Gas Flow Mechanisms Research Articles

GAS PERMEABILITY IN POROUS MEDIA WITH ROUGH SURFACES BY FRACTAL-MONTE CARLO SIMULATIONS

Gas permeability is an important parameter for gas transport in microporous and nanoporous media. A probability model of gas permeability of fractal porous media with rough surfaces is proposed and numerically simulated by the Monte Carlo technique. This model consists of two gas flow mechanisms: the Poiseuille flow and the Knudsen flow, and can be expressed by structural parameters, such as the pore fractal dimension, the tortuosity fractal dimension, the relative roughness and porosity. The validity of the proposed model is investigated through the available experimental data, and a good agreement is obtained. The predicted results indicate that gas permeability of microporous and nanoporous media with rough surfaces decreases with the increase of the relative roughness and the tortuosity fractal dimension, and increases with the increase of porosity and the pore fractal dimension. Our gas permeability model could reveal the physical mechanisms of gas transport in porous media with rough surfaces.

Simulation of the Production Performances of Horizontal Wells with a Fractured Shale Gas Reservoir

The production performances of a well with a shale gas reservoir displaying a complex fracture network are simulated. In particular, a micro-seismic cloud diagram is used to describe the fracture network, and accordingly, a production model is introduced based on a multi-scale flow mechanism. A finite volume method is then exploited for the integration of the model equations. The effects of apparent permeability, conductivity, Langmuir volume, and bottom hole pressure on gas well production are studied accordingly. The simulation results show that ignoring the micro-scale flow mechanism of the shale gas leads to underestimating the well gas production. It is shown that after ten years of production, the cumulative gas production difference between the two scenarios with and without considering the micro-scale flow mechanisms is 19.5%. The greater the fracture conductivity, the higher the initial gas production of the gas well and the cumulative gas production. The larger the Langmuir volume, the higher the gas production rate and the cumulative gas production. With the reduction of the bottom hole pressure, the cumulative gas production increases, but the growth rate gradually decreases.

Experimental study of the radial multi-scale dynamic diffusion model for gas-bearing coal

Purpose. The purpose of this paper is to solve the scientific problem that the classical diffusion model in columnar coal cores cannot accurately describe the whole process of gas diffusion. Methods. The diffusion-percolation experiments were carried out using the laboratory’s homemade experimental equipment with standard ϕ 50mm×100 mm columnar raw coal cores under different air pressures. Findings. The classical diffusion model was used to fit the experimental data. The experiment has found that the classical diffusion model of the columnar coal core can only partially describe the gas diffusion process. The longer the experimental time, the larger the error between the model and the experiment, and the analysis has found that the apparent diffusion coefficient shows decay changes with time. The dynamic diffusion coefficient concept is then proposed in order to con-struct a radial multi-scale dynamic prominent diffusion-percolation model for columnar coal cores. The theoretical curve of the new model nearly coincides with the experimental curve, and the new model can describe the gas diffusion-percolation process of columnar coal cores more accurately. The multi-scale dynamic diffusion-percolation model covers the classical diffusion model. It explains the mechanism of gas diffusion-percolation in multi-scale pores, i.e., at the beginning of the flow, gas flows out from the large external pores first, from the surface inwards. Over time, the pore size through which gas flows gradually becomes smaller, the diffusion resistance gradually increases, and the apparent diffusion coefficient slowly decreases. Originality. This paper proposes a new multi-scale dynamic diffusion-percolation model to compare the old and new model analysis, as well as carefully studying the mechanism of gas flow in coal. Practical implications. This research has important engineering significance for the accuracy of measuring the gas content of coal seams, as well as predicting coal and gas content.

Research on the gas migration trend and mechanism of the transition flow regime in coal based on MRT-LBM simulation

In order to reveal the process and mechanism of gas flow in a low-permeability coal seam, a new multiple-relaxation-time lattice Boltzmann method (MRT-LBM) model of gas migration in coal micro/nanopores based on Langmuir monolayer adsorption theory, the slip boundary scheme and Bosanquet effective viscosity was established. The software MATLAB was used to carry out the simulation study of uniform pore gas flow based on the MRT-LBM model, and the results were compared and verified with the porous anodic alumina membrane gas flow experimental results. On this basis, the gas flow in coal pores with different micro/nanopore sizes considering adsorption was simulated. The results show that the dimensionless permeability coefficient increases with decreasing pore size under the same pressure, which reflects the subsequent enhancement of the microboundary constraint effect and reveals that the pore system becomes the main controlling factor of coal seam permeability within the coal matrix in the middle and late stages of coal seam gas extraction, while the role of the microboundary constraint effect needs to be considered. The gas adsorption layer weakens the pore gas flow capacity, but for pores with a radius greater than 16 nm, the apparent change in permeability caused by the adsorption layer is less than 5%, and the adsorption effect can be ignored. N2, CH4, and CO2 enter the transition flow regime under different pressure conditions; with gas extraction, the gas pressure decreases, and the difference in the gas flow characteristics of the three gases increases.

Study on Apparent Permeability Model for Gas Transport in Shale Inorganic Nanopores

Inorganic nanopores occurring in the shale matrix have strong hydrophilicity and irreducible water (IW) film can be formed on the inner surface of the pores making gas flow mechanisms in the pores more complex. In this paper, the existence of irreducible water (IW) in inorganic pores is considered, and, based on the Knudsen number (Kn) correction in shale pores, a shale gas apparent permeability model of inorganic nano-pores is established. The effect of the Kn correction on the apparent permeability, the ratio of flow with pore radius and the effect of IW on the apparent permeability are assessed. The main conclusions are as follows: (1) at low pressure (less than 10 MPa) and for medium pore size (pore radius range of 10 nm–60 nm), the effect of the Kn correction should be considered; (2) considering the effect of the Kn correction, bulk phase transport replaces surface diffusion more slowly; considering the existence of IW, bulk phase transport replaces surface diffusion more slowly; (3) with increase in pressure, the IW effect on gas apparent permeability decreases. Under low pressure, the IW, where pore size is small, promotes fluid flow, while the IW in the large pores hinders fluid flow. In conditions of ultra-high pressure, the IW promotes gas flow.

Numerical investigation on hydraulic and gas flow response of MSW landfill cover system comprising a geosynthetic clay liner under arid climatic conditions

Municipal solid waste (MSW) landfill cover systems are designed to minimize the infiltration of rainwater into waste and to mitigate the biogas emissions to the atmosphere. In the present study, the efficacy of such a landfill cover system consisting of a Geosynthetic clay liner (GCL)in mitigating the hydraulic flow and gas emissions under arid climatic conditions was investigated critically. For this purpose, the water retention curve (WRC), hydraulic conductivity function, and gas flow characteristics of the chosen GCL were studied through experimental methods in the laboratory. The unsaturated transient state seepage analysis utilizing coupled hydraulic and gas flow mechanisms was performed in the study to assess the performance of the cover system. The results obtained from the experiments were used as input parameters. The effect of drying/desiccation and the self-healing nature of GCL due to climatic changes were also analyzed by exposing the GCL directly to the climatic boundary for one year. It was observed that the GCL present in the chosen cover system effectively functions as a hydraulic barrier in arid climatic conditions. However, during the summer and winter seasons, an increase in gas flow from 0.02 g/h/m2 to 24.7 g/h/m2 was observed, probably due to the drying anddesiccation of GCL. Interestingly, due to the self-healing nature of the GCL, gas flow through the cover system was substantially reduced to 0.02 g/h/m2 during the rainy season. The effect of drying was more pronounced when the GCL was exposed to the climatic condition, leading to an early gas breakthrough and an increase in gas flow from 0.02 g/h/m2 to 957 g/h/m2. The percolation through the cover system remains considerably low throughout the year, mostly due to the unsaturation and low hydraulic conductivity in GCL. A cumulative percolation close to 0.1 m3 was observed at the end of one year in arid climatic conditions.

Structural Design and Simulation of Pneumatic Conveying Line for a Paddy Side-Deep Fertilisation System

To improve the consistency of sectional gas velocities in different rows in the pneumatic conveying line for a paddy side-deep fertilisation system, a new airflow equaliser was designed based on the mechanism of gas flow in ventilation engineering. Subsequently, the effects of key structural parameters and a position parameter of the airflow equaliser on the consistency of sectional gas velocities in different rows were investigated using the method of single-factor tests in Fluent, which provided a reasonable range for the next orthogonal test (notch angle (A): 120–180°, extended length (B): 18–30 mm and distance between adjacent branches (C): 120–160 mm). Thereafter, the parameters were optimised through an orthogonal test, using the coefficient of variation of the consistency of the sectional gas velocities in different rows (CV) as an indicator and using Fluent software. The results revealed that the order of primary and secondary factors was evaluated as B > A × B > C > A, and when A, B and C were selected as 150°, 30 mm and 120 mm, respectively, the consistency of the sectional gas velocities in different rows could get a very effective result (CV = 7.07%). Finally, to validate the feasibility of simulations of the performance of the airflow equalisers and to practically evaluate the contribution of the optimised airflow equalisers to improve the consistency of the sectional gas velocities in different rows, a bench test for the line with the optimised airflow equalisers and a simulation test for the line without the optimised airflow equalisers were carried out and the results showed that the CV values were 9.53%, 20.69%, respectively. It concluded that the optimised airflow equalisers could significantly improve the consistency of the sectional gas velocities in different rows by comparing the CV in the three tests including the simulation test for the line with the optimised airflow equalisers. This research provides a good reference for optimising the pneumatic conveying line for a paddy side-deep fertilisation system.

Gas–Water Two-Phase Flow Characteristics and Flowback Evaluation for Shale Gas Wells

The dynamic characteristics of shale gas wells are complexly affected by the gas–water two-phase flow. Based on the special flow mechanism of gas–water two-phase flow in shale gas reservoir, this paper establishes a mathematical model for gas–water two-phase flow in shale gas multi-stage fractured horizontal wells, introduces the eigenvalue method and orthogonal transformation, and obtains the analytical solution of the two-phase flow model. The gas–water two-phase flow rules and main influence factors of shale gas wells were identified, further combined with the flowback test characteristics and data of the shale gas wells in southern Sichuan, the characteristic parameters for the evaluation of the gas well flowback effect were determined, and an index system was established for the evaluation of shale gas well flowback. The analysis result shows that the shale gas well flowback effect has a good relationship with its production capacity, which is mainly reflected in the flowback characteristic parameters such as gas breakthrough time, gas breakthrough flowback rate, 30 d flowback rate, and maximum production flowback rate. The shale gas wells with lower flowback factors have a better production capacity than those with higher flowback factors. The flowback evaluation index system can accurately forecast the shale gas well production capacity in its initial stage, and furthermore offer guidance to promptly ascertaining the block development potential and formulating the development schemes.

Effect of Pore Geometry on Nanoconfined Bulk-Gas Flow Behavior

In order to improve energy consumption structure in Guizhou province, China, the efficient development of shale gas becomes more and more important. However, the theoretical understanding, towards microscopic gas flow mechanism, is still weak and insufficient. Although many models have been established, one part of them fails to cover all flow mechanisms, and the other part contains several fitting parameters. Moreover, nanopores can be divided into circular pore and slit pore in accordance with pore geometry. For cylindrical nanopores, by fully comparing with the existed models, a new bulk-gas transport model is proposed by weight superposition of slip flow and Knudsen diffusion, in which weight factors are obtained by Wu’s model and Knudsen’s model, respectively. For slit pores, an analytical equation for bulk-gas flow is proposed as well, which is free of any fitting parameters. The reliability of the proposed models has been clarified. The impacts of reservoir pressure, pore scale, and gas flow mechanism on bulk-gas flow behavior in process of shale gas development are analyzed. It is found that gas transport capacity of slit nanopores is significantly higher than that of cylindrical nanopores at the same pore scale. For slit nanopores, the larger the aspect ratio, the stronger bulk-gas transmission capacity. As the established models are free of fitting parameters and can be applied into the entire K n range with sufficient accuracy, the research will greatly benefit shale gas development.

Nanoconfined Bulk-Gas Transport Behavior over a Wide Range of Knudsen Number

Shale matrix, located at Guizhou Province, is rich in nanopores, and gas slippage takes place during shale gas development, resulting in inapplicability of the classical Navier-Stokes equation. Investigation of gas transmission mechanism in nanoscale is helpful to reach a clear understanding of shale gas production performance. On the basis of the Knudsen number, the gas flow mechanism is divided into continuous flow, slippage flow, transition flow, and Knudsen diffusion. Notably, the accurate characterization of transition flow is still challenging up to date. Although there are many established models, they either fail to cover all the flow mechanisms or contain many fitting parameters, the determination of which requires a large number of experimental and molecular simulation data, limiting application of the existing models. Therefore, establishment of bulk-gas transport model over a wide range of Knudsen number without fitting parameters is urgent. First of all, existed theoretical models are compared, and the advantages and disadvantages of previous contributions are analyzed. Weight factors of Knudsen diffusion and slippage flow are obtained, respectively, according to Knudsen’s model and Wu’s model. Then, a model, free of empirical coefficients, is proposed. After that, effects of pore size, reservoir pressure, and temperature on transmission capacity are investigated. As the proposed model does not contain fitting parameters and remains high accuracy over a wide range of Knudsen number, it shares broad application potential, like shale gas production prediction, reserve estimation in Guizhou Province.

Rarefaction Effect, Heat Transfer, and Drag Coefficient for Gas Flow Around Square Cylinder in Transition Flow Regime

Abstract In this work, rarefaction effect, heat transfer, and drag coefficient for gas flow around a square cylinder in transition flow regime are numerically studied using unified gas kinetic scheme (UGKS). To reduce computational cost, a mirror symmetry boundary treatment for UGKS is proposed and applied in this study. It is found that: (1) velocity slip is not obvious on upwind and downwind surfaces of square cylinder due to the tangential gas flow being weak in these surfaces; (2) with the increase of local Knudsen number, velocity slip on upper surface always increases, but temperature jump can decrease, which indicates that Knudsen number is not the decisive parameter to characterize temperature jump; (3) the heat transfer between gas and square cylinder enhances with the increase of inflow Knudsen number and cylinder temperature due to the increase of temperature jump and deteriorates with the increase of inflow Mach number on account of gas stagnation; and (4) the drag coefficient increases with the increase of inflow Knudsen number, the decrease of inflow Mach number, and the increase of cylinder temperature. To further predict the variation of average Nusselt number and drag coefficient, correlations for average Nusselt number and drag coefficient with inflow Mach number ranging from 0.05 to 0.3, inflow Knudsen number ranging from 0.1 to 10, and cylinder temperature ranging from 320 K to 380 K are proposed. This research can improve the understanding for mechanisms of gas flow and heat transfer in micro-electromechanical system (MEMS) devices.

A dual porous and fractures medium CBM numerical model development and analysis

The drilling unloading will produce fractures in the plastic zone around the borehole. However, the fracture is the main seepage channel of coalbed methane. It is not appropriate to ignore the influence of fractures on gas flow, especially in the plastic zone. According to the stress redistribution characteristics after unloading, the fracture field in the plastic zone is considered in the coalbed methane numerical model. A two-dimensional random fracture field is introduced via the Monte Carlo method in the plastic zone, which is based on the central limit theory and box muller method. A dual porous and fracture medium numerical model is proposed, which considers not only gas adsorption and desorption, but also fracture distribution and creep deformation. A comparison between the proposed model and the single porous medium numerical model is carried. The results show that the proposed model can reflect not only the evolution characteristics of gas flow but also the anisotropic influence of fracture. It further explains the evolution mechanism and distribution law of gas flow. Finally, the influence of fracture field parameters (fracture density, fracture length, and fracture width) on gas flow is discussed, which is expected to provide a theoretical basis and reference for the development and research of coalbed methane.

A multi-scale quadruple-continuum model for production evaluation of shale gas reservoirs considering complex gas transfer mechanisms and geomechanics

Shale gas reservoirs are uniquely characterized by extremely low values of permeability, porosity, and complex fracture network which adequately hinders gas transfer and production evaluation. At the reservoir scale, the analysis of complex gas flow and transfer mechanisms from one domain to another is still a challenge. In this regard, a fully coupled multi-scale quadruple-continuum model is proposed, where the gas transfer mechanisms across the four domains (Kerogen, inorganic matrix, natural fractures, and hydraulic fractures) are adopted and the flow terms are corrected to suit real gas and account for shale deformation considering dynamic effective stress. The complexity of modeling gas transfers from the kerogen to the inorganic matrix to fracture system (natural and hydraulic fractures) and the production well is overcome by using the Warren-Root and Vermeulen transfer terms, respectively. The current model matches field data from typical shale reservoirs and accurately predicts gas production. Numerical simulation results show that increasing the kerogen pore volume and stress sensitivity coefficient decreases cumulative gas production. Increasing domain permeability, porosity, number of natural fractures, and the width and length of natural fractures improve the flow of gas, leading to higher cumulative gas production. Moreover, crisscrossing natural fractures and larger stimulated reservoir domain (SRD) could extend the area of contact with the unstimulated reservoir domain (USRD), thus allowing more gas to flow to the production well. Overall, more gas is produced when the four domains are considered than when either of them is ignored. This study introduces a comprehensive understanding of the four domains' primary flow and transfer mechanisms. It provides a practical baseline for evaluating gas production in fractured shale reservoirs.

Mathematical model and field application of aqueous phase trapping release

ABSTRACT Aqueous phase trapping (APT) is a kind of mechanical formation damage, which occurs in tight sandstone gas reservoirs. The occurrence of APT has a dramatically adverse effect on the productivity of gas wells due to the decrease of water saturation and gas permeability near wellbore. In this research, a spontaneous imbibition experiment was carried out. The Lucas–Washburn model was used to describe the imbibition behavior. Then, a mathematical model of release judgment conditions was established based on the imbibition model, taking the threshold pressure gradient into consideration. Finally, combined with the two-phase flow mechanism of gas and water, the release time of APT was deduced. This model was used to calculate the releasing time of APT in actual producing wells, and the difference between the actual release time and predicted release time is only 0.1%, which indicates that the model established in this paper can be applied to predicting release judgment condition and release time in tight sandstone gas reservoirs precisely. The proposed model along with the findings of this research can pave the way for the continuous development of water breakthrough wells in tight sandstone gas reservoirs.

A Model for the Apparent Gas Permeability of Shale Matrix Organic Nanopore Considering Multiple Physical Phenomena

The flow of shale gas in nano scale pores is affected by multiple physical phenomena. At present, the influence of multiple physical phenomena on the transport mechanism of gas in nano-pores is not clear, and a unified mathematical model to describe these multiple physical phenomena is still not available. In this paper, an apparent permeability model was established, after comprehensively considering three gas flow mechanisms in shale matrix organic pores, including viscous slippage Flow, Knudsen diffusion and surface diffusion of adsorbed gas, and real gas effect and confinement effect, and at the same time considering the effects of matrix shrinkage, stress sensitivity, adsorption layer thinning, confinement effect and real gas effect on pore radius. The contribution of three flow mechanisms to apparent permeability under different pore pressure and pore size is analyzed. The effects of adsorption layer thinning, stress sensitivity, matrix shrinkage effect, real gas effect and confinement effect on apparent permeability were also systematically analyzed. The results show that the apparent permeability first decreases and then increases with the decrease of pore pressure. With the decrease of pore pressure, matrix shrinkage, Knudsen diffusion, slippage effect and surface diffusion effect increase gradually. These four effects will not only make up for the permeability loss caused by stress sensitivity and adsorption layer, but also significantly increase the permeability. With the decrease of pore radius, the contribution of slippage flow decreases, and the contributions of Knudsen diffusion and surface diffusion increase gradually. With the decrease of pore radius and the increase of pore pressure, the influence of real gas effect and confinement effect on permeability increases significantly. Considering real gas and confinement effect, the apparent permeability of pores with radius of 5 nm is increased by 13.2%, and the apparent permeability of pores with radius of 1 nm is increased by 61.3%. The apparent permeability model obtained in this paper can provide a theoretical basis for more accurate measurement of permeability of shale matrix and accurate evaluation of productivity of shale gas horizontal wells.

Facets of coalbed methane reservoir in East Bokaro Basin, India

The East Bokaro Basin of the Damodar valley is a potentially prospective CBM (coalbed methane) play having significant cumulative coal seam thickness, in-situ gas content, vitrinite percentage, and adequate thermal maturity. Successful CBM recovery needs a detailed understanding of the organic content, pore structures/networks, storage properties and gas flow mechanism. The present work attempts to systematically investigate East Bokaro coal for organo-petrographic controls on gas content and generation, variations in sorption capacity and saturation, pore mechanisms, cleat intensity, cleat aperture distribution and spacing. The values of in-situ gas, sorption capacity and methane concentration (C1) vary from 3.52 to 30.93 cc/g (dry ash-free basis), 15.40–32.40 cc/g (dry ash-free basis), and 66–93 vol%, respectively. The atomic ratios H/C and O/C indicate that thermally matured coal seams contain type III-IV kerogen positioned in the dry gas window. The decrease of hydrogen-containing liptinite with increasing depth reveals the function of thermal gradient on the cracking of liptinitic compounds with successive evolution of hydrocarbons and the development of a carbon-rich pore matrix. The H/C ratio is also influenced by the increasing content of vitrinite and reflectance values of deeper coal. More than 63 % of desorbed gas was determined from desorption measurement and low sorption time (τ, mainly <10 days). This demonstrates good diffusion characteristics of the studied coal. It shows the tendency of desorbed gas diffusion from pores reaches to cleat-fractures with negligible influence of secondary mineral infillings. The high-pressure sorption studies of methane on various samples indicate substantial open-pore characteristics, supporting adsorption, diffusion and gas release. The relationship of C3/C1 and C2/C1 ratios demonstrates that the hydrocarbons in coal primarily originated from the thermogenic transformation of organic matter. Such an assessment is also supported by the stable isotope (δ13C1) value that ranges between −22.70 ‰ and −57.30 ‰. However, some of the lighter isotope values (<–50 ‰) indicate a mixed origin of gases, which may be due to the influx of fresh-water to coal associated aquifers carrying bacteria received from local drainage. Geochemically and thermally altered dissolved and partially filled pores, shown by SEM photographs, negligibly influence gas sorption, diffusion, and flow mechanism in coalbeds. The pore network model signifying that the studied coal seams are microstructurally different comprises a lateral difference in pore and cleat/fracture.

Petrographic and Geochemical Controls on Methane Genesis, Pore Fractal Attributes, and Sorption of Lower Gondwana Coal of Jharia Basin, India.

The Barakar coal seams of Jharia Basin have been evaluated for the geochemical and petrographic control of coalbed methane (CBM) reservoir characteristics. The coal core samples are analyzed for the total gas content, gas chromatography, stable isotopes (δ13C1), and geochemical, petrographic and vitrinite reflectance. The significant face (1.6–7.6%) and butt (0.9–5.3%) cleat intensities specify the brittle characteristics of coal seams and also favor the gas flow mechanism. The thermal cracking position of hydrocarbon compounds was evaluated, which signifies the excellent source rock potential of coal for gas genesis. The inputs of type III and IV organic matter illustrated by the van Krevelan diagram signify thermally matured coal seams. The low values of sorption time (τ) between 2.1 and 5.6 days designate excellent diffusion characteristics that is favored by the cleat intensities. The values of total gas content and sorption capacity (VL) reveal that moderate saturation indicates a higher gas content, attributed to the seam thickness and thermal maturity. Similarly, the CH4 concentrations (89.4–96.6 vol %) display that the genesis pattern is a function of thermal maturity; however, some samples fall under the mixed type substantiated by the stable isotope (δ13C1) (−25.40 to −64.90‰), emphasizing bacterial hold by seasonal influx of freshwater. The ternary facies diagram (Vmmf, Immf, Lmmf) also supports notable generation of methane gas and storage in the coal seams of the Jharia Basin. The volume percentage of each maceral determined from petrographic study was used to estimate the fraction of conversion (f) of the organic content (0.19–0.97). The values of “f” indicate that the Barakar coal has undergone maximum conversion, which may be attributed to the older early Permian coal and placed at a greater depth after deposition due to the basin sink. The high fraction of conversion and thermal maturity may also be explained due to the existence of volcanic intrusion (sills and dykes). The uniformity in the distribution of functional groups is shown by Fourier transform infrared spectra representing moderate to stronger peaks of aromatic carbon (CO and C=C) between 1750 and 1450 cm–1, which indicates that the presence of a larger total organic carbon content likely validates the removal of aliphatic compounds during gas genesis. The variations in the BET curve have been categorized as H1 hysteresis following the type II adsorption pattern, suggesting that cylindrical pores and some of the coal samples have a type IV H4 hysteresis pattern, characterized as the slit type of pores. The average values of the pore diameter indicate the dominance of mesopores suitable for gas storage and release and hence a major part of the pore volume is contributed by the mesopores having a width mainly between 2.98 and 4.48 nm. The significant role of the meso-macropore network (D1 fractals) in methane storage of the coal matrix is represented by a moderate positive relationship of VL with D1, which accentuated that meso-macropores developed due to devolatilization and dehydration of organic matter and also by geochemical alteration of macerals and minerals formed heterogenetic inner surfaces suitable for gas adsorption. The estimated recoverable resource applying Mavor Pratt methods is 8.78 BCM, which is found to be a more realistic resource value for the studied CBM block.

Pore structure changes induced by hydrate dissociation: An example of the unconsolidated clayey-silty hydrate bearing sediment reservoir in the South China Sea

The natural gas hydrates (NGHs) in the South China Sea are found in unconsolidated clayey-silty sediments, featured by very weak mechanical strength and low permeability. During gas extraction from such unstable, low permeability reservoirs, hydrate dissociation can result in distinct changes in pore properties and flow capability. Thus, it is extremely important to investigate the pore characteristics and gas flow mechanisms in such reservoirs. In this study, five sediment samples including two from the hydrate bearing layer (HBL) and three from the underlying layer (ULL), were taken from an exploration well in the Shenhu area, South China Sea. The pore morphology, pore specific surface area (SSA), total pore volume, porosity, and pore size distribution of these samples have been analyzed to compare the reservoir physical properties of the HBL (after the hydrate has dissociated and disappeared) with the ULL. Results from scanning electron microscopy reveal that the HBL samples without the hydrate present feature larger grain sizes, larger pore apertures and better connectivity than ULL samples. Results of NMR analysis and low temperature N2/CO2-adsorption show that both HBL and ULL samples have high total porosities (42.4%–48.8%) and high SSA (21.17–30.09 m2/g), while the results from centrifugal experiments demonstrate that the producible porosity (defined as the pore volume available for hydrocarbon emplacement) of the HBL samples is about twice (26.6%–30.3%) that of the ULL samples (15.5%–19.6%). The pores in the two sample types are dominated by intra-particle pores of 80 nm, and inter-particle and inter-aggregate pores of 0.2–3 μm. The above differences between HBL and ULL can be used to analyze the pore structure changes occurring as a result of the dissociation of NGHs in the HBL. We assume that on the one hand, formation of NGHs reduces pore pressure, thus resulting in higher effective stress, then compacts the pore space and reduces the total porosity. On the other hand, the release of high-pressure gas during hydrate dissociation could increase the pore aperture and improve pore connectivity, and thus increase the producible porosity of the HBL reservoir. It is also assumed that the flow capacity in the methane production zone in the HBL can be improved by the NGHs dissociation, suggesting that not only the hydrate saturation but also the pore structure changes should be considered when estimating the permeability during the gas production process.

Characterization of anisotropic coal permeability with the effect of sorption-induced deformation and stress

Changes in permeability are related to coalbed methane (CBM) production activities, and to coal mining safety. However, inherent properties, i.g. anisotropy and heterogeneity make the gas flow mechanism more complex. In this work, an anisotropic coal permeability model with synergistic stress compression and gas adsorption was constructed to simulate gas flow behavior in all directions in coal. Changes in coal anisotropic permeability are restricted by the fracture deformation in coal in all directions. Wherein, the fracture deformation in coal is controlled by the effective volume strain coordinated by gas adsorption and stress. The influence of stress can be expressed by employing the generalized Hooke’s law. Then, based on the coal matrix bridge structure, an internal swelling coefficient has been proposed to assess the control of the matrix swelling on fracture width during the gas adsorption process to further obtain the fracture deformation caused by gas adsorption. The anisotropic coal permeability model was verified by laboratory test data under different effective stresses, gas pressures, with respect to their combined influences. Subsequently, based on the experimental conditions of uniaxial strain, the permeability model of anisotropic coal during gas depletion was further deduced, and changes in anisotropic coal permeability under uniaxial strain were also calculated. In addition, the sensitivity of the reduction rate of the elastic modulus, and the internal swelling coefficient was also studied, to reveal the matrix-fracture interaction mechanism.

Investigation of Shale Gas Reserves in The World and A Case Study for Electricity Production from Shale Gas in Turkey

Shale gas reserves, which is globally accepted as unconventional gas resource, scattered around the world and can be used in order to meet the growing energy needs due to limited amounts of conventional resources. Since notable effort has to be put in order to investigate and drill shale gas resources the early decision has to be made carefully, especially considering the economic benefits. Several factors must be considered including the known technically recoverable shale gas amount at the selected region, current technology to drill the shale source and the amount of investment before the extraction of shale gas. In this study, global underground shale gas amount and recent discoveries as well as the potential shale gas areas in Turkey are presented. Benefits of using shale gas for electricity generation and common methods being used during shale gas extraction are studied along with gas and liquid flow mechanisms. At the end, general overview of separation and utilization of shale gas components is schematically presented. In the case study, the potential of electricity generation in the SE Anatolia region in Turkey is estimated with only using methane obtained from the shale gas purification process at the power generation step. According to the estimations, 3337.8 MW electric power may be generated for 50~55 years, at the appropriate gas engine by using 8.5 billion cubic meter shale gas annually.