Abstract
AbstractTo improve our understanding of the complex coupling between circulating fluids and the development of crack damage, we performed flow‐through tests on samples of Etna basalt and Westerly granite that were cyclically loaded by deviatoric stresses. The basalt was naturally microfractured, while the relatively crack‐free Westerly granite was thermally pretreated to 500°C and 800°C to generate microcrack damage. Samples were repeatedly loaded and then unloaded under deviatoric stress paths and ultimately to failure. Permeability and water volume content were measured throughout the loading history together with the differential stress. Permeability decreases at low differential stresses and increases at intermediate differential stresses up to a steady value at failure. We use water volume content as a proxy for fluid storage and show that both permeability and storage evolve with damage and evolution of crack density. We use crack models to represent the evolution of permeability as a function of loading state and are able to independently link it to the observed evolution of deformability, used as an independent measure of crack density.
Highlights
[2] Understanding and predicting the mechanical behavior of volcanic systems and geothermal reservoirs require a detailed knowledge of the physical properties of rocks and their evolution as a function of stress state, fluid flow, temperature, and deformation history
[5] This work explores the influence of incremental crack damage induced by cyclic loading on the mechanical and transport properties of thermally stressed crystalline rocks in shallow crustal conditions that are representative of volcanic edifices and high thermal gradient in the upper crust
[6] In particular, we address questions such as the following: Is there a critical stress or threshold of crack damage that controls the connectivity of the preexisting crack fabric? What determines the rate of permeability change in microcracked rocks and how is this influenced by microcrack formation and reactivation? how do microcracks contribute to porosity and permeability evolution as stress increases to failure?
Summary
[2] Understanding and predicting the mechanical behavior of volcanic systems and geothermal reservoirs require a detailed knowledge of the physical properties of rocks and their evolution as a function of stress state, fluid flow, temperature, and deformation history. [4] Crack damage is strongly controlled by thermal stress histories that influence elastic wave properties [Nasseri et al, 2007], permeability [Simmons and Brace, 1965; Kern, 1982], unconfined compressive strength [Nagaraja Rao and Murthy, 2001], Young’s modulus [Daoying et al, 2006; Homand-Etienne and Houpert, 1989; Takarli and PrinceAgbodjan, 2008], tensile strength [Homand-Etienne and Houpert, 1989], fracture toughness [Nasseri et al, 2007; 2010], and porosity [Glover et al, 1995; Takarli and Prince-Agbodjan, 2008] This significant influence on physical properties is partly explained by the a–b quartz phase transition (~570C) and by differential deformation between grains of contrasting moduli that occur at all temperatures.
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