Seismic low‐frequency effects in monitoring fluid‐saturated reservoirs

Abstract

There is a complex relationship between seismic attributes, including the frequency dependence of reflections and fluid saturation in a reservoir. Observations in both laboratory and field data indicate that reflections from a fluid‐saturated layer have an increased amplitude and delayed traveltime at low frequencies, when compared with reflections from a gas‐saturated layer. Comparison of laboratory‐modeling results with a diffusive‐viscous‐theory model show that low (<5) values of the quality factor Q can explain the observations of frequency dependence. At the field scale, conventional processing of time‐lapse VSP data found minimal changes in seismic response of a gas‐storage reservoir when the reservoir fluid changed from gas to water. Low‐frequency analysis found significant seismic‐reflection‐attribute variation in the range of 15–50 Hz. The field observations agree with effects seen in laboratory data and predicted by the diffusive‐viscous theory. One explanation is that very low values of Q are the result of internal diffusive losses caused by fluid flow. This explanation needs further theoretical investigation. The frequency‐dependent amplitude and phase‐reflection properties presented in this paper can be used for detecting and monitoring fluid‐saturated layers.