A fall in standpipe pressure can indicate a drillstring washout, reduced pump delivery, bad instrumentation, or changed hydraulic context. Pressure is the symptom shared by several failure modes. Diagnosis depends on the expected pressure-flow relationship, supporting channels, operating state, and how the disagreement develops.

Start with the expected hydraulic relationship

Standpipe pressure is not independent of the rest of the operation. For a given circulating configuration, pressure depends on quantities such as:

  • flow rate,
  • mud rheology,
  • drillpipe and BHA geometry,
  • bit nozzle area,
  • hole and casing geometry,
  • motor pressure loss,
  • pipe rotation,
  • eccentricity,
  • and other hydraulic effects.

That means an engineering hydraulic model can provide expected pressure for the current operating condition. SPE-191797 describes a computationally efficient real-time hydraulic model using streaming data with mud, BHA, and wellbore context.[2] The useful comparison is measured versus modeled standpipe pressure. If modeled pressure remains near 4,700 psi while the measurement falls through 4,650, 4,550, 4,400, and 4,250 psi, the residual confirms a departure from expected behavior. It does not yet distinguish a washout from pump degradation; flow-out provides the next discriminating measurement.

Use flow and pressure together

The key difference lies in where the fluid is going. During a drillstring washout, fluid leaks from inside the drillstring into the annulus before reaching the bit. Much of it remains in the circulating system and returns to surface. During pump degradation, actual flow entering the well falls even if the stroke counter still indicates a normal rate.

Failure mode Reported pump activity Standpipe pressure Actual flow out
Drillstring washout Approximately stable Decreases Approximately stable
Pump degradation May appear stable Decreases Decreases after the circulation lag

That makes flow out one of the most useful discriminating measurements. The field methodology described in IADC/SPE-189700 identified flow-out behavior as the primary practical distinction between the two failure modes.[1]

Synthetic engineering traces comparing drillstring washout and mud-pump failure: pump activity remains stable and standpipe pressure decreases in both cases, while flow out stays stable during a washout and decreases during pump failure.

Washouts and pump failures can produce similar pressure behavior; flow-out trends provide an important means of separating them.

The pressure-flow relationship can also be examined directly. IADC/SPE-189700 used a simple hydraulic coefficient of the general form:

$$h = \frac{P}{Q^2}$$

where:

  • \(P\) is standpipe pressure,
  • \(Q\) is flow rate.

This is not intended to replace a complete hydraulic model. It creates a convenient way to monitor whether the relationship between pressure and flow is changing over time. Two versions can be considered:

$$h_{\mathrm{in}} = \frac{P}{Q_{\mathrm{in}}^2}$$

and

$$h_{\mathrm{out}} = \frac{P}{Q_{\mathrm{out}}^2}$$

The coefficients behave differently under the two failures. During a washout, pressure falls while flow out remains approximately constant, so the flow-out coefficient decreases. During pump degradation, pressure and actual flow out fall together, and the coefficient can remain comparatively stable. The published methodology found this behavior useful in differentiating the failures.[1] This illustrates a broader analytics principle:

Derived relationships can expose changes that are difficult to see by looking at raw channels individually.

Synthetic hydraulic coefficient trends comparing a drillstring washout with mud-pump degradation: pressure decreases in both scenarios; flow out remains stable and P/Q² decreases during a washout, while flow out decreases and P/Q² remains comparatively stable during pump degradation.

Monitoring the pressure-to-flow relationship provides additional evidence about whether hydraulic resistance changed or actual pump delivery declined.

Respect missing evidence and operating state

A pressure change while drilling is not necessarily interpreted the same way as a pressure change while:

  • pumps are ramping,
  • pumps are staging,
  • circulating off bottom,
  • reaming,
  • downlinking,
  • making a connection,
  • or performing another operation.

IADC/SPE-189700 incorporated rig activity and pump status into the event interpretation because failure likelihood and expected channel behavior depend on what the rig is actually doing.[1] This prevents another common analytical mistake: comparing measurements generated under different operating conditions as though they represented the same process. For example, a drop in pressure while the pump rate is being intentionally reduced is not evidence of a washout. The diagnosis only becomes interesting when:

  • the operating condition should produce stable hydraulic behavior,
  • but the measurements begin departing from that expectation.

Rig-state detection and event detection are therefore tightly connected. The study also demonstrates the practical limit of diagnostic specificity. Constant pump strokes, decreasing standpipe pressure, and measured pressure below modeled pressure support both a washout and pump degradation. Remove flow-out, and the system may no longer distinguish them. IADC/SPE-189700 demonstrated a case in which unavailable flow-out data made the two probabilities indistinguishable.[1] Mature analytics should be allowed to say:

There is not enough evidence to distinguish the two failure modes.

The answer should not be manufactured simply because software is expected to output one. This is especially relevant to drilling data, where:

  • sensors may fail,
  • channels may not be installed,
  • values may be stale,
  • and some rigs have substantially better instrumentation than others.

A system that reports uncertainty appropriately can be more useful than one that always produces a confident classification.

Separate detection from diagnosis

Detection asks whether something abnormal is happening; diagnosis asks what it is. Declining pressure, apparently stable pump operation, and increasing disagreement with the hydraulic model may establish the first. Determining whether that abnormality is:

  • a washout,
  • pump degradation,
  • a sensor problem,
  • or another hydraulic event

requires more evidence. The published alerting approach recognized this distinction by first identifying the combined likelihood of a washout/pump-failure type event and then using additional flow behavior to help determine which failure was more likely.[1] This is a useful design principle for drilling surveillance generally:

Detect the abnormal condition at the level supported by the evidence. Increase diagnostic specificity only as the evidence allows.

A surveillance system that generates hundreds of hydraulic alerts is not necessarily safer than one that generates ten. The important questions are:

  • Are the alerts meaningful?
  • Do they persist long enough to be operationally relevant?
  • Is the evidence visible?
  • Can the engineer understand why the alert occurred?
  • Does the system distinguish uncertainty from confidence?

The best real-time analytics reduce the amount of raw data an engineer has to continuously watch without hiding the evidence necessary to make a decision.

DrillingMetrics Time Traces showing synchronized depth, hook load, block height, modeled pump pressure, standpipe pressure, total pump output, washout belief, pump-failure belief, and volume channels.

Useful event surveillance should preserve the underlying data needed to investigate why an abnormal condition was identified.

Field evidence and the practical boundary

The methodology published in IADC/SPE-189700 was evaluated across more than 100 onshore wells and deployed across multiple North American rigs.[1] One documented washout case generated several automated alerts before the crew ultimately tripped because of pressure loss. The first alert occurred more than six hours before the trip.[1] That result should not be interpreted as a universal promise that washouts can always be predicted six hours in advance. The more useful observation is that some mechanical failures develop progressively enough to create detectable signatures before the final event.

That creates an opportunity for real-time surveillance. Instead of asking only:

Has the equipment failed?

we can ask:

Is the hydraulic relationship progressively moving away from its normal operating behavior?

That is a fundamentally different surveillance philosophy.

Related technical resources

References

  1. Ambrus, A., Ashok, P., Ramos, D., Chintapalli, A., Susich, A., Thetford, T., Nelson, B., Shahri, M., McNab, J., and Behounek, M. Self-Learning Probabilistic Detection and Alerting of Drillstring Washout and Pump Failure Incidents During Drilling Operations. IADC/SPE-189700-MS, IADC/SPE Drilling Conference and Exhibition, Fort Worth, Texas, 2018.

  2. Shahri, M., Kutlu, B., Thetford, T., Nelson, B., Wilson, T., Behounek, M., Ambrus, A., and Ashok, P. Adopting Physical Models in Real-Time Drilling Application: Wellbore Hydraulics. SPE-191797-MS, SPE Liquids-Rich Basins Conference, North America, Midland, Texas, 2018.