Monte Carlo Simulation on the Effect of Fracture Characteristics on Reduction of Permeability by In-Situ Bacteria Growth

Abstract

Abstract We have simulated the effects of fracture characteristics on the reduction of effective permeability of fractured rocks due to in-situ bacteria growth. A solute is injected continuously to provide food for the growth of in-situ bacteria. We used a power law for fracture length distribution and a fBm for fracture aperture spatial distribution. The results show that in-situ bacteria growth reduces the permeability hyperbolically, but the porosity of backbone fracture does not change significantly. It shows that the reduction of the permeability proceeds faster for smaller values of length exponent and for larger values of Hurst exponent. The fracture length distribution has a stronger effect on the speed of permeability reduction than the aperture spatial distribution. The time needed to reduce permeability is inversely proportional to the hydraulic gradient. Introduction In-situ bacteria growth can plug underground flow paths and thus reduce permeability of a medium. The reduction of permeability by in-situ bacteria growth often has adverse effects on groundwater recharge. It increases injection pressure or decreases injection rates in water injection wells,1 which can cause serious problems in managing underground caverns for hydrocarbons storage.2 On the other hand, the reduction of permeability by in-situ bacteria growth has been used for enhanced oil recovery by petroleum engineers.3,4 The sealing of high permeability zones with a bacterial biofilm would permit an ensuing waterflood to contact bypassed low permeability regions. This can improve oil recovery efficiency by increasing volumetric sweep. Although the change of permeability of a porous medium by bacteria has been researched experimentally5,6 and numerically,7,8 few researches have been performed for a fractured medium. The transport of solutes needed for bacteria growth and the transport of bacteria itself are affected by fracture characteristics such as a fracture length distribution and a fracture aperture spatial distribution.9 In this study we assume that a fracture length distribution follows a power law characterized by the length exponent (a) and a fracture aperture spatial distribution follows a fractional Brownian motion (fBm) characterized by the Hurst exponent (H). Then we simulate numerically the effect of a and H on the reduction of permeability of a fractured medium by in-situ bacteria growth. Procedures Procedures of Numerical Simulation. We need five steps to numerically simulate the effect of fracture characteristics on the reduction of permeability by in-situ bacteria growth:generation of a fBm fracture aperture spatial distribution,generation of a power law fracture length distribution,calculation of flow field and permeability,simulation of transport of the solute, bacteria, and extracellular polymeric substances (EPS) that are reaction byproduct from bacteria, andcalculation of change of fracture aperture by bacteria and EPS.