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Introduction to OILVOL
OILVOL Technical Information
Estimation of Spill Volume in OILVOL
OILVOL Pre-/Post-Processor
OILVOL Requirements


OILVOL estimates the free hydrocarbon volume in a soil and computes the volume of residual NAPL in the saturated and unsaturated zones. To obtain these values, the user enters the depth to air-oil and oil-water interfaces measured in monitoring wells, van Genuchten soil moisture retention parameters (which can be estimated with SOILPARA) and fluid properties. Three-phase constitutive relations are used to obtain the vertical distribution of fluid saturations which are integrated in OILVOL to compute the free product (true product) volume in the soil. OILVOL can also estimate the residual oil volume in the unsaturated and saturated zones after oil skimming. When fluctuations in the fluid tables (air-oil, oil-water) are high, as those resulting from the operation of water and oil recovery wells, and/or when regional ground-water gradients are high and hydrogeology is complex, we recommend the use of the MARS model which estimates transient areal migration, LNAPL/water recovery, and free and trapped NAPL volumes in the vadose/saturated zones.

  • Estimate free hydrocarbon volume from fluid level measurements in monitoring wells.
  • Compute volume of residual NAPL in the saturated and unsaturated zones.


  • Domain size.
  • Monitoring well locations.
  • Air-oil and oil-water interface depths in each monitoring well.
  • Hydrocarbon fluid properties.
  • Soil properties.


  • Total free product.
  • Total residual oil in the saturated and unsaturated zones.
  • Distribution throughout the modeling domain.
  • Contouring interface with SURFER for total oil volume and product thickness distribution.
  • Oil distribution files for aqueous phase transport with BIOF&T.


OILVOL is a computer program for estimating free hydrocarbon volume in the soil following a spill. The elevations of the air-oil table and the oil-water table measured in a network of monitoring wells are used to compute free hydrocarbon volume at the time fluid level observations are taken.

OILVOL also computes the volume of hydrocarbon that can be recovered with skimmers and the residual product volume that is trapped in the unsaturated and saturated zones when the air-oil table falls and the oil-water table rises during skimming, respectively. These estimates are based on the assumption that the lateral migration of the plume is contained and insignificant.

OILVOL divides the problem domain into equally-spaced rectangular grids. The difference between fluid level elevations gives the product thickness at the monitoring well locations, and these are krigged to obtain the product thickness at each node in the rectangular grid of the domain. Using the three-phase constitutive relation between phase saturation and pressure, the vertical distribution of water and oil saturation is computed. Integration of the oil saturation with depth gives the specific oil volume (volume of free oil per unit surface area). The specific oil volume at each grid node is multiplied by the respective nodal surface area to compute the volume of free hydrocarbon at the node, and these are summed to obtain the total free product volume (before skimming) in the soil.


The actual volume of free hydrocarbon per unit area in the soil (true product thickness or specific oil volume) is less than the observed hydrocarbon thickness in the monitoring well. Farr et al. (1990) and Lenhard and Parker (1990) independently reported theoretical methods for estimating specific oil volume from a measured monitoring well thickness. This technique is based on the assumption of vertical equilibrium phase pressure distribution. Fluid table elevations are used to get the vertical distribution of water and oil pressures. Three phase constitutive relations (described in the "More Information" document available below in pdf format) are used to obtain the vertical distribution of water and oil phase saturations.


What is the OILVOL Pre-processor/Post-processor?

OILVOL is a program for estimating oil volume in a network of monitoring wells. The pre-processor/post-processor for OILVOL is an interface to allow users quick access to input parameters and monitoring well data for the numerical model. The processor also automates writing of the input data file for the numerical model and post-processing the text output file.

Using the Pre-processor/Post-processor

OILVOL's interface is a single tabbed notebook designed for easy parameter and data entry. The interface has a simple toolbar across the top of the notebook with speed buttons for opening and saving project files, running the numerical model, viewing output files, and exiting. The main menu along the top of the form has additional options for opening a monitoring well database, or changing the project name. There are also selections for quick viewing of full data files and output files.

Well Data

The first page of the OILVOL tabbed notebook is called Well Data. This page contains monitoring well Name, X location on the domain, Y location on the domain, DAO (depth to the air-oil interface), and DOW (depth to the oil-water interface). X and Y locations for monitoring wells must be converted to the modeling domain where the modeling domain always has a minimum X and minimum Y of zero. The modeling domain is the area of the mesh used for calculation (see Mesh Section below).


Easy monitoring well data entry

The OILVOL numerical model requires all data in the monitoring well database be filled in. X and Y coordinates must be within mesh coordinates defined on the Mesh page. DAO and DOW must be entered even if LNAPL thickness in a well is zero.

Saving Files

There are two files associated with an OILVOL project. The first is the project file with the extension .prj. This file contains parameter data, project title and a link to a monitoring well database. The monitoring well database is an ASCII text file with monitoring well name, X and Y location, DAO and DOW.

Parameter Entry


Soil and fluid parameters are entered on the Parameter Entry page. Each entry box needs to contain at least one value. These parameters will be saved in the OILVOL project file.

Sor is the maximum residual oil saturation for the soil.

Sog is the maximum allowable residual oil saturation in the unsaturated zone.

Sm is the residual water saturation in the soil.

Alpha and n are van Genuchten parameters determined for this soil type.

Bao is the air-oil phase scaling parameter.

Bow is the oil-water phase scaling parameter.


The mesh in OILVOL can have up to 100 columns (number of nodes in the X direction) and 100 rows (number of nodes in the Y direction). Calculations for OILVOL are performed over the rectangular elements. The finer the grid used on the modeling domain, the more accurate the output data will be. The Delta X and Delta Y define the spacing of the mesh.


The Results page prints total free product on the site, total residual product in the unsaturated zone and the total residual oil in the saturated zone.

OILVOL Requirements: Windows 3.x/Windows 95/Windows NT and 4 MB RAM.