Gas Migration Model Research Articles

A Case Study on the Gas Drainage Optimization Based on the Effective Borehole Spacing in Sima Coal Mine

Based on the dynamic expressions of permeability and porosity of the coal seam derived in the paper, a multiphysical field coupling numerical model of gas migration under the interaction of stress field and seepage field was established. The gas drainage project #3 Coal Seam operated by Sima Coal Industry Co., Ltd., was selected as the study object. Taking different drainage time periods in various positions of drainage holes into consideration, combined with the advance situation of the 1207 working face in the Sima Coal Mine, a mixed layout gas drainage scheme featured with the effective borehole spacing was obtained through the COMSOL multiphysics simulation. In addition, a series of field industrial tests were performed to validate the research result, revealing that comprehensively considering the extraction time of coal and optimizing the layout of extraction boreholes can effectively improve the engineering economic benefits.

Modeling for gas dissolution and diffusion in water-intrusion coal seam and its potential applications

In order to determine the gas pressure for water-intrusion borehole, a new method of gas pressure measurement in water-intrusion borehole based on gas solubility was proposed. A mathematical model of gas migration in water-intrusion borehole considering the effective diffusion coefficient was established. The influencing factors, such as water-intrusion radius, diffusion coefficient and solution convection, on the dissolution equilibrium of gas in borehole water were investigated. Finally, the feasibility of the new gas pressure measurement method was discussed. Results showed that the gas diffusion coefficient and water-intrusion radius of coal seam are the key factors influencing the equilibrium duration of gas dissolution. During the accumulation stage of formation water in borehole, the gas is able to quickly dissolve and diffuse in borehole water. The amount of dissolved gas can reach more than 90% of solubility in three days. After the formation water enters the coal seam, the equilibrium time grows sharply because the effective diffusion coefficient of gas in water-saturated coal is much lower than that in pure water. In addition, the water convection causes a higher amount of gas dissolved at the borehole bottom. Finally, the field applicability for the new method of pressure measurement were analyzed by combining the data from previous works. This work provided a new idea for the gas pressure measurement of water-intrusion borehole.

Discrete Dilatant Pathway Modeling of Gas Migration Through Compacted Bentonite Clay

A coupled multiphase fluid flow and discrete fracturing model is applied to simulate bench-scale gas migration experiments on compacted bentonite. The numerical modeling is based on the linking of the multiphase fluid flow simulator TOUGH2 with a Rigid-Body-Spring Network model, which enables a discrete (lattice) representation of elasticity and individual fractures. The evolution of a complex network of dilatant flow paths is modeled through opening and breakage of lattice interface bonds between porous-elastic matrix elements. To achieve a good match with the experimental results, including an abrupt gas breakthrough along with pressure and stress responses, it was necessary to calibrate model parameters for (1) air-entry pressure, (2) shear and tensile failure of lattice interface bonds, (3) moisture swelling/shrinkage effects on stress, and (4) aperture-dependent permeability of dilatant flow paths. Our best-fit conceptual model considers a pervasive network of discrete flow paths propagating from the gas injection point, whereas some of the experimental data indicate the potential for heterogeneous and unstable flow paths.

Numerical analysis of permeability rebound and recovery during coalbed methane extraction: Implications for CO2 injection methods

The coal’s permeability plays a crucial role in coalbed methane (CBM) extraction and coal seam CO2 sequestration. An accurate understanding of permeability rebound and recovery is therefore essential. This study establishes an improved fully coupled gas migration model for CBM extraction. The permeability rebound and recovery times as well as rebound values are proposed to accurately quantify permeability evolution during CBM extraction. The evolution of these three parameters under the influence of different factors are evaluated in detail, such as initial gas pressure, the diffusion coefficient, and the permeability. The results show that the permeability rebound and recovery times increase along with initial gas pressure and the amount over time rises rapidly under high gas pressures. As the initial gas pressure increases, the permeability rebound value decreases. However, initial diffusion coefficient and permeability have a negative trend in permeability rebound, recovery time, and rebound value. These tendencies are particularly large for low initial permeabilities and diffusion coefficients, yet the change in rebound time is smaller than the one in recovery time. Finally, inspired by the relationship between permeability rebound and gas pressure change during CBM extraction, the evolution of coal seam permeability under different CO2 injection method is discussed. A stepwise increasing-pressure CO2 injection method is also proposed, which could effectively increase the volume of CO2 sequestered and reduce project costs. Therefore, our findings shall shed light on improving coal mine safety production and reducing greenhouse gas emission.

An Innovative Finite Tube Method for Coupling of Mine Ventilation Network and Gob Flow Field: Methodology and Application in Risk Analysis

Explosions and fires originated from longwall gob due to the formation of methane-air mixture have been a severe threat to coal miner’s lives. Many numerical studies on coal mine fire and explosion hazards have focused on the airflow in roadways and mine gobs. However, most of these studies isolate the gob from its surrounding roadways, and the network analysis and the CFD method are applied independently to model the two classes of airflows. This approach greatly limits the ability of simulating mine ventilation flow, especially unable to consider the effects of gob boundary conditions on air exchange between the gob and the surrounding airways. An innovative finite tube method (FTM) is developed to couple the one-dimensional mine ventilation network (MVN) and the 2D/3D gob flow field (GFF). In FTM, GFF is discretized into a finite number of flow tubes each of which is formed by any two adjacent stream lines. These tubes, representing the gob’s field flow, connect the MVN into a new coupling network. To solve the coupling model between MVN and GFF, an iterative solution technique is developed in which the MVN analysis is used to evaluate the boundary pressures for GFF simulation and in turn the FTM feeds the GFF results back to the coupling network. Based on the FTM approach, a model for gas migration in gob has been established for delineating the hazard zones of explosive methane concentration and spontaneous combustion. A computer program is developed to implement the FTM simulation. An illustrative example with five flow tubes representing the GFF is created to verify the stability and convergence of the FTM solution process. A simulation example also indicates that the accuracy of FTM is improved by 12% compared with previous method. Results of an application case show that the program is capable of quantitatively evaluating the gob’s risk zones prone for spontaneous combustion and gas explosion as well as performing risk analysis for various ventilation scenarios.

Numerical Modeling of Gas Migration and Hydrate Formation in Heterogeneous Marine Sediments

The formation of marine gas hydrates is controlled by gas migration and accumulation from lower sediments and by the conditions of the hydrate stability zone. Permeability and porosity are important factors to evaluate the gas migration capacity and reservoir sealing capacity, and to determine the distribution of hydrates in the stable region. Based on currently available geological data from field measurements in the Shenhu area of Baiyun Sag in the northern South China Sea, numerical simulations were conducted to estimate the influence of heterogeneities in porosity and permeability on the processes of hydrate formation and accumulation. The simulation results show that: (1) The heterogeneity of the hydrate stability zone will affect the methane migration within it and influence the formation and accumulation of hydrates. This is one of the reasons for the formation of heterogeneous hydrates. (2) When the reservoir is layered heterogeneously, stratified differences in gas lateral migration and hydrate formation will occur in the sediment, and the horizontal distribution range of the hydrate in a high porosity and permeability reservoir is wider. (3) To determine the dominant enrichment area of hydrate in a reservoir, we should consider both vertical and lateral conditions of the sedimentary layer, and the spatial coupling configuration relationships among the hydrate stability region, reservoir space and gas migration and drainage conditions should be considered comprehensively. The results are helpful to further understand the rules of hydrate accumulation in the Shenhu area on the northern slope of the South China Sea, and provide some references for future hydrate exploration and the estimation of reserves.

Coupled heat and mass transfer model of gas migration during well cementing through a hydrate layer in deep-water regions

During well cementing through a hydrate layer, hydrate dissociation occurs because of the heat generated during the cement hydration reaction, which causes gas migration into the wellbore. Given the shortcomings of extant studies, the present study proposes a coupled heat and mass transfer model to investigate the gas migration occurring during well cementing through a hydrate layer. The coupled model considers the complex interactions among the cement sheath thermal model, the hydrate layer thermal model, the multiphase flow in the hydrate layer, and the gas migration model in the cement sheath. The proposed models are validated through three experiments from literature, which exhibit good agreement with the proposed model. In addition, numerical simulations are conducted using a simulation well. According to the simulation results, the heat exchange between the cement sheath and hydrate layer leads to hydrate dissociation, which is the primary cause of gas migration. Gas migration results in a high casing pressure, which poses a serious threat to the safety of the workers in the future. Conclusions and suggestions are drawn from the case study, which can assist engineers to improve the safety of workers.

The hydrogen migration in cryogenic tubes of superconductive accelerator with gas source

In superconductive linear accelerator, the performance and stability can be impacted by gas adsorbed on cryogenic surfaces adversely. The cryogenic devices usually work at 4.2 K or 1.9 K and are refrigerated by normal or super fluid liquid helium, respectively. The purpose of this paper is to study the character of gas migration in cryogenic tubes. Adsorption coefficient for hydrogen at 4.2 K is measured by experimental study. Then, a gas migration model is established based on the experimental results to depict the hydrogen migration process in cryogenic tubes. The experimental results and model analysis indicated that at cryogenic temperature (4.2 K), adsorption coefficient for hydrogen is very close to 1, which is several orders higher than the adsorption coefficient at room temperature, resulting in a unique pressure distribution pattern in cryogenic tubes when compared with the pressure distribution in room temperature tubes. At 4.2 K or 1.9 K, the gas migration process is obviously different from the process at room temperature and is significantly affected by the gas adsorption on the cryogenic surfaces. The model established in this article can be applied not only to hydrogen migration in cryogenic tubes but also to other gas migration in tubes with high adsorption coefficient.

Development of a mathematical model for gas migration (two-phase flow) in natural and engineered barriers for radioactive waste disposal

Abstract In a deep geological repository (DGR) for the long-term containment of radioactive waste, gases could be generated through a number of processes. If gas production exceeds the containment capacity of the engineered barriers or host rock, these gases could migrate through these barriers and potentially expose people and the environment to radioactivity. Expansive soils, such as bentonite-based materials, are currently the preferred choice of seal materials. Understanding the long-term performance of these seals as barriers against gas migration is an important component in the design and long-term safety assessment of a DGR. This study proposes a hydro-mechanical linear poro-elastic visco-capillary mathematical model for advective-diffusive controlled two-phase flow through a low-permeability expansive soil. It is based on the theoretical framework of poromechanics, incorporates Darcy's Law for both the porewater and poregas, and a modified Bishop's effective stress principle. Using the finite element method (FEM), the model was used to numerically simulate 1D flow through a low-permeability expansive soil. The results were verified against experimental results found in the current literature. Parametric studies were performed to determine the influence on the flow behaviour. Based on the results, the mathematical model looks promising and will be improved to model flow through preferential pathways.

Improved model for gas migration velocity of stagnant non-Newtonian fluids in annulus

Currently, analysis of the Sustained Casing Pressure (SCP) tests or other gas migration problems in the wellbore is limited because the mathematical models employed by most researchers are based on Newtonian fluids, which is not consistent with the actual situation that the mud is non-Newtonian fluid in the Annulus. To ensure the operators a realistic and accurate diagnostic test, the improvement of current gas-migration models is of great significance. In this study, a new model of gas migration in non-Newtonian power law fluids was established. This new model incorporates the correlation between swarm bubbles velocity and drag coefficient, and combines the petroleum correlations and the drift-flux model to determine the friction pressure gradient. In addition, the gas slip velocity in non-Newtonian power law fluids is rigorously determined by the proposed calculation procedure using an iteration approach. Furthermore, improvements of the new model are analyzed and discussed in details. During the process of the hypothetical parameters determination (Initial gas chamber length, initial gas concentration in liquid column, mud compressibility, cement permeability, and formation pressure) for simulation, the same values of parameters with previous study are tried to be utilized to minimize the difference. Results in this work indicate that the big discrepancy of matched parameters between the new model and previous model are generated from the mud compressibility, which is attributed to the gas holdup. The higher gas holdup obtained from the new model manifest the validity of proposed model, which is caused by the higher viscosity of non-Newtonian power law fluids. The new model is suitable for relatively low gas holdup (α<0.25), where the bubble flow prevails in annuli. This work provides significant and necessary improvements for gas migration. For the first time, the problem of multi-phase flow in the SCP problem is accomplished by introducing calculation approach of non-Newtonian fluids. This calculation approach can be extended to calculations of other multiphase flow problems in the wellbore.

Simulation investigation of N2-injection enhanced gas drainage: Model development and identification of critical parameters

The performance of pre-gas drainage in low permeable seams especially in CO2 abundant coal seams is not satisfactory. Due to the high affinity of coal to CO2, much lead time is needed for gas drainage boreholes to reduce the coal seam gas content below the threshold limit value (TLV), thus meeting the demand of the scheduled mining activities. In this study, simulations of N2 injection enhanced gas drainage were carried out for CO2-rich coal seams. A binary gas migration model was developed to illustrate gas migration in coal seam. The simulation results matched well with the laboratory test results, indicating that this model could be used to investigate the mechanism of N2 flushing coal seam gas. The parameters affecting the flushing performance were then studied using this model, including coal seam permeability, gas diffusion coefficient, N2 injection pressure andin-situ gas content. These results suggest that coal seam permeability and CO2 diffusion coefficient are the key parameters having significant impacts on the drainage performance whilst N2 diffusion coefficient only has negligible effect. A critical parameter theory was developed to describe the correlation between these parameters (especially coal seam permeability and the diffusion coefficient of coal seam gas). Specifically, in low permeable coal seams, permeability was the critical parameter and the effects of other parameters were insignificant. Similarly, if the diffusion coefficient of coal seam gas was very low, it could become a critical parameter.

Modeling of Gas Migration Through Low-Permeability Clay Rock Using Information on Pressure and Deformation from Fast Air Injection Tests

The characterization of gas migration through low-permeability clay formations has been a focus of R&D programs for radioactive waste disposal, which is also of great importance for shale gas exploration, cap-rock behavior of hydrocarbon reservoirs, and hbox {CO}_{2} sequestration. Laboratory tests have been performed on Opalinus Clay, a Mesozoic claystone that is being investigated in Switzerland as a potential host rock for the storage of nuclear waste. The laboratory program included specific water and air injections tests, as well as oedometer and isotropic compression tests. Undisturbed core samples have been retrieved from a shallow borehole in the Mont Terri Underground Research Laboratory (URL) and from a deep borehole in northern Switzerland. For the shallow cores from Mont Terri URL, largely linear-elastic deformations associated with the gas injection test could be inferred and the change in void ratio was accounted for by the pore compressibility. The corresponding change in permeability was obtained from the results of the water tests, indicating a log-linear relation between permeability and porosity. The derived porosity change and the corresponding change in permeability were implemented in the standard TOUGH2 code, which reproduced the measured gas test results using fitted water retention data derived from laboratory measurements. Similar air injection tests performed on Opalinus Clay cores from the borehole at greater depth showed overall similar behavior, but at lower porosities, lower permeability values, and lower compressibility. These cases indicated nonlinear behavior which was implemented using an effective stress-dependent porosity change and the associated change in permeability. In addition, the anisotropy associated with the bedding planes of the clay formation was considered by assuming different properties for “soft” and “hard” layers to account for storage capacity for the injected gas prior to gas breakthrough. The computed change in the overall porosity could be compared to the measured axial deformation during the gas injection test and was used for calibration of the parameters describing the relationship between the effective stress and porosity, as well as the corresponding change in permeability and capillary pressure.

Modeling of gas migration in water-intrusion coal seam and its inducing factors

Formation water intrusion into the coal seam through cross drainage boreholes seriously affects the gas extraction efficiency. Particularly in the downward boreholes, the formation water cannot be discharged through the drain. Using the conventional seepage model to estimate the effect of gas drainage will cause that the coal seam residual gas pressure is underestimated. In this paper, we construct the gas migration models in a water-intrusion coal seam, which include the gas seepage in original coal seam and the gas diffusion in water-saturated coal seam. By comparing with the field data, the mathematical models are authenticated. Then, the influences of diffusion coefficient, permeability, Henry’s constant and the radius of water-saturated coal seam on the residual gas pressure and gas production are analyzed by numerical calculations. The results show that the gas pressure decreases with the increase of the diffusion coefficient, while increases with the increase of Henry’s constant and the radius of the water-saturated coal seam. The impact of permeability is more complex. In the high permeability coal seam, the overall pressure relief effect is better. However, in the low permeability coal seam, the pressure relief effect around the borehole is better. Finally, the influence mechanism of each factor is analyzed to revel the gas production in the water-intrusion coal seam. The results of the work are of great significance to enrich the theory of gas migration and the prevention and control of mine gas disaster.

Progress on the hydraulic measures for grid slotting and fracking to enhance coal seam permeability

A method of hydraulic grid slotting and hydraulic fracturing was proposed to enhance the permeability of low permeability coal seam in China. Micro-structural development and strength characteristics of coal were analysed to set up the failure criterion of coal containing water and gas, which could describe the destruction rule of coal containing gas under the hydraulic measures more accurately. Based on the theory of transient flow and fluid grid, the numerical calculation model of turbulence formed by high pressure oscillating water jet was used. With the high speed photography test, dynamic evolution and pulsation characteristics of water jet water analysed which laid a foundation for mechanism analysis of rock damage under water jet. Wave equation of oscillating water jet slotting was established and the mechanism of coal damage by the impact stress wave under oscillation jet was revealed. These provide a new method to study the mechanism of porosity and crack damage under high pressure jet. Fracture criterion by jet slotting was established and mechanism of crack development controlled by crack zone between slots was found. The fractures were induced to extend along pre-set direction, instead of being controlled by original stress field. The model of gas migration through coal seams after the hydraulic measures for grid slotting and fracking was established. The key technology and equipment for grid slotting and fracking with high-pressure oscillating jet were developed and applied to coal mines in Chongqing and Henan in China. The results show that the gas permeability of coal seam is enhanced by three orders of magnitude, efficiency of roadway excavation and mining is improved by more than 57% and the cost of gas control is reduced by 50%.

Numerical simulation of gas migration into mining-induced fracture network in the goaf

Gas extraction practice has been proven for the clear majority of coal mines in China to be unfavorable using drill holes in the coal seam. Rather, mining-induced fractures in the goaf should be utilized for gas extraction. To study gas migration in mining-induced fractures, one mining face of 10th Mine in Pingdingshan Coalmine Group in Henan, China, has been selected as the case study for this work. By establishing the mathematical model of gas migration under the influence of coal seam mining, discrete element software UDEC and Multiphysics software COMSOL are employed to model gas migration in mining-induced fractures above the goaf. The results show that as the working face advances, the goaf overburden gradually forms a mining-induced fracture network in the shape of a trapezoid, the size of which increases with the distance of coal face advance. Compared with gas migration in the overburden matrix, the gas flow in the fracture network due to mining is far greater. The largest mining-induced fracture is located at the upper end of the trapezoidal zone, which results in the largest gas flux in the network. When drilling for gas extraction in a mining-induced fracture field, the gas concentration is reduced in the whole region during the process of gas drainage, and the rate of gas concentration drops faster in the fractured zone. It is shown that with gas drainage, the gas flow velocity in the mining-induced fracture network is faster.

Gas migration model based on overburden strata fracture evolution law in three dimensional mine-induced stress conditions

Based on the fracture features obtained from similarity simulation excavation experiment in 3D mine-induced stress condition, the gas migration model in gob area was set up, and numerical simulation using COMSOL Multiphysics software on gas migration law was studied. The research results show that: Gas pressure variety gradient and flow velocity are obviously influenced by fracture distribution shape. Gas concentration distribution presents the rounded rectangle shape in the lower strata, while in higher strata it tends to be a "O shape” morphology which is similar to the fracture form in strata above the gob. Besides, the pressure relief gas mainly accumulates in the working face fracture field, especially in the higher overlying strata. In the compaction field, gas concentration distribution form presents a “saddle” shape morphology in the vertical section, and gas concentration in boundary fracture zone is higher than in the compaction zone. It is remarkable that gas concentration in start-up fractured zone is also relatively high, and with the advance of working face, its gas enrichment degree decreases. The results of the research have an important guiding significance for coal and gas simultaneous extraction.

Deep Coal Mine Methane Drainage in China with Lower Greenhouse Gas Emissions: Insights of Borehole Protection Technology

Climate warming has currently been one of the most important global environmental issues. Coal mine methane (CMM) is a typical greenhouse gas with higher global warming potential and ozone depletion potential than CO2. Unfortunately, it should be noted that the CMM emission increases rapidly with the increasing coal consumption in China. Fast and whole protection technology of borehole is a recently developed method to maintain the drainage efficiency in deep mining level. However, the potential negative impact of screen pipes on CMM drainage efficiency has not been well studied. To investigate this impact, an innovative coal permeability model from elastic to post-failure state was deduced to develop our previous gas migration model. Then redistributed stress, coal permeability and gas pressure around a borehole were studied by implementing the mathematical model into Comsol Multiphysics. Numerical results indicate that the negative impact of screen pipes on drainage subpressure do not affect the drainage efficiency due to the stress redistribution. Engineering application shows that comparing to the traditional borehole protection technology, the gas concentration increases about 120% and the gas flux increases about 110% by using the fast and whole protection technology. The research will provide theoretical foundation of CMM capture and borehole protect technology and is of great significance for the CMM utilization and the global environment.

Gas migration modelling in geological repository module in clay formation and sensitivity analysis

Corrosion of steel canisters disposed of in a geological repository for high-level waste leads to generation and accumulation of hydrogen gas which may significantly affect long-term safety of the repository. Numerical modelling can be used to predict the likely hydraulic evolution of such a disposal facility and to estimate the influence of generated gas on repository safety.This article presents a two-phase flow (not taking into account mechanical coupling) gas migration model developed in the original version of the computer code TOUGH2-MP (USA). An important and new aspect of this model is its capability to introduce a network of very thin interfaces in three-dimensional analysis of gas migration. This model was verified in the international benchmark exercises in the recent European Commission project FORGE (Fate Of Repository GasEs). The analysis was focused on the results of a second benchmark exercise, i.e. hydrogen gas behaviour in a part of a geological repository (module) situated in clay formation, and sensitivity analysis. The results showed that the interfaces, backfill and excavation disturbed zone are the main migration and accumulation media for gaseous hydrogen in the repository module. The peak pressure in the module did not exceed the geological environment pressure at a depth of 500m, and is not sufficient to disturb mechanical stability and functionality of the system of engineered and natural barriers.

Modeling of H2S migration through landfill cover materials

The emission of H2S from landfills in the United States is an emergent problem because measured concentrations within the waste mass and in ambient air have been observed at potentially unsafe levels for on-site workers and at levels that can cause a nuisance and potentially deleterious health impacts to surrounding communities. Though recent research has provided data on H2S concentrations that may be observed at landfills, facility operators and landfill engineers have limited predictive tools to anticipate and plan for potentially harmful H2S emissions. A one-dimensional gas migration model was developed to assist engineers and practitioners better evaluate and predict potential emission levels of H2S based on four factors: concentration of H2S below the landfill surface (C0), advection velocity (v), H2S effective diffusion coefficient (D), and H2S adsorption coefficient of landfill cover soil (μ). Model simulations indicated that H2S migration into the atmosphere can be mitigated by reducing H2S diffusion and advection or using alternative cover soils with a high H2S adsorption coefficient. Laboratory column experiments were conducted to investigate the effects of the four parameters on H2S migration in cover soils and to calculate the adsorption coefficient of different cover materials. The model was validated by comparing results with laboratory column experiments. Based on the results, the laboratory column provides an effective way to estimate the H2S adsorption coefficient, which can then be incorporated into the developed model to predict the depth of cover soil required to reduce emitted H2S concentrations below a desired level.

On persistent primary variables for numerical modeling of gas migration in a nuclear waste repository

Numerical simulation of gas migration driven by compressible two-phase partially miscible flow in porous media is of major importance for safety assessment of deep geological repositories for long-lived high-level nuclear waste. We present modeling of compositional liquid and gas flow for numerical simulations of hydrogen migration in deep geological radioactive waste repository based on persistent primary variables. Two-phase flow is considered, with incompressible liquid and compressible gas, which includes capillary effects, gas dissolution, and diffusivity. After discussing briefly the existing approaches to deal with phase appearance and disappearance problem, including a persistent set of variables already considered in a previous paper (Bourgeat et al., Comput Geosci 13(1):29–42, 2009), we focus on a new variant of the primary variables: dissolved hydrogen mass concentration and liquid pressure. This choice leads to a unique and consistent formulation in liquid saturated and unsaturated regions, which is well adapted to heterogeneous media. We use this new set of variable for numerical simulations and show computational evidences of its adequacy to simulate gas phase appearance and disappearance in different but typical situations for gas migration in an underground radioactive waste repository.