Finite-difference grid for a doublet well in an anisotropic aquifer
The U.S. Geological Survey is modeling hydraulic flow and thermal-energy transport at a two-well injection/ withdrawal system in St. Paul, Minnesota. The design of the finite-difference model grid for the doublet-well system is complicated because the aquifer is anisotropic and the principal axes of transmissivity are not aligned with the axis between the two wells. An analytical solution for flow in a doublet-well system in an infinite anisotropic aquifer was employed in the design of a grid with artificial boundaries placed in the midst of the flow field. Flow-net analysis was used to determine water flux across an cquipotential boundary and to assign approximate flux values at model boundaries. This enabled the simulation of the effects of the entire flow field, although only a small part was modeled. The validity of the flux values at the model boundaries for the isothermal case was tested by simulation of an eight-day injection test of ambient-temperature water. Model-computed pressures compared very favorably with field-observed pressures. The validity of boundary-flux values also was tested for nonisothermal conditions by simulation of injection of 300o F water at 300 gallons per minute for eight days.