Abstract

This paper investigates two-phase flow in microfractured porous media. We obtain relative permeabilities and capillary pressure curves for drainage and imbibition processes using a two-dimensional microfracture network model. The system of microfractures is composed of interconnected parallel plates of varying apertures, lengths and pre-established orientation.

Fluid flow in each microfracture unit is described by the paralle plate flow model. Capillary pressure for individual microfractures is obtained as a function of fracture aperture. The model takes into account capillary and viscous forces, thus the model allows for the analysis of different configurations of fluids on the network obtained with different capillary numbers and viscosity ratio. Relative permeabilities and capillary pressures are obtained as a function of the average saturation of the fluids in the network and the effect of different flow conditions on these curves is presented.

Introduction

Fracture network modeling has been used by several authors, mostly to study tracer flow in naturally fractured systems and to predict tracer breakthrough curves by front tracking techniques. Fractures are modeled as parallel plates and flow is described by Poiseuille's law, with the rate of flow being proportional to the cube of the fracture aperture. Fractures of varying apertures along the flow path have been considered; stochastic distributions of fracture density, length and aperture have been used to build the static fracture network, aiming at a better representation of fracture's nature, as observed in laboratory and field studies. A detailed review of fracture network modeling is presented in Reference [1].

Recently, a different use of fracture network modeling was reported, aiming at investigating and characterizing the dynamics of two-phase flow in microfractured porous systems. The network of microfractures was built by taking into account main features of previous static fracture networks. By allowing viscous and capillary forces, a dynamic flow model was built and used to study flow behavior under drainage conditions. Integration of flow in the Darcy sense, allowed the computation of relative permeabilities under different microfracture arrangements and flow patterns, as established through the capillary number, Ca, and viscosity ratio, M.

It is the purpose of this paper to extend the previous work to study two-phase flow in microfractured systems under imbibition, investigate the the dynamics of flow under such conditions and derive relative permeabilities and capillary pressure curves that would characterize the flow process.

As it has been mentioned in our previous paper, these studies have been motivated by field studies on a fractured reservoir, where discrimination between small- and large-scale fractures was required to match field behavior, in particular concentration data measured in wells during pressure maintenance with nitrogen injection[2]. Proper characterization of multiphase flow in a triple porosity medium formed by matrix, small- and large-scale secondary porosities[3] would require of relative permeability and capillary pressure curves for the small-scale fractures. Thus, this study aims at providing some understanding of the behavior and characterization of two-phase flow in such systems.

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