Stiff String Casing Design with 3D Orientated Casing Wear

Abstract

The forces and stresses along a casing string are modelled using a stiff string torque and drag model. A post analysis of casing loads and stresses, considering wellbore tortuosity and centralisation is performed in addition to the effect of 3D orientated casing wear. A post analysis case study is presented to show the resulting effect on axial, burst, collapse and VME safety factor as well as VME body and connection design envelopes. While running in hole (RIH) a tubular, the industry standard is to assume a smooth wellbore when performing a torque and drag calculation. The drilling process can cause significant local doglegs and ultimately increase the tortuosity of the wellbore. When applying a soft-string torque and drag model, it is possible that the stiffness, radial clearance, and high frequency surveys are not directly considered. The stiff string torque and drag and buckling model can model these effects, as well as the addition of rigid and flexible centralisers. This study involves the comparison of different casing design load cases, using the actual tortuosity of a drilled wellbore considering a 3D orientated casing wear. The results show that there can be noticeable differences in overall axial stress with the addition of tortuosity. The stiff string model can directly calculate the additional bending stresses experienced by the tubular and this additional stress can be particularly prevalent while RIH casing with centralisers and high tortuosity. The reduction in API and VME stress envelope is also quantified using a 3D orientated casing wear model. A better understanding of axial stress state reduces risk of well integrity issues and can pinpoint areas along the casing in which special care must be taken during well intervention. This paper will show the benefits of using a stiff string torque and drag model during casing design. Highly tortuous wellbores, especially ERD and HPHT wells, may exhibit stresses that are vastly different than assumed during preplanning phase. The design API/VME envelope may also be reduced due to casing wear.