Fracture permeability pressure solution

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. 

These measurements provide unusual constraint on the evolving processes. Importantly, they allow the source of dissolved components to be determined we need to discriminate whether the source is from free-face dissolution of the fracture wall, or from stress-mediated dissolution at contacting asperities. This distinction is crucial since these two mechanisms impart opposite effects in the sense of permeability change, under net dissolution free-face dissolution increases permeability, and pressure solution reduces permeability. 

Yasuhara, H., Elsworth, D., and Polak, A. (2004a) The evolution of permeability in a natural fracture significant role of pressure solution. J. Geophys. Res., Vol. 109, B03204,... 

THE EVOLUTION OF PERMEABILITY IN NATURAL FRACTURES - THE COMPETING ROLES OF PRESSURE SOLUTION AND FREE>FACE DISSOLUTION... 

This analysis has been performed for different values of the gas flow rate. The behaviour was similar and the maximum gas pressures achieved are included in Table 2. It can be observed that a flow rate of 0.0576 g/h produces failure, which implies large aperture of the fracture, large permeability increase, sharp drop of the gas injection pressure and, consequently, unstable numerical solution. 

A first possible solution is to lower p to reduce flux of whole mud into the fractures. Note that fissure permeability is strongly aperture dependent, and increasing the pressure in the rock also increases the aperture of the fractures. 

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