Quantification of Asphaltene Flocculation During Miscible CO2 Flooding in the Weyburn Reservoir
Authors: Srivastava R.K. , Huang S.S. , Dyer S.B. , Mourits F.M. 
Published: March 2010

In a CO2 miscible displacement process, asphaltenes from the oil can flocculate in the presence of CO2 and can cause numerous production problems with a detrimental effect on oil recovery. A five-year multiclient research project was initiated in 1988 to evaluate the suitability of CO2 as an enhanced oil recovery (EOR) agent for a southeast Saskatchewan reservoir, Weyburn. The initial asphaltene content of the oils collected from the Weyburn pool was about 5% and suggested the possibility of formation plugging and wettability alteration during a CO2 injection. A laboratory study using a prototype high pressure PVT cell was undertaken to quantify asphaltene flocculation during a CO2 miscible flood in the Weyburn field. A photo metric technique was developed to determine the asphaltene content of the flashed CO2-saturated test oils. The effect on the asphaltene flocculation of operating pressure, CO2 oil contact; time, CO2 concentration, gas contaminants such as N2 and CH4' in CO2, and the presence of formation brine was investigated for; three different oil samples (approximately 29 ºAPI) collected from the pool.

Results of the study indicated that asphaltene began to flocculate at about 42 mol% CO2 concentration. After this onset, there was a linear increase in asphaltene flocculation with CO2 concentration. The asphaltene flocculation pattern seemed to be, relatively insensitive to the operating pressure in the pressure range studied. The normalized asphaltene flocculation data showed a negligible effect of brine and contaminants in CO2 tested. For the experiments conducted in the single-phase region) the effect of CO2 oil contact time on asphaltene flocculation during the 4–19 day period was negligibly small. However, the mixing time required to achieve equilibrium in the two-phase' region was significant.

Introduction

The majority of the Saskatchewan light and medium oil (LMO) reservoirs have reached their economic limit of production under current technology (primary and secondary recovery methods)(1). Miscible flooding with carbon dioxide is shown to be a promising enhanced oil recovery (EOR) technique for southeast Saskatchewan reservoirs (2). Recent research suggests that near miscible (or extremely low interfacial tension) CO2 flooding can result in highly efficient displacement(2). A study(4) undertaken by Saskatchewan Energy and Mines has shown that the successful development of the miscible (or near-miscible) displacement process using CO2 and hydrocarbon gases can lead to a significant increase in Saskatchewan LMO reserves and substantially extend the production life of these pools. Crude oil normally consists of a conglomeration of waxes, resins, asphaltenes, and other semisolid materials. Because of their different physical and chemical characteristics, and when subjected to slight changes in equilibrium conditions, paraffins, resins and asphaltenes often precipitate. One of the problems confronting researchers and field operators is asphaltene deposition during a CO2 miscible flood.

This concern with asphaltene deposition and problems arising from it during CO2 miscible flooding was one aspect of a comprehensive five-year multiclient research project(5). It was started in 1988 at the Saskatchewan Research Council (SRC) to evaluate the suitability of CO2 as an EOR agent for a southeast Saskatchewan reservoir, Weyburn.