Numerical Analysis Of Fracture Propagation Under Temporary Plugging Condition In Natural Fractured Reservoir

Chinese tight oil and gas resources are very rich and have become an important part of domestic energy supply.At present,the development of fractured tight oil and gas reservoirs at home and abroad is usually based on the temporary plugging and fracturing technology in the fracture area,so as to form complex fracture network,to connect the oil and gas enrichment zone near the well area and improve the oil and gas seepage channel.This technology is widely used in field,but the specific applicable conditions are not clear,the type of plugging agent,timing of addition,and addition amount of the temporary plugging agent are difficult to determine,and the fracturing parameters are difficult to optimize.Based on the above problems,this paper will carry out the research on the law of crack propagation under temporary plugging conditions,which will provide effective guidance for selection of wells and layers,and optimization of temporary plugging and fracturing parameters.In this paper,according to the main characteristics of crack propagation under temporary plugging conditions,firstly,the crack propagation is established under the temporary plugging condition considering the fluid flow in the fracture,the induced stress field of the fracture,the hydraulic fracture propagation criterion and the non-orthogonal natural fracture opening criterion under the induced stress.The model then combines the L-M method with the Gauss-Seidel method to solve the coupled fluid pressure and stress field.The accuracy and reliability of the model are verified.Finally,based on the process of fracture operation,the law of hydraulic fracture propagation before plugging,natural cracks opening after plugging,and the further extension of single-stage and multi-level natural cracks after plugging are fully analysized.Before the temporary plugging,the hydraulic crack propagation pattern is symmetrical two-wing line,and the extension direction is the direction of the maximum horizontal principal stress.The induced stress field generated by the surrounding rock in the process of hydraulic fracture propagation is mainly affected by the Young's modulus,the fracture height and the fluid viscosity.The higher the Young's modulus,the higher the liquid viscosity,and the smaller the reservoir thickness,the greater the induced stress generated by hydraulic fracture.The opening direction of the natural cracks after temporary plugging is mainly affected by its approaching angle and position.When the approaching angle is 0?90°,the upper natural crack is easier to open;when the approaching angle is 90°?180°,the lower natural crack is easier to open.The opening angle of the natural fractures on the same side is also affected by the location of natural fractures.The opening range of natural fractures is mainly affected by the difference of horizontal principal stress,the approaching angle and the breakthrough pressure of temporary plugging agent.The smaller the difference of horizontal principal stress,the larger the approaching angle,and the higher the breakthrough pressure,the larger the range of natural fractures that can be opened.After the temporary plugging,the primary natural crack propagation path will gradually deflect along the direction of the maximum horizontal principal stress,and gradually open the secondary natural fractures in the fractured reservoir,resulting in the crack network still tending to extend along the maximum horizontal principal stress direction;Under the induced stress field of hydraulic fracture,the fluid pressure during primary nature crack propagation process will keep decreasing until it reaches a certain distance or meets a secondary natural fracture.The fluid pressure drop during the crack propagation mainly occurs in the unturned nature fractures,and the width of the unturned nature fracture is significantly lower than that of turned nature cracks and hydraulic fractures.The main factors affecting the nature fracture propagation process under temporary plugging conditions are the difference of horizontal principal stress,the nature fractures location,the nature fractures angle,the plugging time and plugging length in the the hydraulic fracture.This paper deeply analyzes the opening direction and open range of natural fractures and the single-stage and multi-level nature fracture propagation process under artificial temporary plugging conditions,and the main factors influencing the nature crack propagation are clearly identified,which provides theoretical support for the application of temporary plugging and fracturing technology.