Simulation And Damage Study Of Fracturing Fluid Flow In Shale Gas Reservoirs

The environmental risks caused by hydraulic fracturing in shale gas development have sparked intense public debate.This is especially true in terms of the fluids used in fracturing that may reach water resources.It is difficult to exclude the possibility of future groundwater contamination based on theoretical analysis alone.In this paper,the damage of fracturing fluid is studied at macro and micro levels by means of theoretical analysis,modeling and numerical modelling.The macro level takes the formation as the study object and focuses on the impact of fracturing fluid diffusion at the bottom of the well and the contamination of shallow groundwater.The micro level takes the fracture as the research object,and mainly studies the residue left after the fracturing fluid is returned to drainage to block the fracture flow channel,reduce the fracture permeability,which affect the time for the fracturing fluid to be completely discharged from the formation and thus increasing the possibility of shallow groundwater contamination.This paper first analyzes the sources as well as the pathways of fracturing fluid damage.In view of the potential upward flow possibility after fracturing fluid injection,the flow control equations of fracturing fluid under the aquifer are investigated to provide the physical basis for subsequent modeling.From the perspective of expanding the possibility of fracturing fluid damage,seven elements that contribute to the upward migration of fracturing fluid are considered and compiled,which are fracturing fluid injection rate,shale formation properties,overburden properties,fracturing fluid properties,fault properties,fracture parameters,and field operation parameters.A model was developed to study fracturing fluid damage in the context of geological data from the Sichuan basin.The elements that contribute to fracturing fluid migration are studied at both macroscopic and microscopic levels,and finally guidance suggestions for flow control are given to achieve the purpose of damage reduction.The results of the macroscopic part of the study show the conditions that are favorable for fracture fluid to drain out of the formation and reduce the damage:Total fracture fluid injection volume ranges from 11×10~6-12×10~6 kilogram per day.High permeability with low porosity shale formations and overburden formations.50 m Pa?s fracture fluid viscosity.No fault presence or fault does not communicate with shale formations and shallow aquifers.Relatively small fracture aperture.Short fracture fluid injection time and 20-50 days shut-in time.Among them,conditions of fracturing fluid injection rate,fracture fluid viscosity,fracture aperture and operation time are controllable and have drastic influence on damage.The simulation results also show that the effect of primary fracture aperture on concentration is greater than that of secondary fractures,and the effect of injection period duration is much greater than that of shut-in period duration.The results of the microscopic part of the study show that complete blockage delays the discharge of fracturing fluid from the formation to a greater extent than partial blockage,and this difference is more obvious in the early stage.The difference in fracture fluid return rate between complete and partial blockage is large,and the contaminants in the fracture fluid will stay in the formation for a longer time because they cannot be discharged in time.In terms of particle size,the simulation results of complete blockage show that the effect of large particle size on fracturing fluid is higher than that of small particle size,but different results are obtained in partial blockage.Among the three scenarios simulated in this paper,the effect on fracturing fluid is the largest when the radius of particle size is medium,and partial blockage will have a more complicated effect on fracturing fluid than complete blockage.