The Study On Asphaltene Precipitation And Reservoir Physical Properties Damage During CO₂ Injection In Tight Oil Reservoirs

With the sustainable growth of oil and gas demand in the world and the continuous decline of conventional oil and gas production,tight oil with great potential has gradually become a new highlight of global oil exploration.Therefore,the development of tight oil reservoir has important strategic significance for China and other countries in energy supply and economic growth.CO₂ injection is an effective development method to improve tight oil recovery.However,CO₂ may lead to the precipitation of asphaltene,which will block the pore throat and damage the reservoir,especially the tight reservoir.Herein,this paper focuses on the Lucaogou formation in Jimsar sag,Xinjiang.Based on the literature research about gas injection in tight reservoir,asphaltene precipitation and reservoir damage evaluation,a series of experiments were carried for the tight oil.Firstly,the basic characteristics of tight oil and CO₂/oil interactions were analyzed.Then,asphaltene precipitation under the effects of pressure and CO₂ concentration was studied.Finally,the physical simulation experiments were designed to reveal the effect of CO₂ on the surface properties of tight core,asphaltene precipitation,and reservoir physical properties changing in tight pore media.The above experiments results showed:?1?The molecular weight of tight oil was 370.The oil was sensitive to temperature and shear rate.The viscosity of tight oil was 20 mPa s at the reservoir temperature?75??.The content of heavy components in tight oil was high,with the asphaltene content of 5%.Meanwhile,the physical properties of the tight oil were improved after the dissolution of CO₂.The viscosity of tight oil decreased with the increase of CO₂ concentration,and the volume factor and gas-oil ratio increased in a nearly linear trend.?2?The static precipitation experiment showed that the precipitation amount of asphaltene increased with the CO₂ concentration.It was firstly increased with the increase of pressure until reaching a certain pressure,after that the precipitation amount of asphaltene gradually decreased.After CO₂ injection,the texture of the asphaltene was notably altered to be smooth,as revealed by SEM observations.?3?The CO₂ huff and puff experiment showed that the higher the soaking incremental pressure was after CO₂ injection,the higher the corresponding recovery would be,the highest recovery was 20.75%.In addition,the oil recovery rate of the multi-round CO₂ huff and puff was better than that of the single round,which was mainly contributed by the former two rounds of huff and puff.The displacement experiment showed that there was a positive correlation between the displacement differential pressure and designed experimental pressure,and the recovery increased with the increase of the maximum displacement differential pressure.Meanwhile,the recovery increased with the increase of the CO₂ PV number and it showed an S-shaped curve.The highest recovery was 45.45%.?4?After CO₂ injection,the porosity and permeability of the tight core were damaged.As the experimental pressure increased,the damage increased.On the whole,the damage in porosity was relatively small,which was evidenced by NMR,while the permeability damage was relatively large due to the tight pore throat blockage caused by asphaltene precipitation and migration with CO₂.So the damage of core physical properties in displacement experiment was more noticeable than that in huff and puff experiment.The maximum permeability reduction in huff and puff experiment was 27%,while that in displacement experiment it ranged from 46%to 63%.?5?The precipitation of asphaltene in the tight core was observed by EDS and SEM analysis.The texture of the asphaltene was altered to be smooth covering the surface of rock particles.Meanwhile,the contact angle experiment showed that the precipitation of asphaltene on the core inlet surface will lead to the change of wettability.