Study On Micro-mechanism Of Precipitation,Aggregation,and Deposition Of Waxes In Waxy Condensates

The rising demand for energy makes the exploitation of oil and gas resources inevitably develop towards low permeability,unconventional and abnormally high-pressure deep layers and deep sea.While ensuring the development and utilization of conventional oil and gas resources,strengthening the exploration and development of condensate oil and gas resources has become a powerful guarantee and way to enhance oil and gas production.Waxes dissolved in the bulk phase as molecules will gradually precipitate,aggregate,and deposit during the production and transportation process of condensate oil and gas resources,reducing the effective circulation area of the wellbore and surface gathering facilities,posing flow assurance challenges,and directly affecting the green and efficient development of condensate gas fields.This paper was based on the production and transportation engineering of deep high-pressure condensate gas wells in China’s TLM oilfield,the temperature and pressure distributions in wellbore were simulated in combination with condensate base property experiments and production static and dynamic data.Molecular dynamics simulations were used to microscopically characterize the diffusion,aggregation,and deposition behavior of wax molecules in the condensates under phase transition conditions.And the microscopic mechanism and influencing factors of wax crystal aggregation and deposition process were revealed.Eventually,the mechanical response model of adhesion-stripping of wax deposits on the wall was established,and the critical mechanical relationship of adhesion-stripping and its corresponding temperature distribution were determined.Considering the phase transition conditions such as temperature and pressure,the kinetic behavior of wax molecules in the presence of condensate gas phase was revealed.The diffusion ability of wax molecules in different systems was as follows: propane > methane-ethane-propane > ethane >methane-ethane > methane.Different from the general effect of temperature on the aggregation and deposition behavior of wax molecules,the effect of pressure on the aggregation behavior of wax molecules was related to the solubility of the gas molecules in the condensates,the pressure effect on the deposition behavior of the wax molecules was related to the carbon number distribution of the condensate gas component,the higher the carbon number,the denser the wax molecular layer deposited on the wall when the pressure decreased.By the contact angle between the condensate cluster and the wall,the process that the waxy condensate undergoes on the wall was explained,which was cluster decomposition→molecular detachment from the cluster and diffusion towards the wall→aggregation and accumulation of molecules above the wall→clusters increasing in spread on the wall→formation of a condensate layer on the wall.And compared to macrocrystalline wax clusters,microcrystalline wax clusters were more agglomerated and the resulting deposits are more solid.As the wax content increased,more wax molecules aggregated in the clusters and it spread less on the wall.The increase in asphaltene content provided more adsorption sites for wax molecules.Meanwhile,waxy condensate clusters underwent varying degrees of decomposition,diffusion,and aggregation and deposition near the walls of different materials.For fiberglass reinforced plastic pipe,718 pipe,HP2-13Cr110 pipe,BG140 V pipe and common pipe,the contact angle formed between them and the clusters decreased sequentially,and the degree of clusters spreading on the wall gradually increased.Although the influence of wall morphology on cluster aggregation and deposition was relatively small,when further considering the roughness characteristics,the degree of cluster adhesion on the wall was related to the surface area fraction of the wall and the height of the groove column.As the surface area fraction increased,the contact angle first increased and then gradually stabilized.Correspondingly,the adhesion of the cluster on the wall gradually weakened.The higher the height of the groove column,the greater the degree of cluster adhesion on the wall,but under simulated conditions,it tended to stabilize when it was greater than 12 ?.Finally,based on the mechanical balance relationship,a wall adhesion-stripping mechanical response model was established.On the basis of the wax crystal micro size distribution and related mechanical property tests,the critical mechanical relationship for different wall and condensate wax deposition characteristics was obtained,and the temperature distribution corresponding to the critical mechanical relationship for different wall and deposition characteristics was identified as 32 ℃～36 ℃.The research systematically reveals the diffusion,aggregation,deposition,and adhesion behavior of wax in waxy condensates at the molecular level,providing a theoretical basis for the design and optimization of efficient production and transportation processes for waxy condensates.