Ch. 32] EARLY STUDIES 585 



development in many pools that had been virtually exhausted by the 

 solution-gas mechanism; it is generally called "water flooding." 



Production by Gravity Drainage 



In certain reservoirs of relatively high permeability and steep dip, 

 oil is able to flow down the dip by gravity alone. Good recoveries 

 can be obtained when this occurs because the oil is not retained to such 

 a large extent by capillary forces and can be recovered from the down- 

 dip wells. The rate of recovery, when gravity drainage is effective, can 

 be increased by reinjecting the produced gas at the top of the structure. 

 This maintains the pressure in the reservoir and results in a more rapid 

 movement of the oil down the dip. The recovery from the Oklahoma 

 City pool would have been much less if the high permeability of the 

 sand had not permitted the oil to run down-dip and accumulate in the 

 lower parts of the reservoir. 



POROSITY AND PERMEABILITY OF OIL RESERVOIRS 



Early Studies 



The first large-scale production of oil and gas occurred in north- 

 western Pennsylvania in 1860. Early speculations regarding the nature 

 of underground reservoirs dealt mainly with crevices or caverns. Some 

 years later Carll, of the Pennsylvania Geological Survey, worked out 

 most of the important features of petroleum reservoirs and their be- 

 havior. Carll (1880) showed that the producing sandstone had a 

 porosity between % and y i5 of its bulk and that it was capable of 

 holding up to 1,000 barrels of oil per acre-foot of rock. He also showed 

 that the aggregate area of the pores was ample to transmit large quan- 

 tities of oil. He also pointed out that the removal of oil from a reser- 

 voir required the admission of some other fluid to take its place, sug- 

 gesting the deliberate injection of water for this purpose. 



Darcy (1856) studied the behavior of sand niters and discovered the 

 law that now bears his name. As later formulated, it states that the 

 flow of water Q through the porous medium per unit cross section is 

 directly proportional to the pressure gradient dp/dx in the fluid. The 

 constant of proportionality is k/fi, where /x is the viscosity of the fluid 

 and k is a constant depending on the matrix through which the fluid 

 flows. 



k dp 



/i ox 



