Precipitation 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. 

Sandstones with little clay cement are a favourable environment for the circulation of silica-bearing deep waters, especially in provinces with a network of faults (Taylor 1950 Heald 1956). In these conditions some silica could precipitate in the available pore space. In the study area, it was particularly important to examine closely ancient faulting, i.e. Triassic and Cretaceous communicating faults which do not seal between different reservoir blocks. However, studies of the relationship between sandstone permeability and the distance of boreholes from fractures and faults have demonstrated that the mean permeability of sandstones from boreholes located within 500 m from faults is practically the same as the mean permeability of sandstones from other boreholes. 

The Middle Jurassic Sandstone Fractured Confined Aquifer. It is above the coal seam with distance of 4.0-54.0 m. The layer consists of pack-sand, sandy mudstone and conglomerate with the average thickness of 180 m. The gravels mainly contain quartzite, quartz sandstone and limestone. The water yield property is relatively weak and this aquifer is the direct water bearing strata. The water quality type is HCOj Na Ca. The permeability coefficient is 0.000405-0.0322 m/d and the specific capacity is about 0.00007-0.00622 L/(s m). This aquifer doesn t have the identical water level with poor connection. In the shallow parts, it is supplied by precipitation and river. Currently, its discharge form is drainage. 

The process by which uranium dispersed in a source rock can be concentrated in favourable structures is illustrated in Fig. 4. The complexing agent that was responsible for transporting the uranium is assumed to be fluoride in this case, which is also dispersed in the source rock. Mobilization of the uranium requires fracture-induced permeability. Once the uranium is dissolved in the solutions that percolate through the fracture systems it can be precipitated as a result of the appropriate change in the physico-chemical conditions of the system. 

Borate cross-linked fracture fluids are also believed to cause less damage to the reservoir and less likely to impair permeability than rival cross-linkers [26,50,68], This is partly due to the fact that borate cross-links can be broken down after fracturing simply by reducing pH. That is not to say that chemical (oxidative) or enzymatic means for effecting cleanup of the reservoir are not required to break down the polymer chains and flush away the fluid residues, but this process is more effective with borates because of the reversible nature of the cross-link bond. Some metal ion cross-linked gels have poor cleanup properties and soluble precipitates can be formed when they react with certain chemical breakers. ... 

Steefel Lichtner (1994) highlighted the need to take flow geometries into account when assessing the effects of host-rock alterations. They modelled diffusive and advective transport processes along a hyperalkaline fluid-filled fracture in marl and also perpendicular to it between fracture and matrix. Dolomite dissolution was found to result in increased permeability parallel to the fracture, and diffusion was responsible for the precipitation of a calcite front in the wall rock, thus isolating the fracture physically and chemically from the rock matrix. This may reduce the effective buffering and sorption capacity of the rock. The mechanisms which affect the transport properties of a host rock are shown in this work to depend on many different factors and may be far more complex than can easily be modelled or simulated in a laboratory. 

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