Description
An investigation was undertaken to test the validity of the parameter Pd (entry pressure) as used by the Brooks-Corey method in predicting the distribution of fluids in porous media. Twelve artificial soils were created from San Diego County sediments. The sediments were composed of grain sizes ranging from that which would move through a number 3 U.S. Standard Sieve to that which was caught in the pan beneath a number 230 U.S. Standard Sieve. Combining the sediments in a number of grain size distribution schemes resulted in the twelve tested samples. Using volumetric pressure plates air-water capillary characteristics were determined for the twelve artificial soils. The investigation compared the Brooks-Corey oil-entry pressures, predicted by scaling the air-water capillary pressure data, to the measured pressure where oil commenced displacing water. Oil-entry into the water saturated soils was monitored in conjunction with the volume of water exiting the soil. Oil-water capillary curves were constructed from the resulting data. Both the Brooks-Corey and van Genuchen models were fitted to both the air-water and oil-water data. In all cases the Brooks-Corey method over predicted the pressure at which oil would first displace water from the porous media while the van Genuchen provided a more accurate description of the soil capillary characteristic in both the air-water and oil-water systems. The results suggest that because the parameter Pd is, in fact, a limit to the validity of the Brooks-Corey expression, application of the method where capillary pressures are less than Pd is incorrect. The results also suggest that there may be problems with the accepted method of scaling capillary pressures between fluid pairs using coefficients based upon interfacial tension pairs.
