Fluid flow in synthetic rough fractures and application to the Hachimantai geothermal hot dry rock test site

Abstract

Fracture profiles from the Hachimantai geothermal hot dry rock (HDR) test site in northern Japan have been measured and analyzed to characterise their geometrical properties. These properties have been used to create a population of numerical synthetic fractures that were tuned to imitate all the geometric and statistical properties of the natural fracture (described in a companion paper). Such fractures have been used as input boundary conditions in three types of modeling. (1) Simple elastic normal closure relating the aperture of the fracture to applied normal load. (2) Hagen‐Poiseuille calculations of fluid transmissivity in the fracture as a function of normal load, fracture fluid pressure, and temperature. (3) Two‐dimensional flow modeling within the rough walled fracture using Reynolds equation. The modeled closure of these fractures provided a realistic relationship between normal and fracture fluid pressure and aperture. When this pressure/aperture relationship was combined with a Hagen‐Poiseuille approach to calculating fluid transmissivity in the fracture, we obtained results which, when compared with data from field transmissivity tests at the Hachimantai site, showed that the functional dependence of fluid transmissivity with fracture fluid pressure was well modeled, but overestimated by a factor of about 2. Reynolds equation flow modeling was carried out in the synthetic fracture to ascertain the extent to which the Hagen‐Poiseuille law was overestimating the transmissivity due to the rough surfaces affecting the fluid flow in the fracture. When the calculations were corrected for this effect, the overestimation in the fluid transmissivity was reduced considerably.