1. J.W. Freeman, J.L. Buster, R.D. Knudsen, US Patent 5,856,257, Philips Petroleum Company, January 5, 1999.
2. W.K. Reagan, T.M. Pettijohn, J.W. Freeman, E.A. Benham, US Patent 5786431, Phillips Petroleum Company, 1998.
3. M.E. Lashier, EP 0780353A1, Phillips Petroleum Company, 1997.
4. W.K. Reagen, B.K. Conroy, Canadian Patent 2020509, Phillips Petroleum Company, 1991.
5. W.K. Reagan, T.M. Pettijohn, J.W. Freeman, US Patent 5523507, Phillips Petroleum Company, 1996.