First Time Lapse Walkaway VSP Monitoring of CO2 WAG EOR Pilot, Challenges and Learnings from Onshore Carbonate Field UAE

Repeated time-lapse Walkaway VSP (vertical seismic profile) were acquired as part of CO2 WAG EOR pilot monitoring in an onshore carbonate field in the UAE. The Baseline survey followed by two monitors were acquired, one after 6 months of water injection and another after 3 years of CO2/Water injection cycles. Objectives were to monitor CO2 and Water fronts between injector and producer, in addition to assess source and receiver repeatability. Feasibility study was performed to estimate 4D effects due to petrophysical changes in the reservoir, resulting from Water and/or Gas injection. After performing the survey design for receiver and source positions, 36 level 3C receivers at 7.6 m spacing array were deployed few hundred meters above the reservoir. Baseline and Monitor-1 survey were acquired with 186 source points at 25 m spacing, with maximum offset of 3000 m from wellhead in both directions along NW-SE line, however, Monitor-2 had 10 source points less in NW direction due to surface restrictions. Excellent data quality with good repeatability was achieved. Final images around reservoir showed no visible seismic 4D changes along Walkaway VSP orientation post injection during period between Baseline and Monitor-1, possibly due to fluid transmission not proceeding along this orientation, or it was too early to detect anomalies in the vicinity. This paper presents Baseline/Monitor-2 processing results. 3C VSP processing was performed while taking into consideration Baseline and Monitor-2 data NRMS (normalized root mean square) and predictability at major processing steps. Customized processing workflow was applied for wavefield separation and deconvolution. VSP geometry is lacking high angle first arrivals to directly estimate overburden shales anisotropy parameters, which was addressed by incorporating the values from literature. Cross-equalization was performed pre-migration i.e. scalars computed on downgoing wavefield and applied on upgoing wavefield. Time-lapse analysis was performed pre-migration i.e. on NMO (normal move out) corrected data after flattening at overburden shales to remove any time shift effects from the overburden. Time shifts were noticed across receivers in the overburden shales in Walkaway VSP and validated by Zero-Offset VSP extracted from the Walkaway VSP. The observed time shifts were small with no amplitude differences on NMO corrected data at the reservoir. In the migrated images, amplitude difference observed were possibly due to these time shifts, these slight time changes are stacked in the migration process and hence compounded with the 4D amplitude signature of the images. Time-lapse feasibility studies are available in the literature however; actual time-lapse seismic surveys are very limited in the UAE. This study will help the operators to deploy borehole seismic technology for time-lapse monitoring in the Middle East carbonate reservoirs. Processing workflow was optimized, highlighting challenges and limitations posed by the survey geometry with future recommendations.