1. A method of operating a computer system to perform reservoir simulation, the method comprising: providing a first model of a portion of earth containing a reservoir, the first model specifying parameters defining the reservoir; computing a first productivity index of the first model for a first set of data points; providing a second model of the reservoir that includes fractures; computing a second productivity index of the second model for a second set of data points; calculating skin based on the computed first and second productivity indexes; and performing regression analysis, using a subset of the second set of data points to obtain skin correlation.