Preliminary experimental validation of the improved ROP using hammer and HPWJ (vertical drilling bench results)

Abstract

Increasing drilling speeds to reduce drastically the costs is the main challenge to promote deep geothermal systems. Traditional rotary techniques show limitations in drilling speed on very hard deep crystalline rocks. This is the challenge pursued by ORCHYD, using the combination of high-pressure water jetting (HPWJ) and percussion to break the rock. A peripheral groove is created by the rotating jet at the bit, reducing the stress concentration at the center, then the rock is crushed by the hammer action.

A test bench has been designed to emulate the realistic down-hole pressure conditions on a hard rock sample. A vertical drill string is connected to a MUDHammer, modified to allow an ultra-high-pressure (UHP) line to pass through its center, down through a custom crossover sub redirecting the pressurized fluid to the periphery of the bit. A nozzle is inserted at the end of the UHP line, at a defined stand-off distance vertical to the top of the rock sample. At the top of the test bench, a custom swivel allows the rotation of the drill string with the UHP line. The hammer is fluid driven and thus, appropriate hydrostatic and confining pressures are applied during the operation. Parameters investigated include chamber (bottom hole) pressure, nozzle orifice diameter, stand-off distance, rotation rate as well as in-situ stress and pore pressure conditions. Experimental results are analyzed in terms of rock breakage mechanisms, depth and shape of the groove and rate of penetration with and without jetting, in order to assess the efficiency of the combined tool.