Poroelastic Effects on Fracture Characterization



Reverse water-level fluctuations have been widely observed in aquitards or aquifers separated from a pumped confined aquifer (Noordbergum effect) immediately after the initiation of pumping. This same reverse fluctuation has been observed in a fractured crystalline-rock aquifer at the Coles Hill uranium site in Virginia in which the reverse water-level response occurs within a pumped fracture and results from an instantaneous strain response to pumping that precedes the pore-pressure response in observation wells of sufficient distance from the pumped well. This response is referred to as the Mandel-Cryer effect. The unique aspect of this water level rise during a controlled 24 h pumping test was that the reverse water levels lasted for approximately 100 min and reached a magnitude of nearly 1 cm prior to a typical drawdown response. The duration and magnitude of the response reflects the poromechanical properties of the fractured host rock and hydraulic properties of the pumped fracture. An axisymmetric flow and deformation model were developed using Biot2 in an effort to simulate the observed water-level response along an assumed 0.5 to 1.0 cm aperture horizontal fracture 176 m from the pumping well and to identify the importance of the poroelastic effect. Results indicate that traditional aquifer-testing methods that ignore the poromechanical response are not significantly different than results that include the response. However, the poroelastic effect allows for more accurate and efficient parameter calibration.