Deviated wells are normally drilled using a steerable system which is a combination of bent sub, motor and MWD allowing both rotary drilling and sliding without tripping for directional changes.

Bottom Hole Assembly

Generally the BHA shall comprise:

• tri-cone bit / PBC bits
• straight housing motor (low torque)
• bent sub
• non-magnetic drill collar
• drill collars (as required for WOB at inclination)
• drilling jar
• HWDP.

Provisional bottomhole assemblies are generally recommended by the Directional Drilling Contractor as part of the Directional Well Programme.

Measurement While Drilling

Consideration should be given to running MWD if a system is available; otherwise a high speed low torque motor is preferred, allowing more accurate prediction of reactive torque without the need for surface toolface readout. However MWD becomes necessary for toolface control when a high torque motor is used.

Mudmotor Operating Practices

Introduction

Several operating practices should be applied to ensure efficiency of mudmotor operations. These practices cover:

• surface checks
• running in hole
• tripping out of hole
• motor at surface.

Surface Checks

• check the bent sub offset to ensure it is as planned
• test the motor and MWD at two or three differing flow rates, record circulating pressures and rates
• lock the hook and ensure bent sub orientation is tracked while running in hole (align the scribe mark on the motor with that of the MWD, if used).

Running In The Hole

• check the string is full every 15 stands, or if a float is used, fill the drill pipe every 15 stands
• ensure that damage to the motor bearings or bit does not occur from hitting hard ledges of formation, particularly with a high angle bent sub
• wash down the last stand, gently tag bottom, and establish the required tool face before commencement of drilling.

Tripping Out Of The Hole

• do not rotate when using a high angle bent sub (greater than 2 degrees)
• additionally circulate bottoms up at the shoe for high angle wells.

Motor At Surface

• inspect the body for signs of wear and damage
• check the play in the bearing section by manipulating the bit box on the motor to evaluate bearing wear based on the play and determine if the motor is to be re-run
• test the motor at the same flow rates used in the surface test and compare circulating pressures.

Note: It is normal to see some reduction in pressure as the motor/stator tolerance increases.

Bit Selection

The increased reactive torque generated by a PDC bit will preclude its use on a low torque motor.

Where PDC bits are used, they should have a diamond reinforced gauge of short length for steerability.

Spiral Vs Straight Blade Stabilisers

All stabilisers should have short blade, barrel profiles to reduce wall contact.

The difference between spiral and straight blades is formation dependent.

High Speed Vs Low Speed Motors

In general there is little ROP improvement with faster motors.

The shorter, high torque, low speed motors enhance steerability and provide longer bearing life for rock bits

However, a MWD system must be used in conjunction with the higher torque motor.