Abstract Wellbores are comprised by a steel casing surrounded by a cement sheath. The microannulus is the typically very small annular or degraded space that may develop between cement and casing that has been identified as a common leakage pathway in wellbore systems. Although data regarding the actual size and character of wellbore microannuli are limited, the hydraulic aperture of the microannulus can be estimated from pressure build up or flow measurements at the wellhead. Such information can be misleading, however, as it represents microannuli as uniform annular gaps along the wellbore. This study aims to provide a quantitative measure of the variability of the microannuli. We generated wellbore-based samples with microannuli between the steel casing and cement, and calculated their hydraulic aperture and permeability by flowing gas through the microannuli. We then injected dyed epoxy into the microannuli, cut the specimens into five circumferential sections per sample, and used microphotographs to measure microannulus aperture size and contact between the steel and the cement to generate microannulus profiles for each section. These measurements are unique as they provide a quantitative measure of the variability of the microannulus with a resolution as low as 3 μm. Aperture sizes were fitted to different statistical distributions, most frequently lognormal and gamma. Capillary entry pressure (CEP) for gas displacing brine in the microannulus was estimated from measured aperture size. CEP estimated from actual aperture size was generally much greater than that estimated from the hydraulic aperture of the entire specimen, resulting in a wide range of values. Measured aperture sizes were used to evaluate possible microannulus repair by estimating the penetration of cementitious materials. The data showed that the repair using cementitious materials is unlikely to be effective for microannuli with a hydraulic aperture

中文翻译：

---

井筒微环的表征

摘要 井筒由被水泥环包围的钢套管组成。微环空是典型的非常小的环形空间或退化空间，可能在水泥和套管之间形成，已被确定为井筒系统中的常见泄漏路径。尽管关于井筒微环空的实际尺寸和特征的数据有限，但微环空的水力孔径可以通过井口的压力积累或流量测量来估计。然而，此类信息可能会产生误导，因为它将微环面表示为沿井眼的均匀环形间隙。本研究旨在提供微环可变性的定量测量。我们在钢套管和水泥之间生成了带有微环的井眼样品，并通过使气体流过微环来计算它们的水力孔径和渗透率。然后我们将染色的环氧树脂注入微环中，将每个样品切成五个圆周部分，并使用显微照片测量微环孔径大小以及钢与水泥之间的接触，以生成每个部分的微环剖面。这些测量是独一无二的，因为它们以低至 3 μm 的分辨率提供了微环可变性的定量测量。孔径大小适合不同的统计分布，最常见的是对数正态分布和伽马分布。微环空气体置换盐水的毛细管入口压力 (CEP) 是根据测量的孔径大小估算的。根据实际孔径尺寸估计的 CEP 通常比根据整个试样的液压孔径估计的 CEP 大得多，从而导致数值范围很广。测量的孔径大小用于通过估计胶结材料的渗透来评估可能的微环修复。数据表明，使用胶结材料进行修复不太可能对带有液压孔的微环面有效