(4). The next three papers extend these views by taking a closer look on parameters that govern hydraulic diffusivity in sandstones and other types of rocks. Specific targets addressed are the influence of differential stress on permeability (5), imaging of the fracture geometry (6), and pressure induced variations in the pore geometry (7). Contributions no. 8 to 10 cover investigations of permeability-porosity relationships during rock evolution (8), of the formation, propagation, and roughness of fractures in a plexi-glass block (9), and pressure oscillation effects of two-phase flow under controlled conditions (10). The subsequent four articles focus on diverse modeling approaches. Issues considered are how the geometry and the mechanical behavior of fractures can be characterized by mathematical expressions (11), how the evolution of permeability in a microcracking rock can be expressed by an analytical model (12), deviations from the cubic law for a fracture of varying aperture (13), and the numerical simulation of scale effects in flow through fractures (14). Three further papers refer to in situ observations, being related to topics as the assessment of in situ permeability from the spatio temporal distribution of an aftershock sequence (15), to the scale dependence of hydraulic pathways in crystalline rock (16), and to the significance of pore pressure - stress coupling in deep tunnels and galleries (17).

The subject of thermo-hydro-mechanical coupled processes in fractured rock masses has close relevance to energy-related deep earth engineering activities, such as enhanced geothermal systems, geological disposal of radioactive waste, sequestration of CO2, long-term disposal of waste water and recovery of hydrocarbons from unconventional reservoirs. Despite great efforts by engineers and researchers, comprehensive understanding of the thermo-hydro-mechanical coupled processes in fractured rock mass remains a great challenge. The discrete element method (DEM), originally developed by Dr. Peter Cundall, has become widely used for the modeling of a rock mass, including its deformation, damage, fracturing and stability. DEM modeling of the coupled thermo-hydro-mechanical processes in fractured rock masses can provide some unique insights, to say the least, for better understanding of those complex issues. The authors of this book have participated in various projects involving DEM modeling of coupled thermo-hydro-mechanical processes during treatment of a rock mass by fluid injection and/or extraction and have provided consulting services to some of the largest oil-and-gas companies in the world. The breadth and depth of our engineering expertise are reflected by its successful applications in the major unconventional plays in the world, including Permian, Marcellus, Bakken, Eagle Ford, Horn River, Chicontepec, Sichuan, Ordos and many more. The unique combination of the state-of-the-art numerical modeling techniques with state-of-the-practice engineering applications makes the presented material relevant and valuable for engineering practice. We believe that it is beneficial to share the advances on this subject and promote some further development.

This book presents the coupled Thermo-Hydro-Mechanical-Chemical (THMC) processes in fractured rocks at varying scales from single fractures to fracture networks. It also discussed the implication and potential application of the advanced understanding of coupled THMC processes in fractured rocks for geotechnical and geo-energy engineering.

This work brings together the results, information and data that emerged from an international cooperative project, DECOVALEX, 1992-1995. This project was concerned with the mathematical and experimental studies of coupled thermo(T) -hydro(H) -mechanical(M) processes in fractured media related to radioactive waste disposal. The book presents, for the first time, the systematic formulation of mathematical models of the coupled T-H-M processes of fractured media, their validation against theoretical bench-mark tests, and experimental studies at both laboratory and field scales. It also presents, for the first time, a comprehensive analysis of continuum, and discrete approaches to the study of the problems of (as well as a complete description of), the computer codes applied to the studies. The first two chapters provide a conceptual introduction to the coupled T-H-M processes in fractured media and the DECOVALEX project. The next seven chapters give a state-of-the-art survey of the constitutive models of rock fractures and formulation of coupled T-H-M phenomena with continuum and discontinuum approaches, and associated numerical methods. A study on the three generic Bench-Mark Test problems and six Test Case problems of laboratory and field experiments are reported in chapters 10 to 18. Chapter 19 contains lessons learned during the project. The research contained in this book will be valuable for designers, practising engineers and national waste management officials who are concerned with planning, design and performance, and safety assessments of radioactive waste repositories. Researchers and postgraduate students working in this field will also find the book of particular relevance.