Reports: ND849621-ND8: Systematic Investigation of the Planar Shape of Rock Fractures using PFC3D Numerical Experiments

Lianyang Zhang, University of Arizona

The planar shape of fractures has a profound effect on the connectivity of fractures and thus on the permeability of fractured reservoirs. It is important to know the planar shape of fractures when characterizing a fractured reservoir. However, the real planar shape of fractures at a site is rarely known since a rock mass is usually inaccessible in three dimensions. Information on the planar shape of rock fractures is limited and often open to more than one interpretation, making it difficult to correctly characterize fractured reservoirs. One simple way commonly used in oil reservoir characterization is to assume a circular disk fracture shape, which often leads to underestimated connection probability. Although extensive studies on the planar shape of rock fractures have been conducted, they are based on the macro-scale behavior of rocks and are empirical in nature. The goal of this project is to investigate the fundamental mechanism of fracture propagation in rock and the factors affecting the planar shape of rock factures, based on numerical simulation with the three dimensional Particle Flow Code (PFC3D). The work includes (1) development and calibration of the numerical model using the experimental data including unconfined compressive strength, tensile strength and stress-strain curves; (2) validation of the calibrated numerical model by comparing the simulated fracturing pattern with the experimental fracturing results of the same rock; and (3) investigation of the planar shape of rock factures and the different factors affecting the fracturing of rocks using the validated numerical model.

During the 2011-2012 period of this project, we mainly worked on the development of a new contact model that can be used in PFC3D to better simulate rock fracturing behavior. So far we have achieved two main accomplishments: 1) systematically study the effect of model scale and particle size distribution on rock behavior using the default PFC3D parallel bond model; and 2) design and implement a new contact model that can take into consideration of shear strength dependent on normal stress, bond type, sliding threshold, residual friction, and controlled moment contribution to the bond maximum stress accumulation. Many researchers have used PFC3D model to simulate rock behavior without noticing the effect of model resolution and particle size distribution on the simulation results. A representative model resolution has to be selected to achieve the consistency of numerical simulation results. This is also a key issue when using PFC3D to study the microscopic behavior of rocks such as crack nucleation and propagation. After reasonable model resolution has been achieved, a numerical model with zones containing different size particles can be used to enhance the calculation efficiency. During the parametric study with the default PFC3D parallel contact model, several critical problems have been discovered: (1) the unconfined compressive strength to tensile strength (UCS/T) ratios based on simulation are much lower than those from laboratory tests, (2) the contact model does not consider the dependence of shear strength on the normal stress, and (3) the bond breakage behavior cannot not satisfactorily represent the rock cracking behavior from laboratory tests. So a new contact model that addresses those limitations is developed and imported to PFC3D. The new contact model adopts the Coulomb failure criteria with a sliding threshold after the maximum shear strength is reached. The new model has already been successfully used to simulate the right UCS/T ratios and triaxial behavior of rocks.

We are currently applying the new contact model to simulate laboratory rock fracturing tests. After that, we will conduct systematic investigation of the planar shape of rock factures and the different factors affecting the fracturing of rocks.

So far, one journal paper has been published in the International Journal of Rock Mechanics and Mining Sciences and the other is about to be submitted. In addition, two conference papers have been presented in the 45th US Rock Mechanics/Geomechanics Symposium in 2011 and in the 46th US Rock Mechanics/Geomechanics Symposium in 2012. We hope to write two to three more papers based on the results of this project.