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

．智力优势：破坏或产生孔隙度的介质之间的竞争控制着裂隙岩石输运特性的演化。化学力学效应导致的渗透率变化发生在中等应力（2mpa）和温度（T80C，以H2O为渗透介质）下，变化迅速（c天），幅度显著（渗透率降低10-2），而且，即使溶解净去除矿物块，也会令人惊讶地导致渗透率降低。尽管有这些观察结果，对于控制裂缝中孔隙生成和破坏之间转换的过程和指标参数，仍在寻求一致的观点。对沉淀和溶解速率的重要控制是由局部剪应力和正应力、化学势场和裂缝的演化拓扑施加的。反过来，这些效应介导了岩石裂缝的输运（渗透率）和力学特性（刚度和抗剪强度）的演变。本研究将阐明这些转化如何随偏应力、温度、流体通量和化学势的路径以及不同流体饱和度的变化而进行。这些影响将通过在双直接剪切加载装置中连续剪切裂缝的流动测试来检验，以返回不断变化的渗透率、刚度和剪切强度的连续测量结果。测试将在控制温度（20-300℃）、流速（0-2 cc/min）、环境应力（0-50 MPa）和控制排量（10-106 nm/s）下进行，速度慢到足以接近裂缝内矿物再分配的速度。记录流动阻抗、矿物质量流出和正常驱替率的历史，将提供体内演化裂缝孔径或孔隙度的三种独立测量。这些观察结果与测试前和测试后的裂缝表面轮廓测量（~ 0（5米））相结合，将提供独特的受限微力学数据，以支持基于过程的模型的开发。将开发颗粒力学模型来表示接触的两个粗糙表面的基本特征，并通过机械和化学过程适应弥合裂缝的凹凸不平的产生和破坏。这些模型必须包含应力介导的溶解、扩散输运、自由面溶解和沉淀等一系列过程，这些过程共同定义了裂缝的力学和输运特征的演变。输运建模将通过欧拉-拉格朗日方法进行，该方法适应平流为主的流动，与颗粒力学模型的不断发展的拓扑结构相结合，既限制了实验观测，又使观测能够扩大到现场尺度。结果将定义关键过程，并限制应变率和流体和质量通量的大小，其中孔隙度的产生优于其破坏，适用于广泛的环境应力，温度和化学势路径。更广泛的影响：除了这些运输过程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的广泛影响外，一些更广泛的影响是显而易见的。这包括工程和地球物理学界之间的跨学科交流，本科生和研究生的广泛培训，以及工程和科学文献中成果的及时展示和出版。

.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.