7. Spatial/Time-lapse Data Management -Invited-
More Sensitive 4D Monitoring for Enhanced Recovery
Rodney W. Calvert
Shell International E and P, USA.
Abstract
Real reservoirs are heterogeneous at all scales and always different and more complex than our reservoir models so production efforts are always sub-optimal and usually leave considerable reserves behind. Our industry typically only recovers 35% of the oil in place and should aim to do better.<p>An important enabler for the successful application of improved recovery methods is an ability to see the reservoir changes during production using 4D time lapse seismic monitoring. Just as good time-lapse photography requires a fixed tripod for the recording camera and constant illumination, now a new generation of seismic monitoring approaches is emerging with fixed receivers and more consistent illumination. <p>This next generation of fixed receiver seismic monitoring technologies will be an important step towards making 4D benefits possible on many more fields, including deeper fields where conventional 4D is not sensitive enough to detect timely production changes. We show two new approaches, permanent Sparse OBC where the receivers are fixed and Virtual Source seismic where both sources and receivers are effectively fixed.<p>Sparse OBC. By placing a line of fixed receivers in an Ocean Bottom Cable (OBC) permanently on the seabed and shooting a perpendicular line across it we obtain a single fold 3D patch. This single fold patch may be acquired very quickly and with care, very repeatably. By superimposing patches from more OBCs and more shot lines it is possible to build up the data fold. We show that marine 4D monitoring may be carried out with a sparse cable layout and sparse shot lines. This enables fast, repeatable survey results that may be acquired affordably and often to give near real time monitoring.<p>Virtual Sources. We have developed an even more effective approach to repeatability and 4D sensitivity that may be applied onshore or offshore. If we fix geophone receivers below troublesome overburden we may measure the seismic signal going through them. For a given receiver we may deconvolve this signal to a desired pulse and by summing over different radiation directions the receiver effectively becomes a controlled source. We may focus this energy at each geophone in turn and receive the results at the other geophones. This recasts the seismic method so that each geophone in turn may be made to act like a Virtual Source with a known controllable downward propagating wavelet. A nice feature is that we can do this without needing to know the source waveform, which may thus be variable, nor anything about the overburden, which may also change. <p>We show promising early results from this new approach and how it differs from Interferometry.
Last modified: Thu May 04 23:31:28 2006