Numerical Modeling of the Effects of Pore Characteristics on the Electric Breakdown of Rock for Plasma Pulse Geo Drilling

Drilling costs can be 80% of geothermal project investment, so decreasing these deep drilling costs substantially reduces overall project costs, contributing to less expensive geothermal electricity or heat generation. Plasma Pulse Geo Drilling (PPGD) is a contactless drilling technique that uses high-voltage pulses to fracture the rock without mechanical abrasion, which may reduce drilling costs by up to 90% of conventional mechanical rotary drilling costs. However, further development of PPGD requires a better understanding of the underlying fundamental physics, specifically the dielectric breakdown of rocks with pore fluids subjected to high-voltage pulses. This paper presents a numerical model to investigate the effects of the pore characteristics (i.e., pore fluid, shape, size, and pressure) on the occurrence of the local electric breakdown (i.e., plasma formation in the pore fluid) inside the granite pores and thus on PPGD efficiency. Investigated are: (i) two pore fluids, consisting of air (gas) or liquid water; (ii) three pore shapes, i.e., ellipses, circles, and squares; (iii) pore sizes ranging from 10 to 150 μm; (iv) pore pressures ranging from 0.1 to 2.5 MPa. The study shows how the investigated pore characteristics affect the local electric breakdown and, consequently, the PPGD process.

New Steering Technology Using Pulsed Arc Plasma Shockwave: Plasma Pulse Steering Technology

Steering drilling technology can achieve precise control of wellbore trajectory, and related technologies have been widely used in the field of petroleum drilling. This paper proposed a new steering drilling technology based on the Pulsed Arc Plasma Shockwave Technology (PAPST),Plasma Pulse Steering Technology (PPST). PAPST transforms electric energy into mechanical energy by discharging electrodes, which can break rock. On the basis of PAPST, PPST can precisely control the discharge time and break the rock in the specified direction at the bottom of the well, so as to realize guided drilling. First, the discharge mechanism and guiding mechanism of the PPST were studied separately. Then, the discharge control model of PPST was established to explain the feasibility of using this technology to achieve drilling guidance. Finally, to verify the actual effect of this technology on rock breaking, an experiment was carried out with self-developed experimental equipment. Through the study of the mechanism and discharge control model of PPST, it is considered that it is feasible to use this technology to achieve guidance in theory. The experimental results show that the sandstone samples were damaged and a large area of pits appeared after the shockwave, and the ultrasonic penetration test results showed that there was damage inside the rock. As the number of impacts increased, the rock damage became more severe and fracture occurred. Therefore, it is feasible to apply PPST to the directional fracture of bottom hole rock. In summary, this technology has very good application prospects. For the first time, this paper proposed the idea of applying PAPST to steering drilling. Through the research on the steering mechanism and the experiment, the feasibility of this technology was proved and the theoretical basis was provided for the application of this technology in the field of oil drilling.

Simulating Plasma Formation in Pores under Short Electric Pulses for Plasma Pulse Geo Drilling (PPGD)

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 and 500 kV in rock pores with a width between 10 and 100 μm. 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 μm 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.