CÁLCULO PÓRTICOS

Today, full-service MWD is very much a niche business, with two major markets:

✔ Long-reach, directional drilling where MWD offers a level of real-time formation evaluation, and directional monitoring unavailable from any other technology, and

✔ Those high budget, high profile exploration programs, on which success makes and breaks both companies and careers. Although less common than in the high-rolling eighties, such programs still exist and no innovation is too expensive, nor too speculative to be left out of the program.

In more practical operations, real-time, multi-sensor MWD can provide a revolution in formation evaluation and drilling optimization. Its contribution is threefold:

✔ Data is provided which is unavailable from any other source, such as true axial force and torque on the bit.

✔ Data is provided in a better or different form than any other source, such as formation resistivity from recently drilled and barely invaded formations.

✔ Data is provided in real-time, near-real-time or (at worst) much more quickly than any other source.

The importance of these improvements is unquestionable (see Nuckols, Cobern, and Couillard, 1985). Yet, in order to gain any effective value from and MWD service, it is important to select sensors, transmission system, surface receiving and evaluation systems that are compatible with each other, with the needs of the exploration program and with people and methods to be utilized at the well site (or in real-time communication with it). Any type of MWD operation adds costs to the exploration program: both its own day rate and other rig and logistic costs incurred in supporting MWD. If the type or timing of MWD data gathering is not matched by the needs or capabilities of the program the service, no matter how well performed, becomes an expensive waste of rig time and people.

In selecting an MWD service, the first consideration should, of course, be to match the data requirements and sensors. If data is needed in real-time it should be gathered by MWD. If it is not needed until later then it can wait for later, cheaper or better measurement techniques.

In addition to the theoretical need there is also the practical capability to handle the data. Having data that is available is only part of the

The On-line Mud Logging Handbook Alun Whittaker

solution. Unless skilled people are available with adequate time and suitable tools, the data will go to waste.

Finally, there are considerations of flexibility of the drilling and evaluation procedures for the well. If the well program cannot accommodate changes dictated by real-time evaluation, then much of the value of real-time data is lost. This should be considered on the basis of two worst cases:

✔ If data gathered in real-time dictates major changes in drilling or evaluation, does the well plan have sufficient flexibility to accommodate these changes without major loss or delays -- or will the real-time data fail to have impact on the current well?

✔ If all or a section of the MWD data is lost due to equipment malfunction, is there a means to make up the data from conventional means without extra expense, delays or re-drilling?

The final question is one of trust. MWD measurements are made in a unique environment. The results of these measurements are inevitably different from those of other, more familiar measurements made at different locations or times. If the people involved in the well can accept this fact, then the MWD data can form part of a complete and integrated log suite. For example, MWD formation logs should not replace line logging. Instead, the data gathered from MWD should assist in optimizing the selection of tools, parameters and intervals for wire-line logging.

Unfortunately, to many people different and inferior are synonymous!

✔ To these people, much MWD data is too different to be usable.

✔ Other MWD data may be used in real-time when it is all that is available.

✔ Later when better, or more familiar (which can – sometimes, but not always – mean the same thing), data becomes available, the MWD data is discarded.

This is a waste in two ways. Money is wasted in repetitive measurement. Evaluation potential is wasted by failing to investigate and learn from the differences and similarities between the data from different measurements and times. Both of these act to reduce the value of MWD.

While we can work to improve the understanding and use of new data sources, we are not on running an academic research program, nor a religious crusade. We must be pragmatic about the reality of well-site activity. The tools available must be those most used and favored by the people who will be drilling the well. If the won't be used, supplying them is a waste of money and an unnecessary complication at the well site.

Next... In This Edition

OK, we've finished talking about all the how-its-done stuff.

From here on, it doesn't matter whether you're ever going out to a rig, or if you've ever even seen one. We're going to talk about what's on a mud log and how you can read it and use it, along with other logs.

Chapter 11 is all about the basic mud log. That the five, or ten, or fifty year-old standard mud log you pull from the log library, or the log you got at the well site last week from Billy Bob's Modern Storm Drain, Septic Tank, and Mud Logging Company.

Chapter 12 is about geo-pressure logs, how the various data gathered from mud logging, drill rig sensors, and other places, is processed and plotted, and can be used to anticipate, plan for, and work through formations with abnormally high (or low) formation pressures.

Finally, Chapter 13, looks at that final question: how can someone, without fifty years experience and a magic eight-ball, draw meaningful (that is reproducible) conclusions from a mud log.

We'll try to give you some voodoo-free tools you can use to estimate formation productivity – the type and mobility of hydrocarbons – from a modern mud log.

But first...

Next ... Time Around

OK, it's let down time again. Once again you get to see the flimsy scaffolding behind this fancy facade.

What follows is an outline of what we used to cover in the Modern Mud Logging class about some of the data that might be available in the mud logging unit, what it means, and how we may be able to use it to help us get more out of (or into) the mud log.

But for now, that's all it is – an outline. Maybe next time through the mill, it will be fleshed out with some content. Until then, you can start filling the gaps yourself, or just treat it as an advertisement, a promise of things to come, if you buy enough copies, and I live ling enough...

At least, in an electronic book it isn't wasting any paper – just a few electrons.

The On-line Mud Logging Handbook Alun Whittaker