Fracture permeability observations

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]. 

The above evidence establishes that fracturing and seismic behavior can extend well into the zone of mid to lower crustal metamorphism at rock pressures of —0.5-1 GPa. Veins preserve a valuable record of this brittle deformation they are fractures in which mineral mass has been deposited. The most common vein-forming minerals are quartz, calcite, and the feldspars, but a huge variety of other minerals are also observed. Fractures tend to focus flow, because they are zones of elevated permeability. Fracture flow is commonly approximated using the well-known expression from fluid mechanics for laminar flow between two parallel plates (e.g., White, 1979). For a set of parallel fractures, the flux is approximated by (e.g., Norton and Knapp, 1977) ... 

In Mesozoic sandstones on Tampen Spur and Haltenbanken, normal faults are usually characterized by grain reorientation and enrichment of clay minerals which suggest that ductile deformation dominates. Abundant quartz cement is only observed along these faults associated with stylolites. Open fractures were not present in the sandstones from Tampen Spur and Haltenbanken. Fractures in these sediments therefore normally represent permeability barriers for fluid flow. 

The general observation for all three rock types is decreasing bulk flow with increasing Oj lo ratio, and some effect of the fracture shear displacement on the bulk flow. For the clay containing YBS, bulk flow is actually reduced by 90% for o la = 1.3. The strongest rock (YBS) shows the most pronounced cross-flow reduction, whereas the weakest rock (LC) shows the least reduction. The explanation for the limited cross-flow reduction for the LC sample lies in the low matrix permeability in combination with its special mineralogical composition (>90% carbonate), which reduces the effect of a low permeability gouge layer on the bulk flow. 

Figure 6 shows the evolution of gas pressure at the injection point and the gas flow rate at the outflow point. It can be observed that after a period of gas pressure increase corresponding to the desaturation of the injection layer, gas starts to penetrate in the medium. Figure 7 shows degree of saturation for both the fracture and the rock matrix. Gas flow takes place through the fracture because permeability is higher than in the matrix and because desaturation is easier than in the matrix. Due to fracture aperture, permeability increases and the air entry pressure decreases. 

Finally, figure 8 shows the evolution of intrinsic permeability as a result of fracture aperture. It can be observed that changes in permeability take place as soon as gas is injected due to the assumed... 

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. 

Interpretations of the observed changes in pressure drop across the sample of fractured Berea sandstone, are ambiguous, largely because the permeability of the matrix is high, relative to the fracture. Hence the source of any permeability change cannot be isolated. This difficulty may be... 

Owing to increasing effective stresses with depth. Young s moduli increase rapidly while permeabilities and porosities decrease rapidly with depth. Note that matrix porosities were not included in the table. At the 750-m depth, the properties were also calculated assuming that only 1/3 of the fractures observed in the... 

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