| Abstract |
Dilation accompanying slip on natural fractures is a key mechanism exploited during stimulation of hot, low permeability rock, to reduce resistance to fluid flow and create an Enhanced Geothermal Systems. The dilation results from the impingement of mismatched asperities on opposing fracture walls during slip that force the fracture walls apart. The roughness of the fracture walls and the magnitude of slip relative to the length-scale of this roughness govern the maximum aperture, its variability and the area of contact that holds the fracture open after slip. During fracture nucleation, intrinsic rock characteristics such as grain and pore dimensions influence the path of the propagating fracture and provide an initial source of roughness. As the fracture repeatedly slips and propagates, this initial roughness is modified by linkage of formerly isolated fractures providing an additional source of roughness. In addition, as slip increases, gouge can form by the grinding or breaking of asperities. Similarly, chemical effects such as dissolution and precipitation of cements may also modify the surface roughness. This paper quantifies the history of repeated slip, fracture surface roughness in the direction of slip, and aperture between fracture initiation and gouge formation in natural fractures in core obtained from approximately 1-1.5 km depth from the Newberry Volcano, Oregon. Core samples from well GEO N-2 in the Newberry Geothermal Field, adjacent to the 55-29 EGS stimulation well, preserve cement layers that record repeated slip and dilation events throughout the lifecycle of a fracture. Individual pairs of fracture surfaces participating in each slip event were distinguished and the topography, associated slip, and aperture were measured to reconstruct the complete history of dilation. These histories document the evolution of roughness and its relationship to dilation from the initial stage of fracture propagation to the onset of asperity destruction and gouge formation. In addition, the distribution of grain and pore dimensions in the rock adjacent to the fracture were measured to correlate the relationship between these intrinsic features and the fracture surface roughness. Initial results suggest that grain and pore dimensions strongly correlate with the roughness of the majority of the fracture surface, but that linkage among fracture segments is a critical source of additional roughness and produces the largest asperities that increase the range and diversity of asperity heights. Gouge production sufficient to significantly alter the roughness develops at larger slip and further modifies the roughness by removing large asperities and reducing the range of asperity heights. These results suggest that the dilation potential of natural fractures is closely linked to slip distance, and that dilation is maximized at slips that minimize gouge production. |