Shale gas and hydraulic fracturing (fracking) Q&A

Safety of shale gas drilling and well integrity

The operator is responsible for ensuring the safety of the well and the site. HSE scrutinises the working practices adopted by operators to ensure operators manage and control safety risks, conforming to the requirements of the Health and Safety at Work etc. Act 1974, and the following regulations made under the Act:

The Borehole Site and Operations Regulations 1995 (BSOR) apply to all oil and gas well operations onshore, including shale gas operations. These regulations are primarily concerned with the health and safety management of the site.

The Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 (DCR) apply to all wells drilled with a view to the extraction of petroleum (whose definition includes shale gas) regardless of whether they are onshore or offshore. These regulations are primarily concerned with well integrity.

The Reporting of Injuries Diseases and Dangerous Occurrences Regulations 1995 (RIDDOR) set out a specific set of Wells Dangerous Occurrences in Schedule 2, Part I, that the Well Operator has to report to HSE. Reporting of well incidents enables the HSE Energy Division (ED) to investigate those that would have an effect on well integrity and ensures the Well Operator secures improvements to his operations. These are:

  • A blowout (ie an uncontrolled flow of well fluids).
  • The use of blow out prevention equipment to control an unplanned flow.
  • The unexpected detection of H2S (hydrogen sulphide).
  • Failure to maintain minimum separation distance between wells.
  • Mechanical failure of any safety critical element of a well.

For the drilling process, HSE initially scrutinises the well design for safety and then monitors progress on the well to determine if the operator is conducting operations as planned. An oil and gas well is a complex engineered construction and the key to well integrity inspection is to ensure that the operator is managing risks effectively throughout the life cycle of the well. To ensure this, HSE uses an inspection and assessment process consisting of the following main elements, all of which utilise HSE's experienced specialist wells inspectors:

  • assessment of well notifications submitted to HSE. This assesses well design prior to construction, a key phase of work where the vast majority of issues likely to have an impact on well integrity will be identified and addressed by the well operator;
  • monitoring of well operations during construction based on weekly operations reports submitted to HSE by the well operators. This ensures the construction phase matches the design intent; and
  • meetings with well operators prior to, and during, the operational phase will be undertaken (including joint meetings with the EA). This will include site inspections to assess well integrity during the operational phase. A programme of inspections and site visits will be established for each well prior to drilling, this may include ad-hoc inspections as deemed appropriate. The following has been established as a minimum in the HSE/EA joint inspection arrangements.

For new or first time shale gas operators the HSE and Environment Agency will:

  • meet them and advise them of their legal duties under the relevant legislation;
  • conduct a joint inspection of the key operations at site including:
    • cementing and verification of cement;
    • mini hydraulic fracture;
    • bleed backmain hydraulic fracture.
  • any such meetings and visits could include other interested parties eg DECC; and
  • any change in the process ie hydraulic fracturing at shallow depth or change of media would entail a refresh of this inspection schedule.

For the short-term future, to establish public confidence in the process the HSE/EA intend, as a minimum to:

  • jointly inspect drilling operations at shale gas borehole sites, paying particularly attention to well integrity and cementing issues;
  • jointly inspect fracking operations.

HSE will also request and review an independent analysis of logging outcomes, used to verify cement job/zonal isolation, during the standard scrutiny of the operator's weekly drilling reports.

Ensuring public safety in the vicinity of fracking sites

The well (borehole) site operator is responsible for ensuring public safety within, and in the direct vicinity of, the work activities, following the industry model code of safe practice. HSE is responsible for regulating this requirement.

Assuring well casing integrity and quality

The integrity of the wells is ensured through a combination of:

  • a well design created by competent personnel in compliance with appropriate health and safety regulations, specifically the Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 (DCR) which apply to all wells drilled with a view to the extraction of petroleum regardless of whether they are onshore or offshore. These regulations are primarily concerned with well integrity;
  • a well design process that has identified any well bore hazards and mitigated them;
  • a review of the well design by an Independent Well Examiner;
  • review of the well design by a HSE Wells inspector against construction Standards in HSE guidance and in the Well Integrity Guidance;
  • construction of the well in compliance with the design by competent personnel with any significant changes subject to the same scrutiny as described above;
  • monitoring of the construction phase of the well and subsequent maintenance by the Independent Well Examiner and by the HSE Wells Inspector.

Maintaining well integrity

Well design and construction operations follow a recognised industry design and construction process (eg the API Guidance Document HF1 – 'Hydraulic Fracturing Operations – Well Construction and Integrity Guidelines'). Such processes ensure that wells have 'safety features' incorporated into their design. Specific design and construction requirements include:

  • a well design based upon a review of the local geology, to plan for any forecast well bore hazards;
  • the size and grade of casing is selected based on the results of a casing design process. The casing design analyses the burst, collapse, tensile and triaxial loads that it may be subjected to;
  • when the type of casing and its setting depths have been selected, the cementation programme is developed in consultation with the well operator's specialist cementing contractor. This design process analyses the rock strength and isolation requirements of the cement slurry so that it is placed as per the well design requirements;
  • drilling conducted so that any loss or gain can be closely monitored within a 'closed' circulating system, and blow-out preventers shut in to control any unplanned flow from the well;
  • casings being cemented in place and closely monitored to ensure correct placement of the cement slurry between the outside of the casing and the well bore. Casings should be cemented back into the previous casing or back to surface for shallow casings. If monitoring of the process indicates that the height of cement may be insufficient, then a cement bond log may be run to verify that there is sufficient cement behind the casing;
  • once cemented, the casing is pressure tested to ensure its integrity;
  • completed wells are monitored at the surface for any annulus pressure (pressure in the spaces between the different strings of casing) to verify ongoing integrity. Additional measurements can be made at depth if there is any doubt about the integrity of the well.

Cement bond logging and monitoring the integrity of the cement bonding

Cement bond logging (CBL) can be a useful means of verifying integrity where there is a single casing. CBL cannot verify the cement integrity through double casing of pipe and cement. Where there is a double casing, the best method and standard industry practice is to monitor the annular pressures. As an additional protection, the industry is recommending surface methane and groundwater monitoring, with any anomalies to be reported to EA, HSE and DECC, and compared with data from the National Baseline Methane Survey, being undertaken by the British Geological Survey.

Ensuring the quality of concrete well casings

The cement specification, testing of the slurry and placement of it in the well follows recognised industry best practice as contained in the following American Petroleum Institute (API) documents:

  • API Guidance Document HF1 – Hydraulic Fracturing Operations – Well Construction and Integrity Guidelines.
  • API Specification 10A (ISO 10426-1:2009) Specification for Cements and Materials for Well Cementing.
  • API Recommended Practice 10B-2 (ISO 10426-2:2003) Testing Well Cements.

Testing well casings

There are a number of tests that operators can use to provide information:

  • pressure tests are conducted on the casings installed in wells to ensure they have pressure integrity. Leak-off or formation integrity tests are also conducted once the bottom of the casing strings have been drilled out to determine what the strength of the rock is;
  • a leak-off test is where the rock is subjected to hydraulic pressure until the drilling fluid begins to leak into the rock and it begins to fracture, this determines the 'leak-off pressure';
  • a formation integrity test is where the rock is subject to a predetermined pressure below the leak off pressure to monitor well integrity. Both of these tests provide the well operator with information on the strength of the rock about to be drilled through, but also provides confirmation that the casing is properly cemented into the section of rock that has just been drilled;
  • a cement bond log of casing strings can also be conducted if there is any doubt about the quality of a cementing operation. This determines where the top of cement is in the casing and confirms that the cement is as designed for specific location and of the appropriate quality. The pressures in the spaces between the casings are routinely monitored throughout the life of the well to ensure that integrity is maintained.

All of the above tests represent standard oilfield practice for well construction and are not particular to shale gas operations.

Casings in the well are typically pressure tested as follows:

  • once they have been installed and cemented;
  • a leak-off test or formation integrity test can be conducted on the rock at the bottom of the casing once it has been drilled out. This is to ensure that the cement bond at the base of the casing is good and to gain information on the strength of the rock at the bottom of the casing;
  • the innermost casing will be pressure tested prior to any hydraulic fracturing operations and after the running of any completion tubing. (The completion tubing is recoverable casing, through which the gas is produced, and incorporates safety devices to prevent or mitigate escape of gas if the well-head is damaged).

Independent monitoring arrangements

Regulation 18 of the Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 requires the Well Operator to set up a Well Examination scheme and appoint a Well Examiner. The Well Examination Scheme and involvement of the Well Examiner is for the complete lifecycle of the well from design through to final plugging and decommissioning. The Well Examiner is an independent competent person who reviews the proposed and actual well operations to confirm they meet the Well Operators policies and procedures, comply with the Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 and follow good industry practice. During assessment and inspection activities, HSE checks that the operator has these arrangements in place.

The Well Operator's well examination scheme requires the operator to send the following documents to his Well Examiner:

  • the well construction programme and any material changes to it;
  • reports on how the well is being constructed;
  • reports on how the well is being monitored; and
  • at the end of the well's life, a plan for how it will be plugged and decommissioned.

The Well Examiner reviews these documents to ensure the complete lifecycle of the well is designed, constructed and operated in line with the Well Operator's policies and procedures, good industry practice and legal compliance.

Shale gas well operators will ask their well examiners to examine certain well integrity and fracturing operations in real time, especially during the early stages of a development, to provide a further level of independent assurance. Such periodic site visits should be made at the discretion of the examiner, in addition to assessing documentary evidence of well integrity, to observe and verify that such operations have been executed satisfactorily in accordance with the approved programme. The frequency and need for such site visits to shale gas operations would reasonably be expected to reduce with time.

Safe use of radioactive sources on site

Radioactive sources are used in oil and gas exploration, but are also extensively used throughout many other industries, including the NHS, paper and steel manufacturing, food irradiation, medical sterilisation and the construction industry. Nuclear well logging tools are robustly built with almost no chance of radioactivity release under normal oilfield operations and stringent regulatory requirements are imposed on the transport, storage, handling, abandonment and eventual disposal of chemical radioactive sources.

International Atomic Energy Agency (IAEA) guidelines, European Union protocols, and national regulatory bodies prescribe standards for the handling of all radioactive sources [IAEA, 2003a; IAEA, 2004; IAEA, 2005; EU, 2009; NRC, 1987; NRC, 1991] to ensure their safe use. The use of ionising radioactive sources in the UK is strictly controlled by the UK radiological regulatory framework, which includes the Ionising Radiations Regulations 1999 enforced by HSE, as well as other legislation enforced by the environmental regulators, Department of Health, and the Office of Nuclear Regulation, all of which have programmes of inspection in place to ensure compliance. Strict security and safety procedures are used for storing these tools and special shielded containers are used for transporting sources. Only authorised personnel following specific rules can access sources of this nature.

In almost all cases nuclear logging tools are owned and operated by oil and gas service companies, who are licensed to use the equipment. Operators would commission service companies to undertake well logging as and when their operations require their use. The use of such sources by appropriately trained personnel in accordance with the prescribed standards will not result in any risk to public health.

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2021-05-04