膨胀水泥预防压裂井中水泥环的力学破坏研究

摘要

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 me-chanical failure of cement sheath with expanding cement was analyzed, and the effect of expanding stress on forces on casing was evalu-ated. 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 com-pressive 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 condi-tions, 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.%针对压裂作业井中水泥环的力学破坏问题，研究利用膨胀水泥来预防该类破坏。利用有限元方法计算了压裂作业时普通水泥环中的最大主应力，并结合水泥环的力学性能分析了其破坏方式，将膨胀应力与最大主应力叠加后得出了膨胀水泥环中的最大主应力，分析了膨胀水泥预防水泥环力学破坏的原理，评价了膨胀应力对套管受力状况的影响。通过室内实验研究了模拟压裂作业时普通水泥环和膨胀水泥环的破坏情况。研究结果表明，压裂作业时普通水泥环的破坏方式为切向拉伸破坏；膨胀水泥产生的膨胀应力可以降低水泥环中的切向拉应力甚至使其转变为压应力，从而可以预防水泥环的力学破坏；膨胀应力会降低压裂作业时套管中的米塞斯应力，不会导致套管的受力情况恶化。实验结果表明，相同模拟压裂条件下，膨胀水泥环未发生破坏，普通水泥环因切向拉伸应力的作用而破坏，验证了使用膨胀水泥预防压裂井水泥环力学破坏的有效性。