Re: Salt domes

Biswajit Banerjee — Thu, 24 Jul 2008 01:54:52 -0400

I was hoping for someone else to chime in before I did. But since there seems to be no comers yet let me give it a shot. Caveat: I've looked at the deformation of salt pillars when mined but never at the behavior of in-situ salt.

Whether you want treat the salt as a non-Newtonian fluid or as a solid depends on the time scale that you're interested in (see Deborah number in Wikipedia and other more reliable sources). Also, it's hard to determine whether a material will fracture if you treat it as a fluid that can essentially wrap around itself many times without cracking - think lava before it cools enough to crack. The solid-fluid intermediate stage is a fascinating but largely unstudied domain in the mechanics literature.

If you're dealing with small timescales, a viscoelastic-plastic model is probably the better way to go. Salt creeps considerably under pressure but doesn't flow as readily as lava.

The interface between the cap rock and the salt is best modeled as a frictional contact surface (when you're using solid mechanics) but as a no-slip bc when you're calling the salt a fluid. Unless there is a thin layer of water at the interface which lubricates the flow, I would expect a significant amount of friction and probably cracking due to that.

I'll try to come up with a better response if I get the time.

-- Biswajit

Using Fluid Mechanics for Geologic Salt Domes?

Charlie Grummon — Mon, 21 Jul 2008 13:22:27 -0400

I was recently presented with a problem concerning the migration of a salt formation in an area of Utah. After spending a little time looking at the given data, I decided it might be a problem that could be approached using fluid mechanics. The idea being that the salt formation, relatively speaking, is highly viscous and is free to respond to acting forces and deform appropriately. Essentially, I was hoping to treat the salt formation almost like the bladder of a water bed reacting to differential loading.

As far as the numbers are concerned, I have data sets that define X and Y coordinates and respective depths for various geological formations. These data points constitute the "tops" of the salt layer as well as layers above and below the salt. Along with these depths I am willing to make very broad and very general assumptions about the salt layer.

1. The salt is incompressible unlike the layers above it, and consequently as the layers above do compress with further deposition, the salt will act as a buoyant body that reacts to differential loading.

2. I would like to consider the salt as frictionless with respect to its upper and lower contacts due to its brittle nature (the salt is lying on what is essentially a sloped plain and so it is free to slide along that slope to achieve equilibrium).

3. The salt's volume is a constant, as there is little evidence of dissolution.

4. The underlying foundation of the salt has remained unchanged since initial deposition.

Anyway, where this leaves me is I can assume what the salt looked like at deposition and what it looks like now. I can find volumes and weights, and I am willing to apply appropriate generalizations and assumptions. From there I was wondering if it would be appropriate to use fluid mechanics to analyze the various forces acting upon the salt by the upper layers, and vice versa. I don't expect to get exact numbers, so much as a good idea about the general behavior of the salt with the final result being a good idea of where to look for areas of high stress and dynamic shifting. Ultimately, I hope to use this knowledge to locate probable regions of fracturing which I could then compare to current data of fracture locations and perhaps get correlation and maybe even forecasting.

I realize this is a mouth full for a forum but I thought it would be worth a try. Anyway, if anyone could give me an idea of the feasibility of perusing this problem and maybe even a few pointers in the right direction it would be greatly appreciated. Thank you for taking an interest and I look forward to your responses.

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Re: Salt domes

Using Fluid Mechanics for Geologic Salt Domes?