The effect of foam on the permeability of porous media to water was studied as a function of foaming agent concentration, specific permeability, pressure gradient, length of a porous medium and its oil saturation. At a given fluid saturation in a porous medium, the permeability to water was found to be the same whether foam was present or not. Foam decreases the permeability to water by developing a higher trapped gas saturation than that obtained by water flooding without foam present. Increasing the concentration of foaming agent increased the trapped gas saturation and thereby decreased the permeability to water. The presence of oil reduced the capability of most foaming agents to decrease the permeability of a porous medium to water. A few surfactants were found to be effective foaming agents even in the presence of oil. These results are similar to those reported in a previous paper on the effect of foam on the permeability of porous media to gas. The effect of foam was found to persist in long porous media at moderately high reservoir temperatures and during the passage of many pore volumes of surfactant-free water. Introduction This paper describes part of a study on. a novel approach in the use of surfactants for oil recovery; the use of foam rather than water to displace oil. Previously it was found that foam can displace oil which normally is not displaced by water. The foam is formed by successively injecting a suitable surfactant solution and gas into a porous medium. Foam appears to have at least two uses in the field:it shows promise as a superior oil recovery agent, andit shows promise as a selective permeability reducing agent. Foam may be very useful in water floods, or in other oil recovery processes, where highly permeable streaks or unfavorable mobility ratios are a problem. A previous paper reported the effect of foam on the permeability of porous media to gas. In the present study the effect of foam on the permeability of porous media to water is reported. The specific objectives of the study were to determine:the effect of foam on the permeability to water in porous media of various specific permeabilities,the effect of foam on the permeability to water in the presence of oil,the effect of foam and crude oil on the trapped gas saturation,the effect of foam on permeability to water at trapped gas saturation,the effect of pressure gradient on the permeability to water under foaming conditions,the persistence of foam during the passage of surfactant-free water through the porous medium, andthe effect of various foaming agents, length of the porous medium and temperature on the permeability reduction caused by foam. EXPERIMENTAL PROCEDURES EQUIPMENT AND MATERIALS The experimental apparatus consisted of consolidated and unconsolidated porous media, wet test meters and constant delivery pumps. The porous media consisted of consolidated sandstone cores (6 to 36 in. long), and unconsolidated sand packs (3 to 30 ft long). The consolidated cores had permeabilities of 32 and 1,000 md and porosities of about 20 per cent. The sand packs had permeabilities of 3,500 to 211,000 md and porosities of about 40 per cent. (Throughout this report a term such as "100 md sand" is used. This term means that the porous medium had a dry, nitrogen permeability of 100 md.)Fluids used in the experiments were distilled water, 1 per cent NaCl solution, aqueous solutions of foaming agents, nitrogen gas, air and crude oil. SPEJ P. 295ˆ