Seismic monitoring of pressure depletion evaluated for a United Kingdom continental-shelf gas reservoir

The possibility of using 4D seismic data for monitoring pressure depletion in the low-porosity, tight gas-bearing Rotliegende sandstones of the UK Southern Gas Basin is investigated. The focus here is on whether fractures in the upper part of the reservoir, known to enhance productivity, can also enhance the time-lapse seismic response. The study uses laboratory data to evaluate core-plug stress sensitivity, published data for the stress behaviour of the fractures, followed by petro-elastic and 4D seismic modelling of both the fractured and unfractured formation. The magnitude of the resultant 4D signatures suggests that production-induced changes in the unfractured sands are unlikely to be observed except perhaps with highly repeatable time-lapse surveys. On the other hand, the presence of fractures could render production effects visible in dedicated 4D acquisition or prestack parallel processed data. If present however, the signature will be sporadic, as fractures in the area are known to exist in clusters. The 4D signature may be enhanced further by certain classes of vertical geological variability and also areas of high reservoir pressure. The strongest evidence of depletion is expected to be time-shifts seen at the base of the Rotliegende reservoir. © 2006 European Association of Geoscientists & Engineers.