Analysis on Causes for Deformation Failure of Double-layer Casing in a Certain High-pressure Gas Well

After the cementing of a certain hi-pressure gas well with the fourth spud Φ196.85 mm casing，the plugexploring process is seriously blocked at the position of 15.50 m depth. And it is identified via further operation that the casing string has its outer surface point-contacted with the inner surface of the Φ273.05 mm casing. Based on the cementing quality analysis，the formation pressure system analysis and the theoretic calculation，it is revealed that the causes for the deformation（blocking）of the Φ196.85 mm casing are as follows. Due to the poor quality of the cementing of the third spud Φ273.05 mm casing string，there exists some gas that goes up through the channel from the lower position，and thus cause the gas from the hi-pressure passing-by gas layer at the lower part goes up， and when the wellhead environment control is closed，the pressure of the air cavity as formed at the wellhead section exceeds the collapse resistance of the Φ273.05 mm casing，and thus gets the said casing collapse deformed，and in turn，the deformed casing gets point contacted with outer surface of the Φ196.85 mm casing，which then leads to the deformation of the latter.