Subsurface Laboratory Methods 311 



It should be noted that these saturations are based on the assumption that 

 no free gas will be produced from the sand face. The key to high gas-oil- 

 ratio production lies in the residual-oil saturations, assuming water will 

 not be produced. 



TABLE 17 

 Residual Oil Saturation of Oil Sands 



The connate-water saturations of oil sands vary appreciably with the 

 permeability and structural position of the well. Clean sands with per- 

 meabilities above 1,000 millidarcys may have connate- water saturations 

 less than 15 percent of the pore-space. 



The maximum connate water that an oil-bearing zone can contain 

 and not produce water varies with the effective permeability to each fluid 

 phase. The limiting or critical water saturation must be determined for 

 each sand in each reservoir. 



The critical oil-water ratio may be used as an index to the probable 

 production. This ratio varies for different sands, and no one ratio has 

 been determined that will apply to all formations. 



Bleeding Cores 



Since cores were first taken, the geologist has always been wary of 

 so-called bleeding cores or cores that show oil or gas seeping from the 

 pores for some time after the core is taken from the barrel. Many oil- 

 bearing sands have been condemned as probably water-productive because 

 of this bleeding. 



In the writer's opinion, cores bleed because the pressure-depletion 

 process is not complete by the time the core is taken from the barrel. 

 Usually bleeding cores are low in permeability, and thus the pressure de- 

 pletion process takes longer. Formations should not be condemned be- 

 cause of bleeding alone; core analyses can often explain this phenomenon. 



Oil-Saturated Limestone 



The analysis of limestone varies somewhat from that of sand or sand- 

 stone in that the permeability characteristic is not so critical, particularly 

 if commercial porosity exists. This discussion is limited to porous and 

 does not concern fractured limestone. 



Care must be exercised in selecting representative samples of lime- 

 stone for analysis because of abrupt changes in porosity. Cores from the 



