JET PULSING MAY ALLOW BETTER HOLE CLEANING

M. S. Bizanti
Louisiana Tech University
Ruston, La.

Jet pulsing, intermittent nozzle flow, by sweeping drill particles across the bottom can improve cutting removal. During each rotation cycle, particles will be swept by the high peak velocities and stabilized during normal velocities. For intermittent nozzle flow, the smaller areas open to flow yield higher impact pressure, higher nozzle velocity, and high bit-hydraulic horsepower.

Bottom hole cleaning is a major factor affecting the rate of penetration. Most of the energy available at the rig surface is expended at the drill bit and, in order to minimize the cost of drilling operations, both energy and time must be conserved.

Bottom hole cleaning is a function of the drilling fluid in removal cuttings from underneath the bit face to prevent bit-balling and floundering. At the bit face, fluid flows through the nozzles, across the bottom, and back to the surface through the annulus. The resultant friction and turbulence of mixing causes a great loss of energy.

Efficient bottom hole cleaning must accomplish:

Washing at the mud-rock interface
Controlling cross flow and avoiding the regrinding of cuttings
Fast-lifting of cuttings around the bit face

BACKGROUND

It is widely accepted that bit hydraulics has a major effect on the rate of penetration. The actual rate of penetration is always equal to or less than the rate achieved if chips are removed as soon as they are generated.1 2

Today's jet bits consume the hydraulic horsepower available at the bit by the fluid velocity that exists from the nozzles that are located at certain distance from the bottom of the hole. The nozzle sizes are comparable to drill bit cone diameter.

It has been established 3 that extension of bit nozzles improved bottom hole cleaning. Sutko4 studied the effect of extended nozzles on bottom hole pressures. Five nozzle extensions were used (0-3.5 in.) and several velocities (90-321 fps) were used.

It was observed that the pressure at large values of distance between the nozzle and hole bottom per nozzle diameter is extremely low. As the distance becomes smaller, the pressure increases correspondingly. The conclusion was that as the number of nozzles is decreased, the rate of penetration should increase.

Jet hydraulics has been studied extensively by several authors. Kendall and Goins' have presented hydraulics programs for maximum jet velocity, maximum jet impact, and maximum hydraulic horsepower. They did not point out or indicate which criteria had the greatest effect on the rate of penetration.

McLean6 7 has written on the improvement of jet bit hydraulics, suggesting maximizing cross flow velocity for maximizing hydraulic horsepower in current jet bit. He described the jet impingement action on the hole bottom and classified it as two mechanisms.

One is an impact-pressure wave in the immediate area of the jet impingement. The second is cross flow which spreads across the bottom away from the pressure wave.

Equation 1 shows the expression developed 6 7 for the cross-flow velocity at the edge of the immediate area of jet impingement

[SEE FORMULA (1)]

Equation 1 was developed for three-nozzle jet bits. When one of the nozzles is blank, the flow rate of the other two nozzles will be increased by 50%. The corresponding increase in cross flow velocity is 22.5%, based on the square root of QnVn, assuming the nozzle velocity is constant for either a two or three-jet system.

This increase in the cross flow velocity will yield a higher chip force and, therefore, will result in improvement in bottom hole cleaning.

NOZZLE PULSATION

From the theoretical standpoint, if a rotating disk is placed inside the drill bit nozzle, then as the drilling fluid flows through the nozzle the disk will rotate around its axis. As the disk rotates, it will provide different nozzle areas for the fluid to flow through. Therefore, the area open to flow changes periodically with respect to the position of the disk.

The rotating disk inside the nozzle will vary the flow area in the nozzle from a maximum (when t he rotating disk is vertical) to a minimum (when the rotating disk is in its horizontal position).

The upstream nozzle velocity (velocity above rotating disk) in field units is given by:

[SEE FORMULA (2)]

The downstream nozzle velocity (velocity below rotating disk) is figured out by:

[SEE FORMULA (3)]

The corresponding pulsing jet impact force and hydraulic horsepower can be evaluated from:

[SEE FORMULA (4)]

and

[SEE FORMULA (5)]

where the nozzle pressure drop is calculated by:

[SEE FORMULA (6)]

TEST PROBLEM

Pump flow rate = 400 gpm Mud weight = 13 ppg Nozzle discharge coefficient = 0.95

Total nozzle area = 0.30 sq in.

Allowing the downstream area to change from 100 to 20% by means of the rotating disk, the jet impact force and the hydraulic horsepower are calculated as shown in Table 1.

The results have shown an increase in jet impact force and the hydraulic horsepower as the nozzle area is reduced. For the case at hand, an 80% reduction in the nozzle area has increased the jet impact force by 124% and the hydraulic horsepower by 400%.

During the disk rotation, the jet velocity, the jet impact force, and the hydraulic horsepower will vary from a minimum to a maximum in each complete disk rotating cycle.

Sufficient flow rate is needed to clean the bit, and that jet velocity is needed to free the drilled chip held by differential pressure to the bottom of the hole. This jet velocity has a greater effect on penetration rate than the flow rate alone; however, for a given hole size, there is a limited range of flow rates that can be used for hole cleaning. Therefore, jet velocity for a given hole size is somewhat dependent upon the hydraulic horsepower at the bottom of the hole.

Earlier investigation 8 has shown that penetration rate is dependent upon the circulation rate and also upon the jet velocity. This requirement gave rise to the maximum (QV,) program commonly called "jet impact" program, justified by its proponents who believe that cutting removal is dependent upon the blasting effect of the jet striking the bottom of the hole.

Later investigators, who were concerned primarily with downhole mud-operated devices, chose to consider hydraulic horsepower which has a greater physical significance to oil field engineers than jet impact. The term "bit hydraulic horsepower" came into usage and was justified by its proponents who believed that cutting removal is dependent upon the amount of fluid energy expended at the bit.

REFERENCES

Johnson, V. E., Jr., et al., "Cavitating and Structured Jets for Mechanical Bits to increase Drilling Rate," ASME Paper No. 82-Pet-13, Energy Technology Conference and Exhibition, New Orleans, Mar. 7-11, 1982.
Maurer, W. C., "The Perfect Cleaning Theory of Rotary Drilling," Journal of Petroleum Technology, November 1962, p. 127.
Van Lingen, N. H., "Bottom Scavenging - A Major Factor Governing Penetration Rates at Depth." Journal of Petroleum Technology, February 1962, p. 187.
Sutko, A. A., and Myers, G. M., "The Effect of Nozzle Size, Number, and Extension on the Pressure Distribution Under a Tricone Bit," Journal of Petroleum Technology, November 1971, p. 1299.
Kendall, H. A., and Goins, W. C. Jr., "Design and Operation of Jet Bit Programs for Maximum Hydraulic Horsepower, Impact Force, or Jet Velocity," Trans. AIME, Vol. 219, 1960, p. 238.
McClean, R. H., "Crossflow and Impact Under Jet Bits," Journal of Petroleum Technology, November 1964, p. 1299.
McClean, R. H., "Velocities, Kinetic Energy, and Shear in Crossflow Under Three-Cone Jet Bits," Journal of Petroleum Technology, December 1965, p. 1443.
Kendall, H. A., "Practical Application of Optimum Hydraulics," Libyan Association of Petroleum Technologists Annual Meeting, Tripoli, Libya, Jan. 24-25, 1968.