Page 257 - Geochemistry of Oil Field Waters

RESEARCH STUDIES 241

tions of salts of different concentrations separated by a semipermeable
membrane will cause water from the lower salt-concentration side to move
through the membrahe to the higher concentration side, producing a greater
pressure on the high-concentration side. The pressure differential is the
osmotic pressure of the system and can account for abnormal pressures
found in some reservoirs.
Reverse osmosis occurs when hydraulic pressure in excess of the osmotic
pressure is applied to the high-concentration side, which forces water
through the membrane to the low-concentration side. The system is not
100% effective and some dissolved solids move through the membrane
(Kimura and Souriragan, 1967).
Such a system requires rather high pressure differentials in nature to
produce the highly concentrated brines found in some formations. The
osmotic pressure could produce pressure differentials in formations, but the
pressure comes to equilibrium as the two solutions equilibrate. The reverse
osmosis system works only as long as the excess hydraulic pressure is
applied. In the absence of the excess hydraulic pressure, the system comes to
equilibrium.
Larson (1967) reported some desalination results for water with reverse
osmosis using cellulose-acetate membranes. With a brackish water containing
about 4,300 mg/l of dissolved solids, input pressure of 42 kg/cm2 and
temperature of 15.g0C, the ion rejection rates were as high as 99.9%. The
rejection order based on the percent rejected was:

Ca+’ + Mg+’ > HC03-2 + SO4-’ > C1-> Na+ > NO3-

Assuming that this mechanism operates in a shale filtration system, the order
of ion concentration on the high brine concentration side would be the
same. The ion concentrations on the fresher water side would be the reverse
or :
NO3- > Na+ > Cl- > SO4-’ + HC03-* > Ca+’ + Mg+’

Other investigators have obtained similar results. For example, Loeb and
Manjikian (1965) found a rejection order of SO4-’ > Mg+’ > Ca+’ > Na+
> HC03- > C1- > NO3-. Michaels et al. (1965) found a rejection order of
Ca+2 > Li+ > Na+ > K+ for the pressure independent portion of salt
transport in cellulose acetate reverse osmosis desalination membranes. This
correlates with the size of the hydrated ion radii because calcium is the
largest and potassium the smallest. Further, this indicates that the pore size
of the membrane is a controlling factor.
The data ‘of Larson (1967) showed that sulfate and carbonate scale
formed on the high-pressure side of the membrane and if not removed would
cause flow to decrease or stop. The pH on the output or fresh-water side of
the membrane decreased.