Changes

A fault-repeated section is sometimes seen in a well ([[:file:M91Ch13FG82.JPG|Figure 4]]). Near-vertical or gently dipping wells cutting reverse faults will show a repeated pattern. A repeat section can also occur where a highly deviated well cuts through a normal fault at a shallower angle than the dip of the fault plane ([[:file:M91Ch13FG82.JPG|Figure 4]]).<ref name=Mulvany_1992>Mulvany, P. S., 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0006/0000/0895.htm A model for classifying and interpreting logs of boreholes that intersect faults in stratified rocks]: AAPG Bulletin, v. 76, no. 6, p. 895–903.</ref>

A fault-repeated section is sometimes seen in a well ([[:file:M91Ch13FG82.JPG|Figure 4]]). Near-vertical or gently dipping wells cutting reverse faults will show a repeated pattern. A repeat section can also occur where a highly deviated well cuts through a normal fault at a shallower angle than the dip of the fault plane ([[:file:M91Ch13FG82.JPG|Figure 4]]).<ref name=Mulvany_1992>Mulvany, P. S., 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0006/0000/0895.htm A model for classifying and interpreting logs of boreholes that intersect faults in stratified rocks]: AAPG Bulletin, v. 76, no. 6, p. 895–903.</ref>

[[file:M91Ch13FG84.JPG|thumb|300px|{{figure number|6}}This faulted top reservoir map from the Staffa field in the UK North Sea is represented by a contoured surface and fault polygons. The fault polygons show the hanging wall and footwall fault cuts for the interpreted surface. The downthrown (hanging wall) side of the fault is indicated by a blocked out symbol (from Gluyas and Underhill<ref name=Gluyasandunderhill_2003>Gluyas, J. G., and J. R. Underhill, 2003, The Staffa field, Block 3/8b, UK North Sea, in J. G. Gluyas and H. M. Hichens, eds., United Kingdom oil and gas fields, commemorative millennium volume: Geological Society (London) Memoir 20, p. 327–333.</ref> Reprinted with permission from the Geological Society.]]

[[file:M91Ch13FG84.JPG|thumb|300px|{{figure number|6}}This faulted top reservoir map from the Staffa field in the UK North Sea is represented by a contoured surface and fault polygons. The fault polygons show the hanging wall and footwall fault cuts for the interpreted surface. The downthrown (hanging wall) side of the fault is indicated by a blocked out symbol (from Gluyas and Underhill).<ref name=Gluyasandunderhill_2003>Gluyas, J. G., and J. R. Underhill, 2003, The Staffa field, Block 3/8b, UK North Sea, in J. G. Gluyas and H. M. Hichens, eds., United Kingdom oil and gas fields, commemorative millennium volume: Geological Society (London) Memoir 20, p. 327–333.</ref> Reprinted with permission from the Geological Society.]]

==Well tests and faults==

==Well tests and faults==

==Fault sealing characteristics==

==Fault sealing characteristics==

Fine grained fault rock will have a higher capillary entry pressure compared to the undeformed host rock. Brown (2003) described how the seal behavior of water-wet fault fill defines three potential zones within a fault.

Fine grained fault rock will have a higher capillary entry pressure compared to the undeformed host rock. Brown<ref name=Brown_2003 /> described how the seal behavior of water-wet fault fill defines three potential zones within a fault.

* A fault can seal because the petroleum phase has insufficient buoyancy pressure to invade and displace water from the fine grained material within the fault rock; this has been termed membrane sealing (Watts, 1987).

* A fault can seal because the petroleum phase has insufficient buoyancy pressure to invade and displace water from the fine grained material within the fault rock; this has been termed membrane sealing.<ref name=Watts_1987 />

* Higher within the petroleum column, the buoyancy pressure can increase to the point at which the oil or gas can invade the fault rock and thus leak through it. However, the fault rock will have a very low permeability, and the rate of leakage can be trivial, even over geological time (Heum, 1996). The fault can then be considered to be effectively lsquosealingrsquo by hydraulic resistance (Watts, 1987).

* Higher within the petroleum column, the buoyancy pressure can increase to the point at which the oil or gas can invade the fault rock and thus leak through it. However, the fault rock will have a very low permeability, and the rate of leakage can be trivial, even over geological time.<ref name=Heum_1996 /> The fault can then be considered to be effectively lsquosealingrsquo by hydraulic resistance.<ref name=Watts_1987 />

* Where an exceptionally thick petroleum column exists, even low-permeability fault rocks can leak at significant rates. This is the zone of fault fill seal failure.

* Where an exceptionally thick petroleum column exists, even low-permeability fault rocks can leak at significant rates. This is the zone of fault fill seal failure.

==Fault seal prediction==

==Fault seal prediction==

Where sealing faults are a key element controlling the fluid flow in a reservoir, they should be characterized for reservoir description and modeling (Fisher and Jolley, 2007). Much of the research to date has come about because of the particular importance of understanding fault behavior in deltaic reservoirs. In deltas deposited over thick and unstable mobile shale intervals, synsedimentary faults are a major element controlling reservoir continuity and size. The faults cut relatively unlithified sediments where the potential for clay smear along the fault planes is high and potentially predictable.

Where sealing faults are a key element controlling the fluid flow in a reservoir, they should be characterized for reservoir description and modeling.<ref name=Fisherandjolley_2007 /> Much of the research to date has come about because of the particular importance of understanding fault behavior in deltaic reservoirs. In deltas deposited over thick and unstable mobile shale intervals, synsedimentary faults are a major element controlling reservoir continuity and size. The faults cut relatively unlithified sediments where the potential for clay smear along the fault planes is high and potentially predictable.

Algorithms are available for predicting the clay smear and shale gouge sealing potential of a fault. The basis for these algorithms is that the chances for clay smear to cause fault seal is controlled by the number and thickness of the shale beds displaced past a particular point on the fault. The thickness of the clay smear within the fault plane will decrease with distance from the source beds and with increasing throw of the fault (Yielding et al., 1997). The method involves taking the sand and shale distribution from a well close to the fault as a template for making the fault seal analysis.

Algorithms are available for predicting the clay smear and shale gouge sealing potential of a fault. The basis for these algorithms is that the chances for clay smear to cause fault seal is controlled by the number and thickness of the shale beds displaced past a particular point on the fault. The thickness of the clay smear within the fault plane will decrease with distance from the source beds and with increasing throw of the fault.<ref name=Yieldingetal_1997 /> The method involves taking the sand and shale distribution from a well close to the fault as a template for making the fault seal analysis.

[[file:M91Ch13FG90.JPG|thumb|300px|{{figure number|12}}Fault seal analysis involves numerical methods of predicting the likelihood of fault seal (from Yielding et al., 1997). ]]

[[file:M91Ch13FG90.JPG|thumb|300px|{{figure number|12}}Fault seal analysis involves numerical methods of predicting the likelihood of fault seal (from Yielding et al.<ref name=Yieldingetal_1997 />). ]]

The clay smear potential is calculated for a particular point on the fault plane as a function of the distance of that point from a shale bed acting as the source for the clay smear and the shale bed thickness (Bouvier et al., 1989; Fulljames et al., 1996) ([[:file:M91Ch13FG90.JPG|Figure 12]]).

The clay smear potential is calculated for a particular point on the fault plane as a function of the distance of that point from a shale bed acting as the source for the clay smear and the shale bed thickness<ref name=Bouvieretal_1989 /> <ref name=Fulliamesetal_1996 /> ([[:file:M91Ch13FG90.JPG|Figure 12]]).

The shale smear factor (SSF) is dependent on the shale bed thickness and the fault throw but not on the smear distance (Lindsay et al., 1993) ([[:file:M91Ch13FG90.JPG|Figure 12]]). Smaller values of the SSF correspond to a more continuous development of smear on the fault plane. A large fault is likely to seal where the SSF is equal to or less than 4 (Farseth, 2006).

The shale smear factor (SSF) is dependent on the shale bed thickness and the fault throw but not on the smear distance (Lindsay et al., 1993) ([[:file:M91Ch13FG90.JPG|Figure 12]]). Smaller values of the SSF correspond to a more continuous development of smear on the fault plane. A large fault is likely to seal where the SSF is equal to or less than 4.<ref name=Farseth_2006 />

The shale gouge ratio works on the assumption that the sealing capacity is related directly to the percentage of shale beds or clay material within the slipped interval (Yielding et al., 1997). The shale gouge ratio is the proportion of the sealing lithology in the rock interval that has slipped past a given point on the fault ([[:file:M91Ch13FG90.JPG|Figure 12]]). To calculate the shale gouge ratio, the proportion of shale and clay in a window equivalent to the throw is measured.

The shale gouge ratio works on the assumption that the sealing capacity is related directly to the percentage of shale beds or clay material within the slipped interval (Yielding et al., 1997). The shale gouge ratio is the proportion of the sealing lithology in the rock interval that has slipped past a given point on the fault ([[:file:M91Ch13FG90.JPG|Figure 12]]). To calculate the shale gouge ratio, the proportion of shale and clay in a window equivalent to the throw is measured.