Time-lapse seismic analysis of pressure depletion in the Southern Gas Basin

In the Southern Gas Basin (SGB) of the North Sea there are many mature gas fields where time-lapse monitoring could be very beneficial in extending production life. However, the conditions are not immediately attractive for time-lapse seismic assessment. This is primarily because the main production effect to be assessed is a pore pressure reduction and frame stiffening because of gas production in tight sandstone reservoirs that also have no real seismic direct hydrocarbon indicators. Modelling, based on laboratory measurements, has shown that such an effect would be small and difficult to detect in seismic data. This paper makes two main contributions. Firstly, this is, to our knowledge, the first time-lapse study in the SGB and involves a real-data assessment of the viability for detecting production in such an environment. Secondly, the feasibility of using markedly different legacies of data in such a study is addressed, including an assessment of the factors influencing the crossmatching. From the latter, it is found that significant, spatially varying time shifts need to be, and are successfully, resolved through 3-D warping. After the warping, the primary factors limiting the crossmatching appear to be residual local phase variations, possibly induced by the differing migration strategies, structure, reverberations and different coherencies of the volumes, caused by differences in acquisition-structure azimuth and acquisition fold. Despite these differences, a time-lapse amplitude signature is observed that is attributable to production. The character of the 4-D amplitude anomalies may also indicate variations in stress sensitivity, e.g. because of zones of fracturing. Additionally, warping-derived time attributes have been highlighted as a potential additional avenue for detection of pressure depletion in such reservoirs. Although the effects are subtle, they may indicate changes in stress/pressure in and around the reservoir because of production. However, to fully resolve the subtle time-lapse effects in such a reservoir, the data differences need to be better addressed, which may be possible by full re-processing and pre-stack analysis, but more likely dedicated 4-D acquisition would be required. ©2006 European Association of Geoscientists & Engineers.