800 Subsurface Geologic Methods 



water determinations are available, estimates can be made based on known 

 determinations in similar formations in on-strike fields. Connate water 

 ranges from 10 to 50 percent of the pore space; and, in estimating, around 

 20 percent in good porous reservoirs and 30 percent in mediocre reservoirs 

 are fairly safe averages. 



In this field, the development has been more recent — all of the wells 

 have electrical logs. Cores from one nearby well were analyzed by the 

 restored-state method in the Texas Company's laboratory; some companies 

 do systematic core determinations on all of their own wells and sometimes 

 on adjacent wells. Three pieces of cores were analyzed on this well and 

 showed connate water content of 35, 32 and 36 percent. Comparison with 

 the electrical log indicates that the higher values came from the normal 

 parts of the sand and that the 32 percent was from the most porous part of 

 the sand. 



It is learned that the Shell Company has made connate water determ- 

 inations in their own laboratory on all the cores taken from their wells 

 on this structure. The valuator has a geologist friend at Shell, and while 

 he cannot see their core analysis reports, he is able to "swab" the informa- 

 tion that they are using 33 percent for connate water. Putting all of this 

 information together, he concludes that 34 percent is probably a safe 

 factor for the interstitial water. 



Fortunately, a nearby well logged the producing sand below the water 

 contact, so that the resistivity of this sand saturated with water {Rq factor) 

 is known. Thus, the Rg factor is known, and from the electrical log the 

 probable connate water content is computed as 35 percent — an amount 

 which appears to confirm the other data. 



With a porosity of 22 percent and connate water of 34 percent, ref- 

 erence to figure 413 shows that there are 1126 barrels of available pore 

 space per acre foot in the reservoir. All kinds of charts, graphs, and short 

 cuts are used by various reservoir engineers. This combination of porosity 

 and connate water is one of the more useful ones and saves much routine 

 calculation in both oil and gas problems. In the present instance, it en- 

 ables one to substitute "1126 barrels of pore space" for the P X (1 — I) 

 part of the formula. Therefore, 5776 acre feet X1126 barrels of available 

 pore space equals 6,503,776 barrels of space under the lease — or, in this 

 instance, 6,503,776 barrels of reservoir oil in place. 



The 5 or shrinkage factor is largely dependent on the amount of dis- 

 solved gas and the gravity of the oil. Gas-oil ratios are regularly deter- 

 mined by various state regulatory bodies and are almost always available. 

 The wells on this lease produce oil of 35° gravity with an average gas-oil 

 ratio of 510:1. Referring to published charts and curves, one finds that 

 the shrinkage factor (sometimes called formation volume factor) for a 

 510:1 ratio and 35° gravity oil will be 78 percent; or that a barrel of oil 

 in the reservoir is only equal to 78/100 of a barrel when it reaches the 

 stock tank and is ready to be sold. This is always a reducing factor but 



