NONAQUEOUS PHASE HYDROCARBON IN A FINE-GRAINED SANDSTONE .1. COMPARISON BETWEEN MEASURED AND PREDICTED SATURATIONS AND MOBILITY

The distribution and mobility of light, nonaqueous phase hydrocarbons in the subsurface is poorly understood, particularly in relation to ob servations of hydrocarbons in monitoring wells. Recent work by Lenhard and Parker (1990) and Farr et al. (1990) resulted in an equilibrium m odel in which the degree of hydrocarbon saturation was shown to be a f unction of the apparent thickness of hydrocarbon in a monitoring well, the height above the oil/water interface, and the capillary pressure/ saturation characteristics of the soil. The current investigation comp ared measured hydrocarbon saturations, hydrocarbon volumes, and hydroc arbon mobility to those predicted by the capillary characteristic mode l of Farr et al. (1990) and Lenhard and Parker (1990). Five monitoring wells were completed in boreholes drilled in a hydrocarbon-contaminat ed area of downtown San Diego, where apparent hydrocarbon thickness va ried from zero to over 1.5 meters. Up to 2.5 meters of continuous core was obtained from each of the boreholes and split into 3 cm intervals . Alternating 3 cm samples were analyzed for total petroleum hydrocarb on, and the remaining samples were used for grain-size analysis and me asurement of capillary characteristic curves. Slug-withdrawal tests we re conducted on the hydrocarbon intervals, and constant-rate discharge tests were conducted on the water-saturated intervals of the monitori ng wells. The results show typical hydrocarbon saturations of 5% to 20 %, with one borehole showing a limited zone of up to 50% saturation. H ydrocarbon mobility (relative permeability) is greatly reduced due to the low saturations, as predicted by capillary theory. These observati ons are consistent with both the capillary model of hydrocarbon distri bution and with the very low recoveries achieved using hydrocarbon ski mming systems.