Abstract

The application of electric energy in drilling boreholes by impingement of electric current flow at high current density under the drill bit and the contact point of rock has been studied.

The science of electric conductivity of the ground and the theory of heating effects of ground electrodes at high current density is the basis of electrical disintegrating drilling. The electrothermal and electrodynamical forces generated act directly under the drill bit to disintegrate the rock.

Electric power has been used or recommended previously to drill boreholes.

An electric drilling technique developed in the Soviet Union based on the condensor discharge principle, utilizes shock waves traveling through liquid to fracture rock. In order to produce rock disintegration, the electrodes are energized in sequence around the periphery of the electric drill bit.

The electric arc drilling idea has been advanced by Drilling Research, Inc. This drilling device consists of two electrodes spaced approximately 1 in. apart. In this case, electric arc substitutes combustion as the energy source. Electric arc drilling might be compared with jet piercing drilling.

A number of patents on electric arc drilling have been issued. It has been proposed in these patents to generate the electric arc between the electrodes and the ground through the application of temperatures high enough to melt the rock. Also, it has been proposed to superimpose an electric potential between the ground and conventional bit, hoping that increased drilling rates will result.

The idea advanced and tested by Electrofrac Corp. using the high current density effect of ground electrode [electric drill bit] is different from the above described methods.

The electrical conductivity of ground consists of [1] electron conductivity, [2] ionic conductivity, and [3] dielectric conductivity.

Rock composition is, in general, heterogeneous and the different components have variable electrical conductivity. With the exception of some of the metallic ores that have good electron conductivity, the ground conductivity is essentially electrolytic.

In drilling with a steady flow of current, it has been necessary to generate strong electric stress concentration at the edges and points of the electric drill bit. The high temperature created by electric current flow combined with chemical decomposition and physical fracturing of mineral aggregates disintegrates the rock under the drill bit.

The differential in thermal stresses is increased by the sudden cooling effect of the electrically heated zone. Drilling fluid [air] has been used as a cooling media. In rotating the electric drill bit, it has been possible to develop an oscillating heating and cooling cycle and regulate it with the rotation and electric load to obtain maximum penetration rates.

Experiments to drill concrete blocks and taconite have been conducted in the laboratory with a 10 KVA - AC transformer. The data of penetration rates in drilling concrete at variable electric load with 1- and 2-in. drill bits is available. The maximum rate of drilling has been 126.3 ft/hr with 10.1 KW load.

Field experiments have been performed with a REICHdrill, manufactured by the Reichdrill Div. of Chicago Pneumatic Tool Co. Shallow drilling of top soil and limestone has been accomplished with electric drilling. Electric power equipment capable of delivering up to 1,200 KVA has been available. In drilling 4–7/8-in. holes, up to 150 KVA load has been used.

Introduction

The drilling of boreholes in rock formation is, at the present time, mainly accomplished by rotary or percussion drilling.

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