Time-Lapse Study and Acquisition of Monitor Survey over Carbonate Gas Fields in Sarawak Water, East Malaysia

Bor Seng Lee, Yip Cheong Kok, Chee Kiong Ngu, and Hua Zhu
Sarawak Shell Berhad, Miri, Malaysia

Time-lapse feasibility study was carried out over two carbonate fields. The study indicated that 4D signal could be observed after 3 years of production over the fields. The 4D signal could be observed if the 4D non-repeatable RMS (NRMS) noise level were 0.2 or less. Without previous 4D monitor survey been acquired in the area, there was an uncertainty on how much repeatability was required to achieve a 4D NRMS noise level of 0.2. A provisional value of 150m for delta source plus delta receiver (ΔS + ΔR) error was used for planning purpose. In addition, the 4D monitor survey could not be acquired at the same month as the base survey. This added another uncertainty to the amount of repeatability that can be achieved in the 4D monitor survey. To overcome these uncertainties and achieve as good repeatability as possible, additional two streamers were deployed and a fair amount of time were allocated to cater for 4D re-shoot of lines if required. The monitor survey started in mid August 2005 and was completed in 20 days. The base survey lines (prime and infill lines) were completed in 15 days while the remaining 5 days were used to acquire 4D re-shoot lines. Feathering angle was used as immediate QC parameter while more comprehensive QC plots were generated after several lines were completed before deciding on 4D re-shoot. 4D repeatability measurement of ΔS + ΔR at 2500m to 2575m offset indicated a better than 10m error were achieved for some of the line while majority of the lines were less than 75m. The survey achieved its 4D repeatability objective and preliminary processing has shown observable 4D signal.