Field operating for fixed cutter drill bits
Bit Handling at Rig Site
- Bit cutting elements with diamond both fixed cutter and roller cone bits, are brittle and susceptible to impact damage. Care should be taken when handling or removing any bit containing diamond cutting elements.
- Do not drop the bit even if it is in the container.
- Use a piece of wood or rubber under the bit face.
- Inspect bit for integrity (cutting elements, pin connection and makeup shoulder).
- Ensure there are no foreign objects or obstructions in the internal fluid passageways.
- Verify TFA on bits with fixed TFA.
- Record bit size, type and serial number.
- Ensure that nozzle series are correct for bit type.
- Gauge the orifice size of every nozzle to ensure proper total flow area (TFA).
- Remove the plastic plug and ensure that O-rings are properly installed and seated.
- If different size nozzles are to be used ensure that the correct sized nozzles are in the correct place. Put bigger size nozzles in the center of the bit.
- Ensure that the threads are clean and greased (any petroleumbased grease). Fixed cutter bits with matrix threads should not be greased.
- Grease the nozzle body below the threads to prevent O-ring damage.
- Screw the nozzle in by hand until snug.
- Use nozzle wrench for final tightening. Excessive force is not necessary and can damage the carbide nozzle.
- If nozzle sizes below 9/32 are to be used, recommend the use of drill pipe screens and / or a float to prevent reverse circulation plugging. Use grasshoppers if necessary.
- Ensure that the appropriate bit breaker is with the bit. Inspect to insure good condition and that it fits properly.
- Remove the bit from the box and place face down on a piece of wood or rubber.
- Engage the bit breaker with the bit and move them onto the rotary table.
- A float above the bit should be installed, especially on extended nozzle roller cone bits, in areas that tend to plug.
- Engage the hanging box connection to the doped threads of the bit pin.
- Proper makeup for small diameter bits is to makeup by hand for several turns, then place in the bit breaker and makeup to the recommended torque.
- Uncover the rotary and locate the bit and breaker onto the breaker holder.
- Makeup applying the recommended torque.
- Makeup torque specifications are from API spec RP7G.
TRIPPING IN THE HOLE
- Identify potential problem areas before tripping. Trip slowly through BOP, doglegs, tight spots, ledges, casing shoes, cementing equipment, etc. Wash and or ream as necessary. Severe problems may require a special ‘cleanup’ run.
- Certain types of fixed cutter bits with low junk slot area can create higher surge and swab pressures than roller cone bits due to more restrictive annular space.
- Local knowledge/practice will typically dictate wash down and reaming procedures. Minimum recommendation is to wash down at least the last joint to bottom at reaming speed with full circulation. Preference is to ream the last stand / 90 feet at reaming speed with full circulation.
- Approach the hole bottom cautiously, monitoring WOB and torque. An increase in WOB or torque will indicate either contact with the hole bottom or fill. Fixed cutter bits will typically show an increase in torque first. Bit is on bottom when torque increases with the WOB. Difference between measured depth and contact point should be depth of fill.
- If fill is present, pick up above the fill and rotate to bottom with full circulation until bottomhole contact is assured. Regardless if fill is present, the pipe should always be reciprocated off-bottom.
- On rotary assemblies, use a maximum of 500 pounds per inch of bit diameter, 40 to 60 rpm.
- On motor assemblies, use a maximum of 500 pounds per inch of bit diameter and the minimum allowable rpm.
- Do not use high WOB when in fill. This could cause the bit to ball.
- Circulate and rotate off-bottom (as close as possible preferably less than 6 in, no more than 1 foot) enough (5 to 15 minutes, application dependent, recommend 15 min as minimum) to ensure the hole bottom is clear of fill or junk.
DRILLING CEMENT PLUGS, FLOAT EQUIPMENT AND CASING SHOES
- When using fixed cutter bits to drill out, ensure that all cementing equipment (plugs, float collars and shoes) is PDC drillable (nonmetallic, rubber, nylon, plastic or cement).
- Recommend the use of non-rotating plugs. Alternatively, it is preferable when cementing to pump some cement on top of the plug to help prevent it from rotating during drill out.
- Using the maximum allowable flow rate to assist cleaning is preferred, but may not be possible with motor assemblies.
- Frequently raising and lowering the bit while continuing circulation and rotation will help keep the bit clear of debris. Flushing after every 1 to 2 in drilled while reciprocating 3 to 4 feet will ensure debris is removed and new material is being drilled. Should the penetration rate decrease suddenly, repeat this step until it resumes.
- Do not spud. Spudding (impacting on the hole-bottom) can damage cutting structure elements on both fixed cutter and roller cone bits. It can also damage the roller cone bearing / seal system.
- Monitor pump pressure to ensure nozzles do not become plugged.
- Change rpm if bouncing or erratic torque is encountered.
Fixed Cutter Bits
- On rotary, use the maximum flow rate with less than 6000 lbf WOB and 60-100 rpm.
- On motor assemblies, drill with less than 6000 lbf WOB and the minimum allowable rotary rpm. Local practice will dictate flow rate as a compromise is needed between providing adequate cleaning and minimum rpm.
- Maintain low and consistent torque.
- Should a plug begin to rotate, set down on plug with no rpm
- Increase WOB until 2000 to 3000 lbs. per inch of bit diameter is reached or alternatively an increase of 300 psi over the normal standpipe occurs.
- Then begin rotation, ending with 40 to 60 rpm.
- Repeat until penetration is achieved and wiper plug is drilled.
Alternative procedure (last resort)
- Rotate bit at 20-40 rpm
- Use 500 pounds / inch of bit diameter
- Alternate using no flow rate for 1 minute to full flow for 30 seconds.
ESTABLISHMENT OF BOTTOMHOLE PATTERN
Bottomhole pattern break-in is considered to be when a new bit achieves uniform cutting structure loading. Proper break-in is critical to durability and ROP.
- After drilling out the casing shoe, establish the bottomhole pattern. There may be some BHA dictated WOB and rpm guidelines until the BHA is below the casing shoe. Optimization of WOB and rpm may have to wait until the BHA or some portion of the BHA has cleared the casing shoe.
- Use extra care establishing a new bottomhole pattern when following a bit with a substantially different bottomhole profile, e.g., a PDC bit following a roller cone bit or vice-versa.
- Roller cone bits typically drill a larger size hole than a fixed cutter bit. Be sure to properly establish the bottomhole pattern when following a roller cone bit in order to insure stability.
- Establishment of bottomhole pattern can be dependent upon factors such as bit design, BHA, etc.
- Although a new bottomhole pattern is created in less than a bit diameter, it is preferred to drill 3 to 5 feet before increasing WOB and rpm.
- For starting parameters, use maximum flow rate, less than 6000 lbs. WOB and 60-100 rpm.
- Maintain low and consistent torque changing operating parameters as needed.
- Take extreme care following a coring operation or bits of different types or profiles. A different existing profile can overload specific cutting elements potentially causing a premature failure
RUNNING THE BIT
- If in or encountering a known hard or abrasive formation of short length with a dull bit, controlled drilling (sacrificing ROP) through the interval may enhance the following bit’s performance. The dull condition and bit type will determine the feasibility. Monitor the ROP and torque, follow the recommendations for evaluating ROP and torque as listed below. Proceed only if ROP and increased risk is acceptable to the customer.
Deviation & Directional Control
- Deviation concerns may override optimum WOB used. Typically minimum WOB is used to control deviation on rotary assemblies.
- Monitor deviation on straight holes. Reduce WOB to maximum allowable to maintain deviation specified by customer. Monitor for vibration. Generally, increase rpm to improve ROP.
- Higher torque and higher speed changes bit walk direction by making the bit attack the formation at different angle due to higher torque. The bit tries to climb the hole wall at a different position at different torque levels. High speed on turbines can make the bit change direction completely.
- Formation changes can instigate both torsional and lateral vibrations. Monitor and adjust accordingly.
- When anticipating a harder/more difficult formation, to prevent impact damage, reduce the rpm maintaining WOB while still in the softer formation to help prevent initiation of lateral or torsional vibrations when the hard formation is encountered. After the formation is encountered, adjust the parameters or perform a drilloff test.
- If an unknown formation (anomaly/transition) is expected, reduce the rpm and the WOB to a minimum accepted level. Establish ROP and torque baselines at these levels. Monitor ROP and torque to determine when formation has been encountered and when through the anomaly.
Operational Parameter Guidelines
- Optimum WOB and rpm determined are for a particular application and can only be continuously used in a homogeneous formation.
- There fore in intervals of various formations, ROP optimization tests will not produce the optimum weight and rotary combination.
- Drill off tests will be necessary anytime the formation changes. Typically a range of WOB and rpm combinations is derived for the interval, e.g., interbedded formations.
PULLING THE BIT
- When economics dictates.
- When ROP dictates.
- Loss of directional characteristics.
- Loss of pump pressure due to a washout.