Research On Characteristics Of Intensifying Gas Desorption And Seepage By Viscoelastic Surfactant Fracturing Fluid

China is rich in CBM(coal bed methane)resources,exploring and using CBM will meet the demand of energy strategy and safe production in coal mines.However,most CBM resources have complicated geology conditions and the permeability is extremely low,which lead to difficult extraction.Hydraulic fracturing is an effective method of increasing the extraction of CBM that has gradually come into use in underground coal mines in recent years.The usually fracturing fluid used in underground coal mines is water which has low viscosity.During the process of hydraulic fracturing,the large filtration rate lead to low efficiency and plenty of water get into the pores and fractures of coal seam.The water lock effect inhibite gas desorption and seepage and restrict CBM extraction effect.Therefore,the developments of new fracturing fluid that can intensify gas desorption and seepage is of great significance for the improvement of underground hydraulic fracturing.Based on the characteristics of CBM occurrence and underground hydraulic fracturing,theoretical analysis,laboratory and field experiments methods were used to study the fluid rheology of fracturing fluids with different ratio and its influence on chemical and physical properties of coal samples.Then the influences on characteristics of gas desorption and seepage was analyzed with laboratory tests and field experiments were carried out.The main conclusions are as follows:(1)Fluid rheology of fracturing fluids with different ratio and its influence on wetting,composition,porosity and functional groups of coal samples were tested and formula of viscoelastic surfactant fracturing fluid suitable for underground hydraulic fracturing was optimally selected.Based on the characteristics of CBM occurrence and underground hydraulic fracturing,the viscosity,fluid rheology,viscoelasticity and effect on coal samples were determined.The component and ratio of viscoelastic surfactant fracturing fluid was selected which can increase the wetting and intensity,reduce the content of clay mineral and increase the porosity of coal samples.The viscoelastic surfactant fracturing fluid was 0.8%CTAC+0.2%Nasal+1%KCl.(2)Gas adsorption model with solid-gas-liquid phase was built and mechanism of viscoelastic surfactant fracturing fluid on gas adsorption and desorption was revealed.Intermolecular force,electrostatic force,structural force of water and viscoelastic surfactant fracturing fluid on the surface of coal sample were compared using surface physical and chemical theory to find that the viscoelastic surfactant fracturing fluid had better surface adsorption capacity.Based on the Lagmuir model of gas-solid interface and Gibbs model of gas-liquid interface,gas adsorption model with solid-gas-liquid phase was built.With the adsorption tests under fracturing fluids,mechanism of viscoelastic surfactant fracturing fluid on gas adsorption and desorption was revealed.Viscoelastic surfactant fracturing fluid increases the adsorption capacity by reducing the contact angel and surface tension.Meanwhile,the increase of capillary force is beneficial to intermal displacement of adsorped gas in pores.Viscoelastic surfactant fracturing fluid reduces the adsorption position and promote gas desorption under the dual role of displacement and competition.(3)Gas seepage model in pores was built and mechanism of viscoelastic surfactant fracturing fluid on gas seepage was revealed.Based on porous media fulid dynamics method,the form of fracturing fluid in the process of flow was analyzed and the gas seepage model in pores was built.In combination with seepage experiments of coal samples saturated with fracturing fluids,influence of viscoelastic surfactant fracturing fluid on gas seepage was revealed.The results show that viscoelastic surfactant fracturing fluid reduces the surface tension and increases coal porosity,so that it promotes gas seepage.With increasing effective stress,permeability of coal samples saturated with fracturing fluids decrease exponentially.Viscoelastic surfactant fracturing fluid increases the permeability and reduces the stress sensitivity coefficient so that it promotes gas seepage and extraction.(4)Effect of viscoelastic surfactant fracturing fluid on the gas extraction was compared with field experiments.The comparative experiments of viscoelastic surfactant fracturing fluid and water were conducted on the N3702 coal face of Yuyang coal mine in Songzao mining area.The results show that because of the gas desorption and seepage promotion by viscoelastic surfactant fracturing fluid,amount of gas extraction was increased by 26.1% in 60 days and gas concentration was held more than 70%.This can lay a solid foundation for the application of viscoelastic surfactant fracturing fluid in underground hydraulic fracturing.