There are no prisoner lists for Libby prison per se.As you know when the COnfederates evacuated Richmond at the end of the war, they burned the majority of their records.
Libby prison was used to warehouse Union commissioned and non-commissioned offiers. Enlisted men were also held here for short stays en route to camps for enlisted prisoners or to prisoner exchange sites.
In what state and regiment did your ancestors serve?This is the first information you will need to attempt to find them.I checked the National Park Service Soldiers & Sailors database and did not find a Brainard Curlbut there were 15 John Curls listed. http://www.itd.nps.gov/cwss/soldiers.cfmhttp://www.itd.nps.gov/cwss/soldiers.cfm
After you determine the regiment (and state) in which he served you can request his Compiled Military Service Record from the National Archives.