Collaborative Research: Using Pore Fluid Pressure Gradients to Test the Relative Importance of Hydrologic Versus Mechanical Heterogeneity in Fracture Formation

伸展裂缝是各种地质环境的常见元素。它们通常被推断为在高孔隙流体压力下形成，并在机械非均质上成核。这类裂缝具有特殊的社会意义，因为它们为水库或含水层中的流体流动或污染物输送提供了途径，但我们仍然缺乏预测它们可能形成的位置和密度的能力。该项目考察了水文性质的空间变化与力学性质的空间变化在控制裂缝形成的地点和时间方面同样重要的假设。因此，岩石的力学行为不仅是其非均质力学特性的函数，也是其传递和维持高孔隙流体压力的非均质能力的函数。该项目正在研究水文非均质性和力学非均质性在控制砂岩（一种常见的天然岩石储层/含水层）伸展裂缝形成中的相对重要性。为了实现这一目标，正在进行实验室实验，在测试样品中产生孔隙流体压力梯度的同时启动水力压裂。边界处应力和流体压力的下降为细层状、胶结良好、细粒、低扩散系数砂岩样品的扩展裂缝的产生创造了瞬态条件。通过将孔隙流体压力梯度实验与广泛的体积和颗粒尺度的力学和水文非均质性表征相结合，裂缝形成可以与岩石的岩石物理特征相关联，这是迈向可预测性的重要一步。测试前分析包括测量水力扩散系数、示踪剂穿透曲线（均匀性程度的代表），以及相对于层理（沉积岩中最常见的物理非均质性）不同方向的孔隙弹性特性。对岩石固体骨架和孔隙网络的详细粒度研究，以及毫米至厘米尺度的渗透率变化，补充了这些体积响应的测量。裂缝为流体（如水或油）进入地下提供了重要的通道。目前对裂缝在地质条件下是如何形成和生长的了解非常有限。本研究试图通过一种新开发的基于实验室的测试方法来研究地下流体在岩石裂缝成因中的作用。通过结合压裂前后岩石的详细分析，可以评估控制压裂时间、位置和强度的重要变量。将这些信息纳入地下模型将使科学家和工程师能够更有效地在地球地下生产或储存流体。

Extension fractures are common elements of a wide range of geologic settings. They are generally inferred to form in response to high pore fluid pressure and to nucleate on mechanical heterogeneities. Such fractures are of particular societal interest where they provide pathways for fluid flow or contaminant transport in reservoirs or aquifers, yet we still lack the capability to predict where they are likely to form, and in what density. This project examines the hypothesis that spatial variations in hydrologic properties are as important as spatial variations in mechanical properties in controlling where and when fractures form. Thus, a rock's mechanical behavior is a function not only of its heterogeneous mechanical properties, but also its heterogeneous ability to transmit and maintain elevated pore fluid pressures. The relative importance of hydrologic and mechanical heterogeneities in controlling the formation of extension fractures in sandstone, a common natural rock reservoir/aquifer is being investigated in this project. To accomplish this goal, laboratory experiments in that initiate hydraulic fractures while producing a pore fluid pressure gradient within a test sample are being carried out. A drop in both stress and fluid pressure at the boundary creates transient conditions conducive to the generation of extension fractures in finely laminated, well cemented, fine-grained, low diffusivity sandstone samples. By integrating pore fluid pressure gradient experiments with extensive bulk and grain-scale characterization of mechanical and hydrologic heterogeneity, fracture formation can be related to the petrophysical characteristics of a rock - an important step toward predictability. Pre-test analyses include the measurement of hydraulic diffusivity, tracer break-through curves (a proxy for degree of homogeneity), and poroelastic properties in different orientations relative to bedding, the most common physical heterogeneity in sedimentary rocks. Measurement of these bulk responses is complemented by detailed, grain-scale study of both the rock's solid framework and the pore network, and mm-cm scale variations in permeability.Fractures provide important pathways for fluids (such as water or oil) to move underground. Current understanding of how fractures form and grow under geologic conditions is extremely limited. This study attempts to better understand the role of subsurface fluids on the genesis of fractures in rock by studying their growth in a newly developed laboratory-based testing method. Through the combination of detailed analyses of the rock prior to and post fracturing the important variables that that control the timing, location, and intensity of fracturing can be evaluated. Incorporation of this information into models of the subsurface will allow scientists and engineers to more effectively produce or store fluids in the sub-surface of the Earth.