In the Netherlands over 1300 onshore wells have been abandoned and in the coming years this number will increase significantly, as will the number of abandoned wells offshore. The sites above these wells will increasingly being used or earmarked for urban development. This raises the need to determine the risks associated with these sites. Specifically, the project is aimed at obtaining a better understanding of the long-term behaviour and interaction of cement, steel, and rock in abandoned oil and gas wells, and how this influences the integrity of the abandoned well.
The project was contracted to EVO Energy Consulting, an oilfield consulting firm based in Texas, which specializes in drilling and completions of oil wells. The work was primarily carried out by Dr. Eric van Oort of EVO Energy Consulting, with Metarock Laboratories in Houston acting as a sub-contractor. This research started in the second half of 2020 and finished second half of 2021.
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In the Netherlands over 1300 onshore wells have been abandoned and in the coming years this number will increase significantly, as will the number of abandoned wells offshore. The sites above these wells will increasingly being used or earmarked for urban development. This raises the need to determine the risks associated with these sites. The older wells in the Netherlands were constructed from the 1940`s onwards and were built and abandoned using traditional steel casing and oilfield cements. The oldest abandoned wells are now nearly 80 years old. While industry assumes that the sealing capability of the casing, cement, and rock formation will not change over time, this assumption is not well founded. In some cases, leak paths were introduced during the construction or production phase; if not addressed during the abandonment phase, these may still exist in the abandoned well as micro-annuli. It is not well understood if these micro-annuli remain open, become larger, or close over time due to rock formation movement and/or mineral or petroleum deposits.
This research question attempts to address this issue. Specifically, the project is aimed at obtaining a better understanding of the long-term behaviour and interaction of cement, steel, and rock in abandoned oil and gas wells, and how this influences the integrity of the abandoned well.
FULL RESEARCH QUESTION
The work involved extensive reading of journal articles, books, and standards from the oil and gas industry, civil engineering, and environmental science, along with many government reports from various countries.
The final report addresses several specific research questions. The first stage of this project was aimed at providing a clear overview of the risk-critical elements and parameters that influence the long-term sealing capacity of boreholes. This goal was achieved, and the results are summarized in Chapter 2 of the final report. The next specific question to be addressed was “how does the permeability of oilfield cement change over a long period of time (e.g. 100 to 500 years) and how does the change affect its sealing effectiveness?” It was concluded that the permeability of the cement itself is sufficiently low (micro-Darcy range) so as to not permit appreciable fluid flow through intact cement. Cement permeability does not seem to increase over time spans of tens of years, although data for time spans of hundreds of years are not available. Question 3 was “How well is the steel casing, surrounded by cement, protected against corrosion? How is the corrosion rate affected by the change in permeability of the cement? Will the encased casing corrode in the long term? Can a corroded casing become a leak path?” The report concluded that steel casing is very well protected from corrosion for long periods of time when competent cement is well-bonded to the casing, but corrosion may occur if there is no cement or poor-quality cement in the annulus. Questions 4 and 5 related to the long-term behavior of the micro-annulus that may exist between the cement and the formation. As pointed out in the report, “a micro-annulus represents a high permeability flow path (k > 10 mD) which bypasses the low-permeability cement matrix, thereby becoming an effective conduit for upward movement of fluids and gases to surface.” Some techniques that have been developed to artificially squeeze the micro-annulus closed are reviewed in the report. It is also emphasized that mud filter-cake must be removed from the wellbore during cement displacement, to avoid the filter cake interfering with the cement bonding to the rock formation, leading to the creation of a micro-annulus. The last specific question related to the possible ability of plastic formations such as rock-salt, claystone, or shale to form an effective seal around the casing. The report concluded that it is now accepted that time-dependent deformation (“creep”) of such rocks can fill the annular space and form an effective pressure-tight barrier. However, it was pointed out that it is imperative that the casing be designed to handle the stresses that will occur when the formation squeezes onto the casing. Although the development of a risk assessment tool was not part of the work scope, the report does contain some general recommendations regarding well leakage risk assessment for onshore wells in the Netherlands.
It was recommended that a simple qualitative scorecard approach with traffic light indicators should be developed, following the approach taken by Watson & Bachu. The limitations of such an approach, particularly due to the lack of data, were also highlighted. It was also recommended that certain quantitative aspects of the risk assessment approaches developed by Heriot-Watt and DNV are worth exploring.
The KEM scientific expert panel evaluated the project.
The quality of the work performed in this project was very high. The literature review was extensive and comprehensive. The final draft report is very well written and effectively structured. Among the conclusions reached in this study are the following:
1. Cements that are typically used to plug abandoned wells have negligible permeability, and are not expected to deteriorate over relevant time scales.
2. Steel casing is also generally protected against corrosion by the cement, unless the cement is compromised by the existence of micro-annuli or other types of deterioration.
3. Micro-annuli may form due to cement shrinkage or from stress and thermal loads imposed during the lifetime of the well. These micro-annuli may serve as channels for the leakage of fluids to the surface. However, various chemical, biological, and physical methods exist to close up these annuli.
4. Rock formations such as salt or shale have the ability to creep and thereby close-up these micro-annuli.
A reflection (in Dutch) on the KEM-18 project results has been published on the SodM website. In October 2022, a follow up study has been published by the State Supervision of Mines on "De integriteit en nazorg van buiten gebruik gestelde olie- en gasputten in Nederland op land".