Measurement while drilling
Measurement while drilling refers to directional-drilling measurements. Initial attempts to provide MWD date back to the 1920s. A MWD operator measures the trajectory of the hole as it is drilled. MWD tools may be capable of taking directional surveys, providing information about the conditions at the drill bit, or taking measurements of formation properties. Data may be transmitted in various ways, including by mud pulse telemetry, electromagnetic telemetry, and wired drill pipe. MWD tools may be semi-permanently mounted in a drill collar, or they may be self-contained and wireline retrievable.
Background
A drilling rig is used to create a borehole or well in the earth's sub-surface, for example in order to extract natural resources such as gas or oil. During such drilling, data is acquired from the drilling rig sensors for a range of purposes such as: decision-support to monitor and manage the smooth operation of drilling; to make detailed records of the geologic formations penetrated by a borehole; to generate operations statistics and performance benchmarks such that improvements can be identified, and to provide well planners with accurate historical operations-performance data with which to perform statistical risk analysis for future well operations.Terminology
The terms measurement while drilling and logging while drilling are not used consistently throughout the industry. Although these terms are related, within the context of this article, the term MWD refers to directional-drilling measurements, e.g. for decision support for the wellbore path, while LWD refers to measurements concerning the geological formations penetrated while drilling.History
Initial attempts to provide MWD and LWD date back to the 1920s, and attempts were made prior to WW2 with mud pulse, wired pipe, acoustic and electromagnetics. JJ Arps produced a working directional and resistivity system in the 1960s. Competing work supported by Mobil, Standard Oil and others in the late 1960s and early 1970s led to multiple viable systems by the early 1970s, with the MWD of Teleco Oilfield Services, systems from Schlumberger Halliburton and BakerHughes. However, the main impetus to development was a decision by the Norwegian Petroleum Directorate to mandate the taking of a directional survey in wells offshore Norway every 100 meters. This decision created an environment where MWD technology had an economic advantage over conventional mechanical TOTCO devices, and lead to rapid developments, including LWD, to add gamma and resistivity, by the early 1980s.Measurement
MWD typically concerns measurement taken of the wellbore inclination from vertical, and also magnetic direction from north. Using basic trigonometry, a three-dimensional plot of the path of the well can be produced.Essentially, a MWD operator measures the trajectory of the hole as it is drilled. This information is then used to drill in a pre-planned direction into the formation which contains the oil, gas, water or condensate. Additional measurements can also be taken of natural gamma ray emissions from the rock; this helps broadly to determine what type of rock formation is being drilled, which in turn helps confirm the real-time location of the wellbore in relation to the presence of different types of known formations.
Density and porosity, rock fluid pressures and other measurements are taken, some using radioactive sources, some using sound, some using electricity, etc.; this can then be used to calculate how freely oil and other fluids can flow through the formation, as well as the volume of hydrocarbons present in the rock and, with other data, the value of the whole reservoir and reservoir reserves.
An MWD downhole tool is also "high-sided" with the bottom hole drilling assembly, enabling the wellbore to be steered in a chosen direction in 3D space known as directional drilling. Directional drillers rely on receiving accurate, quality tested data from the MWD operator to allow them to keep the well safely on the planned trajectory.
Directional survey measurements are taken by three orthogonally mounted accelerometers to measure inclination, and three orthogonally mounted magnetometers which measure direction. Gyroscopic tools may be used to measure azimuth where the survey is measured in a location with disruptive external magnetic influences, inside "casing", for example, where the hole is lined with steel tubulars. These sensors, as well as any additional sensors to measure rock formation density, porosity, pressure or other data, are connected, physically and digitally, to a logic unit which converts the information into binary digits which are then transmitted to surface using "mud pulse telemetry".
This is done by using a downhole "pulser" unit which varies the drilling fluid pressure inside the drillstring according to the chosen MPT: these pressure fluctuations are decoded and displayed on the surface system computers as wave-forms; voltage outputs from the sensors ; specific measurements of gravity or directions from magnetic north, or in other forms, such as sound waves, nuclear wave-forms, etc.
Surface pressure transducers measure these pressure fluctuations and pass an analogue voltage signal to surface computers which digitize the signal. Disruptive frequencies are filtered out and the signal is decoded back into its original data form. For example, a pressure fluctuation of 20psi can be "picked out" of a total mud system pressure of 3,500psi or more.
Downhole electrical and mechanical power is provided by downhole turbine systems, which use the energy of the "mud" flow, battery units, or a combination of both.
Types of information transmitted
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; measurements of inclination, azimuth, and tool face, at appropriate intervals, 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 multi-shot 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.
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
Formation properties
Many MWD tools, either on their own, or in conjunction with separate LWD 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, acoustic-caliper, inclination at the drill bit, 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 LWD tools, which communicate with the MWD tools downhole through internal wires.
Measurement while drilling can be cost-effective in exploration wells, particularly in areas of the Gulf of Mexico where wells are drilled in areas of salt diapirs. The resistivity log will detect penetration into salt, and early detection prevents salt damage to bentonite drilling mud.
Data transmission methods
Mud-pulse telemetry
This is the most common method of data transmission used by MWD tools. Downhole, a valve is operated to restrict the flow of the drilling fluid according to the digital information to be transmitted. This creates pressure fluctuations representing the information. The pressure fluctuations propagate within the drilling fluid towards the surface where they are received from pressure sensors. On the surface, the received pressure signals are processed by computers to reconstruct the information. The technology is available in three varieties: positive pulse, negative pulse, and continuous wave.;Positive pulse
;Negative pulse
;Continuous wave
When underbalanced drilling is used, mud pulse telemetry can become unusable. This is usually because, in 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 pulsed data. In this case, it is necessary to use methods different from mud pulse telemetry, such as electromagnetic waves propagating through the formation or wired drill pipe telemetry.
Current mud-pulse telemetry technology offers a bandwidths of up to 40 bit/s. The data rate drops with increasing length of the wellbore and is typically as low as 0.5 bit/s – 3.0 bit/s. at a depth of 35,000 ft – 40,000 ft.
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 drilling parameters in order to send information can require interruption of the drilling process, which is unfavorable due to the fact that it causes non-productive time.