Critical Water Saturation
One key petrophysical property that has been overlooked when interpreting whether mobile water is present in a reservoir is the critical water saturation (Swcr). This saturation is an estimate of the minimum connate water saturation (Sw) above which the relative permeability to water (Krw) allows water to flow in the reservoir. This value is greater than the irreducible water saturation (Swirr) and defines a range in saturation “above irreducible water” before water will begin to flow in the reservoir. So the new concept is that water becomes mobile at or above Swcr. This is very important as we look at the interpretation of mobile water from NMR Swirr (nSwirr) and resistivity-based saturations (rSw), or saturation height functions (SHF).
In the last post VPPS Byte 1, an example was discussed where the reservoir was at irreducible water saturation. The rSw matched the nSwirr and the well produced hydrocarbon with water-free production. In this post we will discuss the more difficult situation when rSw is greater than Swirr. Does that mean that the well will flow a mixture of hydrocarbon and water, all water (or all hydrocarbon)?
Identification of Mobile Water
Below in figure 1 is an example of a reservoir with apparent mobile water. Without NMR, one would identify this as a high water saturation hydrocarbon bearing reservoir. Adding the nSwirr and NMR bound fluid (nBVI) to the interpretation shows that Swirr is 20 to 30 saturation units (s.u.) lower than the rSw and one would conclude that this is mobile water. But is it?
The two right most tracks identifies the difference between irreducible and connate water in blue. This is the difference between nSwirr and rSw.
Water Displacing Gas Relative Permeability
In figure 2 are water–gas relative permeability measurements from this field. On the figure is marked the saturation end points, Swirr and Sgr. The data are identical in both figures, with relative permeability plotted logarithmic and linear on the left and right respectively. This represents the ultimate range in recoverable hydrocarbon between irreducible and residual. The relative permeability to water (Krw) starts out at zero when the reservoir is at Swirr and increases to a maximum of approximately 18 percent of the absolute permeability at Sgr. Notice in the figure to the right that there is a range where the Krw is essentially zero from Swirr at 38 s.u. until water starts moving at about 55 s.u..
Water Fractional Flow
Going one step further, the fractional flow of water (fw) can be calculated, representing the amount of water flowing in the reservoir with respect to the total fluid flow. Fractional flow takes into account the difference in relative permeability and viscosity of the fluids flowing at any Sw. Figure 3 shows the water fractional flow over the possible saturation range in the reservoir. As in the previous figure, the data are the same in both figures, but plotted logarithmic and linear in the left and right respectively.
THE DEFINITION: Critical water saturation (Swcr) is the minimum water saturation at which point water becomes mobile.
This point has been taken where Sw has a fractional flow of 1 percent, and shown by the vertical red line. Swcr is 58 percent when the relative permeability to water is 1 percent, or 0.01. So a range of 20 s.u. in saturation exists between Swirr and Swcr in the reservoir where water is immobile.
Going back to the reservoir in figure 1, there is a range where rSw is 20 to 30 s.u. greater than nSwirr. With the knowledge that Swcr is 20 s.u. above Swirr, the reservoir is at that point where water just becomes mobile. Unfortunately, figure 3 shows that there is a very small window of approximately 17 s.u. where the water fractional flow ramps up quickly from 1 percent at 58 s.u. to 100 percent at a saturation of 75 s.u.. Not much room for error in either the rSw or nSwirr.
Modified NMR Rules of Thumb from VPPS Byte 1
- Swt > Swcr indicates presence of mobile water, i.e. fw > 1 percent
- Swcr > Swt > Swirr indicates reservoir is at irreducible conditions with no mobile water
- Swt < Swirr not possible in the subsurface. Either something is wrong with either the resistivity-based saturation, or the NMR irreducible saturation computations.
As we attempt to identify mobile water in wells by comparing rSw and nSwirr, one must keep in mind that Swcr gives a tolerance margin to achieve water-free production when there is an appearance of some mobile water.