If "well" means in a good way:
* Badly
* Poorly
* Unsuccessfully
If "well" means healthy:
* Sick
* Ill
* Unwell
If "well" means a deep hole:
* Surface
* Top
* Apex
If "well" means a spring of water:
* Dry (as in a dry well)
Please provide more context if you need a more specific opposite word.