Optimization Evaluation And Damage Mechanism Analysis Of Fracturing Fluid For Tight Sandstone Gas Reservoir

The exploration and development show that China’s tight sandstone oil and gas reserves are abundant.Due to its low permeability,strong heterogeneity and obvious anisotropy,the commonly used mining technology for this type of reservoir is hydraulic fracturing,but the incompatibility and poor performance of fracturing fluid cause serious damage to the reservoir and affect productivity.Therefore,it is of great significance to guide the development of tight sandstone gas reservoirs by optimizing the fracturing fluid suitable for reservoirs,understanding the damage mechanism of fracturing fluid to reservoirs,and proposing corresponding protection measures.In this paper,a block in Tarim is taken as the research object,and the scanning electron microscope experiment,whole rock clay mineral X diffraction,permeability experiment,nuclear magnetic resonance experiment,constant speed mercury injection experiment,fracturing fluid damage experiment and fracturing fluid protection experiment are carried out.The mineral composition and physical properties of the reservoir are tested,and the high performance fracturing fluid is optimized.The damage mechanism of fracturing fluid is analyzed from both internal and external aspects,and the best protective agent is selected on this basis.The conclusions of the study mainly include:（1）The overall permeability of the study area is about 0.0024×10^-3μm²～0.1778×10^-3μm²,the porosity is 1.12%～11.36%,and the reservoir connectivity is poor.The main throat types are necking and flake,which are prone to blockage,resulting in water sensitivity and water lock damage;flaky illite,foliated and vermicular kaolinite,and honeycomb and reticular illite/smectite mixed layers are common in the reservoir,with potential water-sensitive and velocity-sensitive damage.（2）The water sensitivity damage of guanidine gum and slick water fracturing fluid in the study area is moderate to weak.Through the experimental study on the mechanism of water-sensitive damage of fracturing fluid,it can be seen that the water-sensitive damage of fracturing fluid increases with the increase of reticular and flake illite.With the increase of injection volume multiples,the clay minerals in the core swelled or dispersed in water and blocked the throat,and the average water sensitivity damage rate increased by 11%.By changing the multiple of backflow volume,it is confirmed that the change degree of water sensitivity damage is low,about 1.24%;core permeability and porosity have little effect on water sensitivity damage rate.（3）Through the fracturing fluid water sensitivity damage experiment,it can be seen that the water lock damage degree of guanidine gum slick water fracturing fluid in the study area is medium weak and medium,and the water lock damage degree of fracturing fluid is medium weak.The experimental study on the mechanism of fracturing fluid water lock damage shows that the damage of fracturing fluid water lock is positively correlated with the content of illite and illite/smectite mixed layer.A certain volume multiple of flowback can greatly relieve the water lock damage.The initial release rate of flowback is faster,and it is basically unchanged after reaching 35～50 PV.The final water lock relief degree can reach 62.07%on average.Most of the cores with large pore throat ratio play a leading role in small pores.The greater the capillary force to be overcome,the greater the difficulty of backflow,and the higher the final water lock damage rate.（4）The solid phase damage experiment of fracturing fluid shows that the solid phase damage rate of guanidine gum fracturing fluid is distributed between 15%and 25%,and the solid phase damage rate of slick water fracturing fluid is basically lower than 5%.The large particle molecules in the fracturing fluid do not match the porosity of the tight gas reservoir,forming filter cake damage at the end face,and some small particle molecules enter the core matrix and block the throat.The two work together to form the solid phase damage of the fracturing fluid,which cannot be improved by backflow.（5）The high performance anti-swelling agent and water lock agent were selected in the fracturing fluid damage protection experiment,which greatly improved the damage caused by fracturing fluid.The reduction of solid phase damage can be achieved by optimizing the formula.