Veith Becker

Occurrence and origin of methane in groundwater in Alberta (Canada): Gas geochemical and isotopic approaches

To assess potential future impacts on shallow aquifers by leakage of natural gas from unconventional energy resource development it is essential to establish a reliable baseline. Occurrence of methane in shallow groundwater in Alberta between 2006 and 2014 was assessed and was ubiquitous in 186 sampled monitoring wells. Free and dissolved gas sampling and measurement approaches yielded comparable results with low methane concentrations in shallow groundwater, but in 28 samples from 21 wells methane exceeded 10mg/L in dissolved gas and 300,000 ppmv in free gas. Methane concentrations in free and dissolved gas samples were found to increase with well depth and were especially elevated in groundwater obtained from aquifers containing coal seams and shale units. Carbon isotope ratios of methane averaged -69.7 ± 11.1‰ (n=63) in free gas and -65.6 ± 8.9‰ (n=26) in dissolved gas. δ(13)C values were not found to vary with well depth or lithology indicating that methane in Alberta groundwater was derived from a similar source. The low δ(13)C values in concert with average δ(2)HCH4 values of -289 ± 44‰ (n=45) suggest that most methane was of biogenic origin predominantly generated via CO2 reduction. This interpretation is confirmed by dryness parameters typically >500 due to only small amounts of ethane and a lack of propane in most samples. Comparison with mud gas profile carbon isotope data revealed that methane in the investigated shallow groundwater in Alberta is isotopically similar to hydrocarbon gases found in 100-250 meter depths in the WCSB and is currently not sourced from thermogenic hydrocarbon occurrences in deeper portions of the basin. The chemical and isotopic data for methane gas samples obtained from Alberta groundwater provide an excellent baseline against which potential future impact of deeper stray gases on shallow aquifers can be assessed.

A Brief Overview of Isotope Measurements Carried Out at Various CCS Pilot Sites Worldwide

About 1800 geochemical measurements including more than 1000 isotope analyses have been published as a result of geochemical monitoring programs established at several CO2 storage and enhanced oil and gas recovery projects worldwide. These projects are briefly discussed here in order to compare sampling techniques to obtain fluid and gas samples for chemical and isotopic analyses. In all the projects, changes of stable isotope ratios of CO2 and dissolved inorganic carbon in samples obtained pre- and post-injection were used to quantify solubility and ionic trapping of CO2 via stable isotope mass balances. Further applications include, monitoring of underground CO2 migration and early detection of potential CO2 leaks into overlying formations. Other benefits of these stable isotope tracers include a better understanding of water-rock-gas interactions with CO2 under supercritical conditions and often highly saline reservoir fluids that are present in the storage formations. While the results of these projects need further laboratory and experimental confirmation, further increase in field applications of stable isotope tracer techniques are anticipated with the introduction of new portable laser stable isotope mass spectrometers.

Advances in Stable Isotope Monitoring of CO2 Under Elevated Pressures, Temperatures and Salinities: Selected Results from the Project CO2ISO-LABEL

The BMBF project CO2ISO-LABEL (Carbon and Oxygen ISOtopes under extreme conditions LABoratory EvaLuations for CO2-storage monitoring) investigated stable isotope methods in laboratory studies for transferral to carbon capture and storage (CCS) field sites including enhanced gas and oil recovery (EGR and EOR). The isotope composition of injected CO2 and water are useful tracers for migration and water-rock-gas interactions during such operations. However, quantification of carbon and oxygen equilibrium isotope effects at elevated pressures and temperatures are so far scarce. They thus need more investigations under p/T conditions that are characteristic for reservoirs and overlying aquifers. With this, the main objective of the project was to improve stable carbon and oxygen isotope methods for monitoring CO2 storage sites and their impact of injected CO2 on reservoir geochemistry under controlled laboratory settings. An important finding was that isotope fractionations of carbon between CO2 and dissolved inorganic carbon (DIC) were not significantly different from each other in experiments with pure CO2 and pressures between 59 and 190 bar. Furthermore, influences of rock types (limestone, dolomite and sandstone) and fluid salinities were found to be negligible for carbon isotope fractionation between CO2 and DIC. Another finding was that water oxygen isotope ratios changed systematically in response to different CO2/H2O molar ratios in closed system equilibration experiments. This helps to reconstruct the amounts of CO2 that equilibrated with formation waters. Results of the project will enable better assessment of geochemical conditions in underground carbon storage sites or other subsurface systems where large amounts of CO2 interact with water and rocks.