1. A method of operating a computer system to perform a reservoir simulation, the method comprising: generating a first model of a portion of earth containing the reservoir, the first model including a predetermined number of grid-blocks, and being specified by a first set of parameters including at least a permeability of each grid block; computing, for a first predetermined number of iterations, a first productivity index of the first model, the first productivity index at each iteration being computed based on a flow rate of a fluid obtained by changing the permeability of each grid-block by a predetermined amount; generating a second model of the reservoir that includes fractures; computing a second productivity index of the second model by simulating the second model for a second predetermined number of iterations, each iteration including, changing a value of exactly one parameter of a second set of parameters by a predetermined amount; calculating a skin factor based on the computed first productivity index, and the computed second productivity index; and computing, by regression analysis, a correlation of the computed skin factor (S), with the second set of parameters, wherein the second set of parameters includes a matrix permeability (K m ), a fracture permeability (K f ), a fracture half length (X f ), and a fracture width (W), and wherein the correlation is computed as: 
 S=âˆ’ 2.7798+0.1183 K m âˆ’0.0003 X f âˆ’4.4211 Wâˆ’ 0.0477 *L n ( K f )âˆ’0.0634 *L n ( C fd ), where C fd is a dimensionless quantity; selecting a candidate well for stimulation; and stimulating the selected candidate well based on the computed correlation.