12 JANUARY 2024 WorldWide Drilling Resource® MWD and Why it’s Important Adapted from Information by Erdos Miller MWD, measurement while drilling, is literally a system which takes measurements while drilling downhole and sends them to the surface continuously while the hole is being drilled. MWD is of great importance as operators need to have realtime information from the well to help with steering the drill. MWD uses gyroscopes, magnetometers, and accelerometers to determine borehole inclination and azimuth while drilling is being done. The gathered data is transmitted to the surface using mud pulses and electromagnetic telemetry. Once on the surface, the data is decoded, and it can be sent to an off-site location. This allows engineers to make important decisions while drilling. One of the building blocks of the MWD system responsible for taking measurements is the inertial measurement unit (IMU), which is located in the wellbore. Inertial measurement is a general term for electronics which use sensors to calculate position and orientation. Most modern vehicles, from cars to airplanes, have some sort of IMU. Two of the primary measurements the MWD is responsible for are inclination and azimuth: inclination is the angle of the wellbore with respect to vertical, and azimuth is the angle of the wellbore’s direction with respect to North. When the wellbore is completely vertical, the inclination is 0° and the azimuth is undefined. When the wellbore takes a turn, the inclination begins changing from zero and, for most drilling operations, may eventually end up at 90° (horizontal). Additionally, when it changes direction from vertical, the azimuth describes the direction: if it is going North, the azimuth is 0°; if it is going East, the azimuth is 90°, etc. Accelerometers and magnetometers can be used to measure inclination and azimuth (there are conditions when these sensors are not enough). Once the measurements have been taken, the MWD needs to transmit the data to the surface, where it can be interpreted by rig operators. There are two primary methods for communicating information from down the hole to the surface: they include mud pulse telemetry and electromagnetic (EM) telemetry. Mud Pulse Telemetry - During drilling, a special drilling mud is pumped down the tubing and then sent back to the surface. Electronics at the bottom of the hole drive a valve that can constrict and relax the flow of this mud, creating pressure pulses in the fluid circulating in and out of the hole. These pressure pulses are the heart of mud pulse telemetry. EM Telemetry - Most people don’t think of the earth as being a conductor and although it may not be as good of a conductor as a piece of copper, the earth’s conductive properties can be used for data transmission. If you have an EM transmitter at the bottom of the hole, you could stick two probes in the rock and send a smooth, repetitive oscillation signal, called a sinusoidal signal, through the earth and up to the surface. The surface system receives all the data transmitted from down the hole and displays it in a way that is meaningful to the rig operators. If the method of telemetry is mud pulse, the surface system will include a pressure transducer which detects the pressure pulses in the drill fluid and converts the pulses to electrical signals; electronics are designed to convert these electrical signals into ones and zeros. If the MWD at the bottom of the hole did its job right, the ones and zeros received at the surface will be decoded and provide valuable information. In the case of EM telemetry, electronics are designed to detect the sinusoidal signal transmitted from the bottom of the wellbore. The challenge with this is, by the time the signal travels from the bottom of the hole to the surface, it will have lost most of its power to the earth and the electronics need to be able to amplify it in a way where it can be identified amidst the noise which inevitably creeps in. Like mud pulse telemetry, the EM transmitter is responsible for encoding data into the signals transmitted and, when those signals are received at the surface, they will be decoded and provide meaningful information. The Eclipse Touch MWD Surface System offers superior mud pulse detection with a modern touch-based user interface. DIR
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