Ch. 32] PRODUCTION BY SOLUTION GAS DRIVE 581 



theory and experiment, water should be present, for during its accumu- 

 lation the oil must have had to displace water from the pores, and a 

 certain fraction of this water is necessarily retained by capillary forces 

 or otherwise trapped. Later work has shown that the water satura- 

 tion is always appreciable and sometimes amounts to half or more 

 of the pore volume. The acceptance of the presence of water in the 

 sand caused a profound change in geological thinking. It had previ- 

 ously been thought that the oil occupied all the pore volume of the 

 sand, of which only a small fraction was being recovered. If between 

 y 10 and Y 2 of the pore volume is occupied by water, the recovery 

 efficiency is somewhat greater. 



Production by Solution Gas Drive 



When the formation is first penetrated by the drill, the fluids are 

 found to be under normal hydrostatic pressure, that is, a pressure ap- 

 proximately equal to that of a column of salt water extending to the 

 surface. The withdrawal of oil or gas causes immediately a zone of 

 lower pressure among the fluids in the vicinity of the well bore. If 

 the sand near the well contains mostly oil, its effective permeability 

 to oil is high, and the effective permeability to water very low. Con- 

 sequently the flowing stream consists entirely of oil, containing gas in 

 solution. As the oil rises in the well toward the surface, its pressure 

 decreases, and the gas comes out of solution. The gas bubbles expand 

 and lighten the column, so that a froth of oil and gas is expelled at the 

 surface, sometimes with great violence. 



The low-pressure zone around the well bore rapidly expands, and, 

 after a certain amount of oil has been withdrawn, the pressure in the 

 fluids for a considerable distance radially from the well is less than 

 the saturation pressure of the gas. When this occurs, the gas comes 

 out of solution in the pores of the sand. At first the small bubbles 

 move the oil, but later they coalesce and move as a separate flowing 

 phase toward the well. This process has a very deleterious effect on 

 the productivity of the well. The effective permeability of the sand 

 to gas increases rapidly while that to oil decreases, and consequently 

 the gas production of the well increases and the oil production de- 

 creases. The production of fluids has been largely brought about by 

 the pressure gradient maintained by the expansion of the gas originally 

 dissolved in the oil. When this gas is able to travel freely to the well 

 bore, driving only small quantities of oil before it, the efficiency of 

 the recovery mechanism falls off very rapidly. Furthermore, the 

 amount of gas is limited, so that eventually its flow decreases also. 

 The loss of gas from solution causes a volume shrinkage of the oil 

 which is nearly always appreciable and may be large. This decreases 



