Pore pressure fracture permeability

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. 

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. 

Figure 5. Calculated critical pore-pressure for shear slip of fractures versus depth of estimated fractures. The zone of permeable fracture, which are detected by well loggings are indicated by shaded rectangles.

The result in Figure 5 implies that the fluid pressure was transmitted into fractures, and that the pore-pressure around zones 1 and 2 increased up to near maximum fluid pressure in fracturing well. It is reported that the fractures in zones 1 and 2 are permeable under low flow rate (Evans, 2000). Then, the distribution of critical pore-pressure in Figure 5 is reasonable. 

A realistic prediction of the permeability distribution in three dimensions in sedimentary basins seems impossible given the wide ranges of values for different types of sediments and the heterogeneities of the basins. Pore pressures and fluid fluxes in three dimensions can not be modelled reliably. When the fracture pressure is reached at high overpressure, the fluid flow becomes decoupled from the permeability of the rock matrix and is mostly a function of the permeability of very thin hydrofractures. The permeability is then coupled to the fracture spacing and width which again is a function of the fluid flux and the rate of compaction. 

Walsh, J.B., 1981. Effects of pore pressure and confming pressure on fracture permeability. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 18 (5), 429-435. October. 

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