Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.
Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada; The Lyell Centre, Heriot-Watt University, Edinburgh, Scotland EH14 4AS, UK.
Sci Total Environ. 2020 Dec 20;749:141459. doi: 10.1016/j.scitotenv.2020.141459. Epub 2020 Aug 3.
Petroleum resource development has generated a global legacy of millions of active and decommissioned energy wells. Associated with this legacy are concerns about wellbore integrity failure and leakage of fugitive gas into groundwater and atmosphere. The fate of fugitive gas in the shallow subsurface is controlled by sediment heterogeneity, hydrostratigraphy and hydraulic connectivity. We characterized the shallow subsurface at a site in northeastern British Columbia, Canada; a region of extensive petroleum resource development. We collected 13 core profiles, 9 cone-penetrometer profiles, 58 sediment samples and 4 electrical resistivity profiles. At the site, a ~ 12 m thick layer of low-permeability diamict (10 m/s) overlays a more permeable (10 - 10 m/s) but highly heterogeneous sequence of glacigenic sand, clay and silt. We develop a conceptual hydrostratigraphic model for fluid flow in this system in the context of fugitive-gas migration. Driven by buoyancy forces, free-phase gas will move upward through discontinuous permeable zones within the Quaternary sediments, until it encounters lower permeability interbeds where it will pool, flow laterally or become trapped and dissolve into flowing groundwater. The vertical extent of gas migration will be significantly limited by the relatively continuous overlying diamict, a feature common across the Western Canadian Sedimentary Basin. However, intra-till lenses observed embedded within the diamict may provide pathways for gas to move vertically towards ground surface and into the atmosphere. This study provides one of the few investigations examining geological and hydrogeological heterogeneity in the shallow subsurface at scales relevant to gas migration. For glaciated regions with similar surficial geology, such as Western Canada Sedimentary Basin, gas that is released into the subsurface from an energy wellbore, below a surface diamict, will likely migrate laterally away from the wellbore, and be inhibited from reaching ground surface and emitting to atmosphere.
石油资源开发在全球范围内产生了数百万口活跃和废弃的能源井，由此产生了对井筒完整性失效以及逸散性气体泄漏到地下水和大气中的担忧。逸散性气体在浅层地下的命运受沉积物非均质性、水文地层和水力连通性的控制。我们对加拿大不列颠哥伦比亚省东北部的一个地点进行了浅层地下特征描述，该地区是石油资源开发的集中区域。我们采集了 13 个岩芯剖面、9 个圆锥贯入仪剖面、58 个沉积物样本和 4 个电阻率剖面。在该地点，一层厚约 12 米的低渗透性冰碛物（渗透率为 10 米/秒）覆盖在渗透性更高（渗透率为 10-10 米/秒）但高度非均质性的冰川成因砂、粘土和粉砂序列之上。我们根据逸散性气体迁移的情况，为该系统中的流体流动开发了一个概念性的水文地层模型。在浮力的作用下，游离相气体会通过第四纪沉积物中的不连续渗透带向上移动，直到遇到渗透率较低的夹层，在那里气体会聚集、横向流动或被捕获并溶解到流动的地下水中。气体迁移的垂直范围将受到相对连续的上覆冰碛物的显著限制，这是加拿大西部沉积盆地的一个常见特征。然而，在嵌入冰碛物中的内部冰碛透镜可能为气体向上垂直移动到地面并进入大气提供途径。本研究是为数不多的在与气体迁移相关的尺度上对浅层地下地质和水文地质非均质性进行研究的研究之一。对于具有类似地表地质的冰川覆盖地区，例如加拿大西部沉积盆地，如果从一个位于地表冰碛物下方的能源井中释放出的气体进入地下，它很可能会从井筒横向迁移，并受到抑制，无法到达地面并排放到大气中。
