Pressure-depth

Gas density at reservoir conditions is useful for calculation the pressure gradient of the gas when constructing pressure-depth relationships (see Section 5.2.8). 

The density of the oil at reservoir conditions is useful in calculating the gradient of oil and constructing a pressure - depth relationship in the reservoir (see section 5.2.8). 

In Section 5.2.8 we shall look at pressure-depth relationships, and will see that the relationship is a linear function of the density of the fluid. Since water is the one fluid which is always associated with a petroleum reservoir, an understanding of what controls formation water density is required. Additionally, reservoir engineers need to know the fluid properties of the formation water to predict its expansion and movement, which can contribute significantly to the drive mechanism in a reservoir, especially if the volume of water surrounding the hydrocarbon accumulation is large. 

Flence it can be seen that from the density of a fluid, the pressure gradient may be caloulated. Furthermore, the densities of water, oil and gas are so significantly different, that they will show quite different gradients on a pressure-depth plot. 

In abnormally pressured reservoirs, the continuous pressure-depth relationship is interrupted by a sealing layer, below which the pressure changes. If the pressure below the seal is higher than the normal (or hydrostatic) pressure the reservoir is termed overpressured. Extrapolation of the fluid gradient in the overpressured reservoir back to the surface datum would show a pressure greater than one atmosphere. The actual value by which the extrapolated pressure exceeds one atmosphere defines the level of overpressure in the reservoir. Similarly, an underpressured reservoir shows an pressure less than one atmosphere when extrapolated back to the surface datum. 

If a pressure measuring device were run inside the capillary, an oil gradient would be measured in the oil column. A pressure discontinuity would be apparent across the interface (the difference being the capillary pressure), and a water gradient would be measured below the interface. If the device also measured resistivity, a contact would be determined at this interface, and would be described as the oil-water contact (OWC). Note that if oil and water pressure measurements alone were used to construct a pressure-depth plot, and the gradient intercept technigue was used to determine an interface, it is the free water level which would be determined, not the OWC. 

Figure 6. Pressure-depth chart for the Middle Devonian Keg River Formation of northern Alberta. See Figure 5 for lines of section.

Thus Korzhinskiy s (1940, 1957) well known conclusion that there is a systematic increase in activity of carbon dioxide with depth finds new theoretical substantiation. It is to be expected that as pressure (depth) increases, if the temperature remains constant silicate mineral associations will be replaced by carbonate associations even in those cases where the composition of the water-carbon dioxide fluid Tcoj- Hjo)... 

Thermobarometry is the estimation of the equilibrium temperatures and pressures (depths) recorded by mineral chemical equilibria. 

Also, the water content of basalts reduces CO2 solubility therefore, water-rich basalts (with high He/ He ratios e.g., along the shallow Reykjanes Ridge) can have significantly lower CO2 (and helium) compared to water-poor MORB erupted at greater confining pressures/depths (Hilton et al, 2000b). 

The term fluidization is applied to processes in which a loose, porous bed of solids is converted to a fluid system, having the properties of surface leveling, flow, and pressure-depth relationships, by passing the fluid up through the bed. 

Figure 2.7 Pressure-depth relation in thick compacting fine-grained rocks (modified after Magara, 1978. Reprinted by permission of Elsevier Science Publishers BV).

A shallow subsystem, characterized by cross-formational vertical upward flow of groundwater. The groundwater potential increases slightly with depth. The pressure-depth gradient is near hydrostatic to slightly superhydrostatic (Figure 2.10). 

Figure 2.10 Characteristic pressure-depth relations in the three subsystems of burial-induced groundwater flow.

C. Pressure-depth curves for sand and shale units in areas I, II and III... 

The shallow subsystem of burial-induced groundwater flow is characterized by cross-formational vertical upward flow of groundwater and near-hydrostatic pressure-depth gradients. In comparison with hydrostatic conditions, the... 

The dynamic pressure increment, Ap at a certain depth is the difference between the hydrostatic and the hydrodynamic pressure-depth relation at that depth (T6th, 1978). T6th (1978) showed that the dynamic pressure increment is a function of both ground surface elevation and depth of measurement. He proposed the use of a two-dimensional presentation of this relation (the elevation-depth pattern Ap,-z-d) as an analysing tool. The elevation-depth... 

---