Integrated Workflow with Mechanical and Chemical Solutions for Offshore UAE Coiled Tubing Strontium Sulfate Scale Removal

Abstract Sulfate scale formation, caused by the mixing of sulfate-rich seawater with barium or strontium-rich formation water, hinders oil and gas production by reducing flow rates and damaging equipment. Removing sulfate scales from wells is challenging because of their hardness, chemical resistance, strong adherence, and the operational and environmental complexities involved. Additionally, handling them poses safety concerns as they are classified as technologically enhanced naturally occurring radioactive materials (TENORM), requiring strict safety protocols. The primary objective of this project was to remove challenging hard scale for an operator in the UAE offshore oilfields, which impaired production and accessibility for well intervention using coiled tubing (CT) milling solutions and innovative chemistry. The operator faced serious issues with strontium sulfate (SrSO4) accumulation on offshore locations that obstructed production flow, necessitating frequent and costly hydraulic workover unit (HWU) operations for full completion replacement. A scale removal workflow was developed using a 2.75-in. and 2.50-in. outside diameter (OD) diamond mill bit in conjunction with chelating sulfate solvents to address the hard SrSO4 scale. Diamond mill bits, chosen for their superior cutting efficiency and wear resistance, were used alongside a chelating agent (CA) identified through laboratory tests as the most effective dissolver, achieving 100% scale dissolution within a period of 24 hours. Periodic soaking with the CA softened the scale, enhancing the rate of penetration (ROP) from 2 to 5 ft/hr to an average of 20 ft/hr. This approach enabled the removal of SrSO4 scale in two wells, successfully restoring the original well production by and establishing a new precedent for offshore rigless interventions.