A geochemical and multi-isotope modeling approach to determine sources and fate of methane in shallow groundwater above unconventional hydrocarbon reservoirs

Field investigation of the transport and attenuation of fugitive methane in shallow groundwater around an oil and gas well with gas migration

'Fugitive' or 'stray' gas migration from deeper formations due to well bore integrity failure has prompted concern regarding environmental impacts. Unintended methane (CH4) migration can increase greenhouse gas emissions and affect groundwater quality in the critical zone. Although the CH4 transport in shallow aquifers has been investigated at experimental injection sites, no intensive groundwater studies have been published around an oil and gas well that has been leaking for a significant period of time. In this field study, groundwater samples were collected from sixteen groundwater monitoring wells (1.25 m below ground surface) installed around a suspended oil and gas well with decadal scale gas migration (estimated ~0.2 m3/day). Stray CH4 distribution and preferential pathways in the shallow groundwater zone were evaluated though high-resolution profiling of equivalent concentrations of hydrocarbon gases (C1-C6; >85 % CH4 at the study site) and bulk formation electrical conductivity to 6.0 m below ground surface. The highest dissolved CH4 concentration (0.074 mmol/L or 1.18 mg/L) in groundwater (1.25 m bgs) was observed immediately downgradient (1.25 m) of the oil and gas well head. Similarly, high-resolution profiling data also revealed the occurrence of relatively high CH4 concentrations in shallow groundwater along the groundwater flow direction and below fine-grained layers up to 10 m distance from the well head. Microbial DNA analysis from groundwater showed significant community shifts, with the highest relative abundance and diversity of methanotrophs observed in the vicinity of the oil and gas well. This study supports findings from experimental injection and laboratory studies, which also found that significant CH4 transport i) dominantly occurs in the groundwater flow direction, and ii) laterally as free phase below fine-grained layers. The occurrence of CH4 concentrations below saturation after more than two decades of gas migration suggests limited impacts have occurred in the shallow subsurface investigated.

A hydrochemical and multi-isotopic study of groundwater sulfate origin and contribution in the coal mining area

Coal mining cities are universally confronted with the degradation of groundwater quality, and the sulfate pollution of groundwater has become a widely studied environmental problem. In this study, we combined multi-isotope (δ³⁴S, δ¹⁸O-SO₄^2- and ⁸⁷Sr/⁸⁶Sr) approach with hydrochemical technique and a Bayesian mixed model to clarify sources and transformations and to quantitatively assess the contribution of sulfate from potential sources. The concentrations of SO₄^2- in groundwater ranged from 7.7 mg/L to 172.9 mg/L, and the high-value areas were located in coal mining area and residential area. The total values of δ³⁴S and δ¹⁸O-SO₄^2- varied from 10.6‰ to 26.9‰ and 6.9‰ to 14.1‰, respectively, in the groundwater. Analyses of SO₄^2- and Sr isotopes and water chemistry indicated that SO₄^2- in groundwater originated from various sources, such as atmospheric precipitation, sulfide mineral oxidation, evaporite dissolution, sewage and mine drainage. The oxidation of pyrite and bacterial sulfate reduction (BSR) had no significant impact on the stable isotopes of groundwater. At the same time, the calculation results of the Bayesian mixed model showed that the sources of SO₄^2- in groundwater mainly include evaporite dissolution in aquifer and mine drainage in the mixture of shallow and deep groundwater, with high contribution proportions of 39.8 ± 10.9% and 31.9 ± 5.7%, respectively, while the contributions of sewage (13.9 ± 8.5%), atmospheric precipitation (9.6 ± 8.6%) and the oxidation of sulfide (4.7 ± 3.3%) to SO₄^2- were lower. The research results revealed the source of SO₄^2- pollution in shallow groundwater in the coal mine area and provided an important scientific basis for the effective management and protection of groundwater resources.

Assessment of geochemical modeling applications and research hot spots-a year in review

Geochemical modeling has been employed in several fields of science and engineering in recent years. This review seeks to provide an overview of case studies that applied geochemical modeling in the 2019 year, which includes over 250 articles. This review is intended to inform new users on the possibilities that geochemical modeling brings, while also informing existing and past users on its latest developments. The survey of studies was conducted with an emphasis on the modeling techniques, the objective of studies, the prevalent simulated variables and the use of specific software packages. The analysis showed that geochemical modeling is still predominantly employed in experimental projects and in the form of equilibrium modeling. PHREEQC and Visual MINTEQ were recognized as the most popular software packages for simulating a wide range of processes, using equilibrium or other geochemical modeling forms. The study of fluid-rock interactions and pollution and remediation processes can be regarded as the principal geochemical modeling objectives, constituting 37% and 36% of the reviewed studies, respectively. Focusing on fluid-rock interactions, hydrogeochemical processes, carbon capture and storage and enhanced oil recovery have been the main topics examined with geochemical modeling. Assessments of the toxicity of metals in terms of leachate and mobilization, as well as their removal from soil and water systems, have been major topics investigated with the aid of geochemical modeling in terms of pollution and remediation research. It was found that the scholars benefit from geochemical modeling in their research both as a main technique and as an accessory tool. Saturation index, elemental concentration and speciation, mineral mass and composition and pH were among the most common variables modeled in reviewed studies. Geochemical modeling has gained a wider user base in recent years, and many research groups have used it in consecutive studies to deepen knowledge. However, much potential for further dissemination still remains.

Potential Impacts of Shale Gas Development on Inorganic Groundwater Chemistry: Implications for Environmental Baseline Assessment in Shallow Aquifers

The potential contamination of shallow groundwater with inorganic constituents is a major environmental concern associated with shale gas extraction through hydraulic fracturing. However, the impact of shale gas development on groundwater quality is a highly controversial issue. The only way to reliably assess whether groundwater quality has been impacted by shale gas development is to collect pre-development baseline data against which subsequent changes in groundwater quality can be compared. The objective of this paper is to provide a conceptual and methodological framework for establishing a baseline of inorganic groundwater quality in shale gas areas, which is becoming standard practice as a prerequisite for evaluating shale gas development impacts on shallow aquifers. For this purpose, this paper first reviews the potential sources of inorganic contaminants in shallow groundwater from shale gas areas. Then, it reviews the previous baseline studies of groundwater geochemistry in shale gas areas, showing that a comprehensive baseline assessment includes documenting the natural sources of salinity, potential geogenic contamination, and potential anthropogenic influences from legacy contamination and surface land use activities that are not related to shale gas development. Based on this knowledge, best practices are identified in terms of baseline sampling, selection of inorganic baseline parameters, and definition of threshold levels.

Occurrence and origin of groundwater methane in the Stellarton Basin, Nova Scotia, Canada

Groundwater methane (CH₄) in areas of fossil fuel development has been a recent focus of study as high CH₄ concentrations pose water quality concerns and potential explosive hazards. In 2013, a provincial study in Nova Scotia identified areas with elevated groundwater CH₄. However, due to limited data, the specific sources and local distribution of CH₄ in those areas remain unknown. In this study, we examined the Stellarton Basin in central Nova Scotia, Canada, a region with an abundance of coal formations, numerous abandoned coal mines, and an active open pit coal mine. Methane was detected in 94% of water samples that were sampled from 45 private water wells. Six water wells exceeded the 28 mg/L hazard mitigation threshold with CH₄ levels of up to 72.7 mg/L. The δ¹³C_CH4 (-85.5 to -48.5‰) and the δ²H_CH4 (-280 to -88‰) indicated that >95% of samples had CH₄ of microbial origin. However, the detection of ethane (C₂H₆) up to 2.97 mg/L and propane (C₃H₈) up to 0.008 mg/L, as well as the δ¹³C_C2H6 values (-30.1 to -15.6‰) suggested a mixture of microbial CH₄ with trace thermogenic gas, likely migrated from Stellarton coals (δ¹³C_C2H6 of -27.6 to -15.35‰). A mobile greenhouse gas analyzer survey was conducted within the perimeter of residences and off-gassing from taps had atmospheric CH₄ measurements as high as 66 ppmv. This study integrates multiple sampling and monitoring methods to investigate groundwater CH₄ in a coal-bearing region. The findings advance the understanding of the origin and occurrence of CH₄ in complex groundwater systems. The data acquired in this study may be used as a pre-drill baseline for groundwater CH₄ concentrations and origins should coal-bed methane operations in Nova Scotia proceed in the future.

A Probabilistic Approach for Predicting Methane Occurrence in Groundwater

Aqueous geochemistry datasets from regional groundwater monitoring programs can be a major asset for environmental baseline assessment (EBA) in regions with development of natural gases from unconventional hydrocarbon resources. However, they usually do not include crucial parameters for EBA in areas of shale gas development such as methane concentrations. A logistic regression (LR) model was developed to predict the probability of methane occurrence in aquifers in Alberta (Canada). The model was calibrated and tested using geochemistry data including methane concentrations from two groundwater monitoring programs. The LR model correctly predicts methane occurrence in 89.8% (n = 234 samples) and 88.1% (n = 532 samples) of groundwater samples from each monitoring program. Methane concentrations strongly depend on the occurrence of electron donors such as sulfate and to a lesser extent on well depth and the total dissolved solids of groundwater. The model was then applied to a province-wide public health groundwater monitoring program (n = 52,849 samples) providing aqueous geochemistry data but no methane concentrations. This approach allowed the prediction of methane occurrence in regions where no groundwater gas data are available, thereby increasing the resolution of EBA in areas of shale gas development by using basic hydrochemical parameters measured in high-density groundwater monitoring programs.