With the advancement in green technology-based energy generation to achieve net-zero emissions, geothermal energy has been considered one of the options to reduce the carbon footprint from oil/gas production. In this paper, geothermal energy refers to energy generation that involves a high-pressure, high-temperature (HPHT) environment. Both geothermal energy generation and oil and gas production in such environments - which includes the use of steam-flooded wells and geothermal wells are always challenging as a result of operational issues caused by inorganic scale formation. Changes in pH, temperature, pressure, and other factors in such environments can significantly reduce the solubility limit for mineral ions, causing minerals to precipitate from the solution. Calcite (CaCO3), silica (amorphous and colloidal), and poorly crystallized metal silicates are commonly encountered mineral scales in geothermal applications when the system temperature is approximately 200 to 250°C. Most of the commonly used scale inhibitors become inefficient in such geothermal environments as a result of structural disintegration and functional modifications. The problems are more severe in brine that contains divalent metal ions such as calcium and iron at higher concentration levels. This paper discusses various scale inhibitor chemistries that were developed and evaluated for their tolerance and performance at geothermal conditions particularly up to 250°C. The inhibition effect of these chemicals on calcite formation was analyzed in dynamic and static systems. The thermal stabilities of the chemistries were assessed in neat form and formulated product form using Fourier transform infrared spectroscopy (FTIR) analysis, visual inspections, and inhibition performance evaluations through both dynamic and static inhibition tests. The degradation and inhibition efficiencies of the inhibitors were evaluated before and after thermal aging.