Iron Precipitation and Secondary Plugging During Acid Removal of Hematite-Based Filter Cakes

Effective removal of hematite-based filter cakes is crucial for restoring formation permeability in open-hole completions. While hydrochloric acid (HCl) is commonly used for filter cake dissolution in carbonate reservoirs, it can also lead to secondary damage through iron precipitation. This study investigates both the dissolution efficiency of hematite in 10 wt.% HCl and the potential for secondary formation damage during acid-rock interaction. A multi-stage experimental workflow was employed, including solubility testing, inductively coupled plasma (ICP) analysis during calcite addition, and core flooding simulations using Indiana limestone. Solubility results showed that HCl dissolved 45% of the hematite under dynamic conditions at 250°F. ICP analysis revealed significant iron precipitation as the acid became neutralized during reaction with calcite, particularly when pH exceeded 1.5. CT scan and permeability measurements confirmed that while acid soaking improved near-wellbore conditions with a 96% recovery in permeability after the primary damage, the far-zone experienced a 71% reduction in permeability compared to the original undamaged value due to secondary plugging. These results highlight the dual role of HCl in both mitigating and potentially redistributing damage, emphasizing the importance of managing acid volume, pH, and iron stability during filter cake removal treatments.