Abstract

The onset and reversibility of asphaltene precipitation from Athabasca and Cold Lake bitumens were investigated over a range of temperatures and pressures in a mercury-free PVT cell. Asphaltenes were precipitated from bitumen with the addition of n-heptane and they were re-dissolved by adding bitumen to the bitumen-n-heptane mixture. Asphaltene precipitation decreased with increasing temperature and pressure. There was a small hysteresis between the precipitation and re-dissolution of asphaltenes but the hysteresis became negligible as temperature and pressure increased. A combined EOS and regular solution theoryapproach is currently under investigation to model the phase behavior of asphaltenes at high pressures and temperatures.

Introduction

Deposition of asphaltenes is a serious potential problem in the production and processing of petroleum. It can occur in the reservoir, in production tubing, or during transportation and processing. Precipitation of asphaltenes most likely is the first step in asphaltene deposition. To prevent or mitigate asphaltene deposition, it is desirable to predict the onset, amount and reversibility of asphaltene precipitation.

There are several approaches for modeling asphaltene precipitation, however the majority of these methods treat asphaltene precipitation as a reversible process and, therefore, use a thermodynamic framework for modeling such as equations of state (EOS) (1, 2) or regular solution theory (3, 4). EOS are suitable for vapor-liquid equilibrium calculations over a range of temperatures and pressures. They are less suitable for liquid-liquid or liquid-solid equilibrium calculations. Regular solution theory is suitable for liquid-liquid and liquid-solid equilibria but must be adapted to handle changes in temperature and pressure. Regular solution theory requires estimates of molar volume and solubility parameter. In this work, these estimates are to be obtained from the Soave-Redlich-Kwong equation of state (SRK EOS). To test this approach, experimental measurements of asphaltene precipitation are required. In this work, precipitation measurements are made from Athabasca and Cold Lake bitumens diluted with nheptane over a range of temperatures and pressures typically encountered in heavy oil and oil sands processes. The reversibility of asphaltene precipitation is also examined.

EXPERIMENTAL
Chemicals and Materials

Athabasca bitumen was obtained from Syncrude Canada Ltd. and Cold Lake bitumen was obtained from Imperial Oil Ltd. The Athabasca bitumen is an oil sands bitumen that has been processed to remove sand and water. The Cold Lake sample was recovered by steam injection from an underground reservoir and has also been processed to remove sand and water. n-Heptane was btained from Aldrich Chemical Company and was 99%+ pure.

Experimental Methods

Both precipitation and redissolution of asphaltenes were investigated using direct gravimetric measurement of precipitated asphaltenes at various n-heptane-tobitumen (H/B) ratios. Two methods for redissolution are solvent removal and bitumen addition. Solvent removal is not possible in the PVT cell because the cell has only one port for both bitumen and solvent addition (or removal). It is also practically impossible to evaporate the solvent under constant pressure and temperature. Therefore, the bitumen addition method was adopted. In this method, fresh bitumen was added to a bitumen solvent mixture to reduce the solvent-to-bitumen ratio and redissolve precipitated asphaltenes.

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