Stress fracture permeability

Importantly, fracture permeabilities may either reduce or increase, in surprising ways, depending on the paths of stress or chemical potential. We illustrate this behaviour through observations during flow-through tests on samples of varied rock types. These include a fractured porous medium (Berea sandstone), and fractures in both silicic (Arkansas novaculite) [Polak et al., 2003 Yasuhara et al., 2004] and carbonate rocks (Bellefonte Limestone) [Polak et al., 2(X)4]. 

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. 

Eidsvig, U. M. K. (2000). Stress-dependent Permeability in Fractured Rock. In 3" Euroconference on Rock Physics and Rock Mechanics, Bad Honnef, Germany. 

A key parameter for modeling the T-H-M-C processes is the relationship between in situ stress and permeability. Assuming several different relations between local permeability and fracture apertures. we developed two upscaling relationships between normal stress and permeability for fractured rock characterized by fLm. While more rigorous upscaling relationships are desirable, the proposed relationships capture relevant large-scale effects of normal stress changes on rock permeabilities. 

Stress induced permeability change is of crucial importance in various kinds of applications such as nuclear waste disposal in deep geological formations, geothermal energy utilization and underground excavations. In particular, coupling between the stress and permeability is a key element in understanding the nature of flow in the fractured rock (Rutqvist and Stephansson, 2003). This is because fractures, which are the main pathways of fluid flow in fractured hard rocks, are heavily dependent on the stress conditions for their deformations. 

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. 

EFFECT OF THERMAL DEFORMATION ON FRACTURE PERMEABILITY IN STRESSED ROCK MASSES... 

Abstract We analyse the effect of thermal contraction of rock on fracture permeability. The analysis is carried out by using a 2D FEM code which can treat the coupled problem of fluid flow in fractures, elastic and thermal deformation of rock and heat transfer. In the analysis, we assume high-temperature rock with a uniformly-distributed fracture network. The rock is subjected to in-situ confining stresses. Under the conditions, low-temperature fluid is injected into the fracture network. Our results show that even under confining environment, the considerable increase in fracture permeability appears due to thermal deformation of rock, which is caused by the difference in temperature of rock and injected fluid. However, for the increase of fracture permeability, the temperature difference is necessary to be larger than a critical value, STc, which is given as a function of in-situ stresses, pore pressure and elastic properties of rock. 

Min, K.B., Rutqvist, X, Tsang, C.F., et al. 2004. Stress-dependent permeability of fractured rock masses a numerical study. International Journal of Rock Mechanics Mining Sciences 41(2004) 1191-1210. 

Hydroxypropylguar gum gel can be crosslinked with borates [1227], ti-tanates, or zirconates. Borate-crosslinked fluids and linear hydroxyethyl-cellulose gels are the most commonly used fluids for high-permeability fracture treatments. This is for use for hydraulic fracturing fluid under high-temperature and high-shear stress. 

Ahmed, U., et al. "Effect of Stress Distribution on Hydraulic Fracture Geometry A Laboratory Simulation Study in One Meter Cubic Blocks," SPE/DOE paper 11637, 1983 SPE/DOE Symposium on Low Permeability, Denver, March 14 16. 

The PF system creates a fracture network by forcing compressed gas into a formation at pressures that cause stress failure. These fractures increase the formation s permeability. Increased permeability can greatly improve contaminant mass removal rates. PF can also increase the effective area that is influenced by each extraction weU and can intersect new pockets of contamination that were previously trapped in the formation. The ARS PF technology is patented and is commercially available. According to the vendor, it has been used at over 135 federal and private sites in the United States, Canada, Japan, and Belgium. 

The PF technology also has several potential limitations. Fractures do not always propagate in the direction or to the distances expected. Fractures may open new pathways for the unwanted spread of contaminants. Pockets of low permeability may remain after fracturing. Surface heave and stress resulting from the process can create hazards for buildings or other structures at a site. If the moisture content of the contaminated media is not controlled, the formation may swell and close the fractures. PF is not applicable at sites with high natural permeabilities. Fractures will close in soils with low clay content. In addition, PF should not be used in areas of high seismic activity. 

It is unlikely, however, that the lithification of chalk will go on without consolidation, in which the volume of chalk material is reduced in response to a load on the chalk. Consolidation can lead to a reduction in porosity up to about 40%, and an increase in the effective stress (Jones et al., 1984). The increased effective stress is required to instigate the process of pressure solution. Pressure solution provides Ca2+ and HCO3 for early precipitation of calcite cement in the chalk. However, the inherently low permeability of chalk would inhibit the processes of consolidation and pressure solution/cementation unless some permeable pathways are opened up to permit the dissipation of excess pore pressure created by the filling of pore space by calcite cement. Pressure solution will cease if the permeable pathways are blocked by cement. Thus, it appears that the development of fractures, open stylolites and microstylolitic seams (Ekdale et al., 1988) is necessary to permit pressure solution to continue and lead to large rates of Ca2+ and HC03 mobilization. 

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