High-pressure and high-temperature (HPHT) drilling presents significant challenges to conventional drilling fluids due to extreme downhole environments that compromise rheology, suspension integrity, and filtration behavior. This review critically analyzes new developments in HPHT drilling fluid systems, highlighting the impact of advanced additives such as nanoparticles, copolymers, and nanogels on thermal stability, barite suspension, and long-term gel stability. Particular emphasis is placed on barite sag static and dynamic alike, evaluated by metrics such as sag factor (SF) and the viscometer sag shoe test (VSST), with special regard for how innovative copolymer-enriched formulations effectively counteract such problems. Additionally, the incorporation of artificial intelligence (AI) and machine learning (ML), specifically artificial neural networks (ANNs), is examined for their ability to forecast mud behavior under HPHT conditions, optimise additive dosages, and facilitate real-time decision-making. The review also addresses thermal expansion-caused increase in equivalent circulating density (ECD), which can surpass fracture gradients and threaten wellbore integrity. It investigates new control techniques such as thermo-responsive polymers and pressure-sensitive nanogels. Based on recent experimental findings, predictive models, and digital advancements, the article recognises critical gaps in knowledge regarding long-duration thermal aging and dynamic rheology adaptation. It provides future research directions such as explainable AI frameworks, digital twin modeling, and smart fluid design. This article is a one-stop read to realise intelligent, adaptive, and safe HPHT drilling fluid systems.
Singh et al. (Fri,) studied this question.