Three‐dimensional, multiphase simulations are used to analyze migration of methane leakage from a hydrocarbon wellbore. The objective is to evaluate the relevance and importance of coupling fast, advective transport of methane through fractures with slower, diffusive transport in the shale matrix below a freshwater aquifer on water quality assuming dual‐domain mass transfer (DDMT) in the reservoir by using the multiple interacting continua (MINC) as implemented in TOUGH2. The conceptual model includes a methane gas‐phase leak from a wellbore 20–30 m below an aquifer; multiphase, buoyant transport through shale partially saturated with brine; and, after methane leakage reaches groundwater, multiphase transport under varying lateral groundwater flow gradients. Results suggest that DDMT affects the rate of methane reaching groundwater by (i) providing long‐time secondary storage in less‐mobile pore space and (ii) creating larger methane‐plume diameters than those predicted by a single‐domain advection‐diffusion equation. Compared to models without DDMT, these factors combine to increase methane flow rates by an order of magnitude across the base of the aquifer 100 years after leakage begins. In the simulated aquifer, dissolution of gas‐phase plumes leads to bimodal aqueous‐phase methane breakthrough curves in a simulated water well 100 m downstream from leakage, with peak concentrations appearing decades after a 1‐year pulse of leakage. The major implication is that DDMT in the reservoir can explain newly discovered methane concentrations in water wells attributable to older leakage events. Therefore, remediation of abandoned or legacy wells with wellbore integrity loss may be necessary to prevent future incidents of groundwater contamination.

中文翻译：

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双孔储层井筒甲烷泄漏及其在地下水中的运移数值研究

三维多相模拟用于分析碳氢化合物井筒中甲烷泄漏的运移。目的是通过利用储层中的双域传质（DDMT），评估淡水含水层下方页岩基质中甲烷快速，平流传输与慢速扩散扩散耦合的裂缝的相关性和重要性。如TOUGH2中实现的多次交互连续体（MINC）。概念模型包括在含水层下方20–30 m的井眼中发生的甲烷气相泄漏。通过部分充满盐水的页岩进行多相浮力运输；甲烷泄漏到达地下水后，在变化的侧向地下水流量梯度下进行多相传输。结果表明，DDMT通过（i）在流动性较小的孔隙空间中提供长时间的二次存储，以及（ii）产生比单域对流扩散方程所预测的更大的甲烷-气溶胶直径，来影响甲烷到达地下水的速率。与没有DDMT的模型相比，这些因素结合起来，在泄漏开始100年后，使整个储层底部的甲烷流速增加了一个数量级。在模拟含水层中，气相羽流的溶解会导致在泄漏下游100 m的模拟水井中出现双峰水相甲烷突破曲线，峰值浓度出现在一年泄漏脉冲后数十年。主要含义是储层中的DDMT可以解释水井中新发现的甲烷浓度，这可归因于较旧的泄漏事件。因此，