Coal Burial Depth Research Articles

Heat-mass transfer coupling effects in water-ice phase transformation of water-bearing coal frozen with liquid nitrogen

• The equation of coal freezing and water-ice phase transformation under the action of cryogenic LN 2 is derived. • A new discretization method is proposed to solve the equation of coal temperature and water-ice phase transformation. • The water-ice phase transition law of LN 2 flowing through water-bearing coal is studied quantitatively. • The influence of ice water conversion process on temperature distribution in water-bearing coal at low temperature is analyzed. • The law of heat and mass transfer in the process of LN 2 flowing through water-bearing coal is revealed. Liquid nitrogen (LN 2 ) fracturing, as a waterless and environmentally friendly fracturing technology, has a broad application prospect in coalbed methane (CBM) exploitation. Currently, there is a lack of sufficient understanding of the heat mass transfer mechanism of LN 2 fractured coal mass. In this paper, the heat-flow coupling problem of LN 2 frozen coal mass was systematically studied by means of numerical simulation. A fluid-solid coupled multiphase module was mathematically modeled on LN 2 flow, coupling between LN 2 and coal mass temperature, and the variation law of coal temperature and water-ice phase with time and geometric position during LN 2 flow was obtained under the coal seam freeze-thaw simulation analysis program by FORTRAN language. The results showed that the most obvious heat mass transfer occurred in the coal mass area close to the LN 2 inlet and the fluid-solid interface. Along the direction of coal burial depth, the changes of temperature and water-ice phase are broadly divided into rapid change zone, slow change zone and stable zone, and along the horizontal direction of LN 2 flow, the temperature and unfrozen water volume first peaked, then decreased and gradually stabilized, while the ice content was diametrically opposed, which indicating that the change of temperature and water-ice phase have significant anisotropic characteristics, and the hydrothermal coupling effect out to be one of the most important factors affecting the effect of LN 2 fracturing coal. The main findings of this study are the keys to the research of LN 2 fracturing physical mechanisms in CBM reservoirs.

Study on Adsorption Model of Deep Coking Coal Based on Adsorption Potential Theory

With the exhaustion of coal resources in shallow coal seams, many mining areas have moved to deep mining, and the coal storage environment is obviously affected by the mining depth, mainly manifested as the increase of gas pressure and temperature, which makes the adsorption characteristics of deep coal seam gas much more complicated than shallow coal seam. Based on this, this paper chooses Pingdingshan coking coal as the research object, using Hsorb-2600 high-temperature and high-pressure gas adsorption instrument to carry on isothermal adsorption experiment. According to the adsorption theory and the uniqueness of the adsorption characteristics cure, the adsorption model was analyzed and studied. The results show that the predicted curve of coal seam gas adsorption isotherm is in good agreement with the measured curve, the relative error is less than 10%, and the adsorption characteristic curve is logarithmic. At the same time, the model is used to study the variation of adsorbed gas amount with mining depth. The results show that the adsorbed gas amount increases first and then decreases with coal burial depth.

Effective Methane Extraction Radius after High-Pressure Water Jet Slotting

The effective radius of methane extraction after high-pressure water jet slotting is the most important parameter for borehole optimization and extraction time planning. We applied a steady flow model and thermal-hydrological-mechanical (THM) coupling model to calculate the effective radius after high-pressure water jet slotting. Field measurements at the Zhongliangshan coal mine show that both the steady flow model and the THM coupling model can accurately represent the effective radius, and the THM coupling model provides further information regarding extraction time. After that, a variety of factors, including extraction time, coal burial depth, slot radius, initial permeability, and initial methane pressure, are discussed. The effective radius of a slotted borehole is 1.94 times larger than that of a conventional borehole.

Analysis of coalbed methane occurrence in Shuicheng Coalfield, southwestern China

This article is the result of authors’ long-term efforts to evaluate the occurrence of coalbed methane (CBM) for CBM exploration and coal gas disaster prevention in Shuicheng Coalfield (SCC). A relatively large program of exploratory drilling to determine the content of CBM, geological surveys and laboratory experiments, including coal proximate analysis, coal physical properties analysis and methane adsorption were conducted. The results show that the coals from the SCC contain 21.42–41.34% volatile, 0.24–1.44% moisture and 6.27–38.1% ash yield, with coal porosity in the range of 5.97–14.89%. The type of coal structure ranges from II to IV. The sedimentary characteristics, tectonic evolution, geological structure, coal reservoir conditions and hydrodynamics were studied via geological surveys, field research and laboratory tests. The relationships between these factors and gas content are explained and analyzed in this present paper. The formation of the Shuicheng coal basin was formed under the control of the Ziyun fracture and the Luoping fracture. Ejective folds and synclines that evolved from inverted geological structures in the stage of Yanshan movement affect CBM occurrence. Folds generated in coal bearing strata play an important role in methane accumulation in the period of Himalayan movement. A positive relationship is observed between gas content and coal burial depth, coal thickness, and hydrodynamics. CBM easily concentrates because of the separation between coal bearing strata and Maokou enrich aquifer. We divide the SCC into three methane geological units, the Ertang syncline unit, the Dahebian syncline unit and the Xiaohebian syncline unit. Furthermore, gas content spatial distributions were predicted in the SCC. The probability of CBM exploration was evaluated based on coal permeability and gas flow attenuation coefficient. These research results could contribute to CBM exploration and coal gas disaster prevention in the SCC.

Characteristic of In Situ Stress and Its Control on the Coalbed Methane Reservoir Permeability in the Eastern Margin of the Ordos Basin, China

Coalbed methane (CBM) development faces many challenges, among which in situ stress and permeability are two of the most important and fundamental factors. Knowledge of the characteristics of these factors is crucial to CBM exploration and development. Based on measured injection/falloff and in situ stress well test data of 55 CBM wells in the eastern margin of the Ordos Basin, correlations between parameters including initial reservoir pressure, in situ stress, lateral stress coefficient, well test permeability, and burial depth were determined. The distribution of in situ stress was analyzed systematically and its influence on permeability was also addressed. The results indicate that the maximum horizontal principal stress (σ H 10.13–37.84 MPa, average 22.50 MPa), minimum horizontal principal stress (σ h 6.98–26.88 MPa, average 15.04 MPa) and vertical stress (σ v 12.30–35.72 MPa, average 22.48 MPa) all have positive correlations with coal burial depth. Stress ratios (σ H/σ h, σ H/σ v, and σ h/σ v) and lateral stress coefficient slowly attenuated with depth. With increase of horizontal principal stresses, coal reservoir permeability (0.01–3.33 mD, average 0.65 mD) decreases. The permeability variation is basically consistent with change of stress state at a certain burial depth, the essence of which is the deformation and destruction of coal pore structures under the action of stresses. Three types of stress fields exist in the area: in the shallow coal seam at burial depths <700 m, the horizontal principal stress is dominant, revealing a strike slip regime (σ H > σ v > σ h), with average permeability 0.89 mD; from 700 to 1000 m depths, there is a stress transition zone (σ H ≈ σ v > σ h) with average permeability 0.73 mD; in the deep coal seam with burial depths >1000 m, the vertical principal stress is dominant, demonstrating a normal stress regime (σ v > σ H > σ h) with average permeability 0.11 mD.

Study of high-pressure sorption of methane on Chinese coals of different rank

To investigate the sorption and diffusion behavior of deep coals, high-pressure sorption experiments of methane on coals were performed by the volumetric method. The experimental sorption isotherms fit the Langmuir model over the experimental pressure and temperature ranges. The sorption volumes of all coals tested exhibit a typical temperature behavior with a negative exponent decreasing as temperature increases. An approximately linear correlation for the methane Langmuir volume with coal rank was observed. The effect of coal rank on adsorption volume decreases with increasing temperature. The Langmuir pressure decreases initially with coal rank, reaches a minimum pressure corresponding to the maximum vitrinite reflectance at ∼2.2 % and then increases. Studies on the diffusion of methane in coal using a unipore diffusion model showed that the effective diffusion coefficients for the seven coals studied varied from 2.98 to 68.3 × 10−5 s−1. The effective diffusion coefficients of coal at the first pressure step generally increased linearly with increasing temperature, and a complex nonlinear relationship for methane sorption rate with coal rank was observed. Finally, an empirical equation was developed to estimate the sorption capacity of methane on coal of a given rank as a function of the coal burial depth in a time-invariant pressure and temperature field. The sorption capacity of the moisture-equilibrated coal was found to increase with burial depth until it reaches a maximum of 24 cm3/g at ∼1,500 m, followed by a slow decline to 20.5 cm3/g at approximately 3,000 m.

Characteristics of Coal and Gas Outburst within Gas Geological Units in Qidong Mine

The precise mechanism of the coal and gas outburst is still unresolved, but productive practices in Qidong mine have testified that the coal and gas outburst accidents are predominantly associated with the belt of geological structure, the zone of tectonic coal occurrence and the area of the stress concentration. So it is very important to study characteristics of coal bed gas geology for coal bed gas prediction and prevention. Using gas-geology theory and method, the research focused on the factors of coal and gas outburst, such as coal bed gas occurrence, stress distribution, tectonic coal occurrence and magmatic rock presence within Qidong mine. The result shows that: Qidong mine district could be divided into 4 geological units by 3 faults as the boundary, the faults of Weimiao fault, F2, and F5; there are different conditions and main controlling factors of coal bed gas occurrence within four geological units. The unit I of geology: coal bed gas occurrence conditions are simpler, mainly contacted with the coal burial depth; the unit II: the conditions are more complex, predominantly associated with fault structures; the unit III: as a result of the presence of igneous intrusions, coal bed gas content in unit III is the highest within the whole mine; the unit IV: major factor of the conditions is the fold caused by strata uplift, which also leads to a large number of gas emissions. 25 coal and gas outburst accidents all occurred within the unit II of geology.

Evaluation of the reservoir permeability of anthracite coals by geophysical logging data

Permeability is one of the most significant reservoir parameters. It is commonly obtained by experiment, history simulation, injection/falloff well test and geophysical logging. Among these, geophysical logging remains as the most economic and efficient technique in evaluating coal permeability in the vicinity of an open-hole. In this paper, geophysical logging data are used to evaluate the coal reservoir permeability for the No. 3 coal seam in the southern Qinshui Basin (Fanzhuang and Zhengzhuang coal zones). Ideally coal reservoirs consist of coal matrix and fracture networks that can be represented by a model called a collection of sheets. Based on the model, coal reservoir permeability can be quantitatively calculated using the theoretical formula of kf = 8.50 × 10− 4 w2φf, in which fracture width (w) and fracture porosity (φf) were obtained by dual laterolog and density logging data, respectively. Calculative results show that coal reservoir permeability ranged from 0.017 mD to 0.617 mD for the Fanzhuang coal zone and from 0.047 mD to 1.337 mD for the Zhengzhuang coal zone. The permeability decreases with coal burial depth, reflecting variations in penetration capability of coal reservoirs at varying depths. Comparing results with those from injection/falloff well tests, however, shows that the model-calculated permeability is slightly higher. This is expected because the model did not include the influence from coal anisotropy.

Fengfeng Coalfield Gas Occurrence Regularity and Influence Factors

The article describes the basic situation of Fengfeng coalfield. Based on gas geological theory, combined with gas geological data in the actual production process, the coal seam gas occurrence law and the relationship among the gas occurrence, geological structure, coal thickness, coal bed rock and coal burial depth have been studied. This study will have important significance for the mine safety and gas control.