Time-Lapse VSP Monitoring of CO2 WAG Injection in a Carbonate Reservoir: Abu Dhabi Case Study

Time-lapse Walkaway VSP is part of the reservoir monitoring of WAG CO2 EOR pilot to study and monitor the reservoir performance in an onshore carbonate field in Abu Dhabi. In this ongoing time-lapse Walkaway VSP monitoring project, a baseline and three monitor surveys have been acquired so far. This paper discusses results from the Monitor-3 survey. For the Monitor-3 Walkaway VSP acquisition, the source line was extended to 5 km on both sides of the well (i.e. additional 2 km compared to the Baseline survey), to provide the wide P-wave arrival angle coverage required for direct anisotropy estimation in the regional shale overlying the reservoir. The velocity model was updated in the shale layer for the anisotropy parameters. 3C processing workflow followed 4D standards aimed at removing noise and unwanted wave modes from the data preserving the Upgoing P wavefield which was then migrated with the updated anisotropic velocity model. Time-lapse analysis was performed at the target reservoir, a strong increase in amplitude (peak) denotes rock hardening in the target reservoir which corresponds to the injected water in the reservoir during the 6.5 years of CO2 WAG injection. This observation is consistent with the reservoir dynamic model.