Compression of downhole data for SWD applications

Abstract

Summary Seismic-while-drilling (SWD) services efficiently support drilling decisions. They use the vibrations produced by the drill-bit during perforation as a downhole seismic source. The seismic signal is recorded by sensors on the surface and it is processed in order to obtain/update an image of the subsurface around the borehole. To improve the characterization of the source, some sensors have been experimentally installed also downhole, on the drill pipes in close proximity to the bit: data logged downhole have been able to give better quality information. Currently, the main drawback of downhole equipments is the absence of a high bit-rate telemetry system to enable real-time activities. This problem may be solved by employing either an off-line solution, with limited memory capacity up to few hundreds of Mbytes; or an on-line solution with telemetry at a very low bit-rate (few bits/s). However, following the off-line approach with standard acquisition parameters, the internal storage memory would be filled up in just a few hours at high acquisition rates. On the contrary, with the on-line solution only a small portion of the acquired signals (or only alarm information about potentially dangerous events) can be transmitted in real-time to the surface by using conventional mud-pulse telemetry. In this paper, we present a lossy data compression algorithm based on a new representation of downhole data in angle domain, which is suitable for downhole implementation and may be successfully applied to both on-line and off-line solutions. Numerical tests based on real field data achieve compression ratios up to 112:1 without major loss of information. This allows a significant increase in downhole time acquisition and in real-time information that can be transmitted through mud-pulse telemetry.