| The imbalance between the supply and demand of fossil energy and the high proportion of new unconventional reservoir reserves restricts China’s energy development.Nevertheless,the development of unconventional reservoirs incurs high investment costs,high construction risk,and a poor production effect.To reduce the contradiction between the energy supply and demand in China and ensure national energy security and sustainable development,"fracture network stimulation+imbibition replacement" has become an effective technical route to develop unconventional tight reservoirs.In this paper,based on the analysis of fracturing technology requirements and the principle of improving oil and gas recovery,combined with the development ideas of reducing cost and increasing efficiency,safety and environmental protection,a high-performance,low-damage and multi-functional fracturing-oil displacement fracturing fluid system is constructed.This system breaks through the traditional design of a"fracturing fluid + imbibition agent".By optimizing the self-synthesized fracturing fluid thickener,combined with the "horizontal well+volume fracturing technology",the reservoir fracture network transformation is realized,and the thickener degradation liquid is used to realize imbibition replacement of the reservoir and improve the efficiency of oilfield exploitation.The main research work includes the following parts:(1)The amphiphilic functional monomer stearic polyoxyethylene ether acrylate(SGA-n,n=3,5,10,15,40)was synthesized using acryloyl chloride(AC)and alcohol polyoxyethylene ether(SG-n,n represents the number of oxyethylene group segments).Structural characterization confirmed that the target products were synthesized.SGA-n had the effect of solubilization and reducing surface tension.The critical micelle concentration(CMC)of SGA-10 and SGA-15 was approximately 0.003 mol/L,indicating that SGA-n underwent entropy-driven self-assembly aggregation in water.To prepare high-performance amphiphilic water-soluble polymers,using SGA-n as the amphiphilic functional monomer to modify the polymer was crucial.(2)Amphiphilic functional monomer-modified partially hydrolyzed polyacrylamide polymers(PAMS-n,n=3,5,10,15)were synthesized using acrylic acid(AA),acrylamide(AM),and SGA-n(n=3,5,10,15)as the raw materials.The fluorescence spectrum of the pyrene-containing polymer solution confirmed the existence of hydrophobic microdomains in the solution.From the investigation of the polymer solubility,rheology and thermodynamics,with the increase in n,the thermal stability,solubility,thixotropic stress and viscoelasticity of the polymers gradually increased.The viscosity-temperature curves revealed that the viscosities of PAMS-3,PAMS-5,PAMS-10 and PAMS-15 with a mass fraction of 0.8%were 41,47,56 and 84 mPa·s at 140℃.The corresponding viscosity retention rates were 23.51%,28.23%,35.56%and 53.63%,the temperature resistance gradually increased.SEM images revealed that the polymer network structure of PAMS-15 was denser than that of PAMS-10.With the increase in the n of the polymer,the polymer structure gradually evolved from a spatial network structure dominated by hydrophobic association to a spatial network structure coordinated via hydrophobic association and linear entanglement.The macroscopic properties and microstructure evolution mechanism of PAMS-n provided key reference values for the research and preparation of modified polymers,especially high-temperature-resistant thickeners.(3)A hydrophobically associative polymer(HAP)was synthesized using AA,AM,2-acrylamido-2-methylpropanesulfonic acid(AMPS),and docosyl dimethylallyl ammonium chloride(DDAC).Amphiphilic functional monomer-modified HAPs(MHAP-n,n=5,10,15,40)were synthesized using AA,AM,AMPS,DDAC,and SGA-n(n=5,10,15,40).The application performance of MHAP-n was clearly better than that of HAP.In MHAP-n,with the increase in n,the water solubility of polymer molecules increased,XRD showed polymers did not exhibit sharp peaks,but only wide diffusion peaks,which were amorphous.The results obtained from the investigation of the thickening ability,rheology,and salt resistance of the polymer solution revealed that in MHAP-n(n≤15),with the increase in n,the thickening ability,temperature resistance,salt resistance,viscoelasticity,and thixotropic stress gradually increased.MHAP-15 exhibited a salt thickening ability in a NaCl solution,with a high viscosity retention rate in a CaCl2 solution.However,the thickening,temperature resistance,and salt resistance of MHAP-40 decreased,which was attributed to the decrease in the compactness of the polymer space network structure and the increase in the topological sliding behavior due to the strong hydrophilicity of the oxyethylene group segments.The performance differences between polymers preliminarily confirmed that MHAP-15 was more suitable as a thickener for oilfield fracturing.(4)The MHAP-15 fracturing fluid was prepared using MHAP-15 as the thickener,CJSJ-3 as the bactericide,KCl as the clay stabilizer,and the CF-5A fluorocarbon surfactant as the cleanup additive.Results of the viscosity,drag reduction rate,temperature resistance,salt resistance,and rheological properties of the fracturing fluid revealed that the MHAP-15 fracturing fluid was a highly viscoelastic fluid with a good thickening performance and high drag reduction rate.By adjusting the thickener amount,the fracturing fluid can be applied to high-temperature and ultra-high-temperature wells.When the salinity of the simulated formation water reached 8 × 104 mg/L,the viscosity of the MHAP-15 fracturing fluid with thickener mass fractions of 0.6%and 0.8%were 72 and 94 mPa·s,respectively,and the viscosity retention rate was greater than 50%.The MHAP-15 fracturing fluid exhibited good sand suspension performance,a low residue content of the gel breaking fluid,good compatibility with formation water,and low reservoir damage,which can meet the requirements for oilfield fracturing construction.(5)To improve the efficiency of tight oil and gas production,the effect of SGA-15 on the relationship between the polymer degradation liquid and reservoir imbibition replacement was explored.Ammonium persulfate was used as the oxidative degradation agent of the polymer.The hydroxyl radicals generated by the thermal activation of ammonium persulfate can attack the long-chain polymer,degrade partially hydrolyzed polyacrylamide(HPAM),HAP,and the amphiphilic functional monomer-modified HAPs(MHAP-15a and MHAP-15b).MHAP-15a and MHAP-15 are the same polymer,and MHAP-15b contained 0.04 mol/L of the SGA-15 monomer during polymerization.The results obtained from the investigation of the degradation of MHAP-15a by ammonium persulfate under different conditions(such as temperature,pH,salt)revealed that with the increase in the temperature,salt ions,and the acidic and alkaline water environment were beneficial for polymer degradation.The results obtained from the investigation of surface/interfacial tension of the polymer degradation solution,zeta potential of the solid surface of the core powder,wetting force,wettability change,capillary rise height and spontaneous imbibition efficiency,the MHAP-15a and MHAP-15b degradation solution exhibited a stronger wetting force,which could change an oil-wet rock into a water-wet rock,and the capillary rise height was higher.The imbibition replacement efficiency of the MHAP-15b degradation solution was greater than that of MHAP-15a.The imbibition efficiencies of 0.6 wt%HAP,MHAP-15a,and the MHAP-15b degradation solution soaked in a natural core at 30℃ for 265 h were 11.69%,34.89%,and 37.95%,respectively.The imbibition efficiencies of the 0.6 wt%MHAP-15a and MHAP-15b degradation solution soaked in an artificial core at 60℃ for 42 h reached 59.25%and 62.74%,respectively;hence,SGA-15 was introduced in the polymer has a positive effect on improving the imbibition replacement efficiency of the polymer degradation liquid and constructing a fracturing-oil displacement fracturing fluid system.In this paper,through the technical means of“fracture network stimulation+imbibition replacement",high-performance fracturing fluid was used to improve the mining efficiency of oil and gas fields,which has important reference value for the development and progress of multi-functional fracturing fluid. |
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