Shear dilation

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. 

The conclusion that fracture shear dilation is not significant at the DST is pending on measurements during the cooling phase of the DST. 

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... 

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. 

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. 

As it was already mentioned, the above model contains the unknown values of shear dilation angles 0, , which can be predicted for the specified rock by analysing the effect of shears displacement of the syntactic fracture on the fracture dilation. This can be made with analytic-experimental method (it couples the analytical algorithm for fracture mathematical modelling and experimental data for the natural fracture... 

The numerical simulation of the fractured reservoir behaviour can be decomposed into two main parts. Namely, hydraulic fracturing part (here at first the values of the fracture shear dilation angles should be obtained for the typical rock in reservoir), and, after that, the model of the stimulated reservoir is employed for evaluating the heat and mass transfer processes during the production stage of reservoir exploitation. 

Fracture dilation is more pronounced for the fractal surfaces with lower fractal dimension Dj. As soon as the variation of the fracture dilation (which is measured geometrically) due to shear offset is computed, the shear dilation angle ( ), can be obtained as a mean angle between the shear dilation plot tangent and horizontal axis. This value is higher for the surfaces with lower fractal dimension. 

Dilation is a function of the direction of shearing. Dilation increases if the shear displacement increment is in the same direction as the total shear displacement, and decreases if the shear increment is in the opposite direction. 

In Figure 5, the mean aperture decreases obviously with the stress ratio increment when the stress ratio changes from 0.5 to 3.5 in x- and y-directions. When the stress ratio is from 3.5 to 5, however, the change law of mean aperture is opposite to that when the stress ratio is from 0.5 to 3.5. The phenomenon shows that the stress ratio is so big that shear dilation and slip happens in fractures, and then the overall apertures become larger. 

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. 

When it comes to volume strain shown in Figure 8(b), all the specimens show dilatation behavior. The shear dilatancy of specimen with flexible lateral boimdary is much more obvious than specimen with rigid lateral boundary, and the gap between two volume strain curves is bigger and bigger with the increase of axial strain. On the other hand, the reduction of volume strain can be seen in the specimen with rigid lateral boimdary while that of flexible boundary specimen can hardly be seen. 

Zhou, J., Dong, Y., de Pater, C.J., and Zitha, P.L.J. 2010. Experimental Study of the Impact of Shear Dilation and Fracture Behavior During Polymer Injection for Heavy Oil Recovery in Unconsolidated Reservoirs. Paper SPE 137656 presented at the Canadian Unconventional Resources and International Petroleum Conference, Calgary, 19-21 October. DPI 10.2118/137656-MS. 

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