Understanding and mitigating leakage pathways in oil and gas well cements

石油和天然气行业的运营公司越来越关注确保其油井的长期完整性。散装水泥以及水泥/套管和套管/岩石界面很可能是油井在漏失和长期井身完整性方面的薄弱环节。水泥与套管和(或)地层的脱粘，以及随之而来的分层隔离的丧失，是油气井表层套管喷口流动和天然气运移的逃逸甲烷排放的重要因素。了解失效和脱粘的机理以及这如何影响流体在水泥中的流动，对于评估和改善油井中的水泥性能至关重要，无论是在生产中还是在被封堵和废弃的井中。按照本项目的建议，对失效前后的孔隙结构和套管/地层界面进行直接成像和后续建模，将使设计和使用更有效的缓解技术成为可能。水泥与钢材或砂岩粘结的实验观察表明，脱粘可能与近壁区发生的微观结构变化有关，这种变化沿水泥/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裂缝背后的机制尚未完全了解。例如，仍然存在的问题是：为什么近壁区域的结构与大块材料中的结构不同？为什么水泥中的大的径向裂纹主要出现在水泥/岩石界面上？岩石类型(例如，对不透水页岩与透水砂岩地层的水泥胶结质量)对径向裂缝的大小有影响吗？热循环和/或储集层枯竭引起的地应力变化将如何影响裂缝的形成，这些裂缝径向发展到水泥体内或沿界面脱粘？观察到的泄漏是由脱粘、裂缝还是两者兼而有之。此外，没有关于泄漏路径的实际外观或流体如何流经这些泄漏路径的信息。因此，提出了一种综合的实验和数值研究方法。

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.