Measurement While Drilling (MWD) Systems Information
Measurement while drilling (MWD) systems monitor wellbore position, drill bit performance, and drill string orientation. Instruments in a module in the steering tool of the drill string transmit real-time data to the MWD operator. It is related to logging-while-drilling (LWD).
Ruggedized instruments—magnetometers, gyroscopes, and accelerometers that can withstand temperatures of 400° F, pressures of 25,000 psi, and 500 G for 0.5 ms of shock—are integrated into the bottom hole assembly (BHA). The BHA is outfitted with a drill bit, drill collar, and drill stabilizers that are the functional components responsible for borehole shape, direction, and penetration. A BHA often includes a downhill motor and rotary steering system in addition to MWD and LWD systems. Mechanical and electrical power is often provided by a downhole turbine.
MWD strictly quantifies drill string performance. LWD measures aspects related to geological formations, including:
Gamma rays emitted from rock or sediment
Density and photoelectric index
Neutron porosity (to measure hydrogen index in a reservoir)
Caliper (borehole size and shape)
Sonic logging (ability of borehole to transmit sonic waves)
Formation tester and sampler (to determine fluids and potential production)
Nuclear magnetic resonance (to determine a geological formation's porosity and permeability)
Seismic while drilling measurements that help determine optimal borehole path
Drilling engineers use the data from MWD and LWD systems to geosteer the drill string and make informed decisions about borehole path and expected well production in on-shore and off-shore drilling applications. Reducing drilling hazards that could slow the rate of penetration improves drill string productivity. This data is also used to ensure drilling only takes place in authorized zones.
MWD also monitors the operation of the drill bit and drill string, including parameters such as speed and smoothness of bit rotation, downhole vibration and temperature, torque and pressure on drill bit, and flow rate of drilling fluid. Keeping the drill string within its operating specifications maximizes drill string life and performance.
Data Collection and Transmission
Traditionally, wireline logging connects a thin array of instruments located in the drill string to the surface via a durable electric cable. Once a wellbore deviates beyond 60°, traditional wireline instruments can no longer be pushed through the drill string, so MWD technologies are used despite their increased cost.
In MWD systems, data is logged in a solid state memory and is also passed to a logic controller that converts the data to binary. Most often, data is then fed to a pulser unit that fluctuates the pressure of the drilling fluid inside the drill pipe according to a code known as mud pulse telemetry (MPT). Pressure transducers and computers on the surface isolate the positive, negative, and continuous sine wave adjustments in pressure and decode them for operators.
Positive: a valve is toggled to restrict mud flow in the drill pipe to produce an increase in pressure that is identified at the well surface. Data is encoded with line codes of pulse-position modulation.
Negative: a valve is toggled that releases drilling fluid from inside the drill pipe to the annulus to produce a decrease in pressure that is recognized at the surface. Data is encoded with line codes or pulse-position modulation.
Continuous: a valve is gradually toggled to create sinusoidal pressure changes in the drilling fluid. Data is encoded with any digital modulation format, the most common being continuous phase modulation.
MPT bandwidths up to 40 bits per second are common; data rates drop as the wellbore length increases, and at depths of 40,000 ft. can be as low as 1.5 bits per second. Often, not all data can be transmitted via MPT, so additional data can be retrieved from the memory via wireline or after the tool has been tripped out of the hole.
However, during underbalanced drilling compressed gas is injected into the drilling fluids to keep the wellbore pressure lower than the pressure of the formation being drilled. This is done to reduce common challenges of conventional (overbalanced) drilling, such as circulation loss, differential sticking, slow drilling rates, formation damage, and drill bit overheating. The addition of gas increases attenuation of MPT signals so alternative transmission techniques are used.
Electromagnetic telemetry (EMT) integrates an electrical insulator in the drill string that generates an altered voltage difference in components above the insulator and those below it. Data is inserted into the voltage by modulating it. The electrodes of a surface-based dipole antenna are formed by a wire attached to the wellhead and a wire attached to a rod driven into the ground. The voltage difference between the electrodes receives the signal from the drill string that is read by a computer. EMT can also send data to the drill string. While EMT offers data rates up to 10 bits per second, signals degrade quickly based on well depth and formation materials.
Drill pipe with electrical hardware is also available. Data rates higher than 2 megabits per second have been achieved. The downside to this technology is the increased expense, from manufacturing, additional care, and circuit protection.
MWD instruments can be mounted in the drill collar and only serviced at equipped facilities. Collar-mounted MWD systems transfer data quicker and can support more instruments. If the drill string gets stuck, all of the tools must be retrieved together.
MWD systems can also be contained in internal modules connected via wireline. These modules can be retrieved from the drill string as needed, though they must be thin since they are located within the drill string. The small configuration limits the devices the MWD system can support. In the event the drill string gets stuck, either just the MWD or entire drill string can be retrieved.
These parameters are often reported by MWD systems.
Drill depth: below ground or sea level; length of wellbore; depth below drill rig
Weight on the drill bit
Torque on the drill bit, in lb.-ft.
Vibration on the drill string, in G/ms
Rotation speed of the drill string, in RPM
Temperature: operational temperate in Fahrenheit or Celsius
Mud flow volume, in m3/s
Inclination: deviation of the wellbore from vertical, in degrees
Azimuth: orientation of the wellbore to north, in degrees
Since wellbore drilling is both dangerous and expensive, it is a highly standardized industrial process. Standards attaining to MWD and LWD processes include:
ISO DIS 22476-15 -- Geotechnical investigation: MWD
ASTM D5753 -- Guide for geophysical borehole logging
IADC Drilling Manual -- Published by the International Association of Drilling Contractors (IADC)
API RP 31A -- Presenting MWD/LWD data in hardcopy