On the vertical circulation wells in a leaky-confined aquifer

Abstract

Vertical circulation well (VCW) is a new type of groundwater heat pump (GWHP) system that is widely used in low-enthalpy geothermal energy utilization. As much attention has been given to the negative environmental impacts of GWHP, cross-formation flow induced by pumping or injection of hot or cold water in storage aquifer should be studied in advance. In this research, a conceptual model of cross-formation flow induced by a thermal VCW in a leaky-confined aquifer is developed and the mathematical implementation of this model is described in an analytical manner. A semi-analytical solution of the spatiotemporal distribution of hydraulic head in the coupled leaky-confined aquifer (in which the VCW is located) and the overlying unconfined aquifer (which is separated from the leaky-confined aquifer by a thin-layer aquitard) is derived using Laplace-Hankel transforms and the solution is tested against a finite-element numerical solution using COMSOL Multiphysics and an existing solution. The global sensitivity analysis is conducted to quantify and prioritize the hydraulic head response to the change in each of the aquifer parameters. The alteration of hydraulic head distribution in the overlying unconfined aquifer induced by the operation of VCW in the underlying leaky-confined aquifer can change the groundwater flow velocity and direction in the unconfined aquifer, thus may cause unwanted environmental consequences. The result indicated that hydraulic head is most sensitive to the hydraulic conductance (Cm) of the aquitard and most insensitive to the length of injection or pumping section of VCW. The proposed semi-analytical solutions can be used to improve the design of vertical single-well circulation system for groundwater treatment or a geothermal energy engineering. This is the phase I research of the thermal VCW concerning mostly the hydraulic response, and it serves as the basis for the phase II research concerning the thermal transport and environmental impact induced by a thermal VCW in the future.