Keywords: 
• fracturing /
• expanding cement /
• cement sheath /
• mechanical failure /
• maximum principal stress

Abstract: According to mechanical failure of cement sheath in fracturing operations, expanding cement was studied to prevent such failures. The maximum principal stress in common cement sheath during fracturing jobs was calculated using the finite element method, and its failure mode was analyzed with the mechanical performance of cement sheath, then the maximum principal stress in expanding cement sheath was obtained after superimposition of expanding stress with maximum principal stress; the principle of preventing mechanical failure of cement sheath with expanding cement was analyzed, and the effect of expanding stress on forces on casing was evaluated. Through indoor experiments, the failures of ordinary cement sheath and expanding cement sheath in simulated fracturing jobs were studied. The theoretical study results show that the failure mode of ordinary cement sheath during fracturing jobs is tangential tensile failure. The expanding stress produced by expanding cement can reduce the tangential stress in cement sheath, or even make it into compressive stress, hence preventing mechanical failure of cement sheath. The expanding stress can reduce the Mises stress in casing during fracturing, but will not deteriorate the stress on casing. The experimental results show that, under the same simulated fracturing conditions, there is no failure in expanding cement sheath, but there is failure in ordinary cement sheath due to the tangential tensile force, which validates the effectiveness of using expanding cement to prevent mechanical failure of cement sheath in fracturing operations.