Analytical and Numerical Study of Casing Collapse in Iranian Oil Field

Abstract

Casing collapses are frequent events encountered in Iranian oil-producing wells. This phenomenon of casing collapse in wells can reduce and stop oil production. Among the oil fields that witnessed this problem, we have been to the Marun oil field in the southwest of Iran. In south Iran’s oil-rich regions, the Gachsaran Formation has the highest level of plasticity as it contains a large amount of salt units and also exhibits the largest frequency of casing collapse occurrence. The casing collapse occurrence phenomenon might result from the exertion of compression and shear stresses from the formation to the body of casing. In the case of inappropriate cementing, the mentioned stresses are applied in a nonuniform and anisotropic method to the casing. If the respective stresses can overcome the cement strength and then the casing strength, they will make the casing collapse. In the present study, the areas where casing collapse is most likely to happen was determined with the aid of an analytical geomechanical model and by comparing the maximal horizontal stresses against the cement and the casing strengths. Then, via 2-dimensional and numerical simulation of plastic Gachsaran Formation related to the well under analysis by means of the finite-element software ABAQUS, it was attempted to obtain the plastic properties of the formation as well as the numerical values and directions of stresses that might lead to casing collapse occurrence if exerted. The results of the numerical and analytical model indicate the high potential of the plastic Gachsaran Formation in creating a collapse phenomenon. Finally, it was observed that the maximum value of the numerical stresses resulting in the occurrence of casing collapse are along the direction of the maximum horizontal stresses and correspond to the Gachsaran Layer 4–2 possessing the highest salt content.