Understanding and Mitigating Leakage Pathways in Oil and Gas Well Cements

甲烷是一种强效温室气体（GHG），其威力是二氧化碳的25倍。在加拿大，甲烷排放量约占温室气体总排放量的15%。上游石油和天然气工业设施是加拿大最大的甲烷工业排放者，释放的甲烷占该国甲烷总排放量的44%。在上游油气行业中，甲烷排放的很大一部分是由地表套管排气流（SCVF）和气体运移（GM）引起的逸散性泄漏造成的。大块水泥、水泥/套管和套管/岩石界面可能是油气井泄漏和长期井完整性方面的薄弱环节。套管和/或地层上的水泥脱落，以及随后的层间隔离失效，是油气井中SCVF和GM挥发性甲烷排放的重要原因。由于向井中注入相对较冷或较热的流体所引起的热循环引起的热应力，可能会导致脱粘。也可能是由地应力变化引起的机械载荷引起的（多级水力压裂和再压裂、油藏枯竭或二氧化碳注入引起的）；通过化学活性流体；或由上述因素的组合。了解泄漏通道形成的机理对于评估和改善油气井的水泥性能至关重要，无论是生产井还是封堵弃井。因此，提出了对水泥破坏和脱粘机制的综合实验和数值研究，以及这种机制如何影响流体在水泥微观结构中的运动。本项目提出的对水泥孔隙结构、水泥/套管、水泥/地层界面破坏前后的直接成像和后续建模，将允许设计和开发坚固的固井系统，可用于更经济有效地减少逸散性甲烷排放。

Methane is a potent greenhouse gas (GHG) that is 25 times more powerful than carbon dioxide. In Canada, methane emissions make up about 15 percent of the total GHG emissions. Upstream oil and gas industry facilities are Canada's largest industrial emitters of methane, releasing 44% of the country's total methane emissions. Within the upstream oil and gas industry a significant fraction of the methane emission results from fugitive leaks arising from surface casing vent flow (SCVF) and gas migration (GM). Bulk cement as well as cement/casing and casing /rock interfaces are likely to be the weak spots in oil and gas wells with regard to leakage and long term well integrity. Debonding of cement from casing and/or formation, and subsequent loss of zonal isolation, is a significant contributor to fugitive methane emissions from SCVF and GM in oil and gas wells. De-bonding can be caused by thermal stresses due to thermal cycling induced by injection of a relatively cold or hot fluid down the well. It may also be caused by mechanical loads due to changing in-situ stresses (caused by multi-stage hydraulic fracturing and re-fracturing, reservoir depletion or CO2 injection); by chemically active fluids; or by a combination of the above factors. Understanding the mechanism of leakage pathway formation is crucial for assessment and improvement of cement performance in oil and gas wells, whether it is producing or plugged and abandoned. Comprehensive experimental and numerical studies of the mechanisms of cement failure and de-bonding and how this impacts fluid movement through the cement microstructure are, therefore, proposed. Direct imaging and subsequent modeling of the cement pore structure, cement/casing, and cement/formation interfaces pre and post-failure, as proposed in this project, will allow for the design and development of robust cementing systems that can be used for more cost effective mitigation of fugitive methane emissions.