Pressure lithostatic

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... 

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.

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... 

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. 

Overburden pressure is depth dependent and increases with depth. In the literature, overburden pressure has also been referred to as geostatic or lithostatic pressure. 

Fracture pressures are estimated from leak-off tests (LOT), where mud is pumped into the formation until the first evidence of fractures is detected. Leak-off pressure is the pressure at which the formation develops very thin fractures prior to rock failure. A great advantage of this kind of test is that it is an in situ test, thus we do not have to deal with relaxation or unloading problems. Fracture pressures estimated from LOT is in the range of 0.6-0.8 times the litho-static pressure, which is within the fracture pressure domain (0.7-0.9 lithostatic pressure) given by DuRouchet (1981). The overpressures do not reach fracture pressure (LOT) in any of the wells investigated. However, the pore pressure reaches 80% of... 

Fig. 14. Relationships between pore-pressures, the hydrostatic gradient, the fracture pressure gradient (approximation to the minimal horizontal stress, Sf,) and the lithostatic pressure gradient (approximation to the vertical stress, S ). Pore-pressures from sea floor to base Pliocene equals hydrostatic. The yellow, dark blue and red pore-pressure trend-lines represent the pore-pressure versus depth gradients for the Paleocene-Eocene, Mid-late Cretaceous and Upper Jurassic-lowermost Cretaceous, respectively. The portion of the red trend-line below approximately 2550 m MSL equals the maximum reservoir pore-pressure trend-line of Fig. 13 and reflects the counter-pressure of the topseal controlling the pore-pressure distribution of hydraulic compartments II, III and (probably) IV.

Experiments have not been carried out in which the combined effect upon smectite dehydration of increasing temperature, lithostatic pressure, hydrostatic pressure and ionic strength of pore fluid is tested in order to simulate the diagenetic environment. Pairs of these variables have been studied in limited numbers of experiments, some of them difficult to interpret, by Stone and Rowland (1955), Weaver (1959), von Engelhardt and Gaida (1963), Khitarov and Pugin (1966), and Demirel et al. (1970). The accumu-... 

Calculations that attempt to reproduce the observed vertical successions of zeolite minerals seen in various sedimentary piles (e.g., Miyashiro and Shido, 1970) typically assume that hydrostatic and lithostatic pressures are equal and that the pore fluid is pure water. The broad features of the successions can be predicted, but there is a bewildering overlap of zeolite subfacies (Coombs, 1971). In addition to the complexities already noted, it should be recalled that zeolites occur in an unusually rich variety of crystal structures, with several different major cations, extensive cation solid solution, significant Si/Al ratio differences in different structures, and frequent growth and persistence of metastable phases. The bulk chemical composition of the parent-rock is important in determining which zeolite mineral forms, and the Pco of the diagenetic environment may determine whether a non-zeolitic clay—carbonate assemblage persists (Zen, 1961). 

Bruton, C.J. and Helgeson, H.C., 1980. Calculation of the chemical and thermodynamic consequences of differences in fluid and lithostatic pressure on equilibrium constraints in hydrothermal systems. Geol. Soc. Am., Abstr. Progr., 12(7) 394. 

By most general definition a sediment is a collection of particles - the sediment grains -which are loosely deposited on the sea floor and closely packed and consolidated under increasing lithostatic pressure. The voids between the sediment grains - the pores - form the pore space. In... 

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