2,803
edits
Changes
Jump to navigation
Jump to search
Line 99:
Line 99: − + − + − + − + − + − + − + − + − + − + − + − + − + − +
no edit summary
A study on the Big Hole Fault in Utah based on core data showed a significant permeability reduction within the damage zone.<ref name=Shiptonetal_2002>Shipton, Z. K., J. P. Evans, K. R. Robeson, C. B. Forster, and S. Snelgrove, 2002, [http://archives.datapages.com/data/bulletns/2002/05may/0863/0863.htm Structural heterogeneity and permeability in faulted eolian sandstone: Implications for subsurface modeling of fault]s: AAPG Bulletin, v. 86, no. 5, p. 863–883.</ref> Probe permeameter measurements of permeability range from more than 2000 md in the undeformed host sandstone to less than 0.1 md in fault-damaged rocks near the fault. Whole-core tests showed that the permeability of individual deformation bands vary between 0.9 and 1.3 md. The transverse permeability modeled over 5–10-m (16–32-ft)-length scales across the fault zone was estimated as 30–40 md. This is approximately 1–4% of the permeability for the undeformed host rock.
A study on the Big Hole Fault in Utah based on core data showed a significant permeability reduction within the damage zone.<ref name=Shiptonetal_2002>Shipton, Z. K., J. P. Evans, K. R. Robeson, C. B. Forster, and S. Snelgrove, 2002, [http://archives.datapages.com/data/bulletns/2002/05may/0863/0863.htm Structural heterogeneity and permeability in faulted eolian sandstone: Implications for subsurface modeling of fault]s: AAPG Bulletin, v. 86, no. 5, p. 863–883.</ref> Probe permeameter measurements of permeability range from more than 2000 md in the undeformed host sandstone to less than 0.1 md in fault-damaged rocks near the fault. Whole-core tests showed that the permeability of individual deformation bands vary between 0.9 and 1.3 md. The transverse permeability modeled over 5–10-m (16–32-ft)-length scales across the fault zone was estimated as 30–40 md. This is approximately 1–4% of the permeability for the undeformed host rock.
The general consensus in the industry is that damage zones around faults probably baffle flow across them rather than acting as barriers to fluid movement.<ref name=Sternlofetal_2004 /> <ref name=Fossenandbale_2007 /> The exception may be in deep reservoirs with high reservoir temperatures (more than 120°C). Here, accelerated quartz cementation at high temperature can decrease the pore throat diameters in the deformation bands to the extent that they become 100% water wet through capillary action. They thus become effective barriers to oil flow.<ref name=Hesthammeretal_2002 />
The general consensus in the industry is that damage zones around faults probably baffle flow across them rather than acting as barriers to fluid movement.<ref name=Sternlofetal_2004 /> <ref name=Fossenandbale_2007 /> The exception may be in deep reservoirs with high reservoir temperatures (more than 120°C). Here, accelerated quartz cementation at high temperature can decrease the pore throat diameters in the deformation bands to the extent that they become 100% water wet through capillary action. They thus become effective barriers to oil flow.<ref name=Hesthammeretal_2002>Hesthammer, J., P. A. Bjorkum, and L. Watts, 2002, [http://archives.datapages.com/data/bulletns/2002/10oct/1733/1733.htm The effect of temperature on sealing capacity of faults in sandstone reservoirs: Examples from the Gullfaks and Gullfaks Sor fields, North Sea]: AAPG Bulletin, v. 86, no. 10, p. 1733–1751.</ref>
Because of the abundance of low-permeability baffles and poorly connected volumes, production wells drilled in fault damage zones can significantly underperform. For example, wells drilled in fault-damaged zones in the North La Barge Shallow Unit of Wyoming are the poorest producers in the field.<ref name=Miskimins_2003 /> It is generally not a good idea to plan a new well trajectory too close to a large fault because of this.
Because of the abundance of low-permeability baffles and poorly connected volumes, production wells drilled in fault damage zones can significantly underperform. For example, wells drilled in fault-damaged zones in the North La Barge Shallow Unit of Wyoming are the poorest producers in the field.<ref name=Miskimins_2003>Miskimins, J. L., 2003, Analysis of hydrocarbons production in a critically-stressed reservoir: Presented at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, October 5–8, Denver, Colorado, SPE Paper 84457, 8 p.</ref> It is generally not a good idea to plan a new well trajectory too close to a large fault because of this.
==Faults and fluid flow==
==Faults and fluid flow==
Faults can have a significant impact on the fluid flow patterns within a reservoir. They can juxtapose one reservoir interval with another creating the potential for cross flow between the units. It is pragmatic to assume that all sand to sand juxtapositions allow fluid transfer across faults unless proven otherwise.<ref name=Jamesetal_2004 /> Alternatively, juxtaposition of reservoir with nonreservoir rocks can cause the trapping of hydrocarbons against the fault. Deformation and cementation within the fault zone itself can create a zone of zero or very low permeability, which can cause the fault plane to act as a barrier to fluid flow. In some instances, fractures in the fault zone itself can act as conduits for fluid flow.
Faults can have a significant impact on the fluid flow patterns within a reservoir. They can juxtapose one reservoir interval with another creating the potential for cross flow between the units. It is pragmatic to assume that all sand to sand juxtapositions allow fluid transfer across faults unless proven otherwise.<ref name=Jamesetal_2004>James, W. R., L. H. Fairchild, G. P. Nakayama, S. J. Hippler, and P. J. Vrolijk, 2004, [http://archives.datapages.com/data/bulletns/2004/07jul/0885/0885.HTM Fault-seal analysis using a stochastic multi-fault approach]: AAPG Bulletin, v. 88, no. 7, p. 885–904.</ref> Alternatively, juxtaposition of reservoir with nonreservoir rocks can cause the trapping of hydrocarbons against the fault. Deformation and cementation within the fault zone itself can create a zone of zero or very low permeability, which can cause the fault plane to act as a barrier to fluid flow. In some instances, fractures in the fault zone itself can act as conduits for fluid flow.
[[file:M91Ch13FG89.JPG|thumb|300px|{{figure number|11}}Allan diagrams show the reservoir stratigraphy of both the hanging wall and footwall blocks of a fault superimposed along the fault plane. At a glance, it can be seen where reservoir and nonreservoir lithologies are juxtaposed with potential for juxtaposition sealing.]]
[[file:M91Ch13FG89.JPG|thumb|300px|{{figure number|11}}Allan diagrams show the reservoir stratigraphy of both the hanging wall and footwall blocks of a fault superimposed along the fault plane. At a glance, it can be seen where reservoir and nonreservoir lithologies are juxtaposed with potential for juxtaposition sealing.]]
==Allan diagrams==
==Allan diagrams==
Allan diagrams or fault juxtaposition diagrams show the reservoir stratigraphy of both the hanging wall and footwall locations superimposed on the fault plane.<ref name=Alan_1989 /> <ref name=Knipe_1997 /> At a glance, the juxtaposition relationships of the various reservoir units across the fault can be seen ([[:file:M91Ch13FG89.JPG|Figure 11]]). Allan diagrams are useful for the production geologist but are subject to the uncertainty in the input data used. The magnitude of vertical fault displacement estimated from seismic data is prone to error. Additionally, where fault drag is present but not picked up on seismic data, the vertical fault displacement can be overestimated. Complex fault zone architecture can also create large uncertainties in establishing fault juxtaposition relationships.<ref name=Hesthammerandfossen_2000 />
Allan diagrams or fault juxtaposition diagrams show the reservoir stratigraphy of both the hanging wall and footwall locations superimposed on the fault plane.<ref name=Allan_1989>Allan, U. S., 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0007/0800/0803.htm Model for hydrocarbon migration and entrapment within faulted structures]: AAPG Bulletin, v. 73, no. 7, p. 803–811.</ref> <ref name=Knipe_1997>Knipe, R. J., 1997, [http://archives.datapages.com/data/bulletns/1997/02feb/0187/0187.htm Juxtaposition and seal diagrams to help analyze fault seals in hydrocarbon reservoirs]: AAPG Bulletin, v. 81, no. 2, p. 187–195.</ref> At a glance, the juxtaposition relationships of the various reservoir units across the fault can be seen ([[:file:M91Ch13FG89.JPG|Figure 11]]). Allan diagrams are useful for the production geologist but are subject to the uncertainty in the input data used. The magnitude of vertical fault displacement estimated from seismic data is prone to error. Additionally, where fault drag is present but not picked up on seismic data, the vertical fault displacement can be overestimated. Complex fault zone architecture can also create large uncertainties in establishing fault juxtaposition relationships.<ref name=Hesthammerandfossen_2000>Hesthammer, J., and H. Fossen, 2000, Uncertainties associated with fault sealing analysis: Petroleum Geoscience, v. 6, p. 37–45.</ref>
==Fault seal==
==Fault seal==
Estimates can be made using Allan diagrams as to the probability that a fault will seal within a reservoir. In the first instance, fault seal can result from the juxtaposition of reservoir with nonreservoir rock. However, experience from many petroleum provinces has shown that faults can seal even where reservoir quality sand bodies are juxtaposed across a fault. The most common mechanism for sealing results from the incorporation of fine grained or dense material into the fault plane. Five different processes may cause this:<ref name=Mitra_1988 /> <ref name=Fisherandknipe_1998 />
Estimates can be made using Allan diagrams as to the probability that a fault will seal within a reservoir. In the first instance, fault seal can result from the juxtaposition of reservoir with nonreservoir rock. However, experience from many petroleum provinces has shown that faults can seal even where reservoir quality sand bodies are juxtaposed across a fault. The most common mechanism for sealing results from the incorporation of fine grained or dense material into the fault plane. Five different processes may cause this:<ref name=Mitra_1988>Mitra, S., 1988, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0072/0005/0500/0536.htm Effects of deformation mechanisms on reservoir potential in central Appalachian overthrust belt]: AAPG Bulletin, v. 72, no. 5, p. 536–554.</ref> <ref name=Fisherandknipe_1998>Fisher, Q. J., and R. J. Knipe, 1988, Fault sealing processes in siliciclastic sediments, in H. Jones, Q. J. Fisher, and R. J. Knipe, eds., Faulting, fault sealing and fluid flow in hydrocarbon reservoirs: Geological Society (London) Special Publication 147, p. 117–134.</ref>
* Clay smear: Faults in clay-rich sediments are believed to form clay smears by the shearing of mudstone beds into the fault zone.<ref name=Weberetal_1978 /> <ref name=Lehnerandpilaar_1997 />
* Clay smear: Faults in clay-rich sediments are believed to form clay smears by the shearing of mudstone beds into the fault zone.<ref name=Weberetal_1978>Weber, K. J., L. J. Urai, W. F. Pilaar, F. Lehner, and R. G. Precious, 1978, The role of faults in hydrocarbon migration and trapping in Nigerian growth fault structures: 10th Annual Offshore Technology Conference Proceedings, v. 4, p. 2643–2653.</ref> <ref name=Lehnerandpilaar_1997>Lehner, F. K., and F. K. Pilaar, 1997, The emplacement of clay smears in synsedimentary normal faults: Inferences and field observations near Frechen Germany, in P. Moller-Pederson and A. G. Koestler, eds., Hydrocarbon seals: Importance for exploration and production: Norwegian Petroleum Society Special Publication 7, p. 15–38.</ref>
* Cataclasis (shale gouge): Fault movement affecting clean sandstones will cause grain crushing and the breakage of rock in the fault plane, which will form a fault gouge.<ref name=Lindsayetal_1993 />
* Cataclasis (shale gouge): Fault movement affecting clean sandstones will cause grain crushing and the breakage of rock in the fault plane, which will form a fault gouge.<ref name=Lindsayetal_1993>Lindsay, N. G., F. C. Murphy, J. J. Walsh, and J. Watterson, 1993, Outcrop studies of shale smear on fault surfaces, in S. S. Flint and I. D. Bryant, eds., The geological modelling of hydrocarbon reservoirs and outcrop analogs: International Association of Sedimentologists Special Publication 15, p. 113–123.</ref>
* Diagenesis or cementation: Fine grained fault rock and associated open fractures in fault zones can be prone to cementation. Fluids migrating up the fault zone can cause the mineralization of the host rock. It is a common observation to find carbonate-cemented intervals in wells drilled close to faults, whereas wells drilled farther away from the faults do not contain carbonate cements (e.g., Reynolds et al.<ref name=Reynoldsetal_1998 />). This is an indication that the fault zones have acted as the locus for the fluids causing carbonate cementation.
* Diagenesis or cementation: Fine grained fault rock and associated open fractures in fault zones can be prone to cementation. Fluids migrating up the fault zone can cause the mineralization of the host rock. It is a common observation to find carbonate-cemented intervals in wells drilled close to faults, whereas wells drilled farther away from the faults do not contain carbonate cements (e.g., Reynolds et al.<ref name=Reynoldsetal_1998>Reynolds, A. D., et al., 1998, [http://archives.datapages.com/data/bulletns/1998/01jan/0025/0025.htm Implications of outcrop geology for reservoirs in the Neogene productive series: Apsheron Peninsula, Azerbaijan]: AAPG Bulletin, v. 82, no. 1, p. 25–29.</ref>). This is an indication that the fault zones have acted as the locus for the fluids causing carbonate cementation.
* Pore volume collapse: Ductile deformation during fault movement can cause poorly sorted sediments to mix and homogenize with a resultant decrease in porosity.
* Pore volume collapse: Ductile deformation during fault movement can cause poorly sorted sediments to mix and homogenize with a resultant decrease in porosity.
* Grain contact dissolution: Fault zones can act as planes for intergranular grain contact dissolution and subsequent recementation of the dissolved material. This can be an important mechanism for fault sealing in carbonate rocks.<ref name=Peacocketal_1998 />
* Grain contact dissolution: Fault zones can act as planes for intergranular grain contact dissolution and subsequent recementation of the dissolved material. This can be an important mechanism for fault sealing in carbonate rocks.<ref name=Peacocketal_1998>Peacock, D. C. P., Q. J. Fisher, E. J. M. Willemse, and A. Aydin, 1998, The relationship between faults and pressure solution seams in carbonate rocks and the implications for fluid flow, in G. Jones, Q. J. Fisher, and R. J. Knipe, eds., Faulting and fluid flow in hydrocarbon reservoirs: Geological Society (London) Special Publication 147, p. 105–115.</ref>
When investigating fault seal, it is important to look at any faults in the core to determine which type of sealing mechanism may be present.
When investigating fault seal, it is important to look at any faults in the core to determine which type of sealing mechanism may be present.
==Fault sealing characteristics==
==Fault sealing characteristics==
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.
Fine grained fault rock will have a higher capillary entry pressure compared to the undeformed host rock. Brown<ref name=Brown_2003>Brown, A., 2003, [http://archives.datapages.com/data/bulletns/2003/03mar/0381/0381.HTM Capillary effects on fault-fill sealing]: AAPG Bulletin, v. 87, no. 3, p. 381–395.</ref> 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.<ref name=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.<ref name=Heum_1996 /> The fault can then be considered to be effectively lsquosealingrsquo by hydraulic resistance.<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.<ref name=Heum_1996>Heum, O. R., 1996, A fluid dynamic classification of hydrocarbon entrapment: Petroleum Geoscience, v. 2, no. 2, p. 145–158.</ref> The fault can then be considered to be effectively lsquosealingrsquo by hydraulic resistance.<ref name=Watts_1987>Watts, N. L., 1987, Theoretical aspects of cap-rock and fault seals for single and two phase columns: Marine and Petroleum Geology, v. 4, no. 4, p. 274–307.</ref>
* 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.<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.
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>Fisher, Q. J., and S. J. Jolley, 2007, Treatment of faults in production simulation models, in S. J. Jolley, D. Barr, J. J. Walsh, and R. J. Knipe, eds., Structurally complex reservoirs: Geological Society (London) Special Publication 292, p. 219–233.</ref> 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.<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.
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>Yielding, G., B. Freeman, and D. T. Needham, 1997, [http://archives.datapages.com/data/bulletns/1997/06jun/0897/0897.htm Quantitative fault seal prediction]: AAPG Bulletin, v. 81, no. 6, p. 897–917.</ref> 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.<ref name=Yieldingetal_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<ref name=Bouvieretal_1989 /> <ref name=Fulliamesetal_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>Bouvier, J. D., C. H. Kaars-Sijpesteijn, D. F. Kluesner, C. C. Onyejekwe, and R. C. Van der Pal, 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0011/1350/1397.htm Three-dimensional seismic interpretation and fault sealing investigations, Nun River field, Nigeria]: AAPG Bulletin, v. 73, p. 1397–1414.</ref> <ref name=Fulljamesetal_1996>Fulljames, J. R., L. J. J. Zijerveld, and R. C. M. W. Fransen, 1997, Fault seal processes: Systematic analysis of fault seals over geological and production time scales, in P. Moller-Petersen and A. G. Koestler, eds., Hydrocarbon seals, importance for exploration and production: Norwegian Petroleum Society Special Publication 7, p. 51–79.</ref> ([[: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.<ref name=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 />