A new computer code for discrete fracture network modelling

Abstract

The authors describe a comprehensive software package for two- and three-dimensional stochastic rock fracture simulation using marked point processes. Fracture locations can be modelled by a Poisson, a non-homogeneous, a cluster or a Cox point process; fracture geometries and properties are modelled by their respective probability distributions. Virtual sampling tools such as plane, window and scanline sampling are included in the software together with a comprehensive set of statistical tools including histogram analysis, probability plots, rose diagrams and hemispherical projections. The paper describes in detail the theoretical basis of the implementation and provides a case study in rock fracture modelling to demonstrate the application of the software.

Introduction

A rock mass comprises rock material and discontinuities that include pores, fractures, joints, faults and bedding planes. In most engineering applications, these discontinuities are the critical factors that determine the performance of the rock, such as the strength of rock structures or the flow characteristics of rock masses. For example, rock fracture networks are the most significant means of fluid transport through rock masses, especially those at significant depths in the subsurface of the Earth. Applications include hot dry rock (HDR) geothermal energy systems in which artificial reservoirs must be created by stimulating fractures to enable geothermal flow; the evaluation and construction of underground repositories for the safe storage and disposal of hazardous wastes for which potential contaminant flow through surrounding natural fractures must be quantified; underground water transport through aquifers in hydrogeological engineering; and movement of oil and gas in hydrocarbon reservoirs. Rock fractures and fracture networks are also the major decisive factors in the stability of rock slopes and underground openings in civil and mining engineering and in block caving mining applications.

Despite the importance of the applications, it is not feasible to map accurately on an engineering scale the fractures and fracture networks in rock masses not least because accurate field measurement of a single discontinuity is difficult and measurement of all discontinuities is impossible. In practical applications there is, therefore, no observable reality on any meaningful scale and the only realistic approach is via a stochastic model informed by sparse data, which normally come from surveys of analogues, such as rock outcrops, or from direct or indirect observations of the rock mass such as drill cores, borehole imaging, geophysical surveys or seismic monitoring during fracture stimulation.

This paper describes software that can be used to generate 2D and 3D fractures and fracture networks and is based on the authors’ previous work in stochastic modelling of fractures in rock masses (Xu et al., 2003, Xu et al., 2006, Mardia et al., 2007, Dowd et al., 2007). The purpose is to provide the research community with a simple, freely available comprehensive tool for stochastic fracture simulation. The software is generic in design and approach and output can be exported for subsequent flow or mechanical analysis. Some common sampling facilities such as plane, window, scanline and borehole sampling are included in the software. The program also includes basic statistical tools such as histogram and rose diagram analysis, probability plots and hemispherical projections.