Measurement while drilling

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Measurement while drilling is a tool that transmits 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 30ft (ie 10m)to every 500 ft), the location of the wellbore can be calculated.

MWD tools are extremely complex pieces of high- tech electronics.

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. Wireline Technologies in Houston Texas is the industry leader in replacement parts.

[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, pseudo-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

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

Continuous wave telemetry sends the sensory data accumulated by the downhole measurement tool, known as an MWD (measurement while drilling) or LWD (logging while drilling) tool, by phase variations in the rig standpipe in a specific analog signal sequence that can be converted to a digital signal by processors that can measure fluctuations in the medium's pressure. Positive pulse telemetry sends the sensory data by pressure increases. Negative pulse is the same transmission of encoded data using pressure decreases.

All digital data is formed by combining ones and zeros, known as the binary numeral system format. When the MWD (measurement while drilling)/LWD (logging while drilling) tool transmits data, it is in the form of an analog signal that sends ones and zeros. Combining the sequences of numbers into a word, (or symbol, or value or anything else a computer is programmed to look for), returns input for the specific variable the system was expecting at the time. Continued decoding of data will be achieved while the binary return values match the expected variable input data format.

When underbalanced drilling is used, mud pulse telemetry can become unusable. This is because usually in order to reduce the equivalent density of the drilling mud a compressible gas is injected into the mud, drastically reducing its ability to transmit pulsed data. In this case it is necessary to use another method different than mud telemetry, such as electromagnetic waves to sensors located at the surface.

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 then 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 continuous debate among competing oilfield service companies. To ensure success in all drilling 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 (bit/s). Attempts to improve this are experimental.[1]

[edit] Electronic pulse telemetry (EM Pulse Tool)

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 tool then generates voltage differences between the drillstring sections in the pattern of very low frequency (2-12Hz) waves. Data is modulated into these waves through phase alterations, similar to continuous wave mud pulse telemetry. The waves are then detected by the wires on the surface, and decoded by computers.

This system generally offers data rates equivalent to or faster than mud pulse telemetry systems. In addition, many of these tools are also capable of receiving data from the surface in the same way, while mud pulse-based tools rely on changes in the drilling parameters, such as rotation speed of the drillstring or the mud flow rate, to send information from the surface to downhole tools. Making changes to the drilling parameters in order to send information to the tools generally interrupts the drilling process, causing lost time.

Compared to mud pulse telemetry, electronic pulse telemetry is more effective in certain specialized situation, such as underbalanced drilling or when using air as the drilling fluid. However, it generally falls short when drilling exceptionally deep wells, and the signal can lose strength rapidly in some formations, becoming undetectable at only a few thousand feet of depth. As with the various types of mud pulse telemetry, the most appropriate choice depends on the drilling conditions.

[edit] Wired Drill Pipe

Several oilfield service companies are currently developing wired drill pipe systems. These systems use electrical wires built into every component of the drillstring, which carry electrical signals directly to the surface. These systems promise data transmission rates orders of magnitude greater then anything possible with mud pulse or electronic pulse telemetry, both from the downhole tool to the surface, and from the surface to the downhole tool. The IntelliServ [2] wired pipe network, offering data rates upwards of 56,000 bits per second, became commercial in 2006. Representatives from BP America, StatoilHydro, INTEQ, and Schlumberger presented three success stories using this system, both onshore and offshore, at the March, 2008 SPE/IADC Drilling Conference in Orlando, Florida[3].

[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 generally 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 as collar mounted tools, and they are also more limited in their ability to communicate with and supply electrical power to 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.

The ability to retrieve the tool via wireline is often useful. For example, if the drillstring becomes stuck in the hole, then retrieving the tool via wireline will save a substantial amount of money compared to leaving it in the hole with the stuck portion of the drillstring. However, there are some limitations on the process.

[edit] Limitations

Retrieving a tool using wireline is not necessarily faster than pulling the tool out of the hole. For example, if the tool fails at 1,500 ft (460 m) while drilling with a triple rig (able to trip 3 joints of pipe, or about 90 ft (30 m) feet, at a time), then it would generally be faster to pull the tool out of the hole then it would be to rig up wireline and retrieve the tool, especially if the wireline unit must be transported to the rig.

Wireline retrievals also introduce additional risk. If the tool becomes detached from the wireline, then it will fall back down the drillstring. This will generally cause severe damage to the tool and the drillstring components in which it seats, and will require the drillstring to be pulled out of the hole to replace the failed components, thus resulting in a greater total cost then pulling out of the hole in the first place. The wireline gear might also fail to latch onto the tool, or in the case of a severe failure, might bring only a portion of the tool to the surface. This would require the drillstring to be pulled out of the hole to replace the failed components, thus making the wireline operation a waste of time.

[edit] See also

[edit] Notes

  1. ^ High Data Rate MWD Mud Pulse Telemetry (1997). Retrieved on 2006-12-12.
  2. ^ Intelliserv Network (2008). Retrieved on 2008-03-13.
  3. ^ T.H. Ali, et al, SPE/IADC 112636: High Speed Telemetry Drill Pipe Network Optimizes Drilling Dynamics and Wellbore Placement; T.S. Olberg et al, SPE/IADC 112702: The Utilization of the Massive Amount of Real-Time Data Acquired in Wired-Drillpipe Operations; V. Nygard et al, SPE/IADC 112742: A Step Change in Total System Approach Through Wired-Drillpipe Technology (2008). Retrieved on 2008-03-13.