Methodology for Fast Transmissibility Determination in Naturally Fractured Reservoirs

Sarah Vitel. ( 2006 )
in: Proceedings of the 10th European Conference on the Mathematics of Oil Recovery

Abstract

Productivity of fractured reservoirs is mainly affected by the fracture network configuration, which impact on the flow paths cannot be neglected. Today, two main approaches, opposite and complementary, are used to perform flow simulation on fractured reservoirs: (1) the continuum approximations, simple and efficient but approximative in the actual fracture geometry integration, and (2) the discrete model formulations, quite complex to put into practice and much slower, but respectful of the discrete fracture network and thus more accurate. Yet fractured reservoir simulation still needs to balance the speed up of the calculation processes while keeping the fracture system paths accurate. This paper presents a methodology to evaluate equivalent transmissibilities in naturally fractured reservoirs, based on a discrete fracture network. It is divided in two parts: (1) first the co-discretization of the fractures and the simulation grid, (2) then the determination of the equivalent transmissibility between two gridblocks of the simulation grid. The first phase consists in representing the reservoir with a connectivity list. Nodes represent control volumes, holding porosity, volume, pressure and saturation properties; pipes stand for connections between those control volumes, holding transmissibility property. First this list is extracted from the simulation grid and from the fractures that have been clipped by the gridblocks. Then both lists are connected together between each fracture piece and the surrounding gridblock. The second phase consists in computing the interblock transmissibilities from this connectivity list. These transmissibilities are evaluated for each couple of gridblocks in each direction by applying electrical simplification theorems. The discretization tool and the simplification technique make the method really fast. Its performance is demonstrated on a 3D highly fractured reservoir. The main limitation is due to the electricity analogy limitations that does not take into account complex flow mechanisms related to fractures; even though, results show good agreement with those of a fine grid.

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