DOWNHOLE MOTOR SPECIFICALLY DESIGNED FOR DIRECTIONAL AIR DRILLING

Les Shale
Eastman Christensen
Houston

A positive displacement motor (PDM), designed specifically for use with air as the drilling medium, reduces the limitations of air drilling with mud motors.

Conventional motors that run on air rather than mud have a reduced stator life and accelerated damage to bearings which were designed for lubrication with mud. Mud motors are designed to be run using drilling mud as the power source, as a lubricant, and as a heat (friction) dissipater.

The significant differences between drilling mud and air have led to problems in the application of conventional mud motors for air operations.

Most air drilling operations typically use either an air hammer or, for a directional well, a Moineau-type positive displacement mud motor adapted to an air drilling environment. In these operations, bottom hole assembly performance is heavily dependent on motor life and lithology type.1

The successful application of air drilling techniques is determined by formation hardness, the available capacity and pressure rating of the equipment, and the water influx from drilled formations. Because soft formation cuttings are usually too large to be lifted with air at applicable depths, soft formations are not usually suitable for air drilling. However, cuttings from hard formations can be lifted with an adequate volume of air.

Water influx from drilled formations reduces the hole-cleaning efficiency if the formation is drilled only with air. The addition of soaps and other chemicals through mist injection into the drillstring allows air drilling to continue even with significant water influx.3

Because air is compressible, its flow rate changes with pressure. Also, because of its much lower lifting capacity, air requires annular velocities much greater than those for drilling mud. The air volume required to clean the hole is approximately three times greater than the recommended flow rate for a conventional mud motor.4 However, the higher air volumes exceed the recommended flow rates for mud motors, often causing premature motor failure.

Air drilling is constrained by the poor performance of standard mud motors converted for air drilling use. Motor life is typically less than 30 hr, and penetration rates are about half those of rotary drilling. When used with air, standard motors also have a tendency to fail to restart after makeup of tool joint connections. This forces additional trips.

These factors have made directional air drilling too time consuming and therefore too expensive for use on many reservoirs.

AIR MOTOR DEVELOPMENT

Much industry experience using mud motors for directional air drilling has been unsatisfactory. On a well-documented Department of Energy horizontal well, 14 motors were used to drill the curve section, with average motor life of approximately 22.6 hr. Motors have improved some since then, but average motor life has reached only about 30 hr.5

The following are considered necessary in the development of a satisfactory air drilling motor:

The optimum air drilling system should have a mean time between failure of at least 50 hr for each component, including bit, motor, and data transmission system. A minimum motor life of 30 hr should permit drilling of any required curve in one trip.6
The tools should be available in standard sizes.
The system should be able to drill with dry air to avoid formation damage caused by various mists.
The system should have a build rate capability that reaches a minimum of 10/100 ft and should include the option of a variable kickoff sub configuration to provide maximum profile design flexibility.
The desired penetration rate of an air drilling system should approach 30-60 ft/hr.

To combat the difficulties faced by mud motors used with air, a positive displacement motor was developed specifically for use with air or gas as the drilling medium. The tool, designated Mach 1/AD, has a modular design based on the same manufacturing process used for mud motors. This motor design includes a Moineau-type multilobe motor section, an adjustable kickoff sub, a unique universal joint assembly, and a sealed bearing assembly (Fig. 1).

Because a bypass valve is not required for air drilling, it has been replaced by either a top sub or an alignment bent sub. The top sub has an API box up and is used for low-to-medium inclinations (i.e., for build-up rates of 0-12/100 ft). The alignment bent sub also has an API box up and, when used in conjunction with an adjustable kickoff sub, can produce build-up rates of up to 20/100 ft.

The motor section is a 7/8 lobe rotor/stator design, with extremely enlarged chamber volume to meet the requirements of drilling with air or gas. The rotor is hard chromed with a smooth surface finish to help reduce drag and heat buildup from friction.

The universal joint housing is a special deflection sub called an adjustable kickoff sub. The tilt angle of the kickoff sub can be adjusted from 0 to 2.75 at the rig floor (Fig. 2).

This replaces a conventional universal joint with flame-cut lobe configuration. In addition to transferring the eccentric motion of the rotor to the concentric rotation of the drive sub, this universal joint design evenly transmits the produced torque through the bearing assembly to the bit (Fig. 3).

The Mach 1/AD is equipped with a sealed bearing assembly with either a grease or oil fill. Both types of sealed bearing assembly have been used, but neither has yet shown any advantage over the other.

DESIGN EFFICIENCY

Mud motors operate less efficiently (with air) than motors designed specifically for use with air. Because mud motors are designed to be run with liquids, they have a small rotor/stator chamber volume to achieve the necessary proportion between flow rate and speed. Therefore, a relatively high differential pressure is needed to get the required amount of torque.

If this working point is to be reached with air as the drilling medium in a mud motor, large compressors and boosters are needed because of the compressibility of air. A great deal of compression work is stored in that air. However, mud motors can use only the displacement work, and not the expansion work, of the compressed air. This leads to extremely poor efficiency (Fig. 4).

Stalling of the motor can lead to a slow increase in pressure and torque because the drill pipe acts as pressure storage. However, pulling the motor off bottom leads to an expansion of the air and a dramatic increase in speed, with the effect of distorting the motor and causing motor "run away," or overspeed.

To address this problem, several test trials used various nozzles to increase the pressure and subsequently limit flow and speed. However, it was determined that the best solution was to adapt the motor to the operating conditions by optimizing the chamber volume. As a result, even at high flow to maintain hole cleaning, the motor speed is kept low. Only low pressure is required to provide sufficient torque for drilling. That gives this type of air drilling motor several advantages:

No boosters are necessary.
Efficiency improves because of the lower portion of work for volume change related to total compression work (Fig. 4).
The motor does not stall easily because of the more efficient relationship between torque and pressure.
The motor is less likely to overspeed when pulled off bottom because there is lower stored energy in the drill pipe.

DESIGN CONDITIONS

The air drilling motor specifications are based on normal calculations for conventional motors in which geometrical and actual flow parameters determine performance (Table 1). The actual intake volume of the working fluid determines the speed, and this relationship is independent of the static pressure.

When the mud motor, which is a hydrostatic motor, is run with compressive fluid, the actual intake flow at the top of the motor, and therefore the speed, depends very strongly on the static pressure at that point. Either the mass flow or the flow rate at standard conditions can be used as a basis for comparisons.

The air motor design boundary conditions are based on rough estimates. The coldness of the expanding air was assumed to balance the geothermal heat. Therefore, the boundary conditions were calculated with the entire drilling process considered isothermal at 80 F. (300 K.). The data on flow rate and standpipe pressure are based on typical air drilling equipment used on conventional rigs.

The pressure loss in the bearing assembly, float valve, and bit is assumed constant at 58 psi. Another initial assumption is that the pressure loss in the annulus is one third that in the drill pipe. However, the pressure loss in the drill pipe and annulus varies, depending on the depth and diameter of the bore hole. The other assumptions for the boundary calculations included a constant intake flow rate at the compressor of 2,100 scfm and constant standpipe pressure of 350 psi. The maximum allowable differential pressure on the motor section is determined by Equation 1. The absolute pressure of the intake of the motor is determined by Equation 2, which is then used to calculate the flow rate through the motor (Equation 3).

To calculate the speed at no load, it is assumed that a minimum differential pressure on the motor section of 50 psi is required and that the increase in speed is proportional to the increase in flow rate. In a practical application, motor performance is influenced by the effects of pressure storage, dampening, temperature, and flow.

For stalling, the pressure loss in the bearing assembly, bit, and annulus is zero, and the pressure loss of the drill pipe decreases because there is no flow rate through the motor. As a result, the complete standpipe pressure is given as differential pressure on the motor and thus, maximum torque independent of depth.

At no load, the speed will be limited by the throttling effects of the drill pipe, bearing assembly, annulus, and the motor section itself. Therefore, the only values which can be roughly calculated are the extreme values of the working range, but with many simplifying assumptions.

MOTOR PERFORMANCE

The comparison of specifications in Table 1 is based on fluid flow rates. The table shows that the design of the special motor section (the rotor/stator configuration) has enabled the motor to accept the much higher flow rates required for air drilling while still maintaining a low differential pressure. This is essential to ensure that the motor does not "run away" with drilling torque when lifted off bottom.

For a situation with the 6 3/4-in. air motor run with no pressure loss through the drill pipe and annulus, the maximum allowable differential pressure on the motor section is 290 psi. This leads to an absolute pressure on top of the motor of 363 psi, flow rate through the motor of 697 gpm, and a motor speed of 110 rpm.

The Mach 1/AD motor has sufficient pressure losses through the bearing housing assembly, bit, and annulus to prevent it from overspeeding. However, in the case of no load (e.g., when testing the motor on the rig floor), the absolute pressure on top of the motor is 123 psi, resulting in a flow rate of 2,049 gpm and a speed of 323 rpm.

This speed will still be acceptable for the motor but care must be taken to ensure that either the intake flow rate is reduced to 1,000 scfm or that standpipe pressure on top of the motor does not drop below 145 psi (i.e., 160 psia). Thus, this determines the safe maximum speed of 250 rpm. To account for the two different downhole working conditions of no load and stall, the losses of the drill pipe and annulus must be included in the calculation.7 The Mach 1/AD motor can be run on dry air. However, experience has shown that motor performance improves if oil is injected into the air supply in quantities sufficient only to lubricate the motor section (0.5 gal/2-4 hr).

With this low rate, no trace of the oil is evident at the blooey line or on sample cuttings. A number of other lubricants can be used on the motor: soap at 10 gal/15 bbl water, graphite powder at 5 gal/15 bbl water, and gel at 1 gal/15 bbl water.

Initial tests showed that the air motor can achieve double the penetration rate (ROP), accept twice as much weight on bit (WOB), and have twice the motor life of a conventional mud motor in an air drilling application (Fig. 5)

ORIENTATION

Generally, in one run with the adjustable kickoff sub, the air motor can establish the desired direction and inclination for the surface interval of a directional well.

The adjustable kickoff sub places the bend close to the bit and can be adjusted so the motor housing tilt angle can be configured on the rig floor to settings from 0 to 2.5. The resulting dogleg capability can be as high as 12/100 ft. The special design of the adjustable kickoff sub requires no shims to adjust the bent housing angle, so a single motor can achieve a variety of build rates.

If an alignment bent sub is fitted to the top of the motor and used in conjunction with the adjustable kickoff sub, the motor configuration can be used for building angle, as in a fixed angle build motor. Orientation of the motor and drillstring is possible in this configuration, but rotation is not. The build rate possible from this motor configuration is approximately 20/100 ft.

In air drilling operations, conventional survey instruments have to contend with severe conditions. Because there is no drilling fluid to help dampen the effect of vibration and resonance, the survey instrument can fail quickly.

Air operations with mud motors rely primarily upon singleshot surveying to provide the directional information necessary to complete the motor run.

Other forms of instrumentation have been tried, but with limited success. For example, the wire line steering tool has limited application because of vibrational damage, constraints on rig time, and the need to use a side entry sub with the wire line. Measurement while drilling (MWD) and electromagnetic MWD have been somewhat successful in air drilling applications, but vibrational damage and inadequate signal retention at depths over 5,000 ft remain major problems.

For the Mach 1/AD motor, a cartridge data transmission system (CDT) allows orientation of the motor in a particular direction, although it still allows drilling straight ahead with drillstring rotation. This CDT system uses a specially made rugged steering tool which provides continuous surface readout of drift angle, azimuth, and tool face orientation while drilling. A hard wire from the steering tool through the drill pipe and kelly to the surface relays the information to surface computer equipment.

CASE HISTORIES

During the past 3 1/2 years in the eastern U.S., Eastman Christensen has completed approximately 97 air drilling directional jobs using both conventional mud motors with air and the Mach 1/AD air motor. For these applications, drilling area, lithology, formations, and surface equipment were basically the same for both types of motor.

Air drilling techniques have become routine in various oil and gas fields in the Appalachians, Arkansas, the San Juan basin, and in some areas of West Texas. In drilling areas with lost circulation problems, such as the Rocky Mountain region, air and natural gas drilling have been used extensively. And air drilling techniques have been used in some offshore operations to drill through potential lost circulation formations.2

A well in Buchanan County, Va., was air drilled to bleed off methane gas from a coal seam. The objective was to kick off in the 8 3/4-in. hole and build angle along a 50-ft radius to a target area 350 ft from vertical at a total depth of 2,232 ft. The formation drilled with the air drilling motor was primarily sandstone and shale.

The well was kicked off at 711 ft using the Mach 1/AD air motor with the adjustable kickoff sub set to 1.3 to achieve a build rate of 9/100 ft. The well was drilled with a tricone bit with open nozzles, and a singleshot package was used for surveying. The motor was lubricated with soap at 10 gal/15 bbl water.

The motor performed with no problems, building angle at an average 7/100 ft. The air motor drilled to 1,547 ft measured depth (MD) in both oriented and rotary modes. Two trips were necessary: one to change the bit after drilling 240 ft to an inclination of 11 and another to set the kickoff sub to 1.1 to achieve a build rate of 8/100 ft.

The assembly then drilled another 596 ft at an average penetration rate of 30 ft/hr. In both motor configurations (1.3 and 1.1), flow rate was held at 1,675 scfm with a standpipe pressure of 180 psi. WOB ranged around 18,000-20,000 lb., and total time drilling was 33 hr.

A second coal seam well in Buchanan County was air drilled with the same motor. The well was planned to kick off at 759 ft MD in the 8 3/4-in. hole, building angle along a 50-ft radius to a target area 200 ft from vertical. The formation to be air drilled was sandstone and shale.

At 759 ft, the well was kicked off with the adjustable kickoff sub set to 1.1 to achieve a build rate between 6 and 7/100 ft. This well was also drilled with a tricone bit with open nozzles, a singleshot survey package, and soap lubrication at a concentration of 10 gal/15 bbl water.

The motor performed well, building angle at 6/100 ft to a final inclination of 11. The motor was run in both rotary and oriented modes, drilling to 1,235 ft true vertical depth, the planned interval for this motor, without requiring a single trip. The motor drilled a total of 476 ft at an average penetration rate of 39 ft/hr. The flow rate for this run was 1,623 scfm with a standpipe pressure of 200 psi. WOB ranged 17,000-21,000 lb. Total drilling time was 12.25 hr, giving this motor an accumulated tool time of 45.25 hr.

The same air motor was used on a third well at Charleston, W.Va. The objective was to kick off at 3,191 ft MD, build angle to 87.37 at 4,553 ft MD, then conventionally drill horizontally for 2,000 ft, dropping angle at 0.25/100 ft. The Mach 1/AD kicked off in the 8 3/4-in. hole at 3,191 ft MD and drilled through a shale formation. The kickoff sub was set to 1.1 to achieve a build rate of 8/100 ft. The drilling assembly included a tricone M84F bit with three 16/32-in. nozzles and a steering tool for directional orientation. Oil injection at 0.5 gal/joint was used for lubrication.

After drilling 559 ft at an average penetration rate of 30 ft/hr, the motor built inclination to 36.5. The adjustable kickoff sub was then set to 1.3 and a 1 1/2 alignment bent sub was added to achieve a build rate of 9/100 ft in the 8 1/2-in. hole.

On this trip the bit was changed to a tricone M84F with open nozzles. This assembly drilled 618 ft, building inclination to 74 at 4,368 ft, achieving the directional objectives. Penetration rates up to 60 ft/hr were recorded during this run, and the average rate of penetration was 40.9 ft/hr.

On both runs, flow rate averaged 1,900 scfm with standpipe pressure of 250-300 psi. WOB was held to 6,000-8,000 lb. Total drilling time was 44 hr for a total of 89.25 tool hours on this Mach 1/AD. The component inspection showed no excessive wear or damage; the motor was rerunnable.

REFERENCES

Yost II, A.B., Overbey, W.K., and Carden, R,S., "Drilling a 2,000 ft Horizontal Well in Devonian Shale," SPE Paper 16681, presented at the SPE Annual Technical Conference, Dallas, Sept. 27-28, 1987.
Lyons, W. C., Air and Gas Drilling Manual, Gulf Publishing Co., 1984.
Whiteley, M.C., and England, W.P., "Air Drilling Operations Improved by Percussion Bit/Hammer tool Tandem," SPE/IADC Paper 13429 presented at the SPE/IADC Drilling Conference, New Orleans, Mar. 6-8, 1985.
Carden, R.S., "Air drilling has some pluses for horizontal wells," OGJ, Apr. 8, 1991.
Yost II, A.B., "Horizontal Gas Well Promises More Devonian Production," American Oil & Gas Reporter, July 1988.
Bitto, R., "Eastman Christensen Memo-Air Drilling," June 2, 1989.
Grimmer, H., "Eastman Christensen-Drilling Research Center Technical Report, Air Drilling with Navi-Drills," May 28, 1991.