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Influences and Evolution of Fracture Surface Roughness and Its Previous HitDependenceNext Hit on Slip

Abstract

Fluid flux through fractures strongly depends on the variation in fracture aperture. In natural fractures, the size, shape, and Previous HitfrequencyTop of asperities on its surfaces influence the development and retention of aperture. This geometry is referred to as the roughness. At small slip, juxtaposition of mismatched asperities causes fractures to dilate and self-prop; increasing slip eventually leads to the grinding or breaking of asperities. In applications such as Enhanced Geothermal Systems (EGS) or massive hydraulic fracture, rising fluid pressure accompanying leak-off of fluid into the formation can cause natural fractures to slip, dilate, and self-prop. This process is capable of permanently increasing the permeability of the rock mass and thus the accessible fracture surface area, maximizing access to heat or natural porosity of the formation. In this study, core from well GEO N-2 in the hot but impermeability flank of the Newberry Volcano, OR, USA and adjacent to EGS stimulation well 55–29, are examined to quantify sources of roughness, its modification through repeated slip, and history of dilation. Individual slip and dilation events are preserved in these fractures by superposed layers of cement. During fracture nucleation when slip is small, the topography of the fracture surfaces correlates with the grain and pore sizes that provide intrinsic sources of mechanical heterogeneity likely to influence fracture propagation. As the fracture continues to grow accompanying repeated slip, linkage among formerly isolated fractures provides new topographic relief obscuring the correlation with the grain and pore size. At the largest slip, gouge production reduces asperity height and diversity. These attributes are quantified in the population of asperity heights at each stage and through the power spectrum that relates the relative contribution of different wavelength asperities to the overall roughness. This analysis suggests that the dilation potential of natural fractures is closely linked to slip relative to the length-scale of mechanical heterogeneity in the rock due to grains and pores and that dilation is maximized at slips that minimize gouge production. More localized impacts on dilation result from linkage of fractures which add roughness throughout the slip history of the fracture but generate highly localized dilation that channelizes flow.