Fabrication Of Viscosity-reducing Combination Flooding System And Its Displacement Mechanisms In Conventional Heavy Oil Reservoir

Although China is rich in conventional heavy oil resources,the application of conventional water flooding still comes with low development efficiency.That is because the unfavorable mobility ratio of water and oil in single and binary oil displacement systems causes low volumetric sweep efficiency,resulting in low oil recovery factor.Therefore,in order to improve the oil recovery,it is of vital importance to reduce the mobility ratio of water and oil,and improve sweep efficiency and oil displacement efficiency.In this paper,the viscosity-reducing surfactant was developed to reduce oil viscosity,while the high viscoelastic polymer was added to increase the water viscosity,leading to improve sweep efficiency.The ultra-low interfacial tension surfactant was applied to further improve the displacement efficiency.Thus,the viscosity-reducing combination flooding system was fabricated and its displacement mechanisms was investigated,providing strong foundation for chemical flooding application in conventional heavy oil reservoirs.Through physical experiments and numerical simulation methods,combined with the relevant industry standards of chemical flooding for enhance oil recovery,the oil viscosity suitable for polymer flooding and binary combined flooding should not exceed 200 m Pa·s and400-500 m Pa·s respectively in conventional heavy oil reservoirs.When the oil viscosity is higher than the criteria,it is crucial to add the viscosity-reducing components to reduce the viscosity of the oil phase to further improve the recovery efficiency.In this dissertation,the dominant viscosity mechanism of conventional heavy oil was investigated by analyzing the composition,structure and properties of conventional heavy oil,the XRD spectrum of asphaltene,and the relationship between asphaltene concentration and hydrodynamic radius.By using molecular simulation method,the molecular configurations of different types of chemical viscosity reducer and resin asphaltene model compounds were established,and the micro interaction mechanism between chemical viscosity reducer and conventional heavy oil was analyzed.The molecular structure of viscosity-reducing surfactant was designed as follows:the hydrophilic group in the structure contains sulfonate group and 2-7 ethoxy groups,while the lipophilic group has a carbon chain length of 8-16,containing a benzene ring,and the carbon chain in the linking group is less than 5.Polyanionic non amphoteric sulfonate type viscosity-reducing surfactant was synthesized,which can not only form a stable O/W emulsion with heavy oil,but also permeate and disperse between the resin and the asphaltene lamellar molecules.The permeation and dispersion effect can partially break up the aggregates formed by overlapping planes and make the aggregate structure loose.Then the network structure formed between resin and asphaltene is destroyed,which can weaken theπ-πconjugation of aromatics in the aggregates and reduce the proportion of surface accumulation with high interaction energy in the two accumulation modes.It is beneficial to the migration of emulsified oil droplets and the start-up of residual oil,thus further expanding the synergestic space.The oil-water interfacial tension of viscosity-reducing agent was in the range of 10^-1 m N·m^-1,and it could form stable O/W emulsion with common heavy oil in a wide range of oil-water ratio（8:2-2:8）under flowing conditions.Considering porous media seepage,non-Newtonian characteristics of fluid,wide oil-water ratio and other reservoir conditions,the evaluation method of dynamic viscosity reducing in porous media under flowing conditions was established.When the concentration of viscosity-reducing surfactant is 0.3%,the dynamic viscosity-reducing rate can reach more than 80%in the wide oil-water ratio range.Based on optimization selection of the high viscoelastic polymer and the low-tension surfactant,the viscosity-reducing combination flooding system was established.The selected high viscoelastic polymer has a denser network structure and stronger aggregates formed between molecules,which shows higher viscosity and bulk viscoelasticity.The elastic modulus G’is higher than 100 m Pa.The selected interfacial tension between oil and water is ultra-low and can reach 10^-3 m N·m^-1.The design requirements of viscosity-reducing combination flooding system with different crude oil viscosities were clarified.The oil-water interfacial tension ranges from 1.4×10^-3 to 9.3×10^-3 m N·m^-1.It has demonstrated that the viscosity limit of chemical flooding for conventional heavy oil extends to 1000 m Pa·s by viscosity-reducing combination flooding according to the results of core flooding experiments.Through core displacement physical experiments and microscopic visualization experiments,the flooding efficiency of different oil displacement systems for conventional heavy oil and the microscopic displacement characteristics in porous media were investigated.The flooding efficiency difference and displacement mechanism were clarified at core and pore throat scale.Compared with the traditional binary combination flooding system,the viscosity-reducing combination flooding system can increase the viscosity of the displacement phase and reduce the oil-water interfacial tension.Moreover,it is easier to form emulsified oil droplets in pores under weak dynamic conditions,and there are a large number of oil filaments or oil bank,which are of highest ability in improving swept volume and oil displacement efficiency in swept area.The unique synergistic effect can recover the remaining oil effectively,which can provide a new research direction for heavy oil chemical flooding.