PETROLEUM RESOURCES ARNOLD. 281 



although what has come in already may sometimes remain in the oil 

 to a greater or less extent. The appearance of '"edge" water in a 

 well is another matter, for here the oil has been permanently replaced 

 by the water, and, so far as the affected sand is concerned, the well 

 can be considered as no longer productive. " Edge " water sometimes 

 appears in a well in some particular sand, while other producing 

 sands are free from water. In this instance, the " edge" water sand 

 is abandoned and cased off and tlie production continued from the 

 ether sands. 



Most of the water troubles are due to a failure to shut off the " top " 

 water in the process of drilling. Wells, properties, and entire fields 

 have been seriously damaged or entirely ruined by the water, some- 

 times from only a few offending wells. This factor of water is, 

 therefore, one of the most potent in oil production and at the same 

 time the most uncertain, 



METHODS OF ESTIMATING FUTURE SUPPLY. 



Two methods of estimating the future production or supply of oil 

 in any area or field are in use, one known as the saturation method, 

 the other, the production curve method. 



SATURATION JIETHOD.^ 



The saturation method of computation involves finding the cubical 

 contents of the reservoir, determining the degree of porosity of the 

 volume, and then estimating the total, available, and net supply 

 of oil contained under the area in question. By total supply is 

 meant the total quantity of oil in the reservoir; by available supply 

 is meant the quantity that theoretically can be recovered with ordi- 

 nary methods in vogue; net supply is the quantity marketed after 

 deducting for fuel used in development and operation, leakage, and 

 other losses. 



Total supply depends on the volume and porosity of the reservoir 

 and on the volume of free gas and of water which are included in the 

 oil. The first factor usually can be approximated by taking the area 

 involved and multiplying by the average thickness of the oil sand 

 or zone. The porosity can be approximated from outcrop samples or 

 drilling samples of the reservoir. Gas and water contents are un- 

 certain, but in most instances can be disregarded for rough approxi- 

 mations. Gas usually is in solution and the water only in the out- 

 lying edges of the oil pools. Available saturation may range from 

 to, possibly, 80 per cent, depending largely on the gas pressure 

 and other factors, such as grain of reservoir, coherency, etc. From 

 40 to 60 per cent of the total quantity ordinarily is recoverable. Of 



1 This method Is described by Chester W. Washburne, Bull. A. I. M. E., No. 98, February, 

 1915, pp. 469-471. 



