COMPONENTS OF MWD SYSTEM

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7 Feb 2024

Telemetry Channel

Transmission System

Power Source

MWD Sensors

Surface Systems

TELEMETRY CHANNEL

Telemetry Channels are the channels or medium via which the downhole data is transmitted to the surface.

Following are the transmission channels used for data transmission :

Hard Wire Method

Electromagnetic Method

Acaustic Method

Mud Pulse Telemetry

MUD PULSE TELEMETRY

Most of the MWD systems commercially available are based on some form of mud pulse telemetry.

The major components of a mud pulse telemetry system are shown in figure below:

The downhole components are all housed in a nonmagnetic drill collar (NMDC).

The major components are:

(a) a power source to operate the tool : it operates the tool when we plan to take a survey

(b) sensors to measure the required information;

(d) a microprocessor or control system to coordinate the various functions of the tool : it powers up the sensors, stores the information that has been measured and then activates the transmitter to send the data in the form of a coded message.

The surface equipment consists of:

(a) a standpipe pressure transducer to detect variations in pressure and convert these to electrical signals;

(b) an electronic filtering device to reduce or eliminate any interference from rig pumps or downhole motors that may also cause pressure variations;

(d) a rig-floor display to communicate the results to the driller, or plotting devices to produce continuous logs

TRANSMISSION SYSTEM

Here in transmission system I shall concentrate on the transmission via drilling fluid, i.e., the mud pulse telemetry.

The Mud Pulse Telemetry can be further categorized into :

Positive Mud Pulse Telemetry (Maximum use in industry)

Negative Mud Pulse Telemetry

Continuous Wave Telemetry

Positive Mud Pulse Telemetry :

Positive mud pulse telemetry (MPT) uses a hydraulic poppet valve to momentarily restrict the flow of mud through an orifice in the tool to generate an increase in pressure in the form of a positive pulse or pressure wave which travels back to the surface and is detected at the standpipe.

To transmit data to surface, this valve is operated several times, creating a series of pulses that are detected by the transducer, and decoded by the surface computer.

The surface computer initially recognizes a set of reference pulses, which are

followed by the data pulses. The message is decoded by detecting the presence or absence of a pulse within a particular time-frame. This binary code can then be translated into a decimal result. A chart recorder is used to monitor the sequence of pulses.

Negative Mud Pulse Telemetry

Negative MPT uses a controlled valve to vent mud momentarily from the interior of the tool into the annulus. This process generates a decrease in pressure in the form of a negative pulse or pressure wave which travels back to the surface and is detected at the standpipe.

The rapid opening and closing of this valve therefore creates a drop in standpipe that can be detected by the pressure transducer.

Continuous Wave Telemetry

Continuous wave telemetry uses a rotary valve or “mud siren” with a slotted rotor and stator which restricts the mud flow in such a way as to generate a modulating positive pressure wave which travels to the surface and is detected at the standpipe.

One of the discs is stationary while the other is driven by a motor.

The constant speed of the motor creates a regular and continuous variation in pressure that is essentially a standing wave. This wave is used as a carrier to transmit the data to surface. When information is to be transmitted the speed of the motor is reduced so that the phase of the carrier wave is altered (i.e., reversed).

The carrier wave is therefore modulated to represent the data required.

The surface equipment detects these phase shifts in the pressure signal and translates this into a binary code.

This is a more sophisticated telemetry system and offers a higher data rate than the previous two mud pulse methods.