General question about Capillary Pressure
In terms of flow I still struggle to understand the reasons/use of having capillary pressure as an option in ROCKS.
How different is flow when it is disabled?
What are reasons or scenarios where it can be disabled (i.e. scale)?
4 replies
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Alejandro,
The capillary pressure function in the ROCKS block applies allows you to set capillary pressure relationship for any rock type for which the option is activated. Otherwise, the capillary pressure function defaults to the one defined in the RPCAP block. I’m unsure if there is a default behavior if neither is included, but there are ways to set a capillary pressure of 0, if that is what you wish.
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Thanks for your reply.
Yes, I have seem this option for setting capillary pressure to zero and that is what I struggle to understand. How is the flow in such cases, do grid cells immediately invaded by a non-wetting phase? In which scenarios it makes sense to use it?
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In general, fluid flow is driven by pressure gradients, gravity, and capillarity. I think it is most important to include capillary pressure when pressure-driven flow is small. For example, in establishing capillary/gravity equilibrium in the vadose zone above a water table. If pressure-driven flow dominates (for example, near a well), then it may be less critical to accurately represent capillarity. But there could be subtle interplays between pressure-driven flow and capillarity (a heat pipe comes to mind), that you would miss without capillary forces. Also, in my experience, the code has more trouble numerically when Pcap=0, because capillary forces act to smooth out saturation changes. One place I always do use Pcap=0 is when representing an atmospheric boundary condition. If you have a particular problem where you are unsure what capillary pressure strength (or functional form) to use, I recommend trying different values and seeing what effect they have.
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Thanks a lot for the explanation. I am modelling some core-scale flooding experiments where water saturated cores are pressurized by injecting a non-wetting phase at a very slow rate. Even though the flooding is pressure driven, it is at a very slow injection rate. I guess such interplay between capillary driven flow and pressure driven flow can be relevant for this exercise.