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Acoustic Data Link For MWD Telemetry Using Swept Carrier Mud Transmission

IP.com Disclosure Number: IPCOM000124442D
Publication Date: 2005-Apr-20
Document File: 1 page(s) / 27K

Publishing Venue

The IP.com Prior Art Database

Abstract

For the purpose of wireless MWD high data rate telemetry HDRT, a new method is described below. The proposed new acoustic data link uses the wide frequency band between 500Hz and 5kHz. A bit speed of more than 1000 bps is possible at this bandwidth. Due to the fact that the transmission properties of the mud channel become worse with rising frequency a raw data bit speed of ~100bps is feasible. Since a single hop transmission is in the range of ~500m to 1km, repeater stations will be necessary for well-bore telemetry services. This is enabled by using a down-hole transmitter located inside the BHA that sends a binary data stream up the inner bore channel. Repeater stations further amplify and transmit the signal to the surface stepwise. Chirp based acoustic waves propagate through the drill string while drilling. The first transmitter inside the BHA converts ~300W of electric power into acoustic power that radiates in line the inner bore mud channel. The acoustic transducer is a pre-stressed cylindrical column made of piezo-stacks. Alternator, power amplifier with transformer unit and a control unit are located close below the transducer. Acoustic waves couple to the surrounding mud across the circular piston front and the active cylinder surface. Stress waves also couple into the pipe string as reactive forces and as a consequence of the radiation field. A considerable part of the radiated acoustic energy will be dissipated inside the pipe and the surrounding formations. Plane waves partly transmit through the circulating mud with a wave speed of ~1300m/s, through the pipe-string with ~5000m/s (elastic stress waves in steel), and through the annulus and surrounding formations. The well-bore channel acoustically behaves like a multi-path communication channel in communications science. Smart receiver algorithms and special DSP are critical for decoding. Beside drilling and circulating noise, sensors at the upper receiver side monitor attenuated and distorted signal components with different arrival times from the channel paths including reflected signals. The sensors monitor the direct signal components, reflections, reverberation and drilling and circulation noise. Adequate sensors for a receiver station are hydrophones as well as sensitive dynamic accelerometers that measure the axial or radial vibration state of a drill-pipe portion. Wideband communications schemes are a good choice in order to support a bit stream of ~100bps over such a channel. The CTC cooperates with EvoLogics, located at the Technical University in Berlin that successfully developed seawater acoustic modems for shallow water applications. They reached up to 40,000bps with ‘S2C’, Sweep Spread Carrier methods. Learning from the schemes of dolphin communications S2C and BPFSK (Binary Phase Shift Keying) methods were developed and refined. These methods were commonly tested and adapted for an acoustic data link over 4 drillpipes successfully at CTC in 2002. This feasibility-study was conducted to test applicability of these acoustic methods for data telemetry in drill pipes and to evaluate the data rate achievable in a 40 meter long test section using the drilling fluid as the major waveguide. Although the bandwidth was limited by the properties of the given transmitter, the S2C technique could be applied successfully. Two methods, a quaternary chirp-frequency shift keying and a chirp-based differential binary phase shift keying were found to provide reliable results. Respectively, data rates up to 120bps and 140bps were achieved with only a very few errors. Based on the present knowledge, 200 bits per second seem to be feasible over a larger distance. In the next development step, the acoustic data link was full-scale tested over 66 drill-pipes in a vertical well under realistic conditions at BETA. In detail, the new communication schemes are based on wide band frequency methods that uses nonlinear chirps at the transmitter side, coherence and correlation DSP of multi-path signals on the receiver side.

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For the purpose of wireless MWD high data rate telemetry HDRT, a new method is described below. The proposed new acoustic data link uses the wide frequency band between 500Hz and 5kHz. A bit speed of more than 1000 bps is possible at this bandwidth. Due to the fact that the transmission properties of the mud channel become worse with rising frequency a raw data bit speed of ~100bps is feasible. Since a single hop transmission is in the range of ~500m to 1km, repeater stations will be necessary for well-bore telemetry services. This is enabled by using a down-hole transmitter located inside the BHA that sends a binary data stream up the inner bore channel. Repeater stations further amplify and transmit the signal to the surface stepwise. Chirp based acoustic waves propagate through the drill string while drilling. The first transmitter inside the BHA converts ~300W of electric power into acoustic power that radiates in line the inner bore mud channel. The acoustic transducer is a pre-stressed cylindrical column made of piezo-stacks. Alternator, power amplifier with transformer unit and a control unit are located close below the transducer. Acoustic waves couple to the surrounding mud across the circular piston front and the active cylinder surface. Stress waves also couple into the pipe string as reactive forces and as a consequence of the radiation field. A considerable part of the radiated acoustic energy will be dissipated inside the pipe and the surrounding formations. Plane waves partly transmit through the circulating mud with a wave speed of ~1300m/s, through the pipe-string with ~5000m/s (elastic stress waves in steel), and through the annulus and surrounding formations. The well-bore channel acoustically behaves like a multi-path communication channel in communications science. Smart receiver algorithms and special DSP are critical for decoding. Beside drilling and circulating noise, sensors at the upper receiver side monitor attenuated and distorted signal components with different arrival times from the channel paths in...