Equations to calculate collapse strength of defective casing for steam injection wells

• We take into account the yield strength degradation for defective casing.
• We obtain the collapse strength equation with defective casing.
• This equation with defective casing presented is simple and practical.

Casings represent a very high proportion in OCTG (Oil Country Tubular Goods) and also influence the productive life of the wells. Studying the collapse strength of defective casing for steam injection wells can prevent casing collapse failure in advance, as well as improve life of defective casing and increase well production. Based on finite element analysis, the effect of pit shape, pit depth, pit circumferential spread, number of pit and circumferential spacing of pit on collapse strength is conducted, through which pit depth and pit circumferential spread are proven as the main controlling factors which affect the collapse strength of defective casing. A formula to calculate collapse strength of defective casing is obtained for steam injection wells by including the effect of temperature on the degradation of yield strength and compression stress function of a cylinder in the existing collapse strength formula. The formula is amended with results from numerical calculation and proven to be in good agreement with experimental data of collapse strength for defective casing. Therefore collapse strength can be accurately predicted using the proposed collapse strength formula for thermal recovery steam injection wells.