The giant field is a geologically complex, faulted anticlinal structure trending east-west, with dips ranging from 0 to 60 degrees. After the current waterflood operations are completed, it is estimated that about 2/3 of oil originally in place will be left behind unrecovered. The combination of light oil gravity and relatively high reservoir pressures makes these reservoirs good candidates for miscible CO2 EOR. There are separate waterflood developments that have been identified as candidates for CO2 EOR. Results of the CO2 pilot provide further encouraging results that show better than expected CO2 injectivity and vertical sweep than waterflood and good displacement of the remaining oil saturation.
The subsurface team successfully used design of experiments to manage uncertainties in history matching and production forecast. Multiple static realizations honoring the key geological uncertainties were built. Based on ranking criteria developed, there were three static realizations chosen for history matching; that resulted in good history matches obtained for five different realizations. The history match model includes all the 500 wells, 70 years of historical production and water injection. All the history matches were within uncertainty ranges and were technically assured for use in CO2 EOR forecasting. Sector modeling was used to identify and rank different subsurface uncertainties and decisions. Experimental design was used to manage the uncertainties remaining after history match. Hundreds of realizations were simulated that incorporated several integrated development decisions such as injection scheme, development sequence, well spacing, completion strategy and well orientation. Simulation results for all the realizations (with different levels of heterogeneity) indicate that the range of recovery factors varies from 10% (P10) to 24% (P90). The simulated forecasts were also benchmarked against limited analog fields. The production curves are then upscaled to the whole field using a dimensionless curves approach in WellSpring. A methodology to integrate inputs from multiple disciplines (surface, subsurface, wells and CO2 source) was developed to different integrated filed development options.
The complexity, depth, and compartmentalization of the field create challenges that affect all aspects of the CO2 EOR field development including: scope for CO2 recovery, flood design options, data gathering needs, integration with existing waterflood, utilization of existing comingled wells, surface facilities and subsurface modeling for realistic production forecasting. Our work focuses on the selection of field development concepts and determining the economic viability whilst incorporating the best practices and lessons learned from the CO2 EOR industry to ensure top quartile performance.