Field evaluation of the hydromechanical behavior of flat-lying fractures during slug tests

Slug tests were conducted at depths of 20–50 m in a fractured gneiss and axial displacements were measured along the borehole with a portable extensometer in order to evaluate the feasibility of combining pressure and displacement measurements to improve the characterization of fractured rock. Displacements on the order of microns occurred during slug tests when maximum head changes were on the order of meters. Maximum displacement lagged behind the maximum head, and the signals at a given location were repeatable to within reasonable tolerances. Tests were conducted at 12 packed-off depth intervals in a borehole and the signals from each interval were distinctly different from those at other depths. Parameter estimation techniques were used with a hydromechanical model to interpret the field results, and the findings show that there are three transmissive zones in the borehole and the compliance of the fractures generally decreases with depth. The inversion also highlights the importance of considering heterogeneities in the vicinity of the well bore when interpreting hydromechanical tests. Zones of leakage and blockage in the fractures were predicted in the vicinity of the well bore, and their presence had a significant effect on the displacement signals.