Casing Effective Strength Study Under The Condition Of Complex Wellbore

Conventional casing strength design does not consider the influence of nonuniform load and cement sheath,so the paper put forward the casing effective strength calculation method under the condition of complex wellbore.Using this method to checking casing effective strength after cementing,it will provide instructional technical indicators for well stimulation,such as acid fracturing,multi-stage segmental fracturing,etc.;still can be used to analysis deformation mechanism of casing,combining downhole complicated conditions.All the above are the application direction of these research achievements in the field.Hereby,casing strength refers in particular to casing collapsing strength,effective collapsing strength is the practical collapsing capacity under the influence of many factors.Due to the current research of casing strength lack of experimental verification,the paper uses the mechanical simulation test device designed independently for the casing collapsing experiment research.Compared with the hydraulic device,mechanical simulation test device has the advantages of simulating many kinds of nonuniform external load,experimental operation simple and lower cost.Comparing the experimental results with theoretical calculation and finite element analysis,further improve the casing effective strength calculation method under the condition of complex wellbore.Casing collapsing simulation experiment research shows that:the nonuniform load cause casing strength are reduced by 12.8%,even greatly reduce under the partial load.The protection effect of cement sheath is more noticeable,high strength and toughness of cement ring can unload 27.4%under the condition of 100%center casing.Casing offset reduce unloading capacity of cement sheath,compared with casing center degree of 100%,the unloading capacity of cement sheath reduces by 6.9%while casing center degree is 30%,and reduces by 2.9%under the elliptical nonuniform load,that is to say,there is a little effect of casing eccentricity on unloading capacity of cement strength under the non-uniform in-situ stress.Compared with the modulus of elasticity,the yield strength of cement greater effects on unloading capacity of cement sheath.Cementing of casing and cement sheath is almost no influence on unloading capacity of cement sheath.In addition,when the major principal stress directly facing the narrow clearance of cement sheath,the unloading capacity of cement sheath is lower.