Mechanical rockcutting with and without high pressure water jets

Abstract

ABSTRACT A hybrid rock cutting method using water jets to assist the mechanical tools vas investigated. Benefits of using a water jet assist system included: (i) substantial reduction in the bit forces; (ii) reduced rates of bit «ear; (iii) reduced dust production; and (iv) reduction in fractional heating. A series of dry and water jet assisted cutting experiments were conducted with a drag bit in Indiana Limestone. The relative influence of jet parameters in reducing the bit forces was investigated. The optimum performance of the water jet assist system was with a single jet parallel to and about 1 mm ahead of the leading face of the bit. Jet power was the most significant factor in reducing the bit forces. A threshold jet power level had to be exceeded to achieve optimum force reductions. If the jet power was increased beyond this threshold valueno further reduction in bit forces was observed. The influence of bit velocity on bit force reductions was not very significant. An increase in the bit velocity by a factor of four resulted in a six percent decrease* from 50 to 44 percent» ân the force reduction achieved with water jets. Potential benefits of the water jet assist approach to oil well drilling were demonstrated by preliminary experiments combining water jets with FDC bits. Indentation tests were conducted to study the crack growth and chip formation underneath the bit. A sieve analysis was carried out on rock fragments produced during dry and water jet assisted cutting. The surface area of fragments from these cuts was determined. Energetics of the fragmentation process producing these surfaces was studied by using a fracture mechanics approach. It was found that only 10 percent of the applied mechanical energy was used in producing the resulting surface. Based on a comparative review of results and possible mechanisms» an explanation of the mechanics of water jet assisted cutting was developed. The dominant mechanism was continuous removal of the broken rock that formed ahead of the advancing bit. Finally» a theoretical model was introduced for a better understanding of this mechanism.