Ground gas source, migration and hazard :a conceptual model

Abstract

For the rst time in human history the global atmospheric CO2 concentration has surpassed 400 ppm and is projected to reach 600{800 ppm by A.D. 2100. A positive feedback in average global temperatures relating to an increase in CO2 levels in the upper atmosphere has been well-documented. With a global warming potential 25 times that of CO2 over 100 years, CH4 is also considered to be a signi cant greenhouse gas. Understanding and managing the exchange of CO2 and CH4 between the geosphere and atmosphere as a part of the global carbon cycle is consequently an important geotechnical engineering challenge of the 21st Century. A conceptual model of ground gas dynamics in the near-surface was developed with wider implications for the geotechnical engineering industry activities in the nearsurface and deep subsurface. Evidence was collated from historic gas monitoring data, high temporal frequency gas monitoring data, stable isotope analysis, and modelling and measuring gas transport mechanics in order to compile a conceptual model. Historic ground gas monitoring of land ll sites provided an extensive data resource. Routine point measurement of CH4 and CO2 in the sub-surface has not enabled gas transport mechanisms to be fully understood. Emission events may be overlooked by this method. By monitoring gases at high temporal frequency (up to every three minutes) this study has attempted to resolve the relationship between ground gases and the atmosphere. As a result, barometric pumping was proven to be a major control on CH4 and CO2 emissions from the near-surface. Data yielded from a high temporal frequency gas monitoring campaign in 2013, showed that an average pressure gradient of 􀀀0:7 mbar/hr was su cient to induce a gas release from the near-surface comprising 40.1% CH4 and 7.9% CO2. The cycling of air and ground gases was shown to be rapid, often occurring over three hours or less. Crucially, only air was present in the test borehole for 70% of the period indicating the fragility of point measurement strategies. v Gas source is an important aspect of regulation. For sites with multiple anthropogenic environments and/or complex geology could have multiple ground gas sources. In the event of gas emission or leaks, liability is critical. Stable isotope ratio analysis was used to di erentiate between biogenic and thermogenic gas sources. Gases were sampled from municipal land ll sites in Greater Manchester and Cheshire, UK, that indicated multiple gas sources. Land ll are gas CH4 yielded 13 CCH4 􀀀65:0‰ and DCH4 􀀀319‰ while perimeter monitoring wells produced 13 CCH4 􀀀55:0‰ and DCH4 􀀀173‰ which compared with mineshaft vent gas CH4 which yielded 13 CCH4 􀀀50:5‰ and DCH4 􀀀208‰. This suggested that CH4 originating from underlying Coal Measures had been transported via the weathered top-surface of the bedrock or via a fault and had mixed with land ll gas in the perimeter monitoring wells. At the Greater Manchester site, gas sources were di erentiated and it was proven that coal gas was an environmental hazard in the locality. Gas transport was modelled by di usion using Fick's Law. A novel approach to measuring gas transport was conducted using a specially designed `arti cial borehole' experimental apparatus. A closed-system, the arti cial borehole data indicate that gas transport was di usive when subject to an additional 50 mbar pressure in addition to ambient pressure. Fick's Law was shown to predict CO2 di usion transport curves between t = 0 and t = 4 hr. Di usion was shown to be an ef- cient process initially. However, the data did not t the model after t = 24 hr and uniform concentration was achieved through the apparatus at t = 168 hr (1 week). Consequently, advective transport may supersede di usive transport. When the apparatus was sand- lled, the data showed that di usion e ciency was reduced due to an increase in tortuosity of CO2 transport path. These components were used to develop a widely-applicable conceptual model of ground gas dynamics in the near-surface. The data presented here have far-reaching implications for industry, policy makers and regulators; particularly concerning hydraulic fracturing (`fracking'), and carbon capture and storage (CCS). The methodology for high temporal frequency gas monitoring is recommended to establish baseline gas concentration, establish the e ect of industry on the natural system and to detect and quantify any leak and migration of CH4 and CO2. Combined with knowledge of site geology it should be possible to predict lateral gas transport in the near-surface environment.