Fracture Flow and Fracture Cross Flow Experiments
Authors: Makurat Axel , Gutierrez Marte 
Published: December 2007

Based on experimental work conducted at NGI, the following major factors controlling the permeability of single fractures and fracture cross-flow have been identified: the stress dependent fracture aperture (e), the fracture internal flow paths (tortuosity), the fracture surface properties such as roughness (JRC), fracture wail strength (JCS) and the intact rock uniaxial compressive strength (), the fracture normal stress () and shear displacement () related dilation (dn) and gouge formation.

Fracture asperity damage (crushing, gouge formation and pore size reduction) depends strongly on the ratio of the fracture wall compression strength (or intact rock uniaxial compressive strength if the fracture surface is not altered) to the normal stress level (JCS/ and the fracture surface morphology, expressed by the joint roughness coefficient (JRC).

Introduction

Fractures and fractured zones require special attention in reservoir development. They may both help or hinder production. Conductive fracture systems may provide the necessary permeability to drain an oil saturated low permeability rock matrix, but they may also act as flow barriers resulting in compartmentalization of the reservoir. Important factors influencing flow across fractures are capillary pressure, which accounts for the imbibition process, gravity forces, and pressure gradients. Whether fractures act as barriers/seals or conduits will depend mostly on their surface properties (e.g., roughness, mineralogy, strength, infilling), their spatial distribution (including parameters such as length. width, continuity, spacing, orientation and dip) and the state of stress. Fracture sealing can be due to several mechanisms. A common mechanical sealing mechanism for the flow along fractures is the formation of gouge material due to shearing. The fracture cross flow has been shown to be strongly influenced by the reduction of the pore space in the vicinity of the fracture surfaces.

The understanding and prediction of the fluid flow properties (parallel and perpendicular to the fracture plane) of individual fractures and complex fracture systems, and the fluid interaction between fractures and the matrix (and vice versa), requires a constitutive law that describes these properties as a function of stress and strain. NGI's fracture model is formulated as a fully coupled flow and nonlinear deformation fracture behaviour law. The advantages of this law are easily determinable and physically meaningful parameters.

NGI's Fracture Behaviour Model

In the Barton-Bandis BB-model, the two components of fracture deformation, namely normal and shear displacement, are both based on the scale dependent index properties JRC (joint roughness coefficient) and JCS (joint compressive strength). The principal shear strength-displacement-dilation behaviour is described by the following two generalized equations:

(1)

(2)

where

mob = the mobilized friction angle

JRCn(mob) = the full-scale mobilized fracture roughness coefficient at any given displacement

(1 – 3) = the confined asperity strength

r = residual friction angle

= effective normal stress

dn(mob) = full-scale mobilized dilation angle at any given displacement

The mobilized friction angle gives the shear strength of the fracture at any given shear displacement. The key aspects of this formulation are:Friction is mobilized when shearing begins.

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