Simulation of heterogeneous flow in a natural fracture under varying normal stress

The present study considers flow heterogeneity in individual fractures under varying degrees of normal stress. We simulate a laboratory investigation where, using a triaxial loading chamber, the flow conditions through a natural fracture were monitored while the normal stress was increased in a step-wise manner. Simultaneously the mechanical displacement was measured. Fracture surfaces have been characterized using optical laser-profilometry. We use this data as a preliminary basis for determining the fracture aperture distributions. Based on these data the heterogeneous transmissivity distributions for the numerical model are determined, assuming cubic law to be valid locally. The effect of varying stress is simulated by moving the fracture surfaces with respect to each other and thus altering the aperture-contact distribution inside the fracture. An agreement between measured and modeled data was achieved to some extent. However, the transmissivity distributions yielding the best results were at places very discontinuous with narrow channels governing the flow route. The system then becomes very sensitive to the transmissivity properties at the critical locations and the measurement accuracy of the aperture data becomes crucial.