Directional mud motors are the most proven and common method utilized by HDD contractors to complete installations in hard rock formations. Although mud motors are one of the more expensive and sophisticated pieces of equipment used in the industry, drilling contractors can incorporate certain procedures into their drilling program to minimize costs in utilizing motors and maximize motor performance while lowering their cost per foot.
As with other types of mechanical equipment, mud motors should be operated sensibly to provide a cost-effective balance of longevity, performance and dependability. Using a mud motor to ram and cram with high weight loading at high RPM and abrupt stalling and starting in abrasive rubbilized formations is analogous to operating an automobile by popping the clutch, running at redline, driving at high speeds over bumpy roads in a sand storm and sliding in curves. Here are some guidelines to get the most out of a motor.
- During drilling, the motor is slid to steer or rotated to drill straight ahead. After being utilized in the sliding mode, the motor should be backed-off from the rock face a few feet prior to beginning the rotational mode. The driller should closely monitor the torque gage while initiating rotation to ensure the drill string is rotating freely. Rotation of the drill string should be limited to 20 RPM to 30 RPM in curved sections of the bore and not exceed 40 RPM in the straight sections. Rotation speed should not exceed 10 RPM to 20 RPM in severe angled sections of a bore.
Reduces chances of sticking drill string
Assist in cutting removal
Reduce external wear of motor housing
Prevent breakage of internal motor components
Increase drill bit life, especially the gage
Reduce motor repair cost
Lower cost per foot
- During drilling in hard abrasive formations, it is a good practice to ream build sections of the bore by pulling the motor and bit off bottom the entire joint up and back down while rotating the drill string 20 RPM to 30 RPM while pumping. This aids in cuttings removal, reaming the hole smooths the hole and reduces sticking risks. However, in softer sandstones, clays and shales this practice is not recommended due to erosion tendencies of these formations.
- Weight on bit for optimum drilling rates is monitored by differential pressure (the difference between “on bottom” drilling pressure and “off bottom” circulating pressure). Normal operating differential pressure on larger motors (4 3/4 in./119 mm, 6 3/4 in./169 mm, and 8 in./200 mm) should not exceed 400 to 450 psi (28 to 31 bars) and 150 to 250 psi (10 to 17 bars) on smaller motors (2 7/8 in./72 mm, 3 3/8 in./85 mm, and 3 3/4 in./94 mm). If reasonable penetration rates can be achieved at lesser differential pressures then the weight on the bit should be maintained at these differential pressure levels. While using a motor, the best indicator of drilling weight on bit is differential pressure by monitoring the rigs mud pressure gage not the thrust hydraulic pressure gage. The “push” control can be constantly adjusted to maintain a steady pressure differential reading as rock formation is steadily eaten away by the drill bit. Hard rock drills slowly. Be patient.
- Drill with clean mud. A good mud cleaning system not only increases rate of penetration and saves mud costs, but also reduces fluid cutting wear on mud motor internal parts and rig components as well. An analogy is an oil filter to remove grit in an automobile engine. A good cleaning system will remove solids down to 10 to 15 microns. A sand content test will show less than 1/8 (.125) of 1 percent sand.
- Use a well-maintained mud pump of adequate output volume to match mud motor capacity. The mud motor can only convert available hydraulic power from the mud pump into rotation. The mud pump must provide enough gpm (Lpm) and pressure to the motor to get into its “power band” and then must be able to efficiently maintain the gpm (Lpm) level during loading without failing, bypassing, or sucking air. Usually it is most efficient to select a pump large enough to run at 50 to 70 percent of maximum output to avoid constant problems of overworking the pump.
- Plan the bore and drill the bore to minimize sharp turning radii. The lower the bent housing angle on a motor, the lower the stress loading. If possible, do not exceed 2 degrees on a mud motors bent housing. Usually a 1 3/4 or 2 degree motor will provide more than adequate steerability control when utilized with the correct drill bit size and by practicing a “project ahead” program technique during drilling. Remember the closer the bit size ratio is to the motor OD size, the shorter the turning radius achieved from a given bent housing degree.
Lower motor repair cost
Longer motor life
Longer bit life
Easier back reaming operation
Reduced connection fatigue-twist offs
- After using the motor, flush out the mud with a few gallons (liters) of fresh water. Store the motor in a safe place away from heavy mobile equipment. Consider having the motor serviced or swapped out if the next project start date allows time.
- Consider a backup motor-usually available at a lower “stand-by rate”, a backup motor is cheap insurance and can save its cost in shutdown rig time.