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

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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.