UNDERGROUND MIGRATION OF OIL AND GAS 
163 
lary pull also varies with the surface tension of the fluid and is 
at once affected by any change in surface tension A water-air 
surface at ordinary temperatures has a tension of 70 to 75 dynes 
per square centimeter, and so far as water is concerned, all 
tubes of diameter less than 0.508 mm., and all smooth fissures 
less than 0.254 mm. wide are capillary. According to Wash- 
burne,'^ ^ A salty water, such as that commonly found in oil fields, 
having a density of 1.14 at 20°C., would have a surface tension 
of about 79 dynes per centimeter,” while Penns^dvania crude 
oil with a specific gravity of 0.852 displayed a surface tension of 
only 24.1 dynes per centimeter at 20°C. The same author is 
responsible also for the statement ^That all crude oils have low 
surface tension, probably in the neighborhood of 25 dynes, except 
only the oils that have lost all of their light constituents, which 
are of no consequence in problems of deep migration.” Because 
of their low surface tension, only those tubes with a diameter 
of less than about 0.25 mm., and fissures not over 0.13 mm. 
wide are capillary openings with respect to the average crude oil. 
Surface tension decreases with increase in temperature and 
becomes zero at the critical temperature of each substance; 
capillary action is therefore limited to the outer shell of the 
earth and cannot be operative at great depths where tempera- 
tures are high. ^'The effect of pressure on surface tension (and 
therefore upon capillary action) is unknown, but is presumably 
small.”® 
Finally in summing up these physical factors concerning capil- 
larity, it should be noted that although capillary action may 
draw fluids into small passageways within the rocks it cannot 
of itself induce continued flow through these passageways. 
Since it is an expression of surface tension, capillarity is opera- 
tive ‘^only when there is a free liquid surface within the capil- 
lary.” Once the capillary opening is filled with liquid, capillary 
action ceases, unless perchance another liquid of different sur- 
face tension approaches an opening of the tube. 
® C. W. Washbiirne, The capillary concentration of gas and oil, Trans. Am. 
Inst. Min. Eng., vol. 50, pp. 829-858, 1915. 
® John Johnson and L. H. Adams, Observations on the Daubree experiment 
and capillarity in relation to certain geological speculation, Journ. GeoL, vol. 22, 
pp. 1-15, 1914. 
