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Title: Potential for contaminant release from fracturing UK gas prospective black shale formations
Author: Adegbulugbe, Adeolu Oluwatosin
ISNI:       0000 0004 5369 9660
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2015
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This thesis investigates the potential for contaminant releases from three prospective shale gas formations following operational and environmental conditions common to the industrial extraction of shale gas. The overall aim was to assess the likelihood for these potentially toxic contaminants posing a substantial risk to existing water resource and infrastructure. This knowledge is geared towards providing the UK oil and gas industry with a quantitative risk assessment of flowback and produced water quality in the likelihood of the UK involvement in shale gas development. Physical and geochemical characterisations of sampled black shales from three UK prospective Lancashire (LAN), Derbyshire (DRB) and Whitby (CLV) formations are presented to supplement the proposed contaminant release investigation with a record of empirical baseline data of the mineralogical and elemental compositions of the sampled shales. Black shale from Lancashire showed notably larger mean compositions of Cr (131ppm ± 16), Rb (308ppm ± 20), Ni (291ppm ± 21), Zn (378ppm ± 41) and Cu (285ppm ± 43) and Pb (160 ± 17) in comparison with data from two documented world shale averages and the currently producing Marcellus formation in the US. Likewise, quantitative mineralogical data suggest a predominantly phyllosillicate (clay) and silicate (quartz) enriched shale matrix with the exception of the Carboniferous Edale shale with a sizable brittle carbonate content (22.27%). A risk weighing evaluation based on quantified trace elements, bioavailability and severity of toxicity highlighted Cr, Cu, Ni and Zn having the largest risk impacts investigated in the sampled shales. An assessment of the potential for acid rock drainage is assessed by static acid base accounting method and all three shales appear to be potentially acid producing with mean NPR at LAN( 0.66 ± 0.24), DRB ( 0.76 ± 0.19) and CLV( 0.71 ± 0.23). Heavy metal fractionation by sequential chemical extraction show a richly mobile carbonate phase Cr and a thermodynamically unstable Ni and Zn, possibly mobile via microbial mediation in all three sampled shales. Based on these findings, laboratory scale kinetic leaching experiments were developed to simulate the release of the selected high risk heavy metals following weathering in their natural environment and subsequently following anthropogenic disturbances from operational fracture treatments. Natural weathering simulated releases were to serve as baseline data on heavy metal mobility trends for comparison with hydraulic fracture influenced releases. We reported the characteristics of the leachate obtained and a quantification of the potential toxic Elements (PTEs) indicator in both leaching kinetic experiments and these trends allowed a prediction of future releases. Both simulated releases revealed an accelerated release activated in the later stages of leaching (between week 3 and 6) indicating a non-solely diffusion controlled release and an approximate exponential release rate was observed. Results showed a rise from one to three orders of a magnitude increase in PTE release rates from all three black shale types. In the natural weathering simulated releases, Cr recorded the highest release rates of 9.31E-09, 8.7E-09 and 2.86E-08mol/m2/day in the Lancashire, Derbyshire and Whitby black shales respectively. Acid base predictions showed higher percentage pyrite dissolution rate in comparison with carbonate rates in the LAN and DRB shales signifying the possible generation of acid mine drainage (AMD) in the Lancashire and Derbyshire black shales while carbonate dissolution rates sulphur rates exceeding sulphur rates in the CLV shale. Speciation analysis by geochemical modelling in both simulated releases revealed the presence of CrO42-, Cr3+, Cu2+, Ni2+ and Zn2+, as toxic species but suggested the predominant reduction of Cr(VI) to Cr(III) following the acidic characteristics of the resulting leachates. In both simulated studies, predictions of the 5 to 10 year released concentrations all exceed the Environment Agency (EA) environmental quality standard (EQS) for inland surface water and transitional/coastal waters permissible concentration. Our findings suggests that significantly higher released concentrations of investigated PTEs were observed in the fractured simulated leaching kinetic experiment to suggest the increased risks from hydraulic fracturing. However, non-anthropogenic influences alone are capable of releasing toxic concentrations of the investigated trace metals. The influence of additional environmental and operational factors prevalent during the development of shale gas wells were further investigated in a microcosm experiments. In particular, the mobility and release of the investigated PTEs influenced by prevailing anaerobic conditions in the presence of proliferating sulphur reducing bacteria. Our result showed that chemolithrophic bacterium like Thiobacillus denitrificans, is capable of anaerobic nitrate dependent pyrite oxidation and if present during fracture conditions can play a role in heavy metal mobility during fluid rock interaction. Mobilization and release of potentially toxic trace metals such as investigated in the study, is accelerated by operational fracturing conditions such as chemical use and environmental factors such as the anaerobic conditions and bacteria proliferation in fracture wells. Data collected has shown that a significant immobilization of heavy metals can be achieved by the eradication of bacteria in fracture wells and during drilling operations.
Supervisor: Wright, Nigel Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available