is known. Thus, the R factor is known, and from the electric 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, reference 

 to Figure 40-1 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 enables one to substitute 1126 

 barrels of pore space for the P X (1 — /) part of the formula. Therefore, 5776 

 acre-feet X 1126 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 S or shrinkage factor is largely dependent on the amount of dissolved 

 gas and the gravity of the oil. Gas-oil ratios are regularly determined 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 at shallower depths and with under-saturated crudes, it is 

 not too large — at great depth with large amounts of dissolved gas, the neglect of 

 this factor gives greatly exaggerated reserves. 



The last item in the formula is F for recovery factor, which is the per- 

 centage of the inplace oil that the estimator believes is recoverable. It is always 

 an estimate, and all too often just an off-hand guess. It is one of the greatest 

 sources of errors in estimates. Many things go into a recovery factor, and lots 

 of them are not found in the science books. 



Chief among the scientific factors affecting recovery is the type of explusive 

 energy that is bringing the oil to the bore hole, and perhaps removing the oil 

 from other leases. If there is a fully effective water drive, the lease high on the 

 structure will have a very high recovery factor — usually recovering more oil 

 than could be calculated to be under it. This fact is due to the Law of Capture ; 

 the oil belongs not to the lease owner under whose property it is discovered, but 

 to the well owner who produces it (reduces it to capture). Courts are making 

 some feeble efforts to get away from the Law of Capture, but it is still very 

 strong in most jurisdictions and must always be considered in every estimate of 

 recovery. The workings of the Law of Capture are intimately related to the 

 subsurface geology of the property — and afford the geologically minded operator 

 many opportunities to take advantage of, or to protect himself from, natural 

 drainage. 



802 



