Experimental and Theoretical Analysis of Casing Collapse Strength for Shale Gas Wells

Casing full-scale collapse strength evaluation is a key technical index to ensure unconventional oil and gas exploration and development. In order to forecast the casing collapse strength accurately, the paper put forward two constraint modes, such as casing end constraint and unconstraint. The full-scale extrusion test of 5-1/2″ casing with different steel grade and wall thickness were carried out, and the relationship between casing strength and wall thickness, steel grade was established. Aiming at the test results, the key factors affecting the collapse strength were analyzed, meanwhile, a analytical model of collapse strength was built combined the FEM. With the analytical model, the quantitative relationship between casing collapse strength and wall thickness, steel grade, external diameter was identified. The theoretical formula of API standard extrusion strength is revised according to the results of experimental and numerical simulation. The results shown that casing collapse strength obtained by end unconstrained test were more accurate than that of constrained. The revised collapse strength formula could predict the casing property accurately, which provided a reference for shale gas wells casing selection.