Foam systems for underbalanced and geothermal drilling: a critical review of stability challenges and research frontiers

Foam-based drilling fluids are increasingly applied in underbalanced drilling (UBD) and geothermal operations because they enhance cuttings transport, reduce fluid loss, and minimize formation damage, leading to improved drilling efficiency and well productivity. This review examines the variables that control foam stability and rheology under realistic downhole conditions and evaluates recent advances in formulation design, performance testing, and predictive modeling. The analysis consolidates findings from experimental, modeling, and field studies to establish how foam behavior is influenced by physicochemical and operational factors. It is found that high salinity, extreme pH, hydrocarbon influx, and rock–formation interactions are major contributors to foam destabilization, particularly under high-temperature and high-pressure conditions where chemical reactivity and liquid drainage dominate. Foam quality and gas type further govern viscosity, flow regime, and in situ stability, especially when gas–liquid ratios fluctuate during dynamic circulation. In geothermal drilling, conventional surfactant-based foams lose stability above about 120 °C, driving the development of thermally stable formulations incorporating smart surfactants, Janus particles, and crosslinked polymers to maintain performance at elevated temperature and salinity. Field observations confirm the potential of these systems to mitigate circulation losses and pressure variations in fractured and high-enthalpy formations, although formulation compatibility and recyclability remain challenges. Recent progress in machine learning has produced accurate predictive models for foam rheology, stability, and oil tolerance, but their application in UBD workflows is still limited. Overall, this review defines the key destabilization mechanisms, identifies unresolved challenges, and outlines research directions for developing more stable, efficient, and data-informed foam systems for complex UBD and geothermal drilling environments.