Small Airgun Sources for Frequent Low-Cost 4D Reservoir Surveillance

We present the results of two Deepwater field trials to assess the feasibility of using smaller airgun sources for low-cost 4D imaging and reservoir surveillance. The first trial was conducted at a field instrumented with an Ocean Bottom Cable (OBC) Permanent Reservoir Monitoring system. Two nearly identical full shot-patch 4D surveys were acquired with different sized airguns, allowing side-by-side comparison of the resulting images. The second trial was conducted at a different field with Distributed Acoustic Sensing Vertical Seismic Profiles (DAS-VSP). A small shotbox was acquired over a period of 8 days with four different source sizes, one of which was repeated for zero-time 4D. Both field trials demonstrated that the small sources performed well, and may provide adequate data quality for monitoring large-scale changes in a reservoir. Smaller airgun sources may reduce costs of seismic surveys by allowing smaller vessels to be used, and could be a critical element of a frequent but minimal monitoring program at applicable fields. A valuation analysis is developed to understand the impact of the reduced cost and potentially reduced data quality when using smaller sources on the overall value to the reservoir surveillance program.