EFFECTS OF CAPILLARITY ON OIL ACCUMULATION 799 



amount of pore space, and this is greater than in rocks which have 

 varying-sized and angular grains. Most rocks are made up of 

 particles irregular in shape and less than 2 mm. in diameter, con- 

 sequently the movement of underground water must be greatly 

 affected by capillary action, and evidently the forces of static capil- 

 larity must be overbalanced before movement can take place. For 

 that reason a discussion of Poiseuille's law of flow in capillary 

 tubes has been omitted, and the conditions of static capillarity 

 are thought to be of first importance. 



The phenomenon of capillarity — that of a column of liquid 

 rising or being depressed by a small opening — is due to two causes : 

 (1) the surface tension of the liquid, and (2) the fact that the mate- 

 rial of which the tube is composed has a greater or less adhesion 

 for the liquid than the cohesion of the liquid itself. 



Surface tension is the force at the surface of a liquid, which 

 tends to make the liquid contract, and can be expressed by the 

 following formula: 



a) T= ^hq^ 



2-rrr cos a 



where r equals the radius of the tube; h, the height of liquid 

 standing in the tube; q, the density of the liquid; g, the accelera- 

 tion of gravity; and a, the angle of contact between the liquid 

 and the tube. 



Surface tension is a linear function of the absolute temperature, 1 

 and that for water can be expressed by : 



b) T=o.2i(s7o-t) 2 



where / equals the temperature Centigrade. 



Pressure causes some change in surface tension, but presumably 

 small. ''For changes in the properties of water induced by pres- 

 sure of, say, 1,000 atmospheres are usually similar in magnitude 

 and direction to those observed when a relatively small quantity 

 of a salt is dissolved in it; and the surface tension of such dilute 

 (o . 5 N or less) solutions differs by only a small percentage from 

 that of pure water." 3 



1 Knipp, Physical Review, XI, 151. 



3 Johnston and Adams, Journal of Geology, XXII, 9. 3 Ibid. 



