Emission of Inflammable Gases during Development of Hydel Tunnels in Complex Himalayan Terrain of Eastern Bhutan

Countries around the world are exploring clean energy technologies and hydropower projects are one of the best options for generating energy without harming the environment and ecosystem. Bhutan is known for being carbon neutral in terms of greenhouse gas (GHG) emissions. Bhutan is part of the greater Himalayas, characterized by complex terrain and a rich river system. The hilly river network features high-gradient, fast-flowing and perennial rivers favours the construction of hydropower projects. Bhutan aims to reach a total electricity generation capacity of 25 gig watts (GW) by 2040. This includes 20 GW from hydropower and 5 GW from solar. The current hydropower capacity ~ 2.4 GW and under construction ~ 3.1 GW. The hydel projects needs extensive tunneling in the complex hilly terrain comprising the classic Himalayan subdivision into four main zones: the Sub-Himalaya, Lesser Himalaya, Greater Himalaya and Tethyan Himalaya, each bounded by major faults. Out of these, the Lesser Himalayan Sequence, located between the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT), comprises 8 -12 km of Paleoproterozoic to Permian metasediments. This sequence is divided into the lower Paleoproterozoic Shumar and Daling Formations and the upper Cambrian Baxa Group and Permian Gondwana Sequence. The Baxa Group includes quartzites and dolomites, while the Gondwana Sequence features coal-bearing strata. The Gondwana sediments mainly contain carbonaceous shale and coal seam lenses, which have a significant amount of organic carbon. Gases generated primarily methane (CH4) and hydrogen sulfide (H2S) are stored in the coal and shale deposits. These gases can be released during tunneling operations and may accumulate along the tunnel roof, posing an explosion risk if ignited.

There are established safety norms for methane and other flammable gas emissions during tunneling or mining activities. Safety is a key concern, especially because tunnels are often developed using blind headings and forced ventilation systems, which take time to fully flush long tunnels with fresh air. In light of the above, a case study was conducted on the measurement of methane and other gases in the Nyera Amari II Powerhouse at Martshala tunnels in eastern Bhutan. A systematic method was proposed to measure methane and H₂S concentrations within the tunnel. The minor methane layering and negligible methane accumulation was observed in general air body of the tunnel. The methane concentration in 1.5 meter borehole drilled at advance face, roof and floor of the tunnel after plugging for 1 hour ranges from 3.78 to 4.37%; whereas, the methane concentration after plugging the borehole for 48 hours varies from 6.43 to 12.76. Similarly, small amount of ethane (C₂H₆) concentration after plugging the borehole for 1 hour ranges from 0.18 to 0.35 %; whereas, the ethane concentration after plugging the borehole for 48 hours varies from 0.25 to 0.32 %. It also indicates minor changes in ethane percentage with time in any of the boreholes on keeping them plugged for 48 hours. The traces of H2S were observed, probably emitted from intersected shale beds in tunnels. Also, the air samples were collected both before and after rock blasting and analyzed for their composition shown slight increase in methane and H₂S concentration in the general body air. The sources of methane and H₂S were identified through a study of the local geology, the thermal maturity of the shale and coal formations encountered in the tunnels and stable isotope analysis of methane. The range values of stable isotope (δ¹³C₁) of methane in the studied borehole samples indicating possible thermogenic origin signatures (δ¹³C₁< -40 ‰). This has been confirmed through plot of C₂+ and δ¹³C₁, which indicates methane and ethane in borehole gas are of thermogenic to late thermogenic in origin. Conventional electrical equipment (such as cables, bulbs and pumps) was replaced with flame-proof alternatives to ensure safety. Moreover, the methane concentration should be maintained below 10% LEL (0.5% by volume) for safe working. A structured set of safety procedures was implemented to raise awareness among workers operating in the tunnels. Regular monitoring using portable, handheld multi-gas analyzers is recommended to detect and manage gas accumulation. Further, it is recommended that regular check on general body concentration of methane, return air velocity at the tunnel portal and duct discharge end near working face may be made to detect any presence of methane even it be low, in view of small amount of methane observed in the boreholes as a preventive measure against any possible gas hazards. This paper presents the sources of methane and H₂S generation, accumulation and emission in tunnels during excavation. It also outlines the detailed methodology used for their measurement and suggests safety measures to prevent potential explosions in tunnel environments.