Poroelastic effects on fracture characterization.

Water-level fluctuations in aquifers and aquitards have been studied extensively in the context of pumping effects. Established knowledge includes the Noordbergum effect, where reverse fluctuations occur in aquitards due to delayed pressure responses. However, the Mandel-Cryer effect remains less understood in fractured rock systems. Prior research has shown that strain responses can precede pore-pressure changes in such settings. Yet, the specific role of poroelastic effects in fractured crystalline rocks has not been fully resolved. This gap motivated the investigation of how poroelastic behavior influences water-level responses during pumping tests. No prior work had resolved the interplay between fracture geometry and poromechanical properties in such systems. The study aimed to clarify whether traditional aquifer testing methods account for these effects accurately. This paper contributes to understanding how poroelasticity affects fracture characterization in crystalline rock.

The study aimed to explore the Mandel-Cryer effect in a fractured crystalline-rock aquifer at the Coles Hill uranium site in Virginia. Specifically, it sought to determine how poroelastic effects influence water-level responses during pumping tests. The researchers focused on a 24-hour controlled pumping test in which reverse water-level fluctuations were observed. Their goal was to assess the role of poromechanical properties in these fluctuations. They also aimed to compare traditional aquifer-testing methods with those that include poroelastic effects. The study's motivation stemmed from the observation that reverse fluctuations lasted longer and reached measurable magnitudes. This raised questions about the accuracy of conventional methods in capturing such effects. The researchers intended to use modeling to simulate the observed responses and evaluate poroelastic contributions.

The researchers conducted a 24-hour controlled pumping test at the Coles Hill site. They monitored water-level changes in a fractured crystalline-rock aquifer. An axisymmetric flow and deformation model was developed using Biot2 software. The model simulated water-level responses along a horizontal fracture with an assumed aperture of 0.5 to 1.0 cm. The fracture was located 176 m from the pumping well. The model incorporated poromechanical properties of the fractured host rock. The researchers compared results from models that included poroelastic effects with those that did not. This allowed them to assess the significance of poroelastic behavior in fracture characterization.

The study observed reverse water-level fluctuations lasting approximately 100 minutes. These fluctuations reached a magnitude of nearly 1 cm before typical drawdown occurred. The Mandel-Cryer effect was identified as the cause of these fluctuations. The response reflected the poromechanical properties of the fractured host rock. The hydraulic properties of the pumped fracture also influenced the observed behavior. The axisymmetric model successfully simulated the observed water-level responses. Traditional aquifer-testing methods produced results similar to those including poroelastic effects. However, poroelastic modeling enabled more accurate and efficient parameter calibration.

The study found that poroelastic effects significantly influence water-level responses in fractured crystalline-rock aquifers. The Mandel-Cryer effect was confirmed as a key mechanism in the observed reverse fluctuations. The observed duration and magnitude of the response were linked to the poromechanical properties of the rock. The hydraulic properties of the pumped fracture also played a role. Traditional aquifer-testing methods produced results comparable to those including poroelastic effects. However, poroelastic modeling improved the accuracy and efficiency of parameter calibration. The authors suggest that poroelastic effects should be considered in fracture characterization. They propose that such modeling enhances the interpretation of pumping test data in fractured rock systems.