All MWD systems have certain basic similarities
- a downhole system which consists of a power source, sensors, transmitter and control system.
- a telemetry channel (mud column) through which pulses are sent to surface.
- a surface system which detects pulses, decodes the signal and presents results (numerical display, geological log, etc.).
MWD SystemsThe main difference between the 3 MWD systems currently available is the method by which the information is transmitted to surface. All three systems encode the data to be transmitted into a binary code and transmitting this data as a series of pressure pulses up the inside of the drillstring. The process of coding and decoding the data will be described below. The only difference between the systems is the way in which the pressure pulses are generated.
(i) Negative Mud Pulse TelemetryIn all systems fluid must be circulating through the drillstring. In the negative mud pulse system a valve inside the MWD tool opens and allows a small volume of mud to escape from the drill string into the annulus. The opening and closing of this valve creates a small drop in standpipe pressure (50 - 100 psi), which can be detected by a transducer on surface.
(ii) Positive Mud PulseIn the positive mud pulse system a valve inside the MWD tool partially closes, creating a temporary increase in standpipe pressure.
(iii) Frequency Modulation (Mud siren)In the frequency modulation system a standing wave is set up in the mud column by a rotating slotted disc. The phase of this continuous wave can be reversed. The data is transmitted as a series of phase shifts.
Many tools also include the ability to record downhole data for later retrieval at surface. Although this undermines the principle of access to ‘real time’ data it allows the operator to gather large volumes of data (typical petrophysical data) and therefore eliminate expensive electric wireline logging operations.
Power SourcesSince there is no wireline connection to surface all the power required to operate the MWD tool must be generated downhole. This means that either a battery pack or a turbine-alternator must be installed as part of the MWD tool.
The turbine has been the standard method of power generation in the positive pulse and frequency modulation tools. Since less power is required in the negative pulse system batteries have been used.
However, with more sensors being added and higher data rates required, batteries are being replaced with turbines in negative pulse systems also.
Turbines have several advantages over batteries but turbines are more prone to mechanical failure. Filter screens are used to prevent debris in the mud from damaging the turbine