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Oilfield Technology
August
2015
for procurement, logistics, repair and
maintenance.
This article will present improvements
for an MPT system to overcome challenges
associated with the HSMPT channel.
Systemdescription
A communication systemmay include
many components. Information must be
transmitted from a signal source to a sink.
In between are typical system components
that perform source coding, channel coding,
and modulation. After the physical data
transmission, the process is reversed. Details
on single components are described in
Proakis, 2001.
1
Mud pulse data transmission is
performed by opening and closing a valve
in the pulser, which transforms the sensor
readings into pressure fluctuations (modulation). Pressure transducers
(PT) mounted at the surface act as receivers. A signal acquisition and
processing unit removes noise and distortions from the received signal
and demodulates it to create machine- or human-readable information,
such as a log. A system is shown in Figure 2.
HSMPT can be achieved by a shear valve pulser, where a moving
disc is oscillated or rotated against a non-moving stator, creating
a carrier-modulated signal, especially when using higher carrier
frequencies for pass band transmission.
During the transmission, a known training sequence (TS) is
transmitted to adapt improved signal processing by means of comparison
between the sequence and the received signal. Signal attenuation and
distortions originating from the transmission channel can be recognised
and corrected. Such a system is described in more detail in Klotz, 2008.
2
In general, signals from an MPT in a deeper borehole experience
higher attenuation and have higher frequencies. Drilling fluids properties
(e.g. mud weight and plastic viscosity (PV)) can vary significantly from
one application to another. Therefore, the MPT signal passes through
various attenuation levels depending on the mud properties. Fluids with
high PV and high density attenuate the MPT signal more than fluids with
low PV and lowmud weights. Obtaining reliable, high data rates with
MPT is extremely challenging in deep wells containing oil-based mud. In
Shen, 2009
3
, different influence factors on the MPT channel have been
modelled.
The decoding quality of the MPT system strongly depends on
the quality of the calibration of the surface system signal-processing
algorithms. The algorithms must be initially calibrated, and the
calibration must be updated during operations as the drilling conditions
and drilling environment change.
In particular, algorithms must be updated after changes in the
mud/hydraulic systems such as changes in mud weight or hole depth,
and after changes in the downhole or surface noise environment such
as changes of pumps, flow rate, or drilling dynamics. This means that
many updates can be required during the course of a run to maintain
good-quality, high-speed telemetry.
The processing algorithms can be calibrated when the surface system
detects a special TS that is then analysed by the system to derive the
calibration parameters. This TS is pulsed up whenever the pumps are
started. In situations where the surface system cannot detect the special
TS to automatically update the calibration, there are some manual
calibration options.
The calibration parameters are entered into the system. The field
service engineer (FSE) can save and reload these parameters into the
Figure 1.
Locations of HSMPT activities.
Figure 2.
SimplifiedMPT systemdescription.
Figure 3.
TS detection improvement on subset ofmost challenging
conditions; data sets fromwellswhere the application of HSMPT
included higher efforts so far.
