DFN Approach for Fractured Reservoirs: Suitable at local-scale?
DFN approach (a) may easily incorporate field-data;
(b) may render near-realistic fracture networks that may possibly include hydraulically connected fractures, I suppose
(because, two-dimensional fracture network from a top view, covering the areal extent will not be any use; and what we require is the actual three-dimensional fracture network with dip details, which are essentially hydraulically connected); and
(c) may explicitly represent the fracture attributes (but still, getting the actual details on the distributions of fracture length, fracture width, fracture spacing, fracture aperture thickness, fracture dip remain extremely challenging); and in turn DFN approach may work well for a larger field-scale scenario, where the local-scale interaction between low-permeable rock-matrix and high-permeable fracture remains not required.
However, in petroleum reservoirs, all the hydrocarbon has been stored within the low-permeable rock-matrix; and hence, the interaction between local-scale rock-matrix and fracture becomes very crucial in order to expel the oil out of the rock-matrix;
and subsequently,
ensuring the continuity of fluid mass fluxes at the fracture-matrix interface becomes very challenging using any numerical approach at the local-scale.
This transient interaction between fracture and matrix remains very crucial until a hydrodynamic equilibrium is achieved between fracture and matrix. This precision of deducing an accurate fluid mass flux at the fracture-matrix interface that subsequently ensures the fluid mass continuity between fracture and matrix – is what exactly required – and this will help to precisely estimate the RF associated with a fractured reservoir.
So, what do we need to get an improved/precise RF from a Fractured Reservoir?
(a) Reservoir Geo-mechanics (stress and strain - displacement of fracture surfaces with details on zero aperture thickness)
(b) Fluid flow (pressure / saturation)
(c) Role of differential advection and effective diffusion
(d) Scale-dependent heterogeneity (Deducing a reasonable REV)
(e) Nature of hyperbolic dominant PDE
(f) Nature of hyperbolic dominant source term
and, we require much more...........before we can include our investigation on the role of gravity-drainage, imbibition and capillarity.
Of course, we can always approximate the above problem using Equivalent Continuum rather than attacking the problem with multi-continuum. Even with multi-continuum, we can always approximate using dual-porosity rather than using dual-permeability (if the rock-matrix is no more permeable, then, how long will it take for the oil/gas to get expelled out of rock-matrix using concentration gradient?).
Even with dual-porosity, we can always assume a reasonable shape factor that reflects the fluid mass transfer between fracture and matrix; rather than going for actual 'rate limited fluid mass transfer'.
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