Theoretical And Experimental Study On The Casing Collapse Under Non-uniform Load And Working Mechanics Of Casing Repair

Both API 5C3 collapse model and ISO 10400 collapse model are not suitable to calculate the collapse strength of casing under non-uniform load,and there is not an internationally accepted standard which can calculate collapse strength of casing under non-uniform load up to now.The mechanical properties and failure mechanism of casing under non-uniform load are still not clear enough so that it is very difficult to predict the collapse strength of casing accurately.Hence,in the drilling process,the down-hole casing are more easily deformed or collapsed under non-uniform load caused by plastic creep,dislocation,slippage,sand production of stratum,large-scale acidizing and fracturing so that most of the oil and gas fields at home and abroad have the problem of casing deformation,which causes the increasing amount of deformed casings to be repaired.In order to reduce the sticking accident and improve the repair efficiency,it is very important to predict accurately the mechanical power required to repair the deformed casing,so that the theoretical and experimental studies on the casing repair are urgently needed to be conducted.Hence,based on the elastic-plastic theory,curved-beam theory,Hertz contact theory and the slip-line theory,the system studies on the deformation laws,failure mechanism of casing with/without cement sheath under non-uniform load and the working mechanics of deformed casing repair are conducted by combining the theoretical analysis,finite element simulation and the laboratory experiment.The method of evaluating systematically the repairing effect of deformed casing with/without cement sheath repaired by casing swage is formed,which can provide technical support for the deformed casing repair in oil and gas field.The main work in this paper includes:(1)In order to meet the actual service environment,a mechanical model that can calculate casing collapse strength under non-uniform load is established based on the curved beam theory of the elasticity and is derived and solved using displacement method.The accuracy and reliability of this mechanical model is validated by the corresponding test data so that it can be used to predict the collapse strength of casing under non-uniform load.(2)Based Hertz contact theory and slip-line theory,the mechanical model that can calculate the reshaping force required to repair deformed casing by spinning casing swage is established,by which the plastic limit load of casing deformed section under one-side uniform pressure is analyzed in the repairing process,and the reshaping force/bit pressure is determined by combining the structural feature of spinning casing swage and the mechanical properties of casing repair.The accuracy and reliability of mechanical model are validated by the corresponding finite element simulation results and test data so that it can be used to predict the reshaping force required to repair deformed casing.(3)Based on the curved-beam theory and unified strength theory,the mechanical model of pear-shaped casing swage used to repair deformed casing is established,by which the reshaping force/bit pressure is determined by combining the structural feature of pear-shaped casing swage and the mechanical properties of casing repair.According to the model,the effects of expansion amount every time,friction efficient,cone angle and length of equal diameter section on the reshaping force are analyzed.The method of mechanics and kinetics analysis about axial loading and circumferential deformation of deformed casing is formed in the repairing process by using pear-shaped casing swage.The accuracy and reliability of mechanical model are validated by the corresponding finite element simulation results and test data.(4)Based on the elastic-plastic theory,the three-dimensional finite element models compound materials system(deformed casing,cement sheath and stratum)which can simulate the repairing process of cement-casing repaired by spinning casing swage and pear-shaped casing swage are presented,by which the whole repairing process of cement-casing and casing alone with elliptic section has been simulated to study the mechanical properties of deformed casing repair.The stress-strain law and deformation(long axis and short axis)law of deformed casing are analyzed in the repairing process.The reshaping force required to repair the deformed cement-casing and casing alone,the expansion amount of short axis,shortening amount of long axis and the spring-back amount after repairing are obtained by calculation.The finite element method of evaluating the repairing effect of deformed casing is formed.(5)The collapsing experiments are conducted for casing without cement sheath and casing with cement sheath under non-uniform load by using electro hydraulic servo pressure testing machine,by which the hardening character,stress-strain laws,displacement variation laws and collapse failure mechanism of casing are studied.Based on the test data,the effects of cement sheath on the mechanical properties(such as yield load and instability load)are analyzed under non-uniform load.The testing results can provide theoretical support for experimental study on the casing repair.(6)Based on self-developed testing equipment,the full-scale repairing experiment of cement-casing and casing alone are conducted by using spinning casing swage for studying the mechanical properties of deformed casing repair,by which the effects of cement sheath and its degree of damage on the reshaping force and torque required to repairing casing,deformation laws of casing in the repairing process and the damage of spinning casing swage after repairing are analyzed.The reshaping force,torque,expansion amount of short axis,shortening amount of long axis and the spring-back amount of deformed casing after repairing are obtained by calculation.The mechanical properties of deformed casing and spinning casing swage before and after repairing are evaluated,and the structural parameters of spinning casing swage is optimized and designed.The test data is used to validate the theoretical and finite element model of casing repair.The experimental method of evaluating the repairing effect of deformed casing repaired by spinning casing swage has been obtained.(7)The full-scale repairing experiment of cement-casing and casing alone are conducted by using pear-shaped casing swage for studying the mechanical properties of deformed casing repair,by which the effects of cement sheath on the reshaping force required to repairing deformed casing,deformation laws of casing in the repairing process and the damage of pear-shaped casing swage after repairing are analyzed.Based on test data,the reshaping force required to repair deformed casing,expansion amount of short axis,shortening amount of long axis and the spring-back amount of deformed casing after repairing are calculated.The mechanical properties of deformed casing and spinning casing swage before and after repairing are evaluated.The test results are in agreement with the theoretical and simulation results,which indicates that the theoretical and finite element model are accurate and reliable.The experimental method of evaluating the repairing effect of deformed casing repaired by pear-shaped casing swage has been obtained.The theoretical model of calculating casing collapse strength under non-uniform load established in this paper has been validated by the test data so that it can provide an important theoretical reference for collapse strength design of casing under complex environments.The accuracy and reliability of theoretical and finite element models used to calculate the reshaping force of pear-shaped and spinning casing swage have been validated by the test data so that they can not only be used to predict reshaping force/bit pressure and deformation laws of casing after repairing,but also have important reference value to improve the parameter design of actual deformed casing repair.Finally,based on the theoretical-analysis,finite element simulation and laboratory experiment,the method of evaluating systematically the repairing effect of deformed casing repaired by pear-shaped and spinning casing swage.