Measurement While Drilling

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Measurement While Drilling tools are used by Drilling rigs to transmit information in real time from the tool, located near the drill bit, to the surface.

Contents

[edit] Types of Information Sent

[edit] Directional Information

MWD tools are generally capable of taking directional surveys in real time. The tool uses accelerometers and magnetometers to measure the inclination and azimuth of the wellbore at that location, and they then transmit that information to the surface. With a series of surveys at appropriate intervals (anywhere from every 30 feet to every 500 feet), the location of the wellbore can be calculated.

By itself, this information allows operators to prove that their well does not cross into areas that they are not authorized to drill. However, due to the cost of MWD systems, they are not generally used on wells intended to be vertical. Instead, the wells are surveyed after drilling through the use of Multishot Surveying Tools lowered into the drillstring on slickline or wireline.

The primary use of real-time surveys is in Directional Drilling. For the Directional Driller to steer the well towards a target zone, he must know where the well is going, and what the effects of his steering efforts are.

MWD tools also generally provide toolface measurements to aid in directional drilling using downhole mud motors with bent subs or bent housings. For more information on the use of toolface measurements, see Directional Drilling.

[edit] Drilling Mechanics Information

MWD tools can also provide information about the conditions at the drill bit. This may include:

  • Rotational speed of the drillstring
  • Smoothness of that rotation
  • Type and severity of any vibration downhole
  • Downhole temperature
  • Torque and Weight on Bit, measured near the drill bit
  • Mud flow volume

Use of this information can allow the operator to drill the well more efficiently, and to ensure that the MWD tool and any other downhole tools, such as Mud Motors, Rotary Steerable Systems, and LWD tools, do not fail. This information can also give Geologists responsible for the well information about the formation which is being drilled.

[edit] Formation Properties

Many MWD tools, either on their own, or in conjunction with separate Logging While Drilling tools, can take measurements of formation properties. At the surface, these measurements are assembled into a log, similar to one obtained by wireline logging.

LWD tools are able to measure a suite of geological characteristics including- density, porosity, resistivity, psuedo-caliper, inclination at the drill bit (ABI), magnetic resonance and formation pressure.

The MWD tool allows these measurements to be taken and evaluated while the well is being drilled. This makes it possible to perform Geosteering, or Directional Drilling based on measured formation properties, rather than simply drilling into a preset target.

Most MWD tools contain an internal Gamma Ray sensor to measure natural Gamma Ray values. This is because these sensors are compact, inexpensive, reliable, and can take measurements through unmodified drill collars. Other measurements often require separate Logging While Drilling tools, which communicate with the MWD tools downhole through internal wires.

[edit] Data Transmission Methods

[edit] Mud Pulse Telemetry

It has been suggested that Mud pulse telemetry be merged into this article or section. (Discuss)

This is the most common method of data transmission used by MWD tools. It can be divided into three general categories - positive pulse, negative pulse, and continuous wave.

Positive Pulse
Positive Pulse tools operate by briefly interfering (restricting) with the mud flow within the drill pipe. This produces an increase in pressure that can be seen at the surface.
Negative Pulse
Negative pulse tools operate by briefly venting mud from inside the drillpipe out to the annulus. This produces a decrease in pressure that can be seen at the surface.
Continuous Wave
Continuous wave tools operate by generating a sinusoid type wave through the mud within the drilling pipe. The information is contained in the phase variation of this wave, and not the amplitude.

Whichever method is used, these pulses or waves are generated by the tool in specific patterns. They travel up the drill pipe through the drilling mud (slurry) and are detected by surface mud pressure transducers. These signals are decoded by computers on the surface.

Many factors constrain the speed of data transmission from MWD tools, and the best method of data transmission is still the subject of continuoual debate among competing oilfield service companies. To ensure sucsess in all drill environments, each method must be very flexible and robust. Even still, some methods are decidedly better suited for certain drilling conditions, while some are better in others.

Current mud pulse telemetry techniques can transmit data at a rate of around one to ten bits per second (bps). Newer technologies can increase this to thirty bps or higher.[1]

[edit] Electronic Pulse Telemetry

These tools insert an electrical insulator into the drillstring, and then generate a voltage difference between the top part (the main drillstring), and the bottom part (the drill bit, and other tools located below the MWD tool). On the surface, one wire is attached to the wellhead, which makes contact with the drillpipe at the surface, and another is attached to a rod driven into the ground some distance away. The voltage difference that the tool generates can then be detected between these two wires on the surface.

[edit] Retrievable Tools

MWD tools may be semi-permanently mounted in a drill collar (only removable at servicing facilities), or they may be self-contained and wireline retrievable.

Retrievable tools, sometimes known as Slim Tools, can be retrieved and replaced using wireline though the drill string. This allows the tool to be replaced much faster in case of failure, and it allows the tool to be recovered if the drillstring becomes stuck. Retrievable tools must be much smaller, usually about 2 inches or less in diameter, though their length may be 20 feet or more. The small size is necessary for the tool to fit through the drillstring, however, it also limits the tool's capabilities. For example, slim tools are not capable of sending data at the same rates at collar mounted tools, and they are also more limited in their ability to communicate with and power other LWD tools.

Collar-mounted tools, also known as Fat Tools, cannot generally be removed from their drill collar at the wellsite. If the tool fails, the entire drillstring must be pulled out of the hole to replace it. However, without the need to fit through the drillstring, the tool can be larger and more capable.

[edit] Companies with MWD Tools

[edit] Notes

  1. ^ High Data Rate MWD Mud Pulse Telemetry (1997). Retrieved on 2006-12-12.

[edit] External links

[edit] See also

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