Estimation of critical and maximum gas saturation using multiple 4D seismic surveys

Estimates for critical and maximum gas saturation are obtained using time-lapse seismic signatures from multiple surveys shot during gas liberation and dissolution in a producing hydrocarbon reservoir. To aid this process, hydrocarbon gas properties and behaviour are studied, and their relation to the fluid-flow physics is understood using numerical simulation and seismic modelling. It is concluded that for seismic surveys repeated at time intervals of six months or more, the gas saturation distribution during either liberation or dissolution exists in two fixed saturation conditions defined by the critical and the maximum gas saturation. This understanding is then used to interpret seismic data from a turbidite field in the North Sea, which has surveys repeated every 12 to 24 months. We find a critical gas saturation of between 0.6 and 4%, but that the maximum gas saturation is relatively unconstrained. These low critical gas saturation values are consistent with the range of measurements from other similar fields in the open literature.