| Injecting carbon dioxide to develop low-permeability reservoirs could enhance the recovery percent of crude oil reserves.After a period of gas injection development,the carbon dioxide gas entering the formation led to gas channeling due to the difference in gas-liquid mobility,which caused inefficient use of gas.The method of water and gas alternate injection could reduce this phenomenon.However,with the implementation of carbon dioxide flooding and water and gas alternate injection,freezing and block problems often occurred in the well bore of gas injection wells.This article focused on the following research aspects based on the problem of freezing and plugging of supercritical carbon dioxide and water alternate injecting into the well bore,for example the reasons of freezing and block problems,generation process of carbon dioxide hydrate,induction time and interface characteristics of carbon dioxide hydrate generation,the molecular dynamics of carbon dioxide hydrate nucleation and the prevention and control of hydrates in the well bore.（1）Through statistical analysis of freezing wells driven by supercritical carbon dioxide in YSL Oilfield,it was found that this problem was most dominated in double-pipe and concentric tubing injection wells and is mainly affected by shut-in wells or water injection.Study on the adsorption characteristics of clay minerals to formation fluids near injection wells by molecular dynamics simulation.The results showed that carbon dioxide had a intensive adsorption capacity in formations containing kaolinite and silica matrix.The carbon dioxide in the well bore were major affected by the reverse diffusion of residual carbon dioxide near the well bore and changes in bottom hole pressure.The coupling model of fluid flowing and back diffusion for carbon dioxide in the well bore was established.The simulation calculations showed that back diffusion of CO2in the well bore could not be avoided,but the upward migration of CO2bubbles were able to be suppressed by controlling the flow rate.The calculated limit flow rate to prevent upward migration of carbon dioxide in the well bore was 1.53m/s.Taking post-water injection as an example,the limit shut-in time of carbon dioxide reverse diffusion to the bottom hole was from 1.6to 32.3d.The limit shut-in time of post-gas injection was from 20.0 to 30.0d,which mainly affected by permeability,cumulative gas injection volume,and formation depth.（2）By studying the heat and mass transfer processes in the generation of carbon dioxide hydrates combining with the fluid flow and heat transfer processes in the well bore,the carbon dioxide hydrate formation model in the well bore was established and this process was studied taking into account the density and solubility of carbon dioxide at different temperatures and pressures.The results showed that when the initial temperature was higher than the hydrate formation temperature,hydrates were preferentially formed on shaft lining at a lower temperature.Hydrates were prior formed at the interface of liquid carbon dioxide and water under opposite condition.Ambient temperature has a greater impact on the amount of hydrate formation in the well bore.The diffusion of carbon dioxide resulted in the changes in distribution characteristics of the hydrate in the well bore in the later period.The convective diffusion of gas during the hydrate formation process could boost the formation rate of hydrate,which generated faster at the initial moment and gradually decreased with the progress of the reaction.The amount of internal hydrate was relatively stabilized at the reaction of 50-70min,and the volume percentage of hydrate was between20.26%and 54.74%.（3）The microscopic characteristics in the process of hydrate formation and decomposition and the interface characteristics in the process of hydrate formation were studied by microscope observation.It was found that during the hydrate formation process,the pressure had a drop point at the initial time,which represented the beginning of hydrate formation.At this time,the hydrate mainly existed on the tube wall boundary,the two-phase interface and the upper liquid carbon dioxide.The hydrate at the middle position of tube was formed last,and the thinnest hydrates generated at the interface.Under the influence of the solid-liquid surface tension,the liquid water moved upwards to form hydrates in the carbon dioxide phase.Due to the formation of the hydrate,the initial interface position between carbon dioxide and water,carbon dioxide could not diffuse further into the water phase which hindered the follow-up generation of hydrates.（4）Research on prevention measures of hydrates in the well of supercritical carbon dioxide and water alternate injection was carried out to determine the location of carbon dioxide hydrates in the well bore,simulate the effect of different hydrate inhibitors on the inhibition of carbon dioxide hydrates,determine the limit freezing concentration of different inhibitors,and optimize the injection parameters of the hydrate inhibitor based on the simulation results.The results showed that hydrates would no longer be formed when the concentration of the added inhibitor reached a certain level.For field operation,the limit concentrations of methanol,ethanol,and ethylene glycol to inhibit hydrate freezing and blocking were 40%,60%,and 60%,respectively.In the initial state,there was an obvious interface between methanol and formation water in the well bore.After shut-in,the two-phase interface was blurred and the upper inhibitor concentration declined.The effective depth of the inhibitor gradually decreased as the increase of the shut-in time.In the range from 50 to 300days,the safety distance of the inhibitor should be controlled from 54 to 127m below the freezing section.The limit flow rate could not be reached at low injection speed.The restrictor could be set at the bottom of the freezing section,and the minimum size of the throttle valve port was calculated to be between 5.44mm and 13.33mm.The above research results are of great significance for carbon dioxide storage and utilization. |
