Free water level

This property is useful in helping to define the interface between fluids. The intercept between the gas and oil gradients indicates the gas-oil contact (GOG), while the intercept between the oil and water gradients indicates the free water level (FWL) which is related to the oil water contact (OWC) via the transition zone, as described in Section 5.9. 

In a reservoir at initial conditions, an equilibrium exists between buoyancy forces and capillary forces. These forces determine the initial distribution of fluids, and hence the volumes of fluid in place. An understanding of the relationship between these forces is useful in calculating volumetries, and in explaining the difference between free water level (FWL) and oil-water contact (OWC) introduced in the last section. 

The capillary pressure can be related to the height of the interface above the level at which the capillary pressure is zero (called the free water level) by using the hydrostatic pressure equation. Assuming the pressure at the free water level is PI ... 

This is consistent with the observation that the largest difference between the oil-water interface and the free water level (FWL) occurs in the narrowest capillaries, where the capillary pressure is greatest. In the tighter reservoir rocks, which contain the narrower capillaries, the difference between the oil-water interface and the FWL is larger. 

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. 

Finally, it is worth remembering the sequence of events which occur during hydrocarbon accumulation. Initially, the pores in the structure are filled with water. As oil migrates into the structure, it displaces water downwards, and starts with the larger pore throats where lower pressures are required to curve the oil-water interface sufficiently for oil to enter the pore throats. As the process of accumulation continues the pressure difference between the oil and water phases increases above the free water level because of the density difference between the two fluids. As this happens the narrower pore throats begin to fill with oil and the smallest pore throats are the last to be filled. 

Wells on both the eastern and western flanks of the structure are water-wet at depths above the free water level of 9815 ft subsea as... 

Figure 11.2 Illustration of capillary trapping in small (a) and large (b) water-wet pores. When at the same height above the free water level, the radii of the oil-water interfaces will be the same. The larger pores contain less water as a percentage of pore volume.

Have the driller or geologist record free water levels in the borings and note aU significant observations during the actual drilling process. These notes should become part of the final log... 

The determination of the volume of free water is difficult because the free water level may vary across the tank bottom surface. The bottom is often covered by pools of free water or water emulsion impounded by layers of sludge or wax. 

Mesopores, with pore-throat diameters between 0.5 and 5 pm, may contain significant amounts of oil or gas in pores above the free-water level (FWL). 

Water zone the rock is 100% water saturated. Note that the 100% water level is above the free water level as a result of the capillary forces. This position correlates with the displacement pressure (also called threshold or entry pressure). Displacement pressure is the capillary pressure at the top of the water-saturated zone. It is the minimum pressure required for the non-wetting fluid to displace the wetting fluid (water) and enter the largest pores (Jorden and Campbell, 1984). 

Ap is the density difference between wetting and non-wetting fluid, h is the height above the free water level. 

This gives the height above the free water level... 

Knowing the function (equation) for capillary pressure versus saturation, the prediction of water saturation distribution above the free water level for the reservoir is possible. Thus, the fluid distribution can be constructed or estimated from core data, if they are representative for a homogeneous section. 

If the Leverett function is known for each rock type of the profile, the saturation profile—referenced to free water level—can be calculated. In Fig. 2.42, a single /-function was applied to the whole reservoir. The additionally plotted red curve connects layers with similar reservoir properties of about 3600 md. 

Step 2 Transformation of pressure (in the reservoir system) into height above free water level. The reservoir-converted capillary pressure is equivalent the buoyancy pressure in the reservoir. From the water gradient and oil gradient (or gas gradient), the height above the free water level results ... 

Figure 2.44 demmistrates the steps from the laboratoiy capillary pressure measurement to the saturation versus depth estimate for the reservoir. From the laboratory measurement, the equivalent oil in reservoir (buoyancy pressure in the reservoir) gives the height above free water level h. ... 

Macroscopic resistivity anisotropy of laminated sediments has been described by Hagiwara (1994, 1996, 1997), Klein (1996), and Klein et al. (1997). Hagiwara (1994) describes anisotropy as the result of the parallel layering of sand and shale. Klein (1996) and Klein et al. (1997) focussed their investigations on modelling of binary, anisotropic sands they demonstrated the effects of macroporous and microporous layers of different water saturation upon resistivity anisotropy. Mollison et al. (1999) and Schon et al. (1999, 2000) derived a modular tensor model to analyse multicomponent induction measurements in anisotropic formations. Kennedy and Herrick (2003) studied the conductivity anisotropy in shale-free sandstone and derived water saturation values of the two sand fractions related to the height above the free water level. 

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