Abstract

A procedure to model the impact of reservoir CO2 on chemical EOR (polymer/ASP) flood performance has been developed. The work which was initially developed for polymer flood modelling has been further extended and a simple approach has been presented to model the impact of reservoir CO2 on ASP flood performance.

Fatehgarh reservoirs in Aishwariya field, located in Barmer Basin of Rajasthan India, have very high CO2 content in reservoir fluid. The oil is moderately viscous and aqueous based chemical EOR techniques like polymer/ ASP flooding are being considered for production enhancement. The presence of high CO2 may impact aqueous based flooding processes planned in Aishwariya field. Three unconventional mechanisms occur in the reservoir: i). With injection of water, CO2 is stripped out of oil over time, this leads to increase in oil viscosity. ii). Since CO2 decreases pH of the system, polymer in-situ viscosity is decreased. iii). CO2 reacts with sodium carbonate (alkali) in aqueous medium leading to reduction in pH which may have an impact on ASP performance.

An advanced processes simulator was used initially to model the impact of CO2 on polymer flood performance. For simplicity, solution gas was assumed to be entirely composed of CO2. Waterflood followed by polymer injection was simulated in the model. CO2 was defined as an aqueous component instead of conventional oleic component allowing for modelling polymer viscosity dependence on CO2 concentration.

In order to model the impact of CO2 on ASP performance, reaction of CO2 with alkali was included in the model. The ASP simulation run could now capture all three unconventional mechanisms occurring in the reservoir due to presence of CO2: increase in oil viscosity, reduction in polymer viscosity and consumption of alkali due to reaction with CO2. Different simulation runs were then carried out both with and without the reaction of CO2 with alkali and the results were compared. Impact of grid refinement on the flood performance was also studied.

The developed approach could mechanistically capture the impact of CO2 on ASP performance in Aishwariya field. It was also observed that grid refinement has a major impact on the results; fine grid simulation is required to properly evaluate the impact of CO2 on ASP performance as it can appropriately capture impact of dilution and reaction based on CO2 concentration (moles) available.

The paper presents a step by step approach of modelling the impact of in-situ CO2 on ASP flood performance. The suggested approach helped in evaluating the impact of CO2 on polymer/ASP flood performance and adjusting the injection slug design appropriately.

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