Lithostat

Properties and extraction processes Aquifer gas, also referred to as geo-pressured gas or brine gas, is natural gas found dissolved in aquifers, primarily in the form of methane. The solubility of natural gas, and thus the methane content of the water, can vary significantly, and depend on factors, such as the total pressure, temperature, salt content of the water and amount of other gases dissolved. The amount of gas dissolved in underground liquids increases substantially with depth. A general rule is that the deeper the aquifers and the higher the pressure, the higher the gas content. At depths down to 5 km, up to 5 m3 of methane can be dissolved per m3 of water in aquifers under normal hydrostatic pressure (load of water) under lithostatic pressure (load of water and rocks), this factor may increase to more than... 

The system variables are composed of n compositional terms plus ambient variables that are usually two in number temperature and pressure (hydrostatic and/ or lithostatic-isotropic pressure). The variance (F) of the system is readily obtained by subtracting the number of condition equations from the total number of variables (n -I- 2) ... 

Both NaCI and CaCb-rich brines are thought to have circulated at the base of the sandstones at minimal P-T conditions of 150 20°C and 1250 250 bars. The low-saiinity fluid is slightly warmer and is thought to have circulated at depth before being injected at the base of the sandstones. The two brines and the low-salinity fluid were mixed and trapped at the favour of pressure decrease from lithostatic to hydrostatic regime at the time of U deposition. 

Pressure effects on equilibria in liquids or solids are generally less spectacular than temperature effects, at least at the pressures normally encountered in chemical engineering (a few tens of megapascals) or in the environment (hydrostatic pressures in the ocean trenches exceed 100 MPa, but about 40 MPa would be more typical of the ocean floors). Higher lithostatic pressures are, of course, found beneath the Earth s surface, reaching 370 GPa (0.37... 

Table A.5 is the output file for salts in the 4.5- to 5.0-km layer, where the system pressure is 484.5 bars (102 bars km-1 x 4.75 km). The temperature of 268.28 K is the freezing point depression for this particular composition and pressure at 268.27 K, ice forms. The pH of this system is 8.02. The number of independent components is seven. This example deals with lithostatic pressures on solutions dispersed in a regolith, which is fundamentally different from the previous examples (Tables A.2-A.4) that dealt with seawaters.

The size of identified pressurized compartments is reported to vary from small-scale structures, 1-10 km across, to mega-scale compartment complexes of hundreds of kilometers across they have been reported from depths of 2000-5000 m and potentiometric heads were mostly between the local (theoretical) hydrostatic head and the local lithostatic head. Multiple compartments are distributed vertically and laterally. The pressurized compartments are regarded to be closed chemical systems as no through-flow occurs in them. 

Water has a high helium dissolution capacity, and groundwaters in nature contain helium in a concentration range spanning over five orders of magnitude, as seen in Table 14.1. In all these cases the partial helium pressure in the aquifers was orders of magnitude below the hydrostatic pressure, not to mention the lithostatic pressure, ensuring no gas losses took place. 

It is obvious that with this approach, lithostatic pressure (nonhydrostatic pressure or loading pressure) is not a factor that has much effect on mineral equilibrium. 

In addition to the ideas examined, the viewpoint that unequal pressures in general and lithostatic pressure in particular play an important role in metamorphism is widespread. In theoretical works (Barth, 1956 Sobolev, 1961 Fyfe et al., 1962 Semenenko, 1966) the principle of the shifting of equilibria under the influence of excess loading pressure on the solid phases, with the liberation of HjO or COj in metamorphic reactions, has been examined repeatedly. 

In substantiating this principle one proceeds from the fact that lithostatic pressure is transmitted only to the solid phases (minerals), and the pressure of the volatile component is independent and is taken either as constant, or as systematically varying with depth. When P =/= Pf, equilibrium is determined by the well known relationship ... 

Specific thermodynamic calculations support this relationship. In the well known work by Marakushev (1968) a system of mineral equilibria is created, based on the assumption that metamorphism of rocks is accomplished under constant moderate pressure of water ( 1000 bar) and wide variations of pressures on the solid phases (up to 12 kbar), which are determined by depth. However, in this case the relationship between P and total fluid pressure is not specified clearly enough. If it is considered that P — Pf and Pn o certain fraction of the fluid pressure, then a special case of the general relationship adopted by Sobolev (1970) is obtained. But if at any lithostatic pressures water predominates in the fluid and P > Pf Pff, then it is necessary to interpret the physical meaning of lithostatic pressure. This question has been examined in detail by Ostapenko (1977), on the basis of thermodynamic analysis and some experimental data. In particular, it was... 

In all cases in the thermodynamic analysis we considered partial pressures of H2O, CO2, and other volatiles to be independent variables, if they were not related to one another by reactions. In addition the general conclusion was drawn that in thermodynamic calculations of metamorphic reactions it is impossible to assume different isotropic pressures on the solid phases and fluid. Lithostatic (nonhydrostatic) pressure or loading pressure has practically no effect on equilibrium in elastic deformation of rocks. Isotropic pressure equal to fluid pressure in the case of an excess of volatiles should be considered an equilibrium factor in actual natural processes. 

Many techniques ranging from hydrostatic or lithostatic drainage of boreholes in underground openings to very sophisticated pumps and pressurized samplers have been used to obtain fluid... 

The magnitude of hydrostatic pressure relative to lithostatic pressure, however, may influence some chemical reactions during coalification and coal maturation by maintaining contact between reaction products and the starting material (Monthioux, 1988), and/or by allowing reaction products to escape. 

The cavity continues to expand in this manner until the pressure of the gases is balanced by the reaction of the rocks. This reaction can result either from the lithostatic pressure of the rocks, which depends on the explosion depth, or from the cohesive forces of the medium, or from residual stresses of tectonic origin which are the trace of the forces exerted on the medium during the formation of the massif. 

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