Patterson Jeremy R, Cardiff Michael
Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA.
Ground Water. 2023 Nov-Dec;61(6):816-833. doi: 10.1111/gwat.13297. Epub 2023 Feb 16.
Fractured sedimentary bedrock aquifers represent complex flow systems that may contain fast, fracture-dominated flow paths and slower, porous media-dominated flow paths. Thus, characterizing the dynamics of flow and transport through these aquifers remains a fundamental hydrogeologic challenge. Recent studies have demonstrated the utility of a novel hydraulic testing approach, oscillatory flow testing, in field settings to characterize single bedrock fractures embedded in low-porosity sedimentary bedrock. These studies employed an idealized analytical model assuming Darcian flow through a nondeforming, constant-aperture, nonleaky fracture for data interpretation, and reported period-dependent effective fracture flow parameters. Here, we present the application of oscillatory flow testing across a range of frequencies and inter-well spacings on a fracture embedded in poorly cemented sedimentary bedrock with considerable primary porosity at the Field Site for Research in Fractured Sedimentary Rock. Consistent with previous studies, we show an apparent period-dependence in returned flow parameters, with hydraulic diffusivity decreasing and storativity increasing with increasing oscillation period, when assuming an idealized fracture conceptual model. We present simple analyses that examine non-Darcian flow and borehole storage effects as potential test design artifacts and a simple analytical model that examines fluid leakage to the surrounding host rock as a potential hydraulic mechanism that might contribute to the period-dependent flow parameters. These analyses represent a range of conceptual assumptions about fracture behavior during hydraulic testing, none of which account for the measured responses during oscillatory flow testing, leading us to argue that other hydraulic processes (e.g., aperture heterogeneity and/or fracture hydromechanics) are necessary to accurately represent pressure propagation through fractured sedimentary bedrock.
断裂的沉积基岩含水层代表着复杂的水流系统，其中可能包含以裂隙为主导的快速水流路径和以多孔介质为主导的较慢水流路径。因此，描述水流和溶质运移通过这些含水层的动态过程仍然是一项基本的水文地质挑战。最近的研究表明，一种新型的水力测试方法——振荡流测试，在野外环境中可用于表征嵌入低孔隙度沉积基岩中的单个基岩裂隙。这些研究采用了一个理想化的分析模型，假设水流通过一个不变形、孔径恒定、无渗漏的裂隙作达西流动来解释数据，并报告了与周期相关的有效裂隙水流参数。在此，我们展示了在裂隙沉积岩研究现场，对嵌入胶结不良且具有相当原生孔隙度的沉积基岩中的一条裂隙，在一系列频率和井间距条件下进行振荡流测试的应用。与先前的研究一致，当假设一个理想化的裂隙概念模型时，我们发现回流参数存在明显的周期依赖性，即水力扩散系数随振荡周期增加而减小，弹性释水系数随振荡周期增加而增大。我们进行了简单分析，研究非达西流和井筒储存效应作为潜在的测试设计假象，还提出了一个简单的分析模型，研究流体向周围围岩的渗漏作为一种可能导致与周期相关水流参数的潜在水力机制。这些分析代表了关于水力测试期间裂隙行为的一系列概念假设，但没有一个能解释振荡流测试期间测得的响应，这使我们认为，需要其他水力过程（如孔径非均质性和/或裂隙流体力学）来准确描述压力在裂隙沉积基岩中的传播。
