Research On The Characteristics Of CO₂ Displacement And Storage During Supercritical CO₂ Injection In Low Permeability Reservoirs

In the process of CO₂ flooding,different CO₂ injection methods and the heterogeneity of the reservoirs have a significant impact on the effect of CO₂ flooding and storage.In addition,the interactions between injected CO₂,rocks and fluids occur in the reservoir,as a result,asphaltenes and metal inorganic precipitations are generated,leading to pore-throat blockage in the reservoir,resulting in increased resistance to crude oil flow,which in turn affects the oil recovery and CO₂ storage efficiency.Consequently,the study on the differences between the flooding effect and storage efficiency of different CO₂flooding methods in different characteristic reservoirs,as well as the problems of asphaltenes and metal inorganic precipitations and reservoir damage during the CO₂flooding process is an important prerequisite for choosing a reasonable CO₂ flooding method and reducing reservoir damage.In this paper,the target reservoir is the block H of Changqing Oilfield,firstly,the basic physical property parameters of crude oil,formation water and supercritical CO₂ under high temperature and pressure conditions are evaluated.The relationship between the concentration of CO₂ in crude oil and the precipitation of asphaltenes is determined,and the minimum miscible pressure(MMP)of CO₂-crude oil is measured.Aiming at the basic problems such as the effect of CO₂ flooding in different CO₂ flooding methods and different reservoir characteristics,and the damage properties of reservoir caused by asphaltenes and inorganic precipitation generated during the flooding process,five core displacement experiments were performed in this paper.Displacement experiments in multilayer reservoirs with strong macroscopic interlayer heterogeneity show that,after CO₂ flooding,the entire system has a low oil recovery factor,and the 91%produced oil is mainly from the high permeability layer.The timing of CO₂ break-through is late,and the oil recovery factor of entire system is significantly improved,as well as the contribution rate of oil production in medium and low permeability layers.Furthermore,the permeability of the high permeability layer decrease by 16.8%after CO₂ flooding,which is mainly due to asphaltene precipitation.However,after CO₂-WAG flooding,the permeability of each layer is significantly reduced,29.4%,16.8%and 6.9%respectively.Asphaltene precipitation is still the main factor,but permeability decline caused by CO₂-brine-rock interactions is 20.6%in the high permeability layer.Based on the pore radius distribution and mercury intrusion curve,the fractal theory was used to quantitatively evaluate the microcosmic pore-throat characteristics of four cores with similar permeability,and miscible and immiscible CO₂ flooding and CO₂-SAG flooding experiments were performed.The experimental results show that the oil recovery of miscible flooding is 12-17%higher than that of immiscible flooding,and the cores with homogeneous pore-throat structure have values of 18-27%higher.The recovery of crude oil is more significantly affected by the pore-throat structure during immiscible flooding.The permeability of the cores decreased by 7-15%and 4-8%due to the pore-throat blockage by asphaltene precipitation,and the decline of permeability was proportional to the fractal dimension of the pore-throat structure.The wettability indexes of cores after miscible and immiscible flooding decreased by 25-60%and 10-22%,respectively.The oil recovery factors of CO₂-SAG flooding are higher CO₂ flooding by8-14%,and the stronger the heterogeneity of the core pore-throat structure,the greater the oil production improvement.The more homogeneous the pore-throat structure,the higher the pressure decay rate in the CO₂ soaking process.The better the pore-throat structure,the less the permeability decrease due to asphaltene precipitation after CO₂-SAG flooding.With the same oil recovery factor,the damage of the CO₂-SAG flooding to the cores is relatively small,especially for the cores with poor pore-throat structure.The more severe wettability changes due to the CO₂ soaking process cannot be ignored.The efficiency of CO₂ replacement oil of CO₂-WAG and CO₂-SAG flooding is significantly higher than that of CO₂ flooding,and miscible flooding is higher than immiscible flooding.After the CO₂-SAG flooding,the CO₂ storage rate is the highest,and the dissolved CO₂ concentration in the remaining fluid in the reservoir is higher.The efficiency of CO₂storage is highest at CO₂ break-through,and the CO₂-famation water-rock interaction during CO₂injection into the brine layer caused much more damage to the reservoir than the CO₂ flooding process.