Mineral-Scale Control in Subsea Completion

Abstract

Abstract Deepwater production is characterized by large production rates from thousands of feet below the ocean floor. Multiple wells are linked to the flowlines on the ocean floor where the produced brines from different wells commingle. Such production system often increases the challenges of scale prevention and control. In this paper, a new thermodynamic Pitzer theory based computer software, ScaleSoftPitzerT?, will be discussed first. ScaleSoftPitzerT? is a MicrosoftT? ExcelTM based program to predict scale tendency, specifically written for the oil and gas production systems. Secondly, the effect of hydrate inhibitors on sulfate scale formation will be evaluated over a wide range of temperature and salinity typically encountered in the oil field. The hydrate inhibitors can reduce the solubility products of sulfate minerals by more than two orders of magnitude. A new speciation model is proposed to correct the hydrate inhibitor effects in terms of a ionnonelectrolyte mean activity coefficient. Introduction Formation of mineral scale from produced water in oil and gas wells and pipelines is a persistent problem facing the petroleum industry. Lost production, formation damage, and operational expenses due to scale deposits cost hundreds of millions of dollars a year. Scale problems are usually associated with specific field condition and operation schemes. Some environmental factors and common practices of offshore production can potentially exacerbate the scale problems. For example, halite scale is mostly observed in the high TDS brine, due to supersaturation condition caused by a reduction in temperature of the fluid during flow through long pipelines and seawater cooling. The primary reasons for the formation of carbonate scales is usually due to supersaturation condition caused by the elevation of pH of the brine as a result of the escape of CO2 and /or H2S gases out of the brine solution at lower pressure. The common cause for sulfate scale is commingling of different sources of brines either due to breakthrough of injected seawater, incompatible brine or mixing of different brines from different zones of the reservoir formation from different wells. A decrease in temperature tends to cause barite to precipitate. In addition, pressure drops tend to cause all scale minerals to precipitate due to the pressure dependence of the solubility product. In subsea gas wells, hydrate formation is often problematic due to the large temperature drop. Methanol, ethanol, and ethylene glycol are often added in large quantity to control hydrate formation. Such practice has significant adverse effect on scale formation. In the following, the magnitude of scaling problems caused by hydrate inhibitor will be discussed. A new computer program to predict scale tendency and inhibitor need will also be discussed. Scale Prediction The thermodynamic-based solubility of mineral salt is defined as the ion activity product at equilibrium.