Recent Findings from Ongoing Surveillance Using Time-Lapse Walkaway VSP Monitoring of CO2 WAG EOR Project in an Onshore Field in Abu Dhabi

Abstract Time-lapse seismic is an essential tool for reservoir monitoring. However, using 4D seismic in carbonate reservoirs poses some difficulties because of rock's nature. Time-lapse Walkaway VSP surveys were carried out on an onshore carbonate field in Abu Dhabi, which included Baseline and three Monitor surveys. The surveys aimed to track the fluid-front due to the CO2 WAG injection, and to assess repeatability between sources and receivers and associated 4D signal. The recent Monitor-3 survey was acquired after 6.5 years of CO2 WAG injection post the Baseline survey. This paper discusses results from the Monitor-3. The acquisition geometry was same for all the surveys, however for Monitor-3 the source offset was extended to 5km from the wellhead to refine the anisotropy model to improve the final image. The data revealed good repeatability between the Baseline and Monitor-3 surveys. A processing workflow that followed 4D standards effectively improved repeatability during the stages of processing. Final images around the reservoir showed visible seismic changes along Walkaway VSP orientation post injection during the period between Baseline and Monitor-3. The 4D signal is stronger due to the longer period of injection. 3C VSP processing was performed while taking into consideration Baseline and Monitor-3 NRMS and predictability at each processing step. The customized processing workflow that was developed for Monitor-2 was implemented in the current Monitor-3 processing. The extended Walkaway VSP line provided higher arrival angles to estimate overburden Shale Layer anisotropy parameters in the model which improved the final image. 4D signal was observed on the migrated images at the reservoir level which is consistent with the findings of the rock physics model. Time-lapse seismic studies are common in clastic reservoirs, yet they are very limited in carbonates. It is believed that this study will add value in understanding the fluid distribution in the reservoir using seismic methods. This will provide better confidence in considering and deploying time-lapse borehole seismic surveys as an initial part of reservoir monitoring activities plan for Enhanced Oil Recovery (EOR) projects. This paper highlights the improvements and optimizations done in acquiring the Walkaway VSP data as well as data processing and interpretation. This workflow can serve as a guide for future time-lapse Walkaway VSP studies.