Molecular Dynamic Study On Mechanism Of Preformed Particle Gel Treatment For Conformance Control And Surfactant Flooding

Oil is called as "the blood of industry",and is widely used in various fields of people's life.Oil mainly comes from geological exploration,and it's unrenewable of reservoir increases year by year,so selecting an water shutoff agent which is economic and effective and an oil displacement agent which is high heat resistant and salt tolerant oil-production efficiency more effectively.Therefore it is important for developing enhanced oil recovery technology such as expanding swept volume and increasing displacement efficiency.The technology of water shutoff or conformance control and polymeric oil-displacing agent becomes a significant environmental and financial challenge for the whole petroleum industry.Fortunately,Preformed particle(PPG)has been proved to be successful in conformance control for oil reservoirs.The successful gel treatment can effectively reduce channeling or fractures without influencing oil productivity.Alcohol ethoxy carboxylates is a new oil-displacing agent showing high heat resistance and salt tolerance.With the deeper research and computer technology development over the past years,it is important to study the mechanism that water shutoff or conformance control of aggregates and aggregation behavior of compounds.Compared with the traditional experimental methods,Molecular dynamics simulations could be more intuitive for researching the interactions of atoms at the molecular scale,and for understanding the interaction mechanism between the mixtures deeply,therefore it could be helpful and meaningful to the synthesis of new compounds.In the dissertation,a series of research have been carried out molecular dynamic simulations in the following several aspects:the swelling in solution of partially hydrolyzed preformed particle gel(PPG),the "direct pass" transport mechanism of PPG within silica nanopores.We mainly focused on the interactions of atoms and effect of the structure in molecular scale.The important and valuable results in the diss.ertation can be summarized as following:(1)We have performed unit-atoms molecular dynamic simulation to investigate the swelling of PPG.After a 20ns simulation,the volume and radius of gyration of the particle increased rapidly.One reason was speculated as the strong hydration of hydrophilic groups of PPG(i.e.-COO-and-CONH2)in solution.It was shown that the structure of water was strongly modified by the presence of polymer.Then this polymer hydrophilic group induced modification was characterized from dynamic,structure and hydrogen bond aspects to gain a fully understanding of the hydration of PPG on molecular level.The structure of the hydration shell was investigated by SDF(spatial distribution function),RDF(radial distribution function)and distribution of dipole.The RDF and dipole distribution both indicated that O(COO-)induced a more ordered and densely packed hydration shell than O(CONH2)and N(CONH2),which means that O(COO-)has a very strong hydration ability.The dynamics of water around hydrophilic groups,as manifested in the translational and rotational diffusion and residence time of water,is slowed down in the presence of the hydrophilic group.The electrostatic interaction and hydrogen bonds between water and hydrophilic group were speculated to account for their slow mobility.We found that the hydrogen bonds in the vicinity of hydrophilic side groups also become stronger and longer lived by calculating the hydrogen bond residence time.The hydrogen bond network formed by hydration layer water molecules also stabilized the hydration shell according to the dipole reorientation residence time.In brief,the negatively charged center atoms of hydrophilic groups of PPG induced a highly ordered,tightly packed hydration shell around them and bound them with hydrogen bonds and electrostatic 8 attraction which strengthened its hydration ability.(2)At the molecular level,a molecular dynamics simulation on PPG transporting through nanopores was performed to investigate its propagation mechanisms during gel injection.Initially,a silica nanopore was modeled as a finite-length cylindrical pore,in which the inner surface was fully hydroxylated.Then,a swollen PPG with a smaller size was put in.After a long enough simulation,the hydration layer induced by silica pore surface was discussed to study the effect on the transport of PPG.Steered molecular dynamics was then used to mimic the transport of PPG under injection pressure.The results suggested that this hydration layer served as a physical and energy barrier that keeps PPG away from the pore surface by analyzing from radial number density distributions,orientational arrangement,dependence of the diffusive mobility,hydrogen bonding characteristics and potential of mean force.Also,the lubrication of the hydration layer may reduce the resistance that PPG has to overcome while transporting through nanopores.These factors will promote the propagation of PPG within nanopore and reduce the injection pressure.The simulated results were expected to provide the molecular level insights into the mechanism of PPG transporting through nanoporous media or the molecular design of optimized PPG.(3)On the basis of nonequilibrium molecular dynamics simulation,the translocation process of PPG in silica nanopores consisting of two different diameters was investigated.During the simulation,an external pulling force was applied to PPG representing the injection pressure.The simulation results suggest that a synergetic deformation and dehydration of PPG occurs during the translocation from the wide side into the narrow side.The energy barrier of the translocation process mainly result from the conformational energy change of PPG(mainly from the angle bend and dihedral torsion)and the dissociation energy barrier between PPG's hydrophilic groups and water.Furthermore,the nanopore size has a crucial impact on the translocation mechanism of PPG,not only the degree of the deformation and dehydration near the entrance,but also the translocation mechanism after they entered the nanopore.For a nanopore with a large diameter,PPG can reabsorb water to induce a complete hydration layer around it after entry.However,for the nanopore with a small size,the compression from the pore restricts PPG's rehydration ability.Without the screen and lubrication of the hydration layer,the pulling force needed to drive PPG increased rapidly,which means a larger injection pressure in the macroscopic view.The findings are helpful for understanding the translocation process of PPG in porous media on molecular level and,also,will facilitate technology developments for enhancement of recovery efficiency of petroleum.(4)By means of non-equilibrium molecular dynamics simulation,we investigated the translocation mechanism of PPG confined in different silica nanopores.The influence of surface chemistry and chemical heterogeneity of silica nanopore on the translocation process was revealed.As the degree of surface hydroxylation increases and the heterogeneity decreases,the pulling force needed to drive PPG decreases.Energetic analysis implies that the tightly bound hydration layer near nanopore surface which turns the "sliding friction" between PPG and nanopore into "rolling friction" plays a key role during the translocation.We infer that the nanopore's surface(i.e.surface chemistry and heterogeneity)affects the translocation of PPG indirectly by forming different hydration layers.The characteristics of the hydration layer was then quantified and compared by computing density distribution,orientation order parameters,residence correlation functions,diffusion coefficients and hydrogen bond properties.In summary,the results show that surface chemistry and chemical heterogeneity affect the structure and dynamic properties of the hydration layer.Generally,the higher degree of surface hydroxylation,the more densely hydration layer packed.The lower surface heterogeneity,the higher mobility of surface water.With the lubrication of this densely packed,tightly bound and high mobility hydration layer,the translocation of PPG in silica nanopore were largely promoted.(5)We have performed unit-atoms molecular dynamic simulation to investigate the effect of Ca2+ on the surfactant alcohol carboxylates(AEC)and sodium dichloroisocyanurate(SDC).Salt-bridging observed between Ca2+ and nearest neighbor headgroup pair,and the salt-briging reducees the electrostatic interaction of the surfactant micelles,so the combination of the micelle is closer.Ca2+ changes the structre hydration layer around the headgroup of surfactant and leads to a decrease in the number of H-bonds.The potential of mean force(PMF)shows that the energy barriers between the headgroup and Ca2+ and Na+ in the AEC System are higher than those in the SDC system.This indicates that SDC binds the ions easily compared with AEC,and the ions have strong effect on SDC system.