Experiments on granular media have significantly improved our understanding of deformation processes in porous rocks. Laboratory results have lead to fundamental theoretical developments (such as poroelasticity, or rate and state-variable friction) that have found widespread application. This paper presents results from laboratory experiments that help constrain these theories. Data from triaxial deformation experiments on quartz sand aggregates are used to illustrate stress-dependent behavior of poroelastic parameters (e.g. the Biot-Willis and Skempton coefficients). Calculations for these coefficients show systematic variations as effective stress increases, in a manner consistent with measured compressibilities of the aggregate. Data from shear experiments show that frictional strength varies systematically with time and temperature. At temperatures below 450 oC, shear zones exhibit greater cohesive strengths as the time of stationary contact increases (hence, positive healing rates). For conditions exceeding 450 oC, shear zone strength is seen to decrease with contact time (negative healing rates). The results from both volumetric compaction and frictional shear experiments are well described by poroelasticity as well as rate and state-variable friction. The combination of these constitutive relations may provide a powerful tool that can be used in numerical models that couple thermal, mechanical, hydraulic, and temporal processes ? as occur in geothermal systems.
Both crystalline and sedimentary rocks permit the study of deformation processes such as brittle failure, inter- and intra-crystalline plasticity, fluidrock interactions, and in-situ phase alterations. Yet, it is with clastic rocks that we are best able to study these processes together with the effects of consolidation, compaction, storage capacity, and fluid transport. Historically, studies of granular media have helped us understand the role of effective stress on deformation, the influence of consolidation and distortional stress on failure strength and yield behavior (i.e. critical state models), the characteristics of frictional shear, the time-dependent variations of material strength, the evolution of porosity and its relationship to permeability - just to name a few. These processes are fundamental for studies of natural systems (e.g. sediment compaction, fault mechanics, crustal rheology) and a variety of geosystems for which rock mechanics finds application (e.g. mining industry, hydrocarbon industry, water resources, and geothermal fields).
While several mechanisms are likely to alter petrophysical properties, it is important to note that processes associated with mechanical deformation of granular media are readily applicable to a variety of components in geothermal systems ? such as proppants, clastic reservoirs, and fracture properties. Results from deformation experiments on granular media can be directly applied to clastic reservoirs and fracture proppants in order to estimate static strength characteristics, material properties, and geotechnical constants. Also, the temporal evolution of petrophysical properties of these materials can be investigated from laboratory studies of compaction creep and frictional shear in granular media. Yet, laboratory tests routinely indicate that deformation in granular media share many characteristics with deformation processes in crystalline rocks (e.g. frictional behavior) and fractured media (e.g. poroelastic behavior.
