The choice of bit should take into account the degree of re-active torque produced. For instance some PDC bits can make motor tool face control difficult resulting in erratic hole, increased torque and drag and low ROP’s.
The use of motor bearing tri-cone bits is often preferred for these sections.
Design Criteria for Selection of Tri-cone Bits
When selecting tri-cone bits it is necessary to consider:
- the maximum penetration rate required to limit open hole time
- extended bearing life to reduce the frequencies of trips pumping in and out of the hole
- high resistance to cone erosion as greater than normal flow rates are required for hole cleaning
- enhanced gauge protection is required on cutters, shank and shirt tail in order to withstand the increased lateral loading when used with steerable systems.
Design Criteria for Selection of PDC Bits
When selecting PDC bits it is necessary to consider:
- the maximum penetration rate required to limit open hole time
- the small cutter size which provides a reduction in reactive torque and enhances steering capability
- cutters and gauge protection on the crown chamber to facilitate back reaming.
- high cutter density and gauge studs to limit gauge wear.
- optimum gauge length
The following table identifies and lists the common problems encountered when drilling horizontal hole sections and potential preventative measures relating to bit selection.
|
Problems Encountered |
Preventative Measures |
|
Side-Loading using Steerable Systems |
Shorten shank or gauge length Increase gauge protection Opt for rotary drilling if possible |
|
High Torque - Reduced Directional Control |
Use less aggressive PDC Shorten gauge length of PDC bit Increase Number of gauge cutters Opt for Roller Cone Bit |
|
Difficulty Maintaining Hole Angle |
Longer Gauge Length on PDC Bits Lug Pads On Roller Cone Bits Use Steerable system |
|
Excessive Cuttings Bed Generation |
Use Lug Pads on Roller Cone Bits Increase gauge cutters on PDC bits (Both measures to facilitate efficient backreaming operations) |
|
Reduced Bit Life Due To Motor Use |
Use PDC Use Motor Bearing Roller Cone Bits Use Low speed PDM’s Opt for rotary drilling if directional control allows |
Hydraulic Requirements and Practice
Efficient hole cleaning is essential to the success of extended reach drilling. Therefore, all hydraulic programmes should:
- circulate at the highest possible rate within the constraints of ECD on weak formations. Note: Install a bypass nozzle on mudmotors to increase flow area if necessary.
- circulate the hole clean (as much as practical) prior to commencing a trip.Note: A minimum of 1.5 times the bottoms up volume is normally required.
- when POOH, rotate the string and pump at the maximum rate to disturb any generated cuttings beds
- consider using a Hi-Vis/Low-Vis/Hi-Vis sweep regime to facilitate disturbance and removal of cuttings beds
- apply enhanced monitoring of drilling parameters for increases in torque and drag (perform wiper trips as required).