Rathlin Energy (UK) Ltd was given a three-year extension in November to drill a second well near Burton Constable Hall to explore oil and gas reserves.
The activists set up a “monitoring and information” camp at the West Newton A site, at High Fosham, a little over a week ago, which has sparked a regular police presence, and say they will stay “as long as it takes”.
Campaigner Pete Lomas said: “This is a gateway to a gas field – we need to shut that gate and put a padlock on it.”
The small group, who are living in caravans on the roadside, say it was only because of the camp and local residents working together when the first well was drilled over five years ago that several breaches of Environment Agency permits and Health and Safety regulations were reported.
Richard Howarth, of Frack Free East Yorkshire, said: “In these times of austerity, regulators have been cut to the bone and are struggling to monitor in detail.”
The latest exploratory borehole – WNA-2 – involves drilling 2,000 metres down and processes, including an “acid wash and squeeze,” using up to 15 cubic metres of 15 per cent acid solution being squeezed into the rock formation.
Asked what that meant, the Environment Agency said it dissolved the rock and "may also result in opening up new fractures, although very small and close to the well. This may enhance or create new flow paths to enable the well to be more productive."
The Agency says they do not consider this well stimulation or groundwater activity.
A spokesman added: "This is effectively conventional gas exploration. It is not a fracking operation - the two are very different."
But Mr Howarth said: “It is generally accepted that conventional wells flow easily. This confirms that this formation is low permeability and requires stimulation by injection of acid, which is clearly unconventional, as generally accepted by everyone except the UK Government. It is unconventional and unconventional wells are more likely to fail.”
It comes as the agency opened a consultation over an application by Rathlin to vary the conditions of its environmental permit.
A drop-in session will be held at Sproatley Village Hall on Thursday next week so people can find out more about the agency’s role in regulating the oil and gas industry.
Humberside Police said officers are deployed to allow people to go about “their lawful business” and to protect the right of individuals to “take part and exercise their right to peaceful protest”.
Rathlin added that acid squeezes were standard operating practice and were part of the previous environmental permit.