316 Subsurface Geologic Methods 



possible under field conditions to reduce the residual oil to as low a point 

 as that measured in cores in the laboratory. 



It should be noted that the permeability factor does not enter in this 

 equation. This factor is definitely reflected in the length of time required 

 to recover the oil. Actually it is the capacity (permeability in millidarcys 

 times thickness in feet) and the pressure that control the flow rate. The 

 higher the capacity, the more rapid the rate of depletion. 



Solution-Gas-Expansion Recovery 



Recovery by solution-gas expansion is usually the initial type of re- 

 covery from most reservoirs. The gas in solution in the oil expands when 

 a pressure differential is created in the reservoir by producing methods, 

 and oil is forced into the well bore. This type of recovery can occur in 

 water-drive reservoirs when the rate of withdrawal exceeds the rate of 

 water encroachment and a sufficient drop in pressure occurs in the forma- 

 tion surrounding the well bore to release the gas in solution. 



Solution-gas-type reservoirs should never be produced at a rate that 

 will cause free gas to break out of solution in the formation. This free 

 gas restricts the flow of oil and results in high gas-oil-ratio production. 



Reservoirs operating under a solution-gas drive may be expected to 

 produce from 10 to 40 percent of the original oil in place. ^ 



Several methods are available for the calculation of gas-expansion 

 recoveries. One method that employs the basic core-analysis data has been 

 proposed by Johnston ^ and has been used with some eff^ectiveness in fields 

 where it was possible to determine the correlation factor. The correlation 

 factor used for certain California fields was 0.65. The following equation 

 has for its basis the assumption that the gas-filled space in the core at the 

 surface is very nearly the same as that which would result from the normal 

 pressure depletion of a dissolved-gas reservoir. 



G.E.R. = ^"^^"f;;"'^''"^ X 7 J 58 X C.F. 

 r.V.F. 



G.E.R. = Gas-expansion recovery, stock-tank barrels per acre-foot 

 of formation. 

 Po — Porosity, expressed as a decimal. 

 Stw — Total water saturation, expressed as a decimal part of 



the pore space. 

 Sro = Residual oil at reservoir conditions, expressed as a deci- 

 mal part of the pore space. 

 C.F. = Correlation factor, expressed as a decimal. 

 The gas-expansion-recovery equation has been used without the corre- 



' Buckley, S. E., and Craze, R. C, The Development and Control of Oil Reservoirs : Am. Petroleum 

 Inst., Drilling and Production Practice, 1943. 



"Johnston, Norris, Core Analysis Interpretation : Am. Petroleum Inst., Drilling and Production Prac- 

 tice, 1941. 



