Research And The Risk Assessment Of Casing Burst Strength After Casing Wear

What the function 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 function 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, environmental 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.Firstly, this text has consulted the related literature at home and abroad about the wear casing, and investigated the casing’s use in the major oil field to make sure the scope of the worn and the shape of the main worn zone. Secondly,we taken the elastical and plastical analysis to the casing. Thirdly we taken numerical simulation analysis by using the finite element software ABAQUS to the strength of resisting internal pressure of the wearing casing. Finally, we discussed the risk assessment method of the wearing casing.The major research work and main conclusions are as follows:1. Failure under internal pressure in the wearing casing will seriously affect the drilling safety and oilfield stable and high yield, resulting in huge economic losses;2. Radius of curvature of the crescent-shaped wear affect the strength of resisting internal pressure of the casing smally, so it can be neglected;3. By researching the resisting internal pressure of the wearing casing, it can be obtained the formula of the initial resisting internal pressure and the bursting strength in a certain amount of wear;4.The weakest position of wearing casing is in the region between0degrees and27degrees. In this region, the stess concentration phenomenon is very obvious;5. When the wearing casing is affected by the internal and external pressure, it can be covert the pressure to the net internal pressure, and then analyzed.