Floc size of asphaltene as a function of N2 and CO2 concentration in black oil

Asphaltenes are a frequent problem in the oil industry, particularly in CO2 Enhanced Oil Recovery (EOR) projects. After more than three decades the phenomena of asphaltene flocculation and precipitation in EOR field operations remains problematic. This study attempts to address these problems from a new perspective. The use of CO2 in EOR is driven by its ability to increase the mobility of oils into which it can be dissolved. To a considerable extent, CO2 injection in EOR projects for this purpose has become a well-understood engineering practice, and laboratory procedures have been developed to determine optimal CO2 solubility conditions. The secondary effect of asphaltene flocculation induced by CO2 injection has received far less attention. Much research has been conducted to understand this negative aspect of CO2 injection using a number of different theoretical and experimental methods. Solubility theories, phase models, equations of state, and statistical mechanical theories such as SAFT have all been investigated. On the other hand realistic experiments in black oils injected with CO2 are limited and most have focused on “precipitated” asphaltene weight as a function of CO2 mole fractions in solution. The body of this work has not resulted in a reliable field engineering model or practice. This experimental study focuses on the floc size of asphaltene entities as a function of CO2 injected mole fraction, rather than merely asphaltene weight. Floc size distributions in comparison to pore throat dimensions are clearly the most elemental determinants of flow through porous media. Asphaltene entities that stay in suspension and are small enough to pass easily through formation rock pose little difficulty. This is not the case when floc sizes approach pore throat dimensions. This perspective may become essential as formations such as tight shale become targets of CO2 EOR.
In this research, N2 and CO2 have been involved to study the floc size of flocculated asphaltene due to N2 and CO2 flooding and their effect on black oil properties. Floc size distributions are determined by stacked sieve filtering. In the filtration process, newly available ceramic filters have been used in this experiment with sizes of 200 nm, 100 nm, 50 nm, and 30 nm.
Results show that both gasses produce asphaltene floc distributions as a function of dissolved CO2 / N2 as well as changing other black oil properties. N2 produces much more asphaltene than CO2 at lower mole fractions, and its behavior is mirrored in conductivity experiments. CO2 does not follow such a correlation and data suggests CO2 may rearrange floc distributions.