Casing and Tubing Design for Sour Oil and Gas Fields

Abstract In sour environments containing H2S and water, the casing and tubing yield failure criterion is no longer used because environmental fracture is the main failure mode. In this case, a reasonable material selection and design of casing and tubing can reduce or prevent casing and tubing failure accidents. On the basis of meeting the yield criterion, conventional sulfur resistance casing T95 and quasisulfur resistance casing C110 is designed considering the fracture mechanics method. According to environmental fracture toughness K ISSC calculated by NACE method A and/or D, antipressure fracture strength of C110, and T95, casing is predicted by failure assessment diagram under various conditions. Numerical and experimental comparisons show that antipressure fracture strength of C110 casing is less than that of T95 casing, hence it is important for C110 cased boreholes to prevent evoked cracking during postoperation of drilling and completion. So in a sour oil and gas field, casing and tubing design not only meets the yield design criterion, but also meets the fracture mechanics design criterion. The new design method provides an important reference for casing and tubing design and material selection in sour environments containing H2S and water.