Damage Tendency of HCl-Based Solutions During Hematite-Based Filter-Cake Removal in Carbonate Formations

The removal of hematite-based filter cakes in carbonate reservoirs is essential for restoring well productivity in openhole completions. However, secondary damage caused by the precipitation of iron-bearing and calcium-based solids during the removal process can significantly reduce formation permeability, leading to long-term productivity loss. This study presents a comparative and mechanistic evaluation of three removal fluids commonly proposed for hematite-based filter-cake removal: (1) hydrochloric acid (HCl), (2) a mixed system of HCl and oxalic acid, and (3) a combination of HCl and ferrous chloride. Each system was evaluated for both filter-cake dissolution efficiency and secondary damage potential in carbonate formations by tracking the formation and transformation of precipitates generated during fluid-rock interactions. A solubility study was conducted to validate the initial dissolution performance of each fluid, followed by inductively coupled plasma (ICP) analysis to monitor changes in iron and calcium concentrations during carbonate disso-lution. Precipitates formed at the end of each reaction were collected and analyzed using X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) to assess their mineralogy, composition, and morphology. The results confirmed distinct phase formation pathways across the three systems. These include akaganeite (β-FeOOH·Cl) in the HCl-only system, whewellite (calcium oxalate monohydrate) along with lepidocrocite and goethite in the oxalic acid system, and progressive transformation from akaganeite to magnetite (Fe₃O₄) and siderite (FeCO₃) in the ferrous chloride system. Among the three, the HCl-only solution demonstrated the lowest precipitation tendency, forming a single solid phase only after extensive reaction with carbonate, whereas the oxalic acid system produced the highest precipitate mass and phase diversity. The study reveals the need for tailored strategies that minimize secondary damage by bal-ancing effective filter-cake dissolution with controlled fluid-rock interactions to reduce formation plugging risks in carbonate reservoirs. Recommendations for future optimization—such as staged fluid designs and post-treatment chelation—are also discussed.