Subsurface Laboratory Methods 315 



tion, if the porosity and the connate-water data are correct, then the oil in 

 place may be readily calculated. O.I. P. = Pg [1 — 8^0) X 7,758 

 O.I. P. = Oil in place, barrels per acre-foot at reservoir condi- 

 tions, 

 Po = Porosity expressed as a decimal. 

 Sew — Connate-water saturation expressed as a decimal part of 

 the pore space. 

 7,758 = Volume of one acre-foot in barrels. 



The oil-in-place equation is based on the assumption that the reser- 

 voir pores are completely saturated at reservoir conditions. 



Maximum Recoverable Oil 



The volume of oil that may be recovered from a high-pressure reser- 

 voir by an effective water drive and gas expansion when the reservoir 

 pressure is maintained at or near the original saturation pressure is termed 

 the "maximum recoverable oil." 



The principles upon which this recovery calculation is based in- 

 volve the consideration of the flushing and pressure depletion that takes 

 place while cutting and recovering the core. As discussed elsewhere, the 

 core is subjected to water flood while being cut and to a pressure-depletion 

 process while being raised to the surface. These mechanisms occur in 

 reverse order in the normal productive life of the formation. 



The following equation is similar to the oil-in-place equation with 

 the exception of the correction for the residual oil. The residual oil is con- 

 verted into a volume at reservoir conditions and is subtracted from the oil 

 in place because there is no means of producing this oil.® 



M.R.= ^'^^~J-~^''"^ X 7,758 

 F.V.F. 



M.R. = Maximum recovery, stock-tank barrels per acre-foot of 

 formation. 

 Po = Porosity, expressed as a decimal part of the formation 



volume. 

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

 mal part of the pore space. 

 Sew — Connate-water saturation, expressed as a decimal part of 

 the pore space. 

 F.V.F. = Formation-volume factor, decimal expression of the vol- 

 ume that one barrel of stock-tank oil occupies at reser- 

 voir conditions. 

 Although this equation is accepted by most core analysts for calcu- 

 lating the maximum recoverable oil from high-pressure reservoirs, the 

 writer believes that the values obtained are sometimes much too high. 

 The critical factor in this equation is the residual oil, and seldom is it 



' Muskat, M., The Determination of Factors Affecting Reservoir Performance : Am Petroleum Inst. 

 Drilling and Production Practice, 1940. 



