Reliable estimates of post perforation damage skin are important for designing remedial solutions and productivity enhancement operations. Underbalanced perforating (UBP), which is widely used in well completions, induces transient fluid flow that provides an opportunity for quantifying the formation parameters. However, the skin factor can rarely be estimated reliably from pressure data acquired in the current UBP operations if without flowing on surface in sufficient time. The reasons are that (a) the flow rate after an UBP continuously varies during the surge; (b) the skin factor may decrease substantially during the flow period because the mud cake, invaded filtrates, and particulate pore plugging are progressively removed at the vicinity of the sandface region; (c) the crashed layer of the perforation tunnels is cleaned up. The existing pressure transient analysis methods to determine the skin were almost exclusively developed with an assumption that the skin factor remains constant during a test. The conventional analyses do not represent the underlying physics of the surge flow and lead to loss of useful information from the skin variations. Few SPE papers have investigated the variable skin condition with a simple hyperbolic function for a constant rate drawdown. The constant rate drawdown assumption is not suitable for the UBP as the flow rate varies along with the changes in the skin (during clean-up) and the wellbore pressures.We present a general formula to handle the skin variation resulting from the UBP and other surge flow tests. This formula extends the applications of a classic convolution algorithm to include variations in the skin and flow rates. New analytical solutions have been derived for both homogeneous and heterogeneous reservoirs based on the general convolution formula. The new solutions are applied to an integrated interpretation workflow to invert the skin variation as well as other formation parameters from pressure measurements. A field example, where both the skin and flow rate vary, demonstrates a successful application of the new solutions and interpretation methodology. The inferred skin variation allows a complete and detailed quantification of the perforation quality and cleanup efficiency. The results of this work enable the implement of better strategies for well completions and production optimization as well as correct inputs of nodal analysis.