The Invasion Percolation in a Gradient (IPG) model is capable of incorporating all forces ― viscous (in both the wetting and nonwetting fluids), buoyancy, and capillary forces ― relevant to geologic problems of two-phase flow in a porous medium. Calibration of the IPG model to a pre-existing data set of two-phase flow experiments on glass bead packs shows that the characteristic throat radius R_t is about 10% of the site radius R_s, assuming R_s～R_g, the grain size. Two-dimensional model results of the fractal dimension D and the invasion probability of a throat on the interface are in excellent agreement with predictions for limiting cases of Capillary number Ca and Bond number Bo. In moving from two- to three-dimensional media, trapped wetting clusters do not scale with the extent of the invading cluster, remaining under 5 to 8 pores in size.
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