Evaluation of Storage in Fractured-Rock Aquifer Systems

许多重要的当地和区域含水层出现在裂隙和断层的结晶岩石系统中。有效孔隙度和相关储集空间赋存于原生和次生裂隙网络以及基质块体中。人们设计了许多水力试验来量化裂隙系统的流量和渗透率，但由于测量裂缝或基质块在应力条件下的压缩很困难，成本也很高，所以很难准确量化和分配储集空间和孔隙度。在提供应力-应变关系以量化松散抽水盆地中的骨架特定存储特性方面，一个非常有效的装置是井筒引伸仪。我们建议在一个特征良好的裂隙和有断层的结晶岩石含水层系统中安装管道引伸仪，该系统已知包含一个来自逆冲断层的主要裂隙带和一个包含在低渗透率基质中的次生裂隙网络。延伸仪的设计将允许对岩石(和裂缝)的压缩系数为10-11 m2/N进行准确的应变测量，可以监测和有效地消除潮汐、气压和降雨载荷以及温度影响的残余应变。设计的循环水力试验将产生滞后的应力-应变曲线，从而量化原始和次生裂缝的压缩存储特性。使用热脉冲流量计对流量随时间变化的分析模型和类型曲线分析可以提供裂缝导水率和裂缝总储量的估计。结合延伸仪和分析模型分析，可以量化孔隙度和区块水力传导性，以及准确的储量估计。利用ABAQUS进行有限元模拟，可以在双重渗透率/孔隙度系统中使用瞬变参数，这将进一步了解水力特性和在压力条件下的响应。这项拟议的研究的更广泛的影响是重大的，因为裂隙岩中的储存和孔隙度的量化通常是难以捉摸的，即使使用详细且昂贵的含水层测试技术也是如此。结合使用延伸仪数据以及分析和数值模拟，将提供对裂缝所有重要水力特性的详细估计。这项研究中使用和开发的技术可以很容易地应用于所有裂隙岩石环境。这将是美国东部第一台也是唯一一台运行中的管道延伸仪(全国唯一一台破碎的结晶岩)，并将为其他大学和美国地质调查局的研究人员和学生提供重要的数据和机会，否则他们将无法使用该设备或它将产生的应变测量。从引伸仪获得的数据将被纳入弗吉尼亚理工大学的两门水文地质学课程，作为与应变、储存和断裂力学有关的讲座的一部分。它还将供学生实地考察我们在蓝山省继续开发的碎裂岩石研究遗址。

Many important local and regional aquifers occur in fractured and faulted crystalline-rock systems. Effective porosity and related storage occur in primary and secondary fracture networks as well as the matrix block. Many hydraulic tests have been designed to quantify flow and permeability of the fractured system, but accurate quantification and distribution of storage and porosity have been elusive because measuring the compression of the fractures or matrix block under stressed conditions is difficult and costly. One device that has been highly effective for providing stress-strain relations for quantifying skeletal specific storage properties in unconsolidated pumped basins is the borehole pipe extensometer. We propose to install a pipe extensometer in a well characterized fractured and faulted crystalline-rock aquifer system that is known to contain a major fracture zone from thrust faulting and a secondary fracture network contained within a low permeability matrix. The extensometer design will allow for accurate strain measurements for a rock (and fracture) compressibility of 10-1l m2/N. Residual strains from earth tides, barometric and rainfall loading and temperature effects can be monitored and effectively eliminated. Designed cyclical hydraulic tests will yield hysteretic stress-strain curves leading to quantification of primary and secondary fracture compressive storage properties. Analytical modeling of flow verses time using the heatpulse flow meter and type-curve analysis can provide estimates of fracture transmissivity and total fracture storage. The coupled extensometer and analytical modeling analysis can allow for quantification of porosity and block hydraulic conductivity along with accurate storage estimates. Finite-element modeling with ABAQUS allows for the use of transient parameters in a dual permeability/porosity system that will provide further understanding of the hydraulic characteristics and response under stressed conditions.The broader impacts of this proposed research are significant because storage and porosity quantification in fractured rocks is often elusive, even with detailed and costly aquifer testing techniques. The combined use of extensometer data coupled with analytical and numerical modeling will provide detailed estimates of all important hydraulic properties of fractures. The techniques used and developed in this research can be readily applied to all fractured-rock settings. This would represent the first and only operating pipe extensometer in the eastern United States (the only one in the country in fractured crystalline rocks) and would provide important data and opportunities for researchers and students from other universities and the US Geological Survey who would not otherwise have access to this device or the strain measurements it will yield. The data obtained from the extensometer will be incorporated into two hydrogeology courses at Virginia Tech as part of lectures pertaining to strain, storage, and fracture mechanics. It will also be available for student field trips to the Fractured Rock Research site that we are continuing to develop in the Blue Ridge province.