Development of Noble Gas Techniques to Fingerprint Shale Gas and Trace Sources of Hydrocarbon in Groundwater

Abstract

The combined use of hydraulic fracturing (HF) and horizontal drilling has greatly increased the natural gas recovery from shales, so called “shale gas”. This steep increase in shale gas production has caused public concern for fear that enhanced permeability through HF may facilitate migration of natural gas, thereby threatening drinking-water supplies.

My dissertation presents a pioneer study of the entire set of noble gases (He, He, Ar, Kr, Xe) in natural shale gas and associated groundwater. Noble gases are used to fingerprint unconventional shale gas, to characterize natural gas generation, to trace the source of crustal fluids (i.e., natural gas and groundwater), and to assess the mixing and migration of various crustal fluids.

Noble gas signatures of Antrim Shale gas in the Michigan Basin are investigated to assess mixing of deep brine and freshwater recharge. High horizontal and vertical variability in crustal noble gas signatures in the Antrim Shale are observed, which are due to variable noble gas input with brine migration from deeper formations. Noble gas ratios suggest also dominance of thermogenic over biogenic natural gas in this shale formation.

I also place constraints on the source and transport mechanisms of methane found in the shallow Trinity Aquifer within the Barnett Shale footprint in Parker and Hood counties, Texas, using the entire set of noble gases with an emphasis on 84Kr and 132Xe. Dissolved methane concentrations are positively correlated with crustal noble gases and suggest that noble gases and methane originate from a common source, likely the Strawn Group as opposed to the Barnett Shale. Overall, noble gas signatures in the Trinity Aquifer do not support the notion that methane present in these groundwaters migrated from nearby production wells either conventional or using HF techniques. Instead, this study suggests that methane in the Trinity Aquifer originates from noncommercial small gas accumulations in the underlying Strawn Group, which these groundwater wells have reached.

Atmospheric and crustal noble gas signatures for natural gas samples from the deep Barnett Shale and shallow Strawn Group in Texas are presented and compared with those of stray gas (methane) in the Trinity Aquifer. It is apparent that Barnett Shale and Strawn Group natural gas display highly distinct noble gas signatures and point to different evolution histories. Comparison of these production shale gas samples with stray gas in the Trinity aquifer reinforces the notion that stray gas in the Trinity originates in the Strawn Group as opposed to the Barnett Shale. At this stage, our findings do not point specifically to anthropogenic causes (e.g., poor-quality cementing of natural gas production wells either conventional or using HF technology) that would be responsible for the presence of methane in the Trinity Aquifer.