Study On The Rules Of Fracture Initiation And Propagation In Longitudinal Multilayer Hydraulic Fracturing

The beach dam sand reservoir is an important low permeability reservoir.Taking Shengli Oilfield in Dongying area as an example,Dongying is located in the alluvial plain of the Yellow River Delta,mostly sand-shale sedimentary facies.The proven oil and gas reserves of beach dams account for a large proportion in low-permeability reservoirs.The beach dam sand reservoir has the characteristics of thin layer,low permeability,large physical difference,and interaction of sandstone,mudstone and conglomerate.Hydraulic fracturing technology is an effective means to develop low-permeability reservoirs,but there are relatively few studies on silt-fracturing of beach dams.Therefore,the study of hydraulic fracturing to develop beach dam sand reservoirs is of great significance to China's petroleum industry.Based on the investigation and a large amount of reading literature,this paper studies the crack initiation and propagation of longitudinal multi-layer hydraulic fracturing cracks through physical model test and numerical simulation.Because natural cores do not have the stratification conditions of physical model test scale,this paper makes artificial cores with perforation to carry out experimental research.On the basis of on-site coring test,mortar materials with similar lithological parameters to natural cores were prepared before the test,and cubic specimens with a side length of 100 mm were poured layer by layer after fixing casing.Coring of various types of lithologic artificial cores,and testing permeability and physical and mechanical parameters.Hydraulic fracturing true triaxial test system is used to simulate multi-layer fracturing process,and acoustic emission system is used to monitor fracture propagation process.A total of 12 groups of comparative tests were completed to study the effects of displacement,stratification,lithology and perforation conditions on fracture initiation and propagation.The test results show that:(1)Displacement: Displacement mainly affects fracture pressure,fracture propagation time and fracture morphology.The larger the displacement is,the higher the fracture pressure is,the shorter the crack propagation time is,and the smoother the crack surface is.(2)Stratum stratification: Stratification mainly affects fracture propagation time and core fracturing properties.The more the number of layers,the more complex the overall core properties,and the longer the fracture propagation time.(3)Lithology: Fracture pressure of different lithologies varies greatly.Fracture pressure of sandy conglomerate is the highest,while that of mudstone is the lowest.The higher the gravel content,the greater the fracture pressure and the shorter the fracture propagation time.(4)Perforation conditions: The greater the perforation density,the smaller the fracture pressure.The greater the azimuth of perforation,the greater the fracture pressure.In multi-stage perforation fracturing,the perforation density should be kept close to prevent unilateral fracture propagation leading to "up" or "down" of fractures.ABAQUS finite element software is used in the numerical simulation.On the basis of lithological parameters test in the previous article,thin interbedded models of three and five layers of sand and mudstone are established respectively.The cohesive element is used to simulate the cracks,and 24 groups of numerical simulation are completed.The effects of displacement,horizontal stress difference and viscosity on crack initiation and propagation are studied,and the relationships among fracture pressure,crack propagation time,crack width and other factors are analyzed quantitatively.The simulation results show that:(1)Displacement: In accordance with the test results,the larger displacement,the higher fracture pressure,and the shorter completion time of crack propagation.(2)Horizontal stress difference: cracks propagate along the direction of maximum horizontal principal stress.The larger the horizontal stress difference,the smaller the fracture pressure.(3)Viscosity: Increasing the viscosity can effectively reduce the filtration of fracturing fluid in formation,shorten the fracturing operation time and increase the fracture width.The experimental results are in good agreement with the simulation results.The numerical simulation effectively compensates for the deficiency of quantitative analysis of the physical model test.In conclusion,in multi-layer hydraulic fracturing construction,displacement should be controlled.It is suitable to use small displacement to produce cracks and large displacement to extend cracks;When perforating fracturing in multiple intervals,the perforation density should be kept equal;When fracturing occurs at mudstone perforation,fractures can effectively extend and fractured upper and lower sandstone layers.Appropriate increase of fracturing fluid viscosity can effectively improve fracturing efficiency and increase fracture width.