Numerical simulation analysis of casing-in-casing cementing in the horizontal section of shale gas wells

The potential of deep shale gas development is huge, but the frequency of shale gas well casing issuance seriously restricts deep shale gas development. To address severe casing damage issues, the casing-in-casing cementing technique is employed for casing transformation management. In order to ensure the quality of well barrier reconstruction, numerical simulation analysis of casing-in-casing is conducted. A three-dimensional finite element model of the formation-cement ring-bilayer casing combination was established, and the stress effects of the inner casing and outer casing defects of different steel grades and wall thicknesses and different cementing conditions such as cement ring eccentricity and missing cement ring on the bilayer combination casing were investigated by finite element simulation method, and finally the residual resistance to external extrusion of the bilayer combination casing was studied. The results show that the eccentricity of the inner and outer casing has a small effect on the stress of the double-layer combination casing under the complete cement ring sealing. The missing inner cement ring and defective outer casing will produce larger stress concentration on the inner casing, and the stress of the inner casing will increase with the increase of the missing angle, missing thickness and missing length of the inner cement ring and the missing area and missing thickness of the outer casing, and the risk of failure of the double-layer combination casing increases significantly. The residual resistance to external extrusion of the double-layer combination casing increases by about 95.8% compared with that of the single-layer casing; the effect of missing cement ring on the residual resistance to external extrusion of the double-layer combination casing has a maximum decrease rate of about 43.3%, and the effect of cement ring eccentricity factor is the least, with a decrease rate of only about 2.2%. This research result can provide useful guidance for shale gas well casing variation management.