Research On The Factors Affecting The Stability Of Plugging Zone In Lost Circulation Fractures During Drilling Process

Lost circulation is one kind of common downhole complexities in drilling process,most of which are caused by induced fractures or natural fractures,and the most common solution is to plug them by suitable materials.However,according to the field application results,some plugging zones may be destroyed in the drilling process,resulting in low one-time plugging rate and repeated lost circulation and other engineering problems.Therefore,it is of great significance to clarify the instability mechanism of plugging zone and the influence factors during dynamic drilling process for the realization of long-term stable plugging of fracture lost circulation.Aiming at the stability of plugging zone in lost circulation fractures during drilling process,and taking the fracture-plugging zone as the research object,treating the process of the building and breaking under different conditions of the plugging zone in the fracture as the main line,the related influencing factors of plugging zone stability in drilling process are analyzed by the method of the combination of theoretical analysis and numerical simulations in this paper.First of all,the factors affecting the stability of the plugging zone are preliminarily determined by the stress analysis and bearing capacity calculation of the plugging zone.Then,numerical models of fracture-plugging zone with fixed and variable fracture aperture were established based on discrete element and continuous-discontinuous(PFC-FLAC)coupling theory,and a set of research methods for plugging zone stability was established.The breaking time and instability mode of plugging zone were judged mainly by the contact number and particle displacement during simulaion process.Based on the research method,the plugging zone stability influence factors of material and formation are analyzed respectively.Material factors including plugging zone bridging mode and ball friction coefficient,and formation factors including fracture friction coefficient,rock elastic modulus,fracture tip fluid pressure and horizontal minimum in-situ stress.Finally,the engineering factors affecting plugging zone stability are analyzed,which is the wellbore fluctuation pressure generated during drilling process.The stability of plugging zone under different tripping velocity is analyzed by using field data,and the range of tripping velocity to maintain the stability of plugging zone is determined.Suggestions for plugging program and construction parameters in site operation can be provided from this paper,and effective technical countermeasures of preventing plugging zone damage in the drilling process can also be concluded,which is of great significance for the realization of long-term stable plugging of fracture lost circulation in field.