Enhanced Oil Recovery Mechanism And Productivity Evaluation Of CO₂ Water-Alternating-Gas Flooding In Tight Oil Reservoirs

It is abundant and widely distributed for the tight oil reservoirs.That efficiently developing tight oil reservoirs is an important way to increase geological reserves and oil productions in China.The technology of CO₂water-alternating-gas flooding is one of the main ways to enhance oil recovery and it has attracted more and more attentionbeen in the context of the Carbon Peaking and Carbon Neutrality.However,it is complex for the mechanism of CO₂diffusion and mass transfer because it is affected by many factors,such as CO₂state,pore structure and oil-water distribution characteristics in the process of CO₂water-alternating-gas flooding,which can have an effect on describing displacement mechanism finely and evaluating oil productivity accuratly.In order to solve the above problems,the theories of seepage mechanics,phase equilibrium and molecular diffusion are applied to study the diffusion mass transfer and seepage mechanism of CO₂water-alternating-gas flooding for tight reservoirs in this paper.Meanwhile,the seepage mathematical characterization model of CO₂water-alternating-gas flooding is builded and the molecular diffusion and mass transfer is considered in this model,whicn provides the theoretical support for enhanced oil recovery by CO₂water-alternating-gas flooding.Firstly,the diffusion simulation experiments of CO₂water-alternating-gas flooding had been carried out and the laws of CO₂diffusion in porous media had been clarified according to the characteristics of slug injection for CO₂water-alternating-gas flooding.The experimental results showed that it can be divided into two stages for the CO₂diffusion in the process of CO₂water-alternating-gas flooding:stage I and stage II.The core is affected by the water slug in stage I and injected water occupies part of the existing pore space of crude oil,which increased water saturation and inhibited CO₂diffusion.Thus the CO₂diffusion rate was reduced.In stage II,the core was not affected by water flooding and the oil saturation was high.So the CO₂diffusion rate increased.With the increase of permeability,the CO₂diffusion rate increased in stage I and stage II and the influence of water slug on CO₂diffusion was weakened.Secondly,the mathematical characterization model of CO₂diffusion in CO₂water-alternating-gas flooding was established.The influence of water slug and reservoir pore structure on CO₂diffusion were considered in the established model.The simulated results of the model was compared with the experimental results to verify the accuracy of the model.The results showed that the CO₂diffusion coefficient was 0.48×10^-9m²/s in stage I and is 1.02×10^-9m²/s in stage II.With the increase of permeability,the CO₂diffusion coefficient increases in stage I and stage II.Then,the physical simulation experiments of CO₂water-alternating-gas flooding in tight reservoirs were carried out to reveal the displacement mechanism of CO₂water-alternating-gas flooding.Meanwhile,the influencing factors of displacement effect were analyzed.The results showed that the CO₂had not diffuse to tail end and the crude oil viscosity is high.So the oil production is low in the early stage of displacement.In the middle stage of displacement,CO₂had diffused to tail end and CO₂dissolution,which expanded oil volume and decreased oil viscosity.At the same time,CO₂contacted with crude oil and there were mass transfer occurs in CO₂and crude oil.Meanwhile,CO₂continued extracting and extracting the light components of crude oil,which enriched CO₂gradually and eventually formed miscible.So the crude oil production increased.At the later stage of displacement,the dominant channel of gas channeling was formed and the injected CO₂channeled along the dominant channel.In the process of CO₂-water-gas alternating flooding,the drivened oil is main C_25-and the proportion was more than 80%.It is is a process of continuous extraction and extraction and precipitation of heavy components for CO₂water-alternating-gas flooding.Finally,based on the established CO₂diffusion model and the results of oil displacement simulation experiment,a seepage mathematical characterization model of CO₂water-alternating-gas flooding in tight reservoir was established.The established model is dual media and the molecular diffusion was considered in this model considering.The effects of CO₂dissolution viscosity reduction,capillary pressure,starting pressure gradient and other factors were also took into account in this model.It was compared with the numerical simulation software and the results showed that there was a good agreement between the model simulation results and the actual data,which indicated that the model could accurately describe the production characteristics.The model had been applied to optimize the reservoir development parameters.The optimizational results showed that the reasonable injection rate is 12m³/d,the optimal injection cycle is 1 month,and the reasonable gas-water ratio is 0.8:1.Through the combination of theory and experiment,this paper explores the mechanism of enhanced oil recovery for CO₂water-alternating-gas flooding in tight reservoirs.The method of productivity evaluation and prediction for CO₂water-alternating-gas flooding had been formed.And the injection and production parameter optimization and application had been carried out,which provided theoretical support for the efficient development of tight reservoirs and had certain guiding significance for the enhanced oil recovery policy of CO₂water-alternating-gas flooding.