Study On Analysis Of Collapse Resistance And Prevention Of Casing Damage In Perforating Interval

One of important research works for preventing perforated casing from damagesis exactly assessing the surplus resistance to collapse. On the basis of the researchof the stress concentration in the vicinity of the perforations, the main contents ofresearch for perforated casing that has been performed is as follows:The governing differential equation of elastic collapse has been presented onperforated casing. In view of the characteristics of the differential equation, theapproximate differential equation is presented by use of a small parameterε. The generalsolution of the elastic buckling question is given by perturbation method, and formulaof factor of elastic collapsing pressure on perforated casing with different forms of holesand perforation phase angle. The concept of equivalent holes is put forward. A methodto calculate collapsing pressure of casing perforating interval is presented on conditionthat stress concentration is taken into account in the neighborhood of holes; meanwhile,effect of perforation parameters on collapsing pressure of perforated casing is analyzed.On the basis of analysis to only keepε¹ in the governing differential equation of elasticcollapse, the differential equation withε² is solved by perturbation method. The generalsolution of the buckling question withε² is presented. The results of numerical analysisindicate that this is reliable one to evaluate collapsing pressure of casing perforatinginterval, when shot density is less than 20, the effect of perforating on collapsingpressure of casing is small, and when maximum axial dimension of equivalent holesd_Oe＜30mm, shot density D_n＜40 shots/m, it is no matter that the terms withε² indifferential equation have influence on collapsing pressure.After effect of creep formation on a single casing has been studied, on conditionthat stratum is viscoelastic, the further investigation is made about the effect oncombined casings. By means of flexibility factors, Additional load resulting from rockcreeping and general calculation formula of pressure transfer coefficients among casingsare given. The analysis results show that combined casings is efficient to prevent casingform damages, although additional load rises with the increase of the layer number ofcasing. In the light of the characteristics of casing perforating interval, a new method toprevent casing perforating interval from damage is presented, namely, the liner tube. The calculating formula is given on design of the liner tube.