4-D seismic monitoring of an active steamflood

3-D migrations of time-lapse seismic monitor data acquired during steam injection show dramatic and complex changes in the reservoir zone over a wide area, compared to baseline seismic data recorded prior to steam injection. Anticipated large decreases in seismic P-wave velocity Vp near the injection well correlate with the presence of a hot desaturated steam zone. Unanticipated large increases in Vp in an annulus around the steam zone may correspond to a high-pressure cold oil front, in which residual free gas in pore space crosses the bubble point and dissolves into liquid oil. Horizontal and vertical anisotropy in flow directions inferred from these seismic observations correlate with two temperature monitor wells, and in situ measurements of upper and lower reservoir permeability. Since the pressure front propagates out from the injector an order of magnitude faster than either the thermal or steam fronts, monitoring it may be useful for predicting future fluid-flow paths of heated oil, months in advance of actual production.