Pressure hydration

Figure 17-6 is for sweet gases. The presence of hydrogen sulfide and carbon dioxide shifts the lines to the right. This results in an increase in the hydrate temperature for a given pressure or a decrease in hydrate pressure for a given temperature. 

If a better estimate of the mixture formation pressure is needed, the engineer may use the KySi value method in Section 4.2.2 to obtain an estimated hydrate pressure of 1.26 MPa. 

Because the two prediction techniques of this chapter were determined over half a century ago, they apply only to si and sH hydrates, without consideration of the more recent sH, which always contains a heavier component. For structure H equilibrium, only the statistical thermodynamics method of Chapter 5 is available for prediction of hydrate pressure, temperature, and composition. 

With the above data, Equation 4.21 indicates that the hydrate number n may be obtained from a measurement of the increase of the hydrate pressure with an inhibitor present at a given temperature. The fugacity may be calculated for a pure component from any of a number of thermodynamic methods given a temperature and pressure. Only in the case of a pure ideal gas (very low pressure or very high temperature) may the fugacities be replaced with the pressure itself. 

As indicated in Example 4.1, note the dramatic decrease in hydrate pressure caused by a small amount of propane added to methane, due to the structure change (si to sll). At pressures above incipient hydrate formation conditions, sll hydrates are predicted to be present throughout the entire composition range. 

Two final points should be made regarding the hydrate pressure-temperature limits shown in Figures 7.11a,b. The intersection of the phase boundary with the geothermal gradient limits the lower stability depth of hydrates. In Section 7.4.2, it will be shown that this intersection usually coincides with the BSR, an approximate exploration tool, which is caused by a seismic velocity decrease from hydrates above, to gas below the reflector. 

As two planar phyllosilicate surfaces approach one another, hydrated counterions that may be interposed between the surfaces must lose waters of hydration upon compression by the two approaching surfaces [23]. This loss of hydration water has been credited for the oscillatory behavior discussed in Sec. I. C. The nature of the counterion as well as its concentration impact this behavior. Pashley and Quirk [51] reported on the effect of Na, Ca, and La on the hydration pressure observed between two mica surfaces in a force balance apparatus. They noted that the hydration pressure could be fit to a double-exponential with two characteristic decay lengths... 

Below 30°C (85°F) hydrate pressure is a function of the temperature, composition, and water content. 

For water content greater than about 250 lb/MMSCF, hydrate pressure is independent of the water content. 

In recent high-pressure X-ray diffraction, neutron diffraction, and Raman studies on methane hydrate, pressure-induced phase transformations of methane hydrate show that the initial si (MH-sl) successively transforms to the sH (MH-II) at about 0.9 GPa and to the sO (MH-III) at about 2.0 GPa. 

Another aspect of current interest associated with the lipid-water system is the hydration force problem.i -20 When certain lipid bilayers are brought closer than 20-30 A in water or other dipolar solvents, they experience large repulsive forces. This force is called solvation pressure and when the solvent is water, it is called hydration pressure. Experimentally, hydration forces are measured in an osmotic stress (OS) apparatus or surface force apparatus (SFA)2o at different hydration levels. In OS, the water in a multilamellar system is brought to thermodynamic equilibrium with water in a polymer solution of known osmotic pressure. The chemical potential of water in the polymer solution with which the water in the interlamellar water is equilibrated gives the net repulsive pressure between the bilayers. In the SEA, one measures the force between two crossed cylinders of mica coated with lipid bilayers and immersed in solvent. 

The hydration pressure, P (in dynes per centimeter squared or force per unit area), acts between parallel membranes and varies exponentially with a decay distance X (1-10) so that one may write... 

The interactions between identical solid particles in a liquid medium can be one or more of the following (a) attractive, Lifshitz-van der Waals (LW). (b) repulsive, electrostatic interactions (EL> between the electric double layers surrounding the particles in suspension, (c) solvation, or structural forces, coming from the more or less structured layer of solvent molecules around the solid particles, interactions of this type can also be of either attractive (hydrophobic) or repulsive (hydration pressure, usually between hydrophilic particles) nature. Hydrophilic repulsion is usually of very short range and hydrophobic attraction can be of even higher range than the van der Waals one. 

The crystal pressure (Eq. 1) and hydration pressure (Eq. 2) are calculated from the following equations ... 

It is evident from this table that the salts with fewer molecules of hydration exert larger hydration pressures. 

There are three ways whereby salts within a rock can cause its mechanical breakdown by pressure of crystallization, by hydration pressure, and by differential thermal expansion. Under certain conditions, some salts may crystallize or recrystallize to different hydrates that occupy a larger space (being less dense) and exert additional pressure, that is, hydration pressure. The crystallization pressure depends on the temperature and degree of supersaturation of the solution, whereas the hydration pressure depends on the ambient temperature and relative humidity. Calculated crystallization pressures provide an indication of the potential pressures that may develop during crystallization in narrow closed channels (see Chapter 6). Crystallization of freely soluble salts such as sodium chloride, sodium sulphate or sodium... 

We argue that the major contributing pressure in the region 6 < log P < 8 is the hydration pressure due to solvent re-organization. This is consistent with several theoretical treatments [23,54, 55] showing that the hydration pressure should decay exponentially with increasing separation. As outlined below, there are several lines of experimental evidence indicating the existence of a hydration pressure. 

Thus, we argue that the underlying short-range pressure for PC bilayers contains at least two contributions an enthalpically driven, exponentially decaying hydration pressure arising from the work to remove water from the... 

These systems are highly dynamic and apt to regulation by a number of membranebinding ligands such as hormones and growth factors, metabolites, ions, pH, dmgs, proteins, and changes in membrane lipid composition, as well as physical parameters such as membrane potential, osmotic forces and hydration, pressure, and temperature [2],... 

---