Traditional hard rock drilling based on mechanical breakage suffers from high costs incurred from severe tool wear. Therefore, alternative drilling methods based on improved concepts and/or using some non-mechanical agents to enhance/replace the mechanical breakage are presently being searched. One such method uses high voltage electric impulses to break rock. This can happen directly through a rapid rise of the pressure and temperature in the electric breakdown channel between electrodes placed on rock surface. Another method is to actuate the piezoelectric properties of Quartz present in hard crystalline rocks, such as Granite.

In the present work, this latter kind of electric shock induced rock breakage is numerically studied. For this end, a numerical method based on embedded discontinuity finite elements for solving the coupled piezoelectro-mechanical problem is developed. Rock fracture is described by embedded discontinuity approach where a crack with the normal parallel to the first principal direction is introduced into a constant strain triangle element upon violation of the Rankine criterion. Random clusters of finite elements representing the constituent minerals with their respective mechanical and electrical properties describe the rock heterogeneity. In this preliminary study, the thermal effects are ignored.

In the numerical examples, cracking of heterogeneous rock specimens under high voltage DC boundary condition is simulated in axisymmetric conditions. The results demonstrate that voltages of 0.5 to 1 MV are required to induce cracking by piezoelectric actuation of Granite. Therefore, based on these preliminary simulations, this principle seems to be a feasible method to weaken rock and thus facilitate the mechanical breakage.

1 Introduction

Traditional rock drilling is based on mechanical breakage by rotary or percussive drilling methods. However, under the pressure of ever-increasing demands for efficiency and economy non-traditional drilling methods using some non-mechanical agent, such as thermal jets (Rossi et al. 2018), to assist or replace the mechanical breakage have been searched during the last couple of decades. These methods are sorely needed in drilling for geothermal energy in Scandinavia where the bed rock is "cold" Granite and thus wells down to depth of 8 km are required. Such depths naturally incur high costs on purely mechanical drilling due to heavy wear of the drill bits.

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