What fish are in the tank? The problem with shells is that they can raise the alkalinity/pH of the water which isn't always good...especially for your average freshwater tank. They would help provide calcium, though, depending on how old the shells are and if they've been treated with anything.
But if you have lots of other fish in the tank be careful. Shells are best suited for brackish/marine set-ups.
If you do need calcium for the snails then I would just buy a small bottle of liquid calcium (made for marine tanks) and add a tiny amount once in a while. You can also feed calcium-rich leafy greens to the snails such as spinach and kale .