Review on using pH-sensitive microgels as enhanced oil recovery and water shutoff agents: Concepts, recent developments, and future challenges

Excessive water production is one of the main problems of mature reservoirs, which reduces hydrocarbon production and economic reservoir lifetime, and may pose severe environmental issues. Injection of pH-sensitive microgel is one of the most recently developed methods that can be used for both water shut-off and deep conformance control purposes. Deeper penetration in reservoirs, less operational cost because of less injection pressure, less mechanical degradation, low material price, and the reversibility of the swelling process by post-acid washing are among their significant advantages over other conventional gel treatment approaches. Despite all benefits, this gel treatment approach needs an acid pre-flushing step to decrease the pH of porous media prior to the main microgel injection, which increases operational costs. This review investigates the recent progress and challenges involved in acid pre-flushing and pH-sensitive microgel treatment discussing synthesis, characteristics, and mechanisms in both sandstone and carbonate rocks. Moreover, recently developed Darcy-scale models for acid pre-flushing and microgel transport through porous media are also comprehensively reviewed, while a perspective for pore-scale models is considered finally.