In 2007, Total decided to launch a CO2 injection pilot, its objective being to prove the impact of CO2 injection on a mature depleted carbonate field. This field has been produced since 1982, mainly in natural depletion. The reservoir is Lower Cretaceous, low permeability carbonate rock with 20° API oil. The pilot pattern is an inverted 5-spot, with one central injector, 3 producers and one observer, the well spacing ranges from 300 to 700 m.
The main objective of the pilot was to observe and understand the effect of water and CO2 injection in a depleted carbonate reservoir, far below the Minimum Miscibility Pressure (MMP). This has been done in terms of microscopic oil recovery mechanisms, monitoring, and also field scale simulations. The microscopic work comprises secondary and tertiary CO2 SCAL data, and also a lean HC gas flood. PVT acquisitions include CO2 swelling tests, and slim tube data to estimate the MMP. Considering the risk of high mobility of immiscible CO2 in a heterogeneous reservoir, a WAG CO2 injection scheme was chosen and simulated.
This pilot was started in 2008, and has now 3 years of monitoring data. Breakthrough in HC phase was observed on the two proximal producers, but not on the most remote producer. Interestingly, as low salinity water was injected for a 18-month period after the CO2 injection, CO2 breakthrough was also measured in aqueous phase. SCAL data were matched by using alpha-factors (also called transport coefficients) in order to limit the stripping effect of the gas. Matching the 5-spot data enabled to quantify a strong multi-layer effect, as well as dissolution of CO2 in the injected water. From the lessons learned, we improved our understanding and capability to simulate and extrapolate WAG CO2 to other fields and conditions.