Study On The Formation Characteristics Of CO₂/CH₄ Hydrate By Wax Crystal Precipitation In Oil-water System

With the development of deep-sea oil and gas fields,the high-pressure and low-temperature environment provides natural conditions for the formation of hydrates and wax deposition,and it is extremely easy to generate wax and hydrates at the same time to block pipelines.When studying the gas hydrate formation characteristics of the mixed oil and gas in the wax-containing system,if the influence of wax crystals on the hydrate formation is ignored,serious accidents will occur.Therefore,when studying the formation of hydrates in wax-containing systems,the influence of wax crystals on the formation of hydrates should be taken into account to ensure the safe operation of deep-sea oil and gas mixed pipelines.In order to explore the influence of wax crystal precipitation on the phase equilibrium characteristics of gas hydrate formation,the phase equilibrium experiment of CO₂ hydrate and CH₄ hydrate formation under different wax crystal concentrations was carried out,and it was found that(1)the precipitation of wax crystals hydrated carbon dioxide hydrate and methane The phase equilibrium curves of the substances all shifted to the right,and the trend of shifting to the right became more obvious with the increase of wax crystal concentration.At T=281.5K and a wax concentration of 3.5wt%,the phase equilibrium pressures of the CO₂ system and the CH₄ system were reduced by6.5%and 8.06%respectively compared with the wax-free system.(2)The precipitation of wax crystals causes a change in the latent heat of hydrate phase change.In the CO₂+wax+oil+water system,the average latent heat of phase change gradually decreases with the increase of wax crystal concentration,which is the energy required for the formation or decomposition of hydrates.In the CH₄+wax+oil+water system,the average latent heat of phase change increases with the increase of wax crystal concentration.(3)Based on the Chen-Guo model,combined with the SRK equation and the UNIQUAC activity model,the phase equilibrium conditions of CO₂ hydrate and CH₄ hydrate under different wax-containing systems are calculated.The results show that the predicted value of the model is the average absolute value of the experimental results.The deviation is 7.28%,indicating that the model can be used to calculate the hydrate phase equilibrium in wax-containing systems.The effects of different wax crystals on the kinetics of CO₂ hydrate formation were explored,and the following conclusions were drawn:(1)The precipitation of wax crystals provides nucleation points for the nucleation of hydrate crystals,shortens the hydrate induction time,and the concentration of wax crystals increases.The greater the impact,the more obvious.(2)The influence of wax crystal precipitation on the change rate of hydrate induction time is related to the concentration.The wax concentration is in the range of0?2.5wt%,and the promotion effect of wax crystal precipitation on the induction of nucleation gradually increases,while the wax concentration is2.5wt%.In the range of%?3.5wt%,the ability of wax crystal precipitation to promote hydrate-induced nucleation begins to weaken.(3)The precipitation of wax crystals will increase the gas consumption of CO₂ hydrate,and the greater the concentration of wax crystals,the more gas consumption.(4)The influence of wax crystal precipitation on the rapid growth rate of CO₂ hydrate is basically in line with the trend that the greater the wax concentration,the greater the rapid growth rate.Except for P₀=3.9MPa and wax concentration of 3.5wt%,the growth rate decreased slightly.In addition,the kinetics of CH₄ hydrate formation under wax-containing system was studied,and the conclusions are as follows:(1)The influence of wax crystal precipitation on the CH₄ hydrate induction time is consistent with that of CO₂ hydrate,that is,the wax crystal precipitation shortens the hydrate induction time.And the greater the concentration,the more obvious the effect.(2)The effect of wax crystal precipitation on the change rate of CH₄ hydrate induction time is different from that of CO₂ hydrate.In the wax range studied in this paper,the ability of wax crystals to promote the induction time of CH₄hydrate increases with the increase of wax concentration.Large,no weakening trend.(3)The influence of wax crystal precipitation on the gas consumption of CH₄ hydrate.The gas consumption of CH₄ hydrate decreases with the increase of wax concentration.(4)Compared with the wax-free system,the rapid growth rate of CH₄ hydrate in the wax-containing system gradually increases with the increase of wax concentration.Finally,based on the experimental results,the formation mechanisms of the two hydrates are obtained,and the conclusions are as follows:(1)In the wax-containing system,the formation characteristics of the two gas hydrates are basically the same.Both hydrates are formed in the center of the reactor first,and then gradually Diffusion and growth to the inner wall of the kettle.(2)On the one hand,the presence of wax crystals provides attachment points for hydrate nucleation,that is,the presence of wax promotes the formation of hydrates.On the other hand,the precipitated wax crystals will wrap water and gas molecules to affect hydrate nucleation and hinder hydrate nucleation generate.Under the research conditions of this thesis,wax crystals have a dominant effect in promoting the formation of hydrates,showing the phenomenon of promoting the formation of hydrates,and the promotion effect on CO₂ hydrate is greater than that of CH₄ hydrate.