Abstract
Gas Enhanced Oil Recovery (GEOR) is one of the most frequently applied enhanced oil recovery methods to increase oil recovery. Nitrogen is one of the most widely investigated gases due to several advantageous properties. During its interaction with the crude oil, nitrogen can induce asphaltene deposition which can result in severe formation damage. This research investigated asphaltene deposition during nitrogen injection in nano sized pores structure in order to study and compare the impact of asphaltene damage in different pore sizes. In the present study implemented a specially designed filtration vessel in all experiments. Crude oil with viscosity of 19 cp was used to investigate the impact of oil viscosity. The impact of varying filter membrane pores size (450, 100, and 50 nanometers), nitrogen injection pressure (1500, 1250, and 1000 psi), and temperature (70, and 32°C) were investigated. Firstly, the experiment which investigated nitrogen minimum miscibility pressure (MMP) was conducted in which all the nitrogen injection pressures selected were below the nitrogen MMP. Filtration experiments were conducted using 30 ml of crude oil for each experiment. Nitrogen was injected until no oil production was observed. The nitrogen injected was in the supercritical phase throughout all experiments. When varying the filter membrane pore size, which was referred as "heterogenous", an inverse relationship was found between pore size and asphaltene pore plugging; as pore size decreased, asphaltene pore plugging increased significantly. Furthermore, the rate at which the crude oil protruded through the filter membrane decreased at a substantially increased rate compared to the larger pore sizes. When membranes with same pore sizes were used, referred as "uniform", our data indicated that almost the same asphaltene weight percentage in all filter membranes. The uniform pore size allowed the passage of asphaltene particles through all filter membranes and resulted in a similar asphaltene weight percentage. At 1500 psi, the nitrogen was completely immiscible with the oil. This resulted in a larger asphaltene weight percentage compared to smaller pressures. Visualization experiments showed an increase in the asphaltene deposition rate and lighter concentration in higher pressure which confirmed these results. Increasing the temperature resulted in a larger asphaltene instability and less asphaltene concentration, which was evident from the 2.56% asphaltene weight recorded at 32°C, compared to the 1.72% at 70°C in the remaining oil. Soaking time was investigated at 10, 60, and 120 minutes. The results indicated that increased in asphaltene weight parentage in the early soaking times from 10 to 60 minutes. The increase was not significant from 60 to 120 minutes. Hence, soaking times beyond 60 minutes appear to have a small effect. The present research serves to highlight the severity of asphaltene damage, especially in nano pores, mainly present in unconventional reservoirs. By studying the impact of different factors on asphaltene formation damage, asphaltene deposition may be avoided in future applications of nitrogen injection.
