(123b) The Role of Scale in Flow Assurance

The ultra-high temperature (150-250^oC), pressure (1,000-2,000 bar) and TDS (>300,000 mg/L) in deepwater oil and gas production pose significant challenges to flow assurance and scaling control due to limited knowledge of mineral solubility, kinetics and inhibitor efficiency at these extreme conditions.  Brines from multiple wells are collected, mixed in a subsea facility and transported vis subsea flowlines.  Often the oil, gas, and brine composition in each well is quite different and consequently will cause significant scaling.  The objective of this paper is to present an overview of the on-going research at Rice University to predict and control scale during oil production.  Five key areas of research related to flow assurance scale control will be discussed:

1. The challenges of scale prediction at ultra high temperature, pressure, and TDS:  Prediction of scale solubility is currently limited by inadequate understanding of the equilibrium constants and activity coefficients associated with common minerals. In some cases, there is also uncertainty about the mineral phases that need to be considered (i.e. silicates, sulfides, etc.). A new apparatus was built to test scale formation and inhibition at high temperatures and pressures up to 250^oC, pressure up to 1,500 bar and ionic strength up to 6m in solutions with elevated concentrations of mixed electrolytes (e.g., calcium, magnesium, sulfate and carbonate) representing the maximum range of interferences expected (95%CI) in oil and gas wells. Predictions by a thermodynamic model based on Pitzer’s ion interaction theory were updated with these new data to provide reliable predictions of solubilities that are consistent with both experimental and literature data at all conditions tested.

2. Efficient method to study the nucleation kinetics of scale formation and inhibition at these conditions. A rigorous and comprehensive study have been conducted to investigate: (1) the time (minutes to days) dependence of inhibitor thermal (up to 200 °C) degradation, (2) the impact of trace heavy metals, such as nickel and manganese on the degradation of inhibitors, (3) the difference between ageing tests with the presence and absence of core material for adsorption, and (4) the effectiveness of chemical treatment by applying chelating agents to prevent the catalytic degradation caused by trace heavy metals. The results enable a more accurate understanding of thermal degradation kinetics and shed light on potential chemical treatment to enhance inhibitor performance in high temperature environments.

3. Kinetics of nucleation, crystal growth and attachment in the presence and absence of scale inhibitors.  In order to assess scaling risk, a better understanding of scale deposition kinetics on steel surface under realistic and complex oil field condition is needed. In this paper, we introduce the development of a novel CaCO₃ pre-coated (55 µm thick uniform layer of well adhered CaCO₃) steel tubing for studies of CaCO₃ crystal growth kinetics and inhibition kinetics at oilfield condition. This approach provides a relatively stable surface area and eliminates the limits of laboratory batch experiments.