Ozioma Carol Uwakwe

Geothermal energy exploitation has emerged as a critical solution to combat global climate crises, such as reducing CO₂ emissions and climate warming. Scaling is the process of mineral precipitation in fluid pathways and geothermal equipment. It is known to significantly hamper geothermal energy production by decreasing the rates of heat extraction. Numerous research efforts are dedicated to characterising dissolution and precipitation processes, not only to provide know-how for further and safer developments in geothermal energy, but also to adapt such findings to the ever emerging field of geothermal energy recovery. This paper presents an overview of experiments—performed under variable pressure and temperature conditions—with a focus on scaling. We assess the different factors that influence disequilibrium reactions in carbonate rocks, the different experimental setups, and their application to the field. The influence of experimental variables (such as temperature and pressure) on mineral dissolution and precipitation is discussed, and the main learning points from experiments are compared and contrasted. We address techniques for preventing and controlling scaling in geothermal wells based on a comprehensive analysis of experimental studies in carbonate rocks. We propose that the intelligent combination of fieldwork, numerical approaches, and laboratory experience provides a foundation for the success of future work in this field.

Cryogenic cave carbonates (CCCs) are speleothems precipitated from freezing water bodies in caves. A series of scholarly papers have explored natural CCC precipitation kinetics, crystal habit, mineralogy, crystallography and geochemistry. However, cave depositional environments are complex multi-parameter systems, and the inherent difficulty of unravelling the physicochemical parameters of ancient speleothem formation also applies to CCCs. In order to test previous models and concepts, experiments were conducted by cooling calcium-rich bicarbonate water under controlled laboratory conditions. The experiments involved calcite precipitation at different durations (6 to 91 days) and temperatures (1, +0, -0.5, -0.7, -1 and -2 °C). The results show that subtle changes in temperature and calcite saturation index (SIcc) remarkably affect the CCC morphological variability and mineralogy. Rhombohedral calcite crystals precipitated both in experiments with and without ice. In contrast, spherulitic crystals (vaterite) precipitated from near-completely frozen waters with high calcite saturation indices. Experimental water monitoring revealed increased calcite precipitation followed by decreased water conductivity values. The formation of CCCs is accompanied by non-equilibrium reaction kinetics, which tends to enrich the isotopic composition of the parental water. Longer experimental durations lead to increasing CO2 degassing and 13C enrichment of the water. Data from the first experimental work dealing with cryogenic carbonate precipitation and isotope geochemistry, as documented here, shed light on the origin of these peculiar speleothems and provides constraints to interpret morphologies and isotopic compositions of ancient CCCs.