When is the Free Water Level Not the Free Water Level?
The FWL is a?horizontal?surface of zero capillary pressure and is very important as it defines the base for the Water Saturation vs. Height Function (SWHF), which initialises the 3D reservoir model with water and hydrocarbon saturations.
The FWL is the level fluids would separate out in a very wide borehole.?It is the intersection point of formation pressures.
In fields with both gas and oil legs, the FWLG for the gas interval is different from the FWLO for the oil interval. Therefore the base of SWHF for the gas leg (FWLG) is actually between the FWLO and Gas Oil Contact (GOC), as shown in the plot.
The Hydrocarbon Water Contact, is less important, as it is the height where the pore entry pressure is sufficient to allow hydrocarbon to start invading the formation pores. This depends on the local porosity and permeability and is therefore a surface of?variable?height.
Petrophysicist
1 年Here's SPWLA paper: https://www.dhirubhai.net/posts/steve-cuddy-84780210_the-bvw-sw-vs-height-function-activity-6868542400042803201-3RN9
Independent consultant Oil and Gas Industry (Freelancer)
1 年Thanks for sharing
Consultant Petrophysicist at MSK Scientific
1 年I would say FWLg (in the diagram) has no fundamental significance. It might be useful for calculating Pc in the gas leg, but it is really just a geometrical construction. The GOC is fundamentally different to the OWC (at least in a water wet system). The latter is the deepest depth where a non-wetting fluid is encountered. The GOC is just a change to the non-wetting fluid with water still the phase adhering to the grains. I have seen claims that there will be a transition from oil to gas above the GOC that is analogous to the transition zone above the FWL but that is not the case and the change from oil to gas should be sudden. By the same token there is no equivalent of the 'entry pressure' and if the well intersects a GOC, logs will respond to the change at precisely that depth. Of course, there are all sorts of practical issues which spoil this neat picture: for example if a gas charge is displacing oil from the top of the structure, I am sure some oil will remain trapped in dead-end pores. But the idea of a smooth transition from 100% oil and water to 100% gas and water over a zone of several metres is simply wrong.
Petrophysicist at Crested Butte Petrophysical Consultants Actively Engaged in Petrophysics
1 年Your question about the apparent FWL not being a true FWL is very apparent in reservoirs that have been affected by production. That is why we have had to go back to older wells in areas of the field that were not affected by production and perform a FWL search on those wells having capillary pressure parameters instead.
Petrophysicist
1 年Make these 2 plots to see if BVW helps you understand your reservoir better than Sw