Investigating Primary and Secondary Formation Damage in Carbonate Reservoirs through Filter Cake Build-Up and Removal with HCl–Oxalic Acid Solutions

Formation damage during drilling is a major concern. While filter cake deposition protects against fluid invasion, it also reduces near-wellbore permeability. Removal fluids may also trigger secondary damage through precipitation reactions. This study evaluates the primary damage from hematite-based filter cake build-up and the secondary damage associated with removal using an HCl–oxalic acid solution, with emphasis on comparing squeeze versus flowback scenarios. Indiana limestone core plugs were subjected to hematite-weighted mud to build the filter cake. Primary damage was characterized through permeability and resistivity measurements, CT scanning, and mechanical property testing. A 6 wt% HCl + 10 wt% oxalic acid solution saturated with hematite was then injected at half pore volume and soaked. Two modes were tested: (i) squeeze injection into the core and (ii) flowback from the opposite direction. After treatment, each core was sectioned into near- and far-zone halves and re-characterized using the same techniques, supplemented with scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) analysis. Primary damage was confined to the near-wellbore, causing up to one order-of-magnitude permeability reduction, decline in electrical resistivity due to hematite conductivity, and increased rock stiffness. In the squeeze scenario, secondary precipitates migrated deeper, severely impairing far-zone permeability, increasing resistivity, and altering elastic properties. Near-zone wormholes created by acid were plugged, further reducing permeability. In contrast, the flowback scenario limited precipitate invasion, preserved far-zone permeability, and reduced near-zone impairment. Flowback cores maintained higher permeability, exhibited lower resistivity, and showed less stiffening than squeeze. This work quantifies primary and secondary damage mechanisms in carbonates and demonstrates both the risks of HCl–oxalic acid systems and the operational advantage of flowback over squeeze.