The model developed under this contract is a modified version of the deep well disposal model developed by INTERCOMP Resource Development and Engineering, Inc., for the U.S. Geological Survey (A model for calculating effects of liquid waste disposal in deep saline aquifers). The model is a finite-difference numerical solution of the partial differential equations describing
(1) single phase fluid flow in aquifer,
(2) energy transport by convection and conduction, and
(3) contaminant transport dissolved in the fluid by convection and dispersion.
Both the energy and the contaminant transports include molecular diffusion and hydrodynamic dispersion.
The objective of the contract was to modify the original version of the model for more general applications. Some of the major features are as follows:
(1) Fluid density is permitted to be a function of fluid pressure, temperature and contaminant concentration, the viscosity can be described as a function of pressure and temperature.
(2) Aquifer heterogeneities in the hydrological properties can be described on a numerical grid block basis.
(3) Free water surface is permitted to exist in the aquifer for shallow ground water applications.
(4) Contaminant may absorb on rock surface or decay according to a first order reaction. The absorption is described by a linear adsorption isotherm function of rock type in the aquifer.
(5) The model is extremely flexible in providing a wide choice of boundary conditions. These include natural flow in the aquifer, aquifer influence functions around the perimeter of the grid in recognition that the gridded region does not have no-flow boundaries, heat losses into the overlying impermeable strata, and the wellbore heat and pressure drop calculations coupled to the aquifer flow equations.
(6) The model offers the option of selecting as iterative or direct solution technique, and selecting central or backward finite-difference approximations in both time and space.
(7) Virtually any aquifer can be modeled by proper grid block description in three dimensions. In addition, the model is fully transient.
The major limitation of the model arises using second-order correct (central-difference) finite-difference approximation in space. To avoid numerical oscillations in the solution, the user must restrict grid block and time step sizes depending upon the magnitude of the dispersivity.