Geological and hydrogeological investigations in the Colchester Northern Gateway boreholes : February 2020 survey

Boon, D.P.; Butcher, A.; Townsend, B.; Woods, M.A.. 2020 Geological and hydrogeological investigations in the Colchester Northern Gateway boreholes : February 2020 survey. Nottingham, UK, British Geological Survey, 47pp. (OR/20/011) (Unpublished)

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This research report describes borehole geophysical and passive seismic surveys done at the Colchester Northern Gateway (CNG) geothermal district heat network and housing development in Essex, UK, between 10-13 February 2020. The first part of the report introduces the project context, the second part describes survey methods, and the third presents the ‘baseline survey’ results and stratigraphic interpretation. The final section summarises the key learnings, relevance of the findings to future geothermal district heating schemes in the Chalk and London Basin, and makes some recommendations for follow-up monitoring research. Five recently drilled production/injection boreholes at CNG were logged with Calliper, Temperature and Conductivity, Natural Gamma, Optical Televiewer (OPTV) tools. Some of the holes also had Resistivity and Impeller flow tools run but results were inconclusive. A site-wide Tromino H/V passive seismic survey was acquired to characterise the resonant frequency of the subsoils between the boreholes, to aid stratigraphic correlation, profile the concealed chalk ‘rockhead’ surface (i.e. depth of bedrock aquifer), and look for any evidence of the presence of faults/fracture zone, and characterise the thickness of karst and weathering profiles. The main geological formation boundaries were identified based on a combination of interpretation of Natural Gamma logs and correlation of sedimentary features observed in the OPTV images. The eroded top of the Newhaven Chalk is around 72 m below ground level (-24 m OD) in all five boreholes (BH1-5). The high-resolution OPTV images provide rich visual information on the fracture state of the upper ‘productive’ part of the chalk aquifer. A notable feature in BH4 is an 18 m long NW-SE orientated open vertical fracture that persists through the Newhaven Chalk but terminates at the top of the Seaford Chalk. The OPTV data also aids identification of key litho-stratigraphic marker horizons, such as tabular flint bands, hard grounds and marls seams. The Natural Gamma aids identification of key marls, such as the Buckle Marl and Shoreham Marls and the top and base of the London Clay and top of Chalk. Marls and flint bands are useful litho-stratigraphic markers for site- to regional-scale correlation and structural analysis, and for developing hydrogeological and thermo-geological conceptual and observational geo-models to inform the set-up of numerical flow models (e.g. FeFLOW models). The interpretation of the new survey data concluded that the near-surface geology at CNG comprises c.72 m of thin Superficial deposits and Palaeogene sediments resting on Cretaceous Chalk bedrock (White Chalk Sub-group). The two deepest (200 m) boreholes (BH2 and BH4) penetrate 4-5 m of superficial deposits (cover sand and gravels) resting on c.36 m of London Clay, c.8.5 m of Harwich Formation, c.10 m of Lambeth Group, c.12 m of Thanet Formation, c.23 m of slightly karstified and fractured basal Newhaven Chalk, c.59.4 m of Seaford Chalk and terminate within c.31 m of upper Lewis Nodular Chalk Formation. Groundwater temperature profiling in February 2020 suggests a shallow geothermal gradient of around 2.4°C per 100 m in the top 200 m of the Chalk aquifer in the Colchester area. Under natural conditions (i.e. no influence from other subsurface users), groundwater source heat pump schemes can therefore expect seasonally stable ambient source temperatures of around 11.7°C and 14.3°C at 100 m and 200 m depths, respectively, with 130m open loop schemes predicted to yield borehole inflow temperatures of around 12.5°C all year round (assuming no thermal interactions/breakthrough), making open loop ground source heat pump schemes a viable option for low carbon/no carbon heating systems. Pumping tests in the CNG wells (undertaken by the developer Amphora Energy Ltd in c.2019) yielded 2 to 10 l/s. The highest yields were from BH4 & 5 located on the far western side of the CNG development site and were attributed to the switch to a reverse-circulation drilling technique, however the presence of karstified open fractures (BH4) may also be a contributing factor. Open loop GSHP heating capacities of between 60 and 300 kW thermal per 130m well-doublets (depending on local aquifer properties) are therefore probably feasible for similar schemes in the region, assuming a sustainable thermal productivity (ΔTgw) of 7°C (i.e. abstraction at 12.5°C and injection at 5.5°C). Larger yields may be possible using larger diameter boreholes and in more fractured, weathered or karstified parts of the aquifer (i.e. fracture systems associated with the major NW-SE bedrock lineaments described by Woods and Chacksfield (2012)). The passive seismic H/V data collected across the CNG site in 2020, prior to the new house construction, suggests chalk aquifer and seismic rockhead surface in-between the boreholes (BH1-5) is relatively level and planar at an elevation of around -24 m OD, although some karstification may have occurred in the upper 10-20m of the aquifer. This top chalk elevation is consistent with the old borehole records from the nearby Severalls Hospital. From interpretation of the closely-spaced H/V survey results there is no suggestion of a major fault/fracture system within the chalk at the CNG site, although there could be undetected fault systems at other locations in the region and potentially at depth in the area.

Item Type: Publication - Report
Funders/Sponsors: British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed, but not externally peer-reviewed.
Additional Keywords: GroundwaterBGS, Groundwater
Date made live: 24 Mar 2022 14:40 +0 (UTC)

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