Characterization of Hydrologically Active Sheet Fractures

Clemson University, Clemson SC

Investigators

Abstract

0001146 Murdoch Sheet fractures in crystalline rock are an important control of ground water whose occurrence, characteristics, and processes of formation are poorly understood. The objective of this project is to combine information from several well-studied sites in the vicinity of western South Carolina with results from a new borehole test to improve the ability to characterize sheet fractures. The new test will use a straddle packer fitted with a precision borehole extensiometer, similar to devices used to measure rock deformation during geothechnical investigations. The device will be centered on a fracture and then used to conduct either injection or recovery tests. The resulting data will provide fracture aperture, or axial displacement of the borehole, as functions of pressure and the volume of injected (or recovered) fluid. The transient aperture change during an injection-displacement test will provide an additional descriptive record that goes beyond what can be measured during conventional packer tests. Preliminary theoretical analyses have been derived, and additional analyses will be developed, to predict transient aperture, volume and driving pressure, as functions of fracture length, spacing, connectivity and other variables. Those solutions will be invested to estimate characteristics of sheet fractures, such as fracture length and connectivity, which are beyond the resolution of conventional packer tests. Characteristics that can be measured using conventional packer test, such as fracture storativity, will also be evaluated using the new method. The results of the analyses will be compared to findings from field mapping of surface exposures, as well as core descriptions, borehole flowmeter logs, temperature logs, borehole radar, concentional packer tests, and other methods. This study will also investigate natural variations in pore-fluid pressure and fracture aperture due to Earth tides, barometric pressure fluctuations or other effects. Those loadings may contribute to the growth of sheet fractures, and provide insights into conceptual models of sheet fracture formation. In addition, the response of fracture aperture to natural stresses holds the potential for estimating properties, such as hydraulic conductivity, storativity and elastic modulus, without pumping tests. The primary contribution of this research is expected to be the development of a new method for determining mechanical and hydrologic characteristics of gently dipping fractures in crystalline rock. This will advance the ability to site productive wells, stimulate well productivity by hydraulic fracturing, and evaluate risks associated with contaminant migration.