Abstract

Because the topography of the fracture surface evolved in a long-term under different conditions, it exerts a significant influence on the flow behavior. The flow would bypass the contact area between the fracture surfaces under the coupled conditions. In order to study the different characteristic factors which control the flow behavior, this work has studied the hydraulic properties of a single rock fracture by utilizing a granite specimen. The granite specimen was fixed within the triaxial vessel under the confining pressure. Several permeability tests were performed at three different temperatures (20, 60 and 90 °C). The permeability value was evaluated at different times in both short-term and long-term tests. Results show that the permeability values changed regularly in the short-term tests under different confining pressure conditions. In contrast, irregular phenomena of the permeability changing was found in the long-term tests under the constant confining pressure of 3.0 MPa. Especially at 60 °C, the permeability decreased in the early experimental period, but it increased after 60 days and varied with slight oscillation. Moreover, the permeability decrease under a higher temperature can be confirmed in both short- and long-term tests. The reason for the permeability reduction may be considered as the chemical effects under coupled processes. In this study, the effluent from the fractured specimen was also sampled in the long-term tests to evaluate the mineral dissolution. The phenomena that the topography of the fracture surface evolved with time might be controlled by the evolution of the mineral dissolution, which resulted in the permeability variation and changed the contact area between the fracture surfaces. The geometrical alteration in fractures that should have affected the permeability evolution is needed to be further examined.

1. Introduction

It is well known that the mechanical and hydromechanical properties of fractures within rock mass would change under various conditions. The coupled THMC (thermal-hydraulic-mechanical-chemical) processes exert significant influence on the subsurface fluid flow in geological systems (Polak et al., 2004; Min et al., 2009). Several previous experimental works have been investigated on the hydraulic properties of rock fracture under coupled hydro-thermal conditions (Polak et al., 2003; Yasuhara et al., 2006). In these researches, the phenomena that chemical and mechanical compaction can reduce the permeability of the fracture was especially focused. The reduction of the permeability may be explained by the mechanism of pressure solution (Polak et al., 2003; Yasuhara et al., 2006). Yasuhara et al. (2004) performed the permeability tests which proved that the chemical interaction on the fracture asperities and the structure of a fracture might be changed by the pressure solution (Yasuhara et al., 2004; 2006). However, the permeability may not reduce monotonously under higher temperature conditions in the long term even though some researchers claimed that the permeability may decrease at a relative high temperature (Yasuhara et al., 2006; Kohl et al., 1995; Morrow et al., 2001; Barnabe, 1986). The complexed processes may alter the fracture surface roughness and further change the permeability of the fractured rock mass. Nevertheless the fracture surface roughness and the fracture aperture change can hardly be obtained directly. Moreover, there is lack of experimental data about the morphology of fractures at various confining stress (Bing et al., 2004). The relationship between the aperture change and the contact area is also difficult to be measured. Therefore, this work studied the short- and long-term permeability variation at different temperatures. The changing of hydraulic aperture under constant stress and temperature conditions was also investigated.

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