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The AAPG/Datapages Combined Publications Database
Pacific Section of AAPG
Abstract
Laboratory Investigation of the Relationship Between Actual and Apparent Product Thickness in Sands
Abstract
Many people assume that apparent petroleum product thicknesses measured in monitoring wells can be used to determine product volume in the ground. To this end, several published equations relate the apparent thickness to the actual thickness of the layer of mobile product in a contaminated aquifer. Our experiment evaluated these equations.
A large plexiglass tank filled with different sands, water and dyed kerosene was studied photographically. The tank is 4 feet (122 cm) tall, 4 feet (122 cm) wide and 1 foot (30.5 cm) thick. Three well screens with 2, 4, and 6-inch (5, 10, and 15 cm) diameters which have been cut in half lengthwise were attached to one side. The tank was filled in turn with two different medium sands, a well-graded local sand and a screened silica sand. Water levels in the sands were controlled by adjusting the water levels in tanks at each end. The water levels were usually held constant during the experiments, but were lowered and raised to simulate a water table aquifer. Measured volumes of dyed kerosene were added, and actual and apparent product thicknesses were recorded periodically. The pictures taken of the experiments illustrate the physical processes involved as well as the results.
Several conclusions have been reached. Apparent product thickness greatly exceeded the actual thickness. Attempts to relate these quantities by subtracting the capillary fringe thickness as suggested by Hall, Blake and Champlin (1984) and Schiegg (1985) or by dividing by a constant factor (usually around 4) as suggested by CONCAWE (1979) were unsatisfactory. Using PC software by Parker based on multiphase saturation-pressure relationships by Parker and Lenhard appears to be the best choice. In general, however, the equilibrium apparent thickness exceeded the actual thickness more than was predicted by these equations. No simple relationship between real and apparent thicknesses always works. Hence, other methods of estimating actual product thickness should be pursued. Thicknesses in three different diameter monitoring wells varied greatly, but eventually reached equilibrium. This may not occur in the field where wells are farther apart.
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