Previous work

Although the concept of time-lapse reservoir monitoring is relatively recent, a few notable pilot projects have been attempted. Pullin et al. (1987) collected two 3-D seismic surveys before and after a steam pilot at an Athabasca tar sands reservoir site. By comparing time delay and amplitude attenuation difference maps between the two surveys, Pullin et al. seemed to be able to map the presence of steamed versus unsteamed zones. The time delays and amplitude attenuation through steamed sections of reservoir compared reasonably well with prior rock physics laboratory studies of the predicted thermal effects on seismic data.

Greaves and Fulp (1987) designed and analyzed a 3-D seismic monitoring experiment of an in situ combustion (fireflood) EOR process. They collected three 3-D surface seismic data sets: one pre-combustion and two during combustion. They made stacked sections from each survey and computed traveltime and amplitude difference maps comparing each fireflood data set to the pre-fire base survey. They were able to monitor the volume of combustion burn accurately, as well as its sweep efficiency using these maps, as confirmed by subsequent borehole measurements.

More recently, Graebner (1993) presented results of a 3-D marine seismic monitoring survey in the Oseberg field offshore Norway. The objective was to monitor the movement of the gascap during production over time. They compared reservoir fluid flow simulation predictions of the gas-oil contact to the actual seismic observations, and found a seismic amplitude anomaly in the difference section which corresponded to the zone of change predicted by the fluid-flow simulator. Unfortunately, their results may not be conclusive due to significant noise in the final seismic difference sections, and concerns about relative and absolute survey positioning errors from one survey to the next.