Assuring well casing integrity and quality
The Offshore Installations and Wells (Design and Construction) Regulations (DCR) require the well operator to ensure that there is no release of fluids from the well so far as is reasonably practicable. There are also requirements of the Borehole Sites and Operations Regulations (BSOR) that are important for maintaining well integrity.
The integrity of oil and gas wells is ensured through a combination of:
- a well design created by competent personnel in compliance with appropriate health and safety regulations, specifically 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 specialist HSE wells inspector against construction standards established by the industry;
- 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 standard and construction as detailed in UKOOG shale Gas Guidelines and Oil and Gas UK Well Life Cycle Integrity Guidelines together with API Guidance such as 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 drilling mud loss or gain can be closely monitored within a 'closed' circulating system, and pressure test blow-out preventers in place 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.