Subsea Scale Management in Pre-Salt Wells: Challenges and Solutions

Abstract This paper presents a case study on inorganic scale management in Brazil's carbonate pre-salt wells. The reservoir lies in water depths of around 2140 m and produces a light 26° API oil. Formation water is characterized by high levels of strontium and calcium. The main scaling risks are deposition of CaCO3, SrCO3 and FeS from the wellhead to the topside. Desulfated seawater is injected to prevent sulfate scale and souring of the reservoir. Effective inorganic scale management is crucial to maintain productivity and integrity; this study explores the challenges posed by this pre-salt field and presents solutions to mitigate and remediate subsea inorganic scale deposition, ensuring optimal performance and longevity of subsea production systems. During startups after prolonged shutdowns, significant pressure drops were observed, leading to high productivity loss and increased delta pressure in the tubing; this prompted a detailed study to restore productivity. Given that pre-salt wells are primarily carbonate formations with high pressure and high temperature conditions, inorganic scale precipitation was identified as a serious flow assurance issue. Operational conditions were evaluated relative to anti-scale optimum conditions, revealing that subsea fluid temperature and velocity exceeded the limits for conventional anti-scale agents. Operational conditions were evaluated relative to anti-scale optimum conditions, revealing that subsea fluid temperature and velocity exceeded the limits for conventional anti-scale agents. Consequently, a method to clean the inorganic scale and mitigate further precipitation was developed. Chemical cleaning with 15% HCl, a corrosion inhibitor and iron chelate, was selected for inorganic scale removal, followed by acid effluent flowback to the FPSO offspec tank, to restrict reprecipitation areas on topside. To prevent subsea reprecipitation, controlled well startups were implemented to minimize differential pressure. Additional offshore tests ensured the correct inorganic scale inhibitor injection rate, while laboratory and onshore tests qualified the inorganic scale inhibitor for specific conditions of liquid velocity, subsea temperature, and erosional forces on the umbilical. This combined approach of chemical cleaning and operational adjustments restored partially the well productivity and provided a reliable method to mitigate future inorganic scale precipitation. This paper addresses inorganic scale management in Brazil's carbonate pre-salt wells, characterized by a high level of CO2 in the gas phase (40 mol%) and a level of H2S equal to 38 ppm in the water phase. As BSW (Basic Sediment and Water) increases, the risk of subsea inorganic scale precipitation becomes significant for several fields in Brazil. The study provides insights into effective inorganic scale management strategies, contributing to the petroleum industry by offering solutions to mitigate and remediate inorganic scale deposition, ensuring optimal performance and longevity of subsea production systems in the pre-salt region.