Page 85 - Petrophysics

PROPERTIES OF SUBSURFACE FLUIDS 59

developed may have been either fresh water or saline marine water. After
the original deposition, however, the oilfield sedimentary formations
have histories of subsidence, uplift, reburial, erosion, etc. Therefore,
the chemistry of the original water may have been altered by meteoric
water, marine water infiltration at a later time, changes of salt types and
concentrations due to solution of minerals as subsurface waters moved
in response to tectonic events, and precipitation of some salts that may
have exceeded equilibrium concentration limits [25].
The origin of deep saline subsurface waters has not been completely
explained. The most plausible explanation is that they were originally
derived from seawater. If seawater is trapped in an enclosed basin,
it will undergo evaporation, resulting in precipitation of the dissolved
salts. The least soluble salts will precipitate first, leaving a concentrated
brine deficient in some cations and anions when compared to seawater.
The common order of evaporative deposition from seawater in a closed
basin is: calcium carbonate (limestone) > calcium-magnesium carbonate
(dolomite) > calcium sulfate (gypsum) > sodium chloride (halite) >
potassium chloride (sylvite). Dolomite begins to precipitate when the
removal of calcium from solution increases the Mg/Ca ratio. The residual
brines (containing unprecipitated salts at any period) may migrate away
from the basin and leave the evaporites behind, or they may become
the interstitial water of sediments that are rapidly filling the basin [19].
In accumulating marine clastic sediments, aerobic bacteria consume the
free oxygen in the interstitial waters and create an anaerobic environment
in which the anaerobes become active and attack the sulfate ion,
which is the second-most important anion in seawater. The sulfate
is reduced by the bacteria to sulfide, which is liberated as hydrogen
sulfide (marsh gas) [ 191. Thus, the composition of saline oilfield waters,
or brines, is quite different from the average composition of seawater
(Table 2.3). With the exception of sulfate, all of the ions in the Smackover
Formation (carbonate) brine are enriched with respect to seawater.
Several mechanisms of enrichment are possible: (1) the original seawater
may have evaporated if it was trapped in a closed basin; (2) movement
of the waters through beds of clay may have concentrated cations by
acting like a semipermeable membrane allowing water to pass through,
but excluding or retarding the passage of dissolved salts; and (3) mixing
with other subsurface waters containing high salt concentrations. The
content of alkali cations is many times greater in the oilfield brines than
in the water that owes its salinity to the dissolution of salts from the earth,
or to the infiltration of high-salinity waters from other sources.
There are many reactions between the ions that can occur as
the environmental conditions change with burial. Consequently, the
composition of oilfield waters varies greatly from one reservoir