Review of key technologies for ultra-deep carbonate reservoir development: Acid fracturing and challenges

As oil and gas exploration moves into deeper and ultra-deep carbonate reservoirs, ultra-high temperature and high pressure (UHTHP) conditions pose serious challenges for acid fracturing. However, a comprehensive review framework covering the full acid fracturing process under UHTHP conditions is still lacking. This review summarizes recent progress in acid fracturing under UHTHP environments. First, it examines acid-rock reaction kinetics, focusing on acid rheology under UHTHP and its impact on reaction rates, while comparing retardation mechanisms across different acid systems. Second, it reviews the effects of acid etching on the mechanical weakening of carbonate rocks and discusses constitutive models based on damage mechanics. Fracture conductivity prediction methods are also evaluated, with emphasis on their current limitations. Third, the review highlights the role of multi-stage alternating acid fracturing in enhancing non-uniform etching and improving fracture conductivity, especially under high closure stress. Finally, it outlines key research gaps in reaction modeling, mechanical degradation, and acid fracturing optimization under UHTHP conditions. The findings indicate that current kinetic models and conductivity prediction methods are poorly applicable to UHTHP environments, and that acid-induced weakening mechanisms require further investigation. This work provides theoretical insight and practical guidance for the design and optimization of acid fracturing in ultra-deep carbonate reservoirs.