TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA large-scale acetic acid-based stimulation treatment was developed to remove drilling mud filter cake in vertical wells in a newly developed carbonate reservoir. The wellbore stability of this weakly consolidated carbonate formation can easily be reduced by contact with HCl-based acids. Extensive lab testing had indicated that the formation is mechanically weak, becomes brittle upon contact with acids and produces large amounts of fine particles that can cause severe damage. Furthermore, a chloride-free acid formula was required to minimize interference with the pulsed-neutron logs (PNL). A special acid treatment was needed to remove the damage while maintaining the integrity of the formation.Several tests were conducted in the laboratory to evaluate both regular and emulsified acetic acid and non-acid formulae (mainly EDTA). Initially, two acid formulae were selected for field application. The first used 10 wt% acetic acid in an emulsified form, while the second formula was based on 10 wt% regular acetic acid in combination with additives. Further lab and field-testing indicated that the emulsified form might pose a problem in the presence of fine inter-matrix dolomite particles. Therefore, the treatment performed in the field focused on the use of 10 wt% acetic acid with several waterwetting agents (mainly surfactants and mutual solvent).The acetic acid treatment was applied in five vertical wells in this field using a coiled tubing unit. Pressure build-up tests after the treatment indicated that the acid was successful in removing the filter cake in all cases. Neither fine particles, nor any type of emulsion was noted in the well flowback samples. However, well productivity after the treatment was found to be a strong function of the lithofacies of the formation (e.g., chalk versus packstone). Wells with high reservoir quality rock (i.e., packstones with a permeability of 20-30 mD), demonstrated excellent pressure response after the treatment. Flowing wellhead pressures were increased significantly. Wells with low reservoir quality showed a moderate increase following the treatment.