Evolution Of Gas Pressure Research Articles

Gas Evolution Principle during Gas Drainage from a Drill Hole along the Coal Seam and Reasonable Borehole Layout Spacing.

The efficiency of coal seam gas drainage can be further improved by the accurate mastery of the gas evolution principle during gas drainage from the drill hole along the coal seam and reasonable optimization of borehole layout spacing. Based on the actual geological conditions of the no. 2 coal seam in a coal mine in Guizhou, China, and relevant control equations, a fluid-structure interaction model of gas drainage from a drill hole along the coal seam was established in this paper. Besides, numerical simulation research on the gas evolution principle during gas drainage along the coal seam and optimization of the borehole layout spacing was carried out with the COMSOL simulation software; and these were verified in combination with the project site. The results showed that in the early stage of gas drainage the gas pressure in the area near the drill holes decreased significantly. As the gas drainage went on, the degree of influence decreased gradually. During the gas drainage from adjacent drill holes, the gas pressure in the coal body between the holes decreased rapidly, and the migration was obvious. With the increase of the spacing between the drill holes, the drainage superposition effect between these holes gradually decreased until the influence area around a single hole is independently distributed in a circle-like shape, indicating that the optimization and the reasonable spacing of the drill holes are directly related to the effect of the gas drainage. With the increase of drilling spacing, the superposition effect of extraction between the holes gradually decreased until the influence area around a single hole is independently distributed in the shape of a circle, indicating that the optimization of drilling spacing is directly related to the effect of gas drainage. The results of numerical simulation were verified by analyzing the three-dimensional map of the gas pressure during the period of gas drainage at the project site and by comparing and examining the rational borehole layout spacing of the drill hole along the coal seam. The results of this study can be used to determine the spacing of gas extraction boreholes and improve the efficiency of gas extraction in the no. 2 coal seam of a coal mine in Guizhou, China, as well as to provide a reference for the gas pressure evolution, velocity field distribution, the prediction of effective drainage area, and the selection of rational borehole layout spacing during gas drainage.

A fractal power-law approach for investigating the interactions between reservoir microstructure and gas migration

As one of the most significant clean energy sources, the migration characteristics and extraction processes of coalbed methane (CBM) have been extensively studied. The structural distribution and evolutionary characteristics of reservoir fractures, as the main conduit for gas migration, significantly affect the permeability and gas production. However, few models have been able to quantitatively and accurately explore reservoir micro–macro interactions under coupled thermal-fluid–solid effects. This work develops a new highly coupled model based on the widely adopted power-law function to quantify reservoir thermal conduction effect, gas pressure evolution, reservoir deformation, in situ stress, the adsorption–desorption effect, and reservoir microstructure evolution. Three parameters are adopted to quantitatively characterize the reservoir structure: (1) fracture power index αf (to characterize the fracture density), (2) fracture length ratio rf (to characterize the fracture size), and (3) the maximum fracture length l. The results demonstrate that the fractal network is a special kind of network in the power-law length distribution. The proposed power-law seepage model is able to accurately characterize the evolution of reservoir microstructure and the impact of microevolution on extraction under multi-field coupling effects, compared to the traditional power-law model. The proposed model can provide a good theoretical and practical support for the study of CBM migration and extraction.

Gas pressure evolution characteristics of deep true triaxial coal and gas outburst based on acoustic emission monitoring

Coal and gas outburst is one of the geological disasters that seriously threaten the safety of coal mines production. In recent years, with the increase of mining depth, outbursts become frequent. To further explore the occurrence mechanism of deep coal and gas outburst, a self-developed true triaxial coal and gas outburst simulation device was used to simulate the coal and gas outburst at different depths. The results show that with the increase of simulation depth, the critical gas pressure of outburst gradually decreases, and the unit outburst intensity increases sharply. The gas threshold of deep coal and gas outburst is lower. During the incubation and excitation, gas pressure has three special variation rules, namely self-increasing characteristic, stage and instantaneous. In the early incubation, acoustic emission (AE) energy is at a low level, low energy frequency is dominant; in the later incubation, AE energy increases greatly, high energy frequency is dominant. From the perspective of AE energy, a quantitative index that reflects the danger of coal and gas outburst in the incubation is defined, which provides a scientific reference for the prediction and prevention of disasters of coal and gas outburst in deep mining.

Intensive field measurements for characterizing the permeability and methane release with the treatment process of pressure-relief mining

Characterizing the permeability evolution and methane release is of great significance for the safe mining of the high gas outburst seams, as well as coal and gas simultaneous extraction. It contributes to reduce methane emissions from coal mining for greenhouse effect control. Theoretical analysis, laboratory testing, and numerical simulation are widely used methods to characterize the permeability and methane release with the treatment process of pressure-relief mining. However, these methods cannot fully reflect the complexity of filed practice. In this study, we report the effectiveness of protective coal seam (PCS) mining and the pressure-relief area in the protected coal seam (PDCS) based on detailed and integrated field measurements in a Chinese coal mine. To the best of our knowledge, it is the first time to measure the permeability coefficient and gas pressure evolution in the PDCS during the process of PCS longwall mining. The evolution of the permeability coefficient in the pressure-relief area during PCS mining can be divided into four stages: slowly decreasing, sharply increasing, gradually decreasing, and basically stable. The maximum permeability coefficient is 322 times of the initial value and stabilized at 100 times after the goaf compacted. The gas pressure evolution in the PDCS indicates that the strike pressure relief angle is 52.2° at the active longwall face zone, and 59.3° at the installation roadway side. The inclined pressure relief angles at the lower and upper sides of the longwall face are 75° and 78.9°, respectively. The residual gas content and gas pressure of the PDCS in the pressure-relief area are reduced to less than 6 m3/t and within 0.4 MPa, respectively. The field measurements further prove that pressure-relief mining can prevent coal and gas outbursts in PDCSs. The field observations in this paper can serve as benchmark evidence for theoretical analysis and numerical simulations, and also provide insights into realizing safety mining in similar conditions.

Discrimination Method of Gas Pressure Measurement in Coal Seams: Physical Experiment and Model Development.

Coal seam gas pressure is a key parameter of gas accident control and gas drainage. At present, there are some problems in field pressure measurement, such as long period, and there is often a need to drill more holes to ensure the reliability of pressure measurement. In this research, a physical gas pressure measurement experiment in a coal sample borehole was carried out, and a mathematical model of gas pressure evolution with time was constructed. Based on the OpenFOAM platform and C++ language, a numerical solver was developed, and the mathematical model was verified by the data of gas pressure in coal seam boreholes. The results show that the evolution process of gas pressure in coal seam boreholes can be divided into two stages. In the first stage, the gas pressure increases rapidly, and the pressure change rate decreases continuously. In the second stage, the gas pressure is slow and stable, and the pressure change rate tends to 0. The correlation coefficients between the mathematical model and the field-measured data are more than 0.94, and the calculation and prediction accuracy are high. Therefore, the model can be used to verify the field data during pressure measurement, which has better field significance and application value.

Numerical simulation of contact vacuum drying of potato slices

• A mathematical model for contact vacuum drying is numerically solved. • The simulations predict the drying kinetics coupled to the pressure changes in the chamber. • The frequency of active/passive phases depends on the mass flux, which is linked to the free moisture content of the samples. • Vacuum drying reveals to have a larger impact in the first period of drying. In this work, a mathematical model is proposed to simulate the Contact Vacuum Drying of potato slices. Three temperatures were analyzed: 55, 65, and 75 °C with three levels of vacuum pressure: 60, 40, and 20 mmHg. In the macroscopic model, the boundary condition for the moisture conservation is written in terms of water vapor pressure, and it considers the dynamics of the vapor pressure in the chamber. The simulations predict the evolution of gas pressure and the drying kinetics. As drying occurs, the passive regime is prolonged as a result of a decrease in the moisture content. The maximum drying rate (1.4 E-3 kg. water/kg. dry solid·s) corresponds to the drying condition at 75 °C and 20 mmHg, which is the most intensive drying condition. The comparison between the predicted and experimental values showed acceptable agreement. The simulations correctly represent the drying kinetics and the evolution of the gas pressure in the drying chamber.

Chromium-coated cladding analysis under simulated LOCA burst conditions

Of the near-term accident-tolerant fuel concepts identified, chromium coatings have been shown to slow cladding oxidation without altering fuel system geometry or greatly affecting neutronic performance. To identify how coated cladding tubes perform under high-temperature accident conditions, pressurized-tube burst tests were conducted in the Severe Accident Test Station at Oak Ridge National Laboratory. To analyze how the coating affects cladding behavior, coated and uncoated cladding burst tests were simulated by using the BISON fuel performance code. Thermocouple data from these tests were fit into axial and azimuthal profiles and combined to generate 3D cladding surface temperatures, and pressure transducer data were compared until cladding failure. The cladding temperatures at failure and the internal gas pressure evolution show relatively good agreement between the simulation and experiment results. Simulations were then extrapolated to demonstrate the coatings effectiveness to increasing the cladding burst temperature by using a parametric evaluation of the initial tube gas pressure. This work demonstrates the possibility of an increased cladding failure margin under transient conditions due to the addition of chromium coating, and more pertinently, an increased cladding failure margin from more accurate experimental characterization.

Hydrogen gas transfer between a borehole and claystone: experiment and geochemical model

The Hydrogen Transfer experiment, located at the Mont Terri underground rock laboratory in Switzerland, is an in situ experimental study of the interactions and transport of hydrogen injected into a borehole installed within Opalinus Clay, a claystone formation. A Python-based model was developed to analyse and model the experimental data, for diffusion of dissolved gases and solutes in claystone pore water, for thermodynamic modelling of gas–water–solid phase equilibria in the injection interval and for simulations of chemical equilibria and reaction kinetics in claystone and injection interval. The developed model reproduces the temporal evolution of gas pressure, composition and solute concentrations measured in situ with a minimum set of adjustable parameters. The effective diffusion coefficients for dissolved gases in Opalinus Clay derived by the modelling of experimental data were found to be very close to values measured in other experimental studies. It was discovered that an accurate description of the temporal variations in hydrogen injection and temporal changes in the inflow of formation water is essential for modelling of microbially mediated hydrogen consumption in the injection interval.

An experimental study of the mechanism of coal and gas outbursts in the tectonic regions

To reveal the mechanism of coal and gas outbursts in tectonic regions, a gas-driven multi-physics coal and gas outbursts experimental apparatus was developed. Based on the experimental results, the damage characteristics and the mechanism of outbursts of tectonic raw coal under different gas pressures and stress conditions are quantitatively investigated. The experimental results show that the damage to coal samples increases with the increase in gas pressure. When the gas pressure is 2.0 MPa, the damage to the coal sample tends to decrease with increases in the confining and axial stress. When the gas pressure exceeds the critical value of 2.2 MPa, a strong outburst phenomenon occurs, in which the pulverized coal is ejected and the outbursts cavern is blocked. However, when the gas pressure is 2.4 MPa, the coal samples are completely broken down after the tests, and the outbursts intensity becomes larger as the axial stress or the confining stress increases. In addition, the evolution of gas pressure and the degree of fragmentation of coal samples are also discussed quantitatively in the process of coal and gas outbursts. The present study of the mechanism of coal and gas outbursts in the tectonic regions is helpful for the assessment and forecasting of potential outbursts.

Pressure of different gases injected into large-scale coal matrix: Analysis of time–space dependence and prediction using light gradient boosting machine

The pore pressure in the coal seam during gas injection affects the coalbed methane (CBM) recovery and increases the risk of coal mine disasters. However, the limitations of conventional experimental methods hinder the direct characterization of gas pressure evolution during CO2 or N2 injection. In this study, a large-scale coal matrix was used to conduct experiments on different gases, focusing on the time–space dependence of the gas pressure during gas injection and adsorption. Finally, the gas pressure was predicted using a decision tree model based on a light gradient boosting machine (LGBM). The results showed that in the early stage of gas injection, it is difficult to increase the gas pressure in the coal matrix. As the gas pressure increases, the injection of CO2 or N2 becomes increasingly difficult. In the later stage, a low-volume gas injection will lead to higher gas pressure. Because of the different adsorption capacity of CO2 or N2, the decay rate of N2 injection is considerably lower than CO2 injection. The direction parallel to the inlet end produces the highest pressure difference. The size of the CBM seam can affect the pore pressure. As the gas adsorption capacity decreases, the pressure difference in each direction gradually decreases. Further, the LGBM can efficiently predict the pressure changes during gas injection and determines the time–space dependence of the pressure for different gases. The proposed method based on the results and the LGBM model facilitates the understanding of gas migration and pore pressure changes during gas-induced CBM extraction.

Mechanism of multifield coupling-induced outburst in mining-disturbed coal seam

Coal and gas outburst is a typical dynamic disaster occurring during coal mining. To analyze its multi-field coupling mechanism, in this study, a multi-field (including coal stress field, damage field, gas diffusion field and seepage field) coupling model was established based on the heterogeneity of coal, and the mining process of horizontal and coal seam with fault were numerically simulated. Besides, the evolution of gas pressure, von mises stress and permeability under different parameters were analyzed. The results show that the stress relief zone in front of the working face is damaged by the mining disturbance, and the permeability increases sharply due to the full development of internal fractures. The coal in the stress concentration zone is affected by the stress concentration, which leads to the closure of fracture and the sharp decrease of permeability. As a result, the gas pressure gradient between stress relief zone and stress concentration zone increases, thus raising the risk of coal and gas outburst. The front geological structure of working face leads to the formation of multiple stress concentration zones in the coal seam, and the risk of coal and gas outburst increases with the increase of overburden stress. Based on the interaction between geological structure, gas pressure, coal mechanical properties and overburden stress, the mechanism of coal and gas outburst can be revealed. The research results can provide theoretical guidance for disaster prevention of coal and gas outburst.

Evolution characteristics of reservoir parameters during coalbed methane drainage via in-seam horizontal boreholes

In-seam horizontal boreholes are used to collect coalbed methane (CBM). Physical simulations of CBM drainage by in-seam horizontal boreholes are conducted. The evolution of gas pressure and permeability ratio indicate a rapid decline in a short time. As time goes by, the permeability ratio recovers, and the gas pressure declines slowly. In the borehole area, initially, the decline rates of the gas pressure and permeability ratio are positively related to the distance from the borehole, and the shape of the distribution of the gas pressures and permeability ratio is similar to that of multiple rings. In the non-borehole area, the decline rates of the gas pressures and permeability ratio is almost the same at different points. When the borehole spacing is optimum, the interaction effect accelerates the drainage of CBM. The permeability is initially controlled by effective stress and is mainly affected by matrix shrinkage after a period of time.

Modelling of preferential gas flow in heterogeneous and saturated bentonite based on phase field method

Phase field (PF) method is incorporated into the coupled hydromechanical (HM) model in this paper to simulate the preferential gas flow in saturated bentonite. In order to account for the effect of the heterogeneous distribution of HM properties on the development of preferential pathways, a spatially auto-correlated random field is generated and applied to the coupled HM-PF model. A stabilized technique for equal low-order finite element is adopted in this paper to avoid the pore pressure oscillation and to increase the computational efficiency. The simulated results showed that the developed coupled HM-PF model is able to explicitly describe the preferential gas flow in saturated bentonite. Meanwhile, some of the commonly observed behaviors in experiments, such as the nearly saturated state after gas injection, the heterogeneous distribution of water pressure and the build-up of both water pressure and total stress during fracturing process, are successfully captured by the developed model. In addition, four factors, including the Gaussian random field, the coefficient of variation, the boundary condition and the injection rate, have been examined in terms of their effects on the fracturing process. The numerical results showed that these four factors have significant influences on both the fracture pattern and the evolution of gas pressure.

How transport properties of a shale gas reservoir change during extraction: A strain-dependent triple-porosity model

In this study, we consider a shale gas reservoir as a triple-porosity media consisting of organic matrix, inorganic matrix, and fractures. A strain-dependent triple-porosity model is established by accurately expressing the deformation of each medium to reveal the change in the transport properties of a shale gas reservoir during production and to predict the reservoir recovery. The deformation of each medium is based on a combination of mechanical deformation and desorption-induced shrinkage behavior, and fracture aperture as well as the difference of effective stress in each medium were considered. We find that the transport properties of shale gas reservoirs are related to the porosity and diffusion coefficient of the organic matrix, and the porosity and permeability of the fracture system and inorganic matrix. The triple-porosity model is then incorporated into the governing equations for gas flow. The finite element method is used to solve the governing equations. By comparing the simulation result with field data, analytical solution, and previous simulation result, we are able to verify the accuracy of our model. Then, we analyze and reveal in detail how the transport properties of shale gas reservoirs change during extraction through the combined evolution of gas pressure, volumetric strain, and strain induced by the adsorption/desorption of gas in the organic matrix. Finally, we conduct sensitive analysis focusing on how the initial transport properties affect their evolutions. We find that the relationship between mechanical deformation and strain induced by adsorption/desorption is not always competitive during gas extraction.

Mastering of the Filling Stage in Low Pressure Sand Casting Process

In Low Pressure Casting (LPC), the counter gravity filling at low velocity and the protective gas atmosphere above the metal can potentially reduce gas and oxides entrapment in the metal. However, the relationship between the imposed gas pressure evolution and the melt filling dynamics cannot be analytically determined as it is geometry-dependent. This issue is the missing link to master and automate the filling step in LPC process. In this work, the filling dynamics is numerically investigated for different mold geometries and pressure ramps. The simulation, carried out using ANSYS Fluent® simulation software, is combined with an analytical model. As the results are quantitatively predictive of the filling flow, it permits to develop a numerical study, considering different sudden or progressive section changes and pressure ramps. The impact of the different process parameters on the flow dynamics is analyzed, particularly the transition smoothing impact.

The stage evolution characteristics of gas transport during mine gas extraction: Its application in borehole layout for improving gas production

Diffusion and seepage play a significant role in the mine gas extraction, while their influence degree is dynamic with the change of time or location, showing a notable dynamic stage characteristic. Therefore, it is significant to master the conversion node of gas transport for improving the gas production. However, it is difficult to determine the conversion node and master controlling roles during mine gas extraction due to the lack of judgment index. In this work, a dual-porosity model was constructed to describe the gas transport in coal seam. Then, a transfer coefficient ratio between gas diffusion and gas seepage was used to define as the conversion node. Furthermore, our model was validated by comparing with the previous model, showing that our model can better describe the evolution of gas pressure under different stress conditions. The influence of stress, initial permeability and initial diffusion coefficient on the conversion node were investigated. Results showed that the initial permeability shows the most notable influence on the conversion node, followed by the stress. The initial diffusion coefficient has the relatively complex effect on the conversion node depending on the specific reservoir conditions. Finally, the transfer coefficient ratio was used to determine the best distance of boreholes for improving the gas production. The research results are important for CBM and mine gas extraction.

Gas network development in a precompacted bentonite experiment: Evidence of generation and evolution

In a deep geological disposal facility for radioactive waste, precompacted bentonite is proposed as a sealing material for the isolation of boreholes, disposal galleries and deposition holes. The advective movement of repository gas in bentonite has been linked to the development of new porosity and propagation of dilatant pathways. For the first time we present a detailed analysis of stress field data during the generation and evolution of a gas network. A new experimental dataset, from a highly instrumented test, clearly shows the strong coupling between stress, gas pressure and flow in bentonite. Multiple discrete propagation events are observed, demonstrating spatial variability and time-dependency as permeability within the clay develops. Analysis of the stress data before, during and after gas entry indicates a heterogeneous stress field initially develops, resulting from the development of these pathways. The flow network is dynamic and continues to spatially evolve after gas entry, such that permeability under these conditions must be time-dependent in nature. Perturbation of the stress field is significant before all major gas outflow events, presumably resulting from the requirement to propagate an effective gas network before outflow is possible. In contrast, no major flow perturbations are detected which did not correlate with fluctuations in the stress field. The controls on the distribution and geometry of the resulting flow network are unclear, as well as its long-term evolution and stability. These will be beneficial in the assessment of gas pressure evolution as part of safety case development.

SAInt – A novel quasi-dynamic model for assessing security of supply in coupled gas and electricity transmission networks

The integration of renewable energy sources into existing electric power systems is connected with an increased interdependence between natural gas and electricity transmission networks. To analyse this interdependence and its impact on security of supply, we developed a novel quasi-dynamic simulation model and implemented it into the simulation tool SAInt (Scenario Analysis Interface for Energy Systems), the first published software application that allows the combined simulation of gas and electric power systems in a single time frame and simulation environment. The model is composed of a transient hydraulic simulation model for the gas system and an augmented AC-Optimal Power Flow model for the electric power system, which includes a model for dispatchable power system loads and considers time transitional constraints, such as the ramp rate and the start-up time of generation units. Both models take into account the control and constraints of the most relevant facilities present in both systems. The bidirectional interconnection between both systems is considered and established by coupling equations describing the fuel gas offtake for power generation in gas fired power plants, and the electric power supply to LNG terminals and electric driven compressors in gas compressor stations. The resulting system of equations for the combined model are solved in a single simulation time frame. In order to quantify the impact of different contingencies on the operation of the combined system, a number of security of supply parameters are proposed, which can be utilised to compare the impact of different contingencies on security of supply and the effectiveness of countermeasures to mitigate this impact. The capabilities of the combined model and the functionality of the simulation tool SAInt are demonstrated in a case study of a sample gas and power transmission system. Results indicate how the combined simulation of gas and electric power systems can give insight into important and critical information, such as the timing and propagation of contingencies cascading from one system to the other or the grace period to react to these contingencies. Such information can contribute to improving the coordination between gas and power transmission system operators in the event of a disruption, thus, increasing the resilience and the level of security of supply in the combined energy system. The information provided by the combined model cannot be obtained by the traditional co-simulation approach, where both systems are solved in different time frames. Furthermore, the studies stress the importance of using transient gas simulation models for security of supply analysis instead of steady state models, where the time evolution of gas pressure and linepack are not reflected appropriately.

Computational model and time–space evolution of gas pressure for different borehole lengths in coalbed methane drainage

We performed physical simulations of coalbed methane (CBM) drainage in a distressed coal-face zone with different lengths of borehole using a multi-field coupling test system. The relationship between gas pressures at different locations and the shortest distance to the borehole during CBM drainage was determined using the method of l1 data fitting based on particle swarm optimization. We developed a computational model to calculate gas pressures, which vary with time and position, which depend on the evolution of constant parameters with time. Gas pressures at different times and positions were calculated. We analyzed the distribution of gas pressures. The results indicate that the relationship between coalbed gas pressure and the shortest distance to the borehole is an exponential decay function; the gas pressure at all positions declines more rapidly when using a longer borehole; gas pressure is a function of time and position; the gas pressure fields of sections perpendicular to the borehole present as circular rings centered on the hole collar, while those of surfaces parallel to the borehole present as elliptical rings around the borehole; isobaric surfaces are funnel-shaped.

A poroelastic model for evolution of fractured reservoirs during gas production

Coupled processes between matrix deformation and fluid flow in fractured reservoirs such as gas shale are important for predicting fracture permeability change and reservoir production behavior. In contrast to sandstone reservoirs, the coupling between gas desorption and rock matrix/fracture deformation must be accounted for in poroelastic response of gas shale. For this purpose, a constitutive relation is developed and used in this work. The continuity equation of dry gas considers the mass conservation including both free and absorbed phases. The absorbed gas content and the sorption-induced volumetric strain are described through a Langmiur-type equation. A porosity model is developed and utilized in the governing equations, and a discrete system of fractures is included in flow and rock deformation analysis. The resulting field equations are solved using a finite element model based on the dual permeability method (DPM). The model is then used to investigate coupled desorption/diffusion-rock deformation phenomena including fracture deformation (both normal and shear deformation with dilation), and its impact on gas flow in naturally fractured reservoir. Simulation results show that the time evolution of gas pressure, effective stresses, fracture aperture, and permeability are strongly affected by gas desorption during production, especially in the near-wellbore region. Gas desorption retards the influence of the effective stress increase associated with pore pressure reduction during production. The combination of in-situ stress condition and gas desorption mechanism governs the fracture deformation behavior in fractured gas shale reservoirs.