“All penetrated zones with flow potential that have been identified as requiring isolation should be isolated from each other and from surface or seabed, by a minimum of one permanent barrier, or two as appropriate” (Oil and Gas UK Guidelines for the abandonment of wells, Issue 5, July 2015).
Sounds simple; however, the abandonment philosophy of a particular well is as unique as its exploration phase. Factors such as age, lithology, infrastructure condition, location and production lifecycle are some factors that combine to provide the challenge.
Like everything in life, information is key. Wells less than ten years old tend to have good records and have been engineered with abandonment in mind, but those greater than ten years old often have limited information and well abandonment was not usually a consideration in their design.
Some of the challenges that Exceed, which specializes in well management and decommissioning, has experienced in abandonment operations to date include lack of, or no cement behind casings, geological anomalies, lack of tooling for older wells, limited well access, pressurized annuli, challenging budgets – to name but a few.
Let’s look at some of the technical challenges in a little more detail…
That low permeability seal withstood the test of time, the thickness and extent being sufficient to keep those hydrocarbons safely stored until we were ready to extract and use them. Now the hole we created must be plugged, in order to keep the remaining non-economically-producible oil and gas in the reservoir permanently.
Rock to rock is a description that immediately conveys the final requirements of the artificial seal; it must fill the wellbore, bonding with the caprock which was drilled through, and be of a sufficient length to ensure a competent seal.
The most commonly accepted plug material is good quality hard cement, which provides similar physical and chemical properties to the rock it is replacing. It should be impermeable, have a compressive strength comparable to the formation strength at the setting depth, be capable of withstanding both mechanical stresses and chemical conditions in the well, adhere to rock and metal (if required), will not shrink, and can be drilled out in case access is required. Cement, however, will typically be heavier than the fluids in place, such as drilling mud, and cement slurry will slump downwards and displace the mud upwards, leading to contamination, channeling and ultimately, poor sealing capabilities. To prevent this, a tested support is required, typically a mechanical plug set in the casing and tested.
Alternative materials such as resins and grouts are acceptable, as long as their properties are at least equivalent to good cement. Sealing materials used in packers and plugs (rubber, synthetic materials, etc.) will degrade over time and therefore cannot be considered as a permanent barrier for well abandonment.
A weighted fluid such as drilling mud left in place cannot be considered a barrier; it may degrade over time due to settlement of weighting material, influx, cross-flow or chemical processes.
Casing Fully In Place: Where well history or logging confirms that there is good cement in place behind the casing at the caprock depth, proper abandonment can be achieved by simply placing cement inside the casing at that depth. With a tested mechanical plug as a base, the cement should be load tested with a drill bit or mill once set, to prove compressive strength has been achieved. This will also confirm top of cement and hence plug length, which has to comply with legislative requirements.
Casing Partially In Place: Where well history or logging confirms that there is no cement in place behind the casing at the caprock depth, a technique known as ‘Perf, Wash, Cement’ (PWC) may be used to place cement behind the casing. The lack of cement may have been caused by, for example, losses during the original cement placement operation, or contamination of the annular cement which lies adjacent to the caprock. PWC involves perforating the casing using TCP guns, thereby providing a flow path through which to displace the material in place behind the casing. Specialized tools will be placed across the perforations, enabling mud to be forced through the perforations, and for old mud or contaminated cement to be displaced out. Once the area between casing and caprock has been suitably washed, cement will be pumped into place, both outside and inside the casing.
If this technique has to be qualified prior to approval by the operator or governmental department, the internal cement will be drilled out to allow a cement bond log to be run to confirm competent cement in the casing to caprock annulus. In this case, the internal cement will have to be replaced following successful logging. Once this technique is approved, future operations may not require the drill-out and log operation to be carried out again.
As in the previous technique, the internal cement will have to be tagged to confirm compressive strength development, top depth and cement plug length.
No Casing In Place: Where logging confirms that poor cement is in place behind the casing at the caprock depth, but is sufficient to prevent circulation and therefore preclude the use of PWC, section milling would be utilized. This requires a specialized tool to be run to depth, and when mud is pumped downhole a series of knives are activated, and the casing and any cement between that and the caprock is cut away by rotation of the knives. The knives will extend to a maximum such that they cut to the open hole diameter, thereby removing all nonnative material, except the milling mud. A section (hence the name) of casing/ cement of some 60m will typically be removed. Once this operation is completed, an open hole cement plug will be placed, again with support to prevent slumping. Typically, the height of the plug will be about 150m – a total of 60m across the milled section and 90m above – which allows for any contaminated cement to be above the milled depth.
An alternative to the above, mainly for shallow formations, is to cut and pull the casing above top of annulus cement, and place an open hole cement plug at the required depth.
Collapsed Formations: An alternative to cement occurs where a particular formation has collapsed onto the casing, for example in shales or swelling salts. Where these can be pressure tested, they may be approved as permanent abandonment barriers in place of cement.
Other Geological Considerations?
So what are the other geological troublemakers in the well abandonment phase?
Moving salts and other mobile formations can cause significant problems. Exceed have come across several decommissioning projects, mainly in the southern North Sea, where access to the well is not possible via a standard intervention deployment method, due to casings and therefore tubing being crushed by the adjacent formation. This can have a huge effect on the complexity and ultimately the cost of the abandonment, as the lower formation still has to be isolated.
In an ideal world, the barriers in a well would include one double cement barrier above the reservoir and one shallow, environmental double cement barrier. Areas of the well where communication or cross flow between the zones is possible will require additional plugs or barriers. Exceed have come across wells that could potentially require as many as five double cement plugs. This ultimately adds time and cost to the project.
This may all seem straightforward and one would assume an old well file can be opened and the information will simply fall into our laps. If only that were true! Wells are now being considered for abandonment where there is little more information than a hand-drawn sketch. The life cycle data for the well may have been lost through numerous asset transfers while engineers who have worked on the wells have disappeared from the industry.
A fresh set of geological eyes are needed on these assets, guiding the engineers on potential risks, geological issues and possible solutions. The ‘G’ is not at the end of decommissioning, but right at the very beginning. All the issues noted above should be considered at an early stage of planning to prevent any surprises during operations. Contingencies can be planned and engineered in plenty of time, ultimately minimizing the effect on budgets.