Abstract

Deep water oil and gas production is playing an important role in global energy support. With the advancement of deep water offshore production in recent years, there are more chances encountering with the extreme conditions of high temperature (up to 250 °C), high pressure (up to 1,500 bars) and high TDS (up to halite saturated salinity) in the presence of mixed electrolytes. Scale control in such environment requires accurate prediction of scale formation tendency. However, most previous models are not capable of predicting the scale mineral solubilities over such wide ranges of temperature, pressure and ionic strength. Pitzer theory is one of the most advanced thermodynamic models for the predictions of mineral solubility and other thermodynamic properties. However, the published Pitzer models often have limited ranges of applicability and potential inconsistencies with each other. In this paper, the published Pitzer models have been thoroughly reviewed to evaluate the consistencies of Pitzer parameters among these published models. In addition, more than 5,000 density and solubility experimental data published in the past 150 years were collected as the input database for model fitting to extend the applicable ranges. These experimental data were fitted through the determinations of standard partial molar volume (V0¯) of each ion and the virial coefficients for species specific short range interactions as functions of temperature and pressure. The developed model is capable to predict the density of soluble chloride and sulfate salt solutions within ±0.1% relative error, common scale mineral saturation index (SI = lg(IonActivityProduct/Ksp) within ±0.1 units under most conditions, CO2 solubility in NaCl and CaCl2 solutions within 0.7% relative error of 95% confidence interval. The developed model has been incorporated into ScaleSoftPitzer for practical use in the oil and gas fields, along with the flash calculator based on Peng-Robinson EoS, which can accurately predict the species partitioning in oil/water/gas phases.

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