promise of enabling one to make a good estimate of the amount of permeable 

 zone in such a section, although the percentage porosity still must be obtained 

 by coring or estimation. If one has micrologs nearby, he can pretend that the 

 section will be the same elsewhere. If one has only electric logs, he just does his 

 best, remembering that assuming 50 percent of the reef to be porous seems to 

 be about the average used. 



Porosity in producing fields varies from 2 to 40 percent, with the average 

 being between 20 and 25 percent. It is a great variable. The porosity between 

 wells is not known, even though one may have measured it in cores from the 

 producing section of the wells. It is always assumed that the well bores are 

 representative of the reservoir. Performance of the wells themselves is often 

 some help — good, high-capacity wells are not made in thin, nonporous sections. 



Connate or interstitial water (denoted by / in the formula) is nonproducible 

 water contained in the reservoir rock along with the oil and/or gas. It is 

 thought that the formations, when laid down, contained sea water in the available 

 pore spaces and that the reservoir rocks retain this sea water (sometimes called 

 fossil water) until it was driven out by the migrating gas and oil. The gas or 

 oil displace most of the sea water, but not all of it — some of it is held by 

 capillary forces and remains as a coating of water around each sand grain. 

 Producing formations nearly always contain some interstitial water, which oc- 

 cupies part of the pore space and reduces the oil or gas reserve. 



The amount of connate water is extremely difficult to determine. Cores 

 are often hard to secure and are usually contaminated by drilling fluids. Only 

 rarely are cores taken with oil-base mud in order to prevent flushing of the core 

 by water from drilling mud. The "restored state" method gives close approxi- 

 mations by determining how much water a core should retain in the face of gas 

 or oil flushing. In this method, capillary equilibrium similar to that existing in 

 the reservoir is established in the laboratory within the core. Connate water can 

 often be estimated from a study of electric-log resistivities. Connate water varies 

 between and 60 percent; it will average 20 percent in good porous sands and 

 30 percent in tight, impermeable sands. Since it is a reducing factor, it is dealt 

 with as 1 — /; thus, 20 percent connate water is 1 — 20 percent, or a multiplier 

 of 0.80. 



The shrinkage factor is denoted by 5 in the formula. A barrel of reservoir 

 oil shrinks when brought to the lower temperature and pressure at the surface — 

 a barrel of reservoir oil being equal to only a fraction of a barrel of stock-tank 

 oil. This reduction in volume is due almost entirely to the gas coming out of 

 solution when the barrel of oil is brought to the surface — a barrel of oil with 

 hundreds of feet of gas in solution in it occupies more space than that same 

 barrel of oil with the gas removed. The more gas in solution, the greater is the 

 shrinkage; and the deeper the reservoir, the greater is the amount of gas in 

 solution — so that, in general, the deeper the well, the greater the shrinkage 



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