Quantitative characterization of low-rank coal reservoirs in the southern Junggar Basin, NW China: Implications for pore structure evolution around the first coalification jump

Coal macrolithotypes are closely correlated with coal macerals and pore–fracture structures, which greatly influence the changes in gas content and the coal structure. Traditional macrolithotype identification in coalbed methane (CBM) wells mostly depends on core drilling observation, which is expensive, time-consuming, and difficult for broken core extraction. Geophysical logging is a quick and effective method to address this issue. We obtained coal cores from 75 wells in the deep regions of the Jiaozuo Coalfield, northern China, quantitatively analyzed the logging cutoff number corresponding to various macrolithotypes, and established natural γ (GR), deep lateral resistivity (LLD), and γ–γ log (GGL) response rules for each coal macrolithotype. The formation mechanisms of different coal macrolithotypes are discussed from the perspective of coal facies and pore structures. The results show that GGL decreased but GR and LLD increased from bright coal to dull coal. Most coal macrolithotypes can be distinguished based on the established thresholds of various logging curves. However, excessively high or low ash yields significantly affect the validity of identification. The vertical coal macrolithotypes attributed to the peat marsh environment in Shanxi Formation mostly comprise three to six sublayers; dull or semi-dull coals are predominant close to the 2 1 coal seam, and the bright or semi-bright types usually appear in the middle part. The semi-bright and bright coals are usually vitrinite rich, whereas the semi-dull and dull coals are primarily inertinite rich. For pore structure arguments, the highest average specific surface area ( S BET ) and the total pore volume ( V BJH ) are found in bright coals, followed by dull and semi-bright coals; those of semi-dull coals are the lowest. However, S BET and V BJH change significantly for different samples, even though the coal macrolithotype is the same. Therefore, the macrolithotype is not the key factor determining the coal parameters of pore structures. Rapid and effective identification of coal macrolithotypes can help determine the CBM enrichment area, the CBM well location, and the exploration horizon.

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“…Nitrogen adsorption–desorption isotherms with relative pressure ranging from 0.01 to 0.99 at 77 K were derived. 25 …”

Section: Methodsmentioning

confidence: 99%

Coal Macrolithotype Distribution and Its Genetic Analyses in the Deep Jiaozuo Coalfield Using Geophysical Logging Data

et al. 2021

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“…Based on this standard, the coal samples can be classified into four types including bright, semi-bright, semi-dull, and dull coals, with the bright composition at a proportion of >80, 50−80, 20−50, and <20%, respectively. 5 The classification and determination of macroscopic description should be performed on the fresh and vertical section of coal seam, coal core, or coal specimen. First, the coal sample is divided into different layers according to the overall gloss intensity and then the contents of bright composition are estimated layer by layer to finally determine the macroscopic type of coal sample.…”

Section: Geological Settingsmentioning

confidence: 99%

“…The volumes of vitrinite (V), inertinite (I), and exinite (E) in coal samples can be obtained. 5,25 3.2.3. Methane Isothermal Adsorption.…”

Section: Geological Settingsmentioning

confidence: 99%

“…The double pore (pores and fractures) structure in coals is not only an important feature of reservoir structure, but also closely related with the coalbed methane (CBM) storage and migration. , The quantitative characterization of the pore-fracture system is of great significance to CBM exploration and development. The experimental methods for characterizing the pore-fracture structure of coals mainly include the following categories: (1) carbon dioxide (CO 2 ) adsorption: it can effectively detect the micropores and ultra-micropores smaller than 2 nm; (2) low temperature nitrogen(N 2 ) adsorption: it can detect the distribution and fractal characteristics of adsorption pores (pore size ≤100 nm) in coals; − and (3) mercury intrusion porosimetry: it can be used to characterize the distribution and fractal characteristics of pores (3.5 nm < pore size < 10 000 nm) by the relationship between mercury injection pressure and pore size combined with classic geometry models. ,, The above three experiments belong to fluid intrusion detection methods, which can partially destroy the coal structures. In addition, the range of pore size in coals detected by each experiment is different; therefore, the pore structure and fractal characteristics of coals cannot be fully and accurately determined. , In general, traditional methods have limitations in characterizing the primariness and integrity of pore-fracture, but nuclear magnetic resonance (NMR) with its speediness and non-destructive inspection can effectively resolve these shortcomings. , …”

Section: Introductionmentioning

confidence: 99%

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Study on the Applicability of Reservoir Fractal Characterization in Middle–High Rank Coals with NMR: Implications for Pore-Fracture Structure Evolution within the Coalification Process

et al. 2021

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In order to evaluate the applicability of the pore-fracture structure fractal characterizations in coal reservoirs and confirm the internal relationships between the porosity, permeability, coal metamorphic grade, and pore-fracture structure, the pore-fracture features of 21 middle–high rank coal samples from Anhe, Jiaozuo, and Huaibei coalfields in northern China were investigated using a low-field nuclear magnetic resonance (NMR). All the coal samples are characterized by low moisture content (M ad), low and medium ash yield (A ad), and high vitrinite (V) in coal maceral. The adsorption space fractal dimension (D A) is positively correlated with the Langmuir volume (V L) under the three-peak transverse relaxation time (T 2) spectrum. The fractal dimension of all effective T 2 points under saturated water (D NMR) is positively correlated with V L and the adsorption pore volume, but negatively correlated with the volume ratio of seepage pores and fractures. The free flow space fractal dimension (D M) is negatively correlated with the porosity of full saturated water (ΦF) and the porosity of movable water (ΦM). There is a negative correlation between ΦF and the seepage space fractal dimension (D S) in the coal samples with one-peak and two-peak T2 spectra, but a positive correlation can be found with the three-peak T2 spectrum. Therefore, it is necessary to consider the types of T2 spectral peak as a prerequisite to analyze the correlations between pore-fracture parameters and NMR fractal dimensions. With the increase of coal rank, the adsorption pore content, ΦF, and bulk volume immovable (BVI) fraction first increase and then decrease, whereas the seepage pore content, fracture development, bulk volume movable (BVM) fraction, and BVM/BVI first decrease and then increase. The inflection points of these changes correspond to the maximum vitrinite reflectance (R o,max) at 2.6–2.8%, which would be attributed to the third coalification jump. Generally, D A is the fractal dimension representing the coal pore surface, and D S and D M are closely related to the pore structure. Furthermore, D NMR not only represents the roughness of the pore surface but also the complexity of the pore structure.

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“…Due to metamorphism, medium and low-rank coals are mainly small-micropores (10-100 nm), and high-rank coals are mainly micropores (less than 10 nm), so the ability to adsorb methane is different (Hou et al 2020;Xin et al 2019). With the generation of adsorption during the adsorption process, the coal-CH4 adsorption equilibrium system becomes more stable.…”

Section: Desorption Thermodynamic Analysismentioning

confidence: 99%

Study the Effect of Temperature on Methane Desorption Based on Thermodynamics and Kinetics

Gao

Chen

et al. 2020

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Desorption hysteresis is important for primary gas production. Temperature may cause serious change in the methane adsorption/desorption behaviors. In order to study the mechanism of methane desorption and desorption hysteresis, three sets of samples of long flame coal, coking coal, and anthracite were collected, and experiments such as microscopic composition determination, liquid nitrogen adsorption, and isothermal adsorption/desorption were performed. From the perspectives of desorption kinetics, desorption thermodynamics and methane occurrence state, the differences in methane and methane desorption characteristics and the desorption hysteresis mechanism are discussed. The results show that at the same temperature, anthracite (SH3#) has the largest saturated adsorption capacity and residual adsorption capacity, followed by coking coal (SGZ11#), and long -flame coal (DFS4#) is the smallest. As the temperature rises, the theoretical desorption rate and residual adsorption capacity of anthracite (SH3#) and coking coal (SGZ11#) will increase first and then decrease. Temperature and methane desorption are not completely positive effects, and temperature may have a threshold for promoting methane desorption. It is necessary to comprehensively consider the influence of temperature on the activation of gas molecules and the pore structure of coal. Under the premise of a certain temperature, as the pressure increases, the desorption hysteresis rate changes in a logarithmic downward trend, and the methane desorption hysteresis rate in the low pressure stage (P 4MPa) is large, and the methane desorption hysteresis rate in the high-pressure stage (P>4MPa) is lower; During the isobaric adsorption process, the adsorption capacity of anthracite (SH3#) increases the fastest, followed by SGZ11#, and DFS4# is the smallest. In the low-pressure stage (P 4MPa), the adsorption capacity increases significantly with the increase of pressure, but in the high pressure stage (P 4MPa), the adsorption capacity does not change significantly with pressure, but gradually stabilizes. Under the same pressure, the molecular free path of methane increases with temperature. Under the premise of constant temperature, in the low-pressure stage (0<P<4MPa), when the pressure continues to decrease, the free path of methane molecules increases significantly, resulting in a decrease in the diffusion capacity. In the high-pressure stage (4<P<8MPa), when the pressure continues to decrease, the free path of methane molecules does not change significantly; DFS4#, SGZ11#, SH3# sample desorption process of three sets of samples, the intermediate adsorption heat is greater than the isometric adsorption heat during the adsorption process, indicating that the desorption process needs to continuously absorb heat from outside the system. The energy difference produced in the process of adsorption and desorption causes the desorption hysteresis effect. The greater the difference in the isometric heat value of adsorption, the more significant the hysteresis.

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Quantitative characterization of low-rank coal reservoirs in the southern Junggar Basin, NW China: Implications for pore structure evolution around the first coalification jump

Cited by 33 publications

References 47 publications

Coal Macrolithotype Distribution and Its Genetic Analyses in the Deep Jiaozuo Coalfield Using Geophysical Logging Data

Coal Macrolithotype Distribution and Its Genetic Analyses in the Deep Jiaozuo Coalfield Using Geophysical Logging Data

Study on the Applicability of Reservoir Fractal Characterization in Middle–High Rank Coals with NMR: Implications for Pore-Fracture Structure Evolution within the Coalification Process

Study the Effect of Temperature on Methane Desorption Based on Thermodynamics and Kinetics

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