Ultra-High Pressure Jet Assist of Mechanical Drilling

Abstract

Abstract An ultra-high pressure (30,000 psi) jet is added to a conventional TCI roller cone drill bit to increase the rate of penetration (ROP) by a factor of 1-1/2 to 2-1/2 times conventional rates in harder, slower drilling rocks. The ultrahigh pressure, high velocity jet cuts a small kerf in the bottom of the borehole which enhances the mechanical drilling action of the conventional bit. The ultra-high pressure is generated down hole just above the bit with an ultra-high pressure down hole pump (DHP). An attempt is made through a numerical finite element model of the bottom hole stresses with jet kerfing to describe what is believed to be the basic mechanism of jet-assisted drilling. The modeling results suggest a factor of 3 increase in ROP as a practical upper bound for low permeability rocks such as shale. Laboratory tests of full-size drill bits (7-7/8 to 8-3/4 inch diameter) with ultra-high pressure jet assist have shown ROP increases of 2 times the bit ROP without jet assist. The same bits with ultra-high pressure jet assist have been run in the laboratory and down hole in the field with similar results. The effects of pressure, mud weight, and placement of the ultrahigh pressure nozzle on jet-assist ROP are discussed using laboratory and field drilling data. The effectiveness of jet assist is also discussed in terms of shale, limestone, and sandstone rock types using field data. from East Texas. Finally, the effect of overall hydraulic horsepower consumed is discussed. Introduction The goal of ultra-high pressure (UHP), jet-assist drilling is to increase the rate of penetration (ROP) in deeper oil and gas wells, where the rocks become harder and more difficult to drill. Increasing the ROP can result in fewer drilling days, and therefore, more economical drilling of oil and gas wells. During the late 1980s and early 1990s, FlowDril developed the FlowDril System for UHP, jet-assisted drilling. With this system, described by Butler, et al (1990), Cure and Fontana (1991) and by Veenhuizen et al. (1993), about 30 gpm of the mud stream was pressurized with pumps at the surface to as much as 32,000 psi, almost 600 hydraulic horsepower, and conducted to the drill bit through a special dual-conduit drill string. This allowed an independent, high-velocity jet of drilling mud at the bit to be directed at the bottom of the hole to assist the mechanical action of the tricone insert bit. A number of field projects were conducted with this system, 11 in West Texas and 11 in East Texas, totaling about 90,000 feet drilled in 8-3/4 and 7-7/8 inch hole sizes. ROP enhancements achieved were between 1.3 and 3.1 times conventional rates. Small scale laboratory tests of ultra-high pressure jet-assist of roller cone bits were reported by Kolle et al. (1991). ROP enhancements from about 1.2 to as much as a factor of 2.5 were observed. They favorably compared the field drilling results with the FlowDril system with their laboratory jet-assist drilling results. Two potential mechanisms were identified as potential mechanisms contributing to the ROP enhancement; more effective cleaning (supercleaning) due to the presence of the ultra-high pressure jet, and kerfing, or slotting of the hole bottom. They postulated that the effect of the kerf was to relieve the confinement of the surrounding rock, allowing the fractures under an indenter cutter to propagate to the kerf, increasing the volume of rock removed by the indenter. In late 1993, FlowDril and the Gas Research Institute (GRI) began a development program of a UHP down hole pump (DHP) for jet-assist drilling. P. 79^