Assessment of Miscible WAG Injection Performance in a Giant Carbonate Reservoir using Analytical and Numerical Approaches

Abstract

Carbonate reservoirs from Pre-salt layer are responsible for a significant volume of Brazils total oil production. Recovery mechanisms applied are mostly water injection, aiming for pressure maintenance and oil recovery increase through macroscopic oil displacement, complemented by gas injection to dispose produced CO2 rich gas stream into the reservoir. Production strategies were originally built using producers combined with gas and water injectors. Later, based on a serious of technical studies, including the ones described here, most injection wells were converted to water alternating gas (WAG). In the literature, WAG injection is applied mostly to gas injection projects to increase oil recovery and provide mobility control; as a recovery mechanism the WAG process combines the increased microscopic sweep efficiency from the gas with the improved macroscopic sweep efficiency obtained from the water. In this work we perform the screening and evaluation of WAG injection as a recovery mechanism in a heterogeneous carbonate reservoir from the Brazilian pre-salt. For that purpose, we use both analytical and numerical methods, the later using a commercial compositional simulator. The screening indicates that this reservoir is a candidate for WAG injection. Lab data shows thermodynamic miscibility at initial pressure levels and phase behavior observed in experiments is matched to a Peng-Robinson equation of state (EoS). Results from numerical simulation have a good qualitative agreement with analytical results and data from the literature, indicating higher oil recovery for greater gas injection. The increase in oil recovery estimated by numerical simulation is compared with actual data from the literature using dimensionless variables where we observe good agreement of our expectations to previous field results. We conclude that the efficiency of WAG injection, in these reservoirs, relies on factors such as gas availability, miscibility development and flow pattern to be developed due to reservoir heterogeneities (channeling versus dispersive flow).