Comments sought on permit changes at “Gatwick Gusher” oil site in Surrey


Well testing at Horse Hill in 2016. Photo: Eddie Mitchell

A public consultation has opened into an environmental permit application for the Horse Hill oil exploration site in Surrey.

The Environment Agency is seeking comments on proposals for well testing and drilling new wells at the site, known as the Gatwick Gusher, near Horley.

The site operator, Horse Hill Developments Ltd (HHDL), has also applied for planning permission to Surrey County Council. More details and link

The permit consultation began yesterday (22 December) and continues until 2 February 2017.

DrillOrDrop has compiled key information from documents submitted by HHDL to accompany the permit application

Site details

Exploration licence area: PEDL137

Companies behind the project: HHDL, of which UK Oil & Gas Investments Plc holds 48% of the stake and provides management of the licence

Address: Horse Hill, Hookwood, Horley, Surrey RH6 0HN

Location: 300m east of Horse Hill, minor road running off the A217 from Hookwood

Nearest property: Wrays Farm House, 50m from access gate and 300m from well site

Nearest streams: Spencer’s Gill 600m to south drains into the Mole River and is connected to the site. Deanoak Brook, which also joins the Mole River, is 2km from the site and has no direct connection with the site because it is in a different catchment

Other water features: Springs: 470m and 760m to south west, flowing into Spencer’s Gill and two ponds down slope (470m and 670m)

Nearest groundwater source protection zones: 5km to north. No private water supplies registered within 1km of the site boundary

Flood risk: Flood Zone 1 (the lowest risk of flooding)

Nearest wildlife sites: Edolphs Copse Local Nature Reserve (2km to south west) and Glover’s Wood Site of Special Scientific Interest (3km to south west)

History of Horse Hill operations

The Horse Hill-1 well (HH-1) is an S-shaped exploration well drilled into the Triassic formation

  • Spudded: 2 September 2014
  • Well suspended: 4 November 2014
  • Short-term flow testing: 1 February-18 March 2016 of:
    • Upper Kimmeridge Limestone (3,102ft–3,185ft)
    • Middle Kimmeridge Limestone (2,827ft– 2,930ft)
    • Portland Sandstone (2,043–2143ft)


Permits already held: Water Discharge Permit (EPR/BB3691NN) and Radioactive Substance Regulations permit number EPR/AB3498DZ/A001.

Currently seeking: Planning permission from Surrey County Council (SCC Ref 2016/0189) for additional testing and drilling

Expected to apply for: Standard rules permit for temporary oil storage

Proposed operations

Phase 1

  • Extended well tests (EWTs) of Portland Sandstone and Upper and Middle Kimmeridge Limestones in HH-1
  • Short-term test on a four reservoir in the Lower Kimmeridge Limestone of HH-1, including perforation and acidisation
  • During this phase, HHDL said it would apply an acid treatment to the existing perforations in HH-1, using 15% concentration of hydrochloric acid. It would use a hot oil wash to remove build-up of was in the production tubing and casing.

Phase 2

  • Dependent on phase 1 results, drilling and EWT of an appraisal sidetrack from the existing HH-1 well.
  • 15% hydrochloric acid would be circulated along the borehole into what the application called the “natural fracture system”

Phase 3

  • Dependent on phase 1 results, construction of a new well cellar, drilling and EWT of a new deviated HH-2 appraisal well 6m to the north of HH-1
  • The new well would also require acidizing, HHDL said, to remove formation damage. Treatment fluids were likely to include:
    • 8m3 of 3% potassium chloride brine (pre flush)
    • 32-48m3 of 15% hydrochloric acid, corrosion inhibitor and surfactants (acid treatment)
    • 18m3 of 3% potassium chloride brine (displacement fluid used to push the main acid treatment into the formation and overflush)

Phase 4

  • Restoration to agriculture and woodland
  • If the oil flow is commercially viable, HHDL said it intended to apply for further consents for oil production

Flaring and venting

HHDL proposes to use an enclosed gas flare during the short and extended well tests. This will burn gas from the well that has been separated from any oil recovered. HHDL said some gas would be carried over in the oil to storage tanks and this would be naturally vented.


HHDL described environmental risks of the operation with mitigation as “negligible”. But it said:

“because of the relative importance of surface water and noise impacts, a decision has been taken to monitor surface water and noise during the operations to demonstrate the effectiveness of the mitigation measures”.

Surface and ground water

HHDL said

“The risk of any adverse impact to surface water or ground water is highly unlikely from the extractive waste streams”.

The application said the main aquifers near the surface were a distance of more than 400m from the site. Deeper principal or secondary aquifers were either too deep for the groundwater to be accessible or they were hydrogeologically inactive, the documents concluded. They added:

“In these circumstances, the water quality is often poor such that they have no water resource potential”.

The documents said the HH-1 sidetrack may penetrate Purbeck Carbonates aquifer and the the HH-2 well would create a direct connection with the Upper Tunbridge Sands secondary aquifer when this formation was penetrated.

But it added

“any interaction with groundwater will be minimised/negligible due to the proposed drilling and casing methods”.

The documents concluded: “the risk of any adverse impact to surface water or ground water is highly unlikely from the extractive waste streams”.

It proposed mitigation including:

  • Site covered by compacted crushed stone on top of a 1mm thick impermeable geomembrane
  • Daily visual monitoring of membrane for damage or leaks
  • Drainage of the site into an interceptor ditch
  • Contaminated fluids in the interceptor ditch would be removed by tanker for disposal at a waste treatment facility
  • Secondary bunding surrounding the well operation and containers for diesel fuel and chemicals
  • Bunds would be inspected regularly and frequently
  • All produced water and oil collected from EWTs would be stored in three 180barrel tanks which would conform to Environment Agency guidance and stored in bunded area
  • The HH-1 well had been cased, as would the new HH-2 well, ensuring there was no possible fluid pathway to the aquifers
  • The site ditch would be kept empty of water and any foliage during operations to ensure its capacity remained at the total of 500m3.

The permit documents added: “monitoring of the surface water system during the drilling of Horse Hill-1 detected no discernible change in quality during or after completion of the work.”


The documents identified four main types of waste:

  • Non-hazardous: water-based drilling muds, formation cuttings and surplus cement
  • Hazardous: Gas and fluids from the reservoir

It also listed surface pad rain water runoff and general waste


An assessment carried out for HHDL found noise levels at night fell no lower than 35 decibels (dB). Morning noise levels ranged from 40-48dB.

The application said:

“The noise from drilling operations, often 24-hour, will inevitably cause a temporary increase in ambient noise. The predicted noise levels at local dwellings are based on the noisiest rig(s) likely to be used in the Appraisal Project, and alternative rigs may be available for contract at the time. Specific noise control measures will be applied as necessary and appropriate.”

HHDL proposed to install a 6m high acoustic barrier surrounding the south side of the wellpad for 81m.This, it said, would reduce noise levels  by at least 5dB.

But it added:

“The drilling operations proposed are temporary in nature, and the daytime and night-time noise limits proposed barely exceed the ambient sound levels measured. This should be seen in the light of the NPPF [National Planning Policy Framework] Technical Guidance, which is considered appropriate for minerals development sites where drilling operations are temporary.”


Predicted noise levels at nearest properties

  • Daytime temporary operations 7am-7pm Monday to Friday and 9am-1pm Saturday: 65dB LAeq
  • Daytime drilling operations 7am-7pm Monday to Friday and 9am-7pm Saturday 48 dB LAeq
  • Night-time drilling operations 7pm-7am all week 42 dB LAeq

The application said:

“Effective communication with the local population regarding noise issues will be maintained through keeping local residents informed of the type and timing of works and any particularly noisy operations expected or out of hours working; and providing local residents with 24-hour contact details for a site representative in the event that disturbance due to noise from the construction works is perceived.”

Exceeded noise levels or complaints would be recorded and investigated. A further noise survey would be carried out as soon as possible, the application said.

Air quality

The application predicted key sources of pollution included

  • Traffic serving the site (nitrogen dioxide and PM10 and PM2.5 particulates)
  • Exhaust from on-site generators (nitrogen dioxide and PM10 and PM2.5)
  • Fugitive dust
  • Gases from the enclosed flare burning gas separated from oil (volatile organic compounds, carbon monoxide and nitrogen oxides)
  • Venting from on-site oil storage tanks

Proposed mitigation measures were:

  • 5mph speed limit on unpaved ground and 20mph on paved ground
  • Sheets covering lorries carrying dusty materials on and off site
  • Off-road plant to use ultra-low sulphur diesel “wherever practicable”
  • Cleaning pave areas, use of water sprays and suppression
  • Minimising dusty activities on very dry or windy days
  • Monitoring emissions from flare

HHDL said its air quality assessment concluded that the impacts of traffic, flaring of associated gas, and construction dust would not be significant if proposed mitigation were in place. Fugitive emissions were expected to be “very minor and pose low environmental risk”, the company said.


The company proposed it would carry out:

  • Surface water sampling before, during and after the operation at three locations representing upstream, midstream and downstream of the site
  • Noise monitoring at three locations (Five Acres, High Trees and Phoenix Lodge) to verify that noise limits in any new planning permission were met

Other documents

Other documents in the application deal with health and safety systems, procedures in event of a site invasion or protest, security issues or pollution incidents, the composition of drilling muds and chemicals.

HHDL said its health and safety target was:

“Get it right, first time, every time with no accidents, no harm to people or to the environment.”


Environment Agency consultation webpage and links to documents

DrillOrDrop timeline for Horse Hill

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