Quantifying the Physical and Chemical Controls on Permeability Evolution in Sheared Fractures

知识产权：破坏或产生孔隙度的因素之间的竞争控制着断裂岩石的输运性质的演变。由化学机械效应引起的渗透性变化已被证明在适度的应力（2 MPa）和温度（T80 C，以H2O作为渗透物）下发生，并且是快速的（c.天），具有显著的幅度（渗透率降低10-2），并且此外，即使当溶解净去除矿物质时，也令人惊讶地导致渗透率降低。尽管有这些观察结果，仍在寻求控制裂缝中孔隙度生成和破坏之间切换的过程和索引参数的一致观点。沉淀和溶解速率的重要控制施加的局部剪切应力和正应力，化学势场，和不断发展的拓扑结构的断裂。反过来，这些效应介导岩石中裂缝的运输（渗透率）和机械性能（刚度和剪切强度）的演变。这项研究将阐明这些转换的进展与路径的偏应力，温度，流体流量，和化学势，并为不同的流体saturations.These效果将通过流通过测试裂缝连续剪切内的双直接剪切加载装置返回不断变化的渗透率，刚度和剪切强度的连续测量检查。试验将在受控温度（20- 300 ℃）、流速（0-2 cc/min）、环境应力（0-50 MPa）和受控位移速率（10-106 nm/s）下进行，位移速率应足够慢，以接近裂缝内矿物重新分布的速率。记录的流动阻抗、矿物质流出和正常位移速率的历史将提供体内不断变化的裂缝孔径或孔隙度的三个独立测量值。这些观察，锚与测试前和测试后的断裂面轮廓测量~O（5米），将提供独特的约束微观力学数据，以支持基于过程的模型的发展。将开发颗粒力学模型，以代表接触的两个粗糙表面的基本特征，并通过机械和化学过程，以适应出生和破坏的凹凸桥接骨折。这些模型将必然纳入应力介导的溶解，扩散运输，和自由面溶解和沉淀，共同定义的断裂的机械和运输特性的演变的系列过程。运输建模将通过链接欧拉-拉格朗日方法，适应平流为主的流量，与不断发展的拓扑结构的颗粒力学模型的接口，既约束实验观测，并使观测放大到现场规模。结果将定义关键的过程和约束的应变率和流体和质量通量的大小，其中产生的孔隙度竞争其破坏，广泛的环境应力，温度和化学势的路径。 更广泛的影响：这些迁移过程除了对放射性废物的安全掩埋、碳氢化合物、地热流体和饮用水的回收以及对地壳内流体循环的理解产生广泛影响外，还显然产生了一些更广泛的影响。其中包括工程和物理学社区之间的跨学科交流，本科生和研究生的广泛培训，以及及时在工程和科学文献中介绍和出版结果。

.Elsworth0510182Intellectual Merit: The competition between agents that either destroy or generate porosity controls the evolution of the transport properties of fractured rocks. Changes in permeability resulting from chemo-mechanical effects have been shown to occur under modest stresses 2 MPa) and temperatures (T80C, with H2O as the permeant), to be rapid (c. days), of significant magnitude (permeability reductions of 10-2), and moreover, to surprisingly result in permeability reduction even when dissolution net removes mineral mass. Despite these observations, a consistent view of the processes and indexing parameters that control the switching between porosity generation and destruction in fractures is still sought. Important controls on rates of precipitation and dissolution are exerted by local shear and normal stresses, the chemical potential field, and the evolving topology of the fracture. In turn, these effects mediate the evolution of the transport (permeability) and mechanical properties (stiffness and shear strength) of fractures in rock. This study will clarify how these transformations progress with paths of deviatoric stress, temperature, fluid flux, and chemical potential, and for different fluid saturations.These effects will be examined via flow-through tests on fractures continuously sheared within a double direct shear loading apparatus to return continuous measurements of evolving permeabilities, stiffnesses, and shear strengths. Tests will be conducted under controlled temperatures (20-300C), flow rates (0-2 cc/min), ambient stresses (0-50 MPa), and under controlled displacement rates (10-106 nm/s) slow enough to approach rates of mineral redistribution within the fracture. Recorded histories of flow impedance, mineral mass efflux, and normal displacement rate, will provide three independent measurements of evolving fracture aperture or porosity, in vivo. These observations, anchored with pre- and post-test fracture surface profilometry ~O(5 m), will provide uniquely constrained micro-mechanical data to support the development of process-based models. Particulate mechanics models will be developed to represent the essential features of two rough surfaces in contact, and to accommodate the birth and destruction of asperities bridging fractures via mechanical and chemical processes. These models will necessarily incorporate the serial processes of stress-mediated dissolution, diffusive transport, and free-face dissolution and precipitation, which together define the evolution of the mechanical and transport characteristics of the fracture. Transport modeling will be via linked Eulerian-Lagrangian methods that accommodate advection dominated flows, that interface with the evolving topology of the particulate mechanics model, and both constrain experimental observations and enable upscaling of the observations to field scale. Results will define critical processes and constrain the magnitudes of strain rates and fluid and mass fluxes where the generation of porosity out-competes its destruction, for a broad range of ambient stresses, temperatures, and paths of chemical potential. Broader Impacts: In addition to the broad impacts these transport processes have in the safe entombment of radioactive wastes, the recovery of hydrocarbons, geothermal fluids, and potable water, and in the understanding of fluid cycling within the crust, a number of broader impacts are apparent. These include the cross-disciplinary exchange between the engineering and geophysics communities, the broad training of undergraduate and graduate students, and timely presentation and publication of results in the engineering and scientific literature.