Presence of microannuli at the cement interfaces in wellbores could provide a path for fluid leakage. In this study, we combined experimental analysis and numerical modelling to assess microannuli development at casing–cement interfaces. Lab experiments were conducted using two-metre long samples consisting of a 7” casing cemented inside a rock analogue (stiffer, stronger cement) encased with a steel outer shell. Hydraulic aperture of the microannuli was calculated by measuring water flow through the casing–cement interface at various casing pressures and after axial displacements. The results show that once a microannulus forms, it remains open at internal casing pressures as high as 40 MPa. The measured hydraulic apertures were in the range of tens of microns, with residual apertures in the range of 15 to $30\mathrm{\mu }\mathrm{m}$. Axial displacement of the casing did not lead to a significant change in the hydraulic apertures. A numerical model was created with a comparable geometry to the experiments, and used to calculate the mechanical apertures of the microannuli. A relationship is proposed to link the mechanical apertures from the numerical models to the hydraulic apertures measured in large-scale experiments. The findings enable the operators and regulators to improve safe well operation practices by forecasting the conditions that lead to possible loss of zonal isolation and the associated well leakage rates.

井筒中水泥界面处的微环可以为流体泄漏提供途径。在这项研究中，我们结合实验分析和数值模拟来评估套管-水泥界面处的微环空发育。实验室实验是使用两米长的样本进行的，该样本由一个 7 英寸的套管组成，该套管胶结在岩石类似物（更硬、更坚固的水泥）内，并用钢外壳包裹。通过测量在不同套管压力下和轴向位移后通过套管-水泥界面的水流来计算微环空的水力孔径。结果表明，一旦形成微环空，它会在高达 40 MPa 的套管内压力下保持打开状态。测得的液压孔径在几十微米的范围内，剩余孔径在 15 到$30\mathrm{\mu }\mathrm{米}$. 套管的轴向位移不会导致液压孔的显着变化。使用与实验相当的几何形状创建了一个数值模型，并用于计算微环的机械孔径。提出了一种将数值模型中的机械孔径与大规模实验中测量的液压孔径联系起来的关系。调查结果使运营商和监管机构能够通过预测导致可能失去区域隔离和相关井漏率的条件来改进安全的井操作实践。