COUPLED HYDROMECHANICAL-THERMOCHEMICAL PROCESSES IN ROCK FRACTURES Chin-Fu Tsang Earth Sciences Division Lawrence Berkeley Laboratory University of California, Berkeley Abstract. The behavior of rock fractures under various coupled processes is reviewed under sevenbroad cate- gories: (1) hydromechanical processes, (2) thermo- hydrologic processes, (3) hydrochemical processes, (4) thermohydromechanical processes, (5) thermohydrochemi- cal processes, (6) hydromechanical-chemical processes, and (7) thermomechanical-hydrochemical processes. The state-of-the-art and possible directions of furtherresearch in thesecoupledprocesses are discussed. Rock fracture behaviors considered include not only dilation, closure, shear, and joint propagation but also changes in flow permeability and chemical sorption and retardation capabilities. These properties are of critical concernin practical considerations of the rock mass as a mediumto store and isolate chemical and radioactive wastes. Inves- tigations of coupled processes in two-fracture systems and multifracture systems are identified as important areas for future research. The need to consider coupled processes in boreholetestingprocedures involving rock fractures is emphasized. 1. INTRODUCTION Rock fractures may exhibitcoupled behavior, by which we mean deformation andpropagation, as well as changes in hydraulicand chemical properties of rock fractures, undervarious coupled processes. In general, rock masses contain jointsandfaults. Rockjointsaredeformed through dilation, closure, and shear. Under shear deformation, faults arecreated. In thepresent paper we shall not make a distinction between jointsandfaults andshall referto them generally asrockfractures. The processes thatare usually considered in the study of coupled behavior are thermal, hydrological, mechanical, andchemical. Coupling of these processes implies that one process affects theinitiation and progress of another, and so under these coupled processes, rock fracture behavior cannot be predicted by considering each process independently. An example is the occurrence of earthquakes inducedby fluid injection [Healy et al., 1968; Evans, 1966; Hsieh and Bredehoeft, 1981]. Here a hydrological process of injection pressure andfluid flow is coupled with rock mechanics of joint dilation and shear movements. To have a proper understanding of such coupled behavior, researchers have to extend themselves beyond their own discipline and to learn from and cooperate with others in related disciplines. Such multidisciplinary interactions andinvestigations have been fruitful in opening up new areas of researchon rock fractures andassociated subjects. From another angle, new areas of research on coupled processes in rock fractures are also suggested basedon considerations of many important practical problems of current interest. Carefulstudy of these practical problems has pointed to thekey role thatcoupled processes play in their definitions and solutions. Theexample of earthquakes inducedby fluid injection was observed in practical projects in whichlarge amounts of treated waste fluid were disposedof by injection underground[Wesson and Nicholson, 1987]. This phenomenon was alsoobserved in petroleum reservoir operations [Gibbs et al., 1973; Raleigh et al., 1972, 1976].Thus studies of this particular coupled process areneeded to ensure thesafety of these operations. Anotherpracticalarea of much current interestis the isolation of chemical andradioactive wastes by storage in repositories in geologicalformations.One of the main transport mechanisms of these wastes is by solution in fluid that flows from the repositories to the biosphere throughjoints and faults in the formation. Thus this problemis intrinsically related to the couplingof the mechanics of rock joint deformation with hydrologic fluid flow and also with the chemical retardation and reaction Copyright 1991 by theAmerican Geophysical Union. 8755-1209/91/91 RG-01832 $15.00 537 ß Reviews of Geophysics, 29, 4 / November 1991 pages 537-551 Paper number 91RG01832