Additive and alternative materials to cement for well plugging and abandonment: A state-of-the-art review

Data from Rystad Energy shows that, till 2020, the US Gulf of Mexico, the largest oil and gas production field, has a total of 34.4 thousand drilled wells with only 44% have been plugged and abandoned and 19.2 thousand wells are pending to be plugged in the future. Thereinto, 71% of the inventory is more than 40 years old. The US Environmental Protection Agency reported that in 2019, the annual CH 4 emission from these unplugged wells reached 209 Mt and the leakage of fluids could destroy the environment and threaten human health eventually. After the implementation of idle iron policy which requires titleholders to decommission the abandoned wells within specific periods, more wells are urgently to be plugged and abandoned. Cement or cementitious materials as the commonly used materials for well decommissioning serving as physical plugs, always suffer shrinkage, poor resistance to corrosive environment, instability under harsh environments, etc., which will initiate or accelerate fluid leakage. Furthermore, since the well conditions vary largely, the requirements or benchmarks of plugging materials may be completely different, this paper therefore aims to summarise and update the findings of past research and review papers, and list the regulations for well cementing as well as commonly used detection methods, identify available additives of ordinary cement that can modify the mechanical, physical, and chemical properties of cement plugs under specific well conditions, enumerate the potential alternative materials that can replace cement plugs, and eventually help the academia or industry to be familiar with relevant regulations or standards, material requirements, and associated detection technologies, recognise the available possibilities and determine the preferred ones easily. Findings of this paper indicate additives such as pozzolans, fibres, self-healing additives, and nanoparticles can significantly improve the mechanical properties, physical properties, chemical inertness, and durability. Alternatives including bentonite, bismuth, modified in-situ material, geopolymers, resin, and slags also exhibit adequate performance as cement replacement but require more field attempts to further assess their feasibilities.