Groundwater Quality and Unconventional Oil and Gas Development: Current Understanding and Science Needs: Alphabetical Content Listing

Approaches and Technology for Monitoring Groundwater near Shale Gas Wells

Anthony Gorody, Ph.D., P.G.

A Flux Based Approach for Monitoring Hydraulic Fracturing Constituents in Groundwater

Michael D. Annable
A major challenge in documenting the presence or absence of compounds in groundwater related to hydraulic fracturing activities is the lack of high quality baseline data. While no method is capable of generating bassline data after hydraulic fracturing activities have begun in a region of interest, by switching to a flux based measurement approach, flux and mass discharge of constituents can be quantified using a simple mass balance framework to better assess the source of mass and proximity to activities. This allows for further characterization of a potential link to hydraulic fracturing activities. Some potential compounds of interest include benzene, phenanthrenes, naphthalene, 1-methylnapthalene, 2-methylnapthalene, fluorenes, aromatics, ethylene glycol, methanol and methane.

The passive flux meter (PFM) technique is ideally suited to enhance monitoring of compounds related to hydraulic fracturing. The PFM device uses sorbents to accumulate compounds of interest and alcohol tracers pre-equilibrated on a permeable sorbet to quantity groundwater flow. The PFM is placed in a monitoring well screen for a few weeks to characterize solute and water flux during the deployment duration. While most of the compounds listed above have been tested in previous applications, a sorbent for methane is currently being evaluated. Zeolites look like a promising sorbent for methane.

A Spatial Data Approach for Assessing Groundwater Resources, Risks, and Uncertainty

Kelly Rose, Ph.D.
To address questions related groundwater resources coinciding with areas of unconventional oil and gas development, there is a need to improve how we predict the distribution of potable groundwater and constrain uncertainty with those interpretations. Groundwater resources in several regions are stressed due to the high demand for the resource from competing activities, including those utilizing groundwater to hydraulically fracture unconventional hydrocarbon wells. In addition, these activities also raise questions and concerns about potential impacts to groundwater aquifers coinciding with development of unconventional resources. However, the quality and quantity of data available to characterize and predict the distribution of potable groundwater is inconsistent and in many areas poor. This study demonstrates a hybrid deductive-geostatistical approach for assessing depth to base of potable groundwater. The approach utilizes different sources of groundwater composition data to produce a structure contour interpolation of depth to base of potable groundwater. Our approach also produces a quantitative assessment of the uncertainty with that analysis using the Variable Grid Method (VGM), a geospatial-statistical approach. We demonstrate application of this approach in an analysis of depth to base of potable groundwater for West Virginia and Pennsylvania. The VGM uncertainty layer is integrated with the interpolation of the depth to base of potable groundwater, simultaneously providing the prediction of the distribution of potable groundwater and information about the quality of that prediction. This combination of spatial and statistical analysis provides more information to drive decisions about resource distribution, potential risks of impacts and knowledge gaps that may require additional data acquisition before robust conclusions can be made. Ultimately, this geospatial-geostatistical approach seeks to improve science-based decision making as it relates to both groundwater and hydrocarbon resource development.

Assessing the Environmental Impacts on Groundwater Quality in Areas of Unconventional Energy Resource Development

Bernhard Mayer, Ph.D.
To assess potential impacts on shallow aquifers by leakage of fugitive gas or saline water from unconventional energy resource development, it is essential to establish a reliable baseline for shallow groundwater, but also to geochemically “fingerprint” gases and fluids in the intermediate and production zones. Subsequently, the groundwater should be regularly sampled after hydraulic fracturing has occurred to monitor for potential changes or demonstrate the lack thereof. The presentation will review current practices and approaches that have been utilized in Western Canada to assess environmental impacts of unconventional energy resource development and outline their potential and limitations.

EOG Resources' Baseline Groundwater Monitoring Program for the Bakken

Justin Soberaski
EOG Resources (EOG) conducts baseline groundwater monitoring at privately-owned water wells within a 1-mile radius of newly proposed oil and gas wells in North Dakota’s Bakken Region. The baseline program is a voluntary risk management tool that establishes baseline groundwater conditions before drilling. The baseline program is modeled after Wyoming Oil and Gas Conservation Commission (WOGCC) rules and it involves available water source identification, preparing a site specific groundwater sampling plan, collection of groundwater samples, laboratory analysis, data quality assurance, and reporting.

Groundwater samples collected from private well owners are analyzed for most of the parameters specified in the WOGCC rules, which include inorganic compounds, hydrocarbons, and dissolved gases such as methane, ethane, and propane.

Many private water wells in the Bakken are in or near shallow lignite beds. Methane is common in lignite and is often present in shallow groundwater wells where it can be mistaken to be a result of local oil and gas production. EOG completes additional gas analysis if the dissolved methane concentrations are found to exceed 5 mg/L to “fingerprint” or further define the origin/diagenesis of the gas. Fingerprinting includes analyzing for fixed C1-C6 hydrocarbons and stable isotope concentrations of carbon (12C and 13C) and hydrogen (1H and 2H) in the methane.

Barr Engineering performs the sampling for EOG in North Dakota and coordinates with the contract laboratories. The program strengthens landowner relations by demonstrating a commitment to environmentally responsible operations and provides transparency by sharing the data with the landowner if they agree to be part of the program. Some of the logistical challenges in implementing the program include garnering the attention of the landowner, the often remote nature of the work, and making decisions regarding which wells are most appropriate for inclusion in the program.

Evaluating Changes in Freshwater Quality Using Groundwater Monitoring Wells in Areas of Natural-Gas Development

Erica Barth-Naftilan
Impairment of groundwater quality in areas of shale gas extraction has been reported, although causes remain unclear and contested due to inadequate baseline (pre-drill) water-quality data and insufficient knowledge of legacy contamination, industry operations, and local hydrogeology. To help address this issue, we are making chemical and hydrological measurements in freshwater aquifers and streams in Susquehanna County, PA. Through analysis of these measurements, we intend to (i) elucidate the spatial and temporal variations in trace metal concentrations, major ions concentrations, hydrocarbon concentrations, and isotope chemistry prior to commencement of gas extraction activities; (ii) quantify perturbations in groundwater flow that occur naturally or that are attributable to any step in the process of shale gas development; and (iii) evaluate changes (if any) in groundwater chemistry and isotopic composition induced by hydraulic fracturing and other stages of shale gas extraction.

Hydrogeophysical Log Analysis of Four Water-Supply Test Wells along an Appalachian Plateau Topographic Profile

John H. Williams
Hydrogeophysical logs from four 500-feet deep test wells drilled to extract freshwater for hydraulic fracturing of the Marcellus Shale were analyzed to determine the hydrogeologic framework along a topographic profile in southeastern McKean County, Pennsylvania. The wells were completed in sandstone and shale in a typical Appalachian Plateau setting along a 2.5-mile long profile having 400 feet of topographic relief. The hydrogeophysical logs collected from the wells included drill-cutting descriptions; caliper, gamma, and induction; acoustic and optical televiewer; and fluid-resistivity, temperature, and vertical flow under ambient and pumped conditions. The hydrogeophysical log analysis indicated that the test well at the highest elevation penetrated a transmissive fractured zone with high hydraulic head in sandstone of the Pottsville Formation at a depth of 175 feet below land surface. As indicated by aquifer-test results, this test well was hydraulically connected to the next test well downslope almost a mile away. The upper well and the downslope well both penetrated a transmissive fractured zone with low hydraulic head, which most likely provided the hydraulic connection between the wells, near the base of a thick sandstone bed at 330 and 240 feet below land surface, respectively, in the Waverly Group. The test well at the lowest elevation penetrated a high hydraulic-head fractured zone having minimal transmissivity that produced saline water near the base of a thick sandstone bed within the Catskill Formation at 475 feet below land surface. Borehole-wall breakouts were present in shale beds of the Catskill Formation penetrated by the well at 480 to 495 feet below land surface. Hydrogeologic framework characterization from analysis of hydrogeophysical logs, such as those used in this study, can provide important information for the design and installation of groundwater monitoring wells at shale-gas wellpads and for the evaluation of stray-gas migration and chemical spills.

In-line Sampling and Fixed Gas Analyses Help to Evaluate Dissolved Hydrocarbon Concentrations

Anthony Gorody, Ph. D., P.G.
Criticisms regarding the validity of dissolved methane concentrations in groundwater often cite in situ ebullition in the domestic water well environment as a factor affecting results. Such gas losses make it more difficult to confirm declining concentration trends that arise following remedial actions conducted in response to stray gas investigations. In-line groundwater sampling devices capture both dissolved and liberated gases during sampling. Subsequent analysis of both atmospheric and hydrocarbon gases in headspace samples can help resolve environmental variables affecting gas concentration.

Sample data presented here are derived from water wells screened in the Laramie-Fox Hills (LFH) aquifer located within the southwest corner of Weld County, Colorado. Total dissolved gas pressure (TDGP) is mediated by contributions from air derived from the air-water interface, dissolved bacteriogenic CH4, bacterial CO2, and dissolved Ar and N2 from the aquifer recharge zone. Among water well sites at high elevations, TDGP far exceeds saturation causing samples to effervesce. Results of this study show that dissolved hydrocarbon concentrations reported for in-line water sample data are consistently greater than RSK-175 data for samples collected with 40 ml VOA vials.

Elevated bacteriogenic dissolved methane concentrations in the LFH aquifer are highly variable. The dissolved argon concentration in these young aquifer fluids is just as soluble as methane but has a more restricted concentration range. There is a linear and positive correlation between the headspace methane:argon ratio and dissolved methane concentrations. Anomalously high C1:Ar ratios record Ar stripping resulting from either free phase bacteriogenic or stray gas migration bubbling locally within or in close proximity to the water column in the well. Dissolved Ar and N2 concentrations further reveal three dominant mechanisms affecting dissolved gas composition and concentration: well column degassing by gas stripping, re-equilibration at ambient temperatures, and mixing with water near the air-water interface.

Microbial Activity in Hydraulic Fracturing Produced Water from Two Shale Gas Reservoirs

Daniel Lipus
As shale gas has become one of the nation’s most important fossil fuels, it is increasingly important to monitor groundwater near shale gas wells to ensure there is no impact from drilling operations. The microbial activity within these shale gas reservoirs have the potential to alter the groundwater flow and groundwater chemistry through biogeochemical processes such as acid production, sulfate reduction, and biofilm formation. While these microbial communities are expected to impact monitoring strategies, little is known about the dominant microbial processes that will occur.

The goal of this study is to better define the microbiology of two major shale gas reservoirs, the Marcellus Shale in the Appalachian region, and the Bakken Shale in the North Dakota region. Metagenomic analysis was completed for 42 Marcellus Shale produced water samples and 64 Bakken Shale produced water samples. Bakken samples contained microbial loads 3 to 4 orders of magnitude below those observed for Marcellus samples. Microbial community structure analysis using 16S rRNA sequencing suggested samples to be dominated by the order Bacillales, Halanaerobiales, and Pseudomonadales. The role Halanaerobiales was further investigated by reconstructing and annotating a Halanaerobium draft genome. Reconstruction of metabolic pathways revealed Halanaerobium to have the potential for acid production, sulfide production, and biofilm formation, suggesting potential impact on groundwater quality. Data from this study extends the current knowledge of microbial processes within hydraulic fracturing produced water system, and will contribute to the development of better montioring strategies in the future.

Probabilistic Risk Assessment: Perspectives on Groundwater Contamination from Hydraulic Fracturing Activities

Carolyn Rodak
Concerns regarding the impact of unconventional oil and gas development have motivated a number of scientific studies investigating potential groundwater quality risks posed by directional hydraulic fracturing activities. These studies tend to adopt one of two perspectives: site-specific risks such as torn liners and well casing failures, or regional groundwater sampling campaigns investigating whether groundwater quality changes are associated with local hydraulic fracturing activities. Here we explore the use of fault trees to determine the overall probability of groundwater contamination due to hydraulic fracturing activities referred to here as the probability of failure. The potential utility of fault tree analysis for the quantification and communication of contamination risks is approached and discussed from two perspectives: site-specific failure and cumulative community-based failure. Available scientific data is sub-divided by stage within the hydraulic fracturing process, and the unique basic events required for failure. For example, to quantify the risk of an on-site spill we must consider the likelihood, magnitude, and composition of the spill. However, when considering the impact on a community-based groundwater supply, the subsurface fate and transport of the contaminant, as well as the proximity of multiple hydraulic fracturing sites to the community become relevant as the probability of failure is now a combination of all sites hydraulically connected to the groundwater source. In addition, the definition of failure in these two scenarios are different: risks on site focus on the volume of spill while a community perspective targets contaminant MCLs or the cumulative health risk posed by contamination events. Ultimately, this analysis highlights the differences between site-specific risks and collective regional risks as regulation will likely continue to focus on controllable site-specific risks but needs to consider the cumulative impact of these risks.

Recommended Practices for Baseline Sampling of Water Wells in Areas of Shale Gas Development

Stephen Richardson, Ph.D., PE,
Baseline sampling of residential water wells is standard practice for operators in many areas of shale oil and gas development. Data collected from these sampling programs is critical for evaluating whether reported changes in local water quality (e.g., methane, salts, metals) are related to nearby drilling activities, or potentially naturally occurring. However, little guidance is available to operators, regulators, and contractors to support the development of sampling programs and interpretation of data.

Our research project, funded by the Department of Energy, evaluated key sources of variability in baseline sampling results from water wells in Northeastern Pennsylvania. Multiple sampling events were conducted over a two-year period. Measured parameters and analytes included daily water usage, water levels, dissolved gases, stable isotopic analyses of methane, water, and dissolved inorganic carbon, major and trace ions, and field parameters. Results culminated in the development of recommended practices and guidance for dissolved gas sampling methods, well purging practices, and expectations for temporal variations in methane concentrations and methane-geochemical parameter relationships. Key findings and recommendations of this study include:

  1. Sample collection method can significantly affect the resulting methane concentration; a closed-system method yields the most accurate methane concentrations under effervescing conditions.
  2. Purging twice the volume of water in the pressure tank and lines is typically adequate for the collection of baseline samples for analysis of dissolved methane concentrations. Such water volumes are reasonably representative of the water quality regularly consumed by residents.
  3. For wells with dissolved methane concentrations greater than 1 mg/L, changes in concentration greater than two-fold over the two-year period were not commonly observed.
  4. At a subset of wells, changes in methane concentrations were significantly correlated with changes in sodium and other salinity indicators, where parameters varied according to the dynamic mixture of fresh and saline water sources in the wells.

Regional Monitoring/Analysis of the Effects of Oil and Gas Development on Groundwater in California

Matthew K. Landon
The California State Water Resources Control Board and U.S. Geological Survey are collaborating to implement a Regional Monitoring Program (RMP) to determine how oil and gas development has contributed to changes in groundwater quality at regional scales. Although creation of the program was motivated by public concerns regarding oil/gas stimulation treatments (primarily hydraulic fracturing), in California, the effects of stimulation treatments on groundwater resources are difficult to distinguish from those of other past and present oil and gas development activities that include larger volumes of injected fluid and overlapping chemical use for enhanced recovery or wastewater disposal.

During 2016-17, the RMP is analyzing selected priority oilfields in the southern Central Valley to: (1) produce three-dimensional salinity maps, (2) characterize the chemical composition of groundwater and oil-field water, and (3) identify the extent to which fluids from oil and gas development may be moving into protected (total dissolved solids less than 10,000 milligrams per liter) groundwater at regional scales.

Salinity distributions in groundwater near oil fields are being mapped using existing water-sample data, analysis of oil-well borehole geophysical logs, and newly collected airborne and surface electromagnetic data in selected areas to fill data gaps.

The RMP sampling-well networks are designed to evaluate groundwater quality along transects from oil fields into adjacent aquifers and include existing wells supplemented by monitoring-well installation in priority locations. Oil-field water and background regional groundwater are being sampled to characterize end-member compositions that may mix to influence groundwater chemistry near oil fields. The analytes include constituents with different transport characteristics such as dissolved gases (light hydrocarbon and noble gases), inorganic constituents (major, minor, trace elements), and organic compounds, as well as isotopic and groundwater-age tracers. Preliminary results indicate that groundwater quality near oil fields is strongly influenced by the regional hydrogeologic setting, including recharge rates.

Stable Isotope and Radiocarbon Evidence for Biogenic Coalbed Methane in Groundwater Wells in Utica Shale

Amy Townsend-Small
Previous studies have shown that natural gas methane (CH4) was present in groundwater near shale gas wells, but did not have pre-development baseline measurements. We present a time series of groundwater CH4 concentration and isotopic composition through a period of increasing shale gas extraction in the Utica Shale of Ohio. Dissolved CH4 ranged from 0.2 μg L-1 to 25 mg L-1, and stable isotopic measurements indicated a biogenic CH4 source. Radiocarbon dating of CH4 in three samples indicated a fossil biogenic coalbed CH4 source, with one 14C date indicative of modern biogenic CH4. We found no relationship between CH4 concentration in groundwater and proximity to active gas well sites and no significant change in CH4 concentration or isotopic composition in water wells during the study period. Continuous monitoring of private drinking water wells is critical to ensuring residents are not exposed to harmful levels of natural gas or fracking contaminants.

Validating a Discriminant Analysis Model Used to Distinguish Salinity Contamination from Deicers vs Produced Water

Nathaniel Chien
Baseline water quality measurements taken prior to unconventional energy development have been advocated as a method to better understand if groundwater quality has changed following resource extraction. However, direct comparisons of pre- and post-development data can produce equivocal results due in part to other potential sources of groundwater contamination. In previous work, we developed a model that uses linear discriminant analysis (LDA) to identify sources of salinity in groundwater samples based on their geochemical fingerprints. Our approach has the advantage of utilizing solute concentrations commonly available in existing databases, rather than novel tracers typically not measured in routine water chemistry studies. While our model appeared successful, there was no clear way to determine the accuracy because the saline groundwater samples we evaluated did not have known sources of contamination. Following up on that work, we have applied a modified version of the salinity-fingerprinting model to a new dataset of groundwater with known sources of contamination compiled from two studies of groundwater quality in Illinois: Panno et al., Illinois State Geol. Survey, Open File Series 2005-1 and Hwang et al. Environ. & Eng. Geosci., 11: 75-90 (2015). By predicting the source of salinity in groundwater samples for which sources of contamination are known, we were able to validate model predictive-accuracy. Results show high classification accuracy for groundwater samples impacted by formation water and road salt, with diminishing success for other contamination sources. Posterior probabilities, a statistic inherent to LDA, provides a proxy for prediction confidence in cases where the source of contamination is unclear. The results indicate that this model could be used as an additional tool in baseline water quality assessments to identify any major changes in the source of groundwater salinity.

Fate and Transport of Fracking Chemicals and Drilling Fluids in Groundwater, including Natural Attenuation Studies

Peter B. McMahon, Ph.D.

Biodegradability of Organic Compounds Used in Hydraulic Fracturing Fluids and Implications for Compound Fate

Paula Mouser
Biodegradation of organic additives used in fracturing fluids can result in unintended chemical byproducts in shale-produced wastewaters. If accidentally released to the environment, these compounds and their daughter products may have serious implications to human and ecological health. This talk presents biological and chemical information supporting the role of microbial communities in degrading a wide range of organic contaminants both downwell (i.e., high pressure, temperature and salinity conditions) and under surface conditions, highlighting polymers commonly used in Appalachian shale wells.

Fate of Hydraulic Fracturing Chemicals Downhole and after Environmental Release

Jens Blotevogel
Once injected into the deep subsurface, hydraulic fracturing chemicals are subjected to extreme temperatures, pressures, and salinities. This presentation will address the fate and transport of organic frac fluid additives downhole as well as after spillage on agricultural soils and after release into surface waters. Furthermore, the benefits of toxicity testing versus chemical analysis for public health and environmental impact assessment will be discussed.

Methane Dispersion from Leaky Petroleum Wells Into Groundwater: Can Point-Source Well Leaks Cause Large Plumes?

John A. Cherry, Ph.D.
Leakage of fugitive gas, mostly methane, from some oil and gas industry wells is a long-standing, unresolved engineering problem causing concern for groundwater resources and the atmosphere. This played a central role in the banning or placing moratoria on shale development in four Canadian provinces and most European countries. Gas leakage measurement at the wellheads is now common but determination of leaky gas in the subsurface away from wells is rare. Sampling domestic/farm wells in areas of shale development is standard but generates controversy because these wells have many complications. Isotope signatures in Pennsylvania provide evidence of methane from recently drilled gas wells showing up in domestic/farm wells within one kilometer of gas wells. This implies that somehow gas manages to find pathways for buoyancy-driven gas movement with advection of dissolved methane from leakage points along gas wells outward and upward to eventually cause detectable presence in the shallow freshwater zone supplying the household wells. For there to be reasonable likelihood of fugitive methane being detected in these wells, point-source methane from well leaks must undergo strong spreading for the methane to expand into a large plume and the methane must be recalcitrant enough to endure during plume migration. This presentation uses the literature for plumes from NAPL, air sparging and CO2 field injection experiments along with a recent methane injection experiment in the shallow sand aquifer at the Borden field experiment site in Ontario. Here, methane injected at two well points resulted in strong effective dispersion due to process combinations acting on the gaseous and dissolved methane. This experiment sets the stage for the design of methane injection experiments planned for a new field experiment station focused on sedimentary bedrock established by the Containment and Monitoring Institute (CaMI) and the University of Calgary near Brooks, Alberta.

Migration of Hydraulic Fracturing Fluids by Deep-Well Disposal into Fresh-Water Aquifers

Ronald Green, Ph.D., P.G.
Deep-well injection is the most common means of disposal of hydraulic fracturing flowback and produced water. While usually environmentally safe when conducted at appropriate locations and depths using acceptable deep-well disposal technology, fresh-water aquifers can be adversely impacted if the injection horizon is not capable of safely storing the fluids or if injection practices are inadequate. Because many shale gas and tight sand hydrocarbon fields are located at or near conventional oil/gas fields, spent oil/gas formations are often targeted for deep-well disposal due to relatively high permeability and under-pressurization. Risks to safe containment in the injection horizon are encountered, however, when the isolation capacity of the injection horizon is compromised by existing boreholes and wells that were not effectively plugged and abandoned, or if the injection well is not properly constructed, maintained, and monitored. Evaluation of current deep-well disposal practices in the Eagle Ford and Permian Basin has shed light on what factors need to be addressed to be able to adequately assess whether a disposal well will pose unacceptable risks to fresh-water aquifers. Examples of risk assessments due to over-pressurization of injection horizons, improperly plugged and abandoned boreholes and wells, and improperly constructed injection wells are discussed.

UOG Wastewater Leaks and Spills: Fate and Transport of Chemical Constituents

Isabelle M. Cozzarelli
Unconventional oil and gas (UOG) wastewaters containing toxic and radioactive elements derived naturally, or from chemical additives, pose largely unknown risks to water resources and aquatic and human health. Exposure pathways include land application, leaky surface impoundments and pipelines, discharge of treated wastewaters, failures in well casings, or migration through fracture networks. During 2008-2015, recorded spills in North Dakota included over 20-million gallons of waste fluids. As part of a national assessment of the impacts of UOG activities, a team from USGS and collaborators are conducting interdisciplinary studies in regions of active development, including the Bakken formation and the Marcellus Shale. We are studying geochemical alterations of water, soil, and sediment samples from UOG wastewater-impacted sites, targeting potential contaminants of aquatic and human health concern or those compounds that could serve as useful tracers of waste materials in the event of a release to the environment.

In one North Dakota study area 3-million gallons of wastewater (containing 138,000 mg/L TDS, 16.2 mg/L Ba, and >1,000 mg/L Sr) from UOG production in the Williston Basin spilled into a small tributary of the Missouri River. Water quality, sediment, and bioassay samples were collected and analyzed for a broad range of organic and inorganic compounds to assess potential impacts from this spill. Results indicate the presence of inorganic markers of the wastewater and the persistence of geochemical alterations in the creek six months post-spill. Labile Ba and Sr concentrations extracted from sediments were higher downstream from the spill site than upstream. Radium concentrations in sediment were up to 5 times the background concentrations downstream from the spill site. Sequestration of elements from the wastewater spill onto the sediment limits movement in surface water downstream but could provide a long-term source to both groundwater and surface water if geochemical conditions change in the future.

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Sources and Migration of Stray Gas in Groundwater Associated with Shale Gas Development

Daniel Soeder, MS

A Geochemical Context for Stray Gas Investigations in the N. Appalachian Basin

Fred Baldassare, P.G.
More than 2,300 gas and water samples were analyzed in the present study for (1) molecular composition, (2) stable carbon and hydrogen isotope compositions of methane and (3) stable carbon isotope composition of ethane. The samples are from Neogene to Middle Devonian-age strata in a five-county study area in northeastern Pennsylvania. Gas and water samples were collected from (1) 234 gas wells during Mudgas Logging (MGL) programs for wells being drilled to the Marcellus Shale Formation, and (2) 67 private water supply wells during baseline groundwater water-quality testing programs. Regional and local geologic conditions were evaluated from core analyses and published studies.

Evaluation of this geochemical database reveals that microbial, mixed microbial/thermogenic, and thermogenic gases occur in some shallow aquifer systems, and that the gas occurrences pre-date Marcellus Formation drilling activity. The isotope data reveal that thermogenic gases in the Neogene and Upper Devonian strata are typically distinct from gases from deeper Middle Devonian strata (including the Marcellus Fm.).

Defining a specific source for stray natural gas requires the investigation and synthesis of several data types at the site-specific level. Molecular and isotope geochemistry provide evidence of gas origin and evidence of secondary processes that may have affected the gases. Such data provide focus for investigations where the potential sources for stray gas include multiple naturally occurring and anthropogenic gases. Additional investigation to delineate migration pathways and the mechanism of migration are necessary to further constrain and identify specific stray gas source(s).

Analysis of Sustained Casing Annular Pressure in Relation to Stray Gas Assessment

Tom Tomastik, P.G.
A key identifier for assessing well integrity related to gas migration investigations (GMIs) is Sustained Casing Annular Pressure (SCAP). The issue of SCAP has been addressed by regulation in some areas, however, gaining a detailed understanding of this well integrity attribute is complex. Unlike the standard annular pressure test (SAPT), part of a casing-tubing-packer integrity testing, SCAP applies to annular spaces that may be open to uncemented portions of the drilled wellbore, cement pumped while casing strings were set, and the casing strings themselves. Further, accessing these annular spaces to perform pressure build-up (BU) or fall-off (FO) tests can be challenging. In some cases, retrofits to the wellhead may be required to even perform pressure testing.

Beyond physical challenges, pressure testing methods vary throughout the industry as do quality control methods, analysis methods, and interpretation of results. This lack of consistent practices can make interpretation of SCAP challenging. This presentation will discuss the significance of SCAP, it’s prevalence throughout all oil & gas producing areas, methods used for identification of SCAP, quality control measures, interpretation methods, and existing regulatory criteria. Furthermore, the presentation will include actual examples to allow real-world examples in both the United States and Internationally.

Approaches for High Resolution Monitoring for Groundwater Impacts from Shale Gas Development

Beth Parker, PhD
Little is known about the migration and impacts of fugitive gas potentially associated with shale gas development because nearly all the monitoring has relied on sampling of domestic and farm wells without the installation of purpose-focused monitoring systems. Prior to shale gas development, the occurrence/abundance of methane and other gases along with groundwater chemistry and flow regime (e.g. fracture flow) need to be established as background so that impacts of hydraulic fracturing, if any, can be discerned. This presentation outlines an approach being developed using geochemical and isotope analyses on gases obtained from continuous core taken during drilling holes in which depth-discrete, multilevel monitoring systems (MLSs) are installed. The emphasis is on the upper freshwater and underlying intermediate zone to investigate baseline methane occurrence and potential migration in layered sedimentary bedrock. Existing commercially available MLSs are useful and new systems are being developed in combination with fiber optic cables. This project has two components, one based at the Field Research Station being established by the Containment and Monitoring Institute (CaMI) and the University of Calgary near Brooks, Alberta, for multidisciplinary research (CaMI.FRS). Initially there will be injections of CO2 beneath caprock strata at as depth of 300 m below surface as part of a carbon capture and storage monitoring technology research program that later may include methane injection. Groundwater at this site is rich in natural biogenic methane. A farm well has been installed and conventional monitor well clusters are planned so that nearly all approaches to groundwater monitoring can be assessed. The other part of the project is planned for a site in the Yukon where extensive shale gas development is planned and background monitoring is needed. Experience at the CaMI.FRS will guide this endeavour.

Evaluating Shallow Aquifer Vulnerability to Potential Shale Gas Exploration and Development in Eastern Canada

Christine Rivard
In Quebec (eastern Canada), public concerns led to a de facto moratorium in 2010 for the St. Lawrence Lowlands, where the underlying Utica Shale is known to contain significant gas resources. As only a few exploration gas wells have been drilled, this area may still be considered “virgin” with respect to exploitation. In 2012, a multidisciplinary project was initiated to evaluate shallow aquifer vulnerability to eventual shale gas exploration and exploitation in the St-Edouard area (500 km2), near Quebec City. It involved multiple components, including tectonostratigraphy, geophysics, geomechanics, hydrogeology, rock organic geochemistry and an extensive groundwater geochemical study. This project thus attempted to evaluate the potential presence of preferential natural pathways that could allow the migration of fluids from shale gas horizons to shallow aquifers using multiple field evidence. Specific aspects related to groundwater geochemistry were also studied: natural variations of methane concentration and isotopes over time and with sampling techniques. Strategic data were acquired from the study of 30 residential wells, 4 deep shale gas wells and 15 new shallow observation wells drilled into shale.
Results showed that dissolved methane is ubiquitous and that its presence is mostly related to specific aquifer conditions such as long water residence times and absence of oxygen. While there is evidence that small amounts of deep formation brines migrate into shallow aquifers near a normal fault, there is no indication that deep thermogenic gas from the Utica Shale is currently reaching the surface through this fault zone or elsewhere in this region.

Hydrocarbons in Groundwater Overlying the Eagle Ford, Fayetteville, and Haynesville Shale UOG Development Areas

Peter B. McMahon, Ph.D.
Groundwater overlying the Eagle Ford, Fayetteville, and Haynesville Shale unconventional oil and gas (UOG) development areas was analyzed for chemical, isotopic, and groundwater-age tracers to evaluate sources of selected hydrocarbons in groundwater. Methane isotopes and hydrocarbon gas compositions indicate most of the methane detected in these areas was biogenic and produced by the CO2 reduction pathway, not from thermogenic shale gas. Two samples contained methane from the fermentation pathway that could be associated with oil or fuel degradation based on their co-occurrence with fuel hydrocarbons such as ethylbenzene, butane, and MTBE. Benzene was detected at low concentrations (<0.15 µg/L), but at relatively high frequencies (2.4–13.3% of samples), in the study areas. Eight of nine samples containing benzene had mean groundwater ages >2500 years, indicating the benzene was from subsurface sources. One sample from the Fayetteville study area contained benzene that might be from a surface source associated with UOG production activities based on its mean groundwater age (10±2.4 years) and proximity to UOG wells. Overall, our results suggest groundwater in the Fayetteville study area was the most vulnerable to potential surface sources of contamination based on estimates of mean groundwater age and fractions of post-1950s water in the samples.

Surface Casing Pressure, Well Integrity Loss, and Stray Gas Migration in the Wattenberg Field, Colorado

Greg Lackey
The risk of environmental contamination by oil and gas wells depends strongly on the frequency with which they lose integrity. Oil and gas wells with compromised integrity typically exhibit pressure in their outermost annulus (surface casing pressure, SfCP) due to gas accumulation. SfCP is an easily measured but poorly documented gauge of well integrity. Here, we analyze SfCP data from the Colorado Oil and Gas Conservation Commission online database to evaluate the frequency of well integrity loss in the Wattenberg Test Zone (WTZ), within the Wattenberg Field, Colorado. Deviated and horizontal wells were found to exhibit SfCP more frequently than vertical wells. We propose a physically meaningful well-specific critical SfCP criterion, which indicates the potential for a well to induce stray gas migration. We show that 270 of 3,923 wells tested for SfCP in the WTZ exceeded critical SfCP. Critical SfCP is strongly controlled by the depth of the surface casing. Newer horizontal wells, drilled during the unconventional drilling boom, exhibited critical SfCP less often than other wells because they were predominantly constructed with deeper surface casings. Thus, they pose a lower risk for inducing stray gas migration than legacy vertical or deviated wells with surface casings shorter than modern standards.

Welcome and Explanation of Format

William Alley, Ph.D.