Abstract

This study focuses on analyzing thermal effects produced by plasma torch on rock physical parameters, designed to assist operations during geothermal well drilling. Plasma torch can interact with samples and reduce rock strength. In the presented theoretical model it is noticed that elasticity modulus (Young’s modulus) and temperatures play important roles in thermal stress distribution. The results show that at 4500°C with 6MPa compressive loading elasticity modulus is the best situation for wellbore instability. By contrast, at 1500°C with a loading of 38MPa elasticity modulus is a suitable result for stability of the borehole. In experimental work it is found that 7 to 10 seconds plasma torch traction with cylindrical specimen rock samples can generate thermal fractures. Elasticity modulus of all granite and limestone samples was sharply dropped after 10 seconds of plasma torch interaction with each dry rock sample on account of the fact that visible cracks generate on the surface.

1. INTRODUCTION

In recent years, problems associated with drilling oil and gas wells around large urban settlements have become a major limitation to underground drilling, particularly in relation to environmental damages in highly populated or industrialized areas. Therefore, the development of underground drilling mechanisms is of high priority [1- 6]. Hard-rock drilling is an expensive process because the drill string machines and bottom hole assembly must be extremely rugged and powerful to overcome the intact hard rocks [3-10]. We present a method to assist improving hard rock drilling machines, which is achieved by developing techniques to weaken the rock before it is mechanically removed [9-12]. This is obtained by generating uniform holes inside of rock textures during drilling and perforation operation [10- 13]. Recently it has been reported that making of holes inside the intact rocks may not be a suitable method [9- 12]. It is proposed that a proper method to change the hard rock into a loss rock can be achieved by generating fractures into the rock texture [10-13]. The fractures can be generated by generating a high temperature gradient within the rock. Temperature increase causes expansion and eventually breakdown. One way to induce a proper temperature gradient is by using the plasma torch in order to assist the drilling operation. During thermally assisted drilling operation fractures are generated inside of the wellbore and this can reduce the wall strength considerably [19-20]. A plasma arc torch can generate that temperature into the rock and material texture [30- 34].