Carbon Capture and Storage (CCS) as a method for climate mitigation relies on CO₂ being locked up in subsurface reservoirs with a long-term perspective. Active and abandoned wellbores are among the major potential leakage paths for CO₂, and their success as "gate keepers" relies on the quality of well cement. The length requirements and recommended practices for cement sheaths in active wells vary from country to country. In this study, we investigate how stochastic properties of the microannulus may affect the recommended "safe length " of cement sheath, should assigning such length be attempted for CO₂ wells. We demonstrate, by means of a simple numerical model, that variation in the width of the microannulus along the well makes it a challenging task to assign a figure to the "safe cement-sheath length", even though longer cement sheaths do indeed reduce the risk of leakage. Variation in the mean value or standard deviation of the microannulus width by only 10% changes the recommended length of the cement sheath by up to an order of magnitude. This finding sheds new light on the use and added value of more accurate leakage prediction models: due to the uncertainties in and the lack of information about the properties of microannuli, such models should focus on investigating the effects of different operational and in-situ factors on the leakage potential rather than on attempting to accurately predict the leakage flow rate, however tempting such prediction exercise might seem at first glance.

碳捕集与封存（CCS）作为缓解气候变化的一种方法，从长期的角度出发，依赖于将CO ₂锁定在地下储层中。活跃和废弃的井眼是CO ₂潜在的主要泄漏途径之一，它们作为“守门员”的成功取决于井眼水泥的质量。活跃井中水泥套管的长度要求和建议做法因国家而异。在这项研究中，我们调查微环的随机特性如何影响水泥护套的建议“安全长度”，如果尝试为CO ₂分配这样的长度井。我们通过一个简单的数值模型证明，沿井孔的微环宽度的变化使得为“安全水泥鞘长度”分配一个数字是一项艰巨的任务，尽管更长的水泥鞘确实确实会减少有泄漏的危险。微环宽度的平均值或标准偏差仅变化10％，可使水泥护套的推荐长度最多变化一个数量级。这一发现为更准确的泄漏预测模型的使用和增值提供了新的亮点：由于不确定性以及关于微环形物特性的信息的缺乏，此类模型应集中于调查不同操作和原位因素的影响而不是试图准确预测泄漏流量，