04 - Naturally Fractured Reservoirs

04 - Naturally Fractured Reservoirs - 1 2 Naturally...

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Unformatted text preview: 1 2 Naturally fractured reservoirs are often referred to as dual porosity reservoirs. Layered reservoirs with interlayer crossflow may also exhibit similar behavior. Dual porosity reservoirs are characterized by the presence of two media, matrix and fractures. The matrix Occupies most of the volume of the reservoir Has very low permeability Has moderate to high porosity - contains most of the pore space for storing hydrocarbons Is often referred to as primary porosity - developed as intergranular spaces during deposition The fracture system Occupies only a small fraction of the total reservoir volume Has very high permeability - provides a good path for movement of fluids Has very low porosity Is often referred to as secondary porosity - developed through diagenetic processes occurring after deposition 3 4 5 Most of the natural fracture geometries can be represented by one of three idealized geometries: slabs, columns, or cubes. Slab geometry One set of parallel fractures May be oriented horizontally or vertically Column geometry Two mutually orthogonal sets of parallel fractures Fracture spacing may be different in both directions May be oriented horizontally or vertically Cube geometry Three mutually orthogonal sets of parallel fractures Fracture spacing may be different in all three directions 6 7 8 9 10 Darcys law requires both a permeability k and an area A Is fracture permeability based on the smaller cross sectional area of the fractures only, or does it include the total area of the rock? Intrinsic fracture permeability -- based on cross-sectional area of fracture Bulk fracture permeability - k f- based on total cross sectional area of matrix + fractures Nomenclature h f- fracture width, microns h m- fracture spacing, feet k f- bulk fracture permeability, md- intrinsic fracture permeability, md f k ~ f k ~ 11 In spite of the complexity of the physical characteristics of naturally fractured reservoirs, only two distinct types of dual porosity behavior occur, and only two parameters in addition to permeability and porosity are required to specify the pressure response for dual porosity behavior. 12 There are two distinct types of dual porosity behavior, designated by the characteristics of the interporosity flow. In pseudosteady state flow, proposed by Barenblatt et al and by Warren and Root, each matrix block acts as if the entire block were at a uniform pressure. Damage or cementation across the interface between the fractures and the matrix may be responsible for this type of behavior. In pseudosteady state flow, the matrix block shape has absolutely no effect on the shape of the pressure transient response....
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04 - Naturally Fractured Reservoirs - 1 2 Naturally...

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