Study On Microscopic Phase Behavior Of High Viscosity Oil In Porous Media And Wettability Modification Method On Rock Surface

Fossil oil is not only an important clean and efficient energy,but also an exhaustible resource.To produce it reasonably,its microscopic properties must be understood.From microscopic view,oil production process can be described as phase behavior of crude oil in porous media,adsorption of oil molecule onto the rock surfaces and micro-scale flow of fluid in different radius of throat.Among them,some basic problems should be noticed,including phase transition behavior of foamy oil molecule and its influence on flow in heavy oil solution gas drive,the mechanism of surfactant wetting rock surfaces.The corresponding problems were studied comprehensively in this thesis by proposed experimental and simulation methods,and some novel understandings were acquired as follows.Four main phenomena in foamy oil pressure depletion process,including bubble generation,growing,merging and breakup,were studied on micro-scale by micro-model experiments.An important phenomenon was observed that bubble growing mainly occurs in fluid transport process.We can conclude that the interface is stable between oil and gas in foamy oil due to the high content of heavy components of it,which leads to the foamy oil difficult degassing and phase transition slowly.According to the characteristic of bubble nucleation,we conducted classical nucleation theory(CNT)to simulate it,and analysis the affecting factors including interfacial tension(IFT),viscosity,density and bubble diameter by volume of fluid method(VOF).The accuracy of the model and calculation program was verified by the generation of single bubble and the false flow distribution of VOF.In addition,we studied the flow characteristics of foamy oil depletion process and investigated the factors affecting the performance of foamy oil recovery in the real porous media(sandpack)with the assistance of computed tomography(CT)technology to help further the understanding of the mechanism.In particular,a novel method to identify bubble in porous media was first proposed by setting the threshold of saturation image pixel,which could associate the macro seepage characteristics with the micro bubble movement.The adsorption of xylyl-substituted biquaternary ammonium salt Gemini surfactants with different spacer(C₃ and C₆)at quartz,polytetrafluoroethylene(PTFE)and polymethylmethacrylate(PMMA)surfaces has been investigated and the different adsorption parameters such as surface tension,contact angle,adhesional tension,solid–water interfacial tension and work of adhesion have been estimated.The results showed that Gemini surfactants adsorbed gradually at a PFTE–water interface via hydrophobic interactions and the adsorption amounts at the water–air interface were apparently higher than those at the PTFE–water interface due to the steric hindrance effect.However,the contact angle kept constant throughout the experimental concentration range because the decrease in surface tension just counterbalanced the decrease in PFTE–water interfacial tension.Gemini surfactants adsorbed at the PMMA surface via polar interactions between xylyl and functional groups of PMMA before CMC.Similar to PTFE,the increase in PMMA–water interfacial tension compensated the decrease in surface tension and the contact angle also showed a stationary value before the CMC.A bilayer structure of Gemini surfactants will be formed at the PMMA–water interface via hydrophobic interaction and PMMA–water interfacial tension decreased consequently after the CMC,which results in the decrease in contact angle.Gemini surfactants adsorbed at the quartz surface via electrostatic interactions and the surface was hydrophobization before CMC.The increase in quartz–water interfacial tension compensated the decrease in surface tension,which results in the slightly increase in contact angle.A tight adsorption membrane will be formed at the quartz–water interface via the bending of flexible chain after the CMC,which results in the apparent increase in contact angle.it is worth noting that we have established a MD model,and analyzed the whole molecular dynamics process by comparing relative concentration curve of the system,radial distribution function between surfactant and water molecular and mean square displacement of surfactant molecular.The simulation results were consistent with the experimental ones.To summarize,for the research on the foamy oil microscopic phase behavior and rock wettability modification,we have established experimental and simulation method.Some important understandings would provide basic support for the applications of improving oilfield development and enhanced oil recovery technologies.