Modelling fluid flow in rough fractures: Application to the Hachimantai geothermal HDR test site

Abstract

Natural fractures in rocks serve a very important function in the transport of fluid through rocks, as well as in the flow of electrical charge and heat. The first step in understanding these processes must be the complete description and characterisation of the fractures themselves, giving due regard to the often fractal nature of their surfaces. This paper will present a brief analysis of fracture profiles from the Hachimantai geothermal HDR test site in northern Japan, and discuss the implications for fluid flow at this location. The analysis shows that the Hachimantai fracture has a well defined isotropic fractal dimension of 2.305 between the length scales 0.02 mm to 40 mm. The data from the analysis of fracture profiles have been used as input parameters to numerical models of rough fracture surfaces. These surfaces have been used together with a simple fracture closure code to calculate the fluid transmissivity at depth in the Hachimantai fracture. Comparison of the predicted transmissivities produced in this way with those measured by field tests show that the trend and variation of the predicted transmissivities are reproduced well, but are overestimated by a factor representing an overestimation of the fracture aperture of about 1.3.