In porous media, there are a variety of configurations in which three
immiscible fluids can be distributed within a single body. For example
, the fluids may be distributed as concentric rings. Alternatively, tw
o fluids may be dispersed as separate blobs within the third fluid. Th
e configuration governs the mobility of each fluid, and under equilibr
ium conditions is dictated by the three phase capillary pressure of th
e system. If the capillary pressure is altered, the configuration will
change. Capillary pressure can be altered by flow of any phase throug
h the pore body. However, in a three phase system flow cannot be descr
ibed as simply ''drainage'' or ''imbibition.'' Rather, flow must be de
scribed as ''drainage/drainage,'' ''drainage/imbibition'' or ''imbibit
ion/imbibition'' to account for the change in saturation of all three
phases. This paper elaborates on the issues governing three phase capi
llary pressures and presents some first experimental results on oil/wa
ter/gas capillary pressures in a drainage/drainage mode.