Influence of filling medium on the stress on double-layer casing under nonuniform stress

The double-layer casing is an effective means to prevent casing deformation and failure in salt beds, and the filling medium is an important factor influencing the overall mechanical property of double-layer casing. In this paper, the creep constitutive equation of salt bed was determined by taking the casing in the salt bed of the first Member of Shahejie Formation in Fault Block J45 of Jingqiu area as the research object. Based on this, the loading model of formation-cement sheath-outer casing-filling medium-inner casing under nonuniform in-situ stress was established by means of the finite element method. In addition, the distribution of equivalent stress on inner and outer casings at different filling medium thicknesses and types were figured out and comparatively analyzed. It is indicated that the increase of medium thickness can weaken the stress transmission effect, but has less influence on the equivalent stress on the double-layer casing overall. The medium type has greater influence on the equivalent stress on the casing. The equivalent stress on the inner casing with the medium of machine oil and natural rubber is 33.55% and 5.87% of that with the medium of cement, respectively. According to above mentioned laws, it is recommended to adopt the double-layer casing of normal-grade outer casing+liquid medium+low-grade inner casing, which can reduce the cost effectively and ensure the safety of oil and gas production.