I think the big danger would be their leaking through the many wells that will be driven through freshwater aquifers to reach those very deep saline ones. Somebody earlier referred to a pincushion effect. My guess is that we'll have tens of thousands of wells drilled through freshwater aquifers to reach bitumen, and for carbon sequestration, to even deeper levels. I suspect that would be the big danger.
I think if we can get it down without seepage into the saline ones, it would probably stay there.