Study on acid rock dissolution mechanism of low-permeability sandstone reservoirs based on dynamic and static experiments and nuclear magnetic resonance technology

It is crucial for optimizing acid fluid selection and enhancing acidification effect to investigate the acid rock dissolution mechanism of low-permeability reservoirs and clarify the impact of such factors as shale content and acid fluid concentration on dissolution efficiency. Combining two experimental approaches—static dissolution (mineral rock plate at 20 ~ 60 ℃) and dynamic flow (with acid fluid concentration 1% ~ 5%, core permeability (0.1 ~ 1) × 10⁻³ μm², and shale content 8% ~ 44%), the dissolution laws of hydrochloric acid, mud acid, and acetic acid were analyzed under various influencing factors. In addition, nuclear magnetic resonance online monitoring technology was introduced to evaluate the changes in pore configuration before and after acid displacement. The results show that quartz exhibits strong acid resistance(dissolution rate < 3% after 24 hours), while montmorillonite is easily dissolved (dissolution rate of 54.46% after 24 hours). As the temperature rises, the dissolution rate gradually increases. Increases in rock permeability, acid fluid concentration, and shale content all contribute to enhanced acid dissolution. Therefore, high-permeability cores present better acidizing effect than low-permeability cores. After displacement with 5% mud acid, the optimal dissolution effect is achieved, core porosity is increased by 14.39% and permeability improved by 76.30%. Overall, clay minerals (particularly montmorillonite) are key factors influencing acidizing efficiency. The use of 5% mud acid is recommended for acidizing low-permeability sandstone reservoirs with shale content ≥20%. The application of nuclear magnetic resonance-mercury penetration coupling technique improves the accuracy of pore configuration description by over 10%.