A Decade of 4D Seismic Monitoring of Carbonate Gas Reservoirs in Offshore Sarawak, Malaysia
Yee, Shuh-Wen¹; Hague, Paul¹; Chiem, Boon-Hong²
¹Production Geology, Sarawak Shell Berhad, Miri, Malaysia.
²Quantitaitve Interpretation, Sarawak Shell Berhad, Miri, Malaysia.
The use of 4D seismic data to monitor production effects in carbonate reservoirs in the published literature is limited. This is due to the fact that the acoustic response of carbonates has been shown to be highly variable and the applicability of Gassmann's equation for predicting the impact of changes in saturation on the acoustic properties of carbonates is debatable. Of particular relevance to the carbonate gas fields of offshore Sarawak is the fact that Gassmann's equation predicts that water influx into a gas reservoir would produce a relatively minor acoustic response, which might be expected to be swamped by non-repeatable noise on 4D seismic data.
Therefore, Shell has adopted a cautious approach to implementing time-lapse seismic monitoring of the carbonate gas reservoirs of offshore Sarawak. The first attempt was undertaken in 2001, when six repeat 2D seismic lines were acquired over a medium-sized carbonate gas field. Despite the limitations of repeating 2D seismic data, two of the six lines appeared to show a relatively strong coherent seismic amplitude response at both the original gas-water contact and the point which coincided with the logged producing gas-water contact in the wells.
These encouraging results prompted the first full-field repeat seismic survey, covering two smaller subsea fields in 2005. Both fields showed clear amplitude responses with one related to water influx and the other related to gas expansion into the aquifer. Interestingly, the latter signal is much stronger than the former, which is precisely what Gassmann's equation predicts. In addition to the 4D seismic amplitude response, both fields showed clear time-shifts between the base and monitor surveys, which are attributed to seafloor subsidence and stress relaxation in the overburden, due to pressure depletion and compaction in the reservoir and provide evidence that it is possible to use 4D seismic data to monitor the aerial extent of pressure communication in these reservoirs.
Since then, repeat 3D swaths have been acquired over two larger carbonate gas fields and both have displayed clear amplitude signals related to water influx. These surveys have conclusively demonstrated that, despite the ongoing debate about the impact of saturation changes on seismic velocities in carbonates, 4D seismic monitoring of carbonate gas fields can yield meaningful signals, which can be used to optimise further field development and maximise ultimate gas recovery in future.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012