Safety Evaluation And Fracturing Process Analysis Of Casing String On The Old Side-tracking Well

With the increase of development time,the output of oil and gas Wells in the old area of oilfield decreases year by year.In order to maximize the oil and gas reserves in the reservoir,the technology of sidetracking old Wells can effectively solve this problem.However,it is difficult to guarantee the safety of casing string because the service life of casing string in old Wells has been completed.In addition,at present,most developers are still using the traditional API formula for checking the crush strength of casing string.In order to ensure the safety of casing string in service,this paper analyzes and draws some conclusions from the following aspects,with specific contents as follows:(1)Using logging data and images,casing deformation of the whole well interval of 6 Wells was analyzed,weak points of casing were pointed out,and the causes of casing deformation were analyzed.For example,in a well,casing geometry changes due to formation fluid corrosion,or casing deformation due to wellbore bending(2)The load spectrum distribution curve of casing string is established according to the geological design data.By using the method of theoretical calculation,the curves of casing strength against internal pressure and external pressure in 6 Wells are established,and the minimum tensile strength of casing is calculated.The safety factor values of 6 Wells are checked by the method of safety factor.(3)According to the actual use of casing,casing tensile,internal pressure bursting,external collapse and other related tests are designed.The tensile performance of API long round thread is verified by finite element method.The results show that the big end thread will have plastic deformation under the axial stress of 300 MPa,and the casing can withstand the maximum inner pressure of 65.7MPa,and the collapse failure pressure is 35.1MPa.By comprehensive comparison,the service performance of N80 steel casing in sidetracking old Wells can meet the requirements.(4)Based on the K-T model and the extrusion test data of 140 ksi steel grade casing,the parameters of the original model were optimized,and a new casing collapse strength prediction model was established.Compared with the K-T model,the prediction accuracy of the new model for the collapse strength of the same batch of steel pipes is reduced by 0.61%(5)The casing load distribution under fracturing conditions was compared between tieback and non-tieback liner completion methods.In the case of tieback liner completion,the lower end of casing string was subjected to smaller internal pressure.It is recommended to use tieback liner completion to fracture old sidetracked Wells.