The potential environmental impacts on subsurface water resources induced by unconventional gas production are still under debate. Solving the controversy regarding the potential adverse effects of gas leakages on groundwater resources is therefore crucial. In this work, an interesting real‐world case is presented in order to give further insight into methane multiphase and transport behavior in the shallow subsurface, often disregarded compared to the behavior in the deep subsurface. Multiphase flow and solute transport simulations were performed to assess the vulnerability of an existing shallow unconfined aquifer with respect to a hypothetical methane leakage resulting from a well integrity failure of a former deep geothermal well. The analysis showed that migration of gaseous methane through the aquifer under examination can be extremely fast (of the order of a few minutes), occurring predominantly vertically upwards, close to the well. By contrast, dissolved methane migration is largely affected by the groundwater flow field and occurs over larger time scales (of the order of months/years), covering a greater distance from the well. Overall, the real concern for this site in case of gas leakages is the risk of explosion in the close vicinity of the well. Predicted maximum gaseous fluxes (0.89 to 22.60 m³/d) are comparable to those reported for leaking wells, and maximum dissolved methane concentrations may overcome risk mitigation thresholds (7 to 10 mg/L) in a few years. Therefore, surface and subsurface monitoring before decommissioning is strongly advised to ensure the safety of the site.

中文翻译：

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地热井向浅层含水层中甲烷泄漏的数值研究

非常规天然气生产对地下水资源的潜在环境影响仍在争论中。因此，解决有关气体泄漏对地下水资源的潜在不利影响的争议至关重要。在这项工作中，提出了一个有趣的现实世界案例，以便进一步了解浅层地下的甲烷多相和输运行为，而与深层地下的行为相比常常被忽略。进行了多相流和溶质运移模拟，以评估现有的浅层无限制含水层相对于由前深层地热井的井身完整性故障导致的甲烷泄漏的脆弱性。分析表明，气态甲烷通过受测含水层的迁移可能非常快（几分钟左右），主要发生在井附近垂直向上。相比之下，溶解的甲烷迁移在很大程度上受到地下水流场的影响，并发生在较大的时间范围内（数月/年的量级），覆盖距离井口较远。总体而言，如果发生气体泄漏，此地点的真正问题在于井附近爆炸的危险。预计最大气体通量（0.89至22.60 m 距井的距离更大。总体而言，如果发生气体泄漏，此地点的真正问题在于井附近爆炸的风险。预计最大气体通量（0.89至22.60 m 距井的距离更大。总体而言，如果发生气体泄漏，此地点的真正问题在于井附近爆炸的危险。预计最大气体通量（0.89至22.60 m³ / d）与泄漏井的报告值相当，并且最大溶解甲烷浓度可能会在几年内超过风险缓解阈值（7至10 mg / L）。因此，强烈建议在停运前进行地面和地下监测，以确保现场安全。