EFFECTS OF CAPILLARITY ON OIL ACCUMULATION 585 



capillary pressures of oil and water for that size of opening." It 

 would be only (a) the pressure exerted by the water-oil surfaces 

 beneath the oil, less (b) the pressure of the oil-gas surfaces on 

 the hemiglobules of oil that are being forced out of the 

 small capillaries into larger spaces. Since (a) and (b) are of 

 similar magnitude, their difference, amounting only to a frac- 

 tion of an atmosphere, cannot be considered a cause of rock 

 deformation. 



In other words, Mr. McCoy takes a wrong basis for his calcula- 

 tion of capillary deforming force, and this nullifies the calculation 

 at the end of his paper. 



Thirdly, the amount of capillary pressure exerted in a group 

 of tubes of variable size and having many lateral connections, as in 

 rock, cannot exceed the pressure in the largest of these tubes plus 

 the head required for an adjusting flow of gas or liquid through the 

 lateral connections from the finer to the larger pores. If shale is 

 cut by minute open joints or fissures, the low capillary pressures in 

 the latter must limit the effective capillary pressure per square 

 inch in all connecting pores. 



Pores that are completely closed above the point of the highest 

 liquid-gas surface within them would feel the complete pressure 

 produced by that surface. Capillary action in completely inclosed 

 pores would not contribute to the general pressure or affect the 

 problem under discussion. In all connected pores, the general 

 capillary pressure could not rise appreciably above the mini- 

 mum capillary pressure, determined in the largest pores or 

 joints. 



I believe that the second and third arguments reduce the prob- 

 able effective capillary pressure, under the conditions of Mr. Mc- 

 Coy's problem, to a maximum of one or two atmospheres, if the 

 rock is cut by any minute open joints or large pores. Certainly 

 the pressure of 200 atmospheres, which he deduces, is out of the 

 question as a general capillary pressure in rock. 



Fourthly, there is no definite orientation to capillary pressures 

 in rock-pores. They push and pull every way and tend to balance 

 each other. The ideal distribution of water, gas, and oil assumed 

 by Mr. McCoy would cause a small capillary pressure of definite 



