Task 3.5: Well integrity
Overall objective: The main objective of the task is to improve CO2 storage safety and cost-efficiency through developing a more detailed understanding of how, why, where and when leaks develop in/along wells penetrating CO2 storage reservoirs. A special focus will be on CO2 injection wells and abandoned exploration wells.
Well integrity and its importance for CCS
Wells have in numerous scientific studies been denoted the «weak link» of safe and cost-efficient CO2 storage. Whether they are active or abandoned, these wells are all man-made intrusions into the storage reservoir – and their sealing abilities depend on engineering materials like steel and cement. Maintaining well integrity means to prevent leakage in/along wells. This is important throughout the well’s life cycle, from drilling to plugging and abandonment.
- Read more about the importance of Well Integrity for safe CCS in the BIGCCS blog.
Contribution to the overall objectives of BIGCCS
The research results generated in the present task can potentially lead to innovations important for various players in the CCS market, especially operators and service companies. It can lead to development of tailored materials, fluids and strategies for safer and more cost-efficient CO2 well construction, operation, remediation and abandonment. Other potential developments are new ways to secure the integrity of an already plugged exploration well, or methods to transform existing petroleum wells into CO2 injection wells.
Task 3.5 aims to investigate well integrity issues occuring during the whole life-cycle of a well, from the early well construction phase to the time when the well will be permanently plugged and abandoned (P&A’ed). A schematic overview of the research topics we are focusing on is given in the figure below, and the word «sealant» is here a more generic replacement for cement (which is the most common annular sealant and plugging material).
In summary, Task 3.5 aims to investigate:
- Sealant placement: How can a given sealant best be placed in wells to ensure an even annular seal and to avoid mud channels or pockets?
- Sealant bonding: How can we optimize the bonding of a given sealant to its surroundings in a well (rock/steel)?
- Sealant solidification: How can we ensure that leakage paths do not form during sealant solidifcation (e.g. as a result of shrinkage or the development of weak zones near solid walls)?
- CO2 -sealant interactions: How are various selant materials (both annular sealants and plugging materials) affected by exposure to CO2? Can we optimize sealants (structurally or chemically) for use in CO2 wells? Could research-based advice be given on which materials to construct and plug wells with in a large-scale CCS scenario?
- Thermal cycling: How is CO2 well integrity affected by the temperature variations that must be expected in the well during CO2 injection? Do leakage paths appear (e.g. as a result of sealant de-bonding or cracking), and if they do – how can we best predict, prevent and remediate them? Can we determine «safe temperature windows» for a given well type?
- Leakage prediction: Can we estimate leakage rates and consequences of the well integrity failure modes that are observed in the activities above? Can these results be upscaled from laboratory to well scale?
Task 3.5 achievements thus far
Sealant placement simulations
A numerical code has been developed to study sealant placement in wells. This has been used to investigate how cement flows and displaces mud in a well. The effect of throughgoing break-outs and casing eccentricity on cementing quality was studied. All this was presented at the 8th Trondheim CCS conference (TCCS-8) in June 2015.
Using micro-CT, we have visualized and quantified how tightly well cement bonds to various rock formations. The impact of different drilling muds has also been investigated. The work indicates that fluid choices made already during drilling of a well will affect its long-term well integrity. New information has also been provided on the size, three dimensional (3D) geometry and density of interface leakage paths (or «microannuli»). This is important for remediating them efficiently. The experimental methodology is illustrated in the figure below, and ongoing experiments focus on flowing CO2 through the samples.
The interface transition zone (ITZ) is a weak zone forming in cement when it solidifies near solid walls. The defect has long been known to exist in construction engineering. Despite its importance for leakage along active and abandoned wells, this topic has not been studied in detail with respect to well cementing. Using detailed characterization methods, like scanning electron microscopy, in combination with physics-based calculations, we have given an overview of the importance of ITZ for CO2 wells in the journal Construction and Building Materials. We have also been granted funding for a project entited «Closing the gaps in CO2 well plugging», which will study sealant solidifcation in greater detail. This will focus not only on cement, but also on other materials that are candidates of replacing cement as a well plugging material. Some of the first work performed in this new researcher project was presented at TCCS-8. The plans in the new researcher project are also outlined in this poster prepared for CLIMIT Summit 2015.
American researchers have recently observed that CO2 exposure can lead to self-healing of cracks in/along cement. This is because the volume of cement increases upon carbonation, and it indicates that CO2 is an efficient well remediation fluid. We have performed some experiments building on this important observation, and this work was presented at TCCS-8.
We are also using high-resolution micro-CT combined with the Avizo software to determine how CO2 reacts with cement and rock when drilling mud is present on/in these materials. This is important in order to optimize mud choices for prolonging CO2 well integrity. A poster was prepared on this topic and presented at the TCCS-8 conference in Trondheim in June 2015.
This work involves using medical CT data from real downscaled well sections (rock, cement and casing) in ABAQUS. We have found that defects in the annular cement sheath dictate the robustness of a well. The more defects – the lower the resistance towards thermal cycling. This work was presented during the ARMA conference in June 2015, and published in a paper after the conference. A numerical model has also been developed for heat transfer in wells, and this was presented at the SPE Bergen One Day Seminar on April 22nd 2015, with a publication being released directly after the conference. This work explores how different annular sealant materials affect the temperature transfer in the well during CO2 injection. More information on the work performed on thermal cycling can be found in this poster prepared for CLIMIT Summit 2015.
We have established a computational roadmap for how to calculate flow through digitalized micro-CT data using the ABAQUS software. The methodology is illustrated in the figure below. The work has revealed that leakage rates through microannuli can be reduced by reducing the leak path area normal to flow and/or the permeability of the leak path. Simulations have been made to calculate the flow through microannuli along the shale-cement interface, and these calculations have been experimentally verified in the GEUS laboratory. A peer-reviewed publication has been submitted on this work in January 2015. Some of our work on CO2 leakage along interface microannuli will also be presented at the ARMA conference in June 2015, and published in a paper after the conference.
The work performed in this task is a continuation of work performed in BIGCCS SP3 in 2013 (in task 3.3), and it involves experimental cooperation between SINTEF Petroleum Research and GEUS. A KPN project entitled «Ensuring well integrity during CO2 injection» is administrated as a part of the task, and SINTEF Energy Research and Lawrence Livermore National Laboratories are contributing to this work. In January 2015 we will also start up a new researcher project entitled «Closing the gaps in CO2 well plugging». As for the KPN, this project is also administrated as a part of Task 3.5. The partners contributing to the researcher project (in addition to SINTEF Petroleum) are: SINTEF Materials and Chemistry, NTNU, Universite du Maine and Curistec.