In a conventional free-phase recovery system (with static gas phase), oil is trapped in the unsaturated zone when the air-oil table falls and oil is trapped in the saturated zone if the oil-water table rises. In a poorly planned recovery system, 70 percent of the NAPL may be trapped due to fluid tables fluctuations within the cone of depression or in areas beyond the radius of influence if the free phase plume is not contained. Vacuum enhanced recovery increases gradients in water and oil potentials with minimal fluctuations in the fluid tables. Thus vacuum enhanced recovery or bioslurping helps to reduce volume of residual product and enhanced free product recovery, thus reducing cleanup costs.
Groundwater contamination from hydrocarbon spills/leaks is a serious environmental problem. Nonaqueous phase liquids (NAPL) are immiscible fluids that have insignificant solubility in water. NAPLs in subsurface migrate under the influence of capillary, gravity, and buoyancy forces as a separate phase. Light NAPLs (LNAPLs) float and migrate on top of the water table posing a continuous source of contamination to the groundwater. Due to water table fluctuations, some of the NAPL gets trapped in the unsaturated and saturated zones. NAPL trapped in the soil and groundwater acts as a continuing source of groundwater contamination resulting in expensive restoration of these aquifers. MOVER optimizes recovery of LNAPL by vacuum enhanced recovery by increasing gradient in the oil potentials with minimum fluctuations in the fluid tables.
MOVER consists of:
The MOVER flow module invokes an assumption of near-equilibrium conditions in the vertical direction. This reduces the nonlinearity in the constitutive model and transforms a 3-D problem into a 2-D areal problem, thereby drastically reducing computational time for the simulation.
MOVER gives the initial distribution of NAPL specific volume in the domain for BIOF&T which models the aqueous phase transport, computes and updates the temporal and spatial variation in the source during the simulation.
This software is accompanied by a user-friendly pre-processor, Mesh Editor and post-processor. The pre-processor and Mesh Editor can be used to create input data files for MOVER. The pre-processor and Mesh Editor include modules for: mesh generation; allocating heterogeneous and anisotropic soil properties; defining fixed head, flux, source/sink boundary conditions for water, oil, and gas phases; and allocating spatially-variable recharge in the domain. Two-dimensional rectangular or isoparametric quadrilateral elements are permissible to accurately model irregular domain and material boundaries.
Required input for flow analyses consists of initial air-oil table, air-water table distribution, soil hydraulic properties, fluid properties, time integration parameters, boundary conditions and mesh parameters. The van Genuchten constitutive model, along with fluid scaling parameters, is used to compute water and oil phase volumes.
MOVER output includes a list of the input parameters, initial and boundary conditions, and the mesh connectivity. It also includes simulated water, oil, and gas phase pressures, water, oil, and gas phase velocities at each node, total volume of water and oil versus time, and water and oil recovery/injection rates for each sink/source location versus time. Volume of free oil, and residual oil, and their spatial distributions are also printed versus time. Flow simulations can be performed in stages. MOVER creates an auxiliary file at the end of the current stage that can be used to define initial conditions for the next stage.
The sequence of the input parameters and their definitions have been furnished in Appendix D of the document. This section explains the procedure for spatial discretization and mesh generation, defining initial conditions, boundary conditions, and the maximum permissible array dimensions.
Spatial Discretization and Mesh Generation
The MOVER modules allow use of rectangular and isoparametric elements. The element size and shape can be changed to obtain mesh refinement that are necessary to obtain accurate results.
Initial Conditions for Flow
Initial head distribution in the domain for water and oil can be specified by:
1) A bilinear interpolation with heads defined on the left and right boundaries
2) A non-uniform head distribution defined by fluid levels in the monitoring wells
Specified pressure head (type-1) boundary conditions can be defined at selected nodes versus time.
Type-2 (specified flux) and source/sink boundary conditions can be defined by specifying the volumetric rate [L3 T -1] versus time for respective nodes. For a type-2 boundary condition, when flux [L T -1] is known at a node, the user should multiply flux with the area represented by the node in a plane perpendicular to the flux.MOVER WINDOWS INTERFACE What is the MOVER pre-processor? The Windows pre-processor for MOVER is designed to help users create and edit input files for the MOVER numerical model. The pre-processor works in concert with the Mesh Editor to allow users to assign boundary condition schedules, soil types, recharge zones, etc., to the finite element mesh used in the MOVER numerical model. The pre-processor contains all control parameters that determine model run options, initial conditions, monitoring well information, fluid properties, boundary schedule data and soil type definitions as well as serving as a binder for Mesh Editor files. The pre-processor also contains a module for writing input files for the MOVER numerical model and for actually running the numerical model. Using the MOVER pre-processor The MOVER pre-processor runs under Windows 3.X, Windows 95 and Windows NT. The pre-processor uses the familiar tabbed notebook interface to allow quick editing of input files. The main program has two sets of tabs, one along the bottom which separates major sections of the interface, and, on some of the large notebook pages, tabs along the top that separate subsections to make the most use of available screen space. For example, clicking on the bottom tab "Boundary Schedules" takes the user to the boundary schedule notebook. Here there is a tabbed notebook for editing type 1 and type 2 boundary condition schedules.
The Tools selection on the main menu opens a tabbed notebook which includes Cue Cards, Files used in the pre-processor, a numerical model Runner, and a place to determine the location of the Mesh Editor. The files listed in the pre-processor are used to store variables and retrieve data and are generated automatically by the pre-processor and the Mesh Editor.MOVER data files All the MOVER variables for the numerical file are stored in ASCII text files that resemble Windows .ini files. These files are read and written by the MOVER pre-processor and Mesh Editor. They can be interchanged in the pre-processor setup window. For example, a material property file used in an earlier project can be assigned to a new project and all those soils will be available in the new project. A mesh file and all its associated files can be imported in the same manner. The data files can be edited with any ASCII text editor although this is not necessary. This open architecture was designed for future expansions of all DAEM models, or for third-party development of graphical interface tools. What is the Mesh Editor? The Mesh Editor was designed to work with these numerical models to create and edit finite element meshes. The Mesh Editor allows designing irregular quadrilateral meshes in two and three dimensions. Working with a numerical model pre-processor, the Mesh Editor provides a graphical interface for assigning properties to a mesh such as initial concentrations of contaminants, soil properties, boundary conditions, etc. Speed Buttons Pan
Panning allows the mesh to be moved in the Mesh Editor window. This puts the Mesh Editor into its "pan" state. You can then hold down the left mouse button and move the mesh on the screen. Letting go of the left mouse button "drops" the mesh in place. Nodes cannot be selected individually in this state.
The rotate button is the leftmost red button on the tool bar. It puts the Mesh Editor into a "rotate" state where the mouse can be used to rotate the mesh in three dimensions. The mouse will rotate the mesh on the XYZ axis intersection. Rotating the Mesh Editor takes a little getting used to but if you ever get lost, the handy X-Y, Y-Z, and Z-X buttons will snap the mesh back into place.
The rightmost red button called Node Control puts the Mesh Editor in its "editing" state. Now a node or group of nodes can be selected to have values assigned to them, or to be moved in the X, Y and/or Z direction. To edit associations (i.e., soil type, recharge zones, type-1 boundary conditions, etc.) for a node, select the node, then right click the mouse. This will bring up a list of available associations. Clicking on an association will bring up a secondary list of associations that can be assigned to the node.Moving Nodes Nodes can be moved by holding down the Ctrl key (control) and the left mouse button, then moving the cursor on the screen. Nodes move according to the dimension displayed on the screen, so that two dimensional meshes should be in the default X-Y view for node movement. Nodes can only be moved when the Mesh Editor is in its "editing" state. DXF Import Version 1.1 of the DAEM Mesh Editor introduced DXF import. This tool allows for .dxf files to be placed on a mesh. This way, site files in CAD programs can be exported to the Mesh Editor, then used to aid mesh refinement and adjustment.
Post-processor The post-processor is a data parsing tool, graphing package and contour export tool for these numerical models. The post-processor is designed to be a user-friendly tool for quickly discerning model results. Users of these models can also review model text output files for a more detailed view of model results.
This example is to test the accuracy of MOVER to simulate an LNAPL leak into an unconfined aquifer. The domain is 40 m x 40 m, discretized with uniformly spaced 21 rows and 21 columns (x = 2 m, y = 2 m). A leak occurred at the center of the domain (x = 20 m, y = 20 m) at a rate of 1 m 3/day for 20 days. Simulation was performed with an initial time step of 0.001 days, and the time incremental factor and maximum time step size were 1.03 and 0.2 days, respectively. The initial uniform piezometric head (Pao = Paw ) was 100 m throughout the domain. All boundaries were no-flow boundaries for water and oil phase. The water head at the out boundary was fixed at 100m throughout the simulation.
The simulated specific oil volume distribution is shown in the following figure along with the corresponding results from MOFAT. There are some differences in these solutions due to the difference in the formulations of the MOVER and MOFAT models. MOVER is a vertically-integrated areal model while MOFAT is a 2-D vertical slice (Planar or radially-symmetric vertical section) model. Nevertheless, it can be seen in the following figure that both solutions agree reasonably well over the range of the simulation.
Comparison of MOVER with MOFAT
MOVER Requirements: (1,500 nodes) Windows 3.x/Windows95/Windows NT and 8 MB RAM.