Brines undersaturated

Figure 8.8 shows the resulting saturation indices for halite and anhydrite, calculated for the first four samples in Table 8.8. The Debye-Hiickel (B-dot) method, which of course is not intended to be used to model saline fluids, predicts that the minerals are significantly undersaturated in the brine samples. The Harvie-Mpller-Weare model, on the other hand, predicts that halite and anhydrite are near equilibrium with the brine, as we would expect. As usual, we cannot determine whether the remaining discrepancies result from the analytical error, error in the activity model, or error from other sources. 

The subsurface situation, however, with respect to dolomitization is more complex, as shown by the work of Land and Prezbindowski (1981). It appears that in the Cenozoic Gulf Coast basin of the United States, subsurface, Ca-rich saline formation waters may be undersaturated with respect to dolomite, and as the brine moves updip, and is progressively diluted by shallow subsurface waters, the brine may cause dedolomitization of carbonates encountered in the subsurface. 

Model 2. If water, no matter what its original salinity, was in near equilibrium with calcite + dolomite or was oversaturated with dolomite by virtue of a high Mg/Ca ratio at shallow depth (like Cretaceous seawater or a sabkha brine), it could never evolve to its present composition with increasing temperature (burial) by dolomitization because the water is everywhere undersaturated with dolomite today. It is very unlikely that the very low Mg/Ca ratio of the brine could be primary because neither Cretaceous seawater nor any reasonable Cretaceous sabkha brine are likely to have had such a composition. 

Predicting the Ca/Na ratio of the resultant brine accurately is difficult, and depends on the availability and reactivity of plagioclase and whether or not the brine remains in contact with halite -1- quartz -I- plagioclase. If a halite-saturated (or near-halite-saturated) brine flowed into a clastic section lacking halite as a phase, Na removed from the brine by albitization would cause undersaturation with respect to halite but Na could not be replaced by continued halite dissolution. 

At 200°C a brine in equilibrium with albite and K-feldspar should have a Na/K activity ratio (or molality ratio) of 16 (Helgeson, 1972, figs. 6 and 7), less than the value of 26 observed for Edwards brine. Edwards brine is therefore undersaturated with respect to K-feldspars. K-feldspars have been observed to be almost completely removed from Frio sandstones in Brazoria County, Texas, as shallow as 4250 m (Land and Milliken, 1981), and we suggest one possible reason for the potassium composition of Edwards brine is that most of the K-feldspars may have been removed from the Jurassic and Lower Cretaceous sandstone aquifers before the Edwards rocks were as deeply buried as they are today. Thus as the Edwards brine forms, insufficient K-feldspars are present in the discharge conduits, and the brine remains undersaturated. 

Therefore it is possible that a paucity of K-feldspar and dolomite in the pre-Edwards aquifers downdip from our study area is the reason for the undersaturation of the brine with respect to these phases. It should be noted that K-feldspar, quartz and dolomite are extremely rare, or absent entirely from the deep Edwards carbonates themselves. 

To illustrate the use of the equations, we will take the simpler problem of the solubility of anhydrite in a concentrated NaCl solution. In natural solutions, such as evaporitic brines, you can analyze the total Ca content and the total sulfate, along with everything else, but to compare the ion activity product (ftca + so ) solubility product in order to determine if the solution is over-saturated or under-saturated, you need the activity coefficients of Ca and SO4 . This is where Pitzer equations come in. Let s say we want to know if anhydrite is over- or undersaturated in a solution 3m in NaCl and 0.01m in CaS04. For the activity coefficient of Ca + in this solution, equation (17.41) becomes... 

The highest content was found in formation waters at depths of 2-3 km, and in hydrothermal waters from 200-400 to 500-700 ppm. The content of silicic acid in some sodium carbonate-bicarbonate brines can be as high as 2 700 ppm at pH 10 (Jones et al. 1969). However, natural waters usually show a concentration of silicic acid considerably lower than the solubility limit of amorphous silica under the same conditions (Fournier and Rowe 1962 Table 3.5). Consequently, modern subsurface waters are undersaturated with respect to amorphous silica, while marine water is unsaturated also with respect to quartz (Fournier and Rowe 1977). 

Conservation of heat becomes more important as the operating temperature increases. Operators of hot brine processes may use fluidized salt resaturators in which an internal cone holds a recirculating salt slurry. Fresh undersaturated brine enters the cone, and saturated brine overflows to flie bottom section of the vessel for removal. The supply of salt is replenished by addition of flesh slurry near the circulating pump. 

The feed point of the concentrated brine is the receiver tank B (Figure 16.19). The only crystallizer fed from here is the Oslo crystallizer. While the concentrated brine in this receiver is still undersaturated, the Oslo crystallizer can be fed with crystal-free solution all the time - the absolute precondition for the granular production in the Oslo-type crystallizer. 

The feed brine not taken to feed the Oslo-type crystallizer leaves the receiver tank B to receiver tank C by overflow. The fines-containing centrates and the hydro-cydone overflows from the separation stations are collected in the receiver tank C, too, where the still existing undersaturation redissolves the fines, before the resulting mixture is fed in parallel to the FC-type crystallizers. 

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