Understanding and mitigating leakage pathways in oil and gas well cements

石油和天然气工业中的运营公司越来越关注确保其威尔斯井的长期完整性得到维持。散装水泥以及水泥/套管和套管/岩石界面很可能是威尔斯井泄漏和长期井完整性方面的薄弱点。水泥与套管和/或地层的脱粘以及随后的层位隔离损失是油气威尔斯井中表层套管排气流和气体迁移的逃逸甲烷排放的重要因素。了解失效和脱粘的机制以及这如何影响通过水泥的流体运动，对于评估和改善威尔斯井中的水泥性能至关重要，无论是生产井还是堵塞井和废弃井。如本项目所建议的，对孔隙结构和套管/地层界面在故障前后进行直接成像和随后建模，将有助于设计和使用更有效的缓解技术。水泥粘结到由钢或砂岩制成的壁的实验观察表明，脱粘可能与发生在近壁区中的微观结构变化有关，所述近壁区沿沿着水泥/套管和水泥/岩石界面发展。应力导致沿界面和垂直于界面的沿着断裂的机制还有待完全理解。问题仍然存在，例如：为什么在近壁区的结构不同于在体材料？为什么水泥中的大的径向裂纹大多出现在水泥/岩石界面处？岩石类型（例如，水泥对不可渗透页岩与可渗透砂岩地层的粘结质量）是否对径向裂缝的大小有任何影响？由于热循环和/或储层衰竭引起的地应力变化如何影响裂缝的形成，这些裂缝径向地扩展到水泥体中或沿着界面脱粘？观察到的泄漏是否由脱粘、裂纹或两者引起。此外，没有任何信息是什么泄漏路径实际上看起来像或流体如何流过这些泄漏路径。

Operating companies in oil and gas industry are becoming increasingly concerned with ensuring the long term integrity of their wells are maintained. Bulk cement as well as cement/casing and casing /rock interfaces are likely to be the weak spots in wells with regard to leakage and long term well integrity. De-bonding of cement from casing and/or formation, and subsequent loss of zonal isolation, is a significant contributor to fugitive methane emissions from surface casing vent flow and gas migration in oil and gas wells. Understanding the mechanisms of failure and de-bonding and how this impacts fluid movement through the cement is crucial for assessment and improvement of cement performance in wells, whether it is producing or plugged and abandoned. Direct imaging and subsequent modelling of the pore structure and casing/formation interface pre and post-failure, as proposed in this project, will allow for the design and use of more effective mitigation techniques. Experimental observations of cement bonding to wall made of steel or sandstone suggested that de-bonding may be associated with the microstructural changes that takes place in the near-wall zone, which develops along the cement/ casing and cement/rock interfaces. The mechanisms behind stresses inducing fractures along the interface and normal to it are yet to be fully understood. The questions remain for example: Why is the structure in the near-wall region different from that in the bulk material? Why do large radial cracks appear in cement mostly at the cement/rock interface? Does the rock type (e.g. cement bonding quality against impermeable shale vs permeable sandstone formation) has any impact on the magnitude of the radial cracks? How would the change in in-situ stresses due to thermal cycling and/or reservoir depletion effect formation of the cracks developing radially into the cement body or de-bonding along to the interface? Whether the observed leakages are caused by de-bonding, cracks, or both. Furthermore, no information is available on what the leak paths actually look like or how the fluid flows through these leak path. A comprehensive experimental and numerical study is, therefore, proposed.