Sustainable Geothermal Well Cements for Challenging Thermo-Mechanical Conditions

In order to meet the targets set in the Paris agreement and ensure the "green shift", fossil fuels need to be replaced by other sources of energy. An important contribution in this regard will be development of geothermal energy sources. The objective of this GEOTHERMICA project is to develop well cement materials that can withstand the harsh conditions that are found in high-temperature geothermal wells. BNL in the US has developed a Thermal Shock Resistant Cement that is suitable for such conditions. Together with TNO and SINTEF, will this cement material and other suitable candidates be tested and further developed. SINTEF, together with Equinor, will use their competence and experimental know-how within well integrity, developed for petroleum wells, to test these geothermal cements and ensure their suitability for challenging geothermal applications. An official kick-off, marking the startup of the project, was held virtual in 2021. Additionally, the team has been able to organize two workshops with the full project team. One 3 day workshop in the Netherlands hosted by TNO and one 3 day workshop in Norway hosted by Equinor and SINTEF. The project has led to enhanced cooperation between the research institutions, and a final joint publication, which will be published in 2024.

The project contributed in several ways to improving economics and reliability of geothermal energy. For HT geothermal energy production, the project demonstrated that currently used cementing solutions based on Portland cement chemistry are not sustainable in deep geothermal wells with geological gasses. Cement samples exposed for 3 and 9 months in Newberry geothermal well nearly completely carbonated in 3 months and lost their mechanical properties in 9 months. Cement formulations designed and tested in the frame of the project all improved their mechanical properties during the exposure tests. Cement formulations stable under both supercritical CO2 and water environments were designed and successfully tested under laboratory and well conditions in the frame of the project. Function tests on downscaled wellbore configurations with casing-cement-samples and results modeling confirmed better performance of all non-Portland cement designs compared to the performance of Portland-cement-based formulations. The model developed based on the parameters derived from the well-exposed samples proved to be relevant for function tests and can be potentially extended to predicting cement durability under conditions varying from lower (~100oC) to high temperatures (~350oC). The lower-temperature part of the project demonstrated significant improvements in cement performance with addition of economic polypropylene fibers under conditions applicable for low-enthalpy geothermal wells with moderate thermal shock cycling. The fibers eliminated cement cracking under environments with low humidity. The large-scale tests combined with numerical modeling showed high likely hood of tensile cracks in low-enthalpy geothermal wells under cyclic thermal shock conditions, but no significant permeability increase and minimal cement damage. Replacing faulty cement with durable solutions can result in tremendous savings on well repairs or re-drilling, especially costly for HT geothermal well. With the experimental work performed in laboratory and real geothermal well environments, the project clearly showed the failure of the currently used OPC/silica HT blend to survive conditions of deep geothermal wells. It offered alternative durable solutions in a wider range of geothermal well temperatures (100 – 400oC) and environments (dry well – below 100oC hydrothermal – supercritical water – supercritical CO2). By proposing durable solutions for geothermal wells, it offered reduced financial risks making investment in geo-energy more attractive. The work on field applications of the proposed solutions is ongoing.

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