Title:

Matrix-Fracture Transfer Functions for Partially and Completely Immersed Fractures

Authors:

Edgar R. Rangel-German and Anthony R. Kovscek

Conference:

Stanford Geothermal Workshop

Year:

2003

Session:

Reservoir Studies

Language:

English

File Size:

549KB

View File:

Abstract:

Modeling multiphase flow in fractured porous media relies on the accurate description of matrix to fracture transfer of water. The rate of mass transfer between the rock matrix and fractures is significant, and calculation of this rate, within dual-continuum models, depends on matrix-fracture transfer functions incorporating the shape factor. Typically, matrix-to-fracture transfer functions are obtained by assuming all fractures to be instantaneously immersed in water (instantly-filled), with a uniform fracture pressure distribution under pseudo-steady state conditions. The result is constant, time-independent, shape factors. Clearly, this is not necessarily true. Partially immersed fractures and other unsteady-state conditions do not lead to constant shape factors.

A new time-dependent matrix-fracture transfer shape factor formulation and transfer functions for both filling- and instantly-filled fracture transfer are derived based on dimensional analysis. The general shape factor is expressed as the area of the matrix block contacted by the wetting phase divided by the product of the bulk volume of the rock matrix times the distance between the saturation at the fracture and the matrix average saturation. These parameters are obtained by means of either the analytical model for imbibition proposed by Rangel-German and Kovscek (2002) or experimental data (Rangel-German, 2002, for example). One of the advantages of the new shape factor is that it is based on dimensional analysis. This avoids simplifications that may lead to expressions that do not represent the physics of matrix-fracture transfer. The new shape factor carries information of the transient behavior of the water saturation, Sw, and so it leads to more accurate description of the matrix-fracture transfer. Good agreement was found between the experimental data and analytical model, and the modified dual porosity formulation with the new time-dependent shape factor and transfer function.


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