Dilation stress-permeability

Field observations indicate that the stress condition affects the flow characteristics of fractured rock mass (Barton et al., 1995 Ito and Hayashi, 2003 Pusch, 1989). Many laboratory investigations on single fractures also prove that the normal closure and shear dilation can significantly change the transmissivity of fractures (Makurat et al., 1990 Olsson et al., 2001). When it comes to the block scale stress-permeability relationship, the analytical models based on the orthogonal and/or persistent fracture model are available (Bai and Elsworth, 1994). However, analytical solutions do not generally exist for more realistic fracture systems. Furthermore, to the authors knowledge, block-scale study about the effect of shear dilations of fractures on the... 

Sanding processes are accompanied by strong non-linearities. Cohesion loss takes place early in the dilation process because stresses are high and shear distortion occurs. Permeability changes from 0.5 - 5 Darcy to very high values, and properties such as matrix compressibility change from -0.50x10 kPa to infinite when the system is liquefied. Initial fabric eventually is fully destroyed by these plasticity processes. 

Fracture closure/opening caused by changes in normal stress across fractures is the dominating mechanisms for TM-induced changes in fracture permeability, whereas fracture shear dilation does not appear to be significant at the DST. 

Figure 1 presents the three basic mechanisms of stress induced permeability change in fractured rock (a) normal closure/opening, (b) shear dilation/contraction and (c) induced anisotropy due to different orientations of fractures and anisotropic stress condition. 

Secondly, horizontal normal boundary stress is increased incrementally while keeping the vertical normal stress constant to produce increased shear stress in the fractures in various orientations. Through this process, anisotropic permeability and the effect of dilation on the permeability change is observed. 

The second mechanism is dilation/contraction induced permeability change during shear (Figure 1 (b)). Fractures dilate when fracture failure occurs under high differential stresses. This mechanism can play a major role in the flow field since the dilated fracture tend to have much higher transmissivity than the fracture under elastic deformation. 

Figure 7 presents the permeability change with the increasing k ratio simulated by increasing the horizontal boundary stress from 2.5 MPa to 25 MPa with the fixed vertical boundary stress of 5 MPa (the k ratio changes from 0.5 to 5). To evaluate the effect of shear dilation, the results are compared with the pure elastic fracture model that does not consider the failure and dilation. 

When the k ratio is beyond 2.5 some fractures start to fail with continuous dilation and notable differences from elastic models are observed. Interesting aspects from these results are 1) the increase of permeability with the increase of differential stresses, and 2) dilation induced channelling effect. 

The increase of permeability is due to the increased apertures from fracture dilation. For this study, k=2.45 is the starting point of shear failure of fractures inclined at about 33 degrees from the horizontal plane. As horizontal boundary stress increases, the range of possible failure orientation angle also becomes larger and this gives increased permeability. The increase of permeability seems to stabilize after a certain k ratio because the dilation does not continue after the critical shear displacement. 

The results show that all of the aperture, fluid velocity and the permeability of the fractured rock mass decrease with the increment of stress ratio during the stress ratios are enough small K is small than 3.5 in this study). That is to say, the permeability of fractured rock will decrease with effective stress increment when shear dilation can t happen or be considered in fractures. 

However, the aperture, fluid velocity and the permeability increase with stress ratio increment when stress ratios are enough large to cause shear dilation of fractures K is large than 3.5). These results show that the shear-induced flow channeling has very important impact on permeability of fractured rock mass and shouldn t be neglected. 

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