Measurement While Drilling or Estimation While Drilling?

We have become accustomed to calling directional drilling systems MWD — Measurement While Drilling.

But perhaps it is time to ask a simple question:

Does MWD really measure the zenith angle and azimuth?

Let us look at this question not from the perspective of sensor design, algorithms, or software.

But from the perspective of metrology.

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Where Is the Boundary Between Measurement and Estimation?

Imagine an inclinometer test stand — a metrological calibration standard.

We place an instrument on it and set it to a defined position.

Now let us apply a fundamental metrological principle:

If the true value of a measurand cannot be determined directly during the measurement process without knowledge of additional characteristics of the object or measurement conditions, the result belongs to the domain of estimation rather than direct measurement.

For inclinometry, this can be stated even more simply.

If, on an inclinometer test stand, at any location on Earth, the true zenith angle or azimuth can be determined directly from the position of the instrument, without first entering any characteristics of that location, then we have measurement.

If, however, determining the true value requires prior knowledge of the characteristics of the measurement location — for example, the parameters of the gravitational or magnetic field — then we have estimation.

This is not a matter of terminology.

It is a matter of metrology.

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The Wireline Inclinometer

A wireline inclinometer can be placed entirely inside an inclinometer test stand.

Its position is defined directly by the geometry of the calibration standard.

The true zenith angle and azimuth are determined by the actual position of the instrument within the stand.

There is no need to provide the system with characteristics of a specific location on Earth in order to establish the reference position.

In this case, we are dealing with measurement.

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Now Let’s Look at MWD

Here the situation is fundamentally different.

Only part of the MWD system — the directional module — can be placed inside an inclinometer test stand.

But a real MWD system is much more than just the directional module.

It is the entire system operating as part of the bottom-hole assembly and drilling tool.

We cannot place the complete real-world MWD assembly inside a metrological calibration standard.

Nor can we reproduce, within a single calibration standard, the full combination of mechanical, gravitational, magnetic, and dynamic effects that occur during drilling.

And, most importantly, we cannot directly determine in a calibration standard the true zenith angle and azimuth of the complete MWD system under all real operating conditions.

We can measure the characteristics of individual components.

We can calibrate the directional module.

We can perform laboratory tests.

We can develop models.

We can compare results with other systems.

But the final output of the complete MWD system in a real well has to be estimated.

This Is Not About Accuracy

Here an important objection arises:

“But modern MWD systems provide very accurate values.”

Yes.

An estimate can certainly be very accurate.

But that is not the real issue.

The question is:

How can that accuracy be demonstrated metrologically?

To demonstrate measurement error, it must be possible to compare the result with a known true value under conditions representative of the intended field of application.

For a complete MWD system, doing this in full is practically impossible.

We cannot place the entire system, together with the drillstring and drilling tool, inside an accredited reference inclinometer calibration standard and subject it to known zenith and azimuth values under real operating conditions.

Therefore, we can obtain highly convincing experimental data.

We can achieve excellent repeatability.

We can obtain very good agreement with other systems.

We can obtain statistically impressive results.