Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania

Exploration of unconventional natural gas reservoirs such as impermeable shale basins through the use of horizontal drilling and hydraulic fracturing has changed the energy landscape in the USA providing a vast new energy source. The accelerated production of natural gas has triggered a debate concerning the safety and possible environmental impacts of these operations. This study investigates one of the critical aspects of the environmental effects; the possible degradation of water quality in shallow aquifers overlying producing shale formations. The geochemistry of domestic groundwater wells was investigated in aquifers overlying the Fayetteville Shale in north-central Arkansas, where approximately 4000 wells have been drilled since 2004 to extract unconventional natural gas. Monitoring was performed on 127 drinking water wells and the geochemistry of major ions, trace metals, CH4 gas content and its C isotopes (δ13CCH4), and select isotope tracers (δ11B, 87Sr/86Sr, δ2H, δ18O, δ13CDIC) compared to the composition of flowback-water samples directly from Fayetteville Shale gas wells. Dissolved CH4 was detected in 63% of the drinking-water wells (32 of 51 samples), but only six wells exceeded concentrations of 0.5mg CH4/L. The δ13CCH4 of dissolved CH4 ranged from −42.3‰ to −74.7‰, with the most negative values characteristic of a biogenic source also associated with the highest observed CH4 concentrations, with a possible minor contribution of trace amounts of thermogenic CH4. The majority of these values are distinct from the reported thermogenic composition of the Fayetteville Shale gas (δ13CCH4=−35.4‰ to −41.9‰). Based on major element chemistry, four shallow groundwater types were identified: (1) low (<100mg/L) total dissolved solids (TDS), (2) TDS>100mg/L and Ca–HCO3 dominated, (3) TDS>100mg/L and Na–HCO3 dominated, and (4) slightly saline groundwater with TDS>100mg/L and Cl>20mg/L with elevated Br/Cl ratios (>0.001). The Sr (87Sr/86Sr=0.7097–0.7166), C (δ13CDIC=−21.3‰ to −4.7‰), and B (δ11B=3.9–32.9‰) isotopes clearly reflect water–rock interactions within the aquifer rocks, while the stable O and H isotopic composition mimics the local meteoric water composition. Overall, there was a geochemical gradient from low-mineralized recharge water to more evolved Ca–HCO3, and higher-mineralized Na–HCO3 composition generated by a combination of carbonate dissolution, silicate weathering, and reverse base-exchange reactions. The chemical and isotopic compositions of the bulk shallow groundwater samples were distinct from the Na–Cl type Fayetteville flowback/produced waters (TDS ∼10,000–20,000mg/L). Yet, the high Br/Cl variations in a small subset of saline shallow groundwater suggest that they were derived from dilution of saline water similar to the brine in the Fayetteville Shale. Nonetheless, no spatial relationship was found between CH4 and salinity occurrences in shallow drinking water wells with proximity to shale-gas drilling sites. The integration of multiple geochemical and isotopic proxies shows no direct evidence of contamination in shallow drinking-water aquifers associated with natural gas extraction from the Fayetteville Shale.

The expanding use of horizontal drilling and hydraulic fracturing technology to produce oil and gas from tight rock formations has increased public concern about potential impacts on the environment, especially on shallow drinking water aquifers. In eastern Kentucky, horizontal drilling and hydraulic fracturing have been used to develop the Berea Sandstone and the Rogersville Shale. To assess baseline groundwater chemistry and evaluate methane detected in groundwater overlying the Berea and Rogersville plays, we sampled 51 water wells and analyzed the samples for concentrations of major cations and anions, metals, dissolved methane, and other light hydrocarbon gases. In addition, the stable carbon and hydrogen isotopic composition of methane (δ13 C-CH4 and δ2 H-CH4 ) was analyzed for samples with methane concentration exceeding 1 mg/L. Our study indicates that methane is a relatively common constituent in shallow groundwater in eastern Kentucky, where methane was detected in 78% of the sampled wells (40 of 51 wells) with 51% of wells (26 of 51 wells) exhibiting methane concentrations above 1 mg/L. The δ13 C-CH4 and δ2 H-CH4 ranged from -84.0‰ to -58.3‰ and from -246.5‰ to -146.0‰, respectively. Isotopic analysis indicated that dissolved methane was primarily microbial in origin formed through CO2 reduction pathway. Results from this study provide a first assessment of methane in the shallow aquifers in the Berea and Rogersville play areas and can be used as a reference to evaluate potential impacts of future horizontal drilling and hydraulic fracturing activities on groundwater quality in the region.

Chapter 5 - Reactive Transport Modeling of Geological Carbon Storage Associated With CO2 and Brine Leakage

In the past decade, new advances in hydraulic fracturing techniques, in combination with horizontal drilling, have dramatically increased the production of natural gas from tight shale deposits that were previously considered uneconomical to produce. Today, over 1 million oil and gas wells have been hydraulically fractured in the United States, many in areas without a long history of oil and gas development. The lack of existing oil and gas related infrastructure in these areas has presented new challenges for water supply and wastewater disposal. This paper reviews key environmental issues associated with shale gas development and provides an overview of how these issues are currently being addressed by industry and regulatory stakeholders in the United States. Environmental concerns related to shale gas development that are addressed in our evaluation include: Impacts to groundwater qualityWater usageProper disposal of flowback waterAir quality impactsSeismic EventsImpacts of trucking and other transportation Although significant public and regulatory concern has been expressed regarding the potential effects of the hydraulic fracturing process on water resources, micro-seismic mapping and empirical data show that the transport of fracturing fluids or gases through hydraulically induced fractures into shallow drinking water aquifers has not occurred. Rather, in the rare cases when such water quality impacts do occur, they are very localized and are more commonly related to cement integrity problems or casing leaks in the vertical wellbore of shale gas wells. The increased focus on shale gas development has resulted in more environmentally-friendly operations. Specifically, sustainability and water management tools are being implemented successfully by many operators to ensure that potential environmental impacts are being mitigated upfront as part of the planning process. Knowledge of previous technical challenges and the techniques for the implementation of sustainable processes will be important tools for stakeholders in areas where shale gas development is in its early stages, such as Argentina, Mexico, and Colombia.

Underground hydrogen storage (UHS) in geological formations is a key element of the clean energy transition as it enables the decarbonization of the transportation and industrial sectors by decoupling hydrogen production and storage. UHS has many benefits, including low cost, much wider availability, large storage capacity, well-established infrastructure, and increased safety because of geological sealing capabilities. However, the impact of hydrogen (H2) biogeochemical interactions in the presence of subsurface microorganisms is largely neglected from UHS perspectives. These interactions might affect the effectiveness of storage and can even cause H2 to leak into the shallow aquifers. Leakage of H2 into groundwater can change the geochemistry and induce several microbial-driven processes. Microorganisms, such as sulfate-reducers, are naturally abundant in groundwater and consume H2 to produce hydrogen sulfide (H2S), which can contaminate the freshwater drinking groundwater and cause damage to infrastructure. Hydrogen leakage can also trigger microbial reactions responsible for metal mobility, which can impact the water quality. However, the kinetics of these reactions and the temporal impact of hydrogen leakage in groundwater are still unknown. Therefore, a time series hydrogen-groundwater interaction experiment was conducted, and the changes in fluid chemistry and headspace gas composition will be analyzed along with DNA sequencing results to understand the extent and kinetics of biogeochemical reactions that occur if hydrogen leaks into groundwater. In the experiments, Ultra High Purity (UHP) hydrogen gas will be injected into glass vials with groundwater samples, for a designated time period. For each glass-sealed vial, 16S rRNA gene sequencing, IC, ICP-MS, and GC-TCD will be performed. The experiments provide insights into plausible impacts of hydrogen leakage into shallow drinking water aquifers.

Developing a robust geochemical and reactive transport model to evaluate possible sources of arsenic at the CO2 sequestration natural analog site in Chimayo, New Mexico

Mathematicians and geochemists have long realized that compositional data intrinsically exhibit a structure prone to spurious and induced correlations. This paper demonstrates, using the Na–Cl–Br system, that these mathematical problems are exacerbated in the study of sedimentary basin brines by such processes as the evaporation or dissolution of salts owing to their high salinities. Using two published datasets of Na–Cl–Br data for fluids from the Appalachian Basin, it is shown that log concentration and Na/Br versus Cl/Br methods for displaying solute chemistry may lead to misinterpretation of mixing trends between meteoric waters (for example shallow drinking water aquifers) and basinal brines, partially due to spurious mathematical relationships. An alternative approach, based on the isometric log-ratio transformation of molar concentration data, is developed and presented as an alternative method, free from potential numerical problems of the traditional methods. The utility, intuitiveness, and potential for mathematical problems of the three methods are compared and contrasted. Because the Na–Cl–Br system is a useful tool for sourcing solutes and investigating the evolution of basinal brines, results from this research may impact such critical topics as evaluating sources of brine contamination in the environment (possibly related to oil and gas production), evaluating the behavior of fluids in the reservoir during hydraulic fracturing, and tracking movement of fluids as a result of geologic CO2 sequestration.

Shallow groundwater is the primary drinking water source for local communities surrounding Duri Field operations area. To ensure the local community has access to safe drinking water and to comply with Government of Indonesia (GOI) environmental safety regulations, PT. Chevron Pacific Indonesia (PT CPI) established a groundwater monitoring system to ensure that slurry waste injection into deep non-potable water aquifers does not contaminate nearby shallower drinking water aquifers. The prevention system also includes well integrity surveillance and monitoring as a leading indicator and water sampling and analysis as a lagging indicator. Duri Field is in the Rokan block, Riau Province of Indonesia. Oily sand and viscous fluids are byproducts from oil production and treatment processes. The Sand Management Facility (SMF) was developed to manage these solid and liquid wastes by injecting mixed waste slurry under high pressure into deep subsurface formations. The scope of this study included 12 injector wells with two groundwater monitoring wells surrounding each injection well. Stable isotope monitoring technology is used to ensure that deep waste injection slurry is not mixing with and contaminating shallow drinking water resources. Stable isotope (δ2H, δ18O) analysis was used to define the Local Meteoric Water Line (LMWL) of Rokan Watershed defined by equation δ2H = 7.6 δ18O + 10.5 (r2 = 0.921), which is applied as a reference point for isotope studies in SMF area. The stable isotope δ2H samples for groundwater in SMF ranged from −70.5 ‰ to −25.1 ‰ followed by formation and surface waters that respectively ranged from −64.8 ‰ to −48.9 ‰ and from −61.9 ‰ to −20.8 ‰, while δ18O samples ranged from −11.18 ‰ to −2.12 ‰. After four years of monitoring, δ2H and δ18O results indicate that the shallow groundwater samples are coincident with the reference meteoric water line, which implies these samples originate from rainwater. Surface water samples are consistent with water influenced by evaporation processes. Conversely, samples of produced waters exhibit a distinctly different isotope character compared to the shallow water samples. This suggests that there is no connection and mixing between surface and shallow aquifer with the deeper slurry injection zones. Stable isotope analysis has proven to be a successful groundwater monitoring technique and is an enabler for continued safe injection of slurry wastes into the deep subsurface. These efforts have been acknowledged by Kementrian Lingkungan Hidup dan Kehutanan (KLHK) of GOI experts as one of the most advance groundwater monitoring technologies currently in use in the Indonesian oil and gas industry.

In addition to efforts aimed at reducing anthropogenic production of greenhouse gases, geological storage of CO2 is being explored as a strategy to reduce atmospheric greenhouse gas emission and mitigate climate change. Previous studies of the deep subsurface in North America have not fully considered the potential negative effects of CO2 leakage into shallow drinking water aquifers, especially from a microbiological perspective. A test well in the Newark Rift Basin was utilized in two field experiments to investigate patterns of microbial succession following injection of CO2-saturated water into an isolated aquifer interval, simulating a CO2 leakage scenario. A decrease in pH following injection of CO2 saturated aquifer water was accompanied by mobilization of trace elements (e.g. Fe and Mn), and increased bacterial cell concentrations in the recovered water. 16S ribosomal RNA gene sequence libraries from samples collected before and after the test well injection were compared to link variability in geochemistry to changes in aquifer microbiology. Significant changes in microbial composition, compared to background conditions, were found following the test well injections, including a decrease in Proteobacteria, and an increased presence of Firmicutes, Verrucomicrobia and microbial taxa often noted to be associated with iron and sulfate reduction. The concurrence of increased microbial cell concentrations and rapid microbial community succession indicate significant changes in aquifer microbial communities immediately following the experimental CO2 leakage event. Samples collected one year post-injection were similar in cell number to the original background condition and community composition, although not identical, began to revert toward the pre-injection condition, indicating microbial resilience following a leakage disturbance. This study provides a first glimpse into the in situ successional response of microbial communities to CO2 leakage after subsurface injection in the Newark Basin and the potential microbiological impact of CO2 leakage on drinking water resources.

Groundwater geochemistry in bench experiments simulating CO2 leakage from geological storage in the Newark Basin

One concern regarding unconventional hydrocarbon production from organic-rich shale is that hydraulic fracture stimulation could create pathways that allow injected fluids and deep brines from the target formation or adjacent units to migrate upward into shallow drinking water aquifers. This study presents Sr isotope and geochemical data from a well-constrained site in Greene County, Pennsylvania, in which samples were collected before and after hydraulic fracturing of the Middle Devonian Marcellus Shale. Results spanning a 15-month period indicated no significant migration of Marcellus-derived fluids into Upper Devonian/Lower Mississippian units located 900-1200 m above the lateral Marcellus boreholes or into groundwater sampled at a spring near the site. Monitoring the Sr isotope ratio of water from legacy oil and gas wells or drinking water wells can provide a sensitive early warning of upward brine migration for many years after well stimulation.

The challenge of predicting groundwater quality impacts in a CO2 leakage scenario: Results from field, laboratory, and modeling studies at a natural analog site in New Mexico, USA

The Effects of Shale Gas Exploration and Hydraulic Fracturing on the Quality of Water Resources in the United States

Gas fracking: can we safely squeeze the rocks?

Environmental concerns regarding the potential for drinking water contamination in shallow aquifers have accompanied unconventional energy development in the northern Appalachian Basin. These activities have also raised several critical questions about the hydrogeological parameters that control the naturally occurring presence and migration of hydrocarbon gases in shallow aquifers within petroliferous basins. To interrogate these factors, we analyzed the noble gas, dissolved ion, and hydrocarbon gas (molecular and isotopic composition) geochemistry of 98 groundwater samples from south-central New York. All samples were collected ≫1km from unconventional drilling activities and sample locations were intentionally targeted based on their proximity to various types of documented fault systems. In agreement with studies from other petroliferous basins, our results show significant correlations between elevated levels of radiogenic [4 He], thermogenic [CH4 ], and dissolved ions (e.g., Cl, Br, Sr, Ba). In combination, our data suggest that faults have facilitated the transport of exogenous hydrocarbon-rich brines from Devonian source rocks into overlying Upper Devonian aquifer lithologies over geologic time. These data conflict with previous reports, which conclude that hydrodynamic focusing regulates the occurrence of methane and salt in shallow aquifers and leads to elevated levels of these species in restricted flow zones within valley bottoms. Instead, our data suggest that faults in Paleozoic rocks play a fundamental role in gas and brine transport from depth, regulate the distribution of their occurrence in shallow aquifers, and influence the geochemistry of shallow groundwater in this petroliferous basin.