Editing OR/15/066 Fracture propagation

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==Observations of natural hydraulic fracturing==     
 
==Observations of natural hydraulic fracturing==     
 
Richard Davies and co-workers (Davies ''et al.'', 2012<ref name="Davies 2012">Davies, R J, Mathias, S A, Moss, J, Hustoft, S, and Newport, L. (2012). Hydraulic fractures: How far can they go? ''Marine and petroleum geology'', '''37''', pp.1–6.</ref>) published a study on natural hydraulic fractures, which is useful in assessing the geometric extent of induced or stimulated hydraulic fracturing.
 
Richard Davies and co-workers (Davies ''et al.'', 2012<ref name="Davies 2012">Davies, R J, Mathias, S A, Moss, J, Hustoft, S, and Newport, L. (2012). Hydraulic fractures: How far can they go? ''Marine and petroleum geology'', '''37''', pp.1–6.</ref>) published a study on natural hydraulic fractures, which is useful in assessing the geometric extent of induced or stimulated hydraulic fracturing.
Cosgrove (1995)<ref name="Cosgrove 1995">Cosgrove, J W. (1995). The expression of hydraulic fracturing in rocks and sediments. In: ''Fractography: Fracture Topography as a Tool in Fracture Mechanics and Stress Analysis''. Geological Society Special Publication No.'''92''', pp.187–196.</ref> showed that natural hydraulic fractures can be observed in outcrops from the centimetre to metre scale. There are several types of natural hydraulic fracture that have all been extensively studied, including: injectities (e.g. Hurst ''et al.'', 2011<ref name="Hurst 2011">Hurst, A, Scott, A, and Vigorito, M. (2011). Physical characteristics of sand injectites. ''Earth Science Reviews'', 106, pp.215–246.</ref>), igneous dykes (e.g. Polteau ''et al.'', 2008<ref name="Polteau 2008">Polteau, S, Mazzini, A, Galland, O, Planke, S, and Malthe-Sørenssen, A.  (2008). Saucershaped intrusions: occurrences, emplacement and implications. ''Earth and Planetary Science Letters'', '''266''', pp.195–204.</ref>), veins (e.g. Cosgrove, 1995<ref name="Cosgrove 1995"></ref>), coal cleats (e.g. Laubach ''et al.'', 1998<ref name="Laubach 1998">Laubach, S E, Marrett, R A, Olson, J E, and Scott, A R. (1998). Characteristics and origins of coal cleat: a review. ''International Journal of Coal Geology'', 35, pp.175–207.</ref>), and joints (e.g. McConaughy & Engelder, 1999<ref name="McConaughy 1999">McConaughy, D T, and Engelder, T. (1999). Joint interaction with embedded concretions: joint loading configurations inferred from propagation paths. ''Journal of Structural Geology'', 21, pp.1637–1652.</ref>). Savalli & Engelder (2005)<ref name="Savalli 2005"></ref> showed that growth of natural hydraulic fractures could be studied in the Devonian Marcellus formation in the US on the basis of plume lines that occur over a range of scales from centimetre to metre scale. The formation of these natural features is inferred to derive from gas diffusion and expansion within the shale during multiple propagation events.
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Cosgrove (1995)<ref name="Cosgrove 1995">Cosgrove, J W. (1995). The expression of hydraulic fracturing in rocks and sediments. In: ''Fractography: Fracture Topography as a Tool in Fracture Mechanics and Stress Analysis''. Geological Society Special Publication No.'''92''', pp.187–196.</ref> showed that natural hydraulic fractures can be observed in outcrops from the centimetre to metre scale. There are several types of natural hydraulic fracture that have all been extensively studied, including: injectities (e.g. Hurst ''et al.'', 2011<ref name="Hurst 2011">Hurst, A, Scott, A, and Vigorito, M. (2011). Physical characteristics of sand injectites. ''Earth Science Reviews'', 106, pp.215–246.</ref>), igneous dykes (e.g. Polteau ''et al.'', 2008<ref name="Polteau 2008">Polteau, S, Mazzini, A, Galland, O, Planke, S, and Malthe-Sørenssen, A.  (2008). Saucershaped intrusions: occurrences, emplacement and implications. ''Earth and Planetary Science Letters'', '''266''', pp.195–204.</ref>), veins (e.g. Cosgrove, 1995<ref name="Cosgrove 1995"></ref>), coal cleats (e.g. Laubach ''et al.'', 1998<ref name="Laubach 1998">Laubach, S E, Marrett, R A, Olson, J E, and Scott, A R. (1998). Characteristics and origins of coal cleat: a review. ''International Journal of Coal Geology'', 35, pp.175–207.</ref>), and joints (e.g. McConaughy & Engelder, 1999<ref name="McConaughy 1999">McConaughy, D T, and Engelder, T. (1999). Joint interaction with embedded concretions: joint loading configurations inferred from propagation paths. ''Journal of Structural Geology'', 21, pp.1637–1652.</ref>). Savalli & Engelder (2005)<ref name="Savalli 2005">Savalli, L, and Engelder, T. (2005). Mechanisms controlling rupture shape during subcritical growth of joints in layered rock. ''Geological Society of America Bulletin'', 117, pp.436–449.</ref> showed that growth of natural hydraulic fractures could be studied in the Devonian Marcellus formation in the US on the basis of plume lines that occur over a range of scales from centimetre to metre scale. The formation of these natural features is inferred to derive from gas diffusion and expansion within the shale during multiple propagation events.
  
 
The tallest example of natural hydraulic fracture result when they cluster and form chimneys (also termed pipes or blowout pipes). These have been observed to extend vertically for hundreds of metres (e.g. Cartwright ''et al.'', 2007<ref name="Cartwright 2007">Cartwright, J, Huuse, M, and Aplin, A. (2007). Seal bypass systems. ''American Association of Petroleum Geologists Bulletin'', 91, pp.1141–1166.</ref>; Huuse ''et al.'', 2010<ref name="Huuse 2010">Huuse, M, Jackson, C A-J, Van Rensbergen, P, Davies, R J, Flemings, P B, and Dixon, R J. (2010). Subsurface sediment remobilization and fluid flow in sedimentary basins: an overview. ''Basin Research'', '''22''', pp.342–360.</ref>). Their origin is uncertain, but may result from critical pressurisation of aquifers and hydrocarbon accumulations (Zühlsdorff & Spieß, 2004<ref name="Zühlsdorff 2004">Zühlsdorff, L, and Spieß, V. (2004). Three-dimensional seismic characterization of a venting site reveals compelling indications of natural hydraulic fracturing. ''Geology'', '''32''', pp.101–104.</ref>; Cartwright ''et al.'', 2007<ref name="Cartwright 2007"></ref>; Davies & Clarke, 2010<ref name="Davies 2010">Davies, R J, and Clarke, A L. (2010). Storage rather than venting after gas hydrate  dissociation. ''Geology'', '''38''', pp.963–966.</ref>). Chimney development may be followed by fluid driven erosion and collapse of the surrounding rock (Cartwright ''et al.'', 2007<ref name="Cartwright 2007"></ref>). The release and expansion of gas from solution during advective flow may also play a role in development (Brown, 1990<ref name="Brown 1990">Brown, K M. (1990). The nature and hydrogeologic significance of mud diapirs and diatremes for accretionary systems. ''Journal of Geophysical Research: Solid Earth'', '''''95''''', pp.8969–8982.</ref>; Cartwright ''et al.'', 2007<ref name="Cartwright 2007"></ref>). Chimneys are clearly identifiable in seismic data as vertical aligned discontinuities in otherwise continuous units (Cartwright ''et al.'', 2007<ref name="Cartwright 2007"></ref>; Løseth ''et al.'', 2011<ref name="Løseth 2011">Løseth, H, Wensaas, L, Arntsen, B, Hanken, N-M, Basire, C, and Graue, K. (2011). 1000 m long gas blow-out chimneys. ''Marine and Petroleum Geology'', '''28''', pp.1047–1060.</ref>). Davies ''et al. ''(2012)<ref name="Davies 2012"></ref> examined 368 chimneys from offshore Mauritania and showed that the average height was 247 metres, with the tallest chimney being 507 metres. In offshore Namibia 366 chimneys showed an average height of 360 metres, with the tallest being approximately 1,100 metres. In offshore Norway 466 chimneys showed an average height of 338 metres, with a maximum of 880 metres. From comparing natural with induced hydraulic fractures, Davies ''et al. ''(2012)<ref name="Davies 2012"></ref> conclude that the probability of an induced hydraulic fracture extending vertically more than 350 metres is about 1%. It should be noted that their conclusion is based on fracture height statistics alone and the mechanistic basis for fracture height control is not taken into account.
 
The tallest example of natural hydraulic fracture result when they cluster and form chimneys (also termed pipes or blowout pipes). These have been observed to extend vertically for hundreds of metres (e.g. Cartwright ''et al.'', 2007<ref name="Cartwright 2007">Cartwright, J, Huuse, M, and Aplin, A. (2007). Seal bypass systems. ''American Association of Petroleum Geologists Bulletin'', 91, pp.1141–1166.</ref>; Huuse ''et al.'', 2010<ref name="Huuse 2010">Huuse, M, Jackson, C A-J, Van Rensbergen, P, Davies, R J, Flemings, P B, and Dixon, R J. (2010). Subsurface sediment remobilization and fluid flow in sedimentary basins: an overview. ''Basin Research'', '''22''', pp.342–360.</ref>). Their origin is uncertain, but may result from critical pressurisation of aquifers and hydrocarbon accumulations (Zühlsdorff & Spieß, 2004<ref name="Zühlsdorff 2004">Zühlsdorff, L, and Spieß, V. (2004). Three-dimensional seismic characterization of a venting site reveals compelling indications of natural hydraulic fracturing. ''Geology'', '''32''', pp.101–104.</ref>; Cartwright ''et al.'', 2007<ref name="Cartwright 2007"></ref>; Davies & Clarke, 2010<ref name="Davies 2010">Davies, R J, and Clarke, A L. (2010). Storage rather than venting after gas hydrate  dissociation. ''Geology'', '''38''', pp.963–966.</ref>). Chimney development may be followed by fluid driven erosion and collapse of the surrounding rock (Cartwright ''et al.'', 2007<ref name="Cartwright 2007"></ref>). The release and expansion of gas from solution during advective flow may also play a role in development (Brown, 1990<ref name="Brown 1990">Brown, K M. (1990). The nature and hydrogeologic significance of mud diapirs and diatremes for accretionary systems. ''Journal of Geophysical Research: Solid Earth'', '''''95''''', pp.8969–8982.</ref>; Cartwright ''et al.'', 2007<ref name="Cartwright 2007"></ref>). Chimneys are clearly identifiable in seismic data as vertical aligned discontinuities in otherwise continuous units (Cartwright ''et al.'', 2007<ref name="Cartwright 2007"></ref>; Løseth ''et al.'', 2011<ref name="Løseth 2011">Løseth, H, Wensaas, L, Arntsen, B, Hanken, N-M, Basire, C, and Graue, K. (2011). 1000 m long gas blow-out chimneys. ''Marine and Petroleum Geology'', '''28''', pp.1047–1060.</ref>). Davies ''et al. ''(2012)<ref name="Davies 2012"></ref> examined 368 chimneys from offshore Mauritania and showed that the average height was 247 metres, with the tallest chimney being 507 metres. In offshore Namibia 366 chimneys showed an average height of 360 metres, with the tallest being approximately 1,100 metres. In offshore Norway 466 chimneys showed an average height of 338 metres, with a maximum of 880 metres. From comparing natural with induced hydraulic fractures, Davies ''et al. ''(2012)<ref name="Davies 2012"></ref> conclude that the probability of an induced hydraulic fracture extending vertically more than 350 metres is about 1%. It should be noted that their conclusion is based on fracture height statistics alone and the mechanistic basis for fracture height control is not taken into account.

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