm 3 ), with the majority of the trials falling between 2000 and 3900 KJ/cm 2 (Table 5). In our study, the specific power ranged between 2546 kw/cm 2 to kw/cm 2 (mean kw/cm 2 ) Table 5 Specific Energy Sample Power intensity (W) Duration of exposure (s) Specific Power (KW/cm2) ROP (cm/s) Specific Energy (KJ/cm 2 ) C C C C C C C Relation of ROP with Laser Energy A statistically highly significant inverse relationship was observed between ROP and the product of laser power and duration of exposure (laser energy (r ; p 0.001) (Fig. 12). Figure 12 Correlation between laser energy and ROP ROP Estimation and Validation of the Developed Constant ROP Program As previous trials did not estimate the ROP, we calculated ROP from the specific power and specific energy values from previous studies using Maurer equation to compare our results and test the newly developed software. Table 6 summarizes the results of these calculations.

15 14 SPE MS Table 6 Overview of previous laser drilling experiments

16 SPE MS 15 Discussion In the current study, the calculated ROP values were significantly higher than those previously reported. The ROP values recorded with Nd:YAG in the current study almost approached those achieved with superior technologies namely CO 2 and COIL lasers which operate at laser powers in the range of 6 to 10 kw. This discrepancy maybe attributed to the difference in beam diameters used in our study and other studies. The laser device we used generated a smaller beam of a diameter 0.1mm while the previous studies used beam diameters ranging between 3 to 15mm. In our study, the mean specific power ( kw/cm 2 ) far exceeds the specific power in the literature which did not exceed 200 kw/cm 2 (Elahifar et al 2011). The smaller beam used in our study resulted in extreme focusing of the laser energy with ultimate high ROP values. It should be noted that the laser penetration experiments conducted thus far are done under lab conditions rather than real field settings. Neither the beam diameter nor the specific power has yet been optimized. Nevertheless, the results of the different experiments pave the way towards refining the procedure. We attempted to analyze factors that influence ROP in our study. Despite the high ROP values achieved, we assume that the ROP values might have recorded higher values but were dampened by some factors such as sample thickness, laser reflection and fluid saturation. Graves et al (2002) reported that in samples less than 2.5 in (6.35 cm) thick, laser energy tends to crack, weaken, or raise the sample s temperature, rather than removing rock volume. An increase in material thickness from 1 to 3 inches decreases specific energy (increases ROP) by 65% (Graves, 2002). Interestingly, we did not demonstrate cracking although the thickness of our samples was less than 6.35 cm but a temperature increase was observed. Despite the standardization of operating parameters in all trials, laser reflection occurred during trials 7 13 in both saturated and dry samples. The actual impact of laser reflection on ROP could not be evaluated in our study due to the limited reflections observed. One may speculate that as the laser beam is reflected off the surface of the sample, it does not remove rock but reduces ROP. Similarly, we cannot determine an effect of the fluid saturation on ROP due to several confounders such as reflections and power intensity. While it is expected that higher laser powers will yield larger penetration depths and greater ROP values, this relation cannot be verified in the current study. On the contrary, for power levels ranging from 500 to 800 Watts, the penetration depth seems to fluctuate and was not directly affected by the power provided. This may suggest that the mechanism of rock removal was mainly melting rather than spallation. The current study showed that the ROP inversely correlated with laser energy (product of lasing time and laser power). However, since this relation has not been previously reported further studies are needed to confirm our findings. We could not demonstrate correlation between the duration of exposure and ROP. It was expected that as the duration of laser exposure increases, the laser had more time to interact with the rock and resulting in more rock removal. However, since the dominant rock removal mechanism in the current study was probably melting, the previous relation does not hold. Specific energy values achieved in our study were significantly higher compared to those in previous studies (Graves 2002). However, Elahifar et al (2011) reported specific energy values reaching 200 kj/cm 3 with CO 2 lasers. Gahan (2001) using Nd: YAG yielded a specific energy value of 900 kj/cm 3 ; although the majority of his tests remained in the average 2 to 40 kj/cm 3. The specific energy represents the amount of energy needed to remove a unit volume of rock. Thus, a high specific energy value may denote a suboptimal drilling method since the tests were conducted during the melting zone during which rock removal is less efficient than at spallation.. Taken together, the laser drilling technique at the melting phase was less efficient than if performed at the spallation zone but still more efficient than mechanical drilling.

17 16 SPE MS Although some factors were shown to influence ROP in our study, the limited number of experiments performed makes it difficult to reliably interpret the data and characterize the factors that maximize laser ROP. Even though there is a high rate of rock removal, the rock removal was not very efficient and energy utilization isn t optimized. However, avoiding penetration at the melting zone and increasing the laser beam diameter seem good approaches to provide higher ROPs at low specific energy. Conclusions Our trials indicate that laser drilling, specifically with Nd:YAG, can provide faster and more consistent ROP in comparison with conventional drilling bits. The high ROP values achieved by Nd:YAG laser in our study are due to the small laser device beam diameter (.1mm) which generated smaller hole sizes compared to those formed by the conventional mechanical bits. To date, it is difficult to predict the ROP values that could be achieved using laser systems when drilling larger holes in actual fields.we also identified some factors that may influence laser penetration such as rock thickness, laser reflection, fluid saturation, laser beam diameter and duration of exposure. Future studies are needed to define the impact and significance of these factors. Despite some experimental limitations and the operating conditions, our findings lend further credence to the previous published results by proving that laser can penetrate rock with faster rates compared to mechanical drilling. Moreover, to the best of our knowledge, our research was the first to operate under very high specific power values, using Nd:YAG to demonstrate the effect of using high specific power laser beams on different rock samples. The previous studies as well as ours pave the way for future optimization and applications of laser based drilling in the field. The next step past lab testing is to design and optimize the laser bit system. The most difficult part will be to drill a large diameter hole whose diameter is optimistically 2cm (0.787 in) using the laser device. A potential promising solution is to utilize an array of multiple beams after adjusting the best configuration to cover the entire drilling surface Fig. 13. Figure 13 Possible beam arrangements (Sinha et al 2006) Our study continues to support the notion of implementing laser drilling in the oil field and promoting further research to this aspect. While it is a relatively new field that remains to be researched and experimented with, it holds massive potential to save time, money and effort, allowing for much faster drilling operations in the process. Recommendations Limited by the type of lasers available to our disposal; we recommend conduction of future research on a wider range of rock samples (particularly sandstone) under a broader range of specific power values

18 SPE MS 17 using different types of laser (preferably COIL and CO 2 ). With further accumulated data and testing, a benchmark may be set for the rates of penetration obtained by drilling with laser. Additional improvements may also be sought through further testing to study the different aspects of this technology and render it one day practically applicable, such as Performing additional experiments on different reservoir cores. Identifying a solid & reliable relationship between the ROP and specific power (SP) while keeping all other parameters constant. Identifying a solid & reliable relationship between the ROP and duration of exposure while keeping all other parameters constant Identificating a minimum threshold laser power intensity at which penetration occurs for select formations In-depth analysis of parameters that affect laser-rock interaction (e.g. laser beam standoff, angle of incidence, etc.) and their relationship with the rate of penetration Studying the effect of hydrocarbon and water saturations on rock penetration by laser Acknowledgements The authors would like to extend their thanks to Dr. Ahmed Noah, and Dr. Taher El-Fakharany, Mr. Marwan Moussa, The American University in Cairo for their guidance, technical discussions and advice. We would also like to acknowledge the support of the staff at the Central Metallurgical Research and Development Institute (CMRDI) during performance of the laser experiments. Nomenclature A L Cross sectional area of the laser, cm 2 CMRDI Central Metallurgical Research and Development Institute COIL Chemical-Oxygen Iodine Laser E absorbed Absorbed Electromagnetic Waves E incident Incident Electromagnetic Waves E reflected Reflected Electromagnetic Waves E scattered Scattered Electromagnetic Waves Nd:YAG Neodymium: Yttrium Aluminum Garnet P L Power provided by laser, W PENG-AUC : Department of Petroleum Engineering, the American University in Cairo ROP Rate of penetration ft/hr SE Specific energy, kj/cm 3 SOP Standard of Procedure SP Specific power kw/cm 2 References Adeniji, A. W The Applications of Laser Technology in Downhole Operations A Review. Presented at the International Petroleum Technology Conference, Doha, Qatar, January. SPE MS. Bakhtbidar, M., Ghorbankhani, M., Alimohammadi, M., KazemiEsfeh, M. R., & Rezaei, P Application of Laser Technology for Oil and Gas Wells Perforation. Presented at the SPE/IADC Middle East Drilling Technology Conference and Exhibition, Muscat, Oman, October. SPE MS. Bielstrein, W. J., & Cannon, G. E Factors Affecting the Rate of Penetration of Bits. Presented at The Drilling and Production Practice, New York, New York, 1 January.

19 18 SPE MS Bjorndalen, N., Belhaj, H. A., Agha, K. R., & Islam, Numerical Investigation of Laser Drilling. Society of Petroleum Engineers. SPE MS. Elahifar, B., Esmaeili, A., Prohaska, M., & Thonhauser, G An Energy Based Comparison of Alternative Drilling Methods. Presented at the SPE/IADC Middle East Drilling Technology Conference and Exhibition, Muscat, Oman, October. SPE MS / MS Gahan, B. C., Parker, R. A., Batarseh, S., Figueroa, H., Reed, C. B., & Xu, Z Laser Drilling: Determination of Energy Required to Remove Rock. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 30 September-3 October. SPE MS. Graves, R. M., O Brien, D. G StarWars Laser Technology Applied to Drilling and Completing Gas Wells. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, September. SPE MS. Graves, R. M., Araya, A., Gahan, B. C., & Parker, R. A Comparison of Specific Energy Between Drilling With High Power Lasers and Other Drilling Methods. Presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas 29, September-2 October. SPE MS. O Brien, D. G., Graves, R. M., & O Brien, E. A StarWars Laser Technology for Gas Drilling and Completions in the 21st Century. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3 6 October. SPE MS. Pooniwala, S. A Lasers: The Next Bit. Presented at the SPE Eastern Regional Meeting, Canton, Ohio, October. SPE MS. Rabia, H Specific Energy as a Criterion for Bit Selection. J Pet Technol.37(07): doi: /12355-PA Sinha, P., & Gour, A Laser Drilling Research and Application: An Update. Presented at the SPE/IADC Indian Drilling Technology Conference and Exhibition, Mumbai, India, October. SPE MS.