OR/18/012 Specific vulnerability - Revision history

http://earthwise-staging.bgs.ac.uk/index.php?action=history&feed=atom&title=OR%2F18%2F012_Specific_vulnerability OR/18/012 Specific vulnerability - Revision history 2026-04-15T16:23:54Z Revision history for this page on the wiki MediaWiki 1.42.3 http://earthwise-staging.bgs.ac.uk/index.php?title=OR/18/012_Specific_vulnerability&diff=44241&oldid=prev Dbk at 11:25, 3 December 2019 2019-12-03T11:25:40Z

<p></p> <table style="background-color: #fff; color: #202122;" data-mw="interface"> <col class="diff-marker" /> <col class="diff-content" /> <col class="diff-marker" /> <col class="diff-content" /> <tr class="diff-title" lang="en"> <td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">← Older revision</td> <td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 12:25, 3 December 2019</td> </tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l40">Line 40:</td> <td colspan="2" class="diff-lineno">Line 40:</td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Natural gas can be bound within coal seams by adsorption in which gas molecules adhere to the surfaces within the coal. This gas can be extracted in situ, i.e. directly from coal seams (Figure 5.5).</div></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Natural gas can be bound within coal seams by adsorption in which gas molecules adhere to the surfaces within the coal. This gas can be extracted in situ, i.e. directly from coal seams (Figure 5.5).</div></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td></tr> <tr><td class="diff-marker" data-marker="−"></td><td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>For the extraction of CBM a borehole is drilled into the coal seam and water is pumped out in order to lower the pressure in the seam (Jones et al., 2004<ref name="Jones 2004"></ref>). In some cases, particularly where there has previously been mining, coal-bearing strata may already be dewatered (Al-Jubori et al., 2009<ref name="Al-Jubori 2009">AL-JUBORI, A, JOHNSTON, S, BOYER, C, LAMBERT, S W, BUSTOS, O A, PASHIN, J C, and WRAY, A. 2009. Coalbed Methane: Clean Energy for the World. ''Oilfield Review'' Vol.&nbsp;21(2). </ref>). The lowering of pressure allows methane to desorb from the internal surfaces of the coal and diffuse into cleats (fractures within the coal) where it is able to flow, either as free gas or dissolved in water, towards the production well (DECC, 2013b<ref name="DECC 2013b">DECC, 2013b. ''The unconventional hydrocarbon resources of Britain’s onshore basins&nbsp;—&nbsp;coalbed methane''. (London: Department for Energy and Climate Change).</ref>). A good permeability is necessary to allow flow of gas to the production well during CBM production. Bituminous coals can have permeabilities of 1 mD, sometimes up to 30 mD although this is often anisotropic (Jones et al., 2004<ref name="Jones 2004"></ref>). Permeability can be imparted by cleats, and in some cases this may be as high as 100 mD, for example in the San Juan basin in the U.S., where natural production rates are similar to conventional reservoirs (Al-Jubori, 2009<ref name="Al-Jubori 2009"></ref>). While the permeability of coal seams in the UK is likely to be low (Jones et al., 2004<ref name="Jones 2004"></ref>) and decrease with depth (Moore, 2012<ref name="Moore 2012">MOORE, T A. 2012. Coalbed methane: a review. ''International Journal of Coal Geology'', Vol.&nbsp;101, 36–81. </ref>), cleats are common (due to their age) and can increase coal seam permeability (EA, 2014<ref name="EA 2014">ENVIRONMENT AGENCY. 2014. An environmental risk assessment for coal bed, coal mine and abandoned mine methane operations in England. ''Environment Agency, Bristol''. SC130029/R. </ref>). In areas of pre-existing mines, the permeability of coal seams and surrounding strata is increased due to rock collapses associated with longwall mining; this can be up to 160–200 m above and 40–70 m below the worked seam (Jones et al., 2004<ref name="Jones 2004"></ref>).</div></td><td class="diff-marker" data-marker="+"></td><td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>For the extraction of CBM a borehole is drilled into the coal seam and water is pumped out in order to lower the pressure in the seam (Jones et al., 2004<ref name="Jones 2004"><ins style="font-weight: bold; text-decoration: none;">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.</ins></ref>). In some cases, particularly where there has previously been mining, coal-bearing strata may already be dewatered (Al-Jubori et al., 2009<ref name="Al-Jubori 2009">AL-JUBORI, A, JOHNSTON, S, BOYER, C, LAMBERT, S W, BUSTOS, O A, PASHIN, J C, and WRAY, A. 2009. Coalbed Methane: Clean Energy for the World. ''Oilfield Review'' Vol.&nbsp;21(2). </ref>). The lowering of pressure allows methane to desorb from the internal surfaces of the coal and diffuse into cleats (fractures within the coal) where it is able to flow, either as free gas or dissolved in water, towards the production well (DECC, 2013b<ref name="DECC 2013b">DECC, 2013b. ''The unconventional hydrocarbon resources of Britain’s onshore basins&nbsp;—&nbsp;coalbed methane''. (London: Department for Energy and Climate Change).</ref>). A good permeability is necessary to allow flow of gas to the production well during CBM production. Bituminous coals can have permeabilities of 1 mD, sometimes up to 30 mD although this is often anisotropic (Jones et al., 2004<ref name="Jones 2004"></ref>). Permeability can be imparted by cleats, and in some cases this may be as high as 100 mD, for example in the San Juan basin in the U.S., where natural production rates are similar to conventional reservoirs (Al-Jubori, 2009<ref name="Al-Jubori 2009"></ref>). While the permeability of coal seams in the UK is likely to be low (Jones et al., 2004<ref name="Jones 2004"></ref>) and decrease with depth (Moore, 2012<ref name="Moore 2012">MOORE, T A. 2012. Coalbed methane: a review. ''International Journal of Coal Geology'', Vol.&nbsp;101, 36–81. </ref>), cleats are common (due to their age) and can increase coal seam permeability (EA, 2014<ref name="EA 2014">ENVIRONMENT AGENCY. 2014. An environmental risk assessment for coal bed, coal mine and abandoned mine methane operations in England. ''Environment Agency, Bristol''. SC130029/R. </ref>). In areas of pre-existing mines, the permeability of coal seams and surrounding strata is increased due to rock collapses associated with longwall mining; this can be up to 160–200 m above and 40–70 m below the worked seam (Jones et al., 2004<ref name="Jones 2004"></ref>).</div></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Coal mine methane (CMM) and abandoned mine methane (AMM) can be considered as subdivisions of CBM. CMM involves the removal of methane from a working mine to enable safe mining, by capturing it at high concentrations. In the UK, ‘post drainage’ is favoured in which methane is captured from strata above and below worked seams via suction pumps (EA, 2014<ref name="EA 2014"></ref>). For deep, gassy longwall mines, boreholes are drilled at an angle above and sometimes below worked seams (EA, 2014<ref name="EA 2014"></ref>). Boreholes may also enter the seams from inside the mines (Karacan et al., 2011<ref name="Karacan 2011"></div></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Coal mine methane (CMM) and abandoned mine methane (AMM) can be considered as subdivisions of CBM. CMM involves the removal of methane from a working mine to enable safe mining, by capturing it at high concentrations. In the UK, ‘post drainage’ is favoured in which methane is captured from strata above and below worked seams via suction pumps (EA, 2014<ref name="EA 2014"></ref>). For deep, gassy longwall mines, boreholes are drilled at an angle above and sometimes below worked seams (EA, 2014<ref name="EA 2014"></ref>). Boreholes may also enter the seams from inside the mines (Karacan et al., 2011<ref name="Karacan 2011"></div></td></tr> </table>

Dbk http://earthwise-staging.bgs.ac.uk/index.php?title=OR/18/012_Specific_vulnerability&diff=44235&oldid=prev Dbk at 11:21, 3 December 2019 2019-12-03T11:21:07Z

<p></p> <table style="background-color: #fff; color: #202122;" data-mw="interface"> <col class="diff-marker" /> <col class="diff-content" /> <col class="diff-marker" /> <col class="diff-content" /> <tr class="diff-title" lang="en"> <td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">← Older revision</td> <td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 12:21, 3 December 2019</td> </tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l40">Line 40:</td> <td colspan="2" class="diff-lineno">Line 40:</td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Natural gas can be bound within coal seams by adsorption in which gas molecules adhere to the surfaces within the coal. This gas can be extracted in situ, i.e. directly from coal seams (Figure 5.5).</div></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Natural gas can be bound within coal seams by adsorption in which gas molecules adhere to the surfaces within the coal. This gas can be extracted in situ, i.e. directly from coal seams (Figure 5.5).</div></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td></tr> <tr><td class="diff-marker" data-marker="−"></td><td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>For the extraction of CBM a borehole is drilled into the coal seam and water is pumped out in order to lower the pressure in the seam (Jones et al., 2004<ref name="Jones 2004"><del style="font-weight: bold; text-decoration: none;">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. </del></ref>). In some cases, particularly where there has previously been mining, coal-bearing strata may already be dewatered (Al-Jubori et al., 2009<ref name="Al-Jubori 2009">AL-JUBORI, A, JOHNSTON, S, BOYER, C, LAMBERT, S W, BUSTOS, O A, PASHIN, J C, and WRAY, A. 2009. Coalbed Methane: Clean Energy for the World. ''Oilfield Review'' Vol.&nbsp;21(2). </ref>). The lowering of pressure allows methane to desorb from the internal surfaces of the coal and diffuse into cleats (fractures within the coal) where it is able to flow, either as free gas or dissolved in water, towards the production well (DECC, 2013b<ref name="DECC 2013b">DECC, 2013b. ''The unconventional hydrocarbon resources of Britain’s onshore basins&nbsp;—&nbsp;coalbed methane''. (London: Department for Energy and Climate Change).</ref>). A good permeability is necessary to allow flow of gas to the production well during CBM production. Bituminous coals can have permeabilities of 1 mD, sometimes up to 30 mD although this is often anisotropic (Jones et al., 2004<ref name="Jones 2004"><del style="font-weight: bold; text-decoration: none;">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. </del></ref>). Permeability can be imparted by cleats, and in some cases this may be as high as 100 mD, for example in the San Juan basin in the U.S., where natural production rates are similar to conventional reservoirs (Al-Jubori, 2009<ref name="Al-Jubori 2009"></ref>). While the permeability of coal seams in the UK is likely to be low (Jones et al., 2004<ref name="Jones 2004"></ref>) and decrease with depth (Moore, 2012<ref name="Moore 2012">MOORE, T A. 2012. Coalbed methane: a review. ''International Journal of Coal Geology'', Vol.&nbsp;101, 36–81. </ref>), cleats are common (due to their age) and can increase coal seam permeability (EA, 2014<ref name="EA 2014">ENVIRONMENT AGENCY. 2014. An environmental risk assessment for coal bed, coal mine and abandoned mine methane operations in England. ''Environment Agency, Bristol''. SC130029/R. </ref>). In areas of pre-existing mines, the permeability of coal seams and surrounding strata is increased due to rock collapses associated with longwall mining; this can be up to 160–200 m above and 40–70 m below the worked seam (Jones et al., 2004<ref name="Jones 2004"></ref>).</div></td><td class="diff-marker" data-marker="+"></td><td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>For the extraction of CBM a borehole is drilled into the coal seam and water is pumped out in order to lower the pressure in the seam (Jones et al., 2004<ref name="Jones 2004"></ref>). In some cases, particularly where there has previously been mining, coal-bearing strata may already be dewatered (Al-Jubori et al., 2009<ref name="Al-Jubori 2009">AL-JUBORI, A, JOHNSTON, S, BOYER, C, LAMBERT, S W, BUSTOS, O A, PASHIN, J C, and WRAY, A. 2009. Coalbed Methane: Clean Energy for the World. ''Oilfield Review'' Vol.&nbsp;21(2). </ref>). The lowering of pressure allows methane to desorb from the internal surfaces of the coal and diffuse into cleats (fractures within the coal) where it is able to flow, either as free gas or dissolved in water, towards the production well (DECC, 2013b<ref name="DECC 2013b">DECC, 2013b. ''The unconventional hydrocarbon resources of Britain’s onshore basins&nbsp;—&nbsp;coalbed methane''. (London: Department for Energy and Climate Change).</ref>). A good permeability is necessary to allow flow of gas to the production well during CBM production. Bituminous coals can have permeabilities of 1 mD, sometimes up to 30 mD although this is often anisotropic (Jones et al., 2004<ref name="Jones 2004"></ref>). Permeability can be imparted by cleats, and in some cases this may be as high as 100 mD, for example in the San Juan basin in the U.S., where natural production rates are similar to conventional reservoirs (Al-Jubori, 2009<ref name="Al-Jubori 2009"></ref>). While the permeability of coal seams in the UK is likely to be low (Jones et al., 2004<ref name="Jones 2004"></ref>) and decrease with depth (Moore, 2012<ref name="Moore 2012">MOORE, T A. 2012. Coalbed methane: a review. ''International Journal of Coal Geology'', Vol.&nbsp;101, 36–81. </ref>), cleats are common (due to their age) and can increase coal seam permeability (EA, 2014<ref name="EA 2014">ENVIRONMENT AGENCY. 2014. An environmental risk assessment for coal bed, coal mine and abandoned mine methane operations in England. ''Environment Agency, Bristol''. SC130029/R. </ref>). In areas of pre-existing mines, the permeability of coal seams and surrounding strata is increased due to rock collapses associated with longwall mining; this can be up to 160–200 m above and 40–70 m below the worked seam (Jones et al., 2004<ref name="Jones 2004"></ref>).</div></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Coal mine methane (CMM) and abandoned mine methane (AMM) can be considered as subdivisions of CBM. CMM involves the removal of methane from a working mine to enable safe mining, by capturing it at high concentrations. In the UK, ‘post drainage’ is favoured in which methane is captured from strata above and below worked seams via suction pumps (EA, 2014<ref name="EA 2014"></ref>). For deep, gassy longwall mines, boreholes are drilled at an angle above and sometimes below worked seams (EA, 2014<ref name="EA 2014"></ref>). Boreholes may also enter the seams from inside the mines (Karacan et al., 2011<ref name="Karacan 2011"></div></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Coal mine methane (CMM) and abandoned mine methane (AMM) can be considered as subdivisions of CBM. CMM involves the removal of methane from a working mine to enable safe mining, by capturing it at high concentrations. In the UK, ‘post drainage’ is favoured in which methane is captured from strata above and below worked seams via suction pumps (EA, 2014<ref name="EA 2014"></ref>). For deep, gassy longwall mines, boreholes are drilled at an angle above and sometimes below worked seams (EA, 2014<ref name="EA 2014"></ref>). Boreholes may also enter the seams from inside the mines (Karacan et al., 2011<ref name="Karacan 2011"></div></td></tr> </table>

Dbk http://earthwise-staging.bgs.ac.uk/index.php?title=OR/18/012_Specific_vulnerability&diff=44232&oldid=prev Dbk at 11:19, 3 December 2019 2019-12-03T11:19:41Z

<p></p> <table style="background-color: #fff; color: #202122;" data-mw="interface"> <col class="diff-marker" /> <col class="diff-content" /> <col class="diff-marker" /> <col class="diff-content" /> <tr class="diff-title" lang="en"> <td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">← Older revision</td> <td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 12:19, 3 December 2019</td> </tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l117">Line 117:</td> <td colspan="2" class="diff-lineno">Line 117:</td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>In the majority of cases, the receptor will overlie the source and an upwards driving force will be required for contamination. Flewelling and Sharma (2014)<ref name="Flewelling 2014"></ref> and Birdsell et al. (2015)<ref name="Birdsell 2015">BIRDSELL, D T, RAJARAM, H, DEMPSEY, D, and VISWANATHAN, H S. 2015. Hydraulic fracturing fluid migration in the subsurface: A review and expanded modelling results, ''Water Resources Research'' 5, Vol.&nbsp;1, 7159–7188. </ref> suggest that, generally, vertical hydraulic gradients are small and densities of deep fluids are high, preventing upwards migration. Contamination from methane and other light gases is more likely than from heavier ones due to their buoyancy. In England, groundwater flow paths tend to be controlled by topographic flow; from recharge areas in uplands (with high hydraulic head) to discharge areas in lowlands (with low hydraulic heads) (Downing et al., 1987<ref name="Downing 1987">DOWNING, R A, EDMUNDS, W M, and GALE, I N. 1987. Regional groundwater flow in sedimentary basins in the UK. In: Goff, J C, and Williams, B P J (Eds). Fluid Flow in Sedimentary Basins and Aquifers. ''Geological Society Special Publications''. Vol.&nbsp;34, 105–25. </ref>). On a regional scale, this means that there is likely to be a downwards gradient at the margins or sides of a basin, and below OD (Ordnance Datum) there is likely to be an upwards head gradient in the centre of a basin. Other factors to consider include fluid buoyancy, palaeoflow systems and compacting sediments as discussed in Bethke (1989)<ref name="Bethke 1989">BETHKE, C M. 1989. Modeling subsurface flow in sedimentary basins. ''Geologische Rundshau'', Vol.&nbsp;78(1), 129–154. </ref>. There is little evidence of natural overpressurisation reported in England (e.g. DECC, 2013a<ref name="DECC 2013a"></ref>). However, over-pressurised gas was encountered in the Hatfield Moors Gas Field in 1981 (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.&nbsp;59, 671–673.</ref>). Fluids are also known to flow from depth to the surface in some places, such as the hot springs at Bath and Buxton. High hydraulic heads seen at about 1,100 m bgl in the Sellafield area have recently been explained by relict heads from a wet-based ice sheet over the area (Black and Barker, 2016<ref name="Black 2016">BLACK, J H, and BARKER, J A. 2016. The puzzle of high heads beneath the West Cumbrian coast, UK: a possible solution. ''Hydrogeology Journal'', Vol.&nbsp;24, 439–457. </ref>). Often the rate of upwards groundwater movement may be very low, taking in the order of thousands of years in deep basins to reach the surface, making it difficult to identify such flows (e.g. 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>; Vengosh et al., 2014<ref name="Vengosh 2014">VENGOSH, A, JACKSON, R B, WARNER, N, DARRAH, T H, and KONDASH, A A. 2014. Critical Review of the Risks to Water Resources from Unconventional Shale Gas Development and Hydraulic Fracturing in the United States, ''Environmental Science and Technology'' 48, 8334–8348. </ref>).</div></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>In the majority of cases, the receptor will overlie the source and an upwards driving force will be required for contamination. Flewelling and Sharma (2014)<ref name="Flewelling 2014"></ref> and Birdsell et al. (2015)<ref name="Birdsell 2015">BIRDSELL, D T, RAJARAM, H, DEMPSEY, D, and VISWANATHAN, H S. 2015. Hydraulic fracturing fluid migration in the subsurface: A review and expanded modelling results, ''Water Resources Research'' 5, Vol.&nbsp;1, 7159–7188. </ref> suggest that, generally, vertical hydraulic gradients are small and densities of deep fluids are high, preventing upwards migration. Contamination from methane and other light gases is more likely than from heavier ones due to their buoyancy. In England, groundwater flow paths tend to be controlled by topographic flow; from recharge areas in uplands (with high hydraulic head) to discharge areas in lowlands (with low hydraulic heads) (Downing et al., 1987<ref name="Downing 1987">DOWNING, R A, EDMUNDS, W M, and GALE, I N. 1987. Regional groundwater flow in sedimentary basins in the UK. In: Goff, J C, and Williams, B P J (Eds). Fluid Flow in Sedimentary Basins and Aquifers. ''Geological Society Special Publications''. Vol.&nbsp;34, 105–25. </ref>). On a regional scale, this means that there is likely to be a downwards gradient at the margins or sides of a basin, and below OD (Ordnance Datum) there is likely to be an upwards head gradient in the centre of a basin. Other factors to consider include fluid buoyancy, palaeoflow systems and compacting sediments as discussed in Bethke (1989)<ref name="Bethke 1989">BETHKE, C M. 1989. Modeling subsurface flow in sedimentary basins. ''Geologische Rundshau'', Vol.&nbsp;78(1), 129–154. </ref>. There is little evidence of natural overpressurisation reported in England (e.g. DECC, 2013a<ref name="DECC 2013a"></ref>). However, over-pressurised gas was encountered in the Hatfield Moors Gas Field in 1981 (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.&nbsp;59, 671–673.</ref>). Fluids are also known to flow from depth to the surface in some places, such as the hot springs at Bath and Buxton. High hydraulic heads seen at about 1,100 m bgl in the Sellafield area have recently been explained by relict heads from a wet-based ice sheet over the area (Black and Barker, 2016<ref name="Black 2016">BLACK, J H, and BARKER, J A. 2016. The puzzle of high heads beneath the West Cumbrian coast, UK: a possible solution. ''Hydrogeology Journal'', Vol.&nbsp;24, 439–457. </ref>). Often the rate of upwards groundwater movement may be very low, taking in the order of thousands of years in deep basins to reach the surface, making it difficult to identify such flows (e.g. 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>; Vengosh et al., 2014<ref name="Vengosh 2014">VENGOSH, A, JACKSON, R B, WARNER, N, DARRAH, T H, and KONDASH, A A. 2014. Critical Review of the Risks to Water Resources from Unconventional Shale Gas Development and Hydraulic Fracturing in the United States, ''Environmental Science and Technology'' 48, 8334–8348. </ref>).</div></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td></tr> <tr><td class="diff-marker" data-marker="−"></td><td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>Some of the hydrocarbon activities listed above change subsurface pressures and provide an external driving force. During hydraulic fracturing, reservoir pressures are typically increased to about 15 MPa above virgin reservoir pressure, increasing hydraulic head by 1500 m (Brownlow et al., 2016<ref name="Brownlow 2016"><del style="font-weight: bold; text-decoration: none;">BROWNLOW, J W, JAMES, S, and YELDERMAN, JR, J C. 2016. Influence of Hydraulic Fracturing on Overlying Aquifers in the Presence of Leaky Abandoned Wells. ''Groundwater'', Vol.&nbsp;54(6), 781–792.</del></ref>). This pressure increase can drive fluids away from the stimulated zone into the surrounding rock and possibly through preferential flow pathways. However, during production, flow-back occurs and hydraulic heads are relaxed slightly (Brownlow et al., 2016<ref name="Brownlow 2016"></ref>). The time over which high heads are sustained is not well known. Lefebvre et al. (2017)<ref name="Lefebvre 2017"></ref> and others suggest that it is unlikely that overpressures will be maintained after the production of a shale gas reservoir has finished. Brownlow et al. (2016)<ref name="Brownlow 2016"></ref> suggest that the increased heads of 1500 m during hydraulic fracturing decreased to nominal head values after 1 year, and decreased by a further 200 m after 15 years, inducing flow towards the well, consistent with other simulations and observations in the Eagle Ford shale, Texas. It should be noted that head propagation occurs over shorter timescales and greater distances than fluid migration (Brownlow et al., 2016<ref name="Brownlow 2016"></ref>).</div></td><td class="diff-marker" data-marker="+"></td><td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Some of the hydrocarbon activities listed above change subsurface pressures and provide an external driving force. During hydraulic fracturing, reservoir pressures are typically increased to about 15 MPa above virgin reservoir pressure, increasing hydraulic head by 1500 m (Brownlow et al., 2016<ref name="Brownlow 2016"></ref>). This pressure increase can drive fluids away from the stimulated zone into the surrounding rock and possibly through preferential flow pathways. However, during production, flow-back occurs and hydraulic heads are relaxed slightly (Brownlow et al., 2016<ref name="Brownlow 2016"></ref>). The time over which high heads are sustained is not well known. Lefebvre et al. (2017)<ref name="Lefebvre 2017"></ref> and others suggest that it is unlikely that overpressures will be maintained after the production of a shale gas reservoir has finished. Brownlow et al. (2016)<ref name="Brownlow 2016"></ref> suggest that the increased heads of 1500 m during hydraulic fracturing decreased to nominal head values after 1 year, and decreased by a further 200 m after 15 years, inducing flow towards the well, consistent with other simulations and observations in the Eagle Ford shale, Texas. It should be noted that head propagation occurs over shorter timescales and greater distances than fluid migration (Brownlow et al., 2016<ref name="Brownlow 2016"></ref>).</div></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>The dewatering process associated with CBM lowers the water table and can create a zone of depressurisation around the borehole. This can mobilise gas and other contaminants from the source rock; however, generally the pathways will be towards, rather than away from, the borehole.</div></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>The dewatering process associated with CBM lowers the water table and can create a zone of depressurisation around the borehole. This can mobilise gas and other contaminants from the source rock; however, generally the pathways will be towards, rather than away from, the borehole.</div></td></tr> </table>

Dbk http://earthwise-staging.bgs.ac.uk/index.php?title=OR/18/012_Specific_vulnerability&diff=44231&oldid=prev Dbk: /* Driving forces */ 2019-12-03T11:18:43Z

<p><span dir="auto"><span class="autocomment">Driving forces</span></span></p> <table style="background-color: #fff; color: #202122;" data-mw="interface"> <col class="diff-marker" /> <col class="diff-content" /> <col class="diff-marker" /> <col class="diff-content" /> <tr class="diff-title" lang="en"> <td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">← Older revision</td> <td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 12:18, 3 December 2019</td> </tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l115">Line 115:</td> <td colspan="2" class="diff-lineno">Line 115:</td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Whilst the presence of potential pathways and the characteristics of rocks between the source and receptor may contribute to the vulnerability of a receptor, a mechanism of transport is required for contamination to actually occur, i.e. to present a risk. Contaminant transport mechanisms include diffusion and advection. Diffusion is likely to be slow and not very significant with the distances and concentrations of chemicals involved in these processes compared to advection, which can transmit a greater volume of contaminants. Advection requires a driving force to make groundwater flow (e.g. Flewelling and Sharma, 2014<ref name="Flewelling 2014">FLEWELLING, S A, and SHARMA, M. 2014. Constraints on upward migration of hydraulic fracturing fluid and brine. ''Groundwater'', 52(1), 9–19.</ref>).</div></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Whilst the presence of potential pathways and the characteristics of rocks between the source and receptor may contribute to the vulnerability of a receptor, a mechanism of transport is required for contamination to actually occur, i.e. to present a risk. Contaminant transport mechanisms include diffusion and advection. Diffusion is likely to be slow and not very significant with the distances and concentrations of chemicals involved in these processes compared to advection, which can transmit a greater volume of contaminants. Advection requires a driving force to make groundwater flow (e.g. Flewelling and Sharma, 2014<ref name="Flewelling 2014">FLEWELLING, S A, and SHARMA, M. 2014. Constraints on upward migration of hydraulic fracturing fluid and brine. ''Groundwater'', 52(1), 9–19.</ref>).</div></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td></tr> <tr><td class="diff-marker" data-marker="−"></td><td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>In the majority of cases, the receptor will overlie the source and an upwards driving force will be required for contamination. Flewelling and Sharma (2014)<ref name="Flewelling 2014"></ref> and Birdsell et al. (2015)<ref name="Birdsell 2015">BIRDSELL, D T, RAJARAM, H, DEMPSEY, D, and VISWANATHAN, H S. 2015. Hydraulic fracturing fluid migration in the subsurface: A review and expanded modelling results, ''Water Resources Research'' 5, Vol.&nbsp;1, 7159–7188. </ref> suggest that, generally, vertical hydraulic gradients are small and densities of deep fluids are high, preventing upwards migration. Contamination from methane and other light gases is more likely than from heavier ones due to their buoyancy. In England, groundwater flow paths tend to be controlled by topographic flow; from recharge areas in uplands (with high hydraulic head) to discharge areas in lowlands (with low hydraulic heads) (Downing et al., 1987<ref name="Downing 1987">DOWNING, R A, EDMUNDS, W M, and GALE, I N. 1987. Regional groundwater flow in sedimentary basins in the UK. In: Goff, J C, and Williams, B P J (Eds). Fluid Flow in Sedimentary Basins and Aquifers. ''Geological Society Special Publications''. Vol.&nbsp;34, 105–25. </ref>). On a regional scale, this means that there is likely to be a downwards gradient at the margins or sides of a basin, and below OD (Ordnance Datum) there is likely to be an upwards head gradient in the centre of a basin. Other factors to consider include fluid buoyancy, palaeoflow systems and compacting sediments as discussed in Bethke (1989)<ref name="Bethke 1989">BETHKE, C M. 1989. Modeling subsurface flow in sedimentary basins. ''Geologische Rundshau'', Vol.&nbsp;78(1), 129–154. </ref>. There is little evidence of natural overpressurisation reported in England (e.g. DECC, 2013a<ref name="DECC 2013a"><del style="font-weight: bold; text-decoration: none;">DECC, 2013a. ''The hydrocarbon prospectivity of Britain’s onshore basins''. (London: Department for Energy and Climate Change) </del></ref>). However, over-pressurised gas was encountered in the Hatfield Moors Gas Field in 1981 (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.&nbsp;59, 671–673.</ref>). Fluids are also known to flow from depth to the surface in some places, such as the hot springs at Bath and Buxton. High hydraulic heads seen at about 1,100 m bgl in the Sellafield area have recently been explained by relict heads from a wet-based ice sheet over the area (Black and Barker, 2016<ref name="Black 2016">BLACK, J H, and BARKER, J A. 2016. The puzzle of high heads beneath the West Cumbrian coast, UK: a possible solution. ''Hydrogeology Journal'', Vol.&nbsp;24, 439–457. </ref>). Often the rate of upwards groundwater movement may be very low, taking in the order of thousands of years in deep basins to reach the surface, making it difficult to identify such flows (e.g. 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>; Vengosh et al., 2014<ref name="Vengosh 2014">VENGOSH, A, JACKSON, R B, WARNER, N, DARRAH, T H, and KONDASH, A A. 2014. Critical Review of the Risks to Water Resources from Unconventional Shale Gas Development and Hydraulic Fracturing in the United States, ''Environmental Science and Technology'' 48, 8334–8348. </ref>).</div></td><td class="diff-marker" data-marker="+"></td><td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>In the majority of cases, the receptor will overlie the source and an upwards driving force will be required for contamination. Flewelling and Sharma (2014)<ref name="Flewelling 2014"></ref> and Birdsell et al. (2015)<ref name="Birdsell 2015">BIRDSELL, D T, RAJARAM, H, DEMPSEY, D, and VISWANATHAN, H S. 2015. Hydraulic fracturing fluid migration in the subsurface: A review and expanded modelling results, ''Water Resources Research'' 5, Vol.&nbsp;1, 7159–7188. </ref> suggest that, generally, vertical hydraulic gradients are small and densities of deep fluids are high, preventing upwards migration. Contamination from methane and other light gases is more likely than from heavier ones due to their buoyancy. In England, groundwater flow paths tend to be controlled by topographic flow; from recharge areas in uplands (with high hydraulic head) to discharge areas in lowlands (with low hydraulic heads) (Downing et al., 1987<ref name="Downing 1987">DOWNING, R A, EDMUNDS, W M, and GALE, I N. 1987. Regional groundwater flow in sedimentary basins in the UK. In: Goff, J C, and Williams, B P J (Eds). Fluid Flow in Sedimentary Basins and Aquifers. ''Geological Society Special Publications''. Vol.&nbsp;34, 105–25. </ref>). On a regional scale, this means that there is likely to be a downwards gradient at the margins or sides of a basin, and below OD (Ordnance Datum) there is likely to be an upwards head gradient in the centre of a basin. Other factors to consider include fluid buoyancy, palaeoflow systems and compacting sediments as discussed in Bethke (1989)<ref name="Bethke 1989">BETHKE, C M. 1989. Modeling subsurface flow in sedimentary basins. ''Geologische Rundshau'', Vol.&nbsp;78(1), 129–154. </ref>. There is little evidence of natural overpressurisation reported in England (e.g. DECC, 2013a<ref name="DECC 2013a"></ref>). However, over-pressurised gas was encountered in the Hatfield Moors Gas Field in 1981 (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.&nbsp;59, 671–673.</ref>). Fluids are also known to flow from depth to the surface in some places, such as the hot springs at Bath and Buxton. High hydraulic heads seen at about 1,100 m bgl in the Sellafield area have recently been explained by relict heads from a wet-based ice sheet over the area (Black and Barker, 2016<ref name="Black 2016">BLACK, J H, and BARKER, J A. 2016. The puzzle of high heads beneath the West Cumbrian coast, UK: a possible solution. ''Hydrogeology Journal'', Vol.&nbsp;24, 439–457. </ref>). Often the rate of upwards groundwater movement may be very low, taking in the order of thousands of years in deep basins to reach the surface, making it difficult to identify such flows (e.g. 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>; Vengosh et al., 2014<ref name="Vengosh 2014">VENGOSH, A, JACKSON, R B, WARNER, N, DARRAH, T H, and KONDASH, A A. 2014. Critical Review of the Risks to Water Resources from Unconventional Shale Gas Development and Hydraulic Fracturing in the United States, ''Environmental Science and Technology'' 48, 8334–8348. </ref>).</div></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><br></td></tr> <tr><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Some of the hydrocarbon activities listed above change subsurface pressures and provide an external driving force. During hydraulic fracturing, reservoir pressures are typically increased to about 15 MPa above virgin reservoir pressure, increasing hydraulic head by 1500 m (Brownlow et al., 2016<ref name="Brownlow 2016">BROWNLOW, J W, JAMES, S, and YELDERMAN, JR, J C. 2016. Influence of Hydraulic Fracturing on Overlying Aquifers in the Presence of Leaky Abandoned Wells. ''Groundwater'', Vol.&nbsp;54(6), 781–792.</ref>). This pressure increase can drive fluids away from the stimulated zone into the surrounding rock and possibly through preferential flow pathways. However, during production, flow-back occurs and hydraulic heads are relaxed slightly (Brownlow et al., 2016<ref name="Brownlow 2016"></ref>). The time over which high heads are sustained is not well known. Lefebvre et al. (2017)<ref name="Lefebvre 2017"></ref> and others suggest that it is unlikely that overpressures will be maintained after the production of a shale gas reservoir has finished. Brownlow et al. (2016)<ref name="Brownlow 2016"></ref> suggest that the increased heads of 1500 m during hydraulic fracturing decreased to nominal head values after 1 year, and decreased by a further 200 m after 15 years, inducing flow towards the well, consistent with other simulations and observations in the Eagle Ford shale, Texas. It should be noted that head propagation occurs over shorter timescales and greater distances than fluid migration (Brownlow et al., 2016<ref name="Brownlow 2016"></ref>).</div></td><td class="diff-marker"></td><td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Some of the hydrocarbon activities listed above change subsurface pressures and provide an external driving force. During hydraulic fracturing, reservoir pressures are typically increased to about 15 MPa above virgin reservoir pressure, increasing hydraulic head by 1500 m (Brownlow et al., 2016<ref name="Brownlow 2016">BROWNLOW, J W, JAMES, S, and YELDERMAN, JR, J C. 2016. Influence of Hydraulic Fracturing on Overlying Aquifers in the Presence of Leaky Abandoned Wells. ''Groundwater'', Vol.&nbsp;54(6), 781–792.</ref>). This pressure increase can drive fluids away from the stimulated zone into the surrounding rock and possibly through preferential flow pathways. However, during production, flow-back occurs and hydraulic heads are relaxed slightly (Brownlow et al., 2016<ref name="Brownlow 2016"></ref>). The time over which high heads are sustained is not well known. Lefebvre et al. (2017)<ref name="Lefebvre 2017"></ref> and others suggest that it is unlikely that overpressures will be maintained after the production of a shale gas reservoir has finished. Brownlow et al. (2016)<ref name="Brownlow 2016"></ref> suggest that the increased heads of 1500 m during hydraulic fracturing decreased to nominal head values after 1 year, and decreased by a further 200 m after 15 years, inducing flow towards the well, consistent with other simulations and observations in the Eagle Ford shale, Texas. It should be noted that head propagation occurs over shorter timescales and greater distances than fluid migration (Brownlow et al., 2016<ref name="Brownlow 2016"></ref>).</div></td></tr> </table>

Dbk http://earthwise-staging.bgs.ac.uk/index.php?title=OR/18/012_Specific_vulnerability&diff=38415&oldid=prev Dbk: 1 revision imported 2018-10-19T08:55:21Z

<p>1 revision imported</p> <table style="background-color: #fff; color: #202122;" data-mw="interface"> <tr class="diff-title" lang="en"> <td colspan="1" style="background-color: #fff; color: #202122; text-align: center;">← Older revision</td> <td colspan="1" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 09:55, 19 October 2018</td> </tr><tr><td colspan="2" class="diff-notice" lang="en"><div class="mw-diff-empty">(No difference)</div> </td></tr></table>

Dbk http://earthwise-staging.bgs.ac.uk/index.php?title=OR/18/012_Specific_vulnerability&diff=38414&oldid=prev Ajhil: /* Driving forces */ 2018-08-30T14:42:09Z

<p><span dir="auto"><span class="autocomment">Driving forces</span></span></p> <a href="http://earthwise-staging.bgs.ac.uk/index.php?title=OR/18/012_Specific_vulnerability&diff=38414">Show changes</a>

Ajhil