Innosuisse [Grant Nr. 28305.1 PFIW-IW]
Prof. Martin O. Saar, Zurich
The economic generation of electricity typically requires geothermal reservoirs that have temperatures higher than 150 ℃ (). As a result, the injection and production wells can reach 5 to 10 km depth. There is a problem: drilling to such depths tends to occur occurs in hard polycrystalline (basement) rock, such as granite, where the drilling costs grows exponentially with depth (). Thus, such deep drilling is expensive, so that the cost of drilling wells for geothermal projects can account for 30-75% of the total investment costs (). Thus, the price of drilling plays a substantial role that determines the economic viability of the construction of any geothermal power plant.
Conventional mechanical rotary drilling has a high cost because it has a long tripping time, among other factors. Tripping time is the time required for pulling out the entire drillstring out of the well for replacing the worn-out bit, and then insert the drillstring again. Also, the number of tripping cycles increases because the drill bit has low wear resistance, and the drilling rig has several mechanical parts (e.g., rotary table, drillstring, and topdrive) that are prone to failure. As the non-productive time of the operation increases, the drilling cost increases as well, thereby the mechanical drilling is not economically viable for deep drilling. Finding an alternative drilling technology is necessary, but the alternative method must have a drill bit that has high wear resistance. Therefore, the bit will have a longer lifetime, meaning thus, fewer tripping cycles and tripping time. If the tripping time decreases, it will reduce the total budget of the project. For example, Plasma Pulse Geo-Drilling and thermal spallation () are contactless drilling techniques that can afford the previously mentioned criteria as an alternative drilling technology. Also, the necessity of the fast and cheap drilling technology for economic geothermal energy is shown briefly in Vido (1).
Plasma Pulse Geo-Drilling uses a high voltage microseconds-long pulses to disintegrate rock without any mechanical abrasions. The voltage pulse creates plasma channels inside the intact rock and fracks it by mechanical tension. This mechanism is unlike the conventional drilling methods that fragment rock by mechanical compression. As the tensile strength of rock is 5% to 10% of the compression strength, PPGD requires less amount of energy to damage rock than the mechanical rotary drilling. Also, the applied high compression stress on the drill bit itself shortens the lifetime of the bit. Due to the mentioned and promising advantages of PPGD, this project’s objectives are the following:
- Understand the fundamental physics of the PPGD process by numerical modeling.
- Examine the viability of PPGD under high lithostatic pressure conditions that simulate depth up to 5000 m.
Video 1: This video shows the potential of geothermal energy, and the necessity of the fast and cheap drilling technology to enable economic viability. By Professor Martin O. Saar.
The electric breakdown of heterogeneous solids (e.g., rock) is a complex problem that occurs with several mechanisms. The efficiency of the process is also affected by several parameters such as the electric pulse peak, rising time, electrode gap distance, the rock type, and the rock environment (e.g., water-saturation degree, lithostatic pressure, and temperature). However, there is no satisfactory-theoretical explanation of that process; and thus, we need to do so.
The most dominant couple of mechanisms are the following: (1) The internal breakdown mechanism that is initiated by the electric breakdown of the air inside the pores/microcracks of the solid. (2) The thermomechanical breakdown mechanism that occurs because of the thermal stresses that are caused by ohmic heating (; ).
The numerical modeling part of this project’s aim is to study both mechanisms individually; however, coupling both mechanisms is foreseen. Recently, a numerical model that can simulate the thermomechanical mechanism has been investigated by , within the framework of this project. However, the internal breakdown mechanism -plasma approach-, which utilizes the plasma formation inside pores/microcracks, still under study. According to the plasma approach, the rock damage takes place in four phases, as shown in Figure (1).
Figure 1: PPGD four phases: (I) Pore/microcrack plasma formation, (II) micro-cracks formation/expansion, (III) Channel Plasma formation, and (IV) the fragmentation. Dark green markers refer to the pores, natural and induced micro-cracks, which have width ranges from few to few hundreds of microns. Light green background refers to the grains, which is dominant because of the low granite porosity. de and VT stand for the electrodes gap distance and the pulse voltage.
The Plasma Pulse Geo-Drilling (PPGD) technology targets deep drilling, which means drilling will occur in hard, often crystalline, rock (e.g., granite) under high lithostatic pressure (> 300 bar) and high temperature (> 100 ℃). Under ambient conditions, several studies have investigated the efficiency dependence of plasma-based drilling methods on three key parameters: a) pulse voltage, b) electrode gap distance, and c) rock water-saturation (; ; ; ). However, only two experiments have been conducted to investigate the dependence of the plasma-based drilling efficiency on lithostatic pressure (; ). Even though the two experiments show a reduction of efficiency by increasing the lithostatic pressure, there is a discrepancy by one order of magnitude between the results from the two experiments. Also, one of those experiments has not stated all operating conditions (). Thus, we cannot rely on those results to quantify the dependence of the PPGD efficiency on the lithostatic pressure. Furthermore, there appears to be no study investigating the effect of the three above-mentioned key parameters on the PPGD efficiency under high lithostatic pressure. We thus propose the following laboratory experiment study, where the goal is to answer the following two main questions:
- What is the lithostatic pressure effect on the PPGD efficiency mainly in granite (where efficiency is defined as the excavated rock volume per energy unit input)?
- Under high granite lithostatic pressure, what is the effect on the PPGD efficiency of the above-mentioned key parameters: a) voltage pulse, b) electrode gap distance, and c) rock water-saturation?
Fraunhofer Institute for Energy Infrastructures and Geothermal Energy (Fraunhofer IEG) at Bochum has an autoclave that is the so-called “i.BOGS” that can pressurize the sample up to 1200 bar and heat it up to 180 ℃. Therefore, the experimental campaigns of this project will be performed in cooperation with Fraunhofer IEG researcher at Bochum. The Plasma Drill Head and the electric pulse generator are being manufactured by our collaborators in Swiss GeoPower company. The schematic of the experimental equipment is shown in Figure (2). Furthermore, the following experiment constraints exist: 1) The experiment setup (drillable i.BOGS at the Fraunhofer IEG in Bochum) has limitations regarding both the maximum pressure and the maximum voltage it can tolerate. 2) The rock cores we are planning to use should have a large diameter of about 40~cm (to eliminate “short-circuiting” to the surrounding and to enable testing different electrode gaps, particularly for none-demineralized water), which are both time consuming and expensive to prepare. 3) The allowed time for the doctoral project is limited to 3 years (till Sep. 2021).
Figure 2: The experimental setup for testing the PPGD under high pressure/temperature conditions. The left block is the i.BOGS and the right is the Marx generator circuit to create the electric pulse. The connection between the two parts is the Plasma Drill Head and the coaxial cable, which transfer the high voltage pulse from the Marx generator to the rock sample under high pressure/temperature.
Walsh, S.D.C., and D. Vogler Simulating Electropulse Fracture of Granitic Rock International Journal of Rock Mechanics and Mining Sciences, 128, pp. 104238, 2020. [Download PDF] [View Abstract]Electropulse treatments employ a series of high-voltage discharges to break rock into small fragments. As these methods are particularly suited to fracturing hard brittle rocks, electropulse treatments can serve to enhance or substitute for more traditional mechanical approaches to drilling and processing of these materials. Nevertheless, while these treatments have the potential to improve hard-rock operations, the coupled electro-mechanical processes responsible for damaging the rock are poorly described. The lack of accurate models for these processes increases the difficulty of designing, controlling and optimizing tools that employ electropulse treatments and limits their range of application. This paper describes a new modeling method for studying electropulse treatments in geotechnical operations. The multiphysics model simulates the passage of the pulse, electrical breakdown in the rock, and the mechanical response at the grain-scale. It also accounts for the contributions from different minerals and porosities, allowing the effect of material composition to be considered. In so doing, it provides a means to investigate the different physical and operational factors influencing electropulse treatments.