key: cord-0427425-9sm4rlao authors: Kazi, Aamer; Riyaz, Mirza; Tang, Xin; Staack, David; Tai, Bruce title: Specific cutting energy reduction of granite using plasma treatment: A feasibility study for future geothermal drilling date: 2020-12-31 journal: Procedia Manufacturing DOI: 10.1016/j.promfg.2020.05.077 sha: be32ccd2df809f5ffe46ba23986de6bbc1c79d7a doc_id: 427425 cord_uid: 9sm4rlao Abstract Geothermal Energy is one of the most consistent and viable sources of renewable energy. However, harnessing this energy has proved to be a challenge mainly due to the difficulty in drilling hard igneous rock formations that occur in and around geothermal wells. Therefore, an energy efficient method that can drill hard rocks needs to be developed. In this paper, the effect of micro-scale energy delivery, in the form of plasma, to weaken rock by inducing micro-cracks is studied. Two different types of granite (igneous rock) were sampled and used in this study, out of which the one control group was treated with five, 40kV, 80J pulses of plasma while the other was left uncracked. The effect of the cracks is quantified by conducting facing tests on the plasma cracked and uncracked samples and measuring a reduction in the specific cutting energy. Two different methods were used to estimate the specific cutting energy, where the first method considered the entire cutting process and the second method considered only the stable cutting region. The plasma treatment showed a maximum of 65% and a minimum of 15% reduction in specific cutting energy and was regarded as being dependent on mainly the hardness and size of the samples. All results in this study are validated using statistical analysis. With the advent of Global warming and the rise of greenhouse gases due to the excessive use of fossil fuel-based electricity, it has become imperative to search for cleaner alternatives to produce energy. Geothermal Energy has long been regarded as one of those viable alternatives. It is defined as the heat derived from within the sub-surface of the earth [1] . It is one of the better sources of renewable energy because it is independent of the weather and the output is far more consistent [2] . However, there are several physical and economic factors that prevent geothermal energy from competing with conventional fossil fuel-based (Oil and Gas) energy. One of the more significant physical factors is the difference between geothermal and petroleum rock formations. Geothermal formations have primarily igneous rocks, such as granite, while oil formations consist of mainly sedimentary rocks, the former being significantly harder and more abrasive than the latter [3, 4] . The harder rock encountered in geothermal drilling leads to higher tool wear, increased cutting forces and a lower rate of penetration. This increases the cost of production and reduces efficiency of the overall process [5] . In order to overcome these issues, attempts have been made to develop deep geothermal drilling technologies that are economic. Several studies revolve around the use of rock fragmentation techniques as a form of aid to induce cracks in hard rocks before drilling them. Saadati et al. [6] With the advent of Global warming and the rise of greenhouse gases due to the excessive use of fossil fuel-based electricity, it has become imperative to search for cleaner alternatives to produce energy. Geothermal Energy has long been regarded as one of those viable alternatives. It is defined as the heat derived from within the sub-surface of the earth [1] . It is one of the better sources of renewable energy because it is independent of the weather and the output is far more consistent [2] . However, there are several physical and economic factors that prevent geothermal energy from competing with conventional fossil fuel-based (Oil and Gas) energy. One of the more significant physical factors is the difference between geothermal and petroleum rock formations. Geothermal formations have primarily igneous rocks, such as granite, while oil formations consist of mainly sedimentary rocks, the former being significantly harder and more abrasive than the latter [3, 4] . The harder rock encountered in geothermal drilling leads to higher tool wear, increased cutting forces and a lower rate of penetration. This increases the cost of production and reduces efficiency of the overall process [5] . In order to overcome these issues, attempts have been made to develop deep geothermal drilling technologies that are economic. Several studies revolve around the use of rock fragmentation techniques as a form of aid to induce cracks in hard rocks before drilling them. Saadati et al. [6] With the advent of Global warming and the rise of greenhouse gases due to the excessive use of fossil fuel-based electricity, it has become imperative to search for cleaner alternatives to produce energy. Geothermal Energy has long been regarded as one of those viable alternatives. It is defined as the heat derived from within the sub-surface of the earth [1] . It is one of the better sources of renewable energy because it is independent of the weather and the output is far more consistent [2] . However, there are several physical and economic factors that prevent geothermal energy from competing with conventional fossil fuel-based (Oil and Gas) energy. One of the more significant physical factors is the difference between geothermal and petroleum rock formations. Geothermal formations have primarily igneous rocks, such as granite, while oil formations consist of mainly sedimentary rocks, the former being significantly harder and more abrasive than the latter [3, 4] . The harder rock encountered in geothermal drilling leads to higher tool wear, increased cutting forces and a lower rate of penetration. This increases the cost of production and reduces efficiency of the overall process [5] . In order to overcome these issues, attempts have been made to develop deep geothermal drilling technologies that are economic. Several studies revolve around the use of rock fragmentation techniques as a form of aid to induce cracks in hard rocks before drilling them. Saadati et al. [6] With the advent of Global warming and the rise of greenhouse gases due to the excessive use of fossil fuel-based electricity, it has become imperative to search for cleaner alternatives to produce energy. Geothermal Energy has long been regarded as one of those viable alternatives. It is defined as the heat derived from within the sub-surface of the earth [1] . It is one of the better sources of renewable energy because it is independent of the weather and the output is far more consistent [2] . However, there are several physical and economic factors that prevent geothermal energy from competing with conventional fossil fuel-based (Oil and Gas) energy. One of the more significant physical factors is the difference between geothermal and petroleum rock formations. Geothermal formations have primarily igneous rocks, such as granite, while oil formations consist of mainly sedimentary rocks, the former being significantly harder and more abrasive than the latter [3, 4] . The harder rock encountered in geothermal drilling leads to higher tool wear, increased cutting forces and a lower rate of penetration. This increases the cost of production and reduces efficiency of the overall process [5] . In order to overcome these issues, attempts have been made to develop deep geothermal drilling technologies that are economic. Several studies revolve around the use of rock fragmentation techniques as a form of aid to induce cracks in hard rocks before drilling them. Saadati et al. [6] the presence of pre-existing cracks in granite resulted in a reduction of the cutting force required to reach a penetration depth. Waterjet [7] , thermal spallation [8] and high-power laser [9] are some of the current rock fragmentation techniques that use this concept of pre-cracking to improve the drilling process. However, these techniques suffer from various drawbacks, such as a poor area of effect in the case of thermal spallation and high-power laser, and high-pressure working fluid transportation issues in the case of waterjet [10] . Another technique that has been used for rock drilling and demolition for decades is electric pulsed power [11] . It is one of the primary methods of non-mechanical contact rock breakage. However, majority of the applications of this technology, such as Electro-Pulse Boring [12, 13] , require very high voltage, high power, short electric pulses to be effective (500-700 kV, 10 ns, ~1,500J) [14] . Therefore, there is a requirement to develop efficient hard-rock drilling techniques that address these shortcomings. In this paper, the effectiveness of a novel methodology that will leverage micro-scale energy delivery, in the form of plasma, to the rock surface in order to cause fracture was studied. The goal of this research was to develop a more controlled and low energy alternative to enhance rock fragmentation in geothermal drilling. This technology will be used along with traditional drag-bit drilling operations (i.e., a hybrid approach). The technology aims to locally pre-crack rocks using under-liquid low energy (~80J and 40 kV) nanosecond plasma pulses (~ 10 ns). Since the required energy is low, the power delivery is also achievable. Micro-cracks on the rock substrate can be generated by inducing a nanosecond pulsed plasma discharge between two electrodes submerged under water [15] . The electrical discharge between the electrodes generates extreme pressures due to the short timescale of the plasma event. This leads to a cavitation bubble generation in the fluid, which when collapses due to the pressure difference induces a shockwave. This shockwave leads to localized micro-cracks on the rocks which weaken them. The weakened rock becomes easier to drill, which in turn improves the efficiency of the drilling process. The effectiveness of the plasma induced micro-cracks is validated by comparing the specific cutting energy (P s) of plasma-cracked and uncracked granite. Specific cutting energy is defined as the amount of energy required to remove a unit volume of the workpiece material. The hypothesis was that micro-cracks on the granite surface would lead to a reduction in Ps and thus a reduction in cutting forces. Granite is considered for this experiment as it is one of the more common igneous rocks encountered during geothermal drilling. Two different varieties of granite (Colonial White and Westerly) are sampled with different dimensions to test the efficacy of the plasma treatment in two completely distinct scenarios, and to account for the inhomogeneous nature of the rock. The specific cutting energy of each granite is estimated by facing the samples on a CNC lathe, where the power expended during the cutting process and material reduction after the process are monitored. A facing process was considered in order to maintain simplicity of an orthogonal cutting configuration. A full-scale geothermal drilling process was not considered due to the process, environmental and geometric complexities that would be encountered [16] . These experiments were done mainly to showcase the isolated effect of the plasma treatment. The experimental results were statistically validated. This paper is organized as follows: Sample preparation procedure and the methodology used to calculate the specific cutting energy are given in section 2. An introduction of the plasma cracking setup and cutting test setup is given in this section as well. Section 3 presents the specific cutting energy results and section 4 discusses the implications. Lastly, section 5 summarizes the study and draws the conclusions. In order to account for the inhomogeneity of rock and the uncontrollable nature of downhole drilling conditions, two different granite varieties are sampled with different dimensions and faced. Multiple cylindrical samples were cut from Colonial White (sample set 1) and Westerly Granite (sample set 2). The composition of these samples, obtained using X-ray diffraction, is given in Table 1 . The samples were then equally divided into a treated and untreated control group. Each rock sample in the treated control group was cracked using five pulses of plasma in a circular pattern on the rock (40 kV, 80 J each). Five pulses were selected as it was the number of pulses at which the effect of the cracking was observable through visual inspection. The cracks were generated equidistant and at a radial distance of 5 mm from the center of the Colonial White samples, and 7 mm from the center of the Westerly samples. The particulars of the specimens are shown in Table 2 . To crack the rocks, a nanosecond pulsing power circuit, with an air spark switch was used. The schematic of the experimental setup is shown in Fig. 1 . The test setup includes the following: (a) the electrical plasma generation system, (b) the test tank, and (c) the characterization/diagnostic equipment. The electrical plasma generation system includes a current source (Spellman SL300a negative polarity high voltage DC power supply) and a nanosecond pulse generation circuit (100 nF capacitor bank and a 40 MΩ ballast resistor, Fig. 1 ). The air spark gap (shown in Fig. 1.) switch enables tuning of the breakdown voltage in the circuit and hence the submerged plasma discharge voltage. The air spark gap also allows for nanosecond pulsing. The test tank is an acrylic container that enables optical access for various characterizations and contains the test liquid, the test sample and the plasma generation electrodes. The electrodes are fed through a custom 3D printed electrode holder (enables inter-electrode spacing adjustment) For the various electrical, fluid, thermodynamic and emission characterizations, the test setup includes an oscilloscope (LeCroy, waveRunner 204MXi) to facilitate plasma discharge voltage and current measurements through voltage and current probes. The specific cutting energy was determined by performing a facing operation on an EMCO Concept Turn 60 CNC lathe. The rocks were clamped in a 3-jaw chuck and faced using a Tungsten carbide insert with a constant 10° rake angle. An example facing operation on colonial white is shown in Fig. 2 . A facing operation was preferred to a turning operation due to limitations with treatment area. Treating the flat surface was more viable than treating the curved surface since the discharge process required a certain minimum area which could not be obtained from the curved surface. Moreover, the rock samples were procured from granite countertop manufacturers, who hone and resin the surface of the countertop. This is done in order to fill pits, crevices, and holes on the rock surface that can compromise with rock stability. However, the underside of the countertop slab is left unpolished. The plasma tests were conducted on this natural unpolished surface. The power expended during the cut was recorded by using a Fast Response Universal Power Cell (Load Controls Inc., Sturbridge, Massachusetts, US) with a response time of 0.050 s. This cutting data was collected using a NI-USB 6341 data acquisition unit with a sampling rate of 20 Hz and processed on LabView. The cutting parameters are illustrated in Table 3 . The sample rotation speed was selected in order to adhere to the scale of cutting speeds seen in traditional geothermal drilling operations [17] . The feed and depth of cut were selected to maintain machine stability. A tungsten carbide was selected as it is difficult to shape polycrystalline diamond compact (PDC) cutters to have a cutting edge suitable for a facing operation. To minimize the effects of tool wear over time, a new insert is used for every sample tested. The overall cutting test setup is shown in Fig. 3 . In this section, an overview of the methodology used to analyze the raw cutting data is presented. A sample power curve obtained from the facing operation is shown in Fig. 4 . The raw data presented shows an oscillatory trend and is primarily due to two factors: the nature of brittle fracture and the inhomogeneous nature of rock [18] . Figure 4 also illustrates three distinct phases that the cutting data could be divided into. The first phase was the power measurement before the cut began, i.e., idle spindle rotation at 1000 RPM. This average of the idle spindle power is subtracted from the rest of the curve in the result section to show pure cutting power and to discount the machine power. The second phase occurred after the initial contact of the cutting tool with the rock, which resulted in a gradual cutting power increase. The third phase was the stable cutting phase which was defined by a decrease in the cutting power due to a linear decrease in the material removal rate as the cutting tool fed towards the center. This three-phase trend was exhibited by all the rock samples tested in this paper, and a similar trend has been reported in other studies as well [17] . In this investigation, the specific cutting energy was calculated using two different approaches. The general formula for specific cutting energy is given by where P is the machining power and Z is the material removal rate, defined by Eq. (2) for facing. The first method considered the entire duration of the cut, i.e., Phase 2 and Phase 3 of the facing process. The Ps is calculated by the total machining energy divided by the actual volume removed. The total energy was obtained by integrating the power data with time and the volume removed was obtained by the weight change before and after the cutting (assuming a constant density). The result is defined as the apparent specific cutting here since this calculation takes advantage of the extra material removal caused by chipping or fracturing. Occasionally, the excess edge effects can get exaggerated due to the proximity of the plasma treatment with the edge of the specimens. Therefore, for a more conservative estimate of Ps, a second approach is proposed which considers only Phase 3 of the cutting process. This phase occurs closer to the center of the specimen, and hence disregards the edge effects. This gives a value of specific cutting energy referred to as the true specific cutting energy, which is more suitable to gauge the effectiveness of the plasma treatment. This is calculated by first finding the rate of change of cutting power, such that As the cutter feeds towards the center with a constant feed rate in facing, the cutting speed decreases linearly with the decrease in diameter. The cutting speed is a function of the diameter in this process (since rotational speed is kept constant), and is given by From Eqs. 3 and 4, it is proven that power decreases linearly as well. However, due to the oscillatory nature of the raw data, a linear curve fitting was adopted to find the rate of change of cutting power during Phase 3. This is illustrated in Fig. 5 . The slope then can be used to find Ps using both equations. In this section, the apparent and true specific cutting energy results of Colonial White Granite are presented. Figure 6 compares the results of the facing operation between cracked and uncracked granite specimens. It is evident that the cutting power is lower for cracked samples during almost the entire cutting process except the initial contact and toward the center. Figs. 7(a) and 7(b) . From visual observation, the cracked sample exhibited more chipping and loss of material. This is further supported by Fig. 8 , which shows less power (forces) required to cut the rock as it fractures apart during the cutting. Also, the linear descending profile becomes less obvious due to the same reason. The apparent and true specific cutting energy of all six samples were averaged and tabulated in Table 4 . By comparing the means, the apparent specific cutting energy was reduced by 64.9%, while the true specific cutting energy was reduced by 22.7% after the plasma treatment. Again, the more significant reduction in apparent specific cutting is because the calculation accounts for possible chipping and fracturing effects, whereas the true specific cutting energy only represents the stable cutting region. The statistical difference between the uncracked and cracked samples for both cases was confirmed using a paired two sample t-test with a confidence level of 95%. A comparison of the power vs. time curve between uncracked and cracked Westerly granite specimens is shown in Fig. 8 . A reduction in the cutting power is seen as well. However, unlike the Colonial white granite, the power data of cracked Westerly granite does not have major fluctuation and plateau, which indicate a more stable cutting throughout. The rock samples after the cutting process are shown in Figs. 9(a) and 9(b). The severity of edge chipping effect is much lesser than that of Colonial White samples discussed in the previous section. This observation explains the smooth and consistent cutting profiles of both uncrack and cracked samples. The apparent and true specific cutting energy results are given in Table 5 . After plasma treatment, the apparent and true specific cutting energy reduce by 13.7% and 15.5%, respectively. The similar reduction percentages indicate minimal chipping and fracturing effects, but the plasma treatment still weakens the rock in certain way. For this sample set, the statistical difference of the apparent specific cutting energy between uncracked and cracked specimens could be validated at a 95% confidence level. However, a marginally significant difference for the true specific cutting energy was observed (p-value of 0.054). In this section, the data analysis results of both samples, limitations and possible improvements of the testing methodology are discussed. From the composition shown in Table 1 , Colonial white was the harder rock between the two granite varieties. A higher hardness would result in a higher specific cutting energy and this can be supported by comparing the uncracked specific cutting energy results in Sections 3.1 and 3.2. Colonial White granite shows a significant reduction in the apparent specific cutting energy after the plasma treatment because of the increased edge chipping effect. Even though cutting around the edge is neglected, the outcome of the plasma treatment is still significant as there is still a 22.7% reduction. In comparison, the reduction of Westerly granite in both true and apparent specific cutting energy values are comparatively lesser. It can be inferred that an extra pulse or two may be required to make this result statistically significant. However, the practical significance of the treatment cannot be discounted as a reduction of the true specific cutting energy does exist. Comparing the specific cutting energy results between Colonial White and Westerly granite, the plasma treatment had a bigger impact on the former although it was harder. The increased chipping seen in Colonial White proves that the effect of plasma is dependent on more than just the hardness. Perhaps this was a result of the plasma treatment being performed on a smaller surface area. Factors such as a loosely packed grain structure, which facilitates material removal, are possible reasons as well. Taking these factors into account, it is recommended to use the true specific cutting energy analysis for smaller samples, and apparent specific cutting energy for larger samples. This is because the effect of edge chipping is negligible in larger samples, and the apparent specific cutting energy is calculated using direct experiment data. The true specific cutting energy is an analytical approximation. The descending power profiles observed (Phase 3) for both rocks are fairly linear. From Eqs. 3 and 4, this linear relation implies that the specific cutting energy is indeed independent of cutting speed. This means the cutting force is also speed independent, which has been confirmed by other studies on the brittle failure mode of rocks [17, 18] . From an energy perspective, plasma pre-treatment is also a loss of processing energy to trade for cutting energy reduction. In this study, a total of 400J of energy was spent to perform 5 pulses of plasma (80J per pulse). Cutting energy reduction can be calculated by the difference of power curves before and after plasma treatment. In the Colonial White samples, approximately 190J of energy was saved, while in the Westerly samples, 250J was lost. This indicates that an energy saving scenario requires proper plasma pre-treatment. However, it should be noted that the comparison does not consider other energy aspects, such as process time, tool life, and so on. Finally, in this study, a couple of limitations could be addressed in future work. Due to a facing process for this experiment, a tungsten carbide insert was used over a practical PDC insert for rock drilling. PDC inserts are usually manufactured for drag-bit type drilling operations and do not have the requisite cutting edge requirements for facing. Therefore, as a result of using a carbide insert, the presence of tool wear in this experiment had to be accounted for, despite that a new tool was used for every cut. In addition, although the true specific cutting energy reveals the cutting force reduction in a stable cutting condition, a full factorial experiment is necessary to confirm the effects of rock type and size, and their interactions on the plasma-induced chipping and fracturing, which appear to be beneficial to real applications. In this investigation, the effect of micro-scale energy delivery, in terms of plasma pulses, on the specific cutting energy of granite is studied for the development of future geothermal technology. Three major findings are concluded as follows. Firstly, noticeable reduction of specific cutting energy (up to 65%) was confirmed on two different granite samples. Secondly, the plasma treatment was deemed to be dependent on more than just the hardness of the sample. The size and rock grain structure may both play a major role in influencing the extent of edge chipping and fracturing. Finally, between the two methods to calculate the specific cutting energy (namely true and apparent specific cutting energy), the true specific cutting energy is a more conservative approach which potentially eliminates the effect of size. Having shown the effectiveness of plasma-assisted cutting process, the next step of this study is to quantify the extent of cracks to relate the plasma energy input and cutting energy reduction. This will provide a fundamental basis for the design and optimization of this concept at scale. The Status and Future of Geothermal Electric Power Worldwide Status of Geothermal Development and International Cooperation. 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Digest of Technical Papers Electro-pulse boring (EPB): Novel super-deep drilling technology for low cost electricity Electro Pulse Boring: An Experimental Rock-breaking Technology for Low-cost Access to Ubiquitous, Inexhaustible, Autonomous, Ultra-deep (5-10 km) Geothermal Heat Influences on High-Voltage Electro Pulse Boring in Granite Microbubble generation by microplasma in water Rock Cutter Interactions in Linear Rock Cutting Experimental study of force responses in polycrystalline diamond face turning of rock The mechanics of rock cutting This project was funded by the U.S. Department of Energy EERE Geothermal Office, grant number #DE-EE0008605