Bottomhole data are acquired incrementally from sensors located in the drill string near the bit in a drilling well. Measurements may include directional information (hole inclination, azimuth, tool facing), drilling parameters (bottomhole temperature, pressure, torque, weight-on-bit, RPM), rig safety data, formation evaluation and correlation data (formation resistivity, gamma-ray, and sonic logs). Data can be transmitted to the surface in real time by pressure pulses through the mud inside the drill pipe (timed amplitude and phase encoding). Telemetry by conductor cable integrated with the drill pipe or temporary digital recording at the sensor for later wire-line retrieval are alternative data recovery methods. Compare Tomex.
The term Measurement-while-drilling simply refers to the remote collection of borehole data, typically in real time, while the drill string is in place. This phrase is often used to refer to a multitude of different geophysical and directional measurements.
History of MWD
The earliest MWD technology focused on directional measurements such as inclination and azimuth relative to North. These systems were first used in the 1960's but at that time they were only in their infancy. It wasn't until the late 1970's - early 1980's when the use of MWD technology became mainstream. This occurred first in the North Sea where increasing regulations regarding borehole surveys made the use of wireline surveys cost inefficient. This is because wireline based surveys require considerable downtime to trip out and then back into the hole. Furthermore, for the first time wells were being drilled at angles which could not be navigated by traditional borehole tools.  This drove the need for an alternative to wireline logging which could be used to obtain basic, but valuable directional data without putting the drill string on slips. Eventually, geophysical tools measuring gamma ray values, rock densities, and porosity were added to bottom hole assemblies. The collection of geophysical data while drilling is also referred to as Logging-while-drilling (LWD).
MWD and Unconventional Development
Today, the production of unconventional reservoirs using horizontal drilling has continued to bolster the MWD service industry. These high angle wells require regular directional data and tools which can be passed through curve sections into the lateral portion of the well. Geophysical measurements, particularly gamma ray tools, are also commonly used while drilling today’s unconventional reservoirs. Gamma ray data provides valuable insight to geosteerers regarding the lithologic variations within the producing, lateral section of a well. This is particularly important in the case of shale reservoirs, where relatively high gamma ray readings are expected.
MWD data is first acquired by a series of tools and sensors which are placed on the bottom hole assembly (BHA). The challenge is then transmitting the data which has been collected back to the surface with minimal delay in drilling operations. This is done by converting the collected data into binary at the bottom hole assembly. This binary code is most commonly transmitted to the surface using what is called mud pulse telemetry. This uses pressure pulses sent through the drilling mud which can be decoded at the surface giving the original binary code/data. Currently, three different mud pulse telemetry technologies exist, these being positive pulse (pressure increases), negative pulse (pressure decreases), and continuous wave (Sinusoidal waves) telemetries. A simple analog for this is a Morse Code-like signal, consisting of variations in a parameter with time, which carry an encoded message to a receiver. Data is then typically converted into .las files which can be displayed on wireline logs, mud logs, or on MWD logs.
In addition to mud pulse telemetry, less common MWD system utilize other transmission methods such as wired drill strings and electromagnetic signals sent through a conductor on the drill string. Furthermore, surface software systems play a major role in the visualization and utilization of the acquired MWD data. Many service companies develop their own MWD software's in addition to storing data as versatile .las files.
The MWD Service
MWD tools and technicians are typically contracted from 3rd party oil field service companies. Today, it is common to see the MWD service offered in package with directional drillers and sometimes geosteerers. While these individuals are employed by separate contractors, it is still crucial that they communicate effectively with mud loggers, company men, and geology departments to ensure optimal production from the well. This is because 'target' borehole locations are often determined using geophysical data from the MWD tool. These target locations can changed significantly, even as the borehole is drilled.
It is common for surveys to be taken every 30, 60, or 90 feet depending on the length of drill pipe stands, as it is most efficient to conduct surveys at connection when drilling has already stopped momentarily. In most cases, survey data is sent immediately to mud loggers to be placed on the well logs. When the bit direction becomes hard to control it is common to take additional check-shot surveys to avoid sudden changes in borehole direction. MWD data can also be very useful for routine drilling operations. For instance, after running surface or intermediate casing the cement plug must be drilled out. Improper clearing of the plug can create major problems from a relatively routine task. Therefore, MWD measurements which may not be intended for locating cement can be used to locate the plug and avoid tripping in too far. Measurements which may be helpful in this situation include weight on bit. If weight on bit is present and the pipe is not continuing to enter the hole, it is likely you are on bottom (i.e. this plug in this example). This is just one of the many routine drilling applications in which unconventional uses of MWD data may be applied.
The largest downfall to MWD technology is the exposure of expensive technology to an unsecured borehole. It is inevitable the issues such as stuck pipe will occur from time to time when drilling wells in almost any field. When this occurs 'twist-offs' are common where the drill string can separate during attempts to jar it free. When this happens the entire bottom hole assembly, MWD tools included, can be lost to the subsurface. This presents a risk that must be considered before placing too many expensive sensors on a drill string.
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