Analysis of Deformation Performance and Anti-Extrusion Strength of the Expansion Tube Subsidy

As a new mechanical sealing method, the sealing and burst and collapse strength of expansion tube patching technology is difficult to meet the requirements of large-scale volume fracturing with large displacement and high pump pressure when it is applied to the re-fracturing of old shale gas wells. Aiming at these problems, the expansion process of 20G, 316L, and duplex stainless steel (DSS) materials at an expansion rate of 12.7% was analyzed. The influence of casing diameter (10–20 mm), expansion tube wall thickness (2–7 mm), and expansion rate (12.7%–29.2%) on the burst and collapse strength of an expansion tube was studied. The research results are as follows: (1) By comparing the bugle performance and the burst and collapse strength of the expansion tube of different materials, and considering the economical use of the material, the expansion tube of 20G material was selected to be used in the casing after repeated fracturing. (2) Taking the 20G material expansion tube as an example, the driving force increases by 35 kN for every 1 mm increase in the wall thickness of the expansion tube, and 16 kN for every 1% increase in the expansion rate. (3) When the expansion rate is between 12.7% and 15.2%, a small increase in pressure will lead to a significant increase in the stress of the expansion tube. For every 1% increase in the expansion rate, the burst strength of the expansion tube decreases by about 4.8 MPa, and the collapse strength decreases almost linearly with increasing expansion rate. (4) When the wall thickness of the expansion tube is in the range of 4–7 mm, a small increase in pressure will lead to a significant increase in the stress of the expansion tube. For every 1 mm increase in wall thickness, the burst strength of the expansion tube increases by 14 MPa, and the collapse strength increases by about 20 MPa. When the wall thickness is less than 4 mm, the collapse strength of the expansion tube does not exceed 20 MPa, which does not meet the requirements of on-site construction. (5) Under external casing support, the burst strength of the expansion tube will be increased by about 50% after expansion. As the casing diameter increases, the burst strength of the expansion tube decreases, and the larger the expansion rate of the expansion tube and the smaller the wall thickness, the more sensitive to the change of the aperture. This study can provide technical support for repeated fracturing of old shale gas wells