Abstract

In considering the safety performance of CO2 storage in oil reservoir or aquifer and deep geological repository of the nuclear wastes, the groundwater flow and the permeability should be estimated under various stress and thermal conditions.

In this paper, the permeability tests of single tensile fracture of granite cylindrical have been carried out under loading and unloading confining stress processes. In the experiments, the confining stress conditions of from 1.0 to 3.0 MPa and three types of thermal conditions, such as 20, 60, and 90 degree C have been applied to the specimen. The permeability of single fracture has been measured on each confining stress condition.

From the experimental results, the permeability in each thermal condition has decreased under loading process and it has also increased under unloading process. In cases of 20 and 60 degree C conditions, the rates of increase are the same and the reversible performances on loading and unloading process can be found. And, the permeability at each confining stress condition of 60 degree C is lower than that of 20 degree C. In case of 90 degree C, the permeability becomes smaller than that in other thermal conditions at each confining condition. The hysteresis on the loading and unloading process can be found. It is thought that the thermal expansion has occurred in the intact part of rock and the aperture becomes closed in case of 60 and 90 degree C. Moreover, in case of 90 degree C, it is also though that the thermal expansion and the geochemical response has been affected the structure of fracture and the permeability has been changed.

1.
Introduction

Considering the utilization of geothermal energy, the fluid circulated in the rock mass need to be focused. Especially the hydraulic and thermal performance in the low permeable granite basement rock [1].The effective normal stress and permeability acting on the fracture roughness surface has been studied for years [2]. Some research indicates that with increasing the effective stress the permeability is decreasing, and the permeability would be reduced with time due to the fracture asperities deformation and the fracture creep [3]. The differential confining stress has a great influence in the initial coefficient of permeability, and the compaction deformation has less influence than dilation on the monotonic loading creep test [4]. Moreover, in the hydro-thermal processing of short-term permeability test, no minerals dissolution could be seen after the experiment, but when the experimental duration is longer, it need to be given special attention on the issue of dissolution [5]. Some researchers found in a time interval about 400h, the thermal influence reduced the permeability due to the mineral mass removed from the fracture asperities [6]. Although it is important to know the time dependence and mechanical creep behavior under hydro-thermal conditions in many engineering applications, it is difficult to measure the permeability during a long time by using the steady state flow for tight rock, few laboratory works had been conducted [7].

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