A bit-pull recommendation is an economic decision under uncertainty, not a wear alarm. The useful question is whether continuing with the current bit is likely to reach the next objective sooner and with acceptable risk than tripping for an uncertain replacement-bit response.
Establish a contextual degradation baseline
A drilling-performance indicator becomes much more informative when there is an established baseline. Imagine that five previous bit runs through the same formation show a relatively consistent progression:
- low indicator early in the run,
- gradual increase with depth,
- modest rise as normal abrasive wear accumulates.
Now the current bit follows that baseline for several thousand feet and then suddenly departs upward. That change may be more informative than the absolute numerical value itself. SPE-205844 observed this type of behavior in hard, abrasive lateral drilling.[1] Under normal abrasive wear, the wear indicator could increase gradually. In some failure cases, the indicator departed sharply from that baseline, corresponding with accelerated cutter damage. The important concept is not the specific threshold from that field. It is change relative to expected behavior.
Change relative to comparable historical behavior can be more informative than an absolute bit-wear number.
Correct reversible dysfunction first
This is one of the most important operational safeguards in the published field-deployment work. When the advisory indicated that bit effectiveness had become poor, the workflow did not immediately say:
Pull the bit.
Instead, the driller was asked to evaluate whether drilling dysfunctions such as:
- stick-slip,
- bit balling,
- whirl,
could be responsible for the poor performance and attempt to correct them first.[2] That is an important distinction because some poor drilling performance is reversible. Bit damage is not. If ROP recovers after reducing a dysfunction, an unnecessary trip may be avoided. If drilling remains ineffective after the identifiable dysfunction is mitigated, the evidence that the bit itself has reached the end of useful life becomes stronger.
Compare the two paths to TD
Take a hypothetical case: the current bit is drilling at 70 ft/hr, a replacement is expected to drill at 150 ft/hr, and the trip-and-return time is 10 hours.
| Footage remaining | Continue current bit | Trip and replace | Faster path |
|---|---|---|---|
| 500 ft | 7.1 hr | 13.3 hr | Continue |
| 3,000 ft | 42.9 hr | 30.0 hr | Replace, if the forecast holds |
Nothing about the bit changed between the scenarios; only the remaining footage changed. Bit condition therefore cannot determine the decision by itself. The calculation looks precise, but replacement-bit ROP is still a forecast informed by:
- offset performance,
- previous bit runs,
- formation trends,
- BHA history,
- vendor expectations.
But the replacement-bit ROP is still a forecast. Likewise, the exact TD may change. The formation may change. Trip time may be longer or shorter than expected. The current bit may degrade faster than its recent ROP trend suggests. That means a single deterministic calculation can create false confidence. SPE/IADC-214608 addressed this by evaluating a range of possible replacement-bit ROP values and remaining distances to TD rather than requiring one exact forecast.[2] The result can be represented as a decision surface or heat map. That is an excellent way to handle uncertainty.
Instead of saying:
Pull.
the analysis can say:
If the fresh bit is expected to achieve at least this level of performance and more than this much footage remains, replacement becomes increasingly attractive.
That is much more informative. A simplified break-even condition can be derived by equating the two time paths:
$$\frac{D}{ROP_{old}} = T_{trip} + \frac{D}{ROP_{new}}$$
where:
- \(D\) = footage remaining,
- \(ROP_{old}\) = expected current-bit ROP,
- \(ROP_{new}\) = expected new-bit ROP,
- \(T_{trip}\) = total replacement trip time.
Solving for break-even footage:
$$D_{BE} = \frac{T_{trip}} { \left( \frac{1}{ROP_{old}} - \frac{1}{ROP_{new}} \right) }$$
This is a useful engineering expression because it makes the decision logic explicit. For example:
- longer trip time pushes break-even farther away,
- poorer current-bit ROP pulls break-even closer,
- larger expected new-bit improvement pulls break-even closer.
But again, every variable is an estimate. The equation should inform the decision, not disguise uncertainty.
Treat a sensitive answer as uncertain
The replacement-bit assumption deserves similar scrutiny. Using the best ROP ever observed in the field is not a realistic forecast. A more defensible estimate might be conditioned on:
- formation,
- hole size,
- BHA configuration,
- bit design,
- depth interval,
- offset runs,
- mud system,
- and current directional requirements.
Better still, use a distribution rather than a single number. An expected fresh-bit ROP might be represented as P25 = 115 ft/hr, median = 140 ft/hr, and P75 = 165 ft/hr.
Now the bit-pull decision can be tested over a realistic range. If the trip only works economically under the P75 outcome, it is a weak recommendation. If even the P25 outcome favors replacement, the decision is much stronger. This moves the analysis from:
What do we think will happen?
to:
Across the range of outcomes we normally observe, how often does replacement appear advantageous?
That is a much more useful engineering question.
Using a realistic distribution of replacement-bit performance reveals whether a trip recommendation is robust or dependent on an optimistic forecast.
Not every decision needs to be continue or trip now. If:
- bit-effectiveness evidence is deteriorating,
- ROP has fallen,
- but the decision surface is close to break-even,
- and there is still uncertainty about whether stick-slip is contributing.
A reasonable recommendation could be: Continue for one or two stands while changing parameters to mitigate dysfunction and re-evaluate. That creates a third state: monitor / test / reassess. This is important because drilling decisions are sequential. More evidence will arrive. The next stand may show:
- ROP recovery,
- continued degradation,
- increased torque instability,
- or another formation change.
Real-time analytics should take advantage of that. The objective is not to force the earliest possible decision. It is to make the decision when the evidence and economics become sufficiently persuasive.
A practical bit-pull workflow
The workflow should preserve both the mechanical and economic sides of the decision:
| Stage | Decision work |
|---|---|
| Detect and confirm degradation | Review ROP, DOC, WOB, RPM, torque, and wear indicators; require a sustained departure from the contextual baseline. |
| Check context and reversible causes | Test formation change, stick-slip, whirl, bit balling, hydraulics, and parameter selection before attributing the loss to permanent damage. |
| Re-evaluate bit effectiveness | Determine whether ROP recovered or the degradation stabilized after corrective action. |
| Forecast both paths | Estimate current-bit ROP, replacement-trip time, and replacement-bit performance from comparable history. |
| Test uncertainty | Vary distance to TD, both ROP forecasts, trip time, and continued degradation rate. |
| Decide on robustness | Replace when that path wins across most reasonable scenarios; otherwise monitor, test, or gather more evidence. |
Across BHA run 2, depth of cut trends lower while total MSE rises relative to its earlier baseline and the offset well. After a new bit is picked up for BHA run 3, depth of cut returns toward trend and MSE establishes a lower baseline closer to the offset. This behavior is consistent with possible bit damage during the later part of run 2.
Related technical resources
- Interpreting PDC Bit Wear from Surface Drilling Data
- Mechanical Specific Energy: Powerful Metric, Incomplete Diagnosis
- Probabilistic Reasoning for Drilling Dysfunction Detection
References
-
Witt-Doerring, Y., Pastusek, P. P., Ashok, P., and van Oort, E. Quantifying PDC Bit Wear in Real-Time and Establishing an Effective Bit Pull Criterion Using Surface Sensors. SPE-205844-MS, SPE Annual Technical Conference and Exhibition, 2021.
-
Yi, M., Ashok, P., Ramos, D., Pearce, J., Hickin, G., Peroyea, T., White, S., Thetford, T., and Behounek, M. Field Deployment of a Real-Time Bit Pull Advisory System. SPE/IADC-214608-MS, SPE/IADC Middle East Drilling Technology Conference and Exhibition, Abu Dhabi, UAE, 2023.