Almost 60% of global oil reserves are contained in carbonate reservoirs. A large number of those are naturally fractured and oil- or mixed-wet. Waterflooding efficiency is often very low for such reservoirs because of both high fracture-to-matrix permeability contrast and unfavorable wettability. The recovery of the oil trapped in the matrix is then very challenging. To enhance this recovery, several recovery mechanisms can be assessed:
capillary imbibition by using wettability modifiers,
viscous drive by imposing a pressure gradient in the fracture network,
water-oil gravity drainage by using low IFT surfactants formulation.
This paper presents an experimental approach that evaluates different chemical EOR alternatives using one or several of the three previous recovery mechanisms. The experiments consist in injecting an aqueous chemical solution or a foam containing chemical additives into an artificially fractured carbonate core. The imbibition is monitored with a recent CT-scanner allowing the local quantitative monitoring of three phases, including accurate quantification of matrix oil recovery. This paper is mainly focused on the impacts of foaming agents and of wettability modifiers, implemented separately or jointly. The experiments have also been conducted on several cores of different permeability, resulting in various permeability contrasts between matrix and fracture.
A major result concerns the kinetics of oil recovery by chemical additives, that is greatly increased when a viscous drive is applied across the matrix medium via the circulation of foam in the fracture. Experiments in fractured cores of different permeabilities indicate that foam does not penetrate the matrix but drives the chemical aqueous phase into the matrix due to the generated pressure gradient. Detailed analysis of oil mobilization dynamics is provided. These foam flow experiments are compared with former chemical imbibition tests on non-fractured cores, for further insight of the role played by viscous forces. The comparison of tested recovery scenarios leads to conclusions regarding optimal chemical EOR strategies for naturally-fractured carbonate reservoirs with poor secondary recovery prognosis.