Experimental and mathematical model evaluation of asphaltene fractionation based on adsorption in porous media: Part 1. calcite reservoir rock

摘要

This study investigates the properties of asphaltene fractions in porous media and their role in reducing permeability and porosity. Based on the classification of a whole asphaltene deposited in porous media, the fractions can be defined as bulk, hard (h)-adsorbed, normal (n)-adsorbed, and irreversible (i)-adsorbed, which effects on porosity and permeability reduction in the reservoir rock. According to laboratory results for five types of calcite cores with porosity and permeability close to each other, the highest reduction in porosity (55%) and permeability (76%) is related to bulk asphaltene fractions. The n-adsorbed and h-adsorbed asphaltenes have approximately equal influence in reducing the porosity and permeability values in the reservoir rock. The i-adsorbed asphaltene, which has the highest adsorption on the rock surface, has the lowest porosity and permeability decrease due to lower amounts of asphaltene deposits in porous media. The amounts of hydrogen (H), nitrogen (N), sulfur (S), and oxygen (O) are higher in adsorbed fractions of asphaltene. In the case of hadsorbed asphaltene, H has the highest value in the asphaltene fractions (7.13 wt%). Compared with adsorbed asphaltene fractions, N values are higher in n-adsorbed (1.89 wt%) than h-adsorbed (1.41 wt%) fractions. This suggests that the S element plays a more significant role in the adsorption of asphaltene to the calcite surface. Based on the results, asphaltene compounds that adhere to the rock surface contain lighter carbon compounds than whole asphaltene. This means that the peak of the carbon compounds of h-adsorbed asphaltene in each rock sample is C-17. Fourier-transform infrared spectroscopy (FTIR) results show that the highest concentrations of carbonyl, carboxylic acid or derivative groups are found in h-adsorbed and n-adsorbed asphaltenes, respectively. A new experimental model of asphaltene deposition has been presented based on the deposition analysis of each asphaltene fraction in a porous medium under dynamic conditions, which results in a high correlation coefficient for each of the asphaltene classifications in the porosity and permeability reduction model. The reason for the high accuracy and uniqueness of the model is the use of parameters such as the velocity of fluid flow in the pores and the amount of volume injected in the reservoir rock sample, as well as the asphaltene fractions and their polarization ratio.