Risk Assessment Of Casing Burst Strength After Casing Wear

What the fuction of casing is separating the different porosity formation of fluid/pressure system,supporting the unstable layer which is collapsed or broken easily,and maintaining the stability of the borehole in the process of drilling and production.whether the faction of the casing is working effectively,it is determined by the casing burst strength.At present,drilling practice of deep wells,ultra deep wells,directional wells and horizontal wells are becoming more and more.due to the long time drilling and the serious bending of wellhole trajectory,the problem of casing wear is serious.in the stage of drilling,the casing is filled with slurry,so the inner pressure of casing is tremendous.after casing wear,the wall thickness of casing become thinner,so its burst strength turn down.the casing wall will crack easily,causing the slurry is leaking and running from one layer to another layer under the powerful inner pressure.it will not only pollute the producing layer,but also make the well control can’t be carried out safely,leading to endanger the personal safety,enviromental pollution and significant economic losses.so it is a urgent task for us to study the effect of casing well and well cementation defects to the casing burst strength,and then estimating the risk of high pressure wellhole,providing the scientific proof for reasonable casing maintainance and the decision of well control measure from now on.We have studied burst strength of worn casing through FEA in this text,the result of FEA is congsistent with the result of experiment,it shows that the FEA method is feasible.Firstly, we studied the maximum mises stress’s circumferential angle in the external wall of worn casing. Secondly, we studied the trend of the mises stress change with the wear depth under the same internal pressure, the mises stress is located in the0angle of the casing external wall. Thirdly,we studied the trend of the burst strength change with the radius of the drill pipe joint.Fourthly,we studied the trend of the burst strength changed with the worn region area under the same wear depth and we get the empirical formula of the burst strength.Finally,the risk assessment software of the burst strength after casing wear has been developed.And the conclusions have been obtained in this text are as follows:1. While the casing is bursting,the maximum mises stress of the external wall of worn casing is always located at0angle of the casing external wall.2.At a certain worn depth,the effect of the Rd size on the burst strength could be ignored,Rd is the radius of the drill pipe joint.3.At a certain worn depth,the effect of the worn area on the burst strength is very little,and the burst strength of the crescent-shaped wear casing coulded be calculated by uniform wear casing.4.Slotted ring formula exist defects, the modified empirical formula has ideal precision.5.Risk assessment software has been developed,the result computed by risk assessment software matched well with the result computed by ANSYS PSD module,it implied that the result computed by software is believable.