Prior to retirement I was involved in global P&A and had the chance to evaluate different country standards. Also had the chance to review and input standards to countries that are still developing. Like Gordon mentioned, different countries have different standards. In the absence of existing regulation, as the company I was working for has global barrier standard - our standard will be followed. Our global standard is risk based and as a general guideline..on areas our standard is not very specific .. risk assessment is recommended.
Hope the above helps
Permanently abandoned offshore wells are generally not monitored. The challenge with these wells is generally that most of the time the wellhead has been cut below the seabed and finding the well and monitoring becomes complex.
If there are problems with abandoned offshore wells, they are generally discovered due to escaping gas or escaping oil which is spotted at surface or during some other survey.
Currently working on a leaking subsea abandoned well in Africa, where (luckily) the wellhead was left in place. Gas was discovered during an ROV survey of a nearby well. Re-entering an abandoned well and finding the origin of the leak source and isolating the zone is incredible complex and expensive.
Connecting a rig to an abandoned leaking well and safely drilling out cement plugs which potentially can have gas below them takes a great deal of planning. Once connected and re-entered, finding and isolating the producing source zone will be the next challenge. Sometimes these zones are thin gas producing sands that were often not even seen when the well was drilled. These zones can be leaking from annuli behind casing strings where direct access is not possible.
It is critically important that offshore well abandonment is done correctly. Re-entering an abandoned well to fix a leak will be very costly and there is no financial return on that investment. It is important that well abandonment is done right the first time.
A. For how long shall be monitored
It can be as low as 0 and as high as 10 years
Use the practical tools (risk-based analyse, in place local and international legislation)
Risk based analyse shall include:
- A record of issues during the life of the well (no significant issues might not require any monitoring to be carried out post-decommissioning)
- The presence of at least one operational well (the full suite of monitoring is still undertaken at the site and shall stop only when the last well at a site is decommissioned)
- A record of the post-decommissioning pressure test (if tests passed the likelihood is that an adequate seal has been achieved for short to medium period of time)
- Impact of post decommissioning activities on well integrity (new well accidentally drilled through a decommissioned well, the wellbore can be cut at a critical depth horizon due to natural or induced1 fault re-activation)
1 High pressure injected fluids into the formation
Fault reactivation assessments and micro-seismic monitoring should be considered best practice for shale gas wells
- Understanding the long-term performance of materials2 used to plug and abandon wells, particularly where there are hostile chemical environments (e.g. H2S and CO2), or in high temperature wells
2 Corrosion of steel well tubulars / joints and eventual chemical breakdown of the cement
It shall be considered that adverse environmental effects from these slow processes may not be seen for many decades or even hundreds of years.
Local environmental regulations might require regular monitoring for a specific period of time.
B. How often
I will follow also the risk assessment approach
I hope this is useful for you.