Uncertainties arise while interpreting transient tests of horizontal wells. A hybrid approach that uses both analytical and numerical models can circumvent many problems associated with conventional analytical interpretation. The simulator is capable of capturing detailed wellbore phenomena, such as crossflow, phase segregation, and variable storage. The numerical approach is very effective in quantifying vertical permeability and sealing factors of semipermeable barriers. Matching unrecognizable derivative signatures is an advantage of this approach.