OR/17/048 Regions

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Fellgett, M W, Kingdon, A, Williams, J D O, and Gent, C M A. 2017. State of stress across UK regions. British Geological Survey Internal Report, OR/17/048.

The selection of UK regions in which stress field information has been collated or calculated was largely determined by the availability of data and their resource potential for unconventional shale resources, highlighted by Andrews et al. (2013). The three main areas of focus for this study are:

  • East Yorkshire and North Nottinghamshire
  • The Weald
  • Cheshire and Lancashire.

Wells, Quarries and mines in this report are identified by name or well title, additional information is provided in Appendix 1.

Eastern Yorkshire and North Nottinghamshire

The area of interest (AOI) was identified based on the availability of well data and current exploration interest for unconventional resources. This region covers the area between Doncaster, York and Scarborough. Initially 217 wells were identified though 97 mainly water wells were discarded on account of having no useful information. The remaining 120 wells were investigated for stress field information. Of these 120 wells, 54 contained information suitable for stress field characterisation and are shown in Figure 7. See Appendix 1 for details.

Figure 7    Map of East Yorkshire and North Nottinghamshire showing 120 wells in the area of interest which were investigated for stress field information (Blue) and the 54 wells with suitable information to aid stress field characterisation (Red). Contains Ordnance Survey data © Crown copyright and database rights 2017.

The area of East Yorkshire and North Nottinghamshire shown in Figure 7 consists of the Market Weighton Block and a large fault zone known as the Howardian-Flamborough Head Fault Zone (Hawkins and Saul, 2003[1]). Late Devonian rifting led to the subsidence of the Market Weighton Block and deposition of shelf carbonates which continued into the Lower Carboniferous (Dinantian) (Fraser and Gawthorpe, 1990[2]). A series of prograding deltas, originating in the north inundated the area during Westphalian times. This was followed by basin inversion and erosion during the Variscan Orogeny (Fraser and Gawthorpe, 1990[2]). The Market Weighton Block underwent a period of subsidence in Triassic–early Cretaceous times related to rifting in the North Sea (Kent, 1980[3]). In Late Cretaceous times a period of inversion resulted in the erosion of the early Cretaceous and Jurassic strata, before extension resumed in Late Cretaceous times. A key structure in the area is the E–W trending Howardian–Flamborough Head Fault Zone (Kirby and Swallow, 1987[4]), which separates the Market Weighton Block and the Cleveland Basin. This fault zones is the onshore component of the Dowsing Fault zone and was triggered by N–S extension in Late Jurassic to the early Cretaceous (Hawkins and Saul, 2003[1]).

Twenty nine of the 120 wells were drilled by the Coal Authority and were mostly used for constraining density profiles in the post-Carboniferous succession. The remaining 91 hydrocarbon wells include some small oil and gas fields such as Kirby Misperton, Malton and Hatfield Moors.

Vertical stress

To determine the vertical stress, 17 wells were selected (five Coal Authority wells, 12 hydrocarbon wells) from the initial list based on location and top and base depth of density logging. Group or Formation level stratigraphy was recorded for each well allowing average formation densities to be calculated for units including the Mercia Mudstone Group and/or the Sherwood Sandstone Group. Density logs from the hydrocarbons industry typically start below these units as they rest on the Carboniferous targets. Due to the issues highlighted in Vertical stress and the Well data quality control sections, including density logs in non-standard units and poor digitisation due to the quality of the available log plots, four of the wells were removed from the initial wells list. The final vertical stress graph is shown in Figure 8 contains information from the 13 wells used to constrain the vertical stress.

Figure 8    Compilation of vertical stress gradients from East Yorkshire and North Nottinghamshire. Dashed black line represents a gradient of 23 MPakm-1 which is a good fit to the deepest wells in the region.

The black dashed line in Figure 8 represents the stress gradient which has been fitted to the vertical stress data calculated from the density log inversion after Zoback (2003)[5]. The vertical stress gradient shown is between 23 MPakm-1 m and 24 MPakm-1. Due to the consistency of the trend in vertical stress from these wells no other wells from this region were used.

Pore pressure

Repeat Formation Test (RFT) data were available from eight wells. Wells which had drill stem test reports were also recorded but not used in this case due to incomplete reports. The pressure data were converted to MPa and plotted in Figure 9, the majority of the pressure data plot on or near the hydrostatic pressure line of 10 MPakm-1. This indicates that pore pressure is hydrostatic with several outliers. The pore pressure data from this region plot on a gradient of 10.17 MPakm-1.

Figure 9    Compilation of available pressure data for East Yorkshire and North Nottinghamshire. With the exception of three data points from Marishes all of the data plot on or near the hydrostatic pressure gradient of 10 MPakm-1, indicating that pore pressure is hydrostatic.

Minimum horizontal stress

From the 91 hydrocarbon wells studied a variety of files were investigated for any information on the stress field including: company composites, mud logs, EOWRs, log scans etc. These were interrogated for FIT’s and LOT’s.

Of the 84 wells, 44 had FIT’s or LOT’s listed with the equivalent mud weight listed (EMW) and recorded with sufficient metadata to estimate Shmin. This method will only give a regional estimate of Shmin due to the lack of XLOT’s to validate the data. In total there were 64 FIT’s/LOT’s observations from 44 wells for the Eastern Yorkshire and North Nottinghamshire area. Following collection of the LOT data these were plotted alongside the RFT data and the vertical stress measurements (Figure 10).

Figure 10    Compilation of FIT, LOT and RFT data available for the East Yorkshire and North Nottinghamshire region. Sv is the vertical (lithostatic) stress for the region and corresponds to a gradient of 23 MPakm-1 after Figure 8. PP is the pore pressure gradient for the area of 10.17 MPakm-1 after Figure 9. Shmin plotted using the lower bound of the LOT’s after Addis et al (1998). Above 1500 m eleven of the values recorded from both FIT’s and LOT’s plot on or above the Sv line. This may be the result of poor tests or it may indicate that at shallow depths Shmin ≈ Sv.

Maximum horizontal stress

Across the region only four locations had sufficient data to calculate SHMax. Overcoring data were available from North Selby Colliery (Farmer and Kemeny, 1992[6]). Hydraulic fracturing data were available from two Coal Authority wells: Howden Dike and East Lodge (summarised in Table 2). A summary of the two hydraulic fracture reports is provided below.

Table 2     Summary of the hydraulic fracture tests from two UK
Coal Authority Boreholes in East Yorkshire and North Nottinghamshire
Well Title Top Depth (m) Bottom Depth (m) Shmin (σ3) SHMax (σ1) SHMax (σ1)
Howden Dike 799.5 896 15.3 Pa 21 MPa 29.7 MPa
East Lodge 1023 1123 16.6 MPa 26.1 MPa 32.2 MPa

The only other borehole with sufficient data available to calculate SHMax was the Melbourne 1 Coal Bed Methane well in North Yorkshire. Breakouts were observed on borehole imaging at depths of 987–988 m (Figure 11), which reveal a clear SHMax direction of 144.4°. Figure 11 also illustrates DIFs between 984.8 m and 986.87 m.

Figure 11    A 10 m section of borehole imaging from the Melbourne 1 well showing both borehole breakouts and DIFs. Left hand panel; conventional wireline logs including Gamma Ray, Density and P-Wave Velocity. Right hand panel; Unwrapped CMI resistivity image (clockwise from North), breakouts highlighted by green boxes and showing an SHMax direction of 148.2°.

Equation 2 and Equation 3 in Maximum horizontal stress shows how SHMax can be calculated from both breakouts and DIFs. There were no UCS estimates available so SHMax could not be calculated from the breakout zone. However, point load tests were carried out on this core and can be multiplied by 0.8 to give an estimate of tensile strength (Ulusay & Hudson 2007[7]). Equation 3 can therefore be used to calculate SHMax. The list of parameters which were used to calculate SHMax in Equation 3 are presented in Table 3. Due to the lack of temperature information it was not possible to estimate σ∆T and it was discounted. This yielded a SHMax value of 27.22 MPa at 982.08 m.

Table 3    List of parameters from the Melbourne 1 well used
in Equation 3 to calculate SHMax from a unit with a DIF
Parameter Value Units Assumption
Depth 982.08 M
Sv 22.59 MPa Based on 23 MPakm-1
Point Load 2.56 MPa
Tensile strength 2.05 MPa 0.8 * Point Load
Shmin 16.8 MPa Estimated from regional FIT/LOT data
Pore Pressure 9.82 MPa Assumed to be hydrostatic
Mud Weight 11.31 MPa Based on a mud weight of 9.6 ppg from EOWR

The Weald

The Weald study initially focused on vertical stress gradients to establish changes in the state of stress. Some 118 wells drilled for oil/gas purposes were initially identified as potentially containing information to characterise the stress field (Figure 12). LOT and FIT data has not yet be collated for this region. See Appendix 1 for details.

Figure 12    Map of The Weald showing the distribution of the initial 118 wells in the area of interest screened for information to characterise the in-situ stress field (Blue) and the 18 wells used to calculate the vertical stress (Red). Contains Ordnance Survey data © Crown copyright and database rights 2017.

The Weald is a part of the Wessex Basin, a major sedimentary basin complex in southern Britain, formed as a sinistral pull apart basin during episodes of Mesozoic extension (Lake and Karner 1987[8]). In late Cretaceous–Early Tertiary times the basin underwent inversion as a result of Alpine compressional forces (Lake and Karner 1987[8]). The general structure is one of a broad regional basin upwarp (the Weald Anticline or Anticlinorium), upon which are imposed zones of more localised tighter northerly-verging inversion fold pairs, including monoclines with steep to overturned northern limbs. These structures developed along former syndepositional normal faults that underwent reversal of movement during compression (Chadwick, 1986[9]; Chadwick, 1993[10]; Chadwick & Evans, 2005[11]).

Initial stress field characterisation focused on the vertical stress in the core of the anticline to identify any differences in vertical stress gradients. To achieve this 13 wells were selected based on geographical distribution, top and bottom depth on density log and well TD. Figure 13 illustrates the variation in vertical stress gradients across the Weald and shows that the vertical stress at 1 km is in the range of 22–25 MPa. One visible deviation from this trend is Baxter’s Copse 1 below circa 1100 m. This is the result of large washouts within the Kimmeridge Clay section.

Figure 13    Compilation of vertical stress gradients from the Weald based on 13 wells. All of the curves fall in the range of 22–25 MPakm-1. Below 1000 metres there in a sharp change in the profile of Baxters Copse however this is associated with a large wash out rather than any change in the vertical stress gradient and the data from this well below 1100 m are excluded from the regional interpretation.

Cheshire and Lancashire

The Cheshire and Lancashire area was selected based on the availability of well data and current exploration interest for unconventional resources. The initial area of interest included 115 wells, 67 of which were thought to have information to characterise the in-situ stress field. Of these 67 wells only 21 had any information to characterise the stress field (Figure 14). See Appendix 1 for details.

The area of Cheshire and Lancashire with information to characterise the stress field (Figure 14) covers two main sedimentary basins. The Cheshire Basin and the West Lancashire Basin, which is an onshore extension of the East Irish Sea Basin (Smith et al 2005[12]). Both of these basins lie unconformably on Carboniferous strata deposited in the older Pennine and Bowland basins (Kirby et al., 2000[13]; Smith et al., 2005[12]). Much of the Early Carboniferous extension was caused by back-arc spreading created a deep water environment across the whole region (Kirby et al., 2000[13]). This was subsequently infilled by deltas and turbidity flows (in the Namurian–Westphalian) (Kirby et al., 2000[13]). Following uplift and erosion driven by the Variscan orogeny, E–W extension became the dominant tectonic process (Chadwick, 1997[14]). There is little preservation of younger strata in this region though it is thought that extension in these regions continued into the Cretaceous. This was followed by a major phase of uplift and inversion associated with continental convergence to the south (Smith et al., 2005[12]). Detailed descriptions of the structural and stratigraphic history of the areas can be found in Kirby et al. (2000)[13] and Smith et al (2005[12]).

Figure 14    Map of Cheshire and Lancashire showing the distribution of the initial 67 wells in the area of interest thought to contain information to characterise the in-situ stress field (Blue) and the 21 Wells, which contained information to characterise the stress field (Red). Contains Ordnance Survey data © Crown copyright and database rights 2017.

Vertical stress

Studies initially focussed on seven wells in the immediate area surrounding the proposed UK Geoenergy Observatories (UKGEO) southern site in North East Cheshire. The vertical stress gradients derived from two Coal Authority boreholes and three hydrocarbon boreholes are shown in Figure 15. The vertical stress gradient for these wells varies between 23 and 25 MPakm-1. The stress profiles for two wells, Hale and Morley Bridge fell below 23 MPakm-1. This was a result of large washouts and possible errors in the CPS to gcm-3 conversion (see Estimating vertical stress); as a result these wells were removed.

Figure 15    Compilation of vertical stress gradients from the proposed UKGEO Thornton Site and surrounding areas. Dashed black line represents a gradient of 23 MPakm-1. The vertical stress gradients range from 23–25 MPakm-1.

Pore pressure

Pore pressure data from RFT tools was only available for two wells in the area: Blacon West 1, Blacon East 1 with an interpreted DST available for Elswick 1, all measurements are below 700 m depth (Figure 16). In some cases EOWR’s cite pore pressure data collected in the East Irish Sea as the basis for assuming a hydrostatic pore pressure with a gradient of 10 MPakm-1, which is the value for a fluid with a density of 1 gcm-3. The pore pressure gradient for this region is 10.8 MPakm-1.

Figure 16    Compilation of available RFT pressure data for Cheshire and Lancashire. All of the data plot on or near the hydrostatic pressure gradient of 10 MPakm-1. Indicating that pore pressure is hydrostatic.

Minimum horizontal stress

Data from LOT and FIT were available from 18 wells in the region, some of these were recorded as fracture pressure gradients in psift-1. These data were converted to MPa and are shown in Figure 17. The LOT at 850 m plots above the 23 MPakm-1 line is one of the tests recorded as a gradient rather than a discrete value.

Figure 17    Compilation of FIT and LOT data available for Cheshire and Lancashire. Sv (Green) is the vertical stress (lithostatic pressure) for the region and corresponds to a gradient of 23 MPakm-1 after Figure 15. PP is the pore pressure gradient for the area of 10.80 MPakm-1 after Figure 16. Shmin plotted using the lower bound of the LOT’s after Addis et al (1998).

Maximum horizontal stress

SHMax was estimated from borehole breakouts at 16 depths in the Preese Hall 1 well, drilled by Cuadrilla in 2010. Rock property data was estimated from geophysical log data calibrated against rock test data (Baker Hughes, 2011[15]). The results are summarised in Table 4.

Table 4    Summary table of SHMax data calculated using borehole breakouts from the Prees Hall 1 well (based on Baker Hughes, 2011[15]). PPG – Pounds per Gallon
TVD BGL (m) SHMAX (Lower) SHMAX (Upper) Sv SHMAX Orientation Breakout width
PPG (EMW) PPG (EMW) PPG (EMW) Deg Deg
1491.082 22.6 24.83 20.12 173 60
1853.184 20.9 25.7 19.94 173 45
1875.434 23.2 26.96 19.95 173 45
1926.336 25 28 19.98 173 64
2117.446 24.1 27.6 20.13 173 20
2188.464 22.65 25.52 20.17 173 50
2230.526 23.07 25.9 20.19 173 45
2305.812 22.06 23.92 20.24 173 30
2328.062 22.27 24.71 20.25 173 70
2473.757 21.17 24.25 20.34 173 60
2548.433 21.98 23.8 20.38 173 45
2552.09 21.9 23.8 20.38 173 50
2553.31 22.55 24.3 20.38 173 30
2566.721 22.18 23.8 20.39 173 40
2587.142 22.1 24.7 20.4 173 30
2630.424 22.1 25.3 20.41 173 60

Additional maximum horizontal stress data

Due to the paucity of SHMax available across the UK and the lack of SHMax data within the regions themselves (Maximum horizontal stress) the decision was made to incorporate the available legacy SHMax data.

The SHMax values are largely calculated from two techniques; hydraulic fracturing and overcoring. The available data was compiled from peer review papers and published reports. This yielded SHMax information for 25 sites including hydraulic fracturing measurements Carnmenellis Granite in Cornwall (Pine et al., 1983[16]) and overcoring at a quarry in Caithness in Scotland (Figure 18).

Figure 18    Distribution of additional SHMax data from publish literature and Coal Authority reports across the UK: HF – Hydraulic Fracturing, OC – Overcoring. Contains British Geological Survey materials © NERC 2017.

References

  1. ↑ 1.0 1.1 Hawkins, T R W, and Saul, L G H. 2003. Complex extensional faulting of Triassic rocks north of York, North Yorkshire, UK. Proceedings of the Yorkshire Geological Society, 54, 257–267.
  2. ↑ 2.0 2.1 Fraser, A J, and Gawthorpe, R L. Tectono-stratigraphic development and hydrocarbon habitat of the Carboniferous in northern England In: Hardman, R F P, and Brooks, J. (eds), 1990, Tectonic Events Responsible for Britain's Oil and Gas Reserves, Geological Society Special Publication No.55, pp.49–86.
  3. ↑ Kent, P E. 1980. Subsidence and Uplift in East Yorkshire and Lincolnshire: A Double Inversion. Proceedings of the Yorkshire Geological Society, 42(28), 505–524.
  4. ↑ Kirby, G A, and Swallow, P W. 1987. Tectonism and sedimentation in the Flamborough Head region of north-east England. Proceedings of the Yorkshire Geological Society, 46, Part 4: 301–309.
  5. ↑ Zoback, M D, Barton, C A, Brudy, M, Castillo, D A, Finkbeiner, B R, Grollimund, B R, Moos, D B, Peska, P, Ward, C D, and Wiprut, D J. 2003. Determination of stress orientation and magnitude in deep wells. International Journal of Rock Mechanics & Mining Sciences. 40, 1049–1076.
  6. ↑ Farmer, I W, Kemeny, J M. 1992. Deficiencies in rock test data. In: Hudson, J.A. (ed), Rock Characterisation: Proceedings of the International Symposium on Rock Stress, Chester, United Kingdom, 298–303.
  7. ↑ Ulusay, R, and Hudson, J A. 2007. The complete ISRM suggested methods for rock characterization, testing and monitoring: 1974–2006. International Society for Rock Mechanics, 2007. 628p.
  8. ↑ 8.0 8.1 Lake, S D, and Kamer, G D. 1987. The structure and evolution of the Wessex Basin, southern England: an example of inversion tectonics. In: P.A. Ziegler (Editor), Compressional Intra-Plate Deformations in the Alpine Foreland. Tectonophysics, 137: 347–378.
  9. ↑ Chadwick, R A. 1986. Extension Tectonics in the Wessex Basin, Southern England. Journal of the Geological Society, 143, 465–488.
  10. ↑ Chadwick, R A. 1993. Aspects of Basin Inversion in Southern Britain. Journal of the Geological Society, 150, 311–322.
  11. ↑ Chadwick, R A, and Evans, D J. 2005. A seismic atlas of southern Britain: images of subsurface structure. Nottingham, UK, British Geological Survey, 196pp. (British Geological Survey Occasional Publication, 7).
  12. ↑ 12.0 12.1 12.2 12.3 Smith, N J P, Kirby, G A, and Pharaoh, T C. 2005. Structure and evolution of the south-west Pennine Basin and adjacent area. Subsurface Memoir of the British Geological Survey.
  13. ↑ 13.0 13.1 13.2 13.3 Kirby, G A, Baily, H E, Chadwick, R A, Evans, D J, Holliday, D W, Holloway, S, Hulbert, A G, Pharaoh, T C, Smith, N J P, Aitkenhead, N, and Birch, B. 2000. The structure and evolution of the Craven Basin and adjacent areas. Subsurface Memoir of the British Geological Survey.
  14. ↑ Chadwick, R A. Fault analysis of the Cheshire Basin, NW England. In: Meadows, N S, Trueblood, S E, Hardman, M, and Cowan, G. (eds). 1997, Petroleum Geology of the Irish Sea and Adjacent Areas, Geological Society Special Publication No.124, pp.297–313
  15. ↑ 15.0 15.1 Baker Hughes. 2011. Wellbore Failure Analysis and Geomechanical Modelling in the Bowland Shales, Blackpool, UK, GMI. Peliminary Technical Report. Cuadrilla Resources. Last Accessed 04-Feb-2017.
  16. ↑ Pine, R J, Ledingham, P, and Merrifield, C M. 1983. In situ Stress Measurement in the Carnmenellis Granite .2. Hydrofracture Tests at Rosemanowes Quarry to Depths of 2000-M. International Journal of Rock Mechanics and Mining Sciences 20(2): 63–72.
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