Drilling horizontal and highly permeable sandstone acid-sensitive reservoirs with oil based drilling fluids are normally followed by filter cake and associated organic sludge removal treatments. The acid or cleaning recipes should be compatible with the formation minerals, especially when losses are encountered. The objectives of this paper were to conduct a comprehensive evaluation of HCl/formic acid recipe to dissolve oil-based filter cake, characterize and dissolve associated organic sludges, and assess compatibility with highly permeable acid-sensitive sandstone core plugs.

Filter press experiments were conducted to optimize the fluid recipe. Core flood testing was conducted on sandstone core plugs at 160°F. Compatibility with reservoir fluids were assessed using aging cells. TGA was used to identify organic/inorganic composition of sludge samples XRD and ESEM were used to characterize core plugs and sludge samples. ICP analysis was conducted to analyses effluent from coreflood experiments. GC and GC-MS analysis was conducted to identify and characterize sludge samples. Micro CT scan was used to assess the dissolution of rock minerals.

The removal efficiencies of the oil-based filter cake were between 85-100% by weight using HCl/Formic acid recipe. The characterization of the sludge samples revealed the presence of mainly diesel. The inorganic compounds (50% by weight) were mainly quartz with small amounts of calcite, dolomite, kaolinite, microcline, and pyrite. Maximum solubility of nearly 60 wt% was achieved. Core flooding tests of the acid recipe indicated reduction in permeability of core plug. The coreflood effluent analysis indicated dissolution of mainly Ca, Fe, and Mg with small amounts of Al, Si, and Sr with indication of Si-based precipitation. No major indication of precipitation occured. ESEM and EDS spot analysis of the core plug particles showed the sample was comprised Si, O, Fe, S as the main constituents with small amounts of Al. XRD analysis of the core plug after coreflood testing showed the presence of mainly Quartz and small amounts of Microcline, Pyrite, and Palygorskite. The CT scan of core plug before/after coreflooding indicated the acid dissolved rock minerals. There was no clear indication of core damage or solids plugging.

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