FLUID-FLOW IN SYNTHETIC ROUGH FRACTURES AND APPLICATION TO THE HACHIMANTAI GEOTHERMAL HOT DRY ROCK TEST-SITE

Fracture profiles from the Hachimantai geothermal hot dry rock (HDR) t est site in northern Japan have been measured and analyzed to characte rise their geometrical properties. These properties have been used to create a population of numerical synthetic fractures that were tuned t o imitate all the geometric and statistical properties of the natural fracture (described in a companion paper). Such fractures have been us ed as input boundary conditions in three types of modeling. (1) Simple elastic normal closure relating the aperture of the fracture to appli ed normal load. (2) Hagen-Poiseuille calculations of fluid transmissiv ity in the fracture as a function of normal load, fracture fluid press ure, and temperature. (3) Two-dimensional flow modeling within the rou gh walled fracture using Reynolds equation. The modeled closure of the se fractures provided a realistic relationship between normal and frac ture fluid pressure and aperture. When this pressure/aperture relation ship was combined with a Hagen-Poiseuille approach to calculating flui d transmissivity in the fracture, we obtained results which, when comp ared with data from field transmissivity tests at the Hachimantai site , showed that the functional dependence of fluid transmissivity with f racture fluid pressure was well modeled, but overestimated by a factor of about 2. Reynolds equation flow modeling was carried out in the sy nthetic fracture to ascertain the extent to which the Hagen-Poiseuille law was overestimating the transmissivity due to the rough surfaces a ffecting the fluid flow in the fracture. When the calculations were co rrected for this effect, the overestimation in the fluid transmissivit y was reduced considerably.