Gas-bearing Strata Research Articles

THE NATURE OF DARK COLORATION OF DEVONIAN CARBONATE ROCKS (POTENTIALLY PETROLEUM-BEARING WITHIN THE YENISEI-KHATANGA REGIONAL TROUGH) IN THE AMBARDAKH AREA (EASTERN SIBERIA)

Lithological studies of sedimentary rocks are often challenged by manifestations of the superimposed processes which are related neither to the stages of lithogenesis, nor to its zonality/direction. This can be exemplified by skarns in the carbonate-terrigenous sections of the unified Yukta and Nakohoz-Kalargon formations of Devonian age which were studied in the area of Lake Baselak and the Ambardakh River basin during the expedition works. Since the latter contain potentially oil and gas-bearing strata in the area of the Yenisei-Khatanga regional trough, special attention was paid to the dark-colored lithotypes during the research. It was revealed that among the studied samples, the dark color of some of them was of mineral nature, rather than due to the presence of OM. In these samples, the mineral composition was studied in detail, with determination of the association of newly formed minerals, which allowed us to propose hypothesized mechanism and stages of their formation. The dark color of sedimentary rocks has been shown to be not a sufficient criterion for searches for oil and gas accumulation. New results of the field studies of Pre-Triassic sedimentary complexes in the northern part of Kureyka syneclise are also presented.

Numerical and experimental analyses of methane leakage in shield tunnel

Tunnels constructed in gas-bearing strata are affected by the potential leakage of harmful gases, such as methane gas. Based on the basic principles of computational fluid dynamics, a numerical analysis was performed to simulate the ventilation and diffusion of harmful gases in a shield tunnel, and the effect of ventilation airflow speed on the diffusion of harmful gases was evaluated. As the airflow speed increased from 1.8 to 5.4 m/s, the methane emission was diluted, and the methane accumulation was only observed in the area near the methane leakage channels. The influence of increased ventilation airflow velocity was dominant for the ventilation modes with two and four fans. In addition, laboratory tests on methane leakage through segment joints were performed. The results show that the leakage process can be divided into “rapid leakage” and “slight leakage”, depending on the leakage pressure and the state of joint deformation. Based on the numerical and experimental analysis results, a relationship between the safety level and the joint deformation is established, which can be used as guidelines for maintaining utility tunnels.

Gas sealing behavior of gasketed segmental joints in shield tunnels: An experimental and computational study

When shield tunnels pass through gas-bearing strata, gas leakage through could damage tunnel structure and interior equipment during both construction and operation process. Previous studies on sealing behavior of the gasketed joints have focused on the waterproof performance. However, differences in physical characteristics between liquid and gas lead to different permeation properties. This paper presents a combined experimental and computational study to investigate the gas sealing behavior of the gasketed joints in a shield power tunnel project, named as Sutong GIL Utility Tunnel, which passes through soft soil strata rich in high-pressure biogas under Yangtze River. The tests are aimed to measure the air leakage rates through the gasketed joints with various combinations of joint openings and offsets under different pressures. Slight leakage and rapid leakage are defined in the tests, where the leakage rates differ drastically in magnitude. This paper takes critical pressure, which corresponds to the leakage rate of 100 ml∙min−1 per meter joint length, as the index of the capability to resist sealing failure under extreme conditions, takes leakage rate as the index of gas sealing performance during long-term ‘slight leakage’, and evaluates the effects of the joint deformation on the gas sealing behavior from the two aspects. Finite element models of the inner gasket-groove structure were created and set with different joint openings and offsets, loaded with different pressures. Gasket deformation and contact stress under different conditions were studied. Comparison between the numerical results and test results was made to investigate the gas sealing behavior. Based on the existing gas sealing model of rough surfaces, a computational method of leakage rate was developed and validated, which can be used at the stage of long-term ‘slight leakage’. Based on the study, gas sealing behavior of the gasketed joints was analyzed and suggestions of improvement were given.

Study on the long-term performance of shield tunnel passing through the gas-bearing strata

When the shield tunnel passes through the gas-bearing strata, gas and water leakage may occur depending on the sealing performance of the segment joints. This process involves the complex multiphase seepage flow phenomenon in unsaturated soil. In this study, a fully coupled solid-liquid-gas model of the GIL Utility Tunnel was established to investigate the influence of the high-pressure gas on the mechanical properties of the tunnel segments and joints. The constitutive model of the Extended Barcelona Basic Model was implemented to simulate the effect of the seepage process on soil deformation. The results show that significant upward displacement occurred in the gas reservoir and its overlying strata, and the maximum displacement reached 30 mm. In addition, during the leakage of the gas and the water, an increase in the average soil effective stress was observed. It would induce a reduction in the suction and expansion of the yield surface. The tunnel tended to be stable from 20 years onwards, thus the soil deformation due to the water leakage only occurred at the early stage. In addition, the joint opening under the most unfavorable internal force combination was 0.69 mm, and the corresponding bolt stress was 119.5 MPa, which is below the yield limit. The results of this study help to understand the influence of high-pressure gas on tunnel safety and the sealing performance of the joints.

Predicting gas-bearing distribution using DNN based on multi-component seismic data: Quality evaluation using structural and fracture factors

The tight-fractured gas reservoir of the Upper Triassic Xujiahe Formation in the Western Sichuan Depression has low porosity and permeability. This study presents a DNN-based method for identifying gas-bearing strata in tight sandstone. First, multi-component composite seismic attributes are obtained. The strong nonlinear relationships between multi-component composite attributes and gas-bearing reservoirs can be constrained through a DNN. Therefore, we identify and predict the gas-bearing strata using a DNN. Then, sample data are fed into the DNN for training and testing. After optimized network parameters are determined by the performance curves and empirical formulas, the best deep learning gas-bearing prediction model is determined. The composite seismic attributes can then be fed into the model to extrapolate the hydrocarbon-bearing characteristics from known drilling areas to the entire region for predicting the gas reservoir distribution. Finally, we assess the proposed method in terms of the structure and fracture characteristics and predict favorable exploration areas for identifying gas reservoirs.

Gas-bearing reservoir distribution in a volcanic environment: Case of the Yingcheng Formation, China

As a special type of lithologic body, gas-bearing strata, composed of rhyolite, an igneous rock with a volcanic composition, are characterized by deep burial, complex lithology, poor stratification, and a complicated gas-bearing distribution. These characteristics lead to difficulties in the prediction of gas-bearing reservoir parameters and thus restrict the exploration and exploitation of such reservoirs. The unique and complex volcanic model contains different types of facies and subfacies because the volcanic eruption usually experiences different explosion-effusion periods. Our study focuses on the vertical and lateral gas-bearing reservoir distribution in a volcanic environment in the Cretaceous Yingcheng Formation of the Songnan Gas Field, using seismic facies-controlled nonlinear stochastic inversion to integrate the volcanic facies information. First, we built the volcanic facies model for the Songliao Basin to introduce the comprehensive characteristics of the volcanic facies during a volcanic eruption and analyzed the well-tie logging facies and seismic facies to guide the facies analysis of the target layer. Then, based on this facies analysis, we depicted the gas-bearing reservoir in detail through the use of seismic facies-controlled nonlinear random inversion. Finally, we predicted the distribution of the gas-bearing reservoir by the inversion results. The predicted results were found to agree well with the actual drilling data. The inversion method effectively predicts the velocity distribution of the gas-bearing reservoir, which provides a good foundation for the subsequent description of gas-bearing reservoirs in the Yingcheng Formation.

The theory of sedimentation cyclicity of N.B. Vassoevich in the educational and scientific activities of the Petroleum Geology Department of Lomonosov Moscow State University in the XXI century

The article is devoted to the 120th anniversary of Nikolay Bronislavovich Vassoevich (1902–1981) – a famous petroleum geologist, professor, corresponding member of the USSR Academy of Sciences. The article recalls his significant achievements in the field of fundamental geology. The first and main work in this field was his monograph on the methodology for studying the sedimentation cyclicity of flysch strata (1948), with the continuation of the topic in the next book, published in 1951. The scientific heritage of N.B. Vassoevich, who headed the Petroleum Geology Department of Lomonosov Moscow State University in 1963–1981, is preserved and developed in the modern educational and scientific activities of this Department. Teachers, staff and students with deep gratitude and respect honor the memory of N.B. Vassoevich, using his methodological recommendations on sedimentation cyclicity both in the training course of lithofacial analysis and applied in practical work in the study of oil and gas bearing strata of sedimentary rock basins. A huge role in the development of the terminology of this theory is associated with the development of the conceptual base of this geological direction. Professor N.B. Vassoevich had invested continuous work on the creation of systematic and logical classifications according to the composition, structure, genesis and hierarchy of geological sedimentary bodies that make up oil and gas bearing rock complexes.

Theory of gas traps in stope and its application in ground extraction of abandoned mine gas: Part 1 – Gas trap in stope and resources estimation

Scientifically understanding the rules of migration and accumulation of gob gas is the basis for gas development of sealed gobs or abandoned mines. Based on the rules of the movement and failure of surrounding rocks and the migration of gas in the stope, the theory of gas trap in stope is put forward. The theory holds that the strata of continuous deformation zone, coal pillars and their roof and floor strata around the stope, the underlying strata of the floor failure zone, and gas emission zone (GEZ) can be acted as the caprock, the lateral barrier, the bottom barrier and the reservoir respectively, thus forming the gas trap in stope together. The gas trap is a place where abandoned gob methane (AGM) can be prevented from migrating and accumulated. Once there are enough AGM resources in the trap, the gas reservoir can be formed. Mining methods, the layout of the working face, mining space, burial depth and mechanical properties of the surrounding strata of the stope directly affect the formation of the trap. Based on the requirements of waterproof and sealing ability, the thickness and lithology requirements of the strata in the continuous deformation zones as the trap caprock are proposed, and specific calculation and determination methods are given. The influence of the stability of coal pillar, the gas conductivity of overlying and underlying strata of coal pillars, and the overlapping of GEZ in adjacent gob on the formation of traps is studied, and the conditions of using coal pillar and its roof and floor strata around the stope as lateral barriers are proposed. The relationship between mining height, lithology of overlying strata of coal seam，and development height of GEZ is analyzed. The influence of interval between adjacent coal seams on the formation of traps in upper and lower gobs is studied, and the criteria for the formation of traps separately in upper and lower gobs are proposed. After analyzing the function of the reservoir, it is considered that the GEZ should have enough AGM resources and void volume. And both are indispensable. The traps with void volume but not enough AGM resources are of no development value, and the traps with only AGM resources but not enough void volume cannot be exploited with surface borehole. To fully reflect the storage conditions condition and void volume of the reservoir, the quantity and quality of AGM resources in the trap and the water accumulation in the gob, the measurement parameters of traps are proposed, including AGM resource, methane concentration, free gas pressure, void volume and effective void volume in GEZ, effective void volume ratio, residual rate of excavation space, and residual rate of the mining area. The trap can be formed by one gob alone or by multiple gobs. According to the number and spatial location of the gobs, gas traps can be divided into single-stope type, multi stope horizontal type, multi stope vertical type and multi stope mixed type. The source of AGM resources in traps is limited to the resources within the scope of GEZ, mainly including coal pillars, residual coal, adjacent unminable coal seams and gas-bearing strata, as well as free gas in the void of the GEZ. At the same time, according to the characteristics of different kinds of AGM sources, the calculation range and calculation method of AGM resources are proposed.

Key structural periods and Permian tight gas accumulation response in Hangjinqi area, Ordos Basin

Critical tectonic period is a critical time point that characterizes the genetic connection and causal relation (accumulation response) between tectonic effect and oil/gas accumulation in large superposition basins, it is also an important window for analyzing accumulation evolution and accumulation regularity of oil/gas. Based on the systematic review of the tectonic evolution history of the Ordos Basin, the tectonic evolution and deformation sequence since the Mesozoic in Hangjinqi area in the northern part of Ordos Basin were studied through the constraints of apatite fission tracks and other thermal chronological indicators. Two key structural periods including J3-K1 and K2-E1 were proposed. It was believed by combining with the understanding of the Carboniferous-Permian hydrocarbon-generating and reservoir-forming histories that the J3-K1 was the period of design finalization in the Hangjinqi area which formed a currant NE-trending structural framework high in the northeast and low in the southwest during which, lithologic gas reservoirs including the Carboniferous Taiyuan and the Permian Shanxi formations, first member of the Permian Shihezi Formation to the north of the Sanyanjing-Wulan Jilin Temple-Bo'erjiang Haizi fault were formed and corresponding to the period of great amount of hydrocarbon generated and expulsed from source rocks as well as the densification of sandstones. On the other hand, the K2-E1 period was the major stage for fault reactivation and the formation of NE-trending tectonic trap and NW-trending fault that led to the adjustment of early lithologic gas reser-voirs in the south and the formation of structural or structural-lithologic gas reservoirs in the north. The difference of accumulation on each sides of the main fault is obvious. The southern part was formed earlier than the northern part, and was dominated by lithologic gas reservoirs yet the northern part was dominated by structural-lithological gas reservoirs. The southern gas-bearing strata are dominated by deep strata, while those in the north are both shallow and deep ones. In Paleogene, lifting in the north caused loss of gas. The key accumulation and transformation periods transformed with space and time, which transformed accumulation factors differentially and finally constrained the enrichment and depletion of natural gas.

Overview of Lower Cretaceous Achimov Formation: Physical Properties and Their Distribution Pattern in West Siberian Basin, Russia

The Achimov Formation is one of the most important oil- and gas-bearing strata in the West Siberian basin in Russia. The total estimated reserves of this stratum exceed one billion tons. The formation was first explored in 1981, but it remains largely underdeveloped due to its deep burial depth and poor physical properties. Therefore, further research on the genetic mechanisms and distribution characteristics of the reservoirs in the formation can contribute to its further exploitation. The Achimov Formation is dominated by of fine- to medium-grained sandstones interbedded with shale. Based on analysis of well logging data, hand specimens, and previous research, this study analyzed the properties of three members (Ach1, Ach2, and Ach3) of the Achimov Formation and summarized their distribution patterns. Research on reservoir rocks from different oil and gas fields reveals varying physical properties across the formation with permeability and porosity increasing from the northern to central areas and decreasing from the central to the southern areas. Burial depth is one of the major controlling factors for reservoir properties in the formation. Reservoirs in both the northern and southern parts of the formation are buried deeper than those in the central areas, resulting in a disparity in reservoir quality.

Динаміка літогенезу осадових товщ фанерозою Карпато-Чорноморського регіону в аспекті їхньої нафтогазоносності

The objective of this work was to study the environments and processes of ancient sedimentation in the epi- and mesopelagic basins of the Carpathian-Black Sea region and to clarify the conditions of oil and gas basins formation within the study region as well as the main aspects of hydrocarbon generation. The burial history of the basins, some aspects of their fluid regime, issues of lithogenetic record, features of transformation of sedimentary basins into the rock-formation basins and the development of the latter during the Phanerozoic are considered. The spatial and temporal peculiarities of the evolution of epi-mesopelogic systems and their influence on the formation of oil- and gas-bearing strata within the Carpathian-Black Sea region have been studied. It has been established that in the sedimentary basins of the Carpathian-Black Sea continental margin of the Tethys Ocean during the long geological history the different intensity structural and morphological changes took place: changes of the subsidence rate of the basin bottom, inversion uplifts, sedimentation pauses, deformation of the sedimentary fill. This was reflected both in the peculiarities of the development of sedimentary environments and in the processes of substance differentiation with the formation of certain post-sedimentary mineral-structural parageneses. It was proved that discrete processes of differentiated compaction and defluidization of sediments cause a number of deformation phenomena, which can be reflected in the features of the morphology of the sedimentary basin bottom, influencing the nature of sediment transportation and accumulation. On the basis of the conducted investigations a number of practical results were obtained which will allow forming new approaches to criteria of hydrocarbons prospecting, in particular the lithophysical aspect which is concentrated on the reservoir properties of rocks; sedimentary reconstructions and the diversity of cyclicity of the studied sediments as a factor of the establishment of prospective areas, reconstruction of the burial history, which provides an information on the state of transformation of organic matter and hydrocarbons, and therefore the range of prospective depths for oil and gas occurrence.

The Anyue Giant Gas Field in the Sichuan Basin as the Largest Gas Field in Marine Carbonate Deposits from Domestic China

Abstract —The Anyue gas field is located in the middle part of the Sichuan Basin, SW China. It occurs in the oldest marine carbonate strata and is characterized by the highest degree of thermal evolution and the largest gas reserves in China. A significant breakthrough was made with the Gaoshi-1 risk-taking exploration well deployed in 2011. As of 2015, the proven geologic reserves of natural gas were 657.4 billion m3, and the total gas reserves including proven reserves, controlled reserves, and predicted reserves exceeded 1.5 trillion m3. A total of three sets of gas-bearing strata are developed in the Anyue gas field in a descending sequence: The gas reservoir in the Cambrian Longwangmiao Formation (Є1l) is treated as a structure-lithologic gas reservoir; the gas reservoir in Section No. 4 of the Dengying Formation (Z2dn4) is a structure-stratigraphic gas reservoir; and the gas reservoir in Section No. 2 of the Dengying Formation (Z2dn2) is a structural gas reservoir. A comparative analysis of the gas sources has shown that the gas Cambrian of reservoirs was mainly from mudshale of the lower Cambrian Maidiping and Qiongzhusi formations and the gas of the Dengying Formation reservoirs was from mudshale of the Qiongzhusi Formation and mudstone of Section No. 3 of the Dengying Formation. All gas reservoirs are referred to as reservoirs of dry gas with medium or low contents of sulfur and medium contents of CO2. Gas reservoirs in Є1l are characterized by a large burial depth, a high temperature, and a high pressure, while the gas reservoirs in Z2dn2 and Z2dn4 are characterized by an ultralarge depth, a high temperature, and a normal pressure. The accumulation of gas reservoirs is controlled mainly by two factors. The distribution of hydrocarbon generation centers is controlled by the late Sinian–early Cambrian intracratonic rift, which acts as effective updip sealing conditions for gas reservoirs in the Dengying Formation. The late Sinian–early Cambrian Gaoshiti–Moxi paleouplift experienced a long-term inherited development, which controlled the generation and distribution of three sets of large-scale high-quality reservoirs in Z2dn4, Z2dn2, and the Longwangmiao Formation and the generation of three sets of high-quality reservoir–caprock assemblages. An inherited giant structural trap with a longterm stable development is always a favorable zone for petroleum accumulation.

Geochemical conditions of hydrocarbon accumulation in low-permeability shale sequences

The report is devoted to the study of conditions for the formation of organic porosity–void spaces of organic origin formed during transformation of the organic matter into hydrocarbons, and their role in the formation of hydrocarbon accumulation in low-permeability shale strata. The experience of studying and developing known shale formations of the world testifies to the fact that such strata are hybrid phenomena, that is, they are both oiland gas-bearing strata containing traditional and non-traditional accumulations of hydrocarbons. Based on the results of the programmed pyrolysis by the Rock-Eval-6 method, an estimate has been performed to quantify organic (kerogen) porosity. This estimate allows for determining the forecast retention volume of HCs generated during catagenesis.

Using frequency-dependent AVO inversion to predict the “sweet spots” of shale gas reservoirs

Abstract Shale gas is an unconventional hydrocarbon resource with huge potential. Using geophysical methods to predict “sweet spots” is the key to commercial exploration of shale gas. At present AVO technology is one of most widely used geophysical techniques to find shale gas reservoirs with high abundance. However, the effects of the dispersion of actual mediums are neglected by conventional AVO inversion. Recent experiments have shown that in gas-bearing strata, the dispersion may be rather remarkable. In this paper, we develop a frequency-dependent AVO inversion scheme, which includes seismic dispersion in real rock medium, to predict the “sweet spots” of shale gas reservoirs. Application of the inversion has been made to the shale gas in the south of Jiaoshiba area to test the validity. Comparing the results of the frequency-dependent AVO inversion and well production, we find that regions with high dispersion gradients correlate well with places with high production of shale gas. Therefore we conclude that the frequency-dependent AVO may be a workable approach to detect the “sweet spots” of shale gas reservoirs.

Influence of faults and slumping on hydrocarbon migration inferred from 3D seismic attributes: Sanriku-Oki forearc basin, northeast Japan

Abstract Seismic imaging of the Sanriku-Oki forearc basin in the Japan Trench plate convergent margin provides insights into the geological controls on hydrocarbon systems in sediment deposited on the continental slope. We perform a series of seismic attribute analyses on 3D seismic reflection data to better define the influence of geological structures (e.g., faults, slumps and gas chimneys) on hydrocarbon sources, migration pathways and reservoirs. In order to identify coal-bearing strata from the 3D seismic volume, we calculate acoustic impedance distribution by integrating seismic and logging data via waveform inversion. Two Late Oligocene to Early Miocene major coal-bearing strata are characterized as low acoustic impedance strata. Gas-bearing strata characterized by anomalously high reflection strength are widely distributed beneath a bottom simulating reflector (BSR). A greater accumulation of gas occurs in the northeast of the study area, where successive slump deposits and gas chimneys are observed. We propose that a BSR with an underlying gas accumulation in the Sanriku-Oki forearc basin developed as a consequence of the expulsion of gas from Cretaceous to Oligocene coal beds and their subsequent upward migration through gas chimneys and faults. Furthermore, the migration and accumulation of gas and gas hydrate were strongly controlled by a series of porous slump deposits. As chimneys and gas pockets occur above the edges of the slumps, gas migration processes controlled by the slumps could be important in the forearc basin. Our study based on seismic attribute analyses demonstrates that features produced by tectonic movements at plate subduction margins (e.g., faults) and associated instability processes (e.g., slumps) represent important controls on the migration and accumulation of hydrocarbons in forearc basins.

Petrophysical characterisation of tight sandstone gas reservoirs using nuclear magnetic resonance: a case study of the upper Paleozoic strata in the Kangning area, eastern margin of the Ordos Basin, China

ABSTRACTResidual and movable porosity are significant parameters for characterising petrophysical properties, especially in tight reservoirs. Eight tight sandstone samples from the upper Paleozoic gas-bearing strata in the Kangning area, from the eastern margin of the Ordos Basin, were analysed using nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), petrography, and porosity and permeability tests. The lithology and pore types were identified and classified using petrography and SEM. The residual and movable porosity were obtained with NMR. In addition, NMR was used to visualise pore structure and pore size distribution. The results suggest that the upper Paleozoic sandstones in the study area mainly comprise feldspathic litharenite and litharenite. The sandstone porosity and permeability are low, with means of 5.9% and 0.549 mD, respectively. Four pore types exist in the tight sandstones: residual primary pores, grain dissolution pores, micropores (clay-dominated) and microfractures. The T2 spectra under water-saturated conditions correlate with pore size and can be used to distinguish small and large pores based on the transverse relaxation time cutoff value of 10 ms, which corresponds to a pore diameter of 0.232 μm. Small pores account for 72% of the pores in the tight sandstones. The continuous bimodal T2 spectra suggest good connectivity between small and large pores, despite the low porosity and permeability. In this study, the movable porosity of the major tight sandstone gas reservoirs is higher than the residual porosity, which confirms the effective evaluation of movable porosity to tight sandstone reservoirs, based on NMR experiments.

Gas flow analysis for the impact of gob gas ventholes on coalbed methane drainage from a longwall gob

A large amount of methane releasing from the residual coal and the surrounding pressure-relieved gas-bearing strata may accumulate in the longwall gob of gas-bearing coal seam, which is increasingly exploited as a major source of coalbed methane (CBM) development in coal mining areas. Meanwhile, if the gob methane emits into the working space along with air-leakage flow, they may impair the safety and productivity of underground coal mines. Thus, gob gas drainage is of great significance to both the CBM development and the control of gob gas emission. Currently, there exist two major technologies for gob gas drainage–gob gas ventholes (GGVs) drainage and underground drainage (such as high-level suction tunnels, high-located boreholes and buried pipes in the upper-corner). In practical engineering applications, the GGVs have greater advantage over the traditional underground drainage in the extraction flux of gob gas and the control of gob gas emission, but the theoretical mechanism of GGVs has not yet been revealed in previous studies. Consequently, the characteristics of gob gas flow with GGVs drainage needs further research.In this paper, the numerical simulation of gas flow in gob gas reservoirs and the in-situ assessment of GGVs drainage are carried out to investigate the mechanism of the control of gob gas flow with GGVs drainage. Firstly, based on fluid mechanics in porous media the mathematical model of gob gas transport is developed. Then, the gas pressure distribution, gas flow patterns and methane concentration distribution in the gob under the conditions of no drainage measures, drainage with buried pipe in the upper-corner and GGVs in different locations are discussed by conducting numerical simulation, respectively. In addition, the influence of distance between GGV and return airway on the gob methane flow and the strong control scope of GGVs are analyzed in detail. It is found that: 1) GGVs has the “flow-interception effect” on the gas in the deep zone of the gob and the “flow-reversal effect” on the gas in the gob close to the longwall face, which accounts for that GGVs drainage advantage over underground drainage. 2) GGVs can be applied to drain the gas in the deep zone of the gob massively and to control the gob gas migration effectively, so they are a top choice for the extraction of gob gas and the control of gob gas emission. Finally, both GGVs drainage and underground drainage are adopted to extract gob gas in east-102 working face in Weijiadi coal mine. The comparison of the extracted and emitted volume of gob gas with two different technologies shows that, GGV can drain the gob gas with greater flux and can better control the gob gas emission, which validated the prediction of numerical stimulation.

Analysis of gob gas venthole production performances for strata gas control in longwall mining

Longwall mining of coal seams affects a large area of overburden by deforming it and creating stress-relief fractures, as well as bedding plane separations, as the mining face progresses. Stress-relief fractures and bedding plane separations are recognized as major pathways for gas migration from gas-bearing strata into sealed and active areas of the mines. In order for strata gas not to enter and inundate the ventilation system of a mine, gob gas ventholes (GGVs) can be used as a methane control measure. The aim of this paper is to analyze production performances of GGVs drilled over a longwall panel. These boreholes were drilled to control methane emissions from the Pratt group of coals due to stress-relief fracturing and bedding plane separations into a longwall mine operating in the Mary Lee/Blue Creek coal seam of the Upper Pottsville Formation in the Black Warrior Basin, Alabama. During the course of the study, Pratt coal's reservoir properties were integrated with production data of the GGVs. These data were analyzed by using material balance techniques to estimate radius of influence of GGVs, gas-in-place and coal pressures, as well as their variations during mining.The results show that the GGVs drilled to extract gas from the stress-relief zone of the Pratt coal interval is highly effective in removing gas from the Upper Pottsville Formation. The radii of influence of the GGVs were in the order of 330–380m, exceeding the widths of the panels, due to bedding plane separations and stress relieved by fracturing. Material balance analyses indicated that the initial pressure of the Pratt coals, which was around 648KPa when longwall mining started, decreased to approximately 150KPa as the result of strata fracturing and production of released gas. Approximately 70% of the initial gas-in-place within the area of influence of the GGVs was captured during a period of one year.

Account of the Number of Stages and of Cyclicity in the Problem on Acidic Stimulation of an Oil-Bearing Stratum

The problem on the change in the porosity when hydrochloric acid is injected into a carbonate-containing stratum has been solved in a cylindrical coordinate system. Expressions for calculating space-time dependences of the porosity and the density of the acid in the solution during the stimulation of carbonate-containing oil- and gasbearing strata have been obtained. The derived expressions take account of the preceding changes in the porosity and the acid density in the previous stages of stimulation.

A Mechanism Contributing to Subsidence Above Gas Reservoirs and its Application to a Case Study

This paper aims to demonstrate that capillary effects and structural collapse can not be ruled out as significant factors in the development of subsidence occurring above gas fields. These phenomena provide sound explanations for continuing surface settlements when reservoir pore pressures stabilize and for additional settlements occurring even after the end of gas production. Conventional subsidence models fail to simulate this settlement behavior. Capillary effects also explain the lower rock compressibilities observed in gas-bearing strata as compared to the values obtained in the laboratory from fully saturated samples. Taking into account these aspects, the observed subsidence above a reservoir in the North Adriatic basin, Italy, is studied in detail.