Experimental Study On Proppant Slugs For Reducing Pumping Pressure Of Shale Hydraulic Fracturing

In the initial stage of hydraulic fracturing crack propagation in shale reservoir,high injection pressure at wellhead will occur due to hole friction near wellbore and bending fracture.Previous researchers have proposed a slug technology to reduce friction near wells.The proppant slugs technology is based on the principle of pumping sand-carrying fluid with low sand ratio,fine particle size and multiple groups into the formation in the pad fluid stage.which produces diameter polishing and enlarges the hole diameter of the casing hole.At the same time,it cuts the rock fracture hydraulically,reduces the tortuosity of the narrow curved fracture near the well,widens the flow path of the fluid flow into the far well fracture,and then reduces the construction pressure.At present,the application of proppant slugs technology mainly concentrates on reducing fracturing fluid filtration and preventing multi-fracture effects.For the erosion and polishing effect near wells,such as optimizing proppant types and testing based on multi-construction factors,although it has practical application,it lacks quantitative and specialized research.On the basis of literature and field investigation,this paper uses real shale material as target,and through a lot of exploration and demonstration in the early stage,carries out experimental researchs on proppant slugs.The main research contents and conclusions are as follows:(1)Formed a set of laboratory experiment erosion methods for sand-carrying fluid,including device debugging,target processing,characterization method establishment,factor selection,etc.Based on this,it is recognized that tortuous fractures is the main reason for friction suppression;properly increasing the jet velocity of sand-carrying fluid is helpful to break rock and reduce pressure drop(the experimental flow velocity is 110 m/s);the angle of sand-carrying fluid incident on target is different,and the erosion effect is different.The difference is large,and the damage degree of material is the smallest when erosion at zero angle.(2)The influence degree of sand-carrying liquid erosion on pressure drop results is:sand ratio,particle size,erosion time,types of proppants;the influence on hole enlargement rate is:erosion time,particle size,sand ratio,types of proppants.(3)The effects of erosion time on cement paste and rock are different because of their different physical parameters.(4)When erosion with sand-carrying fluid under narrow fractures width,the particle size should be greater than 60 meshes as far as possible,while the sand ratio is relatively insensitive and can be kept within 9%.(5)The mechanism of reducing tortuous friction by erosion with sand-carrying fluid is revealed through numerical simulation analysis.It is shown that when sand-carrying fluid advances in the fractures,the compressive friction will occur at the sudden change of curvature.With the increase of stress,the rock-breaking strength will be gradually reached,and then the friction will begin to decrease when the fluid flow channel is gradually widened.(6)When the grain size of sand-carrying fluid is 80/120 mesh,the intersection point will occur between the erosion and filtering failure results,and the grain size can be used as the optimum slug size for high filtration reservoirs.(7)Based on laboratory experiments,the optimum polishing proppant is quartz sand,and a set of optimum technological parameters is quartz sand,70/140 mesh diameter,9%sand ratio,two groups of slugs for 6 minutes.