This study simulates the effect of in-situ bacteria growth on the reduction of the effective permeability of a fractured medium. The authors use a power law for fracture length distribution and a fractional Brownian motion for hydraulic fracture aperture spatial distribution, which can be characterized by the length exponent (a) and the Hurst exponent (H), respectively. The typical ranges of 1.4 less than or equal to a less than or equal to 2.2 and 0.1 less than or equal to H less than or equal to 0.9 are considered. Nutrients are injected continuously for the growth of in-situ bacteria. The results show that in-situ bacteria growth reduces the permeability hyperbolically, but the backbone porosity does not change significantly. It also shows that the reduction of the permeability proceeds faster for smaller values of a and for larger values of H. The fracture length distribution has a stronger effect on the speed of permeability reduction than the hydraulic fracture aperture spatial distribution. The time needed to reduce permeability is inversely proportional to the hydraulic gradient.