Stray Gas Research Articles

Pathways and Environmental Impacts of Methane Migration: Case Studies in the Marcellus Shale, USA

Gas migration incidents, particularly stream contamination cases, have been rarely investigated and gone through the peer review process, with the exception of three sites in northeast Pennsylvania (Dimock and two Sugar Runs in Lycoming and Bradford counties, respectively) where air emission surveys, dissolved methane measurements, and structural (hydro)geologic interpretations have been used to demonstrate potential environmental impacts due to shale gas operations. In addition to reviewing previously published work from these three sites, we report and analyze unpublished new data trying to determine if a direct relationship between methane migration, stream contamination, and air emissions exists at those sites. Our analysis indicates that subsurface methane migration, stream methane contamination, and air emissions might not be all present or detectable at a faulty/leaky shale gas well. Which of these signs of contamination, if any, exist is largely controlled by the local (hydro)geologic conditions. In each case, the most likely migration pathway was from gas charged zones up well annular spaces to confined permeable formations, then laterally to a direct discharge or by vertically controlled joints to streams, water wells, and the atmosphere. The confining units act as barriers to the buoyant movement of stray gases, allowing subsurface travel of gas for 1–4 km from a leaky gas well. The knowledge we learn from these three sites can guide the future investigations of methane contamination cases in other regions.

Machine learning prediction of methane, ethane, and propane solubility in pure water and electrolyte solutions: Implications for stray gas migration modeling

Hydraulic fracturing is an effective technology for hydrocarbon extraction from unconventional shale and tight gas reservoirs. A potential risk of hydraulic fracturing is the upward migration of stray gas from the deep subsurface to shallow aquifers. The stray gas can dissolve in groundwater leading to chemical and biological reactions, which could negatively affect groundwater quality and contribute to atmospheric emissions. The knowledge of light hydrocarbon solubility in the aqueous environment is essential for the numerical modelling of flow and transport in the subsurface. Herein, we compiled a database containing 2129 experimental data of methane, ethane, and propane solubility in pure water and various electrolyte solutions over wide ranges of operating temperature and pressure. Two machine learning algorithms, namely regression tree (RT) and boosted regression tree (BRT) tuned with a Bayesian optimization algorithm (BO) were employed to determine the solubility of gases. The predictions were compared with the experimental data as well as four well-established thermodynamic models. Our analysis shows that the BRT-BO is sufficiently accurate, and the predicted values agree well with those obtained from the thermodynamic models. The coefficient of determination (R2) between experimental and predicted values is 0.99 and the mean squared error (MSE) is 9.97 × 10−8. The leverage statistical approach further confirmed the validity of the model developed.

Measuring changes in background dissolved gases to evaluate multi-component gas dissolution: Laboratory experiments and implications for stray gas monitoring

Gas emitted from leaking wells is a concern for natural gas production as well as the underground storage of carbon dioxide and hydrogen. Stray gas migration through heterogeneous layers, and its subsequent dissolution, controls the fraction emitted to the atmosphere rather than dissolved in groundwater. Understanding the effects of multiple gas components on gas dissolution is important for risk prediction, and measuring changes in background dissolved gas concentrations may provide a complementary tool to support site investigation. A series of two-dimensional laboratory experiments were performed to measure transient gas saturations and background dissolved gas concentrations during and after gas injection into heterogeneous sand packs. Mass transfer was influenced by both source architecture and groundwater velocity, and steady-state modelling was used to estimate mass transfer coefficients at the scale of the flow cell. The results demonstrate the potential of using background dissolved gases as part of gas migration monitoring strategies.

Discernment of transformer oil stray gassing anomalies using machine learning classification techniques

This work examines the application of machine learning (ML) algorithms to evaluate dissolved gas analysis (DGA) data to quickly identify incipient faults in oil-immersed transformers (OITs). Transformers are pivotal equipment in the transmission and distribution of electrical power. The failure of a particular unit during service may interrupt a massive number of consumers and disrupt commercial activities in that area. Therefore, several monitoring techniques are proposed to ensure that the unit maintains an adequate level of functionality in addition to an extended useful lifespan. DGA is a technique commonly employed for monitoring the state of OITs. The understanding of DGA samples is conversely unsatisfactory from the perspective of evaluating incipient faults and relies mainly on the proficiency of test engineers. In the current work, a multi-classification model that is centered on ML algorithms is demonstrated to have a logical, precise, and perfect understanding of DGA. The proposed model is used to analyze 138 transformer oil (TO) samples that exhibited different stray gassing characteristics in various South African substations. The proposed model combines the design of four ML classifiers and enhances diagnosis accuracy and trust between the transformer manufacturer and power utility. Furthermore, case reports on transformer failure analysis using the proposed model, IEC 60599:2022, and Eskom (Specification—Ref: 240-75661431) standards are presented. In addition, a comparison analysis is conducted in this work against the conventional DGA approaches to validate the proposed model. The proposed model demonstrates the highest degree of accuracy of 87.7%, which was produced by Bagged Trees, followed by Fine KNN with 86.2%, and the third in rank is Quadratic SVM with 84.1%.

Stray gas source determination using forensic geochemical data

There is a standard method for rigorously using geochemical data to identify the source of stray gas. “Stray gas” is defined as gaseous material of undetermined origin found in an area where the gas has impacted the shallow-subsurface or potable water supplies, or has the potential to present a threat to public health and safety. Stray gas is composed of hydrocarbons or other gases, such as CO2. Previous studies have used geochemical data, including molecular and isotopic gas compositions, to empirically constrain the source of stray gas present in samples collected from specific locations (sometimes called “receptors”). On a more granular level than presented previously, we have discussed the interpretive workflow that should be followed to interpret geochemical data in the context of a stray gas investigation. A key concept is that geochemical data should be used impartially to evaluate all potential sources of the stray gas and, where possible, identify the most likely source. Focusing on only a subset of potential gas sources can lead to erroneous conclusions regarding gas sources. Furthermore, to accurately identify the gas source, geochemical data must be interpreted in the context of the site-specific geology, including evidence of previous gas migration events in the area. We illustrate these points using examples from Parker County, Texas, and from Lycoming County, Pennsylvania.

Review on Gassing Tendency of Different Insulating Fluids towards Transformer Applications

This paper reports the critical reviews on the gassing tendency of different insulating fluids along with the precautionary measures to be considered during their fault diagnosis in transformer insulation. The experimental techniques and procedures for identifying the gassing due to electrical and thermal stress along with the stray gassing phenomenon has been elucidated. The different interpretation schemes used for determining the faults in transformers results in unexpected errors when the historical data relating to mineral oil is used for the other alternative fluids. Mineral oil and natural ester show a positive gassing tendency compared to synthetic ester which exhibit a negative gassing tendency. The stray gases are mostly due to breakage of C-C bonds under normal operating temperature of transformer. Among the different hydrocarbons, hydrogen and ethylene are more predominantly formed under lower temperatures. The silicone oil and ester fluids are more stable even under localised hot spots simulated observing a lesser gassing compared to the mineral oil. The impact of additives along with the oxygen and water content in the insulating fluids can lead to the stray gas’s causing confusion towards the identification of actual faults occurring in transformers. Furthermore, the regeneration of insulating fluids using different adsorbents reduces the gassing tendency depending on the number of cycles used for its reclamation.

THE EFFECT OF THERMAL STRESS AND MATERIALS TOWARDS STRAY GASSING FORMATION IN UNINHIBITED AND INHIBITED OIL

Insulation oil is one of the most important components in an oil-insulated transformer. With the use of the dissolved gas analysis (DGA) tool, the condition of the transformer can be studied and predicted to prevent unwanted events from occurring. However, due to the rising of issues related to stray gassing phenomenon that causes false interpretations in the transformer condition, there is a need to find out the factors contributing to the formation of the stray gases in the transformer oil. In this research paper, thermal stress (85℃ and 120℃) on transformer oil with different transformer materials are studied to determine the effect towards the formation of stray gases in the transformer oil. The materials studied include diamond dotted paper, metal plate, coated metal plate, insulation glue and core metal. Both the uninhibited oil and inhibited oil are used for comparison. The metal plate, coated metal plate and core metal were found to be causing the generation of H2 gas at 120℃ while the cellulose ageing of insulation paper is causing the generation of CO and CO2 gases at 85℃ and above. The CH4 gas was found to be generated only at 120℃ and the insulation glue was determined to be the main cause for the C2H4 gas generation at 85℃ and above.

Measurements of Atmospheric Methane Emissions from Stray Gas Migration: A Case Study from the Marcellus Shale.

Understanding emissions of methane from legacy and ongoing shale gas development requires both regional studies that assess the frequency of emissions and case studies that assess causation. We present the first direct measurements of emissions in a case study of a putatively leaking gas well in the largest shale gas play in the United States. We quantify atmospheric methane emissions in farmland >2 km from the nearest shale gas well cited for casing and cementing issues. We find that emissions are highly heterogeneous as they travel long distances in the subsurface. Emissions were measured near observed patches of dead vegetation and methane bubbling from a stream. An eddy covariance flux tower, chamber flux measurements, and a survey of enhancements of the near-surface methane mole fraction were used to quantify emissions and evaluate the spatial and temporal variability. We combined eddy covariance measurements with the survey of the methane mole fraction to estimate total emissions over the study area (2,800 m2). Estimated at ∼6 kg CH4 day-1, emissions were spatially heterogeneous but showed no temporal trends over 6 months. The isotopic signature of the atmospheric CH4 source (δ13CH4) was equal to -29‰, consistent with methane of thermogenic origin and similar to the isotopic signature of the gas reported from the nearest shale gas well. While the magnitude of emissions from the potential leak is modest compared to large emitters identified among shale gas production sites, it is large compared to estimates of emissions from single abandoned wells. Since other areas of emissions have been identified close to this putatively leaking well, our estimate of emissions likely represents only a portion of total emissions from this event. More comprehensive quantification will require more extensive spatial and temporal sampling of the locations of gas migration to the surface as well as an investigation into the mechanisms of subsurface gas migration. This work highlights an example of atmospheric methane emissions from potential stray gas migration at a location far from a well pad, and further research should explore the frequency and mechanisms behind these types of events to inform careful and strategic natural gas development.

Classification of Fault and Stray Gassing in Transformer by Using Duval Pentagon and Machine Learning Algorithms

Classification of Fault and Stray Gassing in Transformer by Using Duval Pentagon and Machine Learning Algorithms

Calculation of Gas Ratios for Use in Stray Gas Migration Investigation and Characterization

Calculation of Gas Ratios for Use in Stray Gas Migration Investigation and Characterization

Temporal changes in domestic water well methane reflect shifting sources of groundwater: Implications for evaluating contamination attributed to shale gas development

Regulatory agencies routinely assess the presence of stray gas release from unconventional gas wells by sampling for methane in nearby groundwater after the well is drilled or if citizens complain about methane in their water. We studied whether methane concentrations in groundwater naturally vary through time in a shale gas basin where unconventional development and hydraulic fracturing has not yet occurred, to test the assumption that pre-drilling observations of well water quality can be reliable measures for assessing impacts of later gas drilling. We collected groundwater samples from 11 domestic wells in New York monthly for 13 months for methane and ion concentrations in a highly gas productive part of the Appalachian basin where fracking has been banned. Changing methane concentrations correlated with changes in chloride and bromide, indicating changing mixtures of shallow freshwater and deeper formation brine extracted by the wells through time. The hydrogeologic setting of a water well can cause variability in methane concentrations that may mimic stray gas but cannot be attributable to gas drilling. For this reason, before and after testing has limited utility to distinguish impacts of gas drilling from other causes of changing methane concentrations unless that testing includes sampling a comprehensive set of ions multiple times prior to drilling.

Electrical Imaging of a Shallow Free-Phase Stray Gas Plume from an Abandoned Exploration Well

Geophysical imaging of free-phase gas (FPG) within aquifers is an emerging method for understanding the mechanisms controlling stray gas migration from oil and gas wells. Crystal Geyser is an unsealed and partially cased well that transports stray CO2 gas to the shallow subsurface. Accumulations of subsurface CO2 FPG near Crystal Geyser have been inferred, but the actual location and dimensions remained unclear. Here, the subsurface FPG distribution surrounding Crystal Geyser was characterized by interpreting 2D electrical resistivity images with previous drilling records and field mapping. An approximately 70-metre-wide FPG plume was located laterally between Crystal Geyser’s conduit and the Little Grand Wash Fault. The FPG plume spanned the vertical extent of approximately 20 to 55 metres below the ground surface, located within the Slick Rock Member sandstone with the relatively low permeability Earthy Member silty sandstone acting as a caprock. The FPG plume was identified from an anomalously high resistivity zone within the Slick Rock Member that was not caused by lateral lithofacies changes or fault displacement. The conceptual FPG migration pathways beneath Crystal Geyser are presented, based on the interpreted FPG distribution from the electrical resistivity images combined with previous site characterization and the principles of buoyant FPG migration. FPG accumulates within the Slick Rock Member by buoyant up-dip migration beneath siltstone capillary barriers of the Earthy Member. FPG leaks to the ground surface within high permeability preferential pathways along the Little Grand Wash Fault and the conduit of Crystal Geyser.

Stray Gassing of Oil in HV Transformers

Currently produced insulating oils tend to generate gases at relatively low temperatures (90–200 °C). This phenomenon, defined by CIGRE as stray gassing (SG), can hinder the diagnostics of oil-filled electrical equipment based on the DGA method, because high amounts of hydrogen H2 and saturated hydrocarbons can sometimes occur in service without an actual fault. The aim and contribution of this article is to present how to distinguish such cases from electrical partial discharges or hot spots in paper, which may be risky for the safe operation of equipment and demand further investigation. Very high levels of H2 and high levels of C2H6 are observed recently in some families of HV transformers in Poland. This article presents DGA results of oil samples taken from these transformers together with results of laboratory tests of stray gassing of these oils performed according to CIGRE procedure. By using Duval Pentagon 2 it is possible to compare stray gassing patterns in the laboratory and in real transformers and evaluate the temperature range of possible defects. Stray gassing/overheating of oil in transformers, within this temperature range and whatever the possible other causes, is not a concern for their safe operation, according to observations published by CIGRE.

Effects of Metal Passivator Degradation on the Dissolved Gases Characteristics of Oil in Oil-immersed Transformers

Sulfur corrosion has been proven to significantly decrease the insulation performance of oil-immersed transformer and the addition of benzotriazole passivator (BTA) to insulating oil has been widely used to control the sulfur corrosion. However, the relevant studies indicate that the addition of metal passivator to oil can cause abnormal generation of dissolved gases in oil. Thus, this study further explores the effects of metal passivator degradation on the dissolved gas characteristics of oil in oil-immersed transformers. Several accelerated aging experiments on oil-paper insulations with and without BTA are conducted. The results indicate that the addition of BTA lead to the generation of abnormal dissolved gases in the oil along with an increase of stray gases, especially the increase of H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , CO and CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . Moreover, an abnormal production of the total hydrocarbon fraction is found, which may interfere with the DGA results. Finally, the degradation of BTA related to abnormal production of stray gases in the insulating oil was preliminarily analyzed and discussed.

A preliminary study on the eco-environmental geological issue of in-situ oil shale mining by a physical model

As traditional methods of oil shale production emitting high levels of pollutants, in-situ exploitation has aroused great concerns. In order to study the effect of in-situ pyrolysis products on the underground environment, an in-situ oil shale exploitation of underground environment impact laboratory simulation system was designed. Based on the hydrogeological condition of oil shale area in Nong'an City, a physical simulation test was conducted. It was found the temperature of surrounding layers continued to be perturbed after heating of the formation had stopped. The time during which the temperature was perturbed was about 4–5 times the heating period. During the simulation test, stray gas migration through fractures and faults was considered a mechanism for groundwater contamination. In the test, the maximum TOC content in aquifer was the value of 97.0 mg/L. The maximum total petroleum hydrocarbon (TPH) content of the simulated formation was 129 mg/kg after oil pyrolysis.

Peculiar Stray Gassing Occurrences in Solar Photovoltaic Transformers during Service

Peculiar Stray Gassing Occurrences in Solar Photovoltaic Transformers during Service

Identification of Stray Gassing of Dodecylbenzene in Bushings

Several high voltage condenser type OIP (oil impregnated paper) bushings used in the electrical industry are filled with dodecylbenzene, because of its ability to absorb hydrogen formed by corona partial discharges in the thick paper insulation of these pieces of equipment. Some of them form large quantities of ethane, raising the concern of overheating faults in their paper insulation, which may be risky for their safe operation in service. The article presents dissolved gas analysis results of oil samples taken from the bushings with high ethane formation, together with results of laboratory tests of stray gassing of dodecylbenzene performed according to CIGRE procedure. By using Duval Pentagon 2 it is possible to compare patterns in the laboratory and in bushings and evaluate the temperature range of possible defects. Stray gassing/overheating of dodecylbenzene in bushings within the stray gassing temperature range and whatever the possible other causes, is not a concern for their safe operation according to observations published by CIGRE.

A review of physical, chemical, and hydrogeologic characteristics of stray gas migration: Implications for investigation and remediation.

Releases of natural gas into groundwater from oil and gas exploration, production, or storage (i.e., "stray gas") can pose a risk to groundwater users and landowners in the form of a fire or explosive hazard. The acute nature of stray gas risk differs from the long-term health risks posed by the ingestion or inhalation of other petroleum hydrocarbons (e.g., benzene). Stray gas also exhibits different fate and transport behaviors in the environment from other hydrocarbon contaminants, including the potential for rapid and extensive transport of free-phase gas through preferential pathways, and the resulting variable and discontinuous spatial distribution of free and dissolved gas phases. While there is extensive guidance on response actions for releases of other hydrocarbons such as benzene, there are relatively few examples available in the technical literature that discuss appropriate response measures for the investigation and remediation of stray gas impacts. This paper describes key considerations in the physical, chemical, and hydrogeological characteristics of stray gas releases and implications for the improved investigation and mitigation of associated risks.

Prospective policy safeguards to mitigate hydrogeological risk pathways in advance of shale gas development in the Karoo basin, South Africa

Policies surrounding energy development are frequently implemented in response to known or perceived problems. South Africa is in a unique position to develop prospective policies that build on knowledge gained from elsewhere. This paper provides a prospective analysis of hydrogeological risk pathways and vulnerability attributes in advance of anticipated shale gas operations in the Karoo Basin of South Africa. The ‘hazard-pathways-receptors’ approach is applied to define the hydrogeologic system in the context of potential sources of water resource contamination. This case study focuses on two critical hydrogeological risk pathways: regional groundwater flow and discrete structural features. Depending on the targeted area (hydro-litho-structural domain, depth of target, presence of aquitard, intrusion ratio), the capacity of deep natural pathways to enable hydraulic fracturing fluids, chemicals, or produced water to reach shallow groundwater, will be reduced or difficult. Deep artesian water could however be intercepted at different depths and, based on past groundwater exploration in the Karoo, water could flow into horizontal fractures or openings characterising the shallow aquifers: sills-ring complexes, lithological contacts, and transgressive fractures across dolerite dykes. However, above-ground water and wastewater management safeguards are needed to protect shallow groundwater from potential water quality degradation due to a) flowback or produced water b) stray gas and/or c) spills or illicit discharges that could introduce contaminants into groundwater resources. This paper describes a systematic approach to evaluate hydrogeologic risk pathways and informs adaptive management policies to protect South African groundwater resources.

Evaluation of dissolved gas analysis and long-term performance of non-edible natural ester

This paper aims at investigating the dissolved gas analysis (DGA) and long-term performance of Environtemp (FR3) and Jatropha Curcas oil (JAT). Thermal stress is applied to both oils in a sealed setup at 150°C for up to 2000 hours and samples are removed and tested at 500 hour intervals. It is observed that FR3 is more prone to stray gassing as compared to JAT. The alterations in the structure of the oils are studied by Fourier-transform infrared spectroscopy (FTIR) and 600 MHz nuclear magnetic resonance (NMR) spectroscopy analyses. The DGA interpretation is done using the Duval triangle, Dornenburg ratios, Rogers ratios and IEC methods. In the proposed study, a regression model for the two types of oils is developed to predict the total dissolved combustible gases with time. The findings show that JAT holds considerable properties of an insulating fluid and is a substitute for mineral insulating oils.