Carrier reservoir

A bare surface of silicon can only exist in fluoride containing solutions. In reality, in these media, the electrode is considered to be passive due to the coverage by Si— terminal bonds. Nevertheless, the interface Si/HF electrolyte constitutes a basic example for the study of electrochemical processes at the Si electrode. In this system, the silicon must be considered both as a charge carrier reservoir in cathodic reactions, and as an electrochemical reactant under anodic polarization. Moreover, one must keep in mind that, according to the standard potential of the element, both anodic and cathodic charge transfers are involved simultaneously (corrosion process) in a wide range of potentials. 

SCLCs require that the contacts on the polymer be able to act as an infinite carrier reservoir with respect to bulk demands this requirement is just an alternative operational description of an ohmic contact. To reiterate, unambiguous determination of mobility directly from steady-state electrical measurements requires ohmic contacts and space-charge-limited conditions. The observation of linear curves ofj versus E is usually of no significance. 

Figure 3.10 Observed groundwater pressures in Jurassic and Triassic carrier-reservoir rocks in the Viking Graben, North Sea (based on data presented by Buhrig, 1989, in Fig. 6, p, 38, Marine and Petroleum Geology, Vol. 6. Reproduced by permission of the publishers, Butterworth Heinemann Ltd. ).

When the source rock is on top of the carrier reservoir rock, the downward expelled separate phase hydrocarbons will initially accumulate along the source rock - carrier rock boundary, as the fine-grained source rock acts as a top seal boundary. Once the critical vertical height of the hydrocarbons has been reached, the hydrocarbons will migrate vertically updip along the source rock - carrier rock boundary. 

Vd = drainage volume = volume of carrier-reservoir rock through which... 

Sr = apparent residual saturation of the carrier-reservoir rock to... 

The process of migration may lead to focussed movement of hydrocarbons into economic accumulations. The secondary migration of hydrocarbons may occur under hydrostatic or hydrodynamic conditions. Under hydrostatic conditions, the hydrocarbons migrate through the water-saturated carrier-reservoir rocks as separate phase hydrocarbons. Under hydrodynamic conditions, the hydrocarbons may be transported in continuous separate phase, in suspension or in aqueous solution. Under both hydrostatic and hydrodynamic conditions, the hydrocarbons ultimately appear as separate phase hydrocarbons before they can accumulate in a trap (Tissot and Welte, 1984). 

The hydrodynamic holding capacity of conventional hydrostatic traps is not only determined by the direct hydrodynamic influence on the sealing characteristics of the barrier rocks, or barrier rocks and faults, but also combined influence of the geometry of the trap and the hydrodynamic condition in the carrier-reservoir rock. This can be illustrated by looking at the hydrodynamic influence on the position of a hydrocarbon accumulation in a conventional hydrostatic trap. 

In areas with non-vertical groundwater flow through the carrier-reservoir rock, the equipotential surfaces for oil and gas are tilted downwards in the direction of the net driving force for groundwater flow. The angle of tilt for the gas or oil equipotential surface is given by Hubbert (1953) (see Figure 5.9) as... 

According to Hubbert (1967), considering hydrodynamic conditions in the carrier-reservoir rock the number of permutations of lithological and hydrodynamic conditions, and of oil and gas densities that can combine to produce hydrocarbon traps is unlimited. Hubbert distinguished two groups of traps ... 

Those that occur in conventionally closed lithological structures (i.e. in hydrostatic traps). In these traps the hydrocarbon-water contact may have any degree of tilt from the horizontal to the maximum dip of the barrier boundary at the downstream side of the closure. Although hydrocarbons may become trapped in the conventional hydrostatic traps of sufficient sealing capacity, the hydrocarbon accumulation is not necessarily present in the same position within the trap, as its actual position depends on the hydrodynamic condition in the carrier-reservoir rock (Figure 5.10). 

Figure 5.11 Cross-section of homoclinally dipping carrier-reservoir rock with variable permeability, showing potential oil migration directions and trapping positions for different directions of groundwater flow (after Hubbert, 1967).

Under hydrostatic conditions, the hydrocarbons will become trapped in the reservoir rock when buoyancy-induced lateral upward hydrocarbon migration in the carrier-reservoir rock is stopped by a capillary pressure boimdary. Hydrostatic trapping positions include structural traps, stratigraphic traps and combination traps. The maximum height of a hydrocarbon column that can be contained in a hydrostatic trap is determined by the sealing capacity and geometry of the rocks, or rocks and faults, that form the trap. 

The present hydrogeological framework of the sedimentary basin, which is characterized by the distribution, thickness and dip of porous and permeable hydrogeological units (aquifers/potential carrier-reservoir rocks, e.g. sands, sandstones, carbonates, fractured rocks) and poorly permeable hydrogeological units (aquitards/potential barrier rocks, e.g. shales, evaporites), and the location of geological structures and tectonic elements of importance for subsurface fluid flow, e.g. permeable or impermeable faults, unconformities... 

At every point in a carrier-reservoir rock, uj,c, which is proportional to the hydrocarbon potential, cam be determined from the elevation z and the value of vj,c. which can be calculated from the groimdwater potential (Figure 8.7). The UVZ mapping procedure results in maps or cross-sections showing hydrocarbon equipotential surfaces in carrier-reservoir rocks from which hydrocarbon migration directions and potential trapping positions can be derived (Figures 8.8 and 8.9). 

Figure 8.9 Map showing potential oil trapping position as determined by the UVZ mapping procedure (z = structure contours of top carrier-reservoir rock vg = groundwater equipotential lines ug = oil equipotential lines (from Dahlberg, 1982. Reprinted by permission of Springer-Verlag).

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