Scientific Evaluation and Interpretation of Baseline Groundwater Well Testing Data Available for Alberta

Transcription

1 Scientific Evaluation and Interpretation of Baseline Groundwater Well Testing Data Available for Alberta Bernhard Mayer, Pauline Humez, Leah Wilson & Michael Nightingale Department of Geoscience University of Calgary Calgary, Alberta, Canada T2N 1N4

2 Introduction Development of unconventional gas resources in Canada is often affected by the public controversy between the rapidly expanding exploitation of unconventional oil and gas resources by industry facilitated by horizontal drilling and hydraulic fracturing and the fear of landowners and parts of the public that these activities may have a negative impact on the quality of groundwater in shallow aquifers in part due to fugitive gas leakage. Figure from Council of Canadian Academies (CCA) shale gas report (2014)

3 Objectives Determine whether gases from shale gas production zones, the intermediate zone, and the shallow groundwater in Western Canada are chemically and isotopically distinct? Assess whether methane currently present in the aquifers has been formed in-situ or whether it has migrated from deeper formations? Well head ready for hydraulic fracturing

4 Unconventional Gas Plays in Western Canada Major shale gas plays: Horn River Montney Duvernay CBM play: Horseshoe Canyon Fm Montney BC Alberta Sask.

5 Key Monitoring Steps for detecting stray gas contamination 1. Widespread sampling and analyses of water and dissolved or free gases from groundwater samples to establish baseline conditions; 2. Collect information of chemical and isotopic compositions of gases from the intermediate zone during drilling of energy wells; 3. Collect information of chemical and isotopic compositions of fluids and gases from the production zone. From CCA shale gas report (2014)

6 Sampling Water samples, produced gases, gases from the intermediate zone, and free & dissolved gases from groundwater samples are obtained and returned to the laboratory in various sampling containers Sampling at shallow groundwater well (picture provided by Don Jones, AITF) Produced gases in stainless steel containers are returned for analysis Mudgas samples in isotubes returned to the laboratory for analysis Free gas sampler in action

7 Sampling and Analyses water and gas samples are analyzed for their chemical compositions (major & minor ions; C 1 -C 5, CO 2 etc.) and Water samples ready for chemical analysis Lab procedures for extracting dissolved gas Gas composition Vegreville, Alberta Sampling for dissolved gases (photo T. Gorody)

8 Isotopic Fingerprinting for the isotopic compositions of methane, ethane and CO 2 (d 13 C CH4, d 2 H CH4, d 13 C C2H6, d 13 C CO2 ). The isotopic composition of methane is expressed as d 13 C and d 2 H values in d 13 C [ ] = ( 13 C/ 12 C sample - 1 ) x C/ 12 C reference d 2 H [ ] = ( 2 H/ 1 H sample - 1 ) x H/ 1 H reference Gas isotope measurements at UofCalgary and can be used to differnetiate between shallow biogenic gas and deeper thermogenic gas

9 Isotopic Fingerprinting of Methane Biogenic Methane d 13 C between -110 and -55 d 2 H between -170 and -400 Thermogenic Methane: d 13 C between -60 and -25 biogenic thermogenic The isotopic composition of methane in shallow groundwater and selected natural gas plays (from: Jackson et al., 2013)

10 Shallow Groundwater Characterization in Alberta: 3 data sources Groundwater Observation Well Network (GOWN) with > 180 dedicated groundwater monitoring wells throughout Alberta; usually completed in single formations; sampled repeatedly since 2006 for water and dissolved and free gas. Baseline Well Water Testing (BWWT) data base; > 15,000 water samples with >1,300 free gas samples from landowner wells. Alberta Health Services (AHS) data base: >78,000 water samples with chemistry data from landowner wells; no gas compositions or isotope data Sampling at shallow groundwater well (picture Don Jones, AITF)

11 Occurrences of Methane in Shallow Aquifers GOWN (n=185) BWWT (n=801) Methane found in many groundwater wells

12 Occurrences of Methane in Shallow Aquifers GOWN BWWT Methane found in many groundwater wells with variable concentrations

13 Source of Methane in Shallow Groundwater Methane concentration versus d 13 C values of methane for 212 free gas samples obtained from 98 GOWN wells; mean d 13 C value of methane: -66.7

14 Source of Methane in Shallow Groundwater Methane concentration versus d 13 C values of methane for 517 free gas samples obtained from 261 BWWT wells mean d 13 C value of methane: -67.3

15 Source of Methane in Shallow Groundwater GOWN free gas has usually a dryness parameter >500 and d 13 C CH4 < -55 confirming a predominantly biogenic origin; only very few samples indicate mixed gas origin: gas migration?

16 Fingerprinting of Produced Gases from Shale Gas Plays in Western Canada Shale gas Montney Shale gas Horn River

17 Produced Gases: Gas Wetness vs. d 13 C of Methane Shallow groundwater: d 13 C CH4 ~ -67, dry Coalbed Methane (CBM): d 13 C CH4-54 to -49 considerable wetness Shale gas: d 13 C CH4 > - 46 variable wetness groundwater CBM shale gas

18 Produced Gases: Gas Wetness vs. d 13 C of Methane Combining the wetness parameter and d 13 C values of methane yields distinct signatures for free gas from shallow groundwater and produced gases from various shale gas plays; Combining both parameters is a very promising approach for monitoring for potential stray gas contamination of shallow aquifers

19 Fingerprinting Gases in the Intermediate Zone obtained mud gas samples during drilling of a deep well analyzed on site with CRDS compared results with laboratory IRMS measurements Figure from Council of Canadian Academies (CCA) shale gas report (2014) adapted with permission from Apache Canada Ltd.

20 Intermediate Zone Carbon Management Canada Research Institutes Field Research Site (CMCRI FRS): - drilled a 530 m deep well in March conducted mud gas on-site analyses of CH 4 concentrations and C isotope ratios of methane Picarro G2201i

21 Fingerprinting Gases in the Intermediate Zone Reliable d 13 C values for methane along the stratigraphic column were obtained in the field d 13 C values of CH 4 in the intermediate zone are isotopically distinct from produced gases Methane in groundwater is mainly formed in upper 150 m Since 2008, Shallow gw well Mayer et al. (2015): Procedia Earth and Planetary Sciences 13: Average gw wells in area

22 Has Methane in Shallow Groundwater Formed In-Situ or Migrated from Below? GOWN: Used aqueous geochemistry data to constrain water types and aquifer redox environments High methane mainly in Na-HCO 3 to Na-HCO 3 -SO 4 type waters Humez et al. (2016), HESS DOI: /hess

23 Has Methane in Shallow Groundwater Formed In-Situ or Migrated from Below? BWWT: Used aqueous geochemistry data to constrain water types and aquifer redox environments High methane mainly found in Na-HCO 3 to Na-HCO 3 -SO 4 type waters

24 The Redox Ladder Concept Reactions E h (V) DG Reduction of O 2 O 2 + 4H + +4e - --> 2H 2 O Reduction of NO 3-2NO 3 + 6H + + 6e - --> N 2 + 3H 2 O Reduction of Mn 4+ MnO 2 + 4H + + 2e - --> Mn 2+ +2H 2 O Reduction of Fe 3+ Fe(OH) 3 + 3H + + e - --> Fe 2+ +3H 2 O Reduction of SO 4 2- SO H + + 8e - --> H 2 S + 4H 2 O Methanogenesis CO 2 + 8H + + 8e - --> CH 4 + 2H 2 O

25 Has Methane in Shallow Groundwater Formed In-Situ or Migrated from Below? GOWN: Elevated methane concentrations only found if - dissolved oxygen is negligible; - dissolved nitrate is negligible, and - dissolved sulfate is negligible indicating that the redox-ladder concept is respected; Suggests that methane has likely formed in-situ in the aquifer

26 Has Methane in Shallow Groundwater Formed In-Situ or Migrated from Below? BWWT: Elevated methane concentrations only found if - dissolved oxygen is negligible; - dissolved nitrate is negligible, and - dissolved sulfate is negligible indicating that the redox-ladder concept is respected; Suggests that methane has likely formed in-situ in the aquifer

27 AHS data well locations This concept can be applied in areas where no groundwater gas data are available 78,952 samples with aqueous geochemical data 67,590 water samples passing QA/QC test (±10% ionic balance) and include groundwater wells, springs, surface water bodies etc. Grand Prairie area includes: >2600 water wells >10 springs

28 PTAC funded project: Groundwater chemistry results from AHS data base are currently assessed in selected shale gas regions AHS data well locations Allows us to identify the redox state of groundwater and predict whether in-situ methane formation in shallow aquifers at baseline conditions is likely to occur. Selected groundwater chemistry data from AHS data base for shallow groundwater wells in the Grand Prairie region

29 Has Methane in Shallow Groundwater Formed In-Situ or Migrated from Below? Current assessment suggests: - most methane formed biogenic in-situ in methanogenic aquifers - most elevated d 13 C CH4 values explained by microbial CH 4 oxidation - very few elevated d 13 C CH4 values that may indicate thermogenic CH 4 migration Humez et al. (2016), HESS DOI: /hess

30 Conclusions Gases from shale gas production zones, the intermediate zone, and from the shallow groundwater zone are isotopically distinct in Western Canada. Generating isotopic mudgas profiles is an important pre-requisite for assessing depths of potential gas migration from the intermediate or the production zones. Methane is naturally occurring in Alberta groundwater, is usually of biogenic origin, and is very often formed in-situ in shallow parts of the stratigraphic column according to water and gas chemistry data. Four wells in Alberta s Montney region ready for hydraulic fracturing

31 Acknowledgements Financial and logistic support is provided by PTAC (through its AUPRF program), Alberta Innovates Energy and Environment Solutions (AI-EES), and by Alberta Environment & Parks (AEP). Additional financial support by Carbon Management Canada is also gratefully acknowledged. Scholarship support to Dr. P. Humez is provided by the University of Calgary Eyes High PDF program. Further financial support is provided by a NSERC strategic project. This support is also gratefully acknowledged.

32 Source of Methane in Shallow Groundwater GOWN Free gas has a dryness parameter >500 and d 13 C CH4 < -55 confirming a predominantly biogenic origin; only very few samples indicate mixed gas origin: gas migration?

33 Has Methane in Shallow Groundwater Formed In-Situ or Migrated from Below? GOWN: Used aqueous geochemistry data to constrain water types and aquifer redox environments High methane mainly in Na-HCO 3 to Na-Cl type waters Humez et al. (2016), HESS DOI: /hess

34 Has Methane in Shallow Groundwater Formed In-Situ or Migrated from Below? BWWT: Used aqueous geochemistry data to constrain water types and aquifer redox environments High methane mainly in Na-HCO 3 to Na-Cl type waters

35 AHS map Provincial overview: bulk data samples with aqueous geochemical data water samples samples with geographically referenced with coordinates among samples (cf. map) filling the QA/QC criteria of ±10% (ionic balance criteria) and include well, spring, surface water bodies etc. Grand Prairie area includes: >2600 water wells >10 springs

36 Has Methane in Shallow Groundwater Formed In-Situ or Migrated from Below? GOWN: Elevated methane concentrations occur predominantly if nitrate and sulfate concentrations are negligible in Na-HCO 3 to Na-Cl type waters Humez et al. (2016), HESS DOI: /hess

37 Has Methane in Shallow Groundwater Formed In-Situ or Migrated from Below? d 13 C values of methane < -55 support biogenic origin d 13 C values of methane > -55 indicate either - migration of thermogenic or mixed gas, or - microbial oxidation of biogenic methane Humez et al. (2016), HESS DOI: /hess