Jan Stammeijer

Quantitative estimation of compaction and velocity changes using 4D impedance and traveltime changes

In some hydrocarbon reservoirs, severe compaction of the reservoir rocks is observed. This compaction is caused by production, and it is often associated with changes in the overburden. Time-lapse (or 4D) seismic data are used to monitor this compaction process. Since the compaction causes changes in both layer thickness and seismic velocities, it is crucial to distinguish between the two effects. Two new seismic methods for monitoring compacting reservoirs are introduced, one based on measured seismic prestack traveltime changes, and the other based on poststack traveltime and amplitude changes. In contrast to earlier methods, these methods do not require additional empirical relationships, such as, for instance, a velocity-porosity relationship. The uncertainties in estimates for compaction and velocity change are expressed in terms of errors in the traveltime and amplitude measurements. These errors are directly related to the quality and repeatability of time-lapse seismic data. For a reservoir at 3000-m depth wit h9mo fcompaction, and assuming a 4D timeshift error of 0.5 ms at near offset and 2 ms at far offset, we find relative uncertainty in the compaction estimate of approximately 50‐60% using traveltime information only.

A New Technique for Pressure - Saturation Separation from Time-Lapse Seismic - Schiehallion Case Study

A technique is shown for estimating pressure and three phase saturation changes from 4D seismic. The technique is a further development of a recently published method which does not require defined rock and fluid physics relationships. A multi-attribute approach is taken, in which 4D seismic attributes are linked (by relationships with a physically reasonable form) to production data, and an optimal combination of seismic attributes are then selected to perform a bayesian inversion separating pressure and saturations. The methodology is applied to the Schiehallion field in the West of Shetlands with encouraging results. It has highlighted saturation changes, previously hidden by the effect of pore pressure on the rock frame, which can be validated against production and tracer data. Additionally, it has shown areas where gas is coming out of solution that have been validated against information acquired in a recently drilled well.

Determination of a seismic and engineering consistent petro-elastic model for time-lapse seismic studies: Application to the Schiehallion Field

Summary A relationship between pressure and saturation changes in the reservoir and the corresponding time-lapse seismic signatures is determined from measurements of the produced fluids and downhole pressures at a number of wells. The results from this process also provide an insight into the governing parameters for the rock and fluid physics model which are then compared directly to conventional petro-elastic predictions. The methodology is applied to the Schiehallion field where it is found that the 4D responses to water replacing oil, and gas breakout, follow the predictions of Gassmann fairly accurately. However, the sensitivity of the seismic to increases in pressure is observed to be roughly half as strong as that estimated from standard core plug measurements.

A petroelastic‐based feasibility study of monitoring pressure depletion in a UKCS gas reservoir

The possibility of using 4D seismic for monitoring pressure depletion in the gas-bearing sands of the Rotliegend Group, Southern Gas Basin, is assessed. On the basis of the rock physics work done, the results suggest that production induced changes to unfractured sands are unlikely to be observed. However, the presence of clusters of fractures can render the 4D signature visible. A favorable response is especially likely for certain distinct facies distributions. The strongest signature of depletion occurs beneath the producing zone, and is due to a combination of reflectivity change and event time displacement.