Improving or Enhancing the oil recovery appears as a challenge for many porous fractured carbonate reservoirs as unfavourable conditions for matrix oil production are often encountered, such as a low permeability or a weak water wettability of the matrix medium. The selection, optimization and risk minimization of a production scenario for such fields goes through a clear understanding of the underlying physical mechanisms, together with a sufficient characterization of the fracture network and the evaluation of determinant matrix properties. Considering the above context, this paper revisits the multiphase spontaneous displacement mechanisms taking place in matrix blocks during water drive production processes, in order to identify the limiting factors and parameters for oil recovery and to open promising research perspectives for Enhanced Oil Recovery (EOR). Firstly, a simple analytical model of matrix-fracture water-oil transfers is presented and validated against published experimental results. This model takes into account the various contributions of co-current and counter-current flows depending on the magnitude of capillary forces and gravity forces. It also elucidates some upscaling issues from the laboratory core scale to the field scale. Then, the mechanistic analytical approach of this paper is used as a framework for the assessment of enhanced oil recovery methods that are worth being considered to improve the production of oil-wet fractured carbonate reservoirs with a poor recovery prognosis. A parametric study is performed under the modified wettability and/or reduced interfacial tension (IFT) conditions that can be established through the injection of chemical agents in the water phase. Results indicate that designing a process to recover the matrix oil at an economic rate often turns out to be a real challenge for such reservoirs. The paper concludes on recommendations for present field EOR assessment studies, and calls for further academic and applied research regarding the optimization of both pore-scale recovery and reservoir-scale recovery.
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