Hydration zones

Moridis, G.J., Collett, T.S., Dallimore, S.R., Satoh, T., Hancock, S., Weatherill, B. 2004. Numerical studies of gas production from several CH4 hydrate zones at the Mallik site, Mackenzie Delta, Canada. Journal of Petroleum Science and Engineering, 43, 219-238. 

Similar to Sites 994 and 995, six indicators of hydrate were present (1) large gas exsolution from cores, (2) high methane sediment concentration, (3) BSRs, (4) low interstitial-water chlorinity, (5) low core temperatures (although IR technology was introduced just after this hydrate leg), and (6) P-wave velocity logs and resistivity logs. As in Sites 994 and 995, there was a discrepancy between the indicated base of the inferred hydrate zone (452 mbsf) and the phase-equilibria stability zone (491 to 524 mbsf). 

To illustrate the hydrate dissociation pressure response, consider the evidence for hydrates from one pressure stimulation test (MDT-2) at 1090 m depth in hydrate Zone C. Figure 7.38 (Hancock et al., 2005b). In this test 0.5 m of wells were temporarily sealed at the top and bottom of the section, before the well was perforated. In the figure three pressure stimulation tests were performed over the first 8 h, with pressure responses shown by the lower line ... 

The first conclusion which is apparent from these results is the absence of strong water-Nafion polymer interactions. The first water molecules which are absorbed correspond to the solvatation of the ions. For water contents larger than about 8 % ( 5 water molecules) one observes a decrease in the energy which can be explained by an elastic deformation of the polymer involving the motion of the hydrophobic chains out of the hydrated zone. The amount of water absorbed at saturation and the corresponding hydration energy strongly depend on the temperature. 

The finding that there are hydrated zones in the Pb02 particles has allowed the following mechanism for the formation of Pb02 particles to be proposed [37,39] ... 

Hydrated zones exchange ions with the solution. Hence, the lead dioxide particle is an open system [39,42]. PbO(OH)2 may also be represented by the formula H2Pb03. Thus, both OH and H" " ions in the gel zones are in equilibrium with the anions and cations of the solution, which facilitates the discharge reaction [44]. It has been established that the crystallites which build up the crystal zones of the formed PAM particles are sized between 25 and 40 nm, whereas the size of the particles themselves varies from 50 nm to about 150nm, and a single particle may contain several crystallites [38]. These particle sizes give a specific surface-area of 5-7m g PAM. 

Fig. 14.4 Left Stability field of pure methane hydrate at normal seawater salinity, as defined by temperature and pressure expressed as water depth. Intersections of the temperature profiles (stippled lines) with the phase boundary (heavy line) define the area of the gas hydrate stability zone (GHSZ). Right Inferred thickness of the gas hydrate zone in sediments at a schematic continental margin assuming a typical geothermal gradient of 28°C km. Typical bottom water temperatures are marked, and range from 18°C on shallow shelf regions to 2°C at the bottom of the continental rise (after Kvenvolden and McMenamin 1980).

Although some details of the seismic reflection properties are not yet fully understood, it appears that the strength and the characteristics of the BSR is determined by the presence of free gas below the gas hydrate zone (Pauli et al. 1996). The presence of free gas represents a very laige change in seismic velocity, and therefore produces a very strong and sharply defined reflection. Theoretical models (Xu and Ruppel... 

The thermodynamically defined gas hydrate zone in sediments of the deep sea is much thicker than in sediments underlying shallow water, such as those in the upper continental slope (see Fig. 

Hesse, R., 2003. Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface - what have we learned in the past decade. Earth-Science Reviews, 61 149-179. 

Mathews, M.A., and von Huene, R., 1985. Site 570 methane hydrate zone, in Orlofsky, S., ed., DSDP Initerum Reports, Volume 84, U.S. Gov. Printing Office, pp. 773-790. 

Electrons enter the polymer networks. Hydrogen ions migrate into the bulk solution. Oxygen atoms leave the active centres and the latter are unblocked. Reaction (2.46) proceeds again in the vacant active centres. The oxygen atoms accumulate in the hydrated zones and recombine according to reaction (2.43) ... 

Chemical Pb02 contains no hydrated zones [36] and hence its capacity is extremely low and falls down to zero after heating. It can he concluded that the electrochemical activity of PbOi is determined by the degree of its hydration. 

Mason, P. E., S. Ansell, and G. W. Neilson, Neutron diffraction studies of electrolytes in null water a direct determination of the first hydration zone of ions/. Phys. Condens. Matter, 2006.18 8437-8447. 

Fig. 1. Hydration zones of a cation in solution. Metal cation Zone A, the primary hydration sphere zone B, the secondary hydration sphere zone C, the disordered water zone D, the bulk water.

The orderliness of arrangement can be regarded as transmitted outwards from the primary zone, so that there is correlation between the first and second shells. The field of the ion is still strong, and highly polarised water molecules in the first shell readily form hydrogen bonds with others further from the ion. Proceeding outwards, however, the influence of the ion must decline and restriction of freedom of the solvent molecules decreases until orientational rather than translational freedom is lost. As a result, the secondary hydration zone is less well defined than the primary zone, but it contributes to the loss of entropy and of enthalpy. Its effect on heat capacity is probably composite. Since weak restriction of freedom will undergo release with increase of temperature there may be a positive contribution. 

Reaction 3.3 is an equilibrium between lead hydroxide and lead dioxide in the presence of water. Then this water can transform the Pb02 particle into an open system. This open Pb02 particle allows for the transfer of ions between the solution outside the particles and the hydrated zones of the particle. Actually, the hydrated zones occupy about 30% of the PAM. Besides the equilibrium between the hydrated and crystal zones, hydrated zones are also in equilibrium with ions in the solution. In this way, the hydrated linear polymer chains containing Pb, O, and OH can be formed, which plays a critical role in both electron and proton conductions. Both high electron and proton conductivities of these polymer-like chains are necessary conditions for the reaction of discharge of Pb02 to proceed. 

Note that the exchange is restricted to near the surface of the glass membrane where it is in contact with the internal or test solutions and where there exists a hydrated zone of the glass as shown in Fig. 5.5. 

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