708 Subsurface Geologic Methods 



of migration and accumulation of oil, of reservoir, and of well behavior 

 are explainable and understandable as a result of these permeability- 

 saturation relations. For example, oil free of water can be produced 

 from reservoir rocks containing considerable amounts of connate water 

 when the effective permeability of the rock to water is trivial at the par- 

 ticular saturation with respect to the wetting phase, water (fig. 378). Like- 

 wise, the gas-oil ratio of wells producing from a gas-from-solution reser- 

 voir is determined chiefly by the amount of natural gas in solution in the 

 oil at the prevailing reservoir pressure during the interval between dis- 

 covery and the time when the gas saturation in the rock is sufficient to 

 permit flow of free gas through it: i.e., that time and that oil and gas 

 saturations when the relative permeability to gas becomes significantly 

 greater than zero. Thereafter, as depletion proceeds, the gas-oil ratio is 

 determined both by the solubility of the gas in the oil and by the relative 

 permeability to gas saturation relation for the particular rock and oil. 

 Consequently, the gas-oil ratio from gas-from-solution drive fields de- 

 creases slowly with depletion for a time to some minimum value because 

 of the decrease in solubility of natural gas in oil with decreasing reservoir 

 pressure. Thereafter, the gas-oil ratio increases ever more and more 

 rapidly in accordance with the inexorable effective permeability-satura- 

 tion relationships for the nonwetting fluid phase (figs. 378, 379, 380, 381, 

 382, and 383) as depletion proceeds. As long as no other mechanism of 

 production is involved, more and more gas in required to produce less and 

 less oil. Consequently, the relative-permeability concept makes possible 

 the solution of many problems in the flow of multiphase fluids, especially 

 on the basis of steady state systems. Technical literature is offering more 

 and more examples of the use of such techniques.^^ 



Capillary Pressure 



Surface tension, interfacial tension, adhesion, and wetting — all are 

 manifestations of the difference in forces to which molecules are sub- 

 jected at the interface between two phases such as between gas and oil or 

 between oil and water. The best known of these manifestations of differ- 

 ences in force is the effect at the gas-liquid interface, which is known 

 best perhaps because it is easily observable. Any molecule, such as that 

 of a liquid, is always subject to the attractive force of all surrounding 

 molecules — those in the liquid and those in the gas. The molecule at the 

 interface between a liquid and a gas is pulled on one side by the liquid 

 and on the other by the gas. Usually the attractive forces differ in 

 magnitude, and the result is a difference in stress. The difference in stress 

 causes the molecules in the interfacial layer to behave something like an 

 elastic membrane in tension. The stress in the surface layer is measurable 

 and is known as the surface tension. Because of interfacial tension, a 

 pressure differential always obtains through the interface, the pressure 



^ Russell, W. L., op. cit. 



