•Heat extraction from EGSs with heterogeneous reservoirs is modeled.•A large number of cases with various heterogeneous reservoirs are simulated.•Effects of reservoir heterogeneity on EGS heat extraction are analyzed.•A method is proposed to properly quantify reservoir heterogeneity.•An equation relating EGS heat extraction to reservoir heterogeneity is established. Strong heterogeneity does exist in engineered heat reservoirs of enhanced geothermal systems (EGSs) due to different rock formation, pre-existence of natural fractures and uncertainties associated with the stimulation processes. Exploring the influence of reservoir heterogeneity on EGS heat extraction is of considerable importance to predict and fully evaluate the performance of EGSs. In this work, we take the reservoir as equivalent to a porous medium and employ a previously self-developed numerical model to conduct a series of simulations of the long-term heat extraction process of EGSs with different reservoir porosity distributions. Based on a detailed analysis to the effects of fluid seepage flow field on the heat extraction performance, we establish a method to quantify the heat extraction performance of EGSs with heterogeneous reservoirs using the seepage distribution data. We numerically test this method by taking a wide variety of heterogeneous reservoirs for EGSs. The method proves to have the capability of predicting the heat extraction performance of EGSs with miscellaneous heterogeneous reservoirs.