Using Seismic While Drilling to Reduce Depth Uncertainty on an Exploration Well Offshore Thailand

Abstract

Abstract Knowing targets depths accurately for Carbonate Exploration wells is critical for a successful well construction process due to the kick / loss scenario if the carbonate is accidentally penetrated. This paper demonstrates the use of innovative Logging-while-drilling seismic technology used offshore Thailand for the first time which furnishes seismic checkshot information whilst drilling to refine depth prediction ahead of the bit for accurate geostopping. Field data will be shown to demonstrate this methodology. Carbonate targets require casing to be set immediately above. Depth uncertainty from the surface seismic is usually in the range of over 50m up to 200m depending on the quality of the surface seismic acquired and the ensuing processing. Lack of offset well velocity information amplifies this uncertainty. By acquiring velocity information while drilling, the depth uncertainty is progressively reduced as drilling proceeds. This is achieved using the seismic while drilling technology, thus avoiding the need for a dedicated intermediate checkshot survey on wireline and saving rig time and cost. The technique consists of a downhole seismic receiver collar forming part of the bottom hole assembly and a surface acoustic source in the form of an airgun cluster. Seismic levels were acquired when drilling entered slips – a natural break in the drilling process and providing an acoustically quiet time. Each seismic level provided a real time seismic waveform which was processed for a Time-Depth pair to update the velocity model and hence refine the target depth prediction. 53 consecutive real time seismic levels were acquired refining the carbonate depth prediction shallower by 30m. The acquisition was transparent to rig operations. The initial error range for the depth was +/− 70m. Casing point was set successfully without penetrating the carbonate. Exploration wells lend themselves well to the use of this service for reducing depth uncertainty.