Drilling Hydraulic Power Optimization in Coiled Tubing Drilling of Hard Rock

Coiled tubing drilling of hard rock for mineral exploration is a new technology being developed by the Deep Exploration Technologies CRC, Australia. Coiled tubing drilling of small holes in hard rock relies on sufficient drilling fluid power for breaking the rock and transporting cuttings up to the surface. High-speed rotating diamond-impregnated bit cuts hard rock into powder form particles, which consumes a large amount of power; meanwhile, cuttings transported through the narrow annular space also cost a lot of hydraulic power. The drilling power supply becomes an issue if water is used as drilling fluid since it is incompressive and can only transmit hydraulic power. In such a case, it is necessary to optimize the use of drilling fluid power for achieving maximum rate of penetration. The drilling power optimization here is different from that of the conventional rotary drilling for oil and gas production. This paper analyses the difference and proposes optimizing the borehole size for achieving the optimum use of drilling fluid power and maximum rate of penetration. An optimization algorithm is developed, and an example is presented.