Modelling Near-Surface Processes

Transcription

1 Modelling Near-Surface Processes Katherine Romanak, Changbing Yang, JP Nicot The University of Texas at Austin Bureau of Economic Geology Gulf Coast Carbon Center DOE, 1999 IEAGHG 2nd Combined Meeting of the Modelling and Monitoring Networks Edinburgh Scotland, July 6-8, 2016

2 Basis For Our Modelling Efforts The near-surface is spatially and temporally complex and dynamic Understanding processes is critical Attribution of anomalies (Dixon and Romanak, 2015, IJGGC) Response to public concerns Leakage quantification BEG s experience in attribution: Cranfield anomaly and Kerr Claim Very difficult Fast accurate attribution is critical for public acceptance The risk of false positives is much greater than the risk of leakage. Source: DOE, 1999: Carbon Sequestration Research and Development

3 Geochemical Assessments Using ratios for geochemical assessments is standard procedure in groundwater geochemistry Severely underutilized in soil gas assessments Concentration comparisons to background Ratios are a powerful tool defining processes. mixing relationships. Soil gases are measured in volume % Full understanding requires knowledge of all gases!

4 Process-Based Soil Gas Ratios Uses simple gas ratios to identify processes. Biologic respiration Methane oxidation Dissolution Leakage No need for years of background. Method can be applied in any environment regardless of variability Romanak et al., 2014, International Journal of Greenhouse Gas Control, 30, Romanak et al., 2012, Geophysical Research Letters, 39 (15).

5 Process-Based Gas Ratio 1 O 2 vs. CO 2 Indicates natural processes that affect CO 2 concentrations Distinguishes among respiration, CH 4 oxidation and dissolution Gives an initial assessment of leakage Leakage

6 Process-Based Gas Ratio - 2 CO 2 vs. N 2 Identifies whether gas has migrated from depth. Indicates whether CO 2 is being added through leakage or lost through dissolution.

7 Playa Lake West Texas

8 Variability in Ratios West Texas Playa Lake, Texas, USA, 3 years What is causing the ubiquitous loss of CO 2 and what does it mean for monitoring? Cranfield, USA, background station, 6 years Can variable gas dissolution explain this or is calcite dissolution required? Kerr Farm, Weyburn Field, Canada, one sampling

9 Numerical Modelling Parameters PHREEQC Code Static batch simulations Decoupled processes STP Single monod kinetic equation for simulating biologic respiration Constant gas volume of 1 liter Porosity 0.3 Varying degrees of water saturation (from 0.1 to 0.9) Saturation = V water V gas + V water Variable gas dissolution/water saturation Carbonate dissolution

10 Respiration Trends With Water Saturation Increasing saturation Respiration CH 4 Oxidation (0.6 saturation) Respiration PB Ratio 1 PB Ratio 2

11 Field Examples West Texas Playa Lake, Texas, USA, 3 years, Floor data. Kerr Farm, Weyburn Field, Canada, One sampling Up to 0.9 saturation Mostly ~ 0.5 saturation

12 Gas Dissolution: 6%O 2, 15%CO 2 Water Saturation from 10% to 90% Without Calcite With Calcite A small amount of dissolution translates to a large change in volume %. Calcite enhances dissolution but is not required for the observed variation in ratios.

13 Sensitivity to Leakage Without calcite 5.5 mmoles 123 mliters With calcite 10 mmoles 220 mliters

14 Ramifications for Monitoring ZERT Controlled Release Experiment, Montana USA Cranfield Background Station, Mississippi, USA Background = respiration ratio

15 15 User-Friendly Environmental Leakage Field Monitoring Universal definition of background Conservative method reduces risk of false positives. Simple data reduction No complex correlations with weather Graphical analysis can be done instantly. Public can understand and implement.

16 Translating Ratios into Surface Flux /content/facilities watch?v=i5stngpeske

17 Summary Simple ratios are historically used in aqueous geochemistry to understand processes but are underutilized in soil gas assessments. A small amount of variable dissolution translates to a large change in volume %. Calcite enhances dissolution but is not required for the observed variation in ratios. Ratios are sensitive to leakage even when dissolution occurs Respiration ratios can be used as a Universal definition of Baseline Ratios may be useful for quantification of surface emissions.

18 Thank you for your attention.! Katherine Romanak

19 6 years of Background Data at Cranfield

20