Hydropulses increase drilling penetration rates

Jack Kollé Mark Marvin
Tempress Technologies Inc.
Kent, Wash.

Tempress Technologies Inc. developed this prototype pulsation device to test flow rates and drilling properties (Fig. 3).

This 20% back-rake cutter bit drilled two samples of Mancos Shale (Fig. 9). [19,264 bytes]

Intense suction-pressure pulses (hydropulses), utilizing a small-scale cycling valve, resulted in increased drilling-penetration rates for test samples of Mancos Shale and Colton Sandstone.

Confining pressure exerted by the mud column in the borehole serves to slow conventional rotary systems because bottom-hole pressures are typically maintained at a value that is equal to, or slightly greater than, the pore pressure of the in situ fluids.

In this case, the confining pressure of the mud increases the strength and plasticity of rock, reducing the efficiency of indentation and shear cutting.¹Thus, the greatest effect of confining pressure occurs in shale, which also accounts for most of the rock encountered during oil and gas drilling activities.

Application theory

Drilling experience has also demonstrated that significant increases in drilling rate can be achieved through underbalanced drilling, accomplished by reducing the amount of weighting material added to the drilling mud, or by using light-weight drilling fluids such as gas or foam.

The problem with underbalanced drilling is that the entire open section of the hole is subject to low pressure. This reduces borehole stability and increases the risk of a gas kick.

An ideal drilling system would create a low-pressure region that is limited to the bottom hole section, where normal or overbalanced conditions are maintained above the bit to control formation fluids.

To create such an environment, the hydropulse system (patent pending) provides a means of generating intense cyclic suction-pressure pulses that overcome the effects of borehole pressure, resulting in increased penetration rates while drilling in deep formations (Fig. 1 [62,624 bytes]).

For future applications, this compact system can be mounted on a conventional drillstring powered with mud hydraulics, or combined with a downhole motor. The suction pulses can be used to enhance the penetration of conventional roller cone, polycrystalline diamond compact (PDC), or diamond bits.

In many situations, such as in horizontal drilling, it is difficult to apply a high mechanical thrust to the bit. The hydraulic thrust generated by a suction-pressure pulse provides a means of drilling with a minimal amount of thrust.

Technical approach

Drilling mud is normally pumped through a drill bit, which then removes the cuttings from the bottom of the hole to transport them to the surface. When directed through high-speed flow courses, the mud flow contains significant kinetic energy that can be converted into a suction-pressure pulse by momentarily interrupting the flow with a valve ( Fig. 2 [75,284 bytes]).

This generates a suction-pressure pulse downstream of the flow interruption. The suction-pulse magnitude can become quite high because of the low compressibility of water-based mud.

The pulse then results in an impulsive thrust on the bit at the same instant the fluid pressure at the bit face is reduced. Suction-pulse drilling results from the simultaneous combination of impulsive bit weights and the reduced confining pressure.

Valve design

The hydropulse system employs a self-cycling poppet valve that generates the appropriate pulse magnitude and time history. The poppet-valve design ensures that flow through the bit is maintained at a constant high velocity up to the instant that the valve closes.

This instantaneously stops the flow of water or mud in a closed conduit, resulting in a suction-pressure pulse downstream of the valve. The magnitude of the pulse can be evaluated by equating the kinetic energy of flow in a closed conduit to the elastic potential energy of the decompressed fluid volume.

If the initial flow velocity is v, the magnitude of the water-hammer pressure pulse is:

where: K£ is the bulk modulus of the fluid and P is the density.² In water with K£ = 2.4 GPa at 35 MPa, the pressure pulse has an amplitude of about 1.5 MPa per m/sec of flow velocity.

The pressure magnitude increases with the square root of the mud density. Flow velocities of 10-20 m/sec are common in the flow courses of carbide-body drill bits, so pressure pulses of 15-30 MPa (2,000-4,000 psi) or more can be generated. The duration of the pressure pulse is determined by the two-way travel time of acoustic waves in the flow conduit.

The speed of sound in water is about 1,500 m/sec. Thus, the duration of a pressure pulse in a high-speed flow course with a length of 1 m would be about 1.3 milliseconds.

Flow cycling valve

Fig. 3 shows a prototype flow cycling valve designed for a flow rate of 2 x 10 ^-4 cu m/sec (3.4 gpm). The discharge from this valve is directed through the pressure drilling test stand shown in Fig. 4 [103,304 bytes].

The flow cycling valve periodically interrupts the flow from the drill to the bit, diverting it to an exhaust port. When the cycling valve poppet is open, flow enters through the drill rod and is discharged through open ports on the bit.

The high-speed flow is directed through flow courses around the bit into a 1-m long suction pressure line. The intensity of the pressure pulse is related to the flow velocity in the suction-pulse line while the pulse duration is related to the length of the line.

High pressure is maintained in the pressure vessel by directing the flow from the suction-pressure line and cycling valve exhaust through an adjustable choke.

Fig. 5 [57,987 bytes] shows an example of a series of suction-pressure pulses generated in a borehole in shale, using a 1-m long suction-pulse flow line. The pressure pulses occur at a cycle rate of 47 hz. Each pulse has a duration of about 1.3 (microsecond) msec.

The hydraulic power associated with each pulse is 0.75 Joules, and the power level is 35 w in this example. Fig. 6 [49,3074 bytes] shows the pressure at a location upstream of the valve. There are no water-hammer pressure spikes because the flow is never interrupted. This means that this cycling valve can be located downstream of a downhole motor without affecting motor performance.

The amplitude spectrum of the suction-pressure pulse signal is shown in Fig. 7 [52,595 bytes]. The pulse energy has strong harmonic peaks at the basic cycle frequency of 47 hz and higher (94 hz, 141 hz). The amplitude level at 1 khz is only 10 db below the peak at 47 hz, indicating that this pulse would provide an intense source of high-frequency seismic energy.

Small-scale tests

A series of small-scale drilling experiments has been carried out to demonstrate the concept of suction-pulse drilling. These experiments involved rotary drilling with a small, 8.7 mm (0.343 in.) diameter drill into a pressurized rock sample. Drill bits with and without mechanical cutting elements were used.

Drilling tests were carried out with and without the application of underbalance pressure pulses in order to allow a direct observation of the effect. As shown in Fig. 8 [50,180 bytes], the application of suction-pressure pulses causes a sharp increase in penetration rate.

Shale, sandstone, and granite samples

Fig. 9 shows a carbide bit that was fabricated by grinding a 20% negative back rake on the carbide. The cutter edge was ground to 90% to provide clearance on the face. The back-rake cutter bit was used to drill two samples of Mancos Shale.

Tests were carried out at two thrust levels. When suction pulses are applied at zero thrust, the penetration rate of the back-rake cutter is essentially the same as with 200 Newtons (N) of thrust. The drilling rate with 200 N thrust shows a step increase by a factor of four at about 10 MPa of suction-pulse pressure.

A similar test was carried out in Colton Sandstone at a relatively low thrust level. The application of 16 MPa suction pulses causes the penetration rate to triple.

An indentor bit with a 120% carbide cutter was used to carry out suction-pulse drilling tests in Sierra White Granite (Fig. 10). The application of 9 MPa suction pulses at a frequency of 20 hz caused the penetration rate to triple in the granite sample.

The drilling rate data obtained during the feasibility study are summarized in Table 1 [47,230 bytes]. All of the drilling tests in shale, sandstone, and granite indicated a significantly increased rate of penetration as a result of the application of suction pulses.

The increased penetration rate was observed at pulse magnitudes of about 10 MPa and a frequency of 20 hz in Mancos Shale. Application of a 10 MPa suction-pressure pulse causes the mechanical load to increase by 250 N for about 1.5 msec.

The pulses do not have the short duration or high-amplitude characteristic of an impact. The pulse rate is 20 hz, so the time-averaged thrust increase is only 8 N. The increased thrust can only account for a small part of the observed effect on drilling rate.

Findings

An analysis of suction-pulse propagation in rock has shown that these pulses will induce effective tensile stresses in the rock that are comparable to or greater than the tensile strength of the rock.3 Thus, it can be concluded that the increased drilling rate results from the combined effects of effective tensile stresses and mechanical indentation.

A preliminary design has been prepared for a full-scale hydropulse system to determine whether a flow cycling valve could be deployed downhole. The hydropulse valve design would fit into a 150-mm diameter (6 in.) subassembly, designed for full-scale, suction-pulse drilling operations.

The valve circuit is similar to the engineering prototype shown in Fig. 2. This valve will operate at 85% volumetric efficiency-that is 15% of the flow is used to actuate the valve and is exhausted.

Specifications for a 150-mm diameter hydropulse drilling system are provided in Table 2. [52,444 bytes] At a flow rate of 300 gpm, the cycling valve would operate at 60 hz. The cycling frequency and suction-pulse magnitude are proportional to flow rate. Intermediate scale systems suited for operation on coiled tubing are also possible.

The pulse power level on the face of a 200-mm diameter bit would be 165 kw/sq m. The subassembly is shown with a conventional PDC bit equipped with jet nozzles. A tri-cone or open-flow diamond bit could also be used with the subassembly.

Surge-and-swab pressures were calculated as a function of mud weight and viscosity using a turbulent Bingham fluid model.⁴ The surge pressures are insignificant compared to normal pressure fluctuations during tripping operations using conventional drilling tools.⁵

Seismic applications

Seismic investigation provides an important tool for characterizing oil and gas reservoirs. Seismic work is normally carried out separately from drilling. The suction-pulse generator provides an intense, periodic acoustic signal that can be used for seismic profiling during the drilling process (seismic-while-drilling).

The suction-pulse drilling system discussed above would generate a pulse power of 7 kw with significant energy at frequencies greater than 1 khz. In this application, a seismic receiver would be located on the surface, in a parallel borehole, or on the drillstring above the bit.

Seismic interpretations of formation properties may be used to locate the bit along with oil or gas-bearing formations. The intense signals generated by the suction-pulse system may be particularly well suited for determining pore pressures in the formation ahead of the bit.

High formation pore pressure or the presence of gas causes strong attenuation of seismic signals.⁶ Attenuation measurements are normally difficult to make because the source signals are relatively weak. Knowledge of formation pore pressure provides enhanced safety during drilling, particularly in deepwater drilling and in gas-pressurized formations.

Acknowledgment

This work was supported under cooperative development agreement No. DE-FC26-97FT34367 with the U.S. Department of Energy-Federal Energy Technology Center. Any opinions, findings, conclusions, or recommendations expressed within the article are those of the authors and do not necessarily reflect the views of the DOE.

References

Kollé J.J., "The effects of pressure and rotary speed on the drag bit drilling strength of deep formations," SPE paper 36434, Proceedings of the 71st Annual Technical Conference and Exhibition, Denver, May 6-11, 1996.
Trostmann, E., Water Hydraulics Control Technology, Marcel Dekker Inc., New York, 1996.
Kollé J.J., "Underbalance pulse drilling rock mechanics analysis," TR-003, Tempress Technologies Inc., Kent, Washington, 1998.
Monicard, R., Drilling Mud and Cement Slurry Rheology Manual, Editions Technip, Paris, 1982.
Bourgoyne Jr., A.T., Chenevert, M.E., Millheim, K.K., and Young Jr., F.S., Applied Drilling Engineering, Society of Petroleum Engineers, Richardson, Tex., 1986.
Mavko, G., and Nur, A., "Wave attenuation in partially saturated rocks," Geophysics, Vol. 44, pp. 161-78, 1979.

The Authors

Jack Kollé is president and cofounder of Tempress Technologies Inc. He received a PhD in geophysics from the University of Washington in 1980 and holds a BA in physics from the University of California at San Diego. Kollé has developed a variety of novel drilling systems and has extensive experience in finite element analysis, fluid mechanics, poroelasticity, flow visualization and engineering systems design. He has authored over 30 papers on subjects ranging from iceberg towing to sediment micromechanics. In addition, Kollé holds three patents for hydraulic pulse generators.

Mark Marvin is a senior engineer who joined Tempress Technologies in November 1997. He received a BS in physics from Central Washington University in 1974 and a BS in mechanical engineering from the University of Washington in 1980. He has 20 years of engineering design and analysis experience in a research and development environment including ultra-high pressure waterjet systems.