Study On The Oil Displacement Mechanism Of Fracturing And Flowback In Tight Sandstone Reservoir

As a key technology for effective utilization and economic development of unconventional oil and gas reservoirs,hydraulic fracturing has been widely used in the development of various unconventional oil and gas reservoirs and has achieved good results.In the process of fracturing fluid injection into the reservoir,it plays a role in supplementing formation energy,communicating cracks and improving reservoir seepage capacity.When returning,it relies on higher formation energy to further carry out crude oil.In order to clarify the mechanism of fracturing flowback oil displacement in tight sandstone reservoirs,this paper uses core casting thin sections,scanning electron microscopy,high-pressure mercury injection and other means to preliminarily characterize the microscopic pore structure of the reservoir in the study area,and obtain various pore parameters;based on the microscopic model displacement system of high temperature and high pressure real core,the visualization experiment of fracturing flowback oil displacement is carried out with a single fracturing fluid system and a variety of additive compound systems.The migration and distribution of crude oil in the dynamic oil displacement process and the action range of fracturing fluid and crude oil before and after adding additives are compared and analyzed.The influence of rock characteristic parameters and experimental parameters on the effect of fracturing flowback oil displacement is quantitatively evaluated.Finally,the mechanism of fracturing flowback oil displacement is revealed from a microscopic perspective.The results show that:（1）In the process of fracturing fluid injection,the more developed the fracture or the larger the permeability,the larger the sweep range of the fracturing fluid,the more the fracturing fluid enters the matrix pores,resulting in a more dispersed distribution of crude oil in the model,mainly manifested as a significant decrease in oil saturation.During the flowback process of fracturing fluid,the more developed the fracture or the greater the permeability,the more conducive to the discharge of fracturing fluid.On the other hand,the greater the contact area between the fracturing fluid and the crude oil during the injection of the fracturing fluid,so that more fracturing fluid and crude oil participate in the flowback.Under the combined action of the two,the flowback fluid carries more crude oil out of the fracture and the surrounding matrix pores,which is mainly manifested in the significant increase of the flowback oil displacement efficiency.（2）In the process of fracturing fluid backflow,the types of backflow and oil displacement in the microscopic model of real sandstone are uniform,snake-like,dendritic and reticular.When the fracturing fluid flows back to the remaining oil state,the remaining oil distribution under the microscope mainly has four forms:cluster-like flake and island-like remaining oil formed by bypass flow,oil film,and oil droplets formed by sticking.（3）When the injection pressure increases from 4MPa to8MPa,the seepage path in the model gradually forms and tends to be stable,and the action range of fracturing fluid reaches the maximum.When the injection pressure increases to10MPa,the oil displacement efficiency remains basically unchanged.By adding surfactant（CHSB）to the fracturing fluid system and supercritical CO₂ assisted fracturing,both of them have a greater effect on the flowback oil displacement efficiency of low permeability and poor fracture reservoirs,and CO₂ has the most significant effect on the recovery of crude oil in such reservoirs.（4）Microscopic pore structure is the main factor affecting the effect of fracturing flowback oil displacement.For reservoirs with relatively large pore throat radius,fracturing fluid is more likely to enter rock pores.The larger the swept volume of fracturing fluid,the higher the recovery of crude oil.For the reservoir with poor pore-throat connectivity and more dispersed pore-throats,it is easy to produce flow-around phenomenon during the flowback process,thus forming island-like and cluster-like sheet-like remaining oil.The crude oil in the rock matrix is more difficult to be expelled,and the crude oil recovery is lower.