Deposition Mechanisms And Reservoir Protection Countermeasures Of A Low-permeability Formation In CO₂ Flooding Process

In the CO2 injection process,the injected CO2 can react with the reservoir rock minerals and fluids(e.g.,brine and crude oil),which may lead to the precipitation of organic(e.g.,asphaltene)and inorganic solids.These reactions may be plugged the pore of the reservoir formation,which may increase the flow resistance of the crude oils.In this paper,taken block H of Jilin oilfield as an example,the basic physical parameters of the reservoir crude oil,brine,and CO2 were determined under the high temperature and the high pressure conditions.The minimum miscibility pressure(MMP)of the CO2-crude oil system was determined by means of the slim-tube apparatus,which was used as a guide to comfirm the miscibility pressure of CO2 injection experiment.In order to solve the problem related to the asphaltene deposition and inorganic deposition during the CO2 injection process,the solid deposition laser detection system,the composite long coreflood setup for the CO2 injection experiment,and the mathematical prediction model of the asphaltene deposition were used to mearsure the phase property of asphaltene deposition,the regularity of asphaltene deposition,and the reservoir damage caused by the asphaltene deposition and inorganic deposition in the CO2 injection process.First,for the testing difficult problem of the thermodynamics parameter of the asphaltene deposition in the live oil,a solid deposition laser detection system was designed in this paper to determine the stability of the asphaltene in the live oil.The results of the asphaltene deposition onset pressures were obtained under various temperatures.Then,this value was used to plot the P-T phase diagram of the asphaltene deposition.The phase diagram is very easy and quick way to obtain the thermodynamics parameter of the asphaltene deposition under the reservoir condition.In addition,the relations of the amount of asphaltene deposition and the CO2 dissolution in the crude oil were determined by the Hirschberg solubility model.It was found that the asphaltene deposition increased with the increase of CO2 dissolution increases in the crude oil when the CO2 fraction in the crude oil exceeds a critical point.Moreover,the critical concentration of CO2 in the crude oil under the miscible condition(25 MPa)was much higher than that under the immiscible condition(15 MPa).Second,the composite long coreflood setup for the CO2 injection experiment was used to investigate the reservoir damage as a result of the asphaltene deposition and inorganic deposition in the CO2 injection process.It is found that,in the CO2 injection process,the reservoir percolation capability reduction caused by the asphaltene deposition is represented the "wave like" mode.The extent of percolation capability reduction along flow direction was first increased and then decreased,and the major damage position of the reservoir was in the middle of the reservoir;The reservoir percolation capability reduction caused by the inorganic deposition was also represented as the trend of first increased then decreased,and the major damage position of the reservoir was in the middle of the reservoir.In addition,the major damage of the reservoir was mainly attributed to the asphaltene deposition under the miscible condition;the major damage of the reservoir was mainly caused by the inorganic deposition under immiscible condition.Final,the chemical inhibitors were investigated comprehensively for the of asphaltene and inorganic depositions.In addition,according to the trend of reservoir damage in the CO2 injection process,the reasonable countermeasure of the reservoir prectection was proposed in this study.