OR/18/012 Intrinsic vulnerability - Revision history

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	==Mines==		==Mines==
	Mines, for coal and other minerals, can create voids in the subsurface which can provide multiple pathways for contaminants over relatively large volumes (Ward et al., 2015<ref name="Ward 2015"></ref>; Monaghan, 2017<ref name="Monaghan 2017">MONAGHAN, A A. 2017. Unconventional energy resources in a crowded subsurface: Reducing uncertainty and developing a separation zone concept for resource estimation and deep 3D subsurface planning using legacy mining data. ''Science of the Total Environment'', Vol. 601–602, 45–56. </ref>). The footprint of voids from coal mines can be 50 000 to 200 000 m<sup>2</sup> in area (Younger, 2016<ref name="Younger 2016"> </ref>). Younger (2016)<ref name="Younger 2016"></ref> states that minewater discharges overwhelmingly occur via anthropogenic mined features such as shafts, adits or boreholes.		Mines, for coal and other minerals, can create voids in the subsurface which can provide multiple pathways for contaminants over relatively large volumes (Ward et al., 2015<ref name="Ward 2015"></ref>; Monaghan, 2017<ref name="Monaghan 2017">MONAGHAN, A A. 2017. Unconventional energy resources in a crowded subsurface: Reducing uncertainty and developing a separation zone concept for resource estimation and deep 3D subsurface planning using legacy mining data. ''Science of the Total Environment'', Vol. 601–602, 45–56. </ref>). The footprint of voids from coal mines can be 50 000 to 200 000 m<sup>2</sup> in area (Younger, 2016<ref name="Younger 2016"></ref>). Younger (2016)<ref name="Younger 2016"></ref> states that minewater discharges overwhelmingly occur via anthropogenic mined features such as shafts, adits or boreholes.

	Mining also impacts the characteristics of the surrounding rock, forming an anthropogenic aquifer (e.g. O’ Dochartaigh et al., 2015<ref name="O’ Dochartaigh 2015">O’ DOCHARTAIGH, B E, MACDONALD, A M, FITZSIMONS, V, and WARD, R. 2015. ''Scotland's aquifers and groundwater bodies''. British Geological Survey Report, 63pp. (OR/15/028) (Unpublished) </ref>). Longwall mining, in which a long wall of coal (3 to 4 km in length, and 400 m in width) is mined in a single slice, allows the mine to collapse within two to three years of coal extraction, forming voids filled with goaf (broken rock) (Younger, 2016<ref name="Younger 2016"></ref>). As a result of the collapse, bed-parallel fractures can form up to 20 m above the roof of the mined seam (or 1/3 of the distance between underground mine roadways which are typically 100 to 200 m). This fractured zone is overlain by a zone of net compression (and reduced permeability) of up to 1/9 of the distance between the roadways which isolates an upper extensional zone of the same thickness (Younger, 2016<ref name="Younger 2016"></ref>). Jones et al. (2004)<ref name="Jones 2004">JONES, N S, HOLLOWAY, S, CREEDY, D P, GARNER, K, SMITH, N J P, BROWNE, M A E, and DURUCAN, S. 2004. UK Coal Resource for New Exploitation Technologies Final Report. ''British Geological Survey Report'' CR/04/015N.</ref> estimate that the permeability of seams and surrounding strata is increased up to 160–200 m above and 40–70 m below worked seams as a result of previous longwall mining. Nevertheless, Younger (2016)<ref name="Younger 2016"></ref> presents the case at Selby Coalfield, Yorkshire, where mines were developed at depth with no connections to shallower workings and ‘complete’ hydraulic isolation from the near-surface hydrogeological environment. Stoop and room mining, in which pillars are left in place and coal mined from around these, can be stable for many years before collapsing (Younger, 2016<ref name="Younger 2016"></ref>).		Mining also impacts the characteristics of the surrounding rock, forming an anthropogenic aquifer (e.g. O’ Dochartaigh et al., 2015<ref name="O’ Dochartaigh 2015">O’ DOCHARTAIGH, B E, MACDONALD, A M, FITZSIMONS, V, and WARD, R. 2015. ''Scotland's aquifers and groundwater bodies''. British Geological Survey Report, 63pp. (OR/15/028) (Unpublished) </ref>). Longwall mining, in which a long wall of coal (3 to 4 km in length, and 400 m in width) is mined in a single slice, allows the mine to collapse within two to three years of coal extraction, forming voids filled with goaf (broken rock) (Younger, 2016<ref name="Younger 2016"></ref>). As a result of the collapse, bed-parallel fractures can form up to 20 m above the roof of the mined seam (or 1/3 of the distance between underground mine roadways which are typically 100 to 200 m). This fractured zone is overlain by a zone of net compression (and reduced permeability) of up to 1/9 of the distance between the roadways which isolates an upper extensional zone of the same thickness (Younger, 2016<ref name="Younger 2016"></ref>). Jones et al. (2004)<ref name="Jones 2004">JONES, N S, HOLLOWAY, S, CREEDY, D P, GARNER, K, SMITH, N J P, BROWNE, M A E, and DURUCAN, S. 2004. UK Coal Resource for New Exploitation Technologies Final Report. ''British Geological Survey Report'' CR/04/015N.</ref> estimate that the permeability of seams and surrounding strata is increased up to 160–200 m above and 40–70 m below worked seams as a result of previous longwall mining. Nevertheless, Younger (2016)<ref name="Younger 2016"></ref> presents the case at Selby Coalfield, Yorkshire, where mines were developed at depth with no connections to shallower workings and ‘complete’ hydraulic isolation from the near-surface hydrogeological environment. Stoop and room mining, in which pillars are left in place and coal mined from around these, can be stable for many years before collapsing (Younger, 2016<ref name="Younger 2016"></ref>).

Dbk: /* Pre-existing boreholes */

2019-12-03T11:16:52Z

Pre-existing boreholes

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	In many areas of hydrocarbon interest, there may be existing boreholes which can provide pathways for contamination if they are not properly sealed (for example the casing or cement) or have had a loss of integrity over time (Jackson et al., 2013a<ref name="Jackson 2013a"></ref>; Ward et al., 2015<ref name="Ward 2015"></ref>). Borehole leakage rates range from 2% to 50% in the UK (Davies et al., 2014<ref name="Davies 2014"></ref>). If abandoned, boreholes might not be monitored and the integrity of their casing will be unknown. In the UK there were 2152 hydrocarbon wells drilled onshore between 1902 and 2013. The ownership of up to 53% of these wells is unclear today and between 50 and 100 are orphaned (Davies et al., 2014<ref name="Davies 2014"></ref>).		In many areas of hydrocarbon interest, there may be existing boreholes which can provide pathways for contamination if they are not properly sealed (for example the casing or cement) or have had a loss of integrity over time (Jackson et al., 2013a<ref name="Jackson 2013a"></ref>; Ward et al., 2015<ref name="Ward 2015"></ref>). Borehole leakage rates range from 2% to 50% in the UK (Davies et al., 2014<ref name="Davies 2014"></ref>). If abandoned, boreholes might not be monitored and the integrity of their casing will be unknown. In the UK there were 2152 hydrocarbon wells drilled onshore between 1902 and 2013. The ownership of up to 53% of these wells is unclear today and between 50 and 100 are orphaned (Davies et al., 2014<ref name="Davies 2014"></ref>).

	Hydraulic fracturing has been shown to impact on adjacent wells (US EPA, 2016<ref name="US EPA 2016"></ref>). In Alberta and British Columbia, 5349 horizontal wells were drilled between 2009 and 2012 and there were 39 reported cases of wellbore connection with existing oil and gas wells, 95% of which were producing in the same geologic unit. Alberta requires that locations of existing oil and gas wells be identified and their capability to sustain increased pressures be verified prior to hydraulic fracturing (Lefebvre, 2017<ref name="Lefebvre 2017"> </ref>).		Hydraulic fracturing has been shown to impact on adjacent wells (US EPA, 2016<ref name="US EPA 2016"></ref>). In Alberta and British Columbia, 5349 horizontal wells were drilled between 2009 and 2012 and there were 39 reported cases of wellbore connection with existing oil and gas wells, 95% of which were producing in the same geologic unit. Alberta requires that locations of existing oil and gas wells be identified and their capability to sustain increased pressures be verified prior to hydraulic fracturing (Lefebvre, 2017<ref name="Lefebvre 2017"></ref>).

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Dbk: /* Faults */

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Faults

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	Faults and fractures have been thought to act as preferential pathways for methane in areas of shale gas exploitation (Warner et al., 2012<ref name="Warner 2012">WARNER, N R, JACKSON, R B, DARRAH, T H, OSBORN, S G, DOWN, A, ZHAO, K, WHITE, A, and VENGOSH, A. 2012. Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania, ''PNAS'', Vol. 109(30), 11961–11966. </ref>; Molofsky et al., 2013<ref name="Molofsky 2013" >MOLOFSKY, L J, CONNOR, J A, WYLIE, A S, WAGNER, T, and FARHAT, S K. 2013. Evaluation of Methane Sources in Groundwater in Northeastern Pennsylvania, ''Groundwater'', Vol. 51(3), 333–349. </ref>; Llewellyn, 2014<ref name="Llewellyn 2014">Llewellyn, G T. 2014. Evidence and mechanisms for Appalachian Basin brine migration into shallow aquifers in NE Pennsylvania, USA. ''Hydrogeology Journal'', 22(5), 1055–1066. </ref> and Moritz et al., 2015<ref name="Moritz 2015">MORITZ, A, HÉLIE, J-F, PINTI, D L, LAROCQUE, M, BARNETCHE, D, RETAILLEAU, S, LEFEBVRE, R, and GÉLINAS, Y. 2015. Methane baseline concentrations and sources in shallow aquifers from the shale gas-prone region of the St. Lawrence lowlands (Quebec, Canada), ''Environmental Science and Technology'', 49, 765–4771. </ref>). Numerical modelling has suggested that permeability and overall volume of the connecting fault or fracture have a greater impact on methane transport than separation distance (Reagan et al. 2015<ref name="Reagan 2015">REAGAN, M T, MORIDIS, G J, KEEN, N D, and JOHNSON, J N. 2015. Numerical simulation of the environmental 603 impact of hydraulic fracturing of tight/shale gas reservoirs on near surface groundwater: Background, 604 base cases, shallow reservoirs, short-term gas and water transport, ''Water Resources Research'', 605 51:2543–2573. </ref>). However, Younger (2016)<ref name="Younger 2016"></ref> states that in the UK there are no known minewater discharges from natural faults although minor seepages are known to occur along natural faults in close proximity to major mine seepages, even if the fault does not deliver the bulk of the flow.		Faults and fractures have been thought to act as preferential pathways for methane in areas of shale gas exploitation (Warner et al., 2012<ref name="Warner 2012">WARNER, N R, JACKSON, R B, DARRAH, T H, OSBORN, S G, DOWN, A, ZHAO, K, WHITE, A, and VENGOSH, A. 2012. Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania, ''PNAS'', Vol. 109(30), 11961–11966. </ref>; Molofsky et al., 2013<ref name="Molofsky 2013" >MOLOFSKY, L J, CONNOR, J A, WYLIE, A S, WAGNER, T, and FARHAT, S K. 2013. Evaluation of Methane Sources in Groundwater in Northeastern Pennsylvania, ''Groundwater'', Vol. 51(3), 333–349. </ref>; Llewellyn, 2014<ref name="Llewellyn 2014">Llewellyn, G T. 2014. Evidence and mechanisms for Appalachian Basin brine migration into shallow aquifers in NE Pennsylvania, USA. ''Hydrogeology Journal'', 22(5), 1055–1066. </ref> and Moritz et al., 2015<ref name="Moritz 2015">MORITZ, A, HÉLIE, J-F, PINTI, D L, LAROCQUE, M, BARNETCHE, D, RETAILLEAU, S, LEFEBVRE, R, and GÉLINAS, Y. 2015. Methane baseline concentrations and sources in shallow aquifers from the shale gas-prone region of the St. Lawrence lowlands (Quebec, Canada), ''Environmental Science and Technology'', 49, 765–4771. </ref>). Numerical modelling has suggested that permeability and overall volume of the connecting fault or fracture have a greater impact on methane transport than separation distance (Reagan et al. 2015<ref name="Reagan 2015">REAGAN, M T, MORIDIS, G J, KEEN, N D, and JOHNSON, J N. 2015. Numerical simulation of the environmental 603 impact of hydraulic fracturing of tight/shale gas reservoirs on near surface groundwater: Background, 604 base cases, shallow reservoirs, short-term gas and water transport, ''Water Resources Research'', 605 51:2543–2573. </ref>). However, Younger (2016)<ref name="Younger 2016"></ref> states that in the UK there are no known minewater discharges from natural faults although minor seepages are known to occur along natural faults in close proximity to major mine seepages, even if the fault does not deliver the bulk of the flow.

	It is thought that a large number of factors might interact to determine whether or not a fault will enhance or hinder fluid flow, including orientation with respect to the regional stress field, lithology, fault throw and deformation history (history of movement and subsequent diagenesis) (e.g. Bense et al., 2013<ref name="Bense 2013">~~BENSE, V F, GLEESON, T, LOVELESS, S E, BOUR, O, and SCIBEK, J. 2013. Fault zone hydrogeology. ''Earth-Science Reviews'', Vol. 127, 171–192.~~ </ref>). Pressure changes surrounding faults, perhaps due to stimulation techniques such as Enhanced Oil Recovery (EOR) or hydraulic fracturing, might also alter the hydraulic behaviour of the fault, for example by fault reactivation, and can also lead to leakage along a fault (e.g. Rinaldi et al., 2014<ref name="Rinaldi 2014">RINALDI, A P, RUTQVIST, J, and CAPPA, F. 2014. Geomechanical effects on CO<sub>2</sub> leakage through fault zones during large-scale underground injection. ''International Journal of Greenhouse Gas Control'', Vol. 20, 117–131. </ref>). Westwood (2017)<ref name="Westwood 2017">WESTWOOD, R F, TOON, S M, and CASSIDY, N J. 2017. A sensitivity analysis of the effect of pumping parameters on hydraulic fracture networks and local stresses during shale gas operations, ''Fuel'', Vol. 203, 843–852. </ref> found that the horizontal ‘respect distance’ (minimum lateral distance that hydraulic fracturing should occur from a pre-existing fault in order not to reactivate it) ranged from 63 to 433 m depending on fracture intensity and failure threshold, based on numerical models of hydraulic fracturing at Preese Hall, Lancashire.		It is thought that a large number of factors might interact to determine whether or not a fault will enhance or hinder fluid flow, including orientation with respect to the regional stress field, lithology, fault throw and deformation history (history of movement and subsequent diagenesis) (e.g. Bense et al., 2013<ref name="Bense 2013"></ref>). Pressure changes surrounding faults, perhaps due to stimulation techniques such as Enhanced Oil Recovery (EOR) or hydraulic fracturing, might also alter the hydraulic behaviour of the fault, for example by fault reactivation, and can also lead to leakage along a fault (e.g. Rinaldi et al., 2014<ref name="Rinaldi 2014">RINALDI, A P, RUTQVIST, J, and CAPPA, F. 2014. Geomechanical effects on CO<sub>2</sub> leakage through fault zones during large-scale underground injection. ''International Journal of Greenhouse Gas Control'', Vol. 20, 117–131. </ref>). Westwood (2017)<ref name="Westwood 2017">WESTWOOD, R F, TOON, S M, and CASSIDY, N J. 2017. A sensitivity analysis of the effect of pumping parameters on hydraulic fracture networks and local stresses during shale gas operations, ''Fuel'', Vol. 203, 843–852. </ref> found that the horizontal ‘respect distance’ (minimum lateral distance that hydraulic fracturing should occur from a pre-existing fault in order not to reactivate it) ranged from 63 to 433 m depending on fracture intensity and failure threshold, based on numerical models of hydraulic fracturing at Preese Hall, Lancashire.

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Dbk: /* Solution features */

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Solution features

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	==Solution features==		==Solution features==
	Dissolution features occur in both carbonate and evaporite rocks. The depth of karst development is highly variable and is related to differences in geology and landscape evolution. In England, karst is quite common at shallow depths in parts of the Chalk and in the Carboniferous Limestone. Cave systems are known to nearly 300 m bgl (e.g. Brants Gill, Yorkshire Dales) (Waltham et al., 1997<ref name="Waltham 1997">WALTHAM, A C, SIMMS, M J, FARRANT, A R, and GOLDIE, H S. 1997. ''Karst and Caves of Great Britain''. (London: Chapman and Hall).</ref>). Karst drainage may have developed at times of lower base levels (sea levels). Some deep caves are formed by water rising up from depth or by geochemical mixing, sulphuric acid dissolution or rising artesian flow through soluble rocks, and are unrelated to modern drainage systems (Farrant, 2008<ref name="Farrant 2008">FARRANT, A, and COOPER, A. 2008. Karst geohazards in the UK: the use of digital data for hazard management. ''Quarterly Journal of Engineering Geology and Hydrogeology'', Vol. 41, 339–356. </ref>). Palaeokarst systems can be inferred at depth in Carboniferous Limestones, for example at the Buxton Springs, where groundwater circulation is inferred to 1500 m bgl (Aitkenhead et al., 2002<ref name="Aitkenhead 2002">AITKENHEAD, N, BARCLAY, W J, BRANDON, A, CHADWICK, R A, CHRISHOLM, M J I, COOPER, A H, and JOHNSON, E W. 2002. ''The Pennines and adjacent areas (4th edition), Regional Geology Guide''. (Nottingham: British Geological Survey).</ref>) and at the Bath Hot Springs, where it is inferred up to 4000 m bgl (Edmunds et al., 2014<ref name="Edmunds 2014">EDMUNDS, W M, DARLING, W G, PURTSCHERT, R, and CORCHO ALVARADO, J A. 2014. Noble gas, CFC and other geochemical evidence for the age and origin of the Bath thermal waters, UK. ''Applied Geochemistry'', Vol. 40, 155–163 </ref>). Palaeokarst is more likely to have developed below an unconformity. The maximum depth of karstification in England is limited by the base of the limestone. Gypsum karst has also formed phreatic cave systems, but the rapid solubility rate of the gypsum means that the karst can evolve on a human time scale (Farrant, 2008<ref name="Farrant 2008">~~FARRANT, A, and COOPER, A. 2008. Karst geohazards in the UK: the use of digital data for hazard management. ''Quarterly Journal of Engineering Geology and Hydrogeology'', Vol. 41, 339–356.~~ </ref>).		Dissolution features occur in both carbonate and evaporite rocks. The depth of karst development is highly variable and is related to differences in geology and landscape evolution. In England, karst is quite common at shallow depths in parts of the Chalk and in the Carboniferous Limestone. Cave systems are known to nearly 300 m bgl (e.g. Brants Gill, Yorkshire Dales) (Waltham et al., 1997<ref name="Waltham 1997">WALTHAM, A C, SIMMS, M J, FARRANT, A R, and GOLDIE, H S. 1997. ''Karst and Caves of Great Britain''. (London: Chapman and Hall).</ref>). Karst drainage may have developed at times of lower base levels (sea levels). Some deep caves are formed by water rising up from depth or by geochemical mixing, sulphuric acid dissolution or rising artesian flow through soluble rocks, and are unrelated to modern drainage systems (Farrant, 2008<ref name="Farrant 2008">FARRANT, A, and COOPER, A. 2008. Karst geohazards in the UK: the use of digital data for hazard management. ''Quarterly Journal of Engineering Geology and Hydrogeology'', Vol. 41, 339–356. </ref>). Palaeokarst systems can be inferred at depth in Carboniferous Limestones, for example at the Buxton Springs, where groundwater circulation is inferred to 1500 m bgl (Aitkenhead et al., 2002<ref name="Aitkenhead 2002">AITKENHEAD, N, BARCLAY, W J, BRANDON, A, CHADWICK, R A, CHRISHOLM, M J I, COOPER, A H, and JOHNSON, E W. 2002. ''The Pennines and adjacent areas (4th edition), Regional Geology Guide''. (Nottingham: British Geological Survey).</ref>) and at the Bath Hot Springs, where it is inferred up to 4000 m bgl (Edmunds et al., 2014<ref name="Edmunds 2014">EDMUNDS, W M, DARLING, W G, PURTSCHERT, R, and CORCHO ALVARADO, J A. 2014. Noble gas, CFC and other geochemical evidence for the age and origin of the Bath thermal waters, UK. ''Applied Geochemistry'', Vol. 40, 155–163 </ref>). Palaeokarst is more likely to have developed below an unconformity. The maximum depth of karstification in England is limited by the base of the limestone. Gypsum karst has also formed phreatic cave systems, but the rapid solubility rate of the gypsum means that the karst can evolve on a human time scale (Farrant, 2008<ref name="Farrant 2008"></ref>).

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Dbk: /* Pre-existing boreholes */

2019-12-03T11:14:39Z

Pre-existing boreholes

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	Boreholes drilled into the subsurface create potential pathways for contaminants to a receptor. While deep hydrocarbon and mineral boreholes are generally completed to prevent leakage, with both steel casing and cement bonding, borehole integrity failures (defects in steel casing, holes in casing joints, mechanical seals and cement e.g. Jackson et al., 2014<ref name="Jackson 2014">JACKSON, R B, VENGOSH, A, CAREY, J W, DAVIES, R, DARRAH, T H, O’SULLIVAN F, PÉTRON G. 2014. The 555 Environmental Costs and Benefits of Fracking, ''Anny. Rev. Environ. Resoure''. 39:327–362, doi 556 10.1146/annrev-environ-031113-144051 </ref>) can occur. 3% of all hydraulic fracturing operations in the USA involved a downhole mechanical integrity failure (US EPA, 2016<ref name="US EPA 2016"></ref>). Davies et al. (2014)<ref name="Davies 2014">DAVIES, R J, ALMOND, S, WARD, R S, JACKSON, R B, ADAMS, C, WORRALL, F, HERRINGSHAW, L G, GLUYAS, J G, and WHITEHEAD, M A. 2014. Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation. ''Marine and Petroleum Geology'', 30(1), 1–16. </ref> present data from around the world for the percentage of boreholes (including production, injection, idle and abandoned boreholes) that have some form of borehole barrier or integrity failure. Percentages range from 1.9% (onshore, nationwide CCS/natural gas storage facilities, dates unknown, including well integrity failure only, described as significant gas loss, for 470 boreholes) to 75% (onshore, operational wells in the Santa Fe Springs Oilfield, discovered 1921, including well integrity failures, leakage based on the observation of gas bubbles seeping to the surface along well casing for more than 50 wells). The probability of borehole integrity failure depends on the quality of completion (which will vary over time), the age of the well (degradation) and the exploitation processes but the high variability in recorded barrier or integrity in Davies et al. (2014)<ref name="Davies 2014"></ref> also reflects differences in classification of failure (e.g. well, single barrier, significant or bubbles), geological setting and importantly regulation (e.g. Thorogood and Younger, 2015<ref name="Thorogood 2015">THOROGOOD, J L, and YOUNGER, P L. 2015. Discussion of ‘Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation’ by R J Davies, S, Almond, R S, Ward, R B, Jackson, C, Adams, F, Worrall, L G, Herringshaw, J G, Gluyas, and M A, Whitehead. (Marine and Petroleum Geology 2014). ''Marine and Petroleum Geology'', Vol. 59, 671–673. </ref>; Davies et al. 2015<ref name="Davies 2015">DAVIES, R J, ALMOND, S, WARD, R S, JACKSON, R B, ADAMS, C, WORRALL, F, HERRINGSHAW, L G, GLUYAS, J G, and WHITEHEAD, M A. 2015. Reply: ‘Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation’. ''Marine and Petroleum Geology'', 59, 674–675.</ref>). Of 143 active wells producing in the UK at the end of 2000, one has evidence of borehole integrity failure (Davies et al., 2014<ref name="Davies 2014"></ref>).		Boreholes drilled into the subsurface create potential pathways for contaminants to a receptor. While deep hydrocarbon and mineral boreholes are generally completed to prevent leakage, with both steel casing and cement bonding, borehole integrity failures (defects in steel casing, holes in casing joints, mechanical seals and cement e.g. Jackson et al., 2014<ref name="Jackson 2014">JACKSON, R B, VENGOSH, A, CAREY, J W, DAVIES, R, DARRAH, T H, O’SULLIVAN F, PÉTRON G. 2014. The 555 Environmental Costs and Benefits of Fracking, ''Anny. Rev. Environ. Resoure''. 39:327–362, doi 556 10.1146/annrev-environ-031113-144051 </ref>) can occur. 3% of all hydraulic fracturing operations in the USA involved a downhole mechanical integrity failure (US EPA, 2016<ref name="US EPA 2016"></ref>). Davies et al. (2014)<ref name="Davies 2014">DAVIES, R J, ALMOND, S, WARD, R S, JACKSON, R B, ADAMS, C, WORRALL, F, HERRINGSHAW, L G, GLUYAS, J G, and WHITEHEAD, M A. 2014. Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation. ''Marine and Petroleum Geology'', 30(1), 1–16. </ref> present data from around the world for the percentage of boreholes (including production, injection, idle and abandoned boreholes) that have some form of borehole barrier or integrity failure. Percentages range from 1.9% (onshore, nationwide CCS/natural gas storage facilities, dates unknown, including well integrity failure only, described as significant gas loss, for 470 boreholes) to 75% (onshore, operational wells in the Santa Fe Springs Oilfield, discovered 1921, including well integrity failures, leakage based on the observation of gas bubbles seeping to the surface along well casing for more than 50 wells). The probability of borehole integrity failure depends on the quality of completion (which will vary over time), the age of the well (degradation) and the exploitation processes but the high variability in recorded barrier or integrity in Davies et al. (2014)<ref name="Davies 2014"></ref> also reflects differences in classification of failure (e.g. well, single barrier, significant or bubbles), geological setting and importantly regulation (e.g. Thorogood and Younger, 2015<ref name="Thorogood 2015">THOROGOOD, J L, and YOUNGER, P L. 2015. Discussion of ‘Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation’ by R J Davies, S, Almond, R S, Ward, R B, Jackson, C, Adams, F, Worrall, L G, Herringshaw, J G, Gluyas, and M A, Whitehead. (Marine and Petroleum Geology 2014). ''Marine and Petroleum Geology'', Vol. 59, 671–673. </ref>; Davies et al. 2015<ref name="Davies 2015">DAVIES, R J, ALMOND, S, WARD, R S, JACKSON, R B, ADAMS, C, WORRALL, F, HERRINGSHAW, L G, GLUYAS, J G, and WHITEHEAD, M A. 2015. Reply: ‘Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation’. ''Marine and Petroleum Geology'', 59, 674–675.</ref>). Of 143 active wells producing in the UK at the end of 2000, one has evidence of borehole integrity failure (Davies et al., 2014<ref name="Davies 2014"></ref>).

	In many areas of hydrocarbon interest, there may be existing boreholes which can provide pathways for contamination if they are not properly sealed (for example the casing or cement) or have had a loss of integrity over time (Jackson et al., 2013a<ref name="Jackson 2013a"></ref>; Ward et al., 2015<ref name="Ward 2015">WARD, R, STUART, M E, and BLOOMFIELD, J P. 2015. The hydrogological aspects of shale gas extraction in the UK. In: ''Issues in Environmental Science and Technology: Fracking''. (London: Royal Society of Chemistry). pp. 122–150. </ref>). Borehole leakage rates range from 2% to 50% in the UK (Davies et al., 2014<ref name="Davies 2014"></ref>). If abandoned, boreholes might not be monitored and the integrity of their casing will be unknown. In the UK there were 2152 hydrocarbon wells drilled onshore between 1902 and 2013. The ownership of up to 53% of these wells is unclear today and between 50 and 100 are orphaned (Davies et al., 2014<ref name="Davies 2014"></ref>).		In many areas of hydrocarbon interest, there may be existing boreholes which can provide pathways for contamination if they are not properly sealed (for example the casing or cement) or have had a loss of integrity over time (Jackson et al., 2013a<ref name="Jackson 2013a"></ref>; Ward et al., 2015<ref name="Ward 2015"></ref>). Borehole leakage rates range from 2% to 50% in the UK (Davies et al., 2014<ref name="Davies 2014"></ref>). If abandoned, boreholes might not be monitored and the integrity of their casing will be unknown. In the UK there were 2152 hydrocarbon wells drilled onshore between 1902 and 2013. The ownership of up to 53% of these wells is unclear today and between 50 and 100 are orphaned (Davies et al., 2014<ref name="Davies 2014"></ref>).

	Hydraulic fracturing has been shown to impact on adjacent wells (US EPA, 2016<ref name="US EPA 2016"></ref>). In Alberta and British Columbia, 5349 horizontal wells were drilled between 2009 and 2012 and there were 39 reported cases of wellbore connection with existing oil and gas wells, 95% of which were producing in the same geologic unit. Alberta requires that locations of existing oil and gas wells be identified and their capability to sustain increased pressures be verified prior to hydraulic fracturing (Lefebvre, 2017<ref name="Lefebvre 2017"> </ref>).		Hydraulic fracturing has been shown to impact on adjacent wells (US EPA, 2016<ref name="US EPA 2016"></ref>). In Alberta and British Columbia, 5349 horizontal wells were drilled between 2009 and 2012 and there were 39 reported cases of wellbore connection with existing oil and gas wells, 95% of which were producing in the same geologic unit. Alberta requires that locations of existing oil and gas wells be identified and their capability to sustain increased pressures be verified prior to hydraulic fracturing (Lefebvre, 2017<ref name="Lefebvre 2017"> </ref>).

Dbk: /* Mines */

2019-12-03T11:14:12Z

Mines

← Older revision		Revision as of 12:14, 3 December 2019
Line 385:		Line 385:

	==Mines==		==Mines==
	Mines, for coal and other minerals, can create voids in the subsurface which can provide multiple pathways for contaminants over relatively large volumes (Ward et al., 2015<ref name="Ward 2015">WARD, R, STUART, M E, and BLOOMFIELD, J P. 2015. The hydrogological aspects of shale gas extraction in the UK. In: ''Issues in Environmental Science and Technology: Fracking''. (London: Royal Society of Chemistry). pp. 122–150.</ref>; Monaghan, 2017<ref name="Monaghan 2017">MONAGHAN, A A. 2017. Unconventional energy resources in a crowded subsurface: Reducing uncertainty and developing a separation zone concept for resource estimation and deep 3D subsurface planning using legacy mining data. ''Science of the Total Environment'', Vol. 601–602, 45–56. </ref>). The footprint of voids from coal mines can be 50 000 to 200 000 m<sup>2</sup> in area (Younger, 2016<ref name="Younger 2016"> </ref>). Younger (2016)<ref name="Younger 2016"></ref> states that minewater discharges overwhelmingly occur via anthropogenic mined features such as shafts, adits or boreholes.		Mines, for coal and other minerals, can create voids in the subsurface which can provide multiple pathways for contaminants over relatively large volumes (Ward et al., 2015<ref name="Ward 2015"></ref>; Monaghan, 2017<ref name="Monaghan 2017">MONAGHAN, A A. 2017. Unconventional energy resources in a crowded subsurface: Reducing uncertainty and developing a separation zone concept for resource estimation and deep 3D subsurface planning using legacy mining data. ''Science of the Total Environment'', Vol. 601–602, 45–56. </ref>). The footprint of voids from coal mines can be 50 000 to 200 000 m<sup>2</sup> in area (Younger, 2016<ref name="Younger 2016"> </ref>). Younger (2016)<ref name="Younger 2016"></ref> states that minewater discharges overwhelmingly occur via anthropogenic mined features such as shafts, adits or boreholes.

	Mining also impacts the characteristics of the surrounding rock, forming an anthropogenic aquifer (e.g. O’ Dochartaigh et al., 2015<ref name="O’ Dochartaigh 2015">O’ DOCHARTAIGH, B E, MACDONALD, A M, FITZSIMONS, V, and WARD, R. 2015. ''Scotland's aquifers and groundwater bodies''. British Geological Survey Report, 63pp. (OR/15/028) (Unpublished) </ref>). Longwall mining, in which a long wall of coal (3 to 4 km in length, and 400 m in width) is mined in a single slice, allows the mine to collapse within two to three years of coal extraction, forming voids filled with goaf (broken rock) (Younger, 2016<ref name="Younger 2016"></ref>). As a result of the collapse, bed-parallel fractures can form up to 20 m above the roof of the mined seam (or 1/3 of the distance between underground mine roadways which are typically 100 to 200 m). This fractured zone is overlain by a zone of net compression (and reduced permeability) of up to 1/9 of the distance between the roadways which isolates an upper extensional zone of the same thickness (Younger, 2016<ref name="Younger 2016"></ref>). Jones et al. (2004)<ref name="Jones 2004">JONES, N S, HOLLOWAY, S, CREEDY, D P, GARNER, K, SMITH, N J P, BROWNE, M A E, and DURUCAN, S. 2004. UK Coal Resource for New Exploitation Technologies Final Report. ''British Geological Survey Report'' CR/04/015N.</ref> estimate that the permeability of seams and surrounding strata is increased up to 160–200 m above and 40–70 m below worked seams as a result of previous longwall mining. Nevertheless, Younger (2016)<ref name="Younger 2016"></ref> presents the case at Selby Coalfield, Yorkshire, where mines were developed at depth with no connections to shallower workings and ‘complete’ hydraulic isolation from the near-surface hydrogeological environment. Stoop and room mining, in which pillars are left in place and coal mined from around these, can be stable for many years before collapsing (Younger, 2016<ref name="Younger 2016"></ref>).		Mining also impacts the characteristics of the surrounding rock, forming an anthropogenic aquifer (e.g. O’ Dochartaigh et al., 2015<ref name="O’ Dochartaigh 2015">O’ DOCHARTAIGH, B E, MACDONALD, A M, FITZSIMONS, V, and WARD, R. 2015. ''Scotland's aquifers and groundwater bodies''. British Geological Survey Report, 63pp. (OR/15/028) (Unpublished) </ref>). Longwall mining, in which a long wall of coal (3 to 4 km in length, and 400 m in width) is mined in a single slice, allows the mine to collapse within two to three years of coal extraction, forming voids filled with goaf (broken rock) (Younger, 2016<ref name="Younger 2016"></ref>). As a result of the collapse, bed-parallel fractures can form up to 20 m above the roof of the mined seam (or 1/3 of the distance between underground mine roadways which are typically 100 to 200 m). This fractured zone is overlain by a zone of net compression (and reduced permeability) of up to 1/9 of the distance between the roadways which isolates an upper extensional zone of the same thickness (Younger, 2016<ref name="Younger 2016"></ref>). Jones et al. (2004)<ref name="Jones 2004">JONES, N S, HOLLOWAY, S, CREEDY, D P, GARNER, K, SMITH, N J P, BROWNE, M A E, and DURUCAN, S. 2004. UK Coal Resource for New Exploitation Technologies Final Report. ''British Geological Survey Report'' CR/04/015N.</ref> estimate that the permeability of seams and surrounding strata is increased up to 160–200 m above and 40–70 m below worked seams as a result of previous longwall mining. Nevertheless, Younger (2016)<ref name="Younger 2016"></ref> presents the case at Selby Coalfield, Yorkshire, where mines were developed at depth with no connections to shallower workings and ‘complete’ hydraulic isolation from the near-surface hydrogeological environment. Stoop and room mining, in which pillars are left in place and coal mined from around these, can be stable for many years before collapsing (Younger, 2016<ref name="Younger 2016"></ref>).

Dbk: /* Pre-existing boreholes */

2019-12-03T11:13:51Z

Pre-existing boreholes

← Older revision		Revision as of 12:13, 3 December 2019
Line 432:		Line 432:
	In many areas of hydrocarbon interest, there may be existing boreholes which can provide pathways for contamination if they are not properly sealed (for example the casing or cement) or have had a loss of integrity over time (Jackson et al., 2013a<ref name="Jackson 2013a"></ref>; Ward et al., 2015<ref name="Ward 2015">WARD, R, STUART, M E, and BLOOMFIELD, J P. 2015. The hydrogological aspects of shale gas extraction in the UK. In: ''Issues in Environmental Science and Technology: Fracking''. (London: Royal Society of Chemistry). pp. 122–150. </ref>). Borehole leakage rates range from 2% to 50% in the UK (Davies et al., 2014<ref name="Davies 2014"></ref>). If abandoned, boreholes might not be monitored and the integrity of their casing will be unknown. In the UK there were 2152 hydrocarbon wells drilled onshore between 1902 and 2013. The ownership of up to 53% of these wells is unclear today and between 50 and 100 are orphaned (Davies et al., 2014<ref name="Davies 2014"></ref>).		In many areas of hydrocarbon interest, there may be existing boreholes which can provide pathways for contamination if they are not properly sealed (for example the casing or cement) or have had a loss of integrity over time (Jackson et al., 2013a<ref name="Jackson 2013a"></ref>; Ward et al., 2015<ref name="Ward 2015">WARD, R, STUART, M E, and BLOOMFIELD, J P. 2015. The hydrogological aspects of shale gas extraction in the UK. In: ''Issues in Environmental Science and Technology: Fracking''. (London: Royal Society of Chemistry). pp. 122–150. </ref>). Borehole leakage rates range from 2% to 50% in the UK (Davies et al., 2014<ref name="Davies 2014"></ref>). If abandoned, boreholes might not be monitored and the integrity of their casing will be unknown. In the UK there were 2152 hydrocarbon wells drilled onshore between 1902 and 2013. The ownership of up to 53% of these wells is unclear today and between 50 and 100 are orphaned (Davies et al., 2014<ref name="Davies 2014"></ref>).

	Hydraulic fracturing has been shown to impact on adjacent wells (US EPA, 2016<ref name="US EPA 2016"></ref>). In Alberta and British Columbia, 5349 horizontal wells were drilled between 2009 and 2012 and there were 39 reported cases of wellbore connection with existing oil and gas wells, 95% of which were producing in the same geologic unit. Alberta requires that locations of existing oil and gas wells be identified and their capability to sustain increased pressures be verified prior to hydraulic fracturing (Lefebvre, 2017<ref name="Lefebvre 2017">~~LEFEBVRE, R. 2017. Potential impacts of shale gas development on groundwater quality. ''Wiley Interdisciplinary Reviews: Water'', Vol. 4 (1). doi: 10.1002/wat2.1188.~~ </ref>).		Hydraulic fracturing has been shown to impact on adjacent wells (US EPA, 2016<ref name="US EPA 2016"></ref>). In Alberta and British Columbia, 5349 horizontal wells were drilled between 2009 and 2012 and there were 39 reported cases of wellbore connection with existing oil and gas wells, 95% of which were producing in the same geologic unit. Alberta requires that locations of existing oil and gas wells be identified and their capability to sustain increased pressures be verified prior to hydraulic fracturing (Lefebvre, 2017<ref name="Lefebvre 2017"> </ref>).

	{\| class="wikitable"		{\| class="wikitable"

Dbk: /* Faults */

2019-12-03T11:13:27Z

Faults

← Older revision		Revision as of 12:13, 3 December 2019
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	==Faults==		==Faults==
	Large volumes of fluids, for example deep brines (Warner et al., 2012<ref name="Warner 2012">WARNER, N R, JACKSON, R B, DARRAH, T H, OSBORN, S G, DOWN, A, ZHAO, K, WHITE, A, and VENGOSH, A. 2012. Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania, ''PNAS'', Vol. 109(30), 11961–11966. </ref>; Llewellyn, 2014<ref name="Llewellyn 2014">Llewellyn, G T. 2014. Evidence and mechanisms for Appalachian Basin brine migration into shallow aquifers in NE Pennsylvania, USA. ''Hydrogeology Journal'', 22(5), 1055–1066. </ref>) and gases (Molofsky et al., 2013<ref name="Molofsky 2013">MOLOFSKY, L J, CONNOR, J A, WYLIE, A S, WAGNER, T, and FARHAT, S K. 2013. Evaluation of Methane Sources in Groundwater in Northeastern Pennsylvania, ''Groundwater'', Vol. 51(3), 333–349. </ref>; Moritz et al., 2015<ref name="Moritz 2015">MORITZ, A, HÉLIE, J-F, PINTI, D L, LAROCQUE, M, BARNETCHE, D, RETAILLEAU, S, LEFEBVRE, R, and GÉLINAS, Y. 2015. Methane baseline concentrations and sources in shallow aquifers from the shale gas-prone region of the St. Lawrence lowlands (Quebec, Canada), ''Environmental Science and Technology'', 49, 765–4771. </ref>), have been shown to migrate vertically through rock masses for large distances (up to 2.4 km (Llewellyn, 2014<ref name="Llewellyn 2014"></ref>)) over long timescales. However, contaminant migration over the large vertical separation distances between deep hydrocarbon source units and shallow receptors (for example shales with an average depth of 2 km in the US, (US EPA, 2016<ref name="US EPA 2016"></ref>) is considered unlikely, or would take a very long time, without preferential flow pathways (Lefebvre, 2017<ref name="Lefebvre 2017">~~LEFEBVRE, R. 2017. Potential impacts of shale gas development on groundwater quality. ''Wiley Interdisciplinary Reviews: Water'', Vol. 4 (1). doi: 10.1002/wat2.1188.~~</ref>). Numerical models by Reagan et al. (2015) have shown that characteristics such as the presence of preferential flow pathways (e.g. faults) and production characteristics might have a greater impact on transport than vertical separation distances.		Large volumes of fluids, for example deep brines (Warner et al., 2012<ref name="Warner 2012">WARNER, N R, JACKSON, R B, DARRAH, T H, OSBORN, S G, DOWN, A, ZHAO, K, WHITE, A, and VENGOSH, A. 2012. Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania, ''PNAS'', Vol. 109(30), 11961–11966. </ref>; Llewellyn, 2014<ref name="Llewellyn 2014">Llewellyn, G T. 2014. Evidence and mechanisms for Appalachian Basin brine migration into shallow aquifers in NE Pennsylvania, USA. ''Hydrogeology Journal'', 22(5), 1055–1066. </ref>) and gases (Molofsky et al., 2013<ref name="Molofsky 2013">MOLOFSKY, L J, CONNOR, J A, WYLIE, A S, WAGNER, T, and FARHAT, S K. 2013. Evaluation of Methane Sources in Groundwater in Northeastern Pennsylvania, ''Groundwater'', Vol. 51(3), 333–349. </ref>; Moritz et al., 2015<ref name="Moritz 2015">MORITZ, A, HÉLIE, J-F, PINTI, D L, LAROCQUE, M, BARNETCHE, D, RETAILLEAU, S, LEFEBVRE, R, and GÉLINAS, Y. 2015. Methane baseline concentrations and sources in shallow aquifers from the shale gas-prone region of the St. Lawrence lowlands (Quebec, Canada), ''Environmental Science and Technology'', 49, 765–4771. </ref>), have been shown to migrate vertically through rock masses for large distances (up to 2.4 km (Llewellyn, 2014<ref name="Llewellyn 2014"></ref>)) over long timescales. However, contaminant migration over the large vertical separation distances between deep hydrocarbon source units and shallow receptors (for example shales with an average depth of 2 km in the US, (US EPA, 2016<ref name="US EPA 2016"></ref>) is considered unlikely, or would take a very long time, without preferential flow pathways (Lefebvre, 2017<ref name="Lefebvre 2017"></ref>). Numerical models by Reagan et al. (2015) have shown that characteristics such as the presence of preferential flow pathways (e.g. faults) and production characteristics might have a greater impact on transport than vertical separation distances.

	Faults are planes of movement along which adjacent blocks of rock strata have moved relative to each other. Faults commonly comprise zones, of up to several tens of metres (or greater) in width, of fractures and fault rock. Faults can enhance or hinder fluid flow, or a combination of both (preventing fluids crossing the fault while at the same time allowing fluids to flow parallel to the fault) (Bense et al., 2013<ref name="Bense 2013">BENSE, V F, GLEESON, T, LOVELESS, S E, BOUR, O, and SCIBEK, J. 2013. Fault zone hydrogeology. ''Earth-Science Reviews'', Vol. 127, 171–192.</ref>). Faults that enhance (are conduits for) fluid flow can allow contaminants to travel along the fault (as a pathway) to a groundwater receptor and can provide vertical pathways through otherwise low permeability bodies of rock. Faults have been found to be conduits for methane (and thermal fluids) even through large thicknesses of shale in British Columbia, Canada (Grasby et al., 2016<ref name="Grasby 2016">GRASBY, S E, FERGUSON, G, BRADY, A, SHARP, C, DUNFIELD, P, and MCMECHAN, M. 2016b. Deep groundwater circulation and associated methane leakage in the northern Canadian Rocky Mountains. ''Applied Geochemistry'', Vol. 68, 10–18.</ref>). Faulting may also bring receptor formations into contact with hydrocarbon source unit formations across the fault zone (known as juxtaposition) and result in laterally variable hydrogeological and rheological properties.		Faults are planes of movement along which adjacent blocks of rock strata have moved relative to each other. Faults commonly comprise zones, of up to several tens of metres (or greater) in width, of fractures and fault rock. Faults can enhance or hinder fluid flow, or a combination of both (preventing fluids crossing the fault while at the same time allowing fluids to flow parallel to the fault) (Bense et al., 2013<ref name="Bense 2013">BENSE, V F, GLEESON, T, LOVELESS, S E, BOUR, O, and SCIBEK, J. 2013. Fault zone hydrogeology. ''Earth-Science Reviews'', Vol. 127, 171–192.</ref>). Faults that enhance (are conduits for) fluid flow can allow contaminants to travel along the fault (as a pathway) to a groundwater receptor and can provide vertical pathways through otherwise low permeability bodies of rock. Faults have been found to be conduits for methane (and thermal fluids) even through large thicknesses of shale in British Columbia, Canada (Grasby et al., 2016<ref name="Grasby 2016">GRASBY, S E, FERGUSON, G, BRADY, A, SHARP, C, DUNFIELD, P, and MCMECHAN, M. 2016b. Deep groundwater circulation and associated methane leakage in the northern Canadian Rocky Mountains. ''Applied Geochemistry'', Vol. 68, 10–18.</ref>). Faulting may also bring receptor formations into contact with hydrocarbon source unit formations across the fault zone (known as juxtaposition) and result in laterally variable hydrogeological and rheological properties.

Dbk: /* Pre-existing boreholes */

2019-12-03T11:12:50Z

Pre-existing boreholes

← Older revision		Revision as of 12:12, 3 December 2019
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	Boreholes drilled into the subsurface create potential pathways for contaminants to a receptor. While deep hydrocarbon and mineral boreholes are generally completed to prevent leakage, with both steel casing and cement bonding, borehole integrity failures (defects in steel casing, holes in casing joints, mechanical seals and cement e.g. Jackson et al., 2014<ref name="Jackson 2014">JACKSON, R B, VENGOSH, A, CAREY, J W, DAVIES, R, DARRAH, T H, O’SULLIVAN F, PÉTRON G. 2014. The 555 Environmental Costs and Benefits of Fracking, ''Anny. Rev. Environ. Resoure''. 39:327–362, doi 556 10.1146/annrev-environ-031113-144051 </ref>) can occur. 3% of all hydraulic fracturing operations in the USA involved a downhole mechanical integrity failure (US EPA, 2016<ref name="US EPA 2016"></ref>). Davies et al. (2014)<ref name="Davies 2014">DAVIES, R J, ALMOND, S, WARD, R S, JACKSON, R B, ADAMS, C, WORRALL, F, HERRINGSHAW, L G, GLUYAS, J G, and WHITEHEAD, M A. 2014. Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation. ''Marine and Petroleum Geology'', 30(1), 1–16. </ref> present data from around the world for the percentage of boreholes (including production, injection, idle and abandoned boreholes) that have some form of borehole barrier or integrity failure. Percentages range from 1.9% (onshore, nationwide CCS/natural gas storage facilities, dates unknown, including well integrity failure only, described as significant gas loss, for 470 boreholes) to 75% (onshore, operational wells in the Santa Fe Springs Oilfield, discovered 1921, including well integrity failures, leakage based on the observation of gas bubbles seeping to the surface along well casing for more than 50 wells). The probability of borehole integrity failure depends on the quality of completion (which will vary over time), the age of the well (degradation) and the exploitation processes but the high variability in recorded barrier or integrity in Davies et al. (2014)<ref name="Davies 2014"></ref> also reflects differences in classification of failure (e.g. well, single barrier, significant or bubbles), geological setting and importantly regulation (e.g. Thorogood and Younger, 2015<ref name="Thorogood 2015">THOROGOOD, J L, and YOUNGER, P L. 2015. Discussion of ‘Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation’ by R J Davies, S, Almond, R S, Ward, R B, Jackson, C, Adams, F, Worrall, L G, Herringshaw, J G, Gluyas, and M A, Whitehead. (Marine and Petroleum Geology 2014). ''Marine and Petroleum Geology'', Vol. 59, 671–673. </ref>; Davies et al. 2015<ref name="Davies 2015">DAVIES, R J, ALMOND, S, WARD, R S, JACKSON, R B, ADAMS, C, WORRALL, F, HERRINGSHAW, L G, GLUYAS, J G, and WHITEHEAD, M A. 2015. Reply: ‘Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation’. ''Marine and Petroleum Geology'', 59, 674–675.</ref>). Of 143 active wells producing in the UK at the end of 2000, one has evidence of borehole integrity failure (Davies et al., 2014<ref name="Davies 2014"></ref>).		Boreholes drilled into the subsurface create potential pathways for contaminants to a receptor. While deep hydrocarbon and mineral boreholes are generally completed to prevent leakage, with both steel casing and cement bonding, borehole integrity failures (defects in steel casing, holes in casing joints, mechanical seals and cement e.g. Jackson et al., 2014<ref name="Jackson 2014">JACKSON, R B, VENGOSH, A, CAREY, J W, DAVIES, R, DARRAH, T H, O’SULLIVAN F, PÉTRON G. 2014. The 555 Environmental Costs and Benefits of Fracking, ''Anny. Rev. Environ. Resoure''. 39:327–362, doi 556 10.1146/annrev-environ-031113-144051 </ref>) can occur. 3% of all hydraulic fracturing operations in the USA involved a downhole mechanical integrity failure (US EPA, 2016<ref name="US EPA 2016"></ref>). Davies et al. (2014)<ref name="Davies 2014">DAVIES, R J, ALMOND, S, WARD, R S, JACKSON, R B, ADAMS, C, WORRALL, F, HERRINGSHAW, L G, GLUYAS, J G, and WHITEHEAD, M A. 2014. Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation. ''Marine and Petroleum Geology'', 30(1), 1–16. </ref> present data from around the world for the percentage of boreholes (including production, injection, idle and abandoned boreholes) that have some form of borehole barrier or integrity failure. Percentages range from 1.9% (onshore, nationwide CCS/natural gas storage facilities, dates unknown, including well integrity failure only, described as significant gas loss, for 470 boreholes) to 75% (onshore, operational wells in the Santa Fe Springs Oilfield, discovered 1921, including well integrity failures, leakage based on the observation of gas bubbles seeping to the surface along well casing for more than 50 wells). The probability of borehole integrity failure depends on the quality of completion (which will vary over time), the age of the well (degradation) and the exploitation processes but the high variability in recorded barrier or integrity in Davies et al. (2014)<ref name="Davies 2014"></ref> also reflects differences in classification of failure (e.g. well, single barrier, significant or bubbles), geological setting and importantly regulation (e.g. Thorogood and Younger, 2015<ref name="Thorogood 2015">THOROGOOD, J L, and YOUNGER, P L. 2015. Discussion of ‘Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation’ by R J Davies, S, Almond, R S, Ward, R B, Jackson, C, Adams, F, Worrall, L G, Herringshaw, J G, Gluyas, and M A, Whitehead. (Marine and Petroleum Geology 2014). ''Marine and Petroleum Geology'', Vol. 59, 671–673. </ref>; Davies et al. 2015<ref name="Davies 2015">DAVIES, R J, ALMOND, S, WARD, R S, JACKSON, R B, ADAMS, C, WORRALL, F, HERRINGSHAW, L G, GLUYAS, J G, and WHITEHEAD, M A. 2015. Reply: ‘Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation’. ''Marine and Petroleum Geology'', 59, 674–675.</ref>). Of 143 active wells producing in the UK at the end of 2000, one has evidence of borehole integrity failure (Davies et al., 2014<ref name="Davies 2014"></ref>).

	In many areas of hydrocarbon interest, there may be existing boreholes which can provide pathways for contamination if they are not properly sealed (for example the casing or cement) or have had a loss of integrity over time (Jackson et al., 2013a<ref name="Jackson 2013a">JACKSON, R E, GORODY, A W, MAYER, B, ROY, J W, RYAN, M C, and VAN STEMPVOORT, D R. 2013a, Groundwater protection and unconventional gas extraction: the critical need for field-based hydrogeological research, ''Groundwater'', Vol. 51(4), 488–510. </ref>; Ward et al., 2015<ref name="Ward 2015">WARD, R, STUART, M E, and BLOOMFIELD, J P. 2015. The hydrogological aspects of shale gas extraction in the UK. In: ''Issues in Environmental Science and Technology: Fracking''. (London: Royal Society of Chemistry). pp. 122–150. </ref>). Borehole leakage rates range from 2% to 50% in the UK (Davies et al., 2014<ref name="Davies 2014"></ref>). If abandoned, boreholes might not be monitored and the integrity of their casing will be unknown. In the UK there were 2152 hydrocarbon wells drilled onshore between 1902 and 2013. The ownership of up to 53% of these wells is unclear today and between 50 and 100 are orphaned (Davies et al., 2014<ref name="Davies 2014"></ref>).		In many areas of hydrocarbon interest, there may be existing boreholes which can provide pathways for contamination if they are not properly sealed (for example the casing or cement) or have had a loss of integrity over time (Jackson et al., 2013a<ref name="Jackson 2013a"></ref>; Ward et al., 2015<ref name="Ward 2015">WARD, R, STUART, M E, and BLOOMFIELD, J P. 2015. The hydrogological aspects of shale gas extraction in the UK. In: ''Issues in Environmental Science and Technology: Fracking''. (London: Royal Society of Chemistry). pp. 122–150. </ref>). Borehole leakage rates range from 2% to 50% in the UK (Davies et al., 2014<ref name="Davies 2014"></ref>). If abandoned, boreholes might not be monitored and the integrity of their casing will be unknown. In the UK there were 2152 hydrocarbon wells drilled onshore between 1902 and 2013. The ownership of up to 53% of these wells is unclear today and between 50 and 100 are orphaned (Davies et al., 2014<ref name="Davies 2014"></ref>).

	Hydraulic fracturing has been shown to impact on adjacent wells (US EPA, 2016<ref name="US EPA 2016"></ref>). In Alberta and British Columbia, 5349 horizontal wells were drilled between 2009 and 2012 and there were 39 reported cases of wellbore connection with existing oil and gas wells, 95% of which were producing in the same geologic unit. Alberta requires that locations of existing oil and gas wells be identified and their capability to sustain increased pressures be verified prior to hydraulic fracturing (Lefebvre, 2017<ref name="Lefebvre 2017">LEFEBVRE, R. 2017. Potential impacts of shale gas development on groundwater quality. ''Wiley Interdisciplinary Reviews: Water'', Vol. 4 (1). doi: 10.1002/wat2.1188. </ref>).		Hydraulic fracturing has been shown to impact on adjacent wells (US EPA, 2016<ref name="US EPA 2016"></ref>). In Alberta and British Columbia, 5349 horizontal wells were drilled between 2009 and 2012 and there were 39 reported cases of wellbore connection with existing oil and gas wells, 95% of which were producing in the same geologic unit. Alberta requires that locations of existing oil and gas wells be identified and their capability to sustain increased pressures be verified prior to hydraulic fracturing (Lefebvre, 2017<ref name="Lefebvre 2017">LEFEBVRE, R. 2017. Potential impacts of shale gas development on groundwater quality. ''Wiley Interdisciplinary Reviews: Water'', Vol. 4 (1). doi: 10.1002/wat2.1188. </ref>).
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	* High = borehole records are kept across England and although it is known that not all borehole records are sent to BGS (e.g. closed loop ground source heat pump holes), this is unlikely to be the case for deeper boreholes, therefore the confidence is high.		* High = borehole records are kept across England and although it is known that not all borehole records are sent to BGS (e.g. closed loop ground source heat pump holes), this is unlikely to be the case for deeper boreholes, therefore the confidence is high.
	* Medium or Low = unlikely due to the available records		* Medium or Low = unlikely due to the available records
	\|}		\|}

	==References==		==References==

Dbk: /* Mines */

2019-12-03T11:12:20Z

Mines

← Older revision		Revision as of 12:12, 3 December 2019
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	\|-		\|-
	\| style="background-color: #ebf1de;" \|		\| style="background-color: #ebf1de;" \|
	This factor accounts for the vertical and lateral proximity of the hydrocarbon activity to mines. The distances correspond to lateral separation distances based on the horizontal extents of high volume hydraulic fractures. Mine shafts can be deeper than the worked coal seams (Monaghan, 2014<ref name="Monaghan 2014">MONAGHAN, A A. 2014. ''The Carboniferous shales of the Midland Valley of Scotland: geology and resource estimation''. (London: British Geological Survey for Department of Energy and Climate Change). 96 pp.</ref>). There are three possible ratings (Table 4.8).		This factor accounts for the vertical and lateral proximity of the hydrocarbon activity to mines. The distances correspond to lateral separation distances based on the horizontal extents of high volume hydraulic fractures. Mine shafts can be deeper than the worked coal seams (Monaghan, 2014<ref name="Monaghan 2014"></ref>). There are three possible ratings (Table 4.8).
	<center>		<center>
	{\| class="wikitable"		{\| class="wikitable"