Effect of Gas Withdrawal upon Reservoir Fluids

Abstract

The withdrawal of gas and/or oil in appreciable quantity from a natural oilreservoir causes the pressure within the reservoir to diminish, and if the oilis completely saturated with gas, this dissolved gas begins to evolve fromsolution. The liberation of gas from solution in oil causes the oil to changein several ways, the principal changes being:reduction in volume or "shrinkage";increase in specific gravity (decrease in degrees A.P.I.);decrease of energy in the oil;increased viscosity;increased surface tension. As the writer's investigations have been concerned more with the first threeitems than with changes in viscosity and surface tension the followingdiscussion relates particularly to the volumetric shrinkage of the oil, to theincreased specific gravity, and to the decreased energy in the oil, each causedby the liberation of dissolved gas. One of the most surprising facts that have developed in the study ofhigh-pressure well-head and" bottom-hole" samples is the greatreduction in volume that occurs in some oils when the naturally dissolved gasis liberated. High-pressure samples taken from well heads in the Oklahoma City, Kettleman Hills and Ventura, Calif., fields decreased in volume from 11.2 to40.5 per cent when the dissolved gas was liberated in reducing the pressure onthe sample to atmospheric. The amount of shrinkage an oil undergoes when gas is liberated depends upon thequantity of gas liberated and also upon its composition. A given volume of theheavier gases, propane and butane for example, will occupy more space in theoil than an equal volume of methane. The reason for this probably is that themolecules of the heavier gases are larger than those of the lighter gases. Whengases are not in solution, and particularly when they are at low pressures, thedistance between the gas molecules is so much greater than the dimensions ofthe molecules themselves that the mere size of the molecules has little effectupon the space occupied by the gas. Avogadro's hypothesis, which in effectstates that all gases under the same conditions have the same number ofmolecules per unit volume, relates to this phenomenon.