Prof. Martin O. Saar, Zurich
Innosuisse [Grant Nr. 28305.1 PFIW-IW]
- GEG group, ETH-Zurich
- Fraunhofer IEG
REFEREED PUBLICATIONS IN JOURNALS
Plasma Pulse Geo Drilling (PPGD) is a contact-less drilling technique, where an electric discharge across a rock sample causes the rock to fracture.
Experimental results have shown PPGD drilling operations are successful if certain electrode spacings, pulse voltages, and pulse rise times are given.
However, the underlying physics of the electric breakdown within the rock, which cause damage in the process, are still poorly understood.
This study presents a novel methodology to numerically study plasma generation for electric pulses between 200 to 500 kV in rock pores with a width between 10 and 100 \(\mu\)m. We further investigate whether the pressure increase, induced by the plasma generation, is sufficient to cause rock fracturing, which is indicative of the onset of drilling success.
We find that rock fracturing occurs in simulations with a 100 \(\mu\)m. pore size and an imposed pulse voltage of approximately 400 kV. Furthermore, pulses with voltages lower than 400 kV induce damage near the electrodes, which expands from pulse to pulse, and eventually, rock fracturing occurs. Additionally, we find that the likelihood for fracturing increases with increasing pore voltage drop, which increases with pore size, electric pulse voltage, and rock effective relative permittivity while being inversely proportional to the rock porosity and pulse rise time.
The cost of deep drilling contributes significantly to the feasibility of economically accessing deep geo-energy resources (i.e., geothermal, oil, and gas) as drilling costs make up the majority of overall project costs (, ). However, mechanical rotary drilling costs are high and increase exponentially with depth as rotary drilling relies on mechanical abrasion (). Therefore, the economic extraction of geothermal energy requires developing cheap and fast drilling methods such as Plasma Pulse Geo Drilling (PPGD) and thermal spallation (, , ).
Plasma Pulse Geo Drilling (PPGD) is a novel contactless drilling technology, which uses high-voltage electric pulses to break away the rock without relying on mechanical abrasion. Several studies have shown that PPGD has lower costs than rotary drilling, as the absence of mechanical abrasion increases the bit lifetime and decreases the number of tripping cycles. Experimentally, found PPGD to be up to 17% cheaper and seven times faster than mechanical rotary drilling. Analytically, and suggested that further research could reduce PPGD drilling costs by 90\% or more of current mechanical rotary drilling costs. Therefore, once developed, PPGD may replace conventional mechanical rotary drilling in several fields, such as geothermal energy extraction.
Plasma Pulse Geo-Drilling uses 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 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.
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 ( cite(id=”Budenstein1980″);; cite(id=”Janet2017″);).
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 cite(id=”Walsh2020″, type=”t”);, 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).
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 ( cite(id=”Kuretz2002″);; cite(id=”Hobejogi2014″);; cite(id=”Lisitsyn1998″);; cite(id=”Inoue1999″);). However, only two experiments have been conducted to investigate the dependence of the plasma-based drilling efficiency on lithostatic pressure ( cite(id=”Vazhov2011″);; cite(id=”Anders2017″);). 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 ( cite(id=”Anders2017″);). 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 surroundings and to enable testing different electrode gaps, particularly for none-demineralized water), which are both times consuming and expensive to prepare. 3) The allowed time for the doctoral project is limited to 3 years (till Sep. 2021).