D block in the South China Sea is challenging to drill due to both high temperature and high pressure (HTHP) concerns. Well D2 is a wild-cat exploration well in this HTHP area. The target formation is HTHP and the safe mud window is narrow. To drill safely, it is required to predict target zone depth accurately and monitor pore pressure ahead of bit while drilling.
Seismic-While-Drilling (SWD) technology was evaluated and applied for the first time in the D2 well of the South China Sea. In this well, with the checkshot data and seisimic waveforms from SWD, an integrated solution was provided, including updates of the time-depth relationship, depth prediction for the high-pressure problem formations, pore pressure monitoring and updated prediction ahead of the bit. All results were updated in real-time while drilling, helping to optimize the mud weight in a tight mud window scenario and to determine the final target depth of the open hole section and the casing depth.
The real-time and memory waveforms were processed and the resultant corridor stack was compared with surface seismic section for marker correlation. The predicted depth of the high pressure zone estimated from the SWD udpates was within 3 meters of the actual depth from drilling data.
Pore pressure and fracture gradient were also estimated in real-time with SWD data and the results were found to be consistent with the pore pressure measurement from wireline formation tester, obtained post drilling. By using the real-time target zone depth prediction and abnormal high pore pressure predictions based on SWD data, the D2 well was successfully completed without severe drilling issues. The casing was set to the proper depth which formed a solid foundation for the safe drilling of the next openhole section.
This case study is the first application of SWD in the sediments of the South China sea, especially within an HTHP environment. The results clearly show the effecacy of SWD in this specific geological environment.