Authors: Lafortune, S; Gouzy, A; Vuidart, I

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Lafortune, S, Gouzy, A & Vuidart, I 2023, 'Forecast and mitigation of the pressure build-up in a depleted mine gas reservoir: case study in a French coal basin', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth,

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Closing mining activities in underground mines leads to stopping water pumping and ventilation. Unflooded voids form reservoirs where dangerous air or toxic gas can appear. If those reservoirs are connected to the surface through adits, shafts, or wells, they not only shape potential migration pathways for gas transfer to surface but also fresh air entrance. Gas can also migrate underground through strata if affected by former underground mining activities (e.g. dewatering, subsidence, fractures). Eventually gas migration between mining voids and surface is mostly driven by temperature and pressure gradients that cyclically change with seasons. Once emitted at surface, mine gas may lead to hazardous situations to people. Related hazards are explosion, suffocation or poisoning. Understanding gas production processes underground and migration to the surface is fundamental to assess post-mining risks at surface. The Nord-Pas-de-Calais (NPdC) basin is the largest coal basin ever exploited in France. A total of 2.4 Gtons of coal have been produced over 270 years. Mining was completed underground, and more than 600 shafts have been documented, although some have not been precisely located. The flooding of the mining voids is ongoing and will not be complete until 2150. Several sectors of the mine gas reservoir are currently at a pressure value below the surface atmospheric pressure due to abandoned mine methane (AMM) exploitation. Today, the depletion limits the mine gas migration from the underground towards the surface. Moreover, water level, gas pressure and gas composition within the reservoir are monitored at about 500 measurement locations, such as former mine shafts, pressure-relief boreholes or piezometers. Since 2019, Ineris and GEODERIS have been revising the gas monitoring plan of the NPdC basin. The purpose is to determine if the monitoring plan requires improvement to control the mine gas emissions at surface. Special consideration is given to pressure-relief boreholes as they are important mitigation measures that prevent pressurization of unflooded sectors of the mine gas reservoir located outside the drainage area related to the AMM production. Here, we detail how the 3D geometric model inherited from the mine operator and since then regularly updated is used to forecast the flooding of the aeraulic connections within the mine gas reservoir and the consecutive gas pressure build-up in the isolated sectors. This makes it possible to plan and prioritize the works to be done to improve the gas monitoring plan of the NPdC basin. The objective is also to anticipate any failure of the pressure-relief boreholes by having increased vigilance on the sectors at the end of flooding.

Keywords: drainage, emissions, flooding, gas, mitigation, monitoring, post-mining, risk prevention

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