Numerical Simulation Analysis of Casing Equivalent Stress of Creep in a Salt-Gypsum Layer in the Gaojiapu Oilfield

Casing damage in the oil or water injection wells in the Gaojiapu oilfield occurred mostly in the salt-gypsum layer of the Shahejie formation. By multi-stage loading triaxial creep test under simulated formation conditions, the salt-gypsum rock steady-state creep behavior under different stresses are attained. Based on the study of stress in the Gaojiapu area, the constitutive relation of steady-state creep rate was established, along with the regression of experimental data to determine the Gaojiapu oilfield salt-gypsum rock rheological parameter. The finite element method was used to establish the coupling mechanics model of the formation and cement sheath–casing, and research on the different thicknesses of the cement sheath, the different elastic modulus of the cement sheath, and different thicknesses of casing conditions were used to get the regularity of the equivalent stress variation of the casing. The results show that the improvement of cementing quality and the increase of casing thickness can improve the ability of casing to resist non-uniform loads. In the instance of the salt-gypsum layer casing failure simulation analysis, it is shown that the outside casing extrusion force can be calculated according to the crustal stress for casing strength design. The study provides a theoretical basis for the prevention of casing damage in the future, and is significant for the design of casing in the salt-gypsum layer.