Dissolved Gases, Salts, and Metals in Appalachian Basin Groundwater: Provenance and Processes in Relation to Oil and Gas Development

Restricted (Penn State Only)
- Author:
- Shaheen, Samuel
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 25, 2024
- Committee Members:
- Donald Fisher, Program Head/Chair
Susan Brantley, Chair & Dissertation Advisor
Max Lloyd, Outside Field Member
Nathaniel Warner, Outside Unit Member
Christopher House, Major Field Member
Eric Roden, Special Signatory - Keywords:
- Groundwater
Methane
Brine
Aqueous geochemistry
Oil and gas
Reactive transport
Machine learning - Abstract:
- As human populations increase worldwide, anthropogenic perturbation of natural systems intensifies. One of the most impactful anthropogenic activities has been the development of oil and gas reservoirs over the last two centuries. This development has produced major changes to many of the Earth’s biogeochemical cycles, including the carbon and water cycles. Impacts to shallow groundwaters because of oil and gas development include the release of deep gases such as methane or deep waters such as basin brines co-produced with hydrocarbons. Fluids can be released during modern oil and gas extraction, which today often utilizes high-volume, high-pressure hydraulic fracturing, or because of the degrading integrity of legacy oil and gas wells. These wells number in the millions worldwide and were often constructed prior to modern regulations designed to prevent fluid migration outside the wellbore. However, the extent of such impacts and the mechanisms through which they occur are often ill-defined. Additionally, any impacts of oil and gas development on groundwater must be distinguished from the natural gases and salt ions that derive from natural geologic processes or other forms of anthropogenic disturbance such as road salting or coal mining. In this thesis, I examine how both contemporary and legacy oil and gas extraction alters shallow groundwater, using the Appalachian Basin of the northeastern United States as a testbed. The Appalachian Basin hosts one of the highest densities of shale gas extraction in the world, and at the same time also is the location of the oldest oil and gas fields in the world. In the first two studies, I assembled the largest dataset of groundwater chemistry analyses from a shale gas play and utilized this unprecedented data availability to investigate the mechanisms and extent of groundwater impacts from shale gas extraction and its overlapping geologic and anthropogenic influences on water chemistry. In Chapter 2, I began by investigating the sources of methane and salt ions in southwestern Pennsylvania, a region where shale gas development was preceded by a long history of coal mining and conventional oil and gas extraction. I discovered that while the frequency of methane migration during shale gas development may be reduced by prior extraction of shallow gases, shale gas development produces “hotspots” where chloride concentrations increase in groundwater nearby shale gas wells. In turn, the number of these hotspots is large enough to produce a detectable regional increase in groundwater chloride concentrations near shale gas wells. This increase is also observable for barium and strontium, and can be distinguished from widespread road salt impacts using machine learning-based endmember separation techniques. Expanding the study area to work across the Marcellus Shale region of Pennsylvania in Chapter 3, I discovered these increases in brine-associated ion concentrations are observable region-wide. When I examine the relationship between these species and wastewater spills on shale gas wellpads, even larger increases in the concentrations of salt ions are observed compared to the increases associated with all shale gas wells. In total, these results suggest wastewater leaks or spills during shale gas development produce isolated areas where the concentrations of brine-related contaminants increase in groundwater nearby shale gas development. These incidents happen with high enough frequency that small, regional increases can be detected in groundwaters nearby shale gas wells across the northern Appalachian Basin. The frequency of spills and leaks in other major shale gas basins are comparable to that of the Appalachian, suggesting that brine releases may be an increasing water quality problem as hydrocarbon basin development continues worldwide. But ongoing leakage of producing oil and gas wells is not the only concern for a basin such as the Appalachian where more wells have been abandoned than are currently operational. Therefore, I also explore how abandoned oil and gas wells in northwestern Pennsylvania can serve as conduits for the migration of deep fluids, as well as the biogeochemical implications when deep subsurface fluids mix with shallow waters in abandoned wellbores. In Chapter 4, I characterize geochemical and isotopic compositions and microbial assemblages of groundwater samples from the world’s oldest oil and gas field. The concentrations and ionic ratios in these samples suggest formation brines migrate upwards with natural gas in some abandoned wellbores in the Appalachian Basin, but are diluted over 50-fold by fresher groundwaters closer to the surface. Isotopic compositions and microbial assemblages suggest the redox disequilibrium induced by this mixing of deep gases and shallow groundwaters promotes methane cycling, including oxidation of the leaking natural gas. Field observations, laboratory experiments, and reactive transport modeling in this study are consistent with the inference that the influx of methane is associated with anaerobic oxidation of methane coupled with metal oxide or sulfate reduction. These reactions can in turn produce or mobilize hazardous species in the water such as arsenic or hydrogen sulfide, creating secondary risks to water resources because of gas migration mediated by abandoned wells. In sum, this thesis advanced our knowledge of the extent of groundwater chemistry alteration during oil and gas extraction while also illuminating the specific mechanisms through which impacts to groundwater may occur. My results indicate water resource impacts occur in “hotspots” where incidents during shale gas extraction or low-integrity wellbores release fluids to shallow groundwater. I discovered that low concentrations of salt ions may be reaching groundwaters nearby shale gas wellpads with a high enough frequency that a regional, albeit small, impact is detectable. While prior work has focused more extensively on methane migration, these trends in groundwater complement a recent nationwide analysis that documented a similar increase in salt ion concentrations in surface waters. My results link these increases for the first time to a plausible mechanism: namely, the leaks or spills of saline wastewaters at wellpads. While regional groundwater impacts during shale gas development are likely driven by isolated leakage incidents, my results also indicate that the degrading integrity of legacy oil and gas wells is promoting the upwards migration of deeper fluids that produce secondary threats to water quality. My results therefore highlight that changes in groundwater chemistry can result from oil and gas extraction both during and after the production lifetimes of wells in basins worldwide.