Abstract
In order to evaluate long-term geothermal reservoir performance, it is important to know how permeability of rocks changes with time under hydrothermal environments. This paper presents experimental results of permeability tests performed on two granites under the conditions of temperature up to 350°C and confining pressure up to 45MPa. Two types of spesimen were used for the experiments; a thick-walled cylinder of the intact rock and a thick-walled cylinder with a throughgoing artificial fracture perpendicular to the cylinder axis. The permeability was measured under the hydrothermal conditions or after the specimen was coolled down, by passing water radially through the specimen. The permeability of the intact samples under the hydrothermal conditions increased with time, and the increasing rate strongly depended on test temperture and flow rate of water. From both porosity distributions and the relations between permeability and porosity, it has been revealed that the water flow occured only through the selected microcracks, and that the width of the microcracks increased during the flow of hot water. Thus the increase of the permeability was caused mainly by the widening of the microcracks due to the rock dissolution. Furthermore, when the pore fluid pressure was greater than the confining pressure, as in the case of radial flow from inner wall, stress corrosion cracking caused the extensive growth of the microcracks, which eventually formed fractures along the rift plane of the granite. The permeability of the artificial fracture decreased initially, and then increased, during the flow of hot water through the fracture. From the reduction of the surface roughness (rms. value) by the test, it was infered that the initial decrease of the permeability was caused by the creep fracture of the surface asperities. In addition, SEM observations of the surfaces showed that some channels developed in the surfaces due to the rock dissolution by hot water, which caused the later increase of the permeability.
