Predicting Deformation in a Fractured Medium as a Result of Stress Rotation Using FEM Analysis

Natural fracture networks consisting of more than one systematic orientation set are common. The relative timing and mechanisms of subsequent development of different fracture sets remain poorly understood, as the changing stress conditions and mechanical properties over geological time are generally complex and difficult to extract from field observations. Numerical mechanical modeling techniques can be used to help understand these mechanisms in an efficient and flexible manner. In this study the finite element technique is employed to investigate the potential for subsequent fracture development in a medium with a set of pre-existing fractures as a result of changes in the ambient tectonic stress orientation.

Numerical experiments are performed on a reservoir-scale model volume that contains one or more large, discrete fractures. Fracture configurations of increasing complexity allow us to test the individual contribution of fracture spacing, relative position and orientation as well as the combined effect of these parameters on the stress and strain distributions in the model. A realistic fracture configuration is adapted from an outcrop analogue case study in southern Jordan. All models are subjected to uniaxial horizontal compression, which is rotated in a step-wise manner . Overburden pressure and gravity are incorporated in full 3-D model realizations to investigate the fractured volume, the type (tensile or shear) and orientation of potentially developing fractures.

Modeling results are presented in terms of the development of deformation zones. Irrespective of the fracture configuration, high equivalent stresses are concentrated at the terminations of the pre-defined fractures and rock failure always initiates here. Propagation of deformation zones from the terminations is preferentially perpendicular to the orientation of compression. For small angles between the applied stress orientation and the strike of the pre-defined fracture strike (0-30°), deformation zones develop under characteristic angles of 60° and 120°. The maximum horizontal principal stress is parallel and the minimum horizontal principal stress perpendicular to the applied stress orientation throughout the model, only slightly deviating in the direct vicinity of the pre-defined fractures. The orientation of developing fractures in the deformation zones will therefore mostly differ from the orientation of the deformation zone itself.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California