Anejo I – Cálculos constructivos
6. ANÁLISIS DE PÓRTICOS
From the available down-hole measurements, transmission schemes and surface systems it is possible to assemble a multitude of MWD systems. Not all of these are, in fact, commercially viable. The following list contains the configurations most commonly encountered in industry.
Directional Survey Services
The minimal level of MWD service requires only measurement of drill string inclination, azimuth and the tool face direction of directional steering tools, when used. The service equipment must readily portable and easy to install and remove since, in low cost production drilling, the service may be used only for complicated steering jobs and then replaced with a cheaper, conventional wire-line directional service for the remainder of the well. It must be compatible with low-tech rigs and, most important of all, either monumentally reliable or capable of being replaced in the drill string without tripping.
Since the service provides only single measurements, not a continuous log, there is little need for any sophisticated data processing, storage or plotting functions. A simple, weather-proofed gauge or liquid-crystal visual display unit is commonly sufficient (see Figure 11 and Figure 19). This is, most often, used to monitor that the well is correctly following a pre-planned well path.
If, on the other hand, the data is to be used, for well planning or course correction, it will be necessary to provided a computer and software to perform the complicated calculations in three-dimensional geometry required for well planning. This software, which may be provided by then MWD contractor, the operator or separate directional drilling contractor will normally run on a networked personal computer, either at the well site, or accessible via the Internet. Alternatively, off-line analyses may use manually entered directional data (there is not so much as to require automatic data acquisition). Reports will be generated as two- or three-dimensional well plots rather than depth-scaled continuous logs (see Figure 20).
Several companies provide this level of service; the majority of them using positive mud pulsing systems. Amongst them are pure MWD service companies and others who combine directional MWD with mud logging or with other services, such as directional well planning, drilling management, wire-line directional measurement and steering tool rental. Obviously, this gives a wide choice in service combinations and competitive pricing.
As I mentioned above, all use positive mud pulsing with similar sensor equipment and performance. Most MWD companies purchase identical directional sensor from the same manufacturer, and offer a very limited form of two-way communication in which the down-hole computer can be turned-off, reset, or switched between various computational modes by applying instructions signaled by drill string movements, or turning the mud pump on and off in a particular sequence.
The major variations in tool design, centering around the reliability question. At on extreme are large, powerful, multiply redundant system which offer very high reliability but they are, of course, as large, and as expensive to use and as cumbersome to rig-up and tear-out as the
multi-sensor MWD tools used in exploration drilling (see below). At the other extreme are systems almost, but not quite, to the point of being expendable. These small light systems are not only easily installed or removed from the drill string, without tripping in a special drill collar, they also light weight enough to be carried easily to and from the well site in a van or helicopter (see Figure 21).
Figure 20: If On-site well-planning or plan modification is undertaken a computer system will be needed to perform the complex calculations and plots needed to determine bottom-hole position and proximity to other wells or lease
boundaries.
The On-line Mud Logging Handbook Alun Whittaker
Figure 21: This small directional MWD tool can be hand-carried to the well site in its suitcase, go-deviled into a conventional drill string, and begin transmitting directional data within a matter of hours from call-out.
Enhanced Directional Services
Directional logging is useful in monitoring a well plan on production wells where a detailed well plan has been designed from good knowledge of the geological structure and target depths. However, on early stage development wells drilled from offshore production
platforms and the rare directional exploration well, knowledge of the section may not be so conclusive. In these cases, there can be need for a directional MWD service with some limited formation logging capability for correlation purposes.
While the simplest directional MWD systems are limited to low end capabilities, the larger systems can be enhanced with natural gamma ray or simple short normal electric log and a surface logging unit capable of producing a rudimentary real-time log, sometimes in conjunction with mud logging. To add this capability requires greater internal complexity: power supply and distribution, processing power and, of course, the ability to operate continuously at higher data rates. It is also necessary, on surface, to have equipment to automatically acquire, process, store and plot the data.
These logs are insufficient for complete formation evaluation but they can be used to maintain real-time well path control relative to both the original drilling plan and to the actual formation tops encountered while drilling. They may particularly effective as in-bed steering tools in long reach,directional wells. When the well path is approximately horizontal following a single, relative thin horizon then, more important than the actual, quantitative bore-hole inclination measurement, is determining whether the well path is within the target horizon, or whether it has transgressed upward, or downward, beyond that horizon. In this circumstance, a real-time Short Normal, or Gamma Ray log may be the most useful directional tool available.
Formation Evaluation Recording Services
Not all exploration data is required instantly as drilling takes place.
While gathering data in real-time may have value, there still may not be any value to using the data in real-time. In many cases, the complexity, cost and failure-probability of a transmission system is simply superfluous to an MWD system (see Coope, and Hendricks, 1984.) In these circumstances, reliability can be improved and costs substantially reduced by dispensing with the transmission system and using a memory-only system.
Where formation evaluation demands can wait a few hours until the end of the bit run, a memory-only MWD system can allow reduced logging costs, or addition of more and better sensors to the system without adding cost.
Yet another use for a memory-only system is for specialized applications requiring either very large volumes of data, or where data
processing demands are complex, variable or subject to operator skill. In these cases, down-hole processing may not be practical and MWD data transmission rates cannot accommodate the data volume in real-time. An obvious example of this type of applications is the detailed analysis of drill string vibration, discussed above. Geophysical applications such as VSP similarly require the gathering of large volumes of data in a short time interval and subjective processing. These types of MWD applications are not compatible with real-time transmission but lose little value when used with a memory-only system.
The On-line Mud Logging Handbook Alun Whittaker
Real-time Formation Evaluation
The top-of-the-line, gold-plated Cadillacs of MWD services are the multi-sensor tools providing a complete suite of drilling, bore-hole and formation sensors (see Teige, Undersrud and Rees, 1984) combined with a powerful system for data acquisition, data analysis and log evaluation.
MWD sensor development is not so far developed that these systems are able to offer a choice of sensor combinations. Most systems provide all available sensors in a single, standard MWD tool. The few choices presently provided are:
✔ The type of formation electrical measurement made. Short normal or focused resistivity measurements may be replaced with
induction or electromagnetic propagation measurements. As with wire-line logging, the choice is made on the basis of the anticipated