Improving hydraulic aperture estimations to inform discrete fracture network models
Monday, September 23, 2019: 1:20 p.m.
Discrete fracture network numerical (DFN) models have long been available to simulate groundwater flow and transport in fractured rock, however these models are rarely applied at field sites because input fracture parameters are difficult to quantify. Because simulation results are strongly sensitive to fracture aperture there is need to decrease errors and uncertainty in aperture estimates. Hydraulic apertures derived using the cubic law using T values from straddle-packer tests are the most practicable way to obtain apertures. Over the last 10 years we have worked on improving our straddle-packer equipment design and test procedures to better understand the fluid mechanics of the water passing through the test equipment and in the fractures intersecting the tested interval. Insights concerning the nature of test conditions are obtained using a multiple-test approach to evaluate the inherent mathematical assumptions used to determine T, including validation of Darcian flow, cross-connection to the open borehole, and the steady flow assumption, to increase confidence in the calculated T values. However, further error in the calculated hydraulic aperture stems from uncertainty in the number of hydraulically active fractures in each test interval, and examples are provided showing how multiple datasets aid in the identification of the active fractures.