Models And Methods To Strengthen Wellbore Pressure Containment By Fracture Plugging In Fractured Reservoirs

Fractured reservoir is one of the hot points of exploration and development as an increasing number of companies move to the exploitation of more and more challenging oil and gas resourcess in deeper,tighter,and more complex reservoirs,which mainly include carbonate reservoir,tight sandstone reservoir and shale reservoirs.Developed fractures are beneficial for the economic and efficient development of fractured reservoirs.However,they will lead to working fluid loss and induce serious formation damage.Therefore,it is necessary to develop effective wellbore strengthening method for fractured reservoir to control working fluid loss and improve wellbore pressure containment,whose importance can never be exaggerated to the timely discover and efficient development of the fractured reservoirs.This paper focused on the working fluid loss prevention and formation damage control in fractured reservoirs.The interaction mechanisms between plugging zone and fracture were taken as the core scientific problems.Along the master line of plugging zone failure,fracture propagation and formation breakdown under high pressure,this paper conducted theoretical analysis,laboratory experiments and computer simulations to achieve our goals.Theoretical analyses of wellbore strengthening effect were carried out to develop the prediction model of wellbore pressure containment after wellbore strengthening in fractured reservoirs.Laboratory experiments including plugging material properties measurement,plugging material optimal selection and plugging effect evaluation were conducted to form the wellbore strengthening methods for fractured reservoirs.Based on the above works,this paper achieves the following progresses.The underlying mechanisms of wellbore breaking after plugging were revealed.Based on the fracture development characteristic of typical fractured reservoirs,the influencing factors of wellbore strengthening effects were analyzed and the plugging breakdown pattern was developed to reveal the wellbore breakdown mechanisms after plugging in fractured reservoirs.Study results show that fracture plugging zone breakdown and fracture propagation are the two major reasons for wellbore breakdown after plugging.Fracture plugging zone strength and fracture propagation pressure have a comprehensive impact on the wellbore pressure containment.Fracture plugging zone strength model was developed.We analyzed the multi-scale structure of fracture plugging zone and conducted stress analysises for plugging zone after it was formed in the fracture.After that we revealed the plugging zone breakdown pattern and mechanism and developed the fracture plugging zone strength model.Study results show that the main acting forces on plugging zone include differential pressure between wellbore pressure and formation pressure,fracture closure stress and fracture face friction stress.And plugging zone friction failure and shearing failure are the two major patterns of fracture plugging zone breakdown,which have a comprehensive impact on the plugging zone stability.Sensitivity analysis results of model parameter show that the geometrical parameters,mechanical parameters and concentration matching parameters are the major parameters that affect the fracture plugging zone strength and stability.Model of fracture propagation pressure considering plugging effect was developed.We analyzed the effects of formation stress,wellbore pressure and fracture pressure on fracture propagation behavoir and developed the physical model for fracture propagation.Considering the formation stress parameters,rock mechanics parameters,fracture geometry parameters and plugging parameters,we developed the fracture propagation pressure model considering plugging effect.Sensitivity analysis results of model parameter show that plugging zone length,plugging zone width,and plugging zone permeability are the major plugging parameters that affect the fracture propagation.Prediction model of wellbore pressure containment after plugging and strengthening for fractured reservoirs was developed.According to the above two models of fracture plugging zone strength and fracture propagation pressure,the prediction model of wellbore pressure containment after plugging for fractured reservoirs was developed considering the wellbore breakdown pattern.Study results show that the wellbore pressure containment is equal to the fracture plugging zone strength when plugging zone permeability is less than the critical permeability of plugging zone.Otherwise,the wellbore pressure containment is controlled by the minimum value of fracture plugging zone strength and fracture propagation pressure.The plugging materials optimal selection principle was summarized.Based on the Prediction model of wellbore pressure containment after plugging and strengthening,we determine the optimal parameters.Using high-temperature high-pressure full diameter core fracture plugging apparatus,we conducted laboratory experiments of plugging material optimal selection,plugging parameters optimizing and plugging effects evaluation and summarized the plugging materials optimal selection principle based on the experimental results.The primary principle is the combination of rigid granule,elastic particle,and fiber.The secondary principle is the optimal selection of geometrical and mechanical parameters and the optimization of concentration and matching properties.The corresponding optimizing parameters include rigid granule D90 degradation rate,plugging zone-fracture face friction coefficient,plugging elastic deformation rate,plugging zone length,plugging zone width,plugging zone porosity and permeability.The wellbore strengthening strategy for fractured reservoirs was developed.According to the prediction model of wellbore pressure containment,we analyzed the applying conditions of fracture plugging strength improvement and fracture propagation pressure enhancing.The integral relations between the above two factors and the wellbore pressure containment after plugging were further determined.Based on the wellbore strengthening methods and plugging material optimal selection principle,we developed the wellbore strengthening strategy and route map for fractured reservoirs.The research results can be applied to prevent formation damage and control working fluid loss during the processes.of drill-in,completion and well workover for tight sandstone,carbonate rock and shale reservoir.