ABSTRACT

During the last decades, CO2 sequestration in oil reservoirs has gained increasing attention for its potential economic benefits deriving from the application of CO2-EOR techniques. Within the framework of the Kyoto Protocol, this process becomes increasingly attractive due to the possibility of coupling enhanced recovery capabilities with carbon capture and storage.

This project focuses on a carbonate CO2-rich field located offshore North Africa. This field is currently produced flaring the associated gas, while future field development foresees the installation of a membrane module treatment. It will separate CO2 from hydrocarbon gas, with the possibility to export processed gas on one side and inject a stream of CO2-rich gas on the other side. This project aims at maximizing field recovery by optimizing the gas injection process. The main objective is to evaluate different gas re-injection strategies which could combine recovery enhancement and CO2 storage. A compositional reservoir model was considered to study the feasibility of injecting part of the produced gas into the reservoir, and evaluate its impact on the recoverable oil reserves and CO2 sequestration capacity of the reservoir.

Simulations were based on a miscibility study involving sour gas and resident oil mixtures. Even though the expected injection stream was found to be supercritical, and therefore liquid-like, at reservoir conditions, the gas-oil density difference is significant and CO2-rich gas displacement could not achieve miscibility conditions.

Numerical results indicated that injection schemes based on highly slanted wells and water alternating gas injection can overcome early gas breakthrough and a considerable amount of gas emissions, providing an improved sweep efficiency, a stable displacement and a significant degree of CO2 retention. Indeed, the incremental oil achieved in the best case is 15% with respect to the reference case without gas injection, and the CO2-rich gas retained in the reservoir is 63% of the total gas injected.

INTRODUCTION

The design of a CO2 injection project and, more generally, a gas injection project, has to take into account the phase behavior of the injected gas and the reservoir fluid mixture. CO2 injection in oil reservoirs is strongly influenced by the phase behavior of CO2 and reservoir oil, which in turn are strongly dependent on reservoir conditions (T, P) and oil and gas composition. Benefits of CO2 injection consist in the reduction of capillary forces that impede the oil flow in the pore spaces, oil swelling and viscosity decrease due to development of mass exchanges between the reservoir oil and injected fluid, and possibility to achieve miscibility conditions (Tzimas et al., 2005; Lake, 1989).

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