.Ch. 32] OIL PRODUCTION BY WATER DRIVE 583 



history of the Northwest Gloyd Extension of the Rodessa Field 

 (Louisiana) , illustrating behavior typical of the solution-gas drive. 



The rapid increase of the effective permeability of the sand to gas 

 and the limited amount of gas available make the recovery efficiency 

 of the dissolved gas drive mechanism very low. When the total liquid 

 saturation has dropped to about 70 percent, of which 50 percent may be 

 oil and 20 percent water, the flow of oil has practically ceased, and 

 the well may produce only a few barrels per day. Thus only from 

 20 to 30 percent of the oil originally in place will have been produced 

 at the exhaustion of the field. 



Oil Production by Water Drive 



In pools where a large aquifer exists below or marginal to the oil 

 zone, the water tends to move into the pool and displace the oil. Water 

 is a more efficient driving agent than gas, partly because it has a more 

 favorable relative permeability relationship to oil, and partly because 

 it has a much higher viscosity. If, therefore, a steady and uniform 

 advance of the edge water or bottom water can be maintained, a much 

 more efficient recovery can be realized; under favorable circumstances 

 as much as 85 percent of the original oil in place. In the past, how- 

 ever, full use has not been made of this mechanism. Usually the drive 

 of the water is caused mainly by its volumetric expansion resulting 

 from the drop in pressure. Unless there are very large amounts of 

 water in the aquifer, so that the additional volume provided by this ex- 

 pansion is roughly equal to the volume of fluids withdrawn, the advance 

 does not continue. Owners of "edge" leases were forced to pump the 

 water out along with the oil or lose their entire production, and this 

 withdrawal is frequently enough to stop completely the encroachment 

 of the water. Recently, with a better understanding of the value of 

 the water drive, it has been possible to take advantage of it in various 

 ways. If production is restricted, as by a state regulating authority, 

 withdrawals from a pool can be held to a point where the advancing 

 water is able to maintain the pressure in the oil reservoir. Figure 2 

 shows the production history of the North Searight Pool (Oklahoma), 

 where the bottom-hole pressure remained high and an efficient recovery 

 was obtained. If the natural advance of the water into the reser- 

 voir is too slow, special water-injection wells may be drilled in the 

 water zone and pressure maintained by pumping additional water into 

 the sand. Water wells may be drilled on a definite pattern between 

 the oil wells, and numerous sources of water may be provided. The 

 latter method has been extensively applied as a secondary recovery 



