Confirmation of Hydraulic, Tracer, and Heat Transfer Characterization of a Fractured Bedrock Using Ground Penetrating Radar

Abstract

Over the past five years multiple characterization techniques have been applied to an experimental field site to elucidate flow channeling in bedrock and its influence on heat transfer. The field site consists of a 10 x 10 meter five-spot borehole pattern isolated in a single bedding plane fracture in sandstone. The Altona Flat Rock site is not a geothermal reservoir but was developed as an inexpensive field laboratory for testing characterization methods applicable to geothermal fields. Periodic hydraulic tests (presented at this workshop in 2010) showed evidence of preferential flow connections among the wells. This connection was confirmed with multiple ionic tracers which showed different sweep efficiency among well pairs (presented at this workshop in 2013). Heated water tracer studies demonstrated that the heat exchange was more efficient in the wells that exhibited greater sweep between well pairs (presented at this workshop in 2012). Finally, surface ground penetrating radar (GPR) was used to map flow in the plane of the fracture by using saline solution to trace flow paths. The implications of the GPR imaging is the focus of this contribution. Reflected radar energy showed a marked change in both amplitude and phase in the presence of saline tracer, allowing flow paths to be imaged. The path followed by the saline tracer was highly anisotropic, presumably following aperture fabrics related to the sandstone depositional environment. Fluid flow, solute migration, and heat transport are all strongly influenced by this anisotropy. These findings highlight the importance of characterizing hydraulic connectivity to predict heat transfer efficiency, even in a single fracture. They also suggest that simple hydraulic testing may provide a cost-effective estimation of preferential flow that is comparable to tracer testing.