Seashell waste as a complete metakaolin substitute in geothermal well cementing grouts: Mechanical and microstructural performance

The high-temperature, high-pressure (HTPT) conditions associated with geothermal wells necessitate the use of cement grouts that are lightweight, flexible, and possess excellent mechanical properties to bolster well integrity and zonal isolation. This study presents an experimental investigation to characterize the mechanical and micro-structural properties of an environmentally sustainable, lightweight, and resilient cement grout formulation composed of different types of finely ground seashell waste powders used as a 100% substitute for metakaolin, commonly employed in well cementing. The objective of this approach is to mitigate environmental impact and enhance the solid-state performance of geothermal well cement. The powders utilised were sourced from shell waste of Crassostrea gigas , Pinctada maxima , Pecten maximus , and Crepidula fornicata , used to develop diverse low-density slurries with approximately 1.2 specific gravity. The seashell powder-based slurries were subsequently compared to a traditional formulation containing metakaolin. In the absence of chemical additives, the findings demonstrated that complete replacement of metakaolin with seashell powder resulted in improved elastic and flexural properties, rendering the slurries suitable for supporting geothermal well steel casings through expansion cycles during thermal variations. At ambient curing conditions, the incorporation of seashell powder reduced the compressive strength of the slurries relative to the industrial reference mix. However, all formulations exceeded the minimum strength required for well casing support. The inclusion of shell powder also induced a slight increase in the thermal conductivity of the slurries under saturated conditions. Consequently, these lightweight formulations containing seashell powder have the potential to enhance zonal isolation in geothermal wells, offering environmentally friendly and cost-efficient benefits.