Experimental Study on Airtightness of Cement Sheath Under Alternating Stress

The airtightness of cement sheath under alternating loading and unloading was studied using self-developed experiment facilities, and the mechanism of airtightness failure of cement sheath under alternating loading and unloading was understood based on the experimental results. Under the action of lower alternating pressure inside the casing, the time for the cement sheath to fail can be long; the action of high alternating pressure, on the other hand, will soon break down the airtightness of the cement sheath. This indicates that low pressure inside the casing string imposes low force on cement sheath which still retains its elasticity, and elastic deformation takes place in the cement sheath under alternating loading and unloading. Under the action of high stresses, on the other hand, the intrinsic micro fractures and deficiencies inside the cement sheath begin to connect with each other, and both elastic deformation and plastic deformation take place inside the cement sheath. As the alternating goes on, the plastic deformation becomes accumulated, to the extent that the plastic deformation does not vanish after unloading. Differences in the deformations of casing string and cement sheath at the interface result in a tensile stress, which, when exceeding the cementation strength at the interface, will cause the airtightness of the cement sheath to fail. This is the so-called low cycle fatigue failure of the airtightness of cement sheath under the action of alternating loading and unloading. Higher internal pressure in the casing string results in higher induced stress inside the cement sheath, larger plastic deformation, higher residual strain and tensile stress at the interface after unloading, and, the less of the number of cycles required to cause the airtightness of the cement sheath to fail.