The theoretical radial stress solution and its experimental verification of expansion cement at annulus interfaces

Abstract

To determine the initial loading state of the casing and cement sheath during cement setting, the theoretical solution of radial stresses at the annulus interfaces is derived considering volume expansion and material parameter variation. During the theoretical derivation, cement setting is divided into many time steps. In each step, the original radii of cement sheath without loading are calculated and the radial stresses are calculated according to the compatible deformation condition among the casing, cement sheath after free expansion and the stratum in this time step. The calculated radial stresses and radii of the cement sheath are used as initial data for the next step. The circumferential strains of casing and stratum are measured during cement setting process. The experimental result verifies the correctness of the theoretical solution. The calculated results and experimental data show that the radial stresses on both inner and outer boundaries of cement sheath are compressive stresses during the cement setting process, and their absolute values increase as the elastic modulus of the rock around the well increases. The higher the cement elastic modulus is, the larger the radial compressive stress on the wellbore will be. On the contrary, the radial compressive stress on the outer boundary of the casing will decrease. The cement material with appropriate elastic modulus should be adopted according to different elastic modulus of stratum, so as to realize loading equilibrium of the oil and gas well structural systems. Key words: cement sheath, cement setting, annulus interface, radial stress, theoretical solution, experimental verification