The Macondo blowout highlighted a need for improved operational management and a rigorous approach to offshore oil and gas well engineering. Four years on, these lessons do not appear to be filtering through to the growing well abandonment sector argues Douglas Nunn. A lax approach to cementation and a preference for cheaper, non-rig abandonment solutions all increase the likelihood of failed well abandonments.
The North Sea is only beginning to get to grips with the financial and technical issues around offshore well abandonment. That is partly because, relative to more mature offshore basins such as the Gulf of Mexico, only a small proportion of North Sea production wells have been abandoned to date. It is also the case that well abandonment is something of a ‘Cinderella’ discipline in well engineering circles, often a low priority in the competition for funding and management resources.
Offshore UK, however, the stakes are high for oil companies planning a well abandonment campaign. Key legislation –- The Offshore Installations and Wells (Design and Construction, etc.) Regulations 1996 – DCR for short – imposes a legal duty on well operators to maintain well integrity throughout the lifecycle of a well, from drilling to abandonment. In the context of well abandonment, well integrity means the permanent isolation of any flow-capable formations both from surface and from each other.
If the well operator gets this wrong, results may include water flows, gas or oil seeps from the seabed, or underground crossflow between formations.
For an illustration of the potential reputation damage of well integrity failures we need look no further than the infamous ‘Gaslands’ video showing a homeowner setting fire to methane entrained in his domestic water supply. In a pointer to a key well abandonment issue, it is of note that such leakage of gas into shallow aquifers is very unlikely to be associated with deep reservoir fracturing; it is far more likely to result from ineffective cementation of surface casings. Major blowouts are extreme examples of such well integrity failures, with the 2009 Montara and 2010 Macondo blowouts both involving failed cementations.
To understand best practise in well abandonment, first consider some basic concepts. The objective of well abandonment is to permanently seal in place formation fluids from several zones. Not just the primary reservoir at the bottom of the well but also any porous, permeable zone throughout the overburden.
Permanent means forever, thousands or millions of years. The phrase used in the Oil & Gas UK well abandonment guidelines is “restoring the caprock” – which means we have to look at the long-term survivability of sealing mechanisms.
The well is not a single hole in the ground, it is more akin to a telescope with up to seven or eight concentric casings, typically ranging from 30 in. in diameter down to perhaps four and a half inches.
When the well is drilled, most of the pressure-containing envelope comprises the steel casings, with cement acting as a backup seal.
With notable exceptions, such as the 2012 North Sea Total Elgin blowout, casing will normally remain intact, barring minor leaks, for the economic life of the well and to the point where the well is abandoned. For the long term, however, we must anticipate that steel well casings will, through corrosion or oxidation, lose their pressure integrity.
The only permanent barrier
The industry has long acknowledged that the only permanent barrier to formation fluid flow is properly-constituted oilwell cement. Achieving annular isolation by cementing between strings of casing is more difficult than might be expected. Good cementation requires:Removal of mud and solids in the annulus.Removal of oily fluids, water-wetting of oil-wet surfaces (cement is water based). Placing good quality cement in the narrow annulus.
These criteria can only be achieved if the casing strings are properly centralised. No wells are vertical. When installed, well casings are often only centralised near the bottom. Uncentralised casings lie on the low side, creating an eccentric annulus and a favoured flow path on the high side for any fluid flow between them. Hence there is a significant probability that even large volumes of cement pumped into an eccentric annulus will not achieve a pressure seal.
The rigorous solution in most cases is to remove the inner string of casing by cutting it as deep as possible and retrieving the pipe above the cut, then setting a cement plug in the void remaining – a technique described as cut & pull’. Cut & pull typically requires a drilling rig whereas the alternative ‘Shoot & squeeze’ method can be done from a floating vessel or platform-mounted well intervention package. Such methods are attractive because they are cheaper and faster than mobilising a drilling rig.
Currently, the well abandonment market appears to be skewed toward cheap and fast. Specialist well abandonment companies tend to focus on non-rig well abandonment while rig owners aim their sales efforts more towards drilling new wells at premium dayrates.
From a drilling engineer’s perspective, the mechanical limitations of rigless well abandonments mean that more rigorous methods such as ‘cut & pull’ are simply not possible.
It is a paradox of well abandonment that the most difficult parts of wells to properly abandon are often non-commercial water or gas sands behind two or more casing strings in the overburden rather than the ‘pay’ or reservoir sections at the bottom of the well. The 2013 Elgin blowout is an example of a gas release from such a zone.
Nonetheless it is imperative that such zones are properly isolated during the abandonment process – failing which, any future release of well fluids is classified as a well integrity failure – and thus a violation of the operator’s legal duty under DCR.
In conclusion I would ask that well operators carefully consider their legal reponsiblities under DCR when planning well abandonment campaigns, and err on the side of caution – that is to say, invest in high quality well engineering and, if necessary, spend more to adopt rigorous techniques for permanent abandonment of all flow-capable subsurface zones. Data is scarce on the incidence of failed offshore well abandonments but the stakes are high in terms of potential environmental damage, corporate and industry reputation.
A classic case of not spoiling the ship for a ha’porth of tar...
About the Author
A chartered mechanical engineer with 32 years’ experience in offshore drilling, Douglas Nunn is managing director of well management company Fraser Offshore Ltd., a subsidiary of Norwegian well management group Acona.
Douglas will be giving an in- depth presentation about How to Make Teams More Effective Throughout the Well Lifecycle at the 2nd Annual Well Integrity & Abandonment Conference, June 3-4, 2014. More than 200 senior well integrity and abandonment executives will be gathering at the Aberdeen Ardoe House Hotel for the largest meeting of its kind. For a detailed technical program and expert speakers list, visit www.decomworld.com/wi.
Adapted by David Bizley
Read the article online at: https://www.oilfieldtechnology.com/drilling-and-production/18032014/macondo_lessons_for_well_abandonment/