Numerical Research On The Propagation Behavior Of Hydraulic Fractures In The Low-Permeability Reservoirs

China is rich in low-permeability oil reservoirs,which depend on hydraulic fracturing technology for development.In Shengli Oilfield,the main types of low-permeability reservoirs are beach bar.glutenite,turbidite and mud shale.among which the fracturing efficiency is closely related to the hydraulic fracturing behavior that is affected by uncontrollable geological factors such as geostress,rock properties,physical properties.natural fractures,etc.and controllable artificial factors such as pump rate,fluid viscocity,perforated condition,bringing in the difficulty for fracture forecast.This paper firstly investigates the hydraulic fracture complexity and the influence of geostress.natural fractures'expores the influence of rock brittleness on fracture propagation behavoir,then focuses on the study of reservoir characteristics,hydraulic fracturing modes of glutenite that is the key object for development in Shengli Oilfield.Subsequently,this papers tries to stimulate complex hydraulic fractures in the huge thick glutenite reservoirs inspired by the complex fractures formed in shale and finally deal with the problems of limited fracture height in the thin layered glutenite reservoirs by numerial simulation.The main work and results are as follows:(1)Numerical simulation based on finite element method integrated with digital image processing technique is applied to reproduce the complex fracture formation.Natural fractures are found to control the fracture path and the natural fracture failure under hydraulic fracture is due to the union action of tensile and shear stresses based on acoustic emission field.Hydraulic fracture branches,simplex fracture network and complex network form in numerical lodels,indicating the denser of natural fractures,the lower of treatment pressure and the more complex of hydraulic fractures.Cases with different geostress differences generate wing fractures,branched fractures and fracture networks,indicating lower geostress difference incurs more complex fractures and it is difficult for reservoirs with geostress difference higher than 1.0 to form complex hydraulic fractures to a certain scale.(2)Influence of rock brittleness of shale and glutenie on hydraulic fracture propagation behavior is numerically investigated based on rock mesomechanics,repectively.Brittle shale is easy to develop dense natural fractures and then form complex hydraulic fractures.Hydraulic fractures prefer to propagate in brittle minerals or reservoirs,not in ductile minerals or reservoirs,due to the higher treatment pressure in ductile minerals or reservoirs.Brittle minerals prefer to produce hydraulic fractures with branches,which is easier to form complex fracture compared with the wing fracture in ductile minerals.A revised brittleness index is proposed based on parameters in a block in Shengli Oilfield and applied to evaluate the fracturing behavior of samples under numerical tests of uniaxial compression and hydraulic fracturing.It shows that the brittle glutenite sample produces multiple fractures in the uniaxial compression test and the ductile produce simplex fracture.Accommpanied with branches,hydraulic fractures in brittle sample propagate faster and require lower treatment pressure than the simplex fracture in ductile sample.Hydraulic fractures prefer to propagate in the brittle rnaterical while stop in the ductile materical.The higher the brittleness is,the larger the hydraulic fracture height is.(3)Glutenite reservoir characteristic is investigated,and then the size effect and hydraulic fracturing behavior of glutenite samples are numerically studied.It shows the size effect of g]utenite strength obviously exists when the sample size is small.Discontinuous bypass fracture is found in the numerical simulation,has many differences with common fractures and not easy to be found in laboratory experiments.The fracturing modes in glutenite samples and their forming conditions are summarized.The research value of fracturing modes in glutenite at laboratory scale for field fracturing lies in the hydraulic fracture complexity.(4)Based on a huge thick glutenite reservoir in Shengli oilfield.the vertical reservoir characteristic and the heterogeneity is analyzed."X"-shaped hydraulic fracture is obtained in numerical simulation and compared with the microseismic monitoring results.The forming mechanism of "X"-shaped fracture is researched and summarized as(?)the heterogeneity caused by weak grave]interfaces and natural fractures and(?)the stress interference of two close hydraulic fractures propagating simultaneously.For comparison,the "X"-shaped fracture is generally less complex than the complex fracture produced in shale.Further numerical studies show a lower geostress difference,a larger pump rate or a lower fluid viscocity can enlarge the horizontal stimulated area of hydraulic fractures and this area is mostly sensitive to geostress difference,then pump rate and last fluid viscocity.(5)The problem of limited fracture height in the thin layered glutenite reservoirs is analyzed.The hydraulic fracture propagation behavior is investigated through numerical simulation based on the cohesive zone model accounting for flow distribution in multiple fractures.The simulation result is compared with the miroseismic monitoring and multipole array sonogram results and gets validated.Influence of interlayer,perforation condition and beddings on fracture height growth are numerically investigated and the results can provide reference to the stimulation design of thin layered glutenite reservoirs with low-permeability.