Mechanics Analysis And Safety Evaluation Of Flexible Drill String In Ultra Short Radius Horizontal Well

Ultra short radius horizontal well is an advanced technology in oil fields.It has the features of lower cost,and greater effect on improving recovery of deposed or ended wells,which now has been widely used in foreign countries.Ultra short radius horizontal well requires a flexible drilling tools for construction,so strength of flexible drill directly affect the drilling success rate.Flexible drill string mainly includes flexible drill rod and guide sieve tube.In order to guarantee the strength of the tool can meet the construction requirements.This paper need to conduct stress analysis.The results can ensure the effective use of flexible drill string and site construction of success.Because of guide sieve tube not only produced random multidirectional collision with the flexible drill,but also contacted with the borehole wall in flexible drill string,which composed a complex dual nonlinear contact problems.The research had the certain difficulty,so the study had important theoretical significance and engineering application value.This paper based on the working state of flexible drill string.Considering the double contact between guide sieve tube and flexible drill,guide sieve tube and borehole wall,the whole nonlinear finite element model of curve sections and horizontal sections were established.Considering the contact properties between joint of the guide sieve tube,the contact between ball head rod and the ball seat,the single finite element model of flexible drill and guide sieve tube was established respectively.Considering the influence of the structure bending rigidity,borehole curvature,well pressure of flexible drill string,the nonlinear mechanics,step-by-step calculation method and numerical calculation methods were established.Through numerical simulation,the stress state and friction resistance of the dangerous sections of the flexible drill in different conditions is obtained which is in curve sections and horizontal sections of the wells under three different radius of curvature.The results show that three kinds of curvature can meet the strength requirement under different working conditions.In this paper,flexible drill rod was conducted torsion of in-house experiment.The limiting torque of single flexible drill rod is 2kN·m by realistic model,experiment and theoretical calculation.Conductor casing was conducted compressive test.The research which got the slot of load and order when closed,were same by comparing the experimental results and numerical simulation.The results verify the rationality of the numerical simulation model,and provide the reasonable parameters for site construction.This paper research describes the reasonable stress state of flexible drill string,provides a theoretical basis for site operation,and has an important project application value.