PDC Drill Bits
PDC drill bits, or fixed cutters, come in either matrix or steel body. One might ask, "What's the difference?" and "Which one do I need?"
Matrix Body PDC
Matrix body fixed cutters contain tungsten carbide material which provides improved wear resistance and erosion. A higher cutter count can also make them heavier set.
In applications of high fluid volume or high sand, matrix body bits offer longer life and multiple bit runs.
Steel Body PDC
High alloy steel comprises the body of steel body fixed cutters, oftentimes making them repairable and good for multiple runs.
They're commonly manufactured with higher blade stand-off and cutter exposure since steel can withstand higher impact than matrix. This design feature increases ROP.
Cost of Matrix Vs. Steel
Drilling a well involves cost - a major factor - and matrix costs more than steel. In addition, the extra pdc cutters add to the overall costs of the pdc drill bit. These two factors make matrix bodies more expensive than steel bodies.
PDC Drill Bit Design
|Initial Cost||Total Depth||Bit Trip Cost|
Below are common aspects of PDC drill bit design that every engineer has to consider:
Cutter Force Balancing
PDC Cutter Technology
Cutter Energy Balancing
Although fixed cutters differ in design aspects from roller cone bits, they share much in common. Both not only have pins that conform to API standards but both also must meet specific plus or minus tolerances during the manufacturing process.
The same dull grading method applies to both fixed cutters and roller cone bits.
Running in the Hole
- In cased hole, take care when entering conductor pipe or lines and when passing through drilled out cementing equipment.
- In open hole, take care with suspected or known tight spots, ledges, doglegs or keyseats. Be especially cautious with increased BHA stiffness because a roller cone bit has some “give” that pdc drill bits do not have.
- When washing to bottom, follow this recommendation for the last two joints of open hole: rotate at 40-60 RPM with a maximum of about 3K WOB. This will ensure the absence of an under gaged hole AND drill any fill that may be present. After tagging bottom, pick up off bottom and pump at full flow rate for about 5 minutes.
- Perform conventional drill-off tests to identify optimum running parameters.
Indications of Unstable Bit Rotation
- Large WOB fluctuations.
- Increased surface torque fluctuation or cycling torque.
- If ROP drops below 20 fph, the formation has a hard stringer or a bit is running unstable.
- Use a roller cone to ream a hole - not a PDC. For extensive reaming, do not run a PDC bit.
- And, use extreme caution and follow this guideline when encountering tight spots: rotate at 60 RPM with no more than 3K WOB.
Rule of Thumb to Improve PDC Drill Bit Performance
To Reduce PDC Cutter Damage
- Use the automatic driller to place the bit in contact with the hole bottom. Hard impact with hole bottom will cause damage to PDC cutters and lead to short bit life.
- Maximize flow rates at all times. Frictional heat due to lack of cooling/cleaning can lead to increased PDC cutter and blade damage.
PDC Bit Hydraulics
- Good PDC hydraulics include maximizing flow rates BEFORE maximizing HSI.
- Use relatively small nozzle sizes if the rig has enough pump to allow for both maximized flow rates and optimized HSI. When using LCM, run large enough nozzles to allow the LCM to pass through.
- Always try to run similar size jet nozzles in a PDC drill bit. Jet nozzles that vary greatly in size can allow cuttings to enter the bigger jet nozzle when the pump turns off. Then, the cuttings will plug the smaller jet nozzles when the pumps turn back on.
Keep WOB to a minimum in the upper section of the production hole. Excessive WOB can lead to early wear flat on cutters, possibly lead to bit balling, and deviation. When ROP slows, try backing off WOB to 8-13k to let the hole clean up, then gradually increase WOB in small increments to desired ROP. Note: Adding weight is not always the answer to slow ROP. Along with backing off weight, try slowing the rotary some, this can help the bit get back in the bottom hole pattern and allow it to take the bite it needs.
Deviation Problems - With a BHA
- To help reduce hole angle, at the bottom of each kelly down, latch the brake handle and allow the bit weight to drill itself off.
- After making the new connection back ream the last 30 feet to bottom prior to drilling more footage.
Deviation Problems - Without a BHA
Increase RPM and decrease bit weight.