The estimation of reservoir heterogeneity through reservoir responses is a challenging task. Well-test analyses provide information about the interwell permeability distribution of a reservoir. To correlate the response of pressure-transient tests with permeability-distribution type, correlated permeability distributions in the x-y plane are generated by use of fractal Brownian motion. A strong heterogeneity effect is an increase in skin effect with increasing fractal dimension of permeability distribution. This additional pseudoskin effect caused by heterogeneity was correlated with the fractal dimension of the permeability distribution. The method can be used to assess reservoir heterogeneity quantitatively by use of the pressure-transient response.