Full-Scale Experiments on Jets in Impermeable Rock Drilling
Authors: Feenstra R. , Van Leeuwen J.J.M. 
Published: December 2007

Abstract
The effect of jets on bit penetration has been investigated by means of a 50-ton drilling machine and 8 1/2 -in. commercial jet bits, drilling under representative bottomhole conditions. The conclusions apply to the drilling of impermeable rock only. Penetration rate is in all cases primarily hampered by a differential pressure effect, known as dynamic holddown. Jet action can reduce this slightly. Major gain in penetration rate, up to 25 or 50 per cent, is already obtained at medium-high jet power. At high bit load, penetration by soft to medium- hard-formation bits is further hampered by bit-balling. Its alleviation requires intensive tooth scavenging, which is best performed by slanted nozzles with trailing, high-velocity, high-volume jets, hitting bottom in front of the teeth. Penetration may then improve by a 100 per cent. At high bit load, bottom-balling limits the penetration of insert-type bits. In this case bottom scavenging is required, which is most effectively performed with high velocity jets from nozzles extended close to bottom. As a result of this measure, penetration may improve 70 per cent in non-friable rock, and less in friable rock. This will make the insert-type bit more versatile. Toothed hard-formation bits suffer at high bit load from both bit- and bottom-balling. Slanted nozzles, possibly much extended, with high-velocity, high-volume jets then have best prospects.
Introduction
For years the use of jet bits in oil well drilling has been increasing, in conjunction with ever greater hydraulic horsepower. To the same extent, the need for efficient use of the horsepower has grown. This paper, it is hoped, will contribute to an understanding of how effective jet action may be obtained. It deals with the effect of jets on phenomena that are known to hamper bit penetration. To study this effect, laboratory drilling tests were performed with 8 1/2-in. commercial bits under bottom-hole pressures representative of deep wells. The investigation has been restricted to the drilling of practically impermeable rock. In the following sections, the increase in bit penetration rates due to jetting is discussed for conditions which made the troublesome phenomena stand out one at a time. A brief definition and interpretation of these phenomena is given in Appendix A. Appendix B gives the results of measurements of fluid velocities along the hole bottom, performed in stationary flow tests. Descriptions of equipment, bits, rocks and mud are collected in Appendix C. Unless otherwise mentioned, drilling conditions were kept standard as given in Table 1.
CUTTING TRANSPORT TO THE ANNULUS
Jet action was for some time believed to merely improve the transport of cuttings from bottom to annulus, resulting in a clean hole bottom. Flow tests performed in 1954 at the Battelle Memorial Institute showed, however, that jet velocities of the order of 40 ft/sec sufficed to clear even large (0.4-in.) chips from bottom in the time between two successive tooth actions. In the field, optimum jet velocities are appreciably higher, indicating that other effects must be involved. This much is confirmed by the results of our drilling experiments in soft rocks performed under atmospheric pressure, i.e., in absence of pressure-differential effects. Fig. 1 shows that variation in the nozzle velocity from 40 to 330 ft/sec had no effect on penetration rate, unless the bit penetrated more than 7 mm per revolution, which is about half the tooth height. At such high rates - 150 ft/hr at 100 rpm-cuttings would be caught in the tooth cavities, leading to bit-balling, which was noticed on inspection of the bit. The jets thus assisted in keeping the tooth cavities clean. This is just perceptibly reflected in the penetration-rate curves of Fig. 1.Much the same results were obtained in hard rocks with hard-formation three-cone bits. The conclusion is that transport of loose cuttings towards the annulus does not require a powerful jet action. In the following sections we shall see that in deep-well drilling other phenomena are of greater importance.
DYNAMIC HOLD-DOWN
It is generally accepted that, in deep-well drilling, bit penetration is hampered primarily by the existence of a difference in pressure between the fluid in the hole and that in the rock at the depth of cutting.
TABLE 1 - STANDARD CONDITIONS
ROTARY SPEED 113 rpmBit size 8 1/2 in. Nozzle size 3 nozzles each 13/32 in. Nozzle velocity 100 m/sec (330 ft/sec)Nozzle position conventionalCirculation rate 1500 liters/min (400 gal/min)Bottom-hole pressure 100 kg/sq cm (1400 psi)
JPT
P. 329ˆ