Risk Management in Deepwater Exploration Well: Reduced Depth Uncertainty Through Acquiring Seismic While Drilling

Abstract

One of the key for successful drilling through carbonates build up structure is accurate time depth information to avoid kick-loss scenario during drilling if the carbonate is accidentally penetrated in the unexpected shallower hole size. Setting the casing point accurately to isolate the high overpressure regime on the overlying shale above the carbonate is required. During Kangean operation, Geostopping using Seismic While Drilling (SWD) technology was applied for setting the 9 5/8" casing before penetrating the Prupuh carbonate formation and updating the pore pressure model. This paper will demonstrate an application of SWD in Indonesia’s deepwater operation of predicting critical carbonate depth enabling well construction on time and on budget. Acquisition of SWD was done during an acoustically ‘quiet’ period, during stand connections while pumps are off. Real-time waveforms are transmitted to surface through the mud telemetry system and sent to processing center via secure connection systematically after each seismic level acquired. Processing was done in real-time providing updated time-depth information, placing bit position on the seismic section in depth domain for refining depth prediction and ultimately Vertical Seismic Profile (VSP) corridor stack containing seismic reflectors along the wellbore and ahead of the bit for look-ahead information. The real-time updated interval velocity was used to update pore pressure model for monitoring the overpressured zone and to adjust mud weight accordingly while drilling to prevent a kick situation. A total of 27 real-time SWD levels were acquired during drilling over interval ~1,800 ftMD. The data was found to be of a high quality that allowed for an accurate well tie of the Prupuh Carbonate top with a final depth prediction. Real time velocity showed that velocity of Cepu shale above the Prupuh carbonate was slower than pre-job velocity model, meaning that the Prupuh carbonate depth is shallower than initial prediction. Real time decision was made to set the casing before Prupuh Carbonate top. The actual Prupuh top was 23 ft deeper than the latest real time Prupuh prediction. This SWD technology allows us to reduce top carbonate uncertainty from +697 ft/-375 ft to become +/− 23 ft and ultimately saved the well from potential catastrophic event. This study demonstrates that SWD technology give us confidence to manage risk in real-time.