Study On The Propagation Mechanism And Effective Development Methods Of Waterflood-induced Fractures In Tight Reservoirs

For tight oil reservoirs,the depletion production after large-scale fracturing of horizontal wells is the main development method with a rapid decline in production.Some domestic oilfields have tried water injection huff and puff,but the waterflooding effect has been significantly worsened or even ineffective after multiple cycles of huff and puff.Therefore,effective energy replenishment methods for tight oil reservoirs are particularly important.This paper studies the water huff and puff of horizontal well in tight oil reservoirs,carries out the study of the principle of water injection-induced fracture propagation,the evaluation of the water injection huff and puff effect of horizontal wells,and the conversion of water injection huff and puff to interfracture area water injection method.On the other hand,this research characterizes the change principle of formation stress in the process of water injection huff and puff,reveals the difference in permeability and pressure of the three media of matrix,fracturing,and natural fractures,calculates the fracture propagation length and closure length,and clarify the seepage flow in the dynamic fracture network formed by natural fracture propagation principle.At the last of this paper,the factors affecting the evaluation of water huff and puff of tight oil reservoir being introduced,and the nine-scale method is used to evaluate the water huff and puff effect.Aiming at the problem of rapid production decline after multiple rounds of water injection in fractured tight oil reservoirs,a method to change the water injection throughput is proposed,the inter-fracture area water injection method is designed,and the injection-production system is optimized.This paper provides a theoretical basis for water flooding and imbibition under the condition of the dynamic fracture network and provides theoretical guidance for the transformation of horizontal well development mode,optimization of production system,and well pattern design.(1)The evolution process of natural fracture propagation induced by water injection during the water injection huff and puff process is described.The changes in the stress field of natural fracture propagation induced by water injection is revealed.characterizes the dynamic fracture propagation process of tight oil reservoirs in the matrix,natural fractures,and fracturing fractures.The changing law of permeability and pressure.The calculation method of dynamic fracture propagation length is proposed,and the fracture propagation length and the fracture closure length of the actual block after the fracture propagation occurs are quantitatively evaluated.(2)Clarify the imbibition characteristics in the dynamic fracture network formed by natural fracture propagation during the water injection huff and puff process.Initially,the injected water mainly produces radial displacement in the fracture network;with the extension of water injection time,both radial displacement and dynamic imbibition occur in the fracture network;reverse static imbibition occurs during the period of stopping injection and simmering wells.The characteristics of linear displacement,radial displacement,dynamic imbibition and static imbibition in dynamic fracture network are analyzed.(3)Taking into account the geological characteristics,fracturing process,and water huff and puff work system,the fracture network development degree and mathematical calculation formula of fracture network development coefficient is put forward,and the development degree of the dynamic fracture network composed of natural fractures and fracturing fractures in tight reservoirs is quantitatively characterized.Four indicators as net fracturing fluid influx,effective water injection rate,throughput replacement rate,and throughput decline rate are defined.Combining with formation energy retention level,6 evaluation indicators,such as oil recovery rate,water cut,water retention rate,and single-stage sanding in horizontal wells,have divided the value of each evaluation index into 3 categories and determine the value boundary of the evaluation index.Based on the nine-scale and expert scoring method,an evaluation method for the development effect of water injection huff and puff for tight oil reservoirs is established,and practical application is carried out.(4)Aiming at the problem of a large number of invalid water injection and production decline caused by multiple rounds of water injection huffs and puffs in fractured tight oil reservoirs,four injection-production methods of changing water injection huffs and puffs to water flooding are proposed.According to the remaining oil distribution characteristics,large-small-large-small cycle water injection,pulse water injection,segmented injection and production,and inter-fracture area water injection are used respectively.As if adding in each injection method Surfactant the interfacial tension will be reduced and the imbibition effect will be enhanced.17 kinds of injection and production systems are designed,and the dynamic fracture network numerical simulation method is adopted.According to the recovery degree index,the order from high to low is: area water injection type reverse seven-point method,segmented injection Mining,Periodic water injection,Pulse water injection.In actual application,the development method can be determined according to the characteristics of the reservoir and the development effect of depletion or water injection huff and puff.