Page 258 - Geochemistry of Oil Field Waters

24 2 ORIGIN OF OILFIELD WATERS

Russell (1933) considered several processes which could produce subsur-
face brines more concentrated than sea water. He concluded that evapora-
tion of the water by natural gas generally is not important, water evapora-
tion in coarse-grained rocks generally is not important, gravitational settling
of dissolved solids is not greatly important, rocks containing considerable
amounts of feldspars and other unstable minerals take up large quantities of
hydration water, clays adsorb bases and later expel them into solution
causing concentration, and osmosis may occur through semipermeable mem-
branes.
DeSitter (1947) noted that oilfield waters are altered as a result of two
prominent diagenetic phases. During the first phase magnesium, calcium,
sulfate, and carbonate precipitate from the original sea water. During the
second phase the concentration of magnesium and calcium ions increases
along with the concentration of other dissolved solids. He reasoned that the
second phase occurred because of filtration through semipermeable shales.
The filtration results because of sediment compaction until a semiper-
meable membrane develops which allows water molecules to pass through
but retards salt ions. Thus, the more concentrated brines are found where
sediment compaction and water flow distance were the greatest. This usually
occurs in the deepest portion of a basin.
McKelvey et al. (1957) forced aqueous saline solutions through ion-
exchange resins and found that the effluent solutions contained less dis-
solved salts than the influent solutions. Effluents from cation-exchange
resins were found to contain Na/K ratios similar to those in the influent;
however, the Mg/Ca ratios were at first higher than in the influent but with
additional squeezing the ratio decreased to much lower values. They
postulated that similar reactions occur during the compaction of sediments
to change the concentrations of constituents dissolved in waters.
Pressures of 7 kg/cm2 to 105 kg/cm2 were applied to force sodium
chloride solutions through cation-exchange membranes. The results indicated
that the membranes desalted the saline solutions, producing a filtrate con-
taining less salt than the influent. This salt filtering effect was attributed to
the electrical properties of the membrane.
Milne et al. (1964) determined the filtering efficiencies of sodium chloride
solutions by bentonite membranes. The filtration efficiencies were 94% at
140 kg/cm2 and 88% at 703 kg/cm2 with 0.5N sodium chloride. Increased
salinity caused less efficient filtration because filtration efficiencies of 94%
for 0.W sodium chloride and 66% for 4N sodium chloride at a pressure of
352 kg/cm2 were obtained. A similar mechanism could operate in the sub-
surface to create concentrated brines.
Young and Low (1965) performed an experiment using natural rock and
demonstrated that osmotic flow of water through shale and siltstone occurs.
The osmotic pressures produced were less than theoretical and they were
attributed to microcracks in the natural rock which caused them to be less
effective than a perfect membrane.