Investigation Of Stability,Rheology And Leakoff Characteristics Of N₂/Liquid CO₂ Foam Fracturing Fluid

Unconventional oil and gas resources are abundant in China and its potential is very huge.Its effective development is crucial to the oil and gas security strategy of China.Hydraulic fracturing is the most widely used fracturing technology in China and is playing a key role in the effective development of unconventional oil and gas.However,conventional hydraulic fracturing showed great water consumption limiting its application in water-deficient area,and it also has pollution risks to the environment.Moreover,the widely used water-base plant gum fracturing fluids are suffering from a series of problems,such as incomplete gel breaking,high residue content,adsorption and plugging in formation,clay expansion,and backflow difficulty,etc.,which affect the performance of fracturing stimulation.Thus,N₂/liquid CO₂ foam fracturing fluid systems,characterized by low water consumption,environmental protection,high efficiency and low damage,were explored in this thesis.The stability,rheology and leakoff characteristics of foams were mainly studied,and the technology of enhancing viscosity and filtration control capacity of N₂/liquid CO₂foam was developed.First,hydrogen fluoride ethers with good solubility,thermal and chemical stability,environment-friendly performance were chosen as the foaming agent of the N₂/liquid CO₂mixed system.The solubility,foamability,foam stability,adsorption,interfacial rheology were measured to study the stability characteristic of N₂/liquid CO₂ foam.Base on the experimental results of foam volume and half-life,dilational viscoelastic interfacial modulus,temperature tolerance,the molecular structure of hydrogen fluoride ether was optimized,and ethoxy-nonafluorobutane was selected as the foaming agent.Foam rheology under the influence of temperature,pressure and foam quality was studied by using the self-designed large-scale tubular rheological test system.Hydrogen fluoride ether stabilised N₂/liquid CO₂foam significantly increased liquid CO₂ viscosity.The foam showed a shear thinning behavior,and the effective viscosity of foam reached the maximum at foam quality of⁸0%.Effect of foam quality,temperature,pressure on foam rheological index n,consistency coefficient k,and friction coefficient were studied.Then the empirical correlations of power-law rheological parameters were established,and showed an average calculation error less than10%.An experiment device for testing dynamic filtration of N₂/liquid CO₂ foam was developed,and the effects of foam quality and core permeability,temperature,pressure on foam filtration were studied.The experimental results showed that the filtration control performance of N₂/liquid CO₂ foam was better than that of pure liquid CO₂,liquid CO₂ with HFE and N₂/liquid CO₂ mixing system;When foam quality increased from 28%to 80%at a filtration pressure drop of 2MPa,the filtration coefficient and spurt filtration volume were reduced significantly.But a continuous foam quality was adverse to filtration control;For foam with a quality of 50%⁸0%,a change of 2 orders of magnitude in permeability resulted in a change of 1 order of magnitude in leakoff coefficient.At high pressure difference,lower initial quality foam?³0%?might show better filtration performance than higher initial quality foam?⁸0%?.The study of core damage after filtration showed N₂/liquid CO₂ foam was extremly clean.The technology of enhancing viscosity and filtration control capacity of N₂/liquid CO₂foam was innovatively developed.Based on the factor sensitivity analysis of foam rheology and filtration,hydrophobic nanosilica dispersions were developed as the enhancer.Nanoparticles absorbed on the interface between liquid enhancer and N₂?or liquid CO₂?and formed a particle"armor",thus improved the dilational viscoelastic interfacial modulus;Because of the enhancement of internal structure and strength,the effective viscosity of foam was increased significantly by adding nanoparticle enhancer,and its temperature sensitivity was decreased.The effective viscosity of foam was still higher than 50mPa?s at 80?;Moreover,flow behaviors in porous meida of foam with nanoparticle enhancer were explored,and flow resistance of Jamming effect and film slipping effect was quantitatively studied.The addition of nanoparticle enhancer deceased foam filtration coefficient by almost one order of magnitude,and the foam filtration sensitivity to temperature,pressure drop,core permeability was reduced.In addition,the experiments of core permeability recovery showed that N₂/liquid CO₂ foam fracturing fluid with nanoparticle enhancer had low-damage property.