Understanding and Managing Environmental Roadblocks to Shale Gas Development: An Analysis of Shallow Gas, NORM, and Trace Metals

Abstract

The main objective of the project was to document occurrences of shallow gas in fresh-water aquifers in Texas either dissolved or free phase and identify controlling processes. A secondary somewhat independent objective was to contribute to the understanding of the nature and variability of flowback and produced water associated with hydraulic fracturing in the context of rock-water interactions.

We undertook a large sampling campaign of aquifers in the footprint of major Texas plays (900+ water samples): Barnett in north-central Texas (555 unique locations), Eagle Ford in South Texas (118 unique locations), Haynesville in East Texas (70 unique locations), and in the Delaware Basin of West Texas (40 unique locations). Most of the wells (2/3) are relatively shallow residential wells sampled at or as close as possible to the wellhead but many wells are irrigation, municipal, or rig-supply wells. All samples were analyzed for major ions, dissolved gases, and, when CH4 > 0.1 mg/L, for methane and light alkanes carbon isotopes and trace elements. The vast majority of wells show some measurable methane and ~100 wells show methane > 0.1 mg/L. A total of ~20 wells have methane concentrations > 10 mg/L, these high concentrations were observed in all plays and present at least a thermogenic component. Some wells, generally with a < 10 mg/L concentration, show a clear microbial origin for methane. A number of samples show mixing between the two origins but also more complex behavior such as methane degradation. Samples with thermogenic methane are generally spatially organized in clusters. Overall the source of the dissolved methane is likely natural sourced from shallow natural gas accumulations in the Barnett Shale, lignite beds associated with a fault in the Haynesville shale, and lignite and degradation of oil and deep organic matter associated with a fractured zone in the Eagle Ford Shale. The Delaware Basin samples show no dissolved methane other than associated to a recent blowout.

We also performed autoclave experiments in controlled conditions exposing shale core fragments to various fluids, examining reacted and unreacted rocks and documenting chemical composition of the evolving fluid through time. The experiments demonstrated that shales undergo typical geochemical processes during hydraulic fracturing such as carbonate and feldspar dissolution as well as ion exchange resulting in an increase in dissolved solids. Observations suggest that rock permeability is increased two to three-fold and that porosity is increased by 50%.

Baseline sampling as it is currently practiced is not sufficient to resolve ambiguity of the source of the dissolved methane even if of thermogenic origin because it still could be natural. Additional analyses such as noble gases and isotopes are needed to better constrain origin of the methane.

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