4D FIXED INSTALLATION SEISMIC
Hostile wells: meeting the borehole seismic challenge presented by deep unconventional reservoirs
William Wills discusses some of the basic exploration challenges faced with characterising deep reservoirs in such a high risk, high cost environment, and the requirements of downhole seismic tools to overcome such issues.
T
he use of borehole seismic recording systems to acquire high resolution data has become common practice
over the last two decades. Te advantages of noise reduction and immunity from the distorting effects of near-surface rocks, especially sediments, have become well known. Downhole vertical seismic profiling (VSP) eliminates both the inferred depth uncertainty presented by surface seismic and time uncertainty (such as from sonic logs) giving an enhanced velocity model around a well.
Borehole recording can be extremely complex; involving gimballed or multiple fixed sensors, special cables, and high gain downhole electronics. Such installations, however, require robust mechanical housing to withstand the extreme pressures, temperatures and damaging borehole fluids encountered at such depths. Deep reservoir exploration often has the challenge of operating in environments such as within subsalt zones. Such complications often manifest as poor seismic resolution
in conventional 3D seismic surveys. Recent seismic wide-azimuth (WAZ) towed streamer acquisition and processing has enhanced the ability to map the subsalt environment. However, certain reservoirs remain below seismic resolution (such as at the deep water Wilcox Trend, Gulf of Mexico). Tese deep wells, which include exploration challenges of well depths often up to 35 000ft, water depths ranging from 8–10 000 ft, and allochthonous salt canopies of the magnitude of 10–20 000 ft thick complicating regional reconstructions and resolution of individual structures, all present a real challenge to determine reservoir quality and integrity. Surface seismic technology is not solely sufficient for such deep reservoir characterisation, even when incorporating WAZ. Te modest angle of incidence from the distal surface (mid 20° range) renders reservoir amplitude analysis as an inappropriate technique, especially within poorly/unevenly illuminated sub-salt environments. Pre-stack depth migrated data
Fig. 1. Schematic of downhole receiver location relative to a hydraulic fracture zone, highlighting ray paths generated from microseismic events.
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