Measurement While Drilling

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Measurement while drilling - Wikipedia, the free encyclopedia Page 1 of 6 Measurement while drilling From Wikipedia, the free encyclopedia Well logging Measurement while drilling is a system developed to make drilling related measurements and transmit information to the surface while drilling the well. MWD tools are conveyed downhole as part of bottom hole assembly (BHA). The tools are either contained inside a drill collar (sonde type) or are built into the collars themseves. MWD systems c
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  Measurement while drilling Well logging Gamma ray loggingSpontaneous potential loggingResistivity loggingDensity loggingSonic loggingCaliper loggingMud loggingLWD/  MWD   From Wikipedia, the free encyclopedia Measurement while drilling is a system developed to make drillingrelated measurements and transmit information to the surface whiledrilling the well. MWD tools are conveyed downhole as part of bottomhole assembly (BHA). The tools are either contained inside a drillcollar (sonde type) or are built into the collars themseves.MWD systems can take measurements of natural gamma ray,directional survey, tool face, borehole pressure, temperature, vibration,shock, torque etc. Some advanced MWD tools can even measureformation pressure and take formation samples. The MWD also provides the telemetry for operatingrotary steering tools (RSTs).The measured results are stored in MWD tools and some of the results can be transmitted digitally tosurface using mud pulser telemetry or other advanced technology.Certain MWD systems have the capability of receiving encoded control commands which are sent byturning on and off mud pumps and/or changing the rotation speed of drill pipe or by other advancedtelemetry technology such as wired pipe. Contents 1 Types of information transmitted ■ 1.1 Directional information ■ 1.2 Drilling mechanics information ■ 1.3 Formation properties ■ 2 Data transmission methods ■ 2.1 Mud pulse telemetry ■ 2.2 Electromagnetic telemetry (EM Tool) ■ 2.3 Wired Drill Pipe ■ 3 Retrievable tools ■ 3.1 Limitations ■ 4 See also ■ 5 Notes ■ Types of information transmitted Directional information MWD tools are generally capable of taking directional surveys in real time. The tool usesaccelerometers and magnetometers to measure the inclination and azimuth of the wellbore at thatPage 1 of 6Measurement while drilling - Wikipedia, the free encyclopedia8/3/2009  location, and they then transmit that information to the surface. With a series of surveys at appropriateintervals (anywhere from every 30ft (ie 10m) to every 500 ft), the location of the wellbore can becalculated.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 arenot authorized to drill. However, due to the cost of MWD systems, they are not generally used on wellsintended to be vertical. Instead, the wells are surveyed after drilling through the use of MultishotSurveying 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 thewell towards a target zone, he must know where the well is going, and what the effects of his steeringefforts are.MWD tools also generally provide toolface measurements to aid in directional drilling using downholemud motors with bent subs or bent housings. For more information on the use of toolface measurements,see Directional Drilling. 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 theMWD tool and any other downhole tools, such as Mud Motors, Rotary Steerable Systems, and LWDtools, are operated within their technical specifications to prevent tool failure. This information also isvaluable to Geologists responsible for the well information about the formation which is being drilled. 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 alog, similar to one obtained by wireline logging.LWD Logging While Drilling tools are able to measure a suite of geological characteristics including-density, porosity, resistivity, pseudo-caliper, inclination at the drill bit (ABI), magnetic resonance andformation 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 formationproperties, rather than simply drilling into a preset target.Most MWD tools contain an internal Gamma Ray sensor to measure natural Gamma Ray values. This isbecause these sensors are compact, inexpensive, reliable, and can take measurements throughPage 2 of 6Measurement while drilling - Wikipedia, the free encyclopedia8/3/2009  unmodified drill collars. Other measurements often require separate Logging While Drilling tools, whichcommunicate with the MWD tools downhole through internal wires. Data transmission methods Mud pulse telemetry This is the most common method of data transmission used by MWD (Measurement While Drilling)tools. Downhole a valve is operated to restrict the flow of the drilling mud (slurry) according to thedigital information to be transmitted. This creates pressure fluctuations representing the information.The pressure fluctuations propagate within the drilling fluid towards surface where they are receivedfrom pressure sensors. On surface the received pressure signals are processed by computers toreconstructs the transmitted information. The technology is available in three varieties -  positive pulse, negative pulse, and continuous wave .Positive PulsePositive Pulse tools briefly close and open the valve to restrict the mud flow within the drill pipe.This produces an increase in pressure that can be seen at surface. Line codes are used to representthe digital information in form of pulses.Negative PulseNegative pulse tools briefly open and close the valve to release mud from inside the drillpipe outto the annulus. This produces a decrease in pressure that can be seen at surface. Line codes areused to represent the digital information in form of pulses.Continuous WaveContinuous wave tools gradually close and open the valve to generate sinusoidal pressurefluctuations within the drilling fluid. Any digital modulation scheme with a continuous phase canbe used to impose the information on a carrier signal. The most widely used modulation scheme iscontinuous phase modulation.When underbalanced drilling is used, mud pulse telemetry can become unusable. This is because usuallyin order to reduce the equivalent density of the drilling mud a compressible gas is injected into the mud.This causes high signal attenuation which drastically reduces the ability of the mud to transmit pulseddata. In this case it is necessary to use methods different from mud pulse telemetry, such aselectromagnetic waves propagating through the formation or wired drill pipe telemetry.Current mud pulse telemetry technology offers a bandwidths of up to 40 bps. [1] The data rate drops withincreasing length of the wellbore and is typically as low as 1.5 bps [2] - 3.0 bps. [3] bits per second at adepth of 35,000 ft - 40,000 ft (10668 m - 12192 m).Surface to down hole communication is typically done via changes to drilling parameters, i.e. change of the rotation speed of the drill string or change of the mud flow rate. Making changes to the drillingparameters in order to send information can require interruption of the drilling process, which isunfavorable due to the fact that it causes non-productive time. Electromagnetic telemetry (EM Tool) These tools incorporate an electrical insulator in the drillstring. To transmit data the tool generates analtered voltage difference between the top part (the main drillstring, above the insulator), and the bottomPage 3 of 6Measurement while drilling - Wikipedia, the free encyclopedia8/3/2009  part (the drill bit, and other tools located below the insulator of the MWD tool). On surface a wire isattached to the wellhead, which makes contact with the drillpipe at the surface. A second wire isattached to a rod driven into the ground some distance away. The wellhead and the ground rod form thetwo electrodes of a dipole antenna. The voltage difference between the two electrodes is the receivesignal that is decoded by a computer.The EM tool generates voltage differences between the drillstring sections in the pattern of very lowfrequency (2-12Hz) waves. The data is imposed on the waves through digital modulation.This system generally offers data rates of up to 10 bits per second. In addition, many of these tools arealso capable of receiving data from the surface in the same way, while mud pulse-based tools rely onchanges in the drilling parameters, such as rotation speed of the drillstring or the mud flow rate, to sendinformation from the surface to downhole tools. Making changes to the drilling parameters in order tosend 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 specializedsituation, such as underbalanced drilling or when using air as drilling fluid. However, it generally fallsshort when drilling exceptionally deep wells, and the signal can lose strength rapidly in certain types of formations, becoming undetectable at only a few thousand feet of depth. Wired Drill Pipe Several oilfield service companies are currently developing wired drill pipe systems. These systems useelectrical wires built into every component of the drillstring, which carry electrical signals directly to thesurface. These systems promise data transmission rates orders of magnitude greater then anythingpossible with mud pulse or electromagnetic telemetry, both from the downhole tool to the surface, andfrom the surface to the downhole tool. The IntelliServ [4] wired pipe network, offering data ratesupwards of 1 megabit per second, became commercial in 2006. Representatives from BP America,StatoilHydro, INTEQ, and Schlumberger presented three success stories using this system, both onshoreand offshore, at the March, 2008 SPE/IADC Drilling Conference in Orlando, Florida [5] . 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 throughthe drill string. This generally allows the tool to be replaced much faster in case of failure, and it allowsthe tool to be recovered if the drillstring becomes stuck. Retrievable tools must be much smaller, usuallyabout 2 inches or less in diameter, though their length may be 20 feet or more. The small size isnecessary for the tool to fit through the drillstring, however, it also limits the tool's capabilities. Forexample, slim tools are not capable of sending data at the same rates as collar mounted tools, and theyare also more limited in their ability to communicate with and supply electrical power to other LWDtools.Collar-mounted tools, also known as Fat Tools , cannot generally be removed from their drill collar atthe 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.Page 4 of 6Measurement while drilling - Wikipedia, the free encyclopedia8/3/2009
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