Influence of Casing Pressure Test on Seal Integrity of Cementing First Interface

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Casing pressure test is an important link of the oil and gas well cementing, but excessive casing pressure test may cause stress failure or plastically deformation of the cement sheath, and generate micro-annulus on the casing-cement sheath cemented surface, then lose seal integrity of the cement sheath. According to the basic theory of elastoplastic mechanics and considering the volume invariance of plastic strain and the influence of elastic strain on volume change, the Mohr-Coulomb criterion is used to establish the casing-cement sheath-surrounding rock combination model. The paper also derived the formula for calculating the micro-annulus of the casing pressure test, and analyzed the influence of the internal pressure of the casing during the pressure test and the elastic modulus of the cement sheath on the radial contact stress of the cement sheath cementation interface after the cementing operation is completed. Results show that: (1) The generation of micro-annulus of casing pressure test is determined by the pressure increase process and the pressure relief process. The pressure increase process may lead the cement sheath into plasticity, and the radial stress at the interface turns into tensile stress and the micro-annulus could appear at the first interface by the decrease of internal pressure during pressure relief process. (2) The varying internal pressure has a great influence on the fatigue failure of the cement sheath, so the continuous multiple casing pressure test should be avoided after the cementing is completed. (3) Under the condition of maintaining the integrity of the cement sheath, increasing the tensile strength of the cement stone and reducing the elastic modulus of the cement stone can improve the pressure bearing capacity of the first interface during the casing pressure test. The model can provide a theoretical basis for the mechanical parameter design of the cement sheath, and provide guidance for on-site construction to reduce or avoid the risk of failure of the first interface seal integrity for the casing pressure test.