Occurrence Of Gas Research Articles

An improved method to characterize the pore-throat structures in tight sandstone reservoirs: Combined high-pressure and rate-controlled mercury injection techniques

The pore-throat size determines the oil and gas occurrence and storage properties of sandstones and is a vital parameter to evaluate reservoir quality. Casting thin sections, field emission scanning electron microscopy, high-pressure mercury injection and rate-controlled mercury injection are used to qualitatively and quantitatively investigate the pore-throat structure characteristics of tight sandstone reservoirs of Xiaoheba Formation in the southeastern Sichuan Basin. The results show that the pore types include intergranular pores, intragranular dissolved pores, matrix pores, intercrystalline pores in clay minerals, and microfractures, and the pore-throat sizes range from the nanoscale to the microscale. The high-pressure mercury injection testing indicates that the pore-throat radius is in range of 0.004–11.017 µm, and the pore-throats with a radius >1 µm account for less than 15%. The rate-controlled mercury injection technique reveals that the tight sandstones with different physical properties have a similar pore size distribution (80–220 µm), but the throat radius and pore throat radius ratio distribution curves exhibit remarkable differences separately. The combination of the high-pressure mercury injection and rate-controlled mercury injection testing used in this work effectively reveals the total pore-throat size distribution in the Xiaoheba sandstones (0.004–260 µm). Moreover, the radius of the pore and the throat is respectively in range of 50–260 µm and 0.004–50 µm. The permeability of the tight sandstones is mostly affected by the small fraction (<40%) of relatively wide pore-throats. For the tight sandstones with good permeability (>0.1 mD), the larger micropores and mesopores exert a great influence on the permeability. In contrast, the permeability is mainly influenced by the larger nanopores. Furthermore, the proportion of narrow pore-throats in tight sandstones increases with reducing permeability. Although the large number of narrow pore-throat (<100 nm) makes a certain contribution to both reservoir porosity and permeability, they have contribution to the former is far more than to the latter.

The distribution and characterization of gas domes in Lumparn Bay, Åland Islands, northern Baltic Sea

Abstract In 2016, approximately 1800 previously unobserved seabed domes were found in high-resolution acoustic soundings of the Lumparn Bay in the Aland Islands, northern Baltic Sea. As similar geological features have only rarely been reported in earlier literature globally, the aim of this study was to characterize the domes, to investigate their geochemical composition, as well as to determine if they had any effects on the composition of the benthic fauna. The domes were generally about 1 m high and had a diameter of 20–30 m, while the largest ones were up 60 m in diameter. The domes resembled the initial processes in pockmark formation where gas is trapped under the sediment surface, doming the sediment upwards. However, no pockmarks were found in the area, indicating that doming does not result in pockmark formation. High sulphur concentrations indicate hydrogen sulphide formation within the dome field area while large zones with acoustic blanketing (acoustic interference) also indicate the occurrence of gas, most likely formed in biogenic processes. Concentrations of chromium, copper, cadmium, zinc, lead, mercury and arsenic showed some differences in character between the domes and reference sites. In general concentrations varied less in the domes and were somewhat higher in the reference sites. Comparison with sediment data from the Gulf of Finland revealed that heavy metal concentrations were on low levels in the Lumparn Bay. The non-metric multidimensional scaling (NMDS) showed some, although not significant, differences between the benthic invertebrate communities of the domes and the reference sites.

A Salgótarjáni és Ózdi paleogén részmedence térképezése szeizmikus és gravitációs mérési adatok alapján, és az eredmények szénhidrogén-földtani vonatkozásai

Az Észak-magyarországi Paleogén-medence perspektivikus szénhidrogének kutatása szempontjából. A kutatási lehetőségek megítéléséhez azonban elengedhetetlen a medencealjzat domborzatának, ill. az üledékkitöltés szintjeinek helyes ábrázolása. A Salgótarjáni- és Ózdi-részmedence területén a medenceperemi kutatófúrások alapján készült, országos léptékű prekainozoos medencealjzat térképek a szeizmikus mélységszelvények alapján pontosíthatók. A Magyar Bányászati és Földtani Szolgálat (MBFSZ) szakemberei szeizmikus, gravitációs és mélyfúrási adatok újrafeldolgozása, újraértelmezése alapján elkészítették a terület részletes, nagyobb felbontású prekainozoos medencealjzat térképét, és az üledékkitöltés vezérszintjeinek térképeit. Az itt bemutatott eredmények alapján a paleogén medence aljzata a medenceperemektől Bárna település alatt (Salgótarjántól 10 km-re K felé) 3400 méter tengerszint alatti mélységig süllyed.
 A geofizikai adatok együttes értelmezése szerint a Salgótarjáni- és az Ózdi-részmedence között jelentős eltérés van mind az aljzat mélysége, mind a medencét kitöltő üledékrétegsor tekintetében. A gravitációs medencemodell két sűrűségmodellel közelíthető, amelyek között egy átmeneti zóna is van. A szeizmikus értelmezés alapján a részmedencék paleogén–alsó-miocén üledékkitöltésében öt vezérszint követhető, a nyugati részen (Salgótarjáni-részmedence) nagy vastagságban találhatók idősebb oligocén üledékek, míg az északkeleti területen (Ózdi részmedence) a fiatalabb oligocén és miocén képződmények dominálnak. A potenciálisan anyakőzetként számontartott összletekről mélység- és vastagságtérképek készültek. Ezek szerint a medence nyugati részének mélyzónája kiterjedtebb, mint azt a korábbi áttekintő térképek ábrázolják. A terület kőolaj és földgáz előfordulásai és indikációi azt jelzik, hogy a Salgótarjáni-részmedencében a szénhidrogének anyakőzetei érettek, a felhalmozódások pedig az anyakőzetekben, vagy azok szűk környezetében lehetnek, mert a szénhidrogén migráció korlátozott. A vitrinit reflexió adatok szerint a medenceperemeken a jelenleg felszín közelben lévő megmintázott kőzetrészek szerves anyaga korábban a jelenleginél mélyebben volt eltemetve, és a kőolajképződés kezdeti fázisának közelébe jutott.
 A Salgótarjáni-részmedencének a korábban várhatótól jelentősebb kiterjedése és mélysége, a várható szénhidrogén anyakőzetek mennyisége és érettsége alapján a terület CH potenciálja jelentősebb lehet a korábbi megítélésnél. A térképek, és a szakirodalomból ismert szénhidrogén-földtani adatok alapján becslés történt az anyakőzet összlet által generált szénhidrogén mennyiségre, amely az értékelés szerint a 100 millió m3-t is meghaladhatja.

Model-based interpretation of hydrogeochemistry and arsenic mobility in a low-enthalpy hydrothermal system

Low-enthalpy geothermal systems are widely spread around the world. Their exploitation for geothermal energy requires an accurate knowledge of the system. Detailed hydrogeochemical characterization is of pivotal importance since geothermal systems are often associated to the occurrence of toxic gases and elements which may contaminate any connected water resources. In this study we demonstrate how increased knowledge of a low-enthalpy geothermal system (Cimino-Vico, Central Italy) can be acquired from the analysis and interpretation of major chemistry and 87Sr/86Sr of local spring waters and groundwaters. With a model-based approach, we assess the main processes governing the major ion composition and arsenic (As) mobility in the system. The occurrence of high concentration of arsenic in the groundwater of the study area is a severe problem for the inhabitants that use the resource for domestic purposes. The system's hydrogeology consists of a shallow highly permeable aquifer, composed of alkaline-potassic volcanic rocks and characterized by fresh waters, a semi-confining layer at the base of the fresh water aquifer, and a deeper thermal aquifer in Mesozoic carbonates and Triassic evaporites. Upwelling of hot waters (up to 63 °C) to the shallow aquifer is related to the presence of faults and fractures in the semi-confining layer. The major chemistry of the deep thermal waters was found to be controlled by dedolomitization, while the fresh waters chemistry is governed by the interaction with the volcanic rocks and the mixing with the upwelling CO2-rich thermal waters. The outcomes of geochemical modeling are consistent with a conceptual model positing that arsenic bound to iron-(hydr)oxides becomes mobilized in the shallow, volcanic aquifer when thermal waters ascend into such shallow aquifer, where they promote the desorption of arsenic in favor of bicarbonate.

Тектоническая дислоцированность мезозойско-кайнозойских отложений как один из основных нефтегазоконтролирующих признаков в северной части Западно-Сибирской плиты

Тектоническая дислоцированность мезозойско-кайнозойских отложений как один из основных нефтегазоконтролирующих признаков в северной части Западно-Сибирской плиты

Occurrence and influence of residual gas released by crush methods on pore structure in Longmaxi shale in Yangtze Plate, Southern China

ABSTRACT The composition of gas released under vacuum by crushing from the gas shale of Longmaxi Formation in Upper Yangtze Plate, Southern China was systematically investigated in this study. The effect of residual gas release on pore structures was checked using low-pressure nitrogen adsorption techniques. The influence of particle size on the determination of pore structure characteristics was considered. Using the Frenkel-Halsey-Hill method from low-pressure nitrogen adsorption data, the fractal dimensions were identified at relative pressures of 0−0.5 and 0.5−1 as D1 and D2, respectively, and the evolution of fractal features related to gas release was also discussed. The results showed that a variety component of residual gas was released from all shale samples, containing hydrocarbon gas of CH4 (29.58% −92.53%), C2H6 (0.97% −2.89%), C3H8 (0.01% −0.65%), and also some non-hydrocarbon gas such as CO2 (3.54% − 67.09%) and N2 (1.88%−8.07%). The total yield of residual gas was in a range from 6.1 μL/g to 17.0 μL/g related to rock weight. The geochemical and mineralogical analysis suggested that the residual gas yield was positively correlated with quartz (R2=0.5480) content. The residual gas released shale sample has a higher surface area of 17.20−25.03 m2/g and the nitrogen adsorption capacity in a range of 27.32−40.86 ml/g that is relatively higher than the original samples (with 9.22−16.30 m2/g and 10.84−17.55 ml/g). Clearer hysteresis loop was observed for the original shale sample in nitrogen adsorption-desorption isotherms than residual gas released sample. Pore structure analysis showed that the proportions of micro-, meso- and macropores were changed as micropores decreased while meso- and macropores increased. The fractal dimensions D1 were in range from 2.5466 to 2.6117 and D2 from 2.6998 to 2.7119 for the residual gas released shale, which is smaller than the original shale. This factor may indicate that the pore in residual gas released shale was more homogeneous than the original shale. The results indicated that both residual gas and their pore space have few contributions to shale gas production and effective reservoir evaluation. The larger fragments samples of granular rather than powdery smaller than 60 mesh fraction of shale seem to be better for performing effective pore structure analysis to the Longmaxi shale.

ИССЛЕДОВАНИЕ ПАРАМЕТРОВ РАБОТЫ ГАЗОРАСПРЕДЕЛИТЕЛЬНОГО МЕХАНИЗМА ПРИ ВИБРОАКУСТИЧЕСКОМ КОНТРОЛЕ В РЕЖИМЕ ТЕСТОВОГО ДИАГНОСТИРОВАНИЯ

Statistical analysis of ICE system faults shows a considerable growth of gas distribution mechanism failures: on average 7-12 %.This is explained by complicacy of gas distribution elements and occurrence of numerous new elements ( hydraulic pushers, hydraulic tensioners, automatic correcting mechanism of gas distribution phases, multivalve systems). However, in practice improvement of the gas distribution mechanism conspicuously surpasses development of new veracious techniques and methods of technical condition control. Development of new techniques and methods of technical condition control over the gas distribution mechanism enables solving of the important task to increase the engine operational resource. The most informative and veracious diagnostic method is vibroacoustic one, which makes it possible to evaluate technical condition of gas distribution mechanism at any stage. In order to diagnose the gas distribution mechanism with consideration of minimum interference and maximum information content, we chose the equipment which includes USB-Avtoscope III (Postolovsky oscillograph), a laptop with data analysis software, a pressure sensor for pressure control in the cylinder and an accelerometer. For prevention of linear expansion of metals , diagnostics was made on the engine with the temperature ranging from +80 to +90° C (measurements which are taken by a measuring probe within this temperature range are identical). The maximum expansion of metal was fixed at 1100 min-1 in 14 sec., at 1300 min-1 - 13 sec., at 1500 min-1 - 10,2 sec. Change of valve expansion clearance from 0,35 mm to 0,7 mm reduces the valve opening phase by 2×15°30′ (i.e. results in considerable delay of valve opening by 15°30′ and acceleration of its closure by 15°30′).

On the potential of gassy marine soils triggering submarine slope instabilities

The development of debris flows and turbidity currents in the course of a submarine slope failure event can cause major damage in offshore infrastructure. Additionally, the tsunamogenic potential of large slope failures at continental margins poses a direct threat to coastal communities. Therefore, the trigger mechanisms of submarine slope failures have been thoroughly investigated in the past. However, the influence of free gas in the sediment, which has been observed close to several slide events, remains unexplained. In order to evaluate the potential of gassy marine soils to precondition or trigger slope failure the mechanical behaviour of gassy soils is assessed based on an extensive literature review. It is found that gas-induced excess pore pressures can lead to liquefaction failure in sands, while cohesive, gassy soils show a less conclusive response. Hence, fine-grained soils and approaches to implement the gas impact into relevant existing constitutive soil models are assessed in greater detail. Concludingly, based on the predominant boundary conditions in failure prone regions at the continental margins, free gas occurrence can be defined as a preconditioning factor rather than as a definite trigger mechanism.

Gas comprehensive prevention and control technology and practice in outburst mine

In view of the present situation of gas occurrence in Xiaoxi coal industry, combined with the problems of high gas pressure, high content and complex geological structure in the gas control, the comprehensive gas prevention and control technology to meet the mining connection is proposed.It mainly includes the pre drainage measures of underground kilometer drill hole, through layer drilling and bedding drilling, comprehensive gas prevention technology of surface well pre drainage of coal seam gas and underground cross layer drilling area outburst prevention. The practice shows that after the implementation of comprehensive gas control measures, the residual gas content and gas pressure of coal seam are greatly reduced, and the gas control effect is remarkable. Through the comprehensive gas control technology of outburst mine, the gas control level of Xiaoxi coal industry has been significantly improved, and the number of times of gas overrun has decreased year by year.

GIS based 3D visualization of subsurface geology and mapping of probable hydrocarbon locales, part of Cauvery Basin, India

The hydrocarbon explorations were mostly guided by conventional geological and geophysical techniques in the past and modern tools like Remote Sensing, GIS, geophysical tomography came into being only during the last 2–3 decades. However, advanced virtues available with GIS, which could provide potential clues in deciphering the deep-seated natural resources were not capitalised deservingly. In this connection, the present article is the outcome of a study carried out in parts of Cauvery Basin, India for deciphering the subsurface hydrocarbon locales using Digital Elevation Modelling (DEM) techniques. The study was accomplished by the following hierarchical steps. (i) In the first step, DEM of gravity, litho tops depth of layered sedimentary rocks of Tertiary period and iso-resistivity data of 50 m depth were generated using Arc-GIS. (ii) It was followed by mapping the surface expressed circular features and the faults. (iii) Then, all the above surface and multi-depth data on the geological structures were integrated using Arc-GIS. (iv) From such an integration, 3D visualised domal structures of probable of hydrocarbon parentage were identified. (v) Finally, these were validated using known oil/gas wells. The study revealed the occurrence of domal structures with encircling peripheral faults from the subsurface to surface level in number of places. The spatial correlation of the known oil and gas occurrences with these 3D visualised domal structures indicated that the peripheral faults and domes have acted respectively as the zones of mobilisation and accumulation of oil and gas.

The occurrence of nano- and micro-scale pores and their controls on the selective migration of gases in the coals of different ranks

The nano- and micro-scale pores distribution of coals are important in determining the gas occurrence mechanism and gas migration behavior in the coal beds. Taking from coals of different ranks in China, the nano- and micro-scale pores distribution characteristics and their controls on the selective migration of gases was studied. It shows that the pore volume of the micro-scale pores presents a decreased trend and evolves towards the nano-scale micropores with the increased coal rank. For coals of different ranks, there is no adsorption energy in the nano-scale mesopores and all the micro-scale pores for CH4, N2, and CO2, where these pores play the roles of gas migration. Instead, the nano-scale micropores provide the sites for gas adsorption. Because of the differences in the adsorption energy for different sizes of pores in the nano-scale micropores section, these pores show selective sorption characteristics and variable gas migration behaviors. In the process of pressure drawdown, the gases in the coals have a dynamic diffusion mode. Of which, the transitional diffusion in the coals evolves from the micro-scale micropores towards the micro-scale transitional pores, mesopores and macropores. Meanwhile, the Fick diffusion in these corresponding pores cannot occurs, which will result in a decreased gas migration ability in the coals. For the CO2 geo-sequestration, the preponderant micro-scale micropores in the high rank coal make more transitional diffusion but less Fick diffusion occurring in the coal, which will result in the injected CO2 difficult to reach the nano-scale micropores for gas adsorption and storage.

Pore Evolution and Formation Mechanism of Organic-Rich Shales in the Whole Process of Hydrocarbon Generation: Study of Artificial and Natural Shale Samples

Shale pore evolution has a dramatic influence on gas occurrence and is of great significance for the evaluation of gas shale reservoirs. To better understand the nanopore evolution during the whole...

A comprehensive re-understanding of the OM-hosted nanopores in the marine Wufeng–Longmaxi shale formation in South China by organic petrology, gas adsorption, and X-ray diffraction studies

The overmature marine Ordovician–Silurian Wufeng–Longmaxi (WF–LMX) formation shale is the most important exploration target for shale gas in South China. In this study, WF–LMX shale samples from Well XY-1 in the Northwestern Guizhou Province (NGP) were selected to comprehensively investigate organic matter (OM) assemblages and OM-hosted nanopores. Petrographic observations under an optical microscope and a scanning electron microscope (SEM) revealed that the OM in the WF–LMX shale is predominately solid bitumen (pyrobitumen), followed by organic zooclasts (predominantly as graptolites), with a minor amount of micrinites (as the heavier conversion products of the oil-prone kerogen macerals). Additionally, the degraded alginites were traced by their bright halo feature in reflected light and blurred fluorescence in blue light when enhancing the exposure intensity, which demonstrates that the previously deposited alginites in the WF–LMX shale were basically converted to hydrocarbons. SEM imaging revealed that the secondary OM pores associated with hydrocarbon generation are pervasive in pyrobitumen, while undeveloped in organic zooclasts and micrinites. Conversely, the good linear relationship between TOC contents and methane absorbed amounts indicates that there must be numerous SEM-invisible pores existed in both the solid bitumen and organic zooclasts in addition to the secondary pores.Aiming to certify and quantify the SEM-invisible pores, the OM were separated from the WF-LMX shale and then were subjected to a series physical and chemical measurements. The combined N2 and CO2 adsorption on the OM isolates of the WF–LMX shale further revealed two pore concentration sections, i.e., pores of 0.33–0.5 nm and 1.2–2.0 nm within the micropore regime, which contributed approximately 82% and 18% to the total surface area and pore volume of the OM, respectively. X-ray diffraction testing further correlated the micropores to the spacing of aromatic rings and macromolecular structural units that constitute the OM. As such, it is the micropores related to the chemical structure of the OM that essentially determine the sorptive capacity of OM and also the WF–LMX bulk shale, whereas the main contribution of SEM-visible OM pores is affording the effective porosity for gas storage. Overall, the clarification of OM assemblages, carriers of secondary OM pores, and existence and origin of OM micropores in the WF–LMX shale provide a further understanding of the generation and occurrence of shale gas in South China.

A study on pore systems of Silurian highly mature marine shale in Southern Sichuan Basin, China

The porosity system in shale, which is a combination of organic-hosted pore and inorganic pore, plays a pivotal role in adsorbing methane for gas shale reservoir. In order to identify the porosity system with different properties and to give a quantitative characterization of pore structure for each individual porosity system, the dual liquid NMR method was conducted on six shale samples from the Lower Silurian Longmaxi Formation in the Southern Sichuan Basin. In shale reservoir, hydrophilic pore and oleophilic pore are respectively related to hydrophilic minerals and oleophilic organic matter. The NMR method detected the signal of water penetrating the water-wetting porosity system when saturated with water, which can reflect the pore size distribution (PSD) of hydrophilic porosity. The PSD of oleophilic pore can also be reflected when saturated with kerosene, vice versa. Generally, the NMR T2 spectra of sample saturated with DI water and kerosene, T2_water and T2_kerosene exhibit bimodal pattern, a dominant peak with a relaxation time range of 0.1–10 ms and a minor peak with a relaxation time ranging 10–100 ms. Generally, higher TOC content corresponds to large amounts of cumulative organic nanoscale pore. The organic-rich shale shows high content of organic-hosted pores and large amounts of inorganic pores which are validated by plane porosity statistics from the observation of SEM photographs. By correlating the two pore structure characterizing methods, the relaxivity of shale can be calculated to be 0.055–0.092 μm/ms when saturated with water. The quantitative estimation of organic-hosted porosity system and inorganic porosity system in the shale will provide a pivotal basis for elucidating gas occurrence states in shale.

Storage, Handling and Safety Procedure for Fuel in Oil & Gas Industry

After Finding the oil & gas occurrences in the subsurface using various methods and tools that were available in Upstream Oil and Gas Industry. Further reaching the reservoir and taking out the Oil & Gas from those to the surface the fuel have to transport and store to get purify for supply to the required user. Here the Midstream peoples plays a vital role in it. The drilled Oil and Gas have to transport from the occurrence to the destination Refinery so it have to be planned well and many safety procedure have to be done to avoid any problem in those transportation and after transporting to the destination it must be maintained in perfect temperature condition and perfect storage tanks either above the ground or the underground .Again there are some safety procedures to be followed which were approved by the Government safety norms. This article deal about the process and procedures in transporting and storage of fuels from upstream to midstream to downstream .Also about the safety precaution and procedure to be followed to have a safe storage and handling.

Hydrogen Sulfide in Wastewater Generated by Hydrogen Induced Cracking Test: Cause and Safety Measures

During the process of treating various wastewater, toxic gas such as hydrogen sulfide is generated due to the chemical reaction among unknown chemicals contained in wastewater, causing an accident that kills or injures workers and requires evacuation of people in neighboring areas. This study tested the reaction of sodium hydrogen sulfide generated during hydrogen induced cracking with sulfuric acid to confirm the occurrence of hydrogen sulphide and proposed safety countermeasures to prevent similar disasters from recurring. Hydrogen sulphide gas used during the process of hydrogen organic crack test reacts with sodium hydroxide in a neutralization tank and a gas scrubber to produce sodium hydrogen sulphide. Highly toxic hydrogen sulphide gases are produced violently when wastewater containing sodium hydrogen sulfide is mixed with sulfuric acid wastewater. In order to prevent the occurrence of harmful gases due to adverse reactions during wastewater treatment, the process of generating consigned wastewater should be well identified in advance, along with the detailed information on the composition and properties of the wastewater. In addition, more attention should be paid to adverse reactions of mixed wastewater.

Determination of Gas Content in Low Permeability and High Gas Coal Seam

The determination of coal seam gas content is of great significance to the study of coal seam gas occurrence law and gas prevention technology. There are direct and indirect methods for the determination of gas content in coal seam. Both direct and indirect methods have their advantages and disadvantages, but the results of direct measurement have more disadvantage than indirect measurement. In this paper, the gas content in No. 1 coal seam of Weijiadi Coal Mine is determined by indirect measurement method. The gas content in No. 1 coal seam is calculated to be 8.1m3/t, which is important for gas geological division and precise gas extraction.

Regional chronostratigraphic framework of sediments build-up in the Benin Basin: a guide for hydrocarbon prospecting

An attempt to understand the structural and sedimentation framework in the Benin Basin is made herein. Unlike in its contiguous basins in the Gulf of Guinea Province, no significant oil and gas discovery has been reported in the basin. The aim here is to provide a chronostratigraphic framework of sediments build-up as a guide to hydrocarbon prospectivity in the basin. Existing biostratigraphic and sedimentologic data as well as few proprietary reports were integrated to establish a chronostratigraphic correlation in six directions within the Nigerian portion of the Benin Basin. Isopach maps of the chronostratigraphic subdivision of the sequences in the basin were also made. Sedimentation in restricted grabens started in the western part of the basin. Only Ise-2 well penetrated the oldest Cretaceous (Neocomian to Albian) sediments in the area of study with a thickness reaching up to 869 m. During the Miocene the focus of sedimentation predominantly shifted towards the offshore in the basin. Significantly a missing Eocene/Oligocene section was observed both on the seismic section and the stratigraphic record of the basin. Only Epiya-1 shows gas occurrence in the Miocene. Based on this work and locations of hydrocarbons recovery in the other part of the basin, exploration effort should target the Cretaceous sediments in the Nigerian portion of the basin.

Assessment of Radon Concentration and Impact on Human Health in a Region Dominated by Abandoned Gold Mine Tailings Dams: A Case from the West Rand Region, South Africa

Radon gas has long been identified as a human carcinogen. The purpose of this study was to understand the occurrence of radon gas in the West Rand region, located west of the city of Johannesburg, South Africa, and to investigate its potential health effects. In this study, the sampling of rocks, tailings, and construction materials was carried out for geochemical analyses. For characterisation of radon, 60 radon monitors (RGMs) were installed in indoor and outdoor environments. The results showed that mine tailings contain high uranium levels, with a maximum of 149.76 ppm and a mean value of 48.87 ppm. The radon levels in the area ranged between 32 Bq/m3 and 1069 Bq/m3 and thus, exceed the typical outdoor radon levels of about 10 Bq/m3. Significantly high values were obtained from gold tailings dams. In indoor environments, radon concentration ranged up to a maximum of 174 Bq/m3, which is above the recommended value of 100 Bq/m3. The effective doses calculated from the obtained radon levels showed a maximum of 16 mSv/y, which is above the recommended value of 1 mSv/y proposed for public exposure. The estimated doses have an enormous potential to pose high lung cancer risks to the populations residing nearby tailings.

Influence of lamprophyre sills on coal metamorphism, coalbed gas composition and coalbed gas occurrence in the Tongxin Minefield, Datong Coalfield, China

Magmatic intrusions in coal-bearing strata can significantly change the physical and chemical properties of the intruded coal. Lamprophyre intrusions are extensive in coal seams 3–5 and 8 of the Carboniferous-Permian coal-bearing strata in the southeastern and southwestern regions of the Tongxin Minefield, Datong Coalfield, China. They appear primarily as sills of varying magnitudes, with thicknesses ranging from 0.15 to 23.51 m. In this area, coals can be classified as unaffected (normal geothermal gradient) and intrusion-affected. Unaffected coals are of high-volatile bituminous rank, with values of maximum vitrinite reflectance ranging from 0.7 to 0.9%, whereas intrusion-affected coals have values of maximum vitrinite reflectance ranging from 0.9 to 7.2%. A marked decrease in the degree of coalification can be observed with increasing distance from the lamprophyre intrusions. Near the intrusion-coal contacts, high-volatile bituminous coals have been thermally altered to form natural coke. The carbonate contents in intrusion-affected coals (0.2–46.1%) are significantly higher than those of the unaffected coal (0–2.0%). The coalbed gases of coal seams 3–5 and 8 are highly variable in both their molecular compositions and overall gas contents. Their CO2 volumes vary from 0.01 to 6.55 m3/t (daf); higher CO2 values are found in the coals affected by intrusions, and roughly decrease with increasing distance from the lamprophyre igneous intrusions toward the central region of the minefield. These variations in gas compositions are mainly related to magmatic activity. The coalbed gas contents in coal seams 3–5 and 8 range from 1.48 to 16.43 m3/t (on a dry, ash-free basis). The positive correlation observed between gas content and the ratio of CO2/CH4 + CO2 occurs only in intrusion-affected coals. Lamprophyre intrusions have negative effects on CH4 content in the study area. Since CO2 has a higher adsorption capacity than CH4, C2H6, and N2, the injection of gaseous CO2 derived from magmatic intrusions can effectively displace CH4-rich reservoir gas. In addition, there is a positive correlation between ash content and gas content on a dry, ash-free basis. Ash content is positively correlated with the ratio of CO2/CH4 + CO2 occurring only in intrusion-affected coals. Massive secondary carbonates that filled in the cleats act as small-scale impermeable seals, which provide a sealing effect both on gas contents and gas compositions during uplift.