Feasibility Modeling and a VSP-Repeatability Test Demonstrate that Time-Lapse Reservoir Monitoring of Thin Layers During Gas and Wag Injection is Possible in Middle East Carbonates with Both Land-Vibrator and Marine-Air Gun Sources
William Soroka1*, Suaad Khouri1**, Samer Marmash1, Ahmed Dawoud1, Saleh Barakat2, Ross Jenkins3, and Maria Ribeiro1
3Baker Atlas, UAE
The ability to proactively adjust and enhance the sweep efficiency of an injection project has the potential to improve recovery and avoid costly problems. Rock-physics feasibility modeling results suggested that when sufficient water replaces in-situ oil or a free gas layer develops during a WAG or gas injection, 4-D anomalies can result. The ability to observe 4-D responses due to reservoir saturation changes in relatively thin layers depends on the seismic images’ vertical resolution and the level of background noise. Modeling suggested that seismic data, with a maximum frequency of 100 Hertz or greater, is required for 30 ft or thinner layers. To achieve the needed high-frequency and low background noise levels, a time-lapse vertical seismic profile (VSP) was determined to be the most appropriate monitoring technology. Results from 2-D VSP repeatability tests successfully demonstrated that high-resolution VSP data can be collected and are repeatable in the Abu Dhabi carbonate environment. Similar quality results were observed between a marine air gun and land vibrator VSP. The results from this study showed that background noise levels below 1.0% of the original VSP amplitudes can be achieved. Both the marine and land VSP results were observed to achieve a maximum frequency of around 100 Hertz. Both the P-wave and converted shear wave signals were found to be of good quality and repeatable. The positive results from the repeatability pilot and feasibility modeling study indicate that time-lapse 3-D VSPs could provide a useful means for monitoring reservoir saturation changes in carbonate fields. The results of this study are encouraging and support the idea of using time-lapse 3-D VSP images to monitor saturation and pressure changes in carbonate reservoirs.
AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain