Effect Of Fracturing Fluid On Oil And Water Flow Law In Micro-fracture Of Tight Reservoir

Hydraulic fracturing is a necessary means for the development of tight reservoirs.In the fracturing construction,the composite fracturing fluid system is mainly used,and after the injection of the gel system to form the main crack,the slick water/linear glue system is injected into the complex micro-fracture network.Multi-stage fracturing of horizontal well in tight reservoir needs a great amount of fracturing fluid,at the same time,the low return rate leads to a large number of slick water/linear glue systems remaining in the micro-cracks after fracturing,while the polymer drag-reducing agent is an important component of slick water/linear glue,so the retention of fracturing fluid will affect the characteristics of oil/water flow in micro-fracture.Because the polymer has the characteristic of selective water shutoff,it is necessary to explore whether the mechanism of the polymer in conventional reservoir to reduce the oil-water permeability can be introduced into the tight reservoir,so as to correctly guide the fracturing construction and efficient development of this kind of reservoir.In this paper,through indoor evaluation experiment,the interaction between fracturing fluid and tight sandstone is studied from the test of wetting angle and filtration,and the influence of different factors such as injection velocity,crack opening degree and polymer concentration of the fracturing fluid to the oil/water phase permeability is analyzed.And the following understandings are obtained:By evaluating the wetting angle of tight sandstone after fracturing fluid treatment and the filtration performance of tight sandstone surface,it is revealed that the fracturing fluid retention causes the rock surface to change from hydrophilic to weak hydrophilic and even neutral wetting,which can effectively reduce the further filtration of fracturing fluid to rock matrix.The experimental results of fracturing fluid filtration show that the filtration curve of ultra-low permeability cores can exhibit three-section characteristics of large,medium and small polymer molecules,but the filtration curve of tight sandstone cores can only present the stable filtration phase of small molecule polymer filtration.Visualized simulation of the fluid flow in tight reservoir fractured model shows that,in the first round of the injection,adhesion or retention of the polymer solution has reached saturation.For the micro-cracks with specific opening degree,the flow characteristics of the polymer solution have the critical molecular weight,and when the molecular weight exceeds the threshold,the flow law changes.The higher molecular weight,the higher Polymer viscoelastic index is,and the stronger the resistance effect of polymer viscoelasticity is.The influence of the retention of polymer to water phase resistance is greater than the degree of the influence to oil phase flow resistance.Through laboratory evaluation experiments,we deeply analyzed the influence of oil and water injection rate,fracture width and polymer agent concentration on the residual resistance coefficient of oil phase and water phase.And established a mathematical evaluation model of residual resistance coefficient based on that.The results show that the residual resistance coefficient of oil phase and water phase decreases with the increase of shear rate(fluid velocity),and shows a good power law relationship at low injection velocity.Under the same injection condition,the residual resistance coefficient of the water phase is greater than the residual resistance coefficient of the oil phase.It is shown that fracturing fluid retention preferentially reduces water phase permeability in micro-cracks Under the same fluid velocity and polymer concentration,the residual resistance coefficient of oil phase and water phase decreases with the increase of crack width.The higher the concentration of polymer drag reducer,the greater the residual resistance coefficient of water phase and oil phase.