Ensuring well integrity during CO2 injection
Project period2014 – 2016
Wells have in numerous scientific publications been denoted the "weak link" of safe and cost-efficient CO2 Capture and Storage (CCS). Whether they are active or abandoned, these wells are all man made intrusions into the storage reservoir and their sealing abilities depend on degradable materials like steel and cement. To ensure that stored CO2 remains underground in a long-term perspective, it is necessary to advance current well technologies, procedures and materials.
As opposed to normal petroleum wells, wells penetrating CO2 storage sites can be exposed to very low temperatures. This can be the case e.g. during injection, or if a leak develops in the well. A prerequisite for maintaining well integrity during CCS is therefore to understand how wells can be constructed (tailor-built or modified) to withstand low temperatures. Especially the strong temperature variations are a concern, as it is well-known from the petroleum industry that thermal cycling can have detrimental effects on well integrity. Repeated heating and cooling of the well will cause materials to expand and contract, and especially the annular sealant material (typically cement) is likely to de-bond and crack radially. This will create leak paths for formation fluids. Few experimental studies have been performed on the effect of thermal cycling on well integrity so far, and no such studies have been performed in the temperature range relevant for CO2 injection wells.
To fill this knowledge gap, the current project aims to study, through numerical modelling and experiments, when, why, where and how well integrity is lost when a well is repeatedly cooled down and heated up, and how such detrimental downhole temperature cycles arise. The deliverables will be new knowledge about well integrity of CO2 injection wells, as well as specific recommendations on material selection, well design and operational parameters for optimal maintenance of well integrity in CO2 injection wells.
During the first 2.5 years of the project, focus has been on performing a series of experiments to investigate how cement adheres to steel and rock. This work has led to the development of a new method for measuring the tensile strength of cement interfaces, which will be presented at a conference during the summer. The goal is to understand how to ensure good cement bonding even in the presence of drilling fluids or CO2. We have also developed a method for simulating leakage through small channels in/along well cement. This can be used to predict how defects in cement affect long-term well integrity. A larger experimental campaign has also been run to study how wells withstand exposure towards low temperatures and strong temperature variations. The results of this work has been used to improve an open-source software for well integrity prediction developed by the international partner in the project, Lawrence Livermore National Laboratory. The good news is that both experiments and simulations indicate that wells are robust towards both low temperatures and strong temperature variations. If the rock and cement is saturated with water, fracturing will be a problem if the water in the pores is allowed to freeze (and thus expand).
In parallel with the experimental work, models of heat transfer and CO2 flow in wells have been developed and coupled. We have used this model to study how the thermal properties of well construction materials affect heat transfer in the well, and for simulating vertical flow of CO2 up along a well in a blow-out situation. The models are developed with input from experimental work, so they are in good agreement with measurements made in the laboratory.