Understanding and Mitigating Leakage Pathways in Oil and Gas Well Cements

甲烷是一种强有力的温室气体(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，排放了该国44%的甲烷排放总量。在上游石油和天然气工业中，甲烷排放的很大一部分是由地面套管通风口流动(SCVF)和天然气运移(GM)引起的逃逸泄漏造成的。散装水泥以及水泥/套管和套管/岩石界面很可能是油气井在泄漏和长期井身完整性方面的薄弱环节。水泥从套管和(或)地层中脱粘，以及随之而来的层间隔离的丧失，是油气井SCVF和GM逃逸甲烷排放的一个重要因素。脱粘可能是由于向井下注入相对较冷或较热的流体而引起的热循环引起的热应力造成的。它也可能是由地应力变化引起的机械载荷(由多阶段水力压裂和再压裂、油层枯竭或二氧化碳注入引起)、由化学活性流体引起，或由上述因素综合引起。了解渗漏通道的形成机理，对于评价和改善油气井的固井性能，无论是生产还是封堵报废都是至关重要的。因此，对水泥失效和脱粘的机理以及这如何影响流体在水泥微观结构中的运动提出了全面的实验和数值研究。如本项目所建议的，对水泥孔结构、水泥/套管和水泥/地层界面失效前后的直接成像和后续建模将允许设计和开发坚固固井系统，该固井系统可用于更具成本效益的减少逃逸甲烷排放。

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.