Shale ductility

Results and conclusions The fault zones at Frechen contain clay fillings of up to 1 m in thickness, derived from extremely plastic shale source beds and smeared out over distances as much as 70 m in dip direction. The generation of substantial smears requires slow fault displacement rates and sufficient shale ductility. When a thick shale source bed is traversed by a normal fault, it is first flexed and eventually disrupted by a pull-apart mechanism that creates room for the emplacement of thick clay smears. Simple theoretical considerations suggest that the source bed thickness to some power n + 1 > 2 may be a key parameter in the ranking of seal quality. The length of continuous smears increases with source bed thickness, but will ultimately be controlled by the smearing process. The latter remains to be investigated. 

Fig. 4. Three stages in the disruption of a ductile shale bed by an upward propagating normal fault pull-apart allowing the emplacement of massive clay smears.

Fig. 5. Mohr-circle construction of precursory R- and R -shears that form early during upward-propagation of normal fault through ductile shale bed. The maximum shear stress is assumed to occur on fault-parallel planes the shear orientations are obtained through the use of...

The terms brittle and ductile are used in the literature in different ways. Here, the following definitions are adopted a brittle shale will increase its permeability by developing dilatant fractures (Fig. 2), whereas a ductile shale is able to undergo plastic deformation without increasing its permeability (it will... 

Levels of mechanical compaction are least in the Avalon/Ben Nevis Sandstone, as indicated by relatively loose packing. Pressure solution contacts are absent except between quartz grains and fossil or limestone fragments (see Fig. 17D). The presence of a high percentage of sedimentary rock fragments (average 10%), which include ductile shale clasts. 

The CDM has two additional features that allow it to represent fracture in rocks. First, there is a brittle/ductile transition pressure. Above this pressure, the rock behaves as an elastic/plastic ductile solid, the failure surface is independent of the level of damage, and the damage is not allowed to increase, even if the failure surface is exceeded. Second, the CDM allows for non-vanishing plastic volume strain to approximate the dilatancy observed in certain laboratory experiments on oil shale. 

Fickian diffusion also alters borehole yielding processes (Geilikman et al. 1996) the ionic change Na Ca leads to shrinkage of ductile smectitic shales. In stiff shale, a AVA of much less than 0.01 will cause stresses to change by many MPa, leading to a self-reinforcing coupled process ... 

Ductile smectitic shales, moderate -salinity pore fluids, high porosity, intact... 

The conservative strategy is to place an intermediate casing string seated at the boundary between ductile and fractured shales, having drilled the ductile shale with oil-based drilling fluid, then converting to a water-based fluid to drill out of the... 

The process of pressure-porosity diffusion, the fourth solution mentioned in Section 7, requires solid-fluid coupling to exist. If the pore liquid cannot flow, as is likely in the case of ductile smectitic shales (Dusseault 2003), or if the liquid is present as a fully occluded phase so that it is in the form of liquid bubbles in the solid mineral, as in a vesicular basalt, the wave cannot exist. This also... 

Ductile unfractured shale (i.e. no natural fractures) can be drilled effectively using OBM. It is prohibited in some areas for environmental reasons and is far more expensive than WBM, so land-based drilling tends to use WBM. 

The shale s ability to fracture is of special interest (King, 2010). Mechanical behaviour of shale is important in hydraulic fracture stimulation where brittleness is an indication of the ability to not only initiate and place a hydraulic fracture, but also maintain its long-term hydraulic conductivity (Harris et al., 2011). Brittle shale may be more easily fractured, opening flow paths. Ductile shales may require more proppants (King, 2010). Two types of criterion are used ... 

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