Joint Stiffness and Deformation Behaviour of Discontinuous Rock

Abstract

Some rock masses are characterized by joints, fractures and other planes of weakness which reduce the strength and deformation properties of rock structure. Under different loading conditions, joints with weaker than normal and shear deformation strength undergo a relatively higher strain than intact rock. Because permeability of jointed rock masses in fractured reservoirs is a strong function of joint aperture size, one may expect a major change in the permeability when subjected to confining load variation. Therefore, it is important to establish the relation between the stress-strain of the jointed rock mass and the reservoir permeability. This relation is particularly important to model hydraulic fracturing and productivity decline in tight gas wells. In this paper, a new relation is proposed to model prepeak shear stiffness of the joint based on the conventional joint surface parameters and the confining load. Furthermore, constitutive matrices for evaluating deformation behaviour of a single-joint and regularly jointed rock are presented as the results of an analytical study. Based on the concept of joint stiffness, an equivalent stiffness for regularly jointed rock masses was derived, assuming that the deformation of the jointed rock mass equals the sum of the deformation of the rock matrix and the joints. Finite element technique is used to numerically model the deformation behaviour of the jointed rock under various loading conditions. The applicability of the constitutive model to represent jointed rock mass was confirmed from comparison of the numerical results with some of the existing experimental data. The model presented here will be the key element for integrated geomechanical modelling of tight gas wells, naturally fracture reservoirs and other fracturing processes in stress-sensitive reservoirs.