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 Ribeiro^1
1ADCO
²VSFusion, Bahrain
³Baker Atlas, UAE
*[email protected]
**[email protected]

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