Abstract :
The diffusion coefficient (D) of He in the Carnmenellis granite, recovered from the Rosemanowes hot dry rock (HDR) geothermal reservoir, is experimentally determined in the laboratory in a temperature range of 100–300°C. Temperature variation of D fits an Arrhenius plot, but yields an activation energy for He release from rock significantly lower than the value for feldspar or quartz, suggesting that most of the He in the reservoir granite resides within grain boundaries and jointing cement and may be easily released.
The reservoir surface area (S) is estimated based on the laboratory determined value of D from granite and the measured4He contents of circulation fluids from RH 15 well. In the first year of reservoir circulation,4He-based reservoir surface area was about twice that based on222Rn. It increased and stabilized at about 6 times after 2 a. The excess He release from newly opened up fracture surfaces within the reservoir during its expansion is shown to be responsible for the temporal increase in the He-based reservoir surface area over that of Rn.
