There are two main sources of the water that floods productive oil or gassands. The water may rise from the lower depths of the producing stratum, or itmay come from beds above or below the oil-bearing formation. Usually therecovery of oil is decreased by water entering the oil sands, and mostoil-field waters, especially those of deep wells, tend to foster the formationof an emulsion, which is expensive to treat. This paper deals with a method ofdistinguishing between waters that encroach upon oil-bearing beds from sourcesin the same stratum and waters that reach the oil sands from horizonsabove.
In order to remedy effectually water difficulties in oil and gas wells, it isabsolutely necessary to determine the source of the invading water. Sometimesevidence such as the structural relations between the strata penetrated byneighboring wells and those found in the well under investigation is notadequate; also, data from drill logs and well records, from mechanical testsmade on the wells by plugs, testers, drilling tools, etc., and from tests bychemical indicators such as eosin and Venetian red, may be unsatisfactory, forsuch data may fail to locate the source of the infiltrating waters. As a resortin such cases, the application of chemical analyses, that is, the comparison ofan analysis of the water in question with that of typical waters from thevarious water horizons in that particular district has in certain instancesproved of value. This method has been used to advantage in the WestsideCoalinga field of California, where the source of water may be fairlydefinitely determined, from its composition.
The chief conclusions concerning the chemistry of the oil-field waters inCalifornia by G. S. Rogers are: Oil-field water is not necessarily salty, as isgenerally believed, and may not be even slightly salty to the taste. The degreeof concentration of chloride in such water is governed primarily by localconditions and is not affected by the position of the water in relation to oil.Sulfate diminishes in amount as the oil zone is approached and finallydisappears.