Effect of increased propensity for non-aqueous carbon dioxide formation on wormholing in carbonate acidizing:An acid flooding studyOA

Acid–rock reactions in carbonate reservoirs generate CO_(2),whose phase behavior significantly influences wormhole propagation and,consequently,matrix acidizing efficiency.In this study,backpressure-controlled core flooding experiments were conducted on>99%calcite plugs using 1 wt%HCl at ambient temperature.Injection rate(1–8 mL/min)and backpressure(0.1–8.2 MPa)were systematically varied,and wormhole development was quantified through pore volume to breakthrough(PVBT),CT imaging,and a Wormhole Propagation Index(WPI).The results show that(1)Despite non-monotonic behavior observed in individual cores,likely attributable to rock heterogeneity,averaged trends consistently demonstrated enhanced wormhole efficiency with reduced backpressure.(2)Reduced backpressure generally promoted wormhole propagation,resulting in thicker,less-branched channels and a reduced pore volume to breakthrough.(3)The most significant reduction in PVBT(∼15%)was observed at 8 mL/min when backpressure decreased from 8.2 MPa to 2.7 MPa.The operational significance of this reduction,however,is contingent upon specific economic and operational constraints.(4)Optimum injection rates of approximately 4.4–5.6 mL/min were identified,reflecting a balance between reaction kinetics and transport as characterized by the Damköhler number.(5)Mechanistically,non-aqueous CO_(2)generated at low backpressure may redistribute flow,shield reactive surfaces,and enhance local mass transfer,thereby promoting the formation of thicker and more efficient wormholes.These results suggest that strategies favoring non-aqueous CO_(2)formation(e.g.,solubility-reducing additives,foam-acid systems,or chemical/thermal adjustments)could enhance wormholing efficiency in carbonate acidizing.However,extrapolation of these findings beyond the specific rocks and fluids tested necessitates further investigation.