Underbalanced Drilling: General Issues
Underbalanced Directional Drilling
Directional Drilling equipment can impose limitations on underbalanced drilling. Hydraulically operated tools cannot be used in UBD wells, and if a gasified system is used the MWD pulse systems may not work. Certain motors and other directional equipment may be prone to failure as a result of the rubber components becoming impregnated with the gas used. The potential for explosive decompression of rubber components needs to be considered when selecting equipment.
The higher Torque and Drag seen in UBD wells (as much as 20%) may also prevent certain trajectories from being drilled underbalanced. The higher torque is caused by the reduced buoyancy combined with the lack of filter cake on the borehole wall.
Positive Displacement Motors (PDMs), or downhole motors, are designed for drilling mud and not air, although some have been adapted to air. Currently there is no reliable MWD system that will work consistently with a motor and air drilling, although an Electromagnetic MWD (EMWD) will work in an air environment. When used with a downhole motor, the motor vibrations quickly damage the EMWD because there is no hydraulic dampening of the vibrations from the motor. The EMWD will work with rotary BHAs since the vibration is much less. Even steering tools experience frequent failures from motor vibration; steering tools will work better if mist is added to the air because the fluid helps to damp the vibrations downhole.
