www.saltworkconsultants.com Salty MattersJohn Warren - Tuesday March 31, 2020 Brine density & persistence: Part 1 Physical properties of a brine Introduction Seawater Temperature - Salinity (lines of equal density in g/cm3) Evaporite sediments precipitate in a brine and can dissolve 20 into a brine. The formative brine is always saline (more sa- 1.0245 line than freshwater) and the levels of salinity in the brine 1.250 will increase or decrease depending on the relative rate of 15 1.0255 1.0260 fluid input to output in the location where the evaporite mass is accumulating at the earth's surface or residing in 1.0265 the subsurface. That is, rates of fluid input to loss to and 10 1.0270 from an evaporite geobody change both in the at-surface 1.0275 1.0280 depositional environment and in the subsurface envi- (°C) Temperature 5 ronment. More saline brines have higher concentrations 1.0285 of ionic constituents in the aqueous host than less saline brines. Accordingly, more saline brines have somewhat 0 1.0290 higher densities and lower specific heat capacities than less saline brines. 33.5 34.0 34.5 35.0 35.5 36.0 36.5 Salinity (PSU) The higher density of a free-standing brine or porosi- Figure 1. Relationships between seawater salinity (PSU = practical salinity ty-held brine compared to an overlying fresher, less-dense units), temperature and density. free-standing brine layer or surrounding fresher pore fluids in non-evaporite sediment can drive geochemical stability, interaction and alteration across time frames measured in months to hundreds of millions of years, from the time of deposition through diagenesis and uplift. The distinct Brine Stage Mineral Precipitate Salinity Evap. H2O Density (‰) (%) (gm/cc) properties of evaporite-associated brine also influence the loss generation of viable oil and gas accumulations and ore Marine or Skeletal & inorgan- 35-37‰ 1x 0 1.040 deposits. Over the next four articles, we consider brine in Sea euhaline ic carbonate this context of short-term and longterm pore fluid stability Mesohaline or Alkaline earth 37 to 1-4x 0-75 1.040- and alteration, starting with this article discussing various vitahaline carbonates 140‰ 1.100 physical properties of evaporite and cryogenic brines. Penesaline CaSO4 140 to 4-7 75-85 1.10- (gypsum/anhydrite) 250‰ 1.214 Marine brine chemistry and density CaSO ± Halite 250 to 7-11x 85-90 1.214- 4 Today the chemical make-up and the proportions of the 350‰ 1.126 Hypersaline major ions in seawater are near-constant in all the world's Supersaline Halite (NaCl) >350‰ >11x >90 >1.126 oceans. Ions dissolved ion seawater are dominated by Na Bittern salts Extreme >60x ≈99 >1.290 and Cl, with lesser amounts of SO4, Mg, Ca, K, CO3 and (K-Mg salts) (‰ vary) HCO3. Using the brine classification of Eugster and Har- Table 1. Salinity-classification tying together mineral paragenesis and die (1978), modern seawater is a Na–(Mg)–Cl–(SO ) wa- brine properties, including density range, based precipitation series with 4 concentration of modern seawater (after Usiglio, 1949). Hypersaline ter, with an average density of 1.03 gm/cc and a salinity of is defined as >35‰. Compared to this mineral-based classification, 35 ± 2‰ (Table 1). Oceanwide, the density of seawater is biologists working in saline waters use a somewhat different hierarchy; not a constant but varies slightly according to interactions fresh water (less than 1‰), subsaline (1-3‰), hyposaline (3-20‰), mesosaline (20-50‰), and hypersaline (greater than 50‰). Geohy- with salinity and temperature. Density of surface seawa- 3 drologists tend to refer to fresh water as less than 1‰, brackish water ter ranges from about 1020 to 1029 kg/m , depending on as 1-10‰, saline water as 10-100‰, and brine as greater than 100‰. temperature and salinity. At a temperature of 25 °C, a sa- Page 1 www.saltworkconsultants.com Degree of evaporation 0 10 20 30 40 50 60 70 80 90 100 1.4 Mirabilite F F F F F F FF F F 150 1.3 F F F F FFF F FF FF F F F F FFFF FF Ikaite & 1.2 F gypsum? FF 100 F F F Salinity (psu) 1.1 F Density (gm/cc) F F FF F 50 1.0 Halite onset K salts -2 -4 -6 -8 -10 -12 A. Gypsum onset MgSO4 C. Temperature (°C) CONCENTRATION MECHANISM Evaporation Cooling/Freezing Mirabilite Hydrohalite 100% Seawater (cryogenic) (cryogenic) to brine Seawater to 2.5 Aragonite cryogenic Cl CaCO3 brine 2.0 Gypsum Na CaSO4.2H2O 1.5 SO 10% 4 Mg Halite Mirabilite 1.0 NaCl NaSO4.10H2O K Ca 0.5 SO4 0 Freezing Hydrohalite Bittern salts -0.5 Ca Evaporation 1% NaCl.2H2O sequence Precipitation Kainite, -1.0 Carnallite, (g/kg) concentration Log K-salts & Bishote Gypsum Halite Sylvite, MgCl2.6H2O -1.5 Percentage of original solution remaining of original Percentage Bischote, (evaporite) (evaporite) etc. Antarcticite -2.0 CaCl2.6H2O 0 5 10 15 20 25 30 B. D. Degree of concentration Figure 2. Marine brine evolution. A)Evaporation pathway of modern seawater, showing how density increases. B) Evaporation series where sea- water is concentrated by solar heating (evaporation) versus brine freezing (cryogenesis). C) Cryogenic brine salinity increases with decreasing temperature. D) Ionic evolution and proportions as well as main salts precipitated in a concentrating seawater brine upon its freezing or evaporation (see Salty Matters May 18, 2019, for details on reference sources). linity of 35‰ and 1 atm pressure, the density of seawater sequence of marine precipitates was first documented by is 1023.6 kg/m3. Deep in the ocean, under high pressure, Usiglio (1849) and is still used to classify marine water seawater can attain a density of 1050 kg/m3 or higher. As salinities based on the associated mineral suits (Table 1). temperature increases the density of seawater decreases. As seawater concentrates, the first mineral to precipitate Similarly, colder seawater tends to be denser than warm- is CaCO3, usually as aragonite. This begins in mesohaline er seawater and so warmer seawater masses tends to float waters where the brine reaches twice the concentration of atop colder masses. This sets up pycnoclines or density in- seawater (40 to 60‰) and achieves a density ≈1.10 gm/ terfaces in the open ocean (see Warren et al., in press, for a cc. As the brine continues to concentrate and approach- paper dealing with the implications of marine pycnoclines es four to five times the concentration of seawater, that is and the associated passage of internal waves in creating of 130 to 160‰, gypsum precipitates from penesaline waters some types of hydrocarbon reservoir sands in SE Asia). with densities around 1.13 gm/cc (Figures 2a, b, 3a). At 10 to 12 times the original seawater concentration (340 For this series of articles, we are more interested in the to 360‰) and densities around 1.22 gm/cc, halite drops geochemical and geophysical effects related to increases in out of supersaline marine waters. If seawater desiccates brine density, tied to evaporative concentration, freezing, completely, the greatest volume of salt extracted from sea- or dilution of a brine, both at the depositional surface and water is halite (Figure 3a). After halite, the bittern salts in the diagenetic realm. (potassium or magnesium sulphates/chlorides) precipitate When seawater evaporates, a predictable suite of prima- from supersaline waters at concentrations that are more ry evaporite minerals crystallise from increasingly con- than 70-90 times that of the original seawater. Carnallite centrated hypersaline waters (Figure 2a; Table 1). The and epsomite are the dominant bittern precipitates from Page 2 www.saltworkconsultants.com 400 400 12 At 25°C Salinity 10 Pure water = 1 cp 300 Aragonite 300 Mercury = 1.5 cp 8 Milk = 3 cp Gypsum Linseed oil = 28 cp ecipitated r Halite 6 Olive oil = 84 cp 200 200 2 iscosity (cp) 4 V MgCl Salinity (‰) Dead Sea 2 crystalliser Minerals p 100 100 brines (g/ml of evaporated brine) (g/ml of evaporated NaCl 0 0 1.14 1.22 1.30 1.38 A. 1.0 1.1 Density 1.2 1.3 B. Density Figure 3. Brine evolution. A) Pre-bittern mineral precipitation sequence in a concentrating marine brine (replotted from Table 1 in Briggs, 1958). B) Viscosity increases with brine concentration (replotted from Karcz and Zak, 1987). a modern marine brine source. Brine density by this stage of concentration is more than 1.30 gm/ cc and brine viscosity and feel approach that of 4.0 olive oil (Figure 3b). As a brine freezes, cryogenic salts can form. Cryogenic brines and associated salts require tem- 3.5 peratures at or below the freezing point of the liquid phase. These salts crystallise from a cold, near-freez- ing, residual brine as it concentrates via the loss of 3.0 its liquid phase, which is converting/solidifying to ice (Figure 2c, d). As cryogenic brine concentration in- creases, the freezing temperature decreases and min- Specific heat capacity (kJ/kg.K) erals such as ikaite, hydrohalite, mirabilite, epsomite, 0 50 100 150 200 250 300 potash bitterns and antarcticite can crystallise from A. Salinity (g/l) the freezing brine (Figure 2d; See Salty Matters May 720 18, 2019). Brine freezing ends when the phase chem- 180°C istry attains the eutectic point. This is the point when 700 all compounds (including H2O) pass to the solid state. Depending on the initial mineralization and compos- 100°C tion of the brine, the eutectic point is reached between 680 -21 and -54 °C. 660 70°C Specific heat and thermal conduc- tivity 640 Specific heat of any evaporating brine decreases 620 40°C as the salinity increases (Kaufmann 1960).