Study On Drag Reduction Mechanism Of Slick-Water Fracturing Fluid

The efficient exploitation of unconventional oil and gas resources is of great significance to China’s energy security.Large scale slick water volume fracturing is the most effective technical method for unconventional reservoir treatment.With the extended scale of fracturing stimulation,the energy loss caused by the wellbore friction greatly weakens the hydraulic fracturing effect,thus proposing stricter requirement for the drag reduction performance of slick water fracturing fluid.At present,there is a great disagreement in the drag reduction performance evaluation among the experimental calculation merely based on drag reduction rate and the practice in field application.And the mechanism study of drag reduction place emphasis on the macroscopic flow law rather than the microstructure of the drag reducer,which cannot provide guidance for performance improvement,structural optimization and on-site selection of drag reducer.In order to solve these problems,this paper carried out experimental evaluation of drag reduction performance,microstructure analysis of drag reducer and mathematical modelling of friction resistance.A high precision loop friction test system was developed to analyze the variation of drag reduction performance by concentration,temperature,salinity and shear flow.By summarizing and analyzing the microstructure of drag reducer,a microstructure characterization method based on the size,shape and thickness of mesh was established.And a set of experimental research methods were formed,which consist of the collection of indoor flow test samples,the preparation of the microstructure samples of drag reducer,and the observation of the microstructure of the drag reducer under the electron microscope.An effective characterization method for the microstructure of drag reducer was set up,which solves the problem that the structure of drag reducer cannot be described quantitatively.When the concentration of drag reducer increases（volume concentration up to 0.05%）,the drag reducers get connected to each other to form a network structure.This structure has certain deformation capacity and strong stability,and can partially store energy and withstand shear failure,which is the main reason for the improvement of drag reduction performance.Meanwhile,there is a critical point(15000s^-1)in the drag reduction performance.The destruction of the mesh structure of drag reducer was the main factor leading to the decrease of drag reduction performance.Through the viscoelastic analysis of the solution,the effective relationship between the microstructure of the drag reducer and the drag reduction performance is built up,deepening the understanding of the viscoelastic drag reduction mechanism.According to the mathematical modelling quantitative analysis of microstructure and the solution viscoelasticity,the contribution of viscoelasticity accounts for 61.8%of the friction resistance,making it the main control factor of drag reduction performance.Hydraulic fracturing field test achieved good operation based on newly established theoretical analysis and experimental research results.The drag reduction rate used in the experiment increases from 65%to 82%under 3m³/min displacement,which provides theoretical guidance for performance optimization,structure improvement and field selection of drag reducer.