When extracting natural gas from shale and tight rock formations, hydraulic fracture stimulation (or fraccing) is necessary.
Critical to the protection of ground and surface water during this activity is the design and integrity of the well.
This information sheet provides an overview of the design and regulations that ensure well integrity.
How is the well designed to maintain integrity?
The goal of well design is to:
- make sure the gas (or oil) is safely held inside the well to allow extraction to the surface
- contain and transport hydraulic fracturing fluids injected into the well, flow-back fluid and produced water
- keep water out of a well
Several layers of steel and cement create a protective barrier between the well and the various rock formations. The intention is to isolate the interior of the well from the outside environment so that communication of fluids, either from inside to outside or outside to inside, does not occur.
A series of small perforations (holes) are then created through the steel casing and cement to create a path for gas to flow from the gas bearing rock into the well.
Maintaining well integrity also assists in preventing well ‘blowouts’. Blowouts, although rare involve a sudden and uncontrolled escape of fluids and can occur above or below the surface.
Western Australia applies international standards for well design and integrity. The recommended onshore petroleum casing design includes:
- Conductor casing: prevents loose surface sediment from collapsing into the well and protects shallow surface aquifers. This casing is approximately 50m deep and is cemented to the surface.
- Surface casing: key purpose is to protect groundwater. It is set below the potable aquifers and is where the blowout preventer is installed on the well. It is usually set at 800m deep and is cemented to the surface.
- Intermediate casing: this is optional and is usually used for deeper wells to manage hole conditions when drilling to the target formation. Cementing procedures must meet international standards.
- Production casing or liner: this is the final casing set for a production well. Casings run from total depth to the surface and liners run from total depth to an appropriate overlap inside the previous casing. Cementing procedures must meet international standards.
- Production tubing: commonly installed inside the casing to act as a conduit during oil or gas production.
How is the integrity of the well tested and monitored?
A number of safeguards and measures are put in place to ensure there are no uncontrolled leaks or loss of control in a well. The Department of Mines, Industry Regulation and Safety (DMIRS) requires all completed wells to be tested to ensure cement is sealed and bonded correctly. To ensure well integrity during production life, wells are also pressure tested far in excess of operating pressures.
Pressure testing makes sure the cementing and casing can take the pressures involved in activities such as hydraulic fracture stimulation. Companies are also required to send a probe down the well to see through the sides of the well, similar to an x-ray, and examine if the cement at the sides of the well has properly bonded to the rock and steel casing. This process is called cement bond logging.
Petroleum companies must undertake real-time monitoring of pressures and drilling during operations, and report these to DMIRS daily. DMIRS engineers review these reports and conduct site audits and inspections to ensure activities are conducted in accordance with the approved plans and that best practice management is being implemented. In addition, all petroleum operators in WA must self-audit their activities to ensure environmental impacts and, risks are managed appropriately and are continuously reduced as low as reasonably practicable’.
What processes are in place to regulate well integrity?
Prior to conducting any drilling activities companies must submit an application to drill a petroleum well for assessment to DMP. The application must include the drilling program which details the geological prognosis and the method the company intends to use to drill the well, an Environment Plan and a Safety Management Plan. The basis of design is examined by DMP before any approval can be granted.
Any further activity or change to previously approved activities during the life of the well must be approved by DMP. This includes hydraulic stimulation or other intervention in the well. For each activity a separate program will be submitted for assessment.
Before a well can be decommissioned, the operator must submit to DMP a decommissioning plan detailing the intended procedure to close the well, including cement and mechanical plugs, removal of the wellhead and any other surface facilities.
As with all petroleum operations, approval is only given if it is demonstrated that the program is in accordance with government regulations and industry best practice, standards and codes.
The well is then sealed with a series of cement and/or mechanical plugs, decommissioned and the site is rehabilitated.
Where can I find more information?
Read other Petroleum information sheets on related subjects such as hydraulic fracture stimulation, chemical use, and water use and management.