The gravity discussed here is that of the clean oil produced. Gravities offluids containing water, free or in the form of emulsion, sand, drilling mud ormineral salts of a no bituminous nature do not represent the gravity of crudeoil as considered in this paper.
During recent years it has become a frequent occurrence for operators to becalled upon to explain to landowners the reasons for variations in gravity onshipments of oil from wells in which they are interested as royalty owners. Inregard to single well leases it has been noted that marked changes in gravityfrequently have occurred for no apparent reason. Data collected on the subjectand correlated with production methods reveal interesting facts. It is thepurpose of this paper to present some of the highlights on the subject.
These gravity changes in the natural crude oil are with a few exceptions theresult of the liberation under varying conditions of the volatile hydrocarbonsfrom the natural oil-gas solution contained within the pool. The subject of theliberation of gases from natural oil-gas solution has been investigated by BenE. Lindsly, of the U. S. Bureau of Mines, and the observations recorded hereare in the main consistent with his conclusions.
Investigators have recognized two methods of liberation of gas from solutionflash and differential. In the latter method the vapors are removed from theliquid as rapidly as formed and have no opportunity to come to a condition ofequilibrium with the oil. In flash liberation an equilibrium condition betweenthe oil and the gas exists at the pressure of separation. Mr. Lindsly has foundthat the method of liberating the gas from the oil has a pronounced effect onthe final gravity and that differential liberation results in a higher A.P.I.gravity due to the higher ratio of gasoline producing hydrocarbons left in theoil.
Under actual production practices with high velocities in flow strings, it isour contention that conditions of perfect equilibrium between gas and the oilrarely exist, and that gas separation is made under varying conditions, sometimes approaching that of equilibrium and at other times departing fromit.