Synthesis And Solution Self-assembly Of Long Chain Alkyl Hydrophobic Polymers And Its Wettability

In the process of tight oil development,the fracturing fluid carries proppant to achieve the purpose of reservoir fracture network transformation.The performance of the fracturing fluid directly affects the development degree of the reservoir,and the sliding water fracturing fluid has been popularized and applied due to its low friction,high sand carrying and low damage performance characteristics.Macromolecular polyacrylamide is generally used as the main additive of sliding water fracturing fluid.The type and structural characteristics of polyacrylamide directly affect the performance of fracturing fluid.Due to its unique molecular structure,hydrophobically associating polymers give good viscoelasticity to the solution.As a sliding water fracturing fluid,hydrophobically associating polymers are characterized by low cost,low friction and high sand carrying.The properties of commonly used hydrophobically associating polymers are affected by the types of ions and hydrophobic groups.Cationic hydrophobic associating polymer has strong associating ability,but its dissolving ability is poor,so it is difficult to meet the technical requirement of fast dissolving and thickening of fracturing fluid.Anionic hydrophobic associating polymer has good solubility and strong association,but it is sensitive to salt ions,especially divalent salt ions,which requires higher water quality and is limited in application and promotion.Amphoteric hydrophobic associating polymers have strong associating ability and certain salt resistance,and have the potential to be used in slippery water fracturing fluid.The differences in the types of hydrophobic side chains in the molecular structure lead to obvious differences in the associating ability,salt resistance and synergistic ability of such polymers with surfactants.Therefore,it is particularly important to develop an associative polymer with good dissolving and thickening properties,outstanding salt tolerance,strong solution association,and suitable for sliding water fracturing fluid.Based on the above background,the modified polymers of different hydrophobic monomers were firstly prepared,and the target functional hydrophobic monomers were screened through the structure of the products and the properties of the solution.Secondly,the structure of the selected hydrophobic monomer was changed,and a series of long-chain alkyl amphoteric polymers were synthesized to find the law of gradual change affecting the structure and solution performance of the associative polymer and the influence law of the ratio change of complex hydrophobic carbon chain structure on the product performance was studied.At the same time,multi-branch amphoteric associative polymers were synthesized to study the molecular structure and solution properties,and surfactant induced assembly was conducted to explore the assembly process and mechanism of self-assembled gels,and to explore the wettability and wettability mechanism of self-assembled gels after degradation.Finally,the preliminary experiment of associating polymer production was carried out and used in the field test of sliding water fracturing fluid.Firstly,Molecular design and synthesis,structural characterization and solution performance of hydrophobic associative polymers.The series of associating polymers are all amorphous.The more complex the hydrophobic chain structure is,the more tightly the polymer macromolecules intertwine,and the higher the thermodynamic decomposition temperature is.The hydrophobic chain length of EHMA,SMA and DOAAC increased,the intermolecular association enhanced,and the phase state was more stable.The molecular weight of the product was affected by the increase in the chain length of benzene ring group,EHMA and SMA structure in the monomer structure,the monomer polymerization competition rate was reduced,the copolymerization ability was enhanced,and the molecular weight was decreased,while the molecular weight of DOAAC-PAM was improved,the water content of modified products of different hydrophobic monomers was similar,and the polymerization process was mature.AMPS-PAM has weak association ability due to the influence of hydrophobic chain length and rigid groups.The CAC values of EHMA-PAM,SMA-PAM and DOAAC-PAM were 0.8%,0.5%and 0.4%,respectively.Doaac-pam is a amphoteric associative polymer with fast dissolution rate,strong thickening ability and associative use in aqueous solution,and high viscosity retention rate and good salt tolerance in salt ion solution.The excellent solution performance and low CAC value indicate that the polymer modified by long chain alkyl hydrophobic has the potential to be used as water fracturing fluid.Secondly,series of hydrophobic modified polymer SGP were prepared and its structure and solution properties were carried out.The molecular chain structure,crystal type,molecular weight and water content of the product are basically the same.The association of DDAAC-PAM,DMAAC-PAM and DHAAC-PAM is weak,showing a viscous structure.DOAAC-PAM and DD22AAC-PAM have strong association characteristics and show elastomer structure.With the increase of the R value of the hydrophobic chain,the linearity degree of macromolecules decreased,the degree of irregular winding increased,the group skeleton size widened,the association structure was formed,and the critical association micellar concentration decreased.Ddaac-pam has excellent surface interface activity,while DOAAC-PAM and DD22AAC-PAM have super association.Thirdly,the polymer FGP was prepared and the structural characterization and performance were tested.In the FGP structure,the arrangement of hydrophobic chains on the main molecules changes,CAC value decreases,molecular chains recombine in aqueous solution,the arrangement of hydrophobic carbon chains changes,association kinetic energy increases,and the density of microregions increases.FGP has obvious increase in apparent viscosity,excellent temperature and shear resistance,strong thixotropy,and high viscosity retention rate in saline.Macromolecules are degraded into small molecule structure like surfactant structure,and small molecules in solution synergistically reduce the solution table/interfacial tension and improve the surface interface activity.Fourthly,the FSGP gel system was prepared and the exploration of the stimulating assembly effect of CTAB was explorated.CTAB induces molecular assembly of hydrophobic chain,and the structure of "hydrophobic microzone" disappears.Micellar charge repulsion forces enhance the tension between molecular chains,forming a "fishing net" structure,and the solution self-assembly structure tends to balance.Influenced by the number of macromolecules and CTAB,the gel structure changed and the self-assembled structure tended to be stable.The mean square radius of rotation and the hydrodynamic radius decreased first and then increased.As the number of macromolecular chains increased,the aggregation number(N)of CTAB micelles increased.FSGP is superior to FGP in temperature resistance,shear resistance,thixotropy,viscoelasticity and salt resistance.The wetting ability of FSGP solution was stronger than that of FGP,and the surfactant structure formed by macromolecule fracture played a synergistic effect with CTAB.Fifthly,the wettability mechanism of FSGP system was studied and the performance of solution was analyzed.After FSGP is destroyed,the structure is changed and the "fishing net"structure disappeared.The assembly points are accumulated.The higher the concentration is,the more obvious the accumulation,showing the micelle aggregation characteristics of surfactant.The molecules in FSGP gel breaker forms "stick" micelles.The capillary tube of FSGP has strong imprinting ability,fast imprinting speed in CT experiment and deep imprinting depth.The contact angle on the sandstone surface is 146°,realizing wettability alternation.FSGP system is non-emulsified with crude oil and the water precipitation rate is 100%.The critical micelle concentration(CMC)of FSGP is low and the surface tension(γLV)is low.Adhesion force(γLvcosθ)is lowest near the CMC and the adhesion work(WA)also has the minimum value.The molecular adsorption resistance is low and there is a bimolecular adsorption.The wettability reversal mechanism of FSGP is "imbibition-adsorption-stripping-adsorption-wettability reversal".Finally,the fracturing fluid of FSGP was tested in changqing oil field.The fracturing fluid system of FSGP shows stable performance,low friction,high sand transport and repeated use.During fracturing,the construction pressure is generally reduced,which reduced the energy consumption of the construction equipment.After the fracturing fluid entered the formation,the gel is broken sufficiently and the gel breaker entered the formation through self-absorption.The reservoir wettability is improved.The initial production of the test wells after construction is 6.3 t/d.The initial test well produced 10.5 t/d oil and then 12.64 t/d oil six months later indicating that FSGP system could self-absorb into the reservoir and change the wettability of the reservoir,resulting in a substantial increase in oil production.After the construction of the horizontal well,the initial oil production is 27.6 t/d and the water cut is 51.8%.At the later stage,the production increased gradually with oil production of 31 t/d and water cut of 31.2%.The increase in oil production indicates a change in the formation wettability.