Fran Asks PERM:
I’ve been out of school for 40 years but once thought I knew a lot about MVTs (Mississippi Valley Type Lead/Zinc Deposits). We operated under a hypothesis that two fluids were involved: a metalliferous basinal brine and a sulphurous trap in a dolostone ‘reef’. However we were constrained to believe that two fluids of differing densities (T) would not mix and therefore the minerals resulted from an electrical cell driving the process.
Is it allowed to make the generalisation that two fluids of differing densities cannot mix in porous media?
Dr. Jonathan Bryan from PERM Answers:
There are a couple of things that will control mixing of fluids in porous media. First is the presence of gravity: like you mention the higher density fluid will tend to sit below the lower density fluid. But the other thing that will govern fluid distributions is the nature of the fluids themselves:
- Are the two fluids miscible? Like are they both different brines? If so, then both waters have solubility in one another, i.e. based on concentration gradients. This is how in gases, for example, you get mixing of different gases even though their densities may be different: each gas will tend to go from an area of high concentration to an area of low concentration to reach some sort of equilibrium.
- Are the two fluids immiscible (e.g. they don’t mix, like oil and water)? In that case, there is still wettability of the rock (capillary forces) playing a role in fluid distributions. Depending on which fluid the rock is more easily in contact with, this “wetting” fluid will tend to be sucked up into the porous medium, and balanced by the downward pull of gravity.
So unfortunately, we can’t assume that just because two fluids have different density this will keep them apart. It will help for sure, but we also need to also consider the balance between gravity (fluid density difference) and other forces (diffusion or capillarity) that will be present.
Dr. Jonathan Bryan