Viscoelastic Surfactant Fracturing Fluids For Use In Coal Seams: Effect Mechanism Of Hydrocarbon Chain And Mixing Of VES On Pores And Fractures Of Coal

Coalbed methane(CBM)is the associated resource of coal,and co-mining of coal and coalbed methane is a major demand of the national energy strategy.The geological conditions of coal mines in China are complex,and low permeability coal seams account for about 80%.Hydraulic fracturing technology is an effective measure to increase the permeability of coal seams.Fracturing fluids are the key to the success of hydraulic fracturing operations.The fracturing fluids used in underground coal mines are required to have low filtration loss and can promote gas desorption and migration.Therefore,the viscosity,temperature resistance,water loss,wettability,the effect on the pores and fractures,fracturing effect are important indicators for formulating,optimizing and evaluating the fracturing fluids.Due to differences in geological conditions and construction techniques,traditional fracturing fluids for oil and gas reservoirs and coal mine grounds fracturing are not suitable for underground coal mine fracturing.The Viscoelastic surfactant(VES)fracturing fluids have good viscoelasticity and environmental friendliness and can be used for underground coal mine fracturing.The performance of VES fracturing fluids and their effect on pores and fractures are affected by the hydrophilic group(type)and hydrophobic group(hydrocarbon group)of VES and the formulation(mixing of VES).However,the mechanism of hydrophilic group and hydrophobic group of VES and mixing of VES on the performance of VES fracturing fluids used in underground coal mines and the effect on pores and fractures of coal are still lack of research.Combination of laboratory experiments and theoretical analysis were adopted.Cationic,anionic and zwitterionic VES were selected.The influence law of the hydrocarbon chain of VES hydrophobic group and mixing of VES on the performance of VES fracturing fluids were explored.The sensitivity and influence mechanism of VES fracturing fluids with the different hydrophobic hydrocarbon chain lengths of VES and mixing of VES as the main agent on coal multi-scale pores and fractures were analyzed.The main research conclusions are as follows.(1)Influence law and mechanism of hydrocarbon chain length of VES and the mixing of cationic-zwitterionic VES,anionic-zwitterionic VES on the micelle self-assembly structure,ability to bind water molecules,wettability,viscosity,temperature resistance were obtained.The VES molecules with longer hydrocarbon chain have strong aggregation,which makes them bound more water molecules.The maximum viscosity of VES fracturing fluids formulated with STAC can be approximately twice that of similar fluids formulated with CTAC and the amount of STAC required to reach this maximum viscosity is 0.19 wt%less than the amount of CTAC needed to reach the CTAC fluid's maximum viscosity.The viscosity of VES fracturing fluid with CAB added is 10 mPa·s higher than that of a fluid without CAB.The mixing of cationic-zwitterionic VES,anionic-zwitterionic VES is beneficial to increase the viscosity of VES fracturing fluid,and there is an optimal ratio to reach the maximum viscosity.(2)The influence law of VES fracturing fluids with the different hydrocarbon chain length of VES and mixing of VES as the main agent on coal multi-scale pores and fractures of coal samples were revealed.The best formula for VES fracturing fluid to improve pores and fractures of different sizes was preferred.The VES fracturing fluid with the mixing of anionic-zwitterionic VES as the main agent reduces the porosity in the range of 14 nm–1?m by approximately 90%,and increases the porosity in the range of 125–180?m(Tashan)and 70–180?m(Nanchuan)by approximately 100%.The VES fracturing fluid with the mixing of cationic-zwitterionic VES as the main agent increases the average pore width of nanopore in the 2–300 nm range(Tashan:570.44%,Nanchuan:158.38%).The influence law of coal seam temperature on the pores of coal treated by VES fracturing fluid were revealed.The optimum temperature range for VES fracturing fluid increasing the porosity,pore volume and pore area of coal was proposed.The effect of temperature on micropores is greater than that of transition pores.(3)The influence mechanism of VES fracturing fluids with the different hydrocarbon chain length of VES and mixing of VES as the main agent on the pores and fractures characteristics of coal were revealed.VES fracturing fluids with cationic VES and the mixing of cationic-zwitterionic VES as the main agent are beneficial to the dissolution of calcite,pyrite,illite and montmorillonite.VES the fracturing fluids can enhance the electrostatic repulsion between the pore wall and the coal particles,which was beneficial to the removal of particles on the coal surface.The VES fracturing fluid with the mixing of cationic-zwitterionic VES as the main agent is most beneficial to expose more hydrophilic functional groups on the surface of coal and minerals,increase wettability,and thus affect nanopores.The VES fracturing fluid with the mixing of anionic-zwitterionic VES as the main agent can increase the spread of the aromatic layer L_a in the aromatic crystal nucleus of coal and reduce the spacing between layers d₀₀₂,thereby increasing the pore volume of the micropores.The temperature changes the wettability of VES fracturing fluids on the coal surface by changing the surface tension of the fracturing fluid and the number of oxygen-containing functional groups of coal surface,thereby affecting the removal of coal particles and mineral impurities in the pores.(4)The fracture propagation law of coal samples fractured by VES fracturing fluids with different formulations at different temperatures was obtained.The VES fracturing fluids with cationic VES as the main agent form a star-shaped(*)fracture network.The effect of the fracture is not affected by temperature,and it is easy to cause micro-cracks and pores in the center of the specimen.The VES fracturing fluids with the mixing of anionic-zwitterionic VES as the main agent were easy to dissolve or remove mineral impurities to increase the original pores or generate new pores.And the pores and fractures formed were mostly distributed around the positions containing mineral impurities.The research results of this paper enrich the relevant theories of the application of VES fracturing fluids in coal seams,which can provide theoretical support for the optimization of VES fracturing fluid formulations and large-scale application on site,and help to improve the coalbed methane extraction effect.