798 Subsurface Geologic Methods 



tain some interstitial water, Avhich occupies 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 approximations by determining how much water a 

 core should retain in the face of gas or oil flushing. In this method, capil- 

 lary 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 electrical 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 — I; thus, 20 percent connate water is 1—20 percent, or a 

 multiplier of 0.80. 



The shrinkage factor is denoted by S 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 en- 

 tirely to the gas coming out of solution when the barrel of oil is brought 

 to the surface — a barrel of oil witli 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 factor. The shrinkage is 

 usually measured in the laboratory or estimated from the gas-oil ratio 

 of the wells and the gravity of the oil. This shrinkage is a substantial 

 reducing factor where ratios in excess of 2000:1 of solution gas are 

 encountered; the factor may be 50 percent or less. At great depths it 

 takes over two barrels of such reservoir oil to make one barrel of stock- 

 tank oil. 



The recovery factor is designated as F in the formula. Knowing how 

 much oil or gas is in the reservoir, how much of this is going to be 

 producible — how much will be left in the reservoir when the field is 

 abandoned? This is alivays an estimate; no one can know. The recovery 

 is influenced by many things: by the amount of gas in solution, the vis- 

 cosity of the oil, the rate of production, the primary expulsive energy, 

 the price of the oil, and a number of other things. Recovery factors vary 

 widely, but generally are within the following ranges: 20 to 40 percent 

 of the oil in place for dissolved-gas-drive fields, 30 to 70 percent for ex- 

 panding gas cap plus gravity, and 50 to 80 percent for fully eff^ective 

 water drive. 



A Hypothetical Case 



An example always best illustrates the actual workings of such a 

 formula, and aff"ords an opportunity to point out the dependence of a 



