Aluminum Scaling and Formation Damage due to Regular Mud Acid Treatment

Abstract

Abstract Accurate formation damage identification significantly improves the success of a remedial stimulation treatment. In an oil-producing well in Central Saudi Arabia, the electrical submersible pump (ESP) used to lift the oil from the well was damaged and needed to be replaced. Production engineers observed a deposit on the impeller of the pump. The deposit was also suspected of plugging the formation and significantly reducing oil production. Laboratory analyses of the scale deposit by XRD, XRF, and SEM revealed an amorphous material containing a high concentration of aluminum. After extensive examination, it was determined that the only possible source of aluminum was a previous HF stimulation. Examination of the well history showed that a typical 12% HCl-3% HF mud acid stimulation treatment was conducted during the original drilling and testing of the well. 9F NMR spectroscopy indicated that the deposit contained F/Al molar ratio of 0.4, consistent with precipitation from completely spent HF fluids. Solubility showed that 70 to 85 wt% of the deposit could be dissolved with 15 wt% HCl, if it could be contacted. A retarded HF acid based on aluminum chloride was designed to remove the deposit and improve well productivity. A wellbore clean-out with 10 wt% HCl was conducted to remove very near wellbore deposits. In addition, a large HCl preflush, twice the volume of the HF stage, was pumped before the retarded HF to remove deposits within the formation. All acid fluids contained 2–5 wt% acetic acid and 50 lbs/Mgal of citric acid to prevent precipitation of aluminum and iron. Special efforts were made to recover spent acid fluids. Return fluid samples were collected throughout the flowback. Complete fluid analyses are discussed along with their implications. Considerable acid spending was observed from dissolution of the aluminum deposits and carbonate minerals. The presence of carbonates suggests precipitation of aluminum within the formation matrix. The retarded HF treatment design successfully removed aluminum precipitation damage from the previous HF treatment. Production of the well stabilized at nearly twice the pre-stimulation levels with no further re-occurrence of the aluminum scaling problems. P. 539