580 russell-dickey. POROSITY AND PERMEABILITY [Ch. 32 



to govern decisions regarding the spacing of wells and optimum pro- 

 duction rates, which are of very great economic importance. It is 

 obvious, for example, that often in the past well spacings have been 

 closer than can be economically justified, and production rates have 

 sometimes been too fast for efficient recovery. Laboratory investiga- 

 tions of the permeability and capillary properties of oil and gas reser- 

 voirs are needed to guide petroleum-production operations and are 

 therefore proceeding actively in many industrial and academic labora- 

 tories. 



PETROLEUM RESERVOIRS 



Oil pools are found in geological formations of widely different age, 

 character, and structural configuration. All commercial pools, how- 

 ever, are found in a porous reservoir rock, enclosed, at least over its 

 upper surface, by a caprock such as shale or dense limestone that is im- 

 permeable to oil or gas. Some oil reservoirs are filled almost entirely 

 with oil. In others the uppermost portion of the reservoir is filled with 

 gas, and even when gas caps are absent there is nearly always some gas 

 dissolved in the oil. Some reservoirs contain only gas consisting of 

 methane with varying amounts of heavier hydrocarbons that can be 

 condensed to form gasoline. When these are abundant, the reservoir is 

 called a condensate pool. Many condensate pools have a narrow rim 

 of liquid oil down-dip around their margins. 



Water in Oil Reservoirs 



The porous reservoir rock nearly always contains at least some 

 water, segregated from the oil at a lower level. In some reservoirs this 

 water-saturated rock extends long distances below and beyond the 

 limits of the oil pool, whereas in others the porous zone is restricted 

 laterally and filled almost completely with oil. The water is believed 

 to be the buried remains of the sea in which the sediments were de- 

 posited, but it is generally more concentrated than sea water. The 

 concentration in a given horizon usually increases with depth of burial. 

 Oil-field brines vary widely in composition but generally contain more 

 calcium, less magnesium, and much less sulphate than sea water in pro- 

 portion to their, sodium and chlorine content. 



Until recently it was believed that the oil-bearing portion of the 

 reservoir contained only oil, because wells completed above the water- 

 oil contact produced no water. Fettke (1927) , who was one of the first 

 to determine the oil content of sand, reported the presence of water 

 also, which was generally believed to have entered the sand during 

 the drilling of the well. It was later proved (Schilthuis, 1938) that 

 water is present in oil and gas sands. Indeed, on the basis of both 



