Sealing Analysis of Cement Plug in Offshore Abandoned Wells

Abstract

A three-dimensional finite element model of stratum-cement ring-casing-cement plug was established for the failure analysis of the cement plug seal in the abandoned oil and gas wells. The mechanical parameters, length, bottom fluid pressure and casing swaging length of the cement plug under non-uniform ground stress conditions were analyzed. The results showed that when the bottom of the cement plug was subjected to fluid pressure, the stress at the interface between the cement plug and the casing increased, and thereby the cement plug at the bottom and the cementation of the casing failed, resulting in a the decrease in the sealing performance of the cement plug, which may be sealed under fluid corrosion. As the modulus of elasticity and the radius of the cement plug increased, the cement plug stress and the cement failure length increased. As the cement plug length increased, the cement plug stress and the cement failure length decreased, while Poisson's ratio for the cement plug stress and the cement failure length increased. The increase of the bottom fluid pressure could increase the cement plug stress and the cementation failure length. In the abandoned well, where the casing was forged and then grinded after the casing was forged, the length of the casing milling increased, the plug stress of cement reduced. These findings can provide insightful potentials for the parameters of cement plugs when the cement plugs are closed in the offshore oil and gas wells.