An Analysis of the Evaporation Method for Determining Interstitial Water

A theoretical study of the principles of evaporation of liquids from large pores in petroleum reservoir rock core samples has resulted in the development of a working hypothesis to explain the application of evaporation measurements to the determination of minimum interstitial-water content. Rate-determining factors that control kinetics of evaporation processes have been considered in evolving the new hypothesis. Variations in temperature during evaporation experiments are found to have a negligibly small effect on evaporation interstitial-water measurements. Determination of complete capillary-pressure curves by evaporation measurements is discussed briefly. Theoretical calculations indicate that minimum interstitial-water saturations determined by the evaporation method should correspond quantitatively with water saturations above transition zones in petroleum reservoirs. Experimental comparisons between the evaporation method and the well-accepted porous-disk method demonstrate excellent agreement. Introduction A new procedure for determining the "irreducible minimum"interstitial-water content of petroleum reservoir rock has been described recently by Messer. The essential novelty of Messer's experimental approach, as compared with methods previously proposed, lies in the displacement of a saturating liquid by evaporation rather than by the pressure of a second immiscible fluid or centrifugal force. Probably the major advantage of the evaporation process, especially when applied to routine core analyses, is its speed. In addition, it is applicable to drill cuttings. A third advantage, not suggested by Messer, results from using a saturating liquid known to wet the laboratory-extracted core with a zero contact angle. Experience with the evaporation method has demonstrated that about 20 core samples may be run in eight hours. The porous-disk procedure of Rose and Bruce and the mercury pressuring method of Purcell permit the analysis of about two samples per day if the use of four cells each holding eight samples is assumed in the case of the former, and one piece of apparatus in the case of the latter. The centrifugal method of Slobod, Chambers, and Prehn is as rapid as the evaporation method when applied to "looser" cores of moderately high permeability, but is as slow as the other methods when relatively"tight" cores having permeabilities of a few millidarcys are analyzed.