Miscellaneous Subsurface Methods 709 



being greater on the concave side — i.e., the nonwetting side — by an amount 

 in accordance with the following equation: 



Apc = y{~ + ~) (16) 



ri rz 



in which the terms have the following significance: 

 Apo — difference in pressure, psi 

 y = interfacial (surface) tension, pounds per inch 

 ri and r2 = radii of curvature in any two mutually perpendicular 

 directions. This difference in pressure is inversely proportional to the 

 mean radius of curvature of the interface and may be either negative or 

 positive. The algebraic sign simply indicates that fluid phase in which 

 the pressure is lower, this phase always being that which wets the surface 

 of the rock. Furthermore, every student of science knows that if one end 

 of a glass capillary tube is under water and the other is open to the air, 

 water rises in the tube to a height above the free-water level so that at 

 constant temperature 



1 1 y 



^Pc= iPio- Pa) h = y {— + —) -2~ (17) 



Ti r2 r 



in which the symbols have the following meaning: 

 Apc = capillary pressure, psi 



y = interfacial tension between fluids, lbs. per foot 

 Pn= density of the water, pounds per cubic foot 

 Pa = density of the air, pounds per cubic foot 



ri and r^ = radii of curvature of two mutually perpendicular planes 

 h = height of water column in the tube, feet 

 r = mean radius of curvature 



The pressure difference across the interface in a capillary system fre- 

 quently is called "capillary pressure." The capillary pressure caused 

 by the surface tension of pure water in a glass capillary tube 1.0 X 10"^ 

 inches in diameter is about 17 psi, which is equivalent to a capillary 

 rise of about 40 feet. Similar effects are observed with systems composed 

 of liquids and porous media modified only in degree, and not in kind, 

 by the increased complexity of the geometry of such systems. 



When water is in contact with another immiscible liquid — -for ex- 

 ample, oil — capillary pressure is developed as a result of the water 

 striving to maintain a surface of minimum area, just as water does in 

 the presence of a gas. The variation of grain size, bonding material, and 

 mineral composition, particularly in the smaller grains such as clay 

 and shale that may be in the pores of sedimentary rocks and between 

 larger grains, aff'ects the magnitude of the capillary pressure in a par- 

 ticular rock. Furthermore, the capillary pressure of a particular rock 

 at any horizontal plane varies with the distance above the free-water 



