1130 



EXPLORATION GEOPHYSICS 



Fluid Density and Subsurface Pressures. — When wells of high pro- 

 ductivity are pumped at rates considerably less than their potential, the 



fluid separates into its components when equilib- 

 rium is reached. The effects of this gravitational 

 separation are illustrated in Figure 702. Because 

 the pump is removing the fluid at the same rate 

 that it is flowing into the well (the condition 

 obtaining when equilibrium is reached and the 

 fluid level is stationary), the fluid below the pro- 

 ducing horizon is essentially of the same com- 

 position as that leaving the formation. In the 

 annulus above the producing horizon, where 

 direct flow of fluid from the formation is not 

 taking place, a quiescent fluid condition exists. 

 The oil-gas phase will comprise the upper por- 

 tion of the fluid column, while the water phase 

 will comprise the lower portion of the column. 

 The degree to which this separation takes place 

 will be chiefly dependent upon: (a) time during 

 which equilibrium has existed and (b) the 

 stability of the oil-water mixture in that par- 

 ticular well. When measurements are made in 

 wells producing appreciable quantities of gas, 

 the work must be conducted so as to compensate 

 for the changes in density. 



The fluid-pressure gradient in the annulus 

 of a well depends on the producing conditions 

 and on the effective area of the annulus. For given producing condi- 

 tions, the pressure gradient in the annulus is practically constant from 

 the top of the fluid to the perforations, if there is no change in casing 

 size. It has been found that the gradient varies from about 0.05 to 0.4 

 pounds per square inch pressure per foot of depth below the fluid level, 

 depending upon the gas-oil ratio. Often, the density decreases almost pro- 

 portionally to the effective area of the annulus. The reason for this will 

 be apparent when it is recalled that the gas-oil ratio remains relatively 

 constant at any given rate of production. As this gas is being evolved, it 

 bubbles up through the oil in the annulus. The smaller the area of the 

 annulus, the greater must be the percentage of gas present, with a result- 

 ant decrease in the weight of the fluid column, i.e., its effective density. 

 Referring to Figure 702, the density in the liner will be much less than in 

 the upper cased part of the hole, due to the change in area of the annulus. 



Fig. 702. 



Illustration of 

 gravitational separation of oil 

 from fluid produced by a well. 

 (Jakosky, A.I.M.E. Petroleum 

 Technology, Tech. Pub. 1058, 

 1939.) 



Determinations of the Fluid Density or Fluid-Pressure Gradient 



To determine the average fluid density or fluid-pressure gradient in a 

 well, measurements may be made of the variation in fluid level as a function 



