Questions about acidisation with regard to well stimulation, reinjection wells

Emily Mott made this Freedom of Information request to Environment Agency

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Dear Environment Agency,

I have a few questions with regard to environmental regulations for the unconventional well stimuation technique of acidisation.

I understand acid washes have been used for a long time to clean corrosive wells and prepare for drilling. I also understand that the process of acidisation has been used for many years to stimulate well flow.
Although some of the principles remains the same, additives are different and the science of how to extract oil improves and yet the science of how it impacts our environment and human health is unknown and doesn’t move as rapidly.

I can not find any regulation in place for the current proposed unconventional method of stimulating deep, horizontal wells which is completely different from an “acid wash”. When I recently
asked DECC similar question about acidisation , officials responded with regulations about fracking fluids.

What is your definition of acid wash?

What is your definition of acidisation?

What kind of chemicals are used in acidisation?

Have you conducted any studies on subsurface release mechanisms including acid wormhole pathways in the rock formation leading to aquifers, fault pathways leading to aquifers, deteriorated abandoned wells leaking into the subsurface, and the failure of production or disposal wells?

What distinguishes an acid wash from matrix acidisation?

What distinguishes an acid wash from acid fracturing with regard to regulation?

What distinguishes flow back fluids from deep well stimulation by acidisation from hydraulic fracturing flowback?

Can I correctly assume that waste fluids from deep well acid stimulation and fracking fluids are regulated in the same way?

How is waste water from matrix acidisation and/or acid fracturing classified?
A few industry reports show that the pH of returning waste is mainly between 0 and 3 for the “first few hours". It is unknown how much of the chemicals returns to the surface for acidizing, but recent data by operators show that the volume of recovered fluids collected after matrix acidisation is 50%- 60% (this would be contrary to what we read about waste water in your directives)

Can this be stored in a reinjection well on site?

Is flowback fluid from deep well acidisation tested for chemicals, pH, toxicity before being reinjected?

Are operators required to list how many times a well has been stimulated by acidisation?

Do you study the fate and transport of the chemicals, their transformations, synergistic and cumulative effects, as well as routes of exposure?

What is the EA's definition of acid wash with regard to the standard waste management plan?
What is the EA;s definition of deep well acidisation (matrix and fracturing) with regard to the standard waste management plan?

Would waste water from deep well stimulation by acidisation require a bespoke permit?

What is your definition of conventional vs. unconventional well stimulation techniques?

Many thanks for answering these important questions.

Yours faithfully,
Emily Mott

Enquiries, Unit, Environment Agency

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Emily Mott left an annotation ()

Response from Environment Agency:

Our ref: NR27849

Date: 18 November 2016

Dear Ms Mott,

Thank you for your enquiry which we received on 31 October. We respond to requests under the Freedom of Information Act 2000 and Environmental Information Regulations 2004.

Your enquiry falls under the regulatory remit of the Environment Agency and therefore we are also responding on behalf of the Department for Business, Energy and Industrial Strategy (BEIS) who received the same request in October. Please find our response below.

What is your definition of acid wash?
Acid washing is the localised use of acid solutions in the geological formation to remove scale or similar deposits from perforations and well-completion components. This treatment may be used to repair formation blinding and help restore the natural porosity of the formation. Acid-wash treatments generally do not include injection of treatment fluid into the reservoir formation.

Acid washing is a standard technique which has been used by the water supply industry as well as the oil and gas industry for many years. It is considered to be a standard activity because the acid, which is used in dilution and in small quantities, will react with the rock formation and be rendered neutral and therefore is not a risk to the water environment. This activity has a long history of being carried out safely and without impact on the environment.

What is your definition of acidisation?
The Environment Agency does not have a specified definition of acidisation. However this again is a standard reservoir or aquifer treatment that has a long history of use in both the water supply industry and the oil and gas industry.

The term refers to the use of dilute acid to improve permeability in an aquifer or reservoir formation to enable enhanced production fluids. The treatment is performed at pressures below the fracture pressure of the formation which allows the acid to penetrate the formation but without causing damage to the formation.

What kind of chemicals are used in acidisation?
The most commonly used acid is Hydrochloric acid, which would be diluted to around 15% strength.

Hydrofluoric acid is only used in limited situations such as use in sandstone settings. Use of this acid for acid washing will require a bespoke environmental permit application.

Have you conducted any studies on subsurface release mechanisms including acid wormhole pathways in the rock formation leading to aquifers, fault pathways leading to aquifers, deteriorated abandoned wells leaking into the subsurface, and the failure of production or disposal wells?
The Environment Agency is working in collaboration with other organisations and universities to review evidence and assess the risks from these types of occurrences.

What distinguishes an acid wash from matrix acidisation?
As per the details in the answers above, the acid wash is used in the well and the formation immediately adjacent to the well. The treatment is not intended to inject fluids into the formation.

Acidisation is a treatment that requires injection into the formation with the intention of increasing the interconnectivity for a distance away from the well.

For the purposes of regulation the Environment Agency regulates the acid wash using a Standard Rules Permit and an acidisation treatment would require a bespoke environmental permit. The application for bespoke environmental permits is published on gov.uk for public consultation.

What distinguishes flow back fluids from deep well stimulation by acidisation from hydraulic fracturing flowback?
We have assumed that by deep you are thinking of shale gas equivalent depths, so greater than 1000m below groundwater level. Please let us know if this is in correct.

At such depths any stimulation process involving the injection of a fluid that is then returned to surface will produce a fluid that has a very high mineral content that will be dependent on the composition of that formation that has been stimulated. This would be true of a fracture producing or non-fracture producing treatment.

Can I correctly assume that waste fluids from deep well acid stimulation and fracking fluids are regulated in the same way?
Yes. Fluids returned from deep formations following acid stimulation or hydraulic fracturing for oil and gas are classified as mining wastes and are therefore covered by the Mining Waste Regulations. This means that a Mining Waste Permit is required to carry out this operation, and it will cover the production, storage, reuse or disposal of the fluid. Radioactive substances permits may also be required for these activities. Applications for bespoke environmental permits are subject to public consultation on gov.uk

How is waste water from matrix acidisation and/or acid fracturing classified?
If the waste water is derived from operations carried out by the oil and gas industry it is classified as a Mining Waste, please see our answer above.

A few industry reports show that the pH of returning waste is mainly between 0 and 3 for the “first few hours". It is unknown how much of the chemicals returns to the surface for acidizing, but recent data by operators show that the volume of recovered fluids collected after matrix acidisation is 50%- 60% (this would be contrary to what we read about waste water in your directives). If you are looking at our documents regarding the returned fluids from hydraulic fracturing this is probably not comparable to the data you are looking at regarding matrix acidisation which is a different operation. If you wish to point us to the data you are looking at we may be able to comment more on this.
The percentage of returned fluid from any formation stimulation method used will be determined by the geophysical properties of that specific formation. Desiccated formations will return less fluid than those with a higher moisture content for example.
Can this be stored in a reinjection well on site?
We have assumed in this question you are asking if flow back fluids from hydraulic fracturing or acidisation can be stored in a reinjection well, but if this is not the case please tell us.

No, flowback fluids cannot be stored in re-injection wells, this is for the following reasons:
• Reinjection wells are points of disposal and are not used for storage.
• Reinjection wells in England have been and are being used to inject produced waters from the conventional oil and gas industry back to formations from which they have been derived. During the recovery of oil and gas from conventional reservoirs natural formation water is also produced, this water is highly saline and will still contain traces of hydrocarbon after the oil and gas is extracted, so the most environmentally friendly disposal route for this water is to re-inject it back into formations that have had hydrocarbon extracted from them. This reinjection can also be used to enhance the production of oil or gas from that reservoir.
• Flow back water from shale hydraulic fracturing is mostly composed of the fluids that were injected to carry out the fracturing activity, and are not derived from the natural rock formations. Shale formations are not porous and the flow back fluid will not be replacing fluids that have been derived naturally from that formation.
• The Environment Agency has taken a position that flowback fluid should not be injected back to the ground because it has not been proven to be the Best Available Technique for the disposal of this water. Shale formations are not porous and the flow back fluid will not be replacing fluids that have been derived naturally from that formation.

Is flowback fluid from deep well acidisation tested for chemicals, pH, toxicity before being re-injected?
Flow back fluids from deep shale stimulations would not be considered acceptable to be re-injected. Flow back fluids will need to be tested by the operator of a site prior to any reuse or disposal off site.

Are operators required to list how many times a well has been stimulated by acidisation?
If an operator is wishing to carry out stimulation activities they are required to apply for an environmental permit to cover any Mining Waste and groundwater activities.

Any applications for such permits will have to state in detail the methods and processes they are wishing to carry out, which would include the number of stimulations they wanted to undertake as part of that application. The applications will need to be accompanied by a detailed hydrogeological risk assessment to ensure that any environmental risks are fully considered and addressed.

Do you study the fate and transport of the chemicals, their transformations, synergistic and cumulative effects, as well as routes of exposure?
Yes, these factors are being taken into account along with other areas of study and research underway in collaboration with Universities and other organisations.

What is the EA's definition of acid wash with regard to the standard waste management plan?
The definition that is set out in the Standard Waste Management Plan which forms part of the Standard Rules Permit states:
“In the drilling and construction of any borehole the drilling process can cause a certain level of formation blinding. This blinding results in a reduction in the level of the natural porosity of the rock formation near to the well bore and is referred to as skin damage.

To repair this formation blinding and restore the natural porosity of the formation a low pressure acid wash may be applied. The acid wash treatment will be composed of a 15% hydrochloric acid (HCl) solution. This acid will be neutralised by its interaction with the rock formation and form mineral salts, water and carbon dioxide which will be reverse circulated out of the formation for recovery at surface. Between 5m3 to 15m3 of HCI may be pumped into the formation during the operation, with all the acid wash fluids being recovered to surface.

Hydrochloric acid solutions (15%) are used during the acid wash process and in volumes which ensure they are neutralised within the formation. The back circulated flush fluid will also contain rock particles and drilling muds.”

What is the EA’s definition of deep well acidisation (matrix and fracturing) with regard to the standard waste management plan?
As stated in the answer to the second question above the Environment Agency does not have a specific definition of deep well acidisation either for matrix enhancement or for fracturing. Neither of these activities would be allowed under a Standard Rules Permit.

These activities require a bespoke environmental permit that would be based on the detailed description of the activity that was being proposed.

Would waste water from deep well stimulation by acidisation require a bespoke permit?
Yes, the waste water would be part of an activity that required a bespoke environmental permit.

What is your definition of conventional vs. unconventional well stimulation techniques?
The Environment Agency does not hold definitions for conventional and unconventional stimulation techniques. The Environment Agency will permit activities that constitute Mining Waste activities, Groundwater activities and Radioactive Substances activities based on the detailed description of the activity being carried out, not with reference to whether they may be regarded as conventional or unconventional activities.

Please get in touch if you have any further queries or contact us within two months if you’d like us to review the information we have sent.

Yours sincerely

Onshore Oil and Gas Team