802 A. W. McCOY 



Capillary pressure of 300 atmospheres means that water will 

 enter the pore spaces above static water level until the pressure 

 in the pore tubes, due to the weight of the column of liquid above 

 or otherwise, is equivalent to 300 atmospheres pressure; or that, 

 if the water is held back by a gas or liquid of less surface tension, it 

 will accumulate a pressure in the said gas or liquid proportional 

 to the difference in capillary pressures for that temperature and 

 size of opening. 



The following assumptions have been made for a hypothetical 

 problem: (1) there exists a cavity or series of connected open- 

 ings, larger than 0.5 mm., under a strip of rock 10,000 ft. wide 

 and 1,000 ft. thick. The openings in the rock above are as 

 small as 0.01 micron, and filled with water; (2) the material 

 below the cavity is an oil shale in which the openings are 

 0.01 micron, and that water is in the lower part of this shale 

 under sufficient head to make it rise to the level of the bottom 

 of the cavity. 



The water will drive the oil into the open cavity with a pressure 

 equal to the difference in the capillary pressures of oil and water 

 for that size of opening. This amount for the given temperature 

 of 1 5 C. and openings of 0.0 1 micron is approximately 200 atmos- 

 pheres, or about 400,000 lb. per sq. ft. The weight of the rock 

 column above is approximately 150,000 lb. per sq. ft.; and that of 

 the full water column would be less than 62,000 lb., because the 

 column cannot possibly act upon a full square foot, but only upon 

 the area of pore space, for convenience say 50,000 lb. Now the 

 resultant pressure upon the rock above the cavity is 400,000 minus 

 (150,000 plus 50,000), or 200,000 lb. per sq. ft. 



This pressure acts as upon a beam fixed at both ends. The 

 capillary water above prevents the rising of the oil into the rock, 

 but in turn affords no downward pressure on the oil in the opening, 

 other than the weight of the hydrostatic column, as has been 

 accounted for in the above assumptions. 



The deflection for a beam fixed at both ends with a uniform 



load may be expressed by the following formula : 



, wi 4 

 d = — 



■ 3 84£I' 



