JAG Rentals

PDM Motors

(Click here for Specifications Chart)

Air or Two-Phase Drilling

The Griffith-Vector multi-lobe motors may be used for drilling with air or two-phase drilling fluids as the circulating medium. Two-phase drilling fluids can be defined as follows:

Mist: occurs when the liquid fraction is less than 2.5% at down hole conditions. In this case the liquid stays as droplets with in the gas.

Foam: occurs when the liquid fraction is between 2.5% and 25% at down hole conditions. Foams are typically specified as "% foam quality". Foam quality is the volume fraction of the gas (i.e. 70% foam quality is 70% gas and 30% liquid, by volume).

Aerated mud: occurs when the liquid fraction is greater than 25% at down hole conditions. In this case the gas stays as bubbles within the liquid.

Since there are large volumes of oxygen present in air drilling, corrosion of the drill string can be a concern. Passivating (oxidizing) inhibitors should be used to minimize this corrosion

Selection / Setup

The critical issue with using a motor in an air or two-phase drilling application is minimizing the temperature generated within the stator elastomer liner. This can be accomplished as follows:

Run oversize stators if temperatures dictate.

Use the lowest foam quality possible (i.e. the highest amount of liquid).

Minimize the RPM.

Minimize times with no circulation.

All of the above, in addition to the down hole temperature, interact to determine the life of the stator. Failure of the stator elastomer liner is accelerated by internal heat build-up. As the liner heats up it expands, causing an increase in the interference fit that in turn causes more friction and further heat build-up. Hard spots then develop in the liner and eventually chunking occurs. The problem is further compounded by the poor thermal conductivity of air compared to a drilling fluid.

There are rotors and stators available that are designed specifically for air or two-phase drilling. These accommodate high flow rates and run at lower pressure drops and should be selected if available. Please contact Griffith-Vector for availability and application of these power sections.

When air drilling power sections are not available use power sections that run the slowest at the same flow rate. Ideally, use high performance power sections (more stages) because the differential pressure per stage required to produce a given torque is less. This lower differential pressure will be less damaging to the stator elastomer liner. Although the motor will produce the same torque at a given differential pressure when using air, the maximum obtainable differential pressure will be less due to the extra slippage that occurs with a gas. Consequently, the maximum obtainable torque (and stall) will be lower.

A crossover sub should be used above the PDM drive rather than the dump sub because airflow will not cause the dump sub to close. Alternatively the dump sub ports can be blanked off.

While drilling with air or two-phase fluid, the fluid density in the annulus may be higher than in the bore due to the cuttings in the annulus. When a connection is being made the pressure is reduced in the bore that can result in cuttings entering the bore. This can damage to the motor or plug the bit. A float valve should therefore be used when drilling with air or two-phase fluids to prevent this. A small hole is sometimes drilled in the flapper to allow pressure to equalize if the drill string does become plugged between the float valve and the bit.

Temperature

Typically, standard fit motors can be used except where the down hole temperature dictates that an oversize stator is required. Static bottom hole temperature up to 225°F (107°C) is considered standard. The air expands as it exits the rotor stator and then the bit nozzles. The resulting pressure drop has a cooling effect that can increase the life of the stator.

Volume Requirements

In a standard air drilling application, the required airflow rate (in SCFM) for proper motor operation is typically three to four times the maximum motor liquid flow rate (in GPM). This rule applies for motor exit pressures up to approximately 300 psi.

As the motor exit pressure increases, the airflow rate required to achieve the same motor operating speed (rpm) also increases. This is common in some under-balanced applications. In these cases the required air flow rate increases. Consult Griffith-Vector engineering in these cases.

If higher flow rates are desired, the rotor can be fitted with a nozzle to bypass a portion of the flow. However, the motor becomes even more sensitive to stall if the rotor bypass is used.

Operation

In general, a motor driven by air, mist or foam should be started while on bottom. It should not be allowed to run freely before tagging bottom because this can cause high shock loads as the bit tags bottom which may damage the motor. Ideally, the motor should be started with fluid first.

A motor is much more torque sensitive when using air, mists, or foams, than with liquids and consequently is more susceptible to stalling.

When drilling operations are to be stopped, let the motor drill off as the compressors and boosters are being shut down. Picking up "off bottom" prior to equalizing the pressure can permit the air compressed in the drill string to expand, over-speeding and possibly damaging the motor. It can also cause the internal connections to back off.

Drilling with Nitrogen (N2)

Air itself consists of approximately 78% nitrogen. The density of nitrogen is approximately 3% less than that of air at standard temperature and pressure. Functionally, the motors will run the same on nitrogen as air. However, the explosive decompression, as discussed below, is more severe with nitrogen.

Since nitrogen is an inert gas the nitrile sealing components within the motor, including the stator, are not affected chemically by nitrogen. However, any sealing compound will absorb nitrogen (as well as other gases) to some extent while under pressure. If the pressure has been applied long enough and the pressure is released too quickly, the gas does not have sufficient time to be expelled from the nitrile and explosive decompression can occur, resulting in blistering. This is typically not a problem with continuous pressure drops across the motor of 400 psi (3,000 kPa) and less.

After drilling for extended periods with nitrogen, stator damage can be expected.

For more detailed recommendations, consult a JAG representative.