Surface slip

Several processes are used in the porcelain enameling industry regardless of the metal being coated. These processes, discussed below, include preparation of the enamel slip, surface preparation of the base material, and enamel application and firing to fuse the coating to the metal.3 6... 

The term film is also applied to sheets of cellophane, polyethylene, polyvinylidene chloride, etc., used for wrapping and packaging of food products, meats, and poultry (especially shrink films that are stretched before application). These function as a moisture vapor harrier. Plaslic lilms are also used as slip surfaces in concrete structures such as airstrips, ice rinks, and highways. Photographic film is made from cellulose acetate. 

The use of a rotating vane has become very popular as a simple to use technique that allows slip to be overcome (33,34). Alderman et al (35) used the vane method to determine the yield stress, yield strain and shear modulus of bentonite gels. In the latter work it is interesting to note that a typical toique/time plot exhibits a maximum torque (related to yield stress of the sample) after which the torque is observed to decrease with time. The fall in torque beyond the maximum point was described loosely as being a transition from a gel-like to a fluid-like behavior. However, it may also be caused by the development of a slip surface within the bulk material. Indeed, by the use of the marker line technique, Plucinski et al (15) found that in parallel plate fixtures and in slow steady shear motion, the onset of slip in mayonnaises coincided with the onset of decrease in torque (Fig. 8). These authors found slip to be present for... 

Colloid Mill Colloid mills are rotor-stator systems that can be used to reduce the particle size distribution of both liquid dispersions (emulsions) and solid dispersions (suspensions). The emulsion or suspension is pumped through a narrow gap that is formed by the rotating inner cone and the stationary outer cone. The width of the annulus can be adjusted by changing the relative position of the two cones. The principal size reduction in colloid mills is due to the high shear forces that are caused by the velocity difference between the rotor and the stator surfaces. To increase wall friction and reduce slip, surfaces are usually not smooth but are roughened or toothed, which, in turn, changes the flow conditions from laminar to turbulent, thereby increasing the shear forces in the annulus. 

The last observation could indeed have major practical implications as a guiding principle in assessing fault trap prospects. It is therefore of some interest that Lindsay et al. (1993) have observed clay smears in tectonic faults that affected a Westphalian sand/shale sequences after lithification. These smears were apparently formed by abrasion of indurated shales. In this process, the surface of a sandstone becomes coated by a thin veneer of abraded material in much the same way as the surface of sandpaper. This veneer may run continuously along polished slip surfaces, but - as Lindsay et al. have documented in their study - with increasing fault displacement and de-... 

In their simplest form, brittle faults consist of a single zone of intense deformation which macroscopically is seen as a slip surface and/or a zone of fault rock. More generally, fault zones have complex geometries with multiple slip surfaces and/or deformation zones. The most common pattern in complex fault zones observed at outcrop is a fault zone bounded by a pair of sub-parallel slip surfaces. In three dimensions, fault zones bounded by paired slip surfaces alternate both laterally and up/down dip with areas of only one slip surface. Within this overall framework, a range of fault rocks is irregularly distributed as spatially impersistent sheets and lenses. 

Data for characterisation of faults in the subsurface are limited to two sources, seismics and wells. Seismic reflection data allow the displacement distribution over a fault surface to be mapped while well and core data may allow determination of fault rock types and deformation mechanisms at specific points, in addition to characterising the lithologies of the host sequence. It is evident from outcrop studies that the internal geometries of fault zones are usually complex, in terms of the numbers of individual slip surfaces, the partitioning of slip between them and in the distribution of different fault rocks, all of which vary over a fault surface. This 3-D complexity of fault zone structure may not be apparent from either seismic or core data but is nevertheless crucial to the bulk hydraulic properties of a fault. 

Brittle fault zones comprise discrete slip surface(s) and fault rocks. There is a general positive correlation between fault displacement and the thickness and complexity of the fault zones (Robertson, 1983 Hull, 1988). Complex fault zones generally comprise multiple slip surfaces or zones of intense shear (Childs et al., 1996). The simplest and most common multi-slip fault zones observed in outcrop are structures with two discrete bounding slip surfaces, enclosing fault rock which may vary from intensely deformed to virtually undeformed (Koestler and Ehrmann, 1991 Childs et al., 1996). Where sufficient data are available, areas of a fault zone with the paired slip surface geometry can be seen to alternate with areas with a... 

Fig. I. Cartoon illustrating the asperity bifurcation model of fault zone widening. An irregularity on a fault surface (grey fill) in (a) is sheared off by the formation of a new slip surface in (b). Subsequent fault movement may result in deformation of the newly formed slip surface bounded lens.

Asperity bifurcation is due to the shearing off of fault surface irregularities by the formation of new slip surfaces. These irregularities may occur anywhere on a fault surface and on any scale. Irregular... 

Fig. 2. Successive stages of the tip-line bifurcation process of fault zone widening and generation of paired bounding slip surfaces (see text). The tip-line of a fault surface (e), part of which is shown shaded in (a)-(d), propagates upwards through a rock volume. The area shown in (a)-(d) is indicated by the rectangle in (e). With fault growth the elliptical tip-line bounding the fault surface propagates radially to the successive positions, a-d, shown in (e). The lines labelled I-III in (a) indicate successive positions of the fault surface tipline.

Measurement of fault zone and fault rock thicknesses in complex fault zones (Fig. 4) can be very subjective (Fvans, 1990). In particular, the distinction in either outcrop or core between a single multi-slip surface fault zone and two or more individual faults is dependent on the distances between slip surfaces relative to their displacements, their relative orientations, the deformation state of the intervening rock and the larger scale context. Slip surfaces which at one scale of observation appear as separate faults may, with a more extended view, be clearly seen to be part of a single fault zone. As this problem can occur on any scale of observation and is effectively intractable, it should be borne in mind when assessing fault zone thickness data. 

The bifurcation mechanisms for formation of multi-slip fault zones suggest that maximum fault zone thickness will often correspond to the strike-normal distance between the traces of two overlapping slip surfaces (Fig. 2c). Fault overlaps and their breached equivalents occur on faults of all sizes as do, by implication, paired and multi-slip surface fault zones. Complex and paired slip surface fault zone structures will occur on scales below that resolvable by even high quality seismic data (lateral resolution is no better than 50-100 m at North Sea reservoir depths). The possible impact of sub-seismic complexity and paired slip surfaces on connectivity and sealing across faults offsetting an Upper Brent type sequence are briefly considered below. 

Fig. 5a shows two fault zones which offset an Upper Brent sequence, each with an aggregate displacement of ca. 40 m. On the scale of observation, fault zone A comprises a single slip surface, while fault zone B comprises two parallel slip surfaces each of which accommodates about half of the total displacement. In this case the paired slip surfaces are separated by ca. 15 m of rock with low shear strain, as indicated by bedding re-orientation. Two slip surfaces would not be distinguished even with good quality seismic data. Across-fault Juxtapositions calculated on the basis of a single slip surface would be valid in the case of fault A, but invalid for fault B. The consequences of incorrect Juxtapositions on fault B are illustrated in Fig. 5b-d), for the Upper Brent sequence shown in Fig. 5a. 

Connectivities for fault zones comprising two slip surfaces with equal displacements are shown in Fig. 5d for aggregate throw values over the range 0-84 m an aggregate throw of 40 m is represented by two slip surfaces each with a throw of 20 m. The connectivity for paired slip surfaces is derived from Fig. 5b as follows. For each reservoir unit within the fault zone and bounded by slip surfaces, the connectivity across each slip surface is measured and the minimum of these taken as the net connectivity for that unit. The process is repeated for all reservoir units within the fault zone and the aggregate of the net connectivities represents the connectivity of the fault zone. 

For the Brent sequence shown here, across-fault connectivity is higher for a single slip surface than paired slip surfaces for all throws less than 80 m. For this sequence the across-fault sandstone connectivity of fault zone A is 23% of the gross reservoir thickness, and that of fault zone B is 14%. The difference in connectivity between the two fault zones is relatively small in this example. In general, however, the... 

Connectivity curves for a single slip surface are shown in Fig. 5c for a range of SGR cut-off values, for the Upper Brent sequence shown in Fig. 5b. The forms of the connectivity curves are very different. For an SGR cut-off = 20 there is no connectivity for throws >33 m. For an SGR cut-off = 30, there is no connectivity for throws from 40 to 60 m, but 8% connectivity at a throw of 65 m. For an SGR cutoff = 40, there is a sharp increase in connectivity at 45 m when the Tarbert and Lower Ness reservoirs become juxtaposed. 

Fault surface bifurcation processes result in areas of a fault zone with paired bounding slip surfaces alternating with areas with only a single slip surface. Either laterally or up-/down-dip, the two slip surfaces of fault zone B may give way to a single slip surface. Similarly, it is unlikely that fault zone A is characterised everywhere by only a single slip surface. As... 

---