Pore-fracture Structure Research Articles

Investigation on CT characterization of pore structure in nylon-uncured rubber composite from a microscopic view

In order to construct the geometric models characterizing the real micro pore-fracture structures of nylon-uncured rubber composite, and further compare the distribution law in the pore-fracture of solid (nylon)-gas (pore) two-phases with that of solid (nylon)-viscoelastic body (rubber)-gas (pore) three-phases composite, in this paper, the X-ray three-dimensional (3D) microscope is applied for the nylon material and nylon-rubber composite respectively. By employing the 3D visualization software (Avizo), three-dimensional reconstruction and pore-fracture network model is realized, where the quantitative statistics and comparative analysis are carried out. The results demonstrate that the pore/throat number of nylon material accounting for 20.8%/33.9% are the largest when the pore/throat radius is in the range of 3–4 μm/1–2 μm, respectively, however, the pore/throat number of nylon-rubber composite with the radius 3–4 μm/1–2 μm occupies merely 5.49%/11.3%. Furthermore, the average pore radius of nylon material is believed as larger than that of nylon-rubber composite based on the pore network model, where the pore/throat surface area and pore/throat volume have perfect consistent patterns with that of pore radius. This work will offer a theoretical basis for the investigation of gas seepage capability discrepancy between the solid (nylon) one-phase and solid (nylon)-viscoelastic body (rubber) two-phases.

Experimental study on the evolution of pore-fracture structures and mechanism of permeability enhancement in coal under cyclic thermal shock

Heat injection is a new method of recovering enhanced coalbed methane (ECBM). In this study, the impact of a sudden increase in ambient temperature on the permeability of coal was studied, and the application prospects of multiple thermal effects in coal fracturing and permeability enhancement were explored. The permeability and development of pore-fracture structures in coal were tested before and after cyclic thermal shock. The evolution characteristics of the pore distribution were evaluated, and an empirical formula for estimating the permeability growth of coal was established. Digital image processing technology and fractal theory were used to analyse changes in macroscopic fractures. The results indicates that cyclic thermal shock could effectively promote the generation and expansion of permeability pores, effectively connect the relatively independent fracture structures, and break and expand the microfractures and pore groups to form an interwoven fracture network. The increase in cyclic thermal shock resulted in an increase of 0.92%, 1.13%, and 16.15% in total porosity increment, the porosity increment of seepage pores and the proportion increment of seepage pores, respectively, after seven thermal shocks. The permeability of coal exhibited a logarithmic growth pattern with an increase in the number of thermal shock cycles; the average permeability increased by 100.02% after seven cycles. Based on the theory of thermodynamics, the influencing mechanism of cyclic thermal shock on the evolution of pore-fracture structures was explored. Differences were observed in the thermal expansion coefficients of minerals in coal, and thermal stress was generated under cyclic thermal shock, which may lead to damage, promoting fracture formation and increasing permeability. With an increase in the number of thermal shock cycles, the efficiency of permeability enhancement gradually decreased, which suggests that the reservoir conditions should be combined and mining plan should be reasonably set in ECBM extraction using multiple thermal injection techniques.

Reconstruction and seepage simulation of a coal pore-fracture network based on CT technology.

The distribution of multiscale pores and fractures in coal and rock is an important basis for reflecting the capacity of fluid flow in coal seam seepage passages. Accurate extraction and qualitative and quantitative analysis of pore-fracture structures are helpful in revealing the flow characteristics of fluid in seepage channels. The relationship between pore and fracture connectivity can provide a scientific reference for optimizing coal seam water injection parameters. Therefore, to analyse the change in permeability caused by the variability in the coal pore-fracture network structure, a CT scanning technique was used to scan coal samples from the Leijia District, Fuxin. A total of 720 sets of original images were collected, a median filter was used to filter out the noise in the obtained images, and to form the basis of a model, the reconstruction and analysis of the three-dimensional pore-fracture morphology of coal samples were carried out. A pore-fracture network model of the coal body was extracted at different scales. Using the maximum sphere algorithm combined with the coordination number, the effect of different quantitative relationships between pore size and pore throat channel permeability was studied. Avizo software was used to simulate the flow path of fluid in the seepage channels. The change trend of the fluid velocity between different seepage channels was discussed. The results of the pore-fracture network models at different scales show that the pore-fracture structure is nonuniform and vertically connected, and the pores are connected at connecting points. The pore size distribution ranges from 104 μm to 9425 μm. The pore throat channel length distribution ranges from 4206 μm to 48073 μm. The size of the coordination number determines the connectivity and thus the porosity of the coal seam. The more connected pore channels there are, the larger the pore diameters and the stronger the percolation ability. During flow in the seepage channels of the coal, the velocity range is divided into a low-speed region, medium-speed region and high-speed region. The fluid seepage in the coal seam is driven by the following factors: pore connectivity > pore and pore throat dimensions > pore and pore throat structure distribution. Ultimately, the pore radius and pore connectivity directly affect the permeability of the coal seam.

Fluid Performance in Coal Reservoirs: A Comprehensive Review

The fluids in coal reservoirs mainly consist of different gases and liquids, which show different physical properties, occurrence behaviors, and transport characteristics in the pore-fracture system of coal. In this study, the basic characteristics of fluids in coal reservoirs are firstly reviewed, consisting of coalbed methane (CBM) components and physical properties of CBM/coalbed water. The complex pore-fracture system mainly provides the enrichment space and flow path for fluids, which have been qualitatively and quantitatively characterized by various methods in recent years. Subsequently, this study has summarized CBM adsorption/desorption behaviors and models, the CBM diffusion-seepage process and models, and gas-water two-phase flow characteristics of coal reservoirs. Reviewed studies also include the effects of internal factors (such as coal metamorphism, petrographic constituents, macroscopic types, and pore structure) and external factors (such as pressure, temperature, and moisture content) on CBM adsorption/desorption and diffusion behaviors, and the relationship between three main effects (effective stress, gas slippage effect, and coal matrix shrinkage effect) and the CBM seepage process. Moreover, we also discuss in depth the implication of fluid occurrence and transport characteristics in coal reservoirs for CBM production. This review is aimed at proposing some potential research directions in future studies, which mainly includes the control mechanism of the microscopic dynamics of fluids on CBM enrichment/storage; enhancing CBM desorption/seepage rate; and the synergistic effect of multiple spaces, multilevel flow fields, and multiphase flow in coal reservoirs. From this review, we have a deeper understanding of the occurrence and transport characteristics of fluids in pore-fracture structures of coal and the implication of fluid performance for CBM production. The findings of this study can help towards a better understanding of gas-water production principles in coal reservoirs and enhancing CBM recovery.

Fractal permeability model for dual-porosity media embedded with natural tortuous fractures

Coal is a dual-porosity medium with an internal pore-fracture structure that serves as a channel for the migration of coalbed methane. Understanding the relationship between permeability and pore-fracture structure characteristics is therefore of great significance to accurately predict coalbed methane productivity. In this work, we analyze the pore-fracture structure characteristics of real coal samples and establish a new fractal permeability model of dual-porosity media embedded with natural tortuous fractures. Compared with the traditional dual-porosity medium model with embedded parallel plane fractures, this model not only considers the characteristics of pore tortuosity but also those of fracture tortuosity. The results show that the predicted permeability values obtained from the proposed model are 1–2 orders of magnitude lower than those of the traditional model, which implies that the latter overestimates coal seam permeability. The coal matrix permeability is approximately 13 orders of magnitude lower than the fracture permeability, and the latter mainly controls the overall coal seam permeability. Gas flow analysis of the coal seam is conducted in COMSOL Multiphysics, in which a fracture structure image is transformed to the computational domain. We use the results to discuss the relationships between coal permeability and fracture geometric characteristic parameters. The results show that the porosity ϕf and fractal dimension Dl of the fracture substantially affect coal seam permeability and follow a strong power-law relationship. The numerical simulation results are in good agreement with the theoretical predicted values (the average error is about 11.97%), which verifies the accuracy of the proposed model.

Simulation of coal microstructure characteristics under temperature-pressure coupling based on micro-computer tomography

The distribution of pore-fracture structure in coal directly affects the storage and migration characteristics of fluids. Simultaneously, under the condition of high buried depth, the coal structure is further changed by the temperature-pressure coupling. To study the pore size distribution of coal and the deformation characteristics of different pore-fracture structures under temperature-pressure coupling conditions, six different coal structure models are constructed using three-dimensional computer tomography reconstruction in this work. The box-counting algorithm is used to calculate the fractal dimensions of the pore-fracture structure and pore clusters in different pore sizes of coal. In the meantime, the fractal characteristics of the pore structure in different pore sizes are analyzed. Based on the coal structure model, a linear elastic deformation model under the temperature-pressure coupling is constructed. Moreover, the relationship of porosity and fractal dimension with the deformation of models is studied. The research results show that the porosity positively correlated with fractal dimension. The changes of pore volume fraction and fractal dimension in each pore size distribution range are consistent. Under the temperature-pressure coupling condition, both the model porosity and fractal dimension exhibit a quadratic function relationship with deformation. The deformation analysis of the three-dimensional models and two-dimensional cross-sections show that as the porosity and fractal dimension increase, the coal body deformation exhibits a trend of first decreasing and subsequent increasing. Low fractal dimension and high porosity could increase the model deformation. The research results can afford a theoretical basis for the study of coal structure characteristics under complex conditions.

Quantitative visualization and characteristics of gas flow in 3D pore-fracture system of tight rock based on Lattice Boltzmann simulation

The geometric complexity of the 3D pore-fracture system of tight rock at the pore-scale is a primary challenge for the prediction of gas flow. In this paper, the 3D pore-fracture systems of tight rock are reconstructed based on the representation unit volume (REV) of the intact tight sandstone and fractal fracture. A 3D regularized lattice Boltzmann model that considered the gas slippage effect is employed to simulate gas flow in the 3D pore-fracture systems. Effects of the micro-fracture morphology and gas rarefaction effect on gas flow and permeability of the pore-fracture systems are examined and investigated. The simulation results indicate that the gas flow behaviors in the pore-fracture system are strongly related to the above two factors. Gas flow behaviors in the pore-fracture structure is more sensitive at the low-pressure condition and gas rarefaction effect are more sensitive to the pore-fracture system with the highly rough fracture or the low fracture connectivity.

Experimental Investigation of CS2 Extraction to Enhance the Permeability of Coal

To examine the permeability enhancement effect of the carbon disulfide (CS2) extraction on block coal samples, in this study, different cylindrical samples of anthracite coal were selected to conduct CS2 extraction experiments. Comprehensive measurement techniques, including Fourier transform infrared spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, industrial computed tomography, and permeability measurements, were employed to analyze the variations of in the chemical structure, microcrystalline structure, pore–fracture structure, and coal permeability before and after the CS2 extraction treatment. The results showed that CS2 extraction can improve the connectivity of multi-scale migration channels, with which the coal permeability can be enhanced. Also, it was found that the permeability enhancement performance by the CS2 extraction is related to the coal anisotropy, as the coal permeability in the vertical bedding direction is smaller than that in the parallel bedding direction before and after CS2 extraction. Also, the CS2 extraction had a time effect on enhancing the coal permeability, and the permeability enhancement was more significant after a longer period of CS2 treatment. It can be predicted that the permeability can be enhanced up to more than 1 mD when the extraction time exceeds 6 h. Also, we concluded that CS2 fracturing is worth exploring by the on-going studies to enhance the permeability and CBM recovery in low-permeability reservoirs.

Experimental Study on the Change of the Pore-Fracture Structure in Mining-Disturbed Coal-Series Strata: An Implication for CBM Development in Abandoned Mines

After a coal mine is abandoned, a large amount of coalbed methane (CBM) resources is still accumulated in the overlying and underlying coal strata. The structure of the coal/rock pore fracture, whi...

Pore-fracture alteration of different rank coals: Implications for CO2 sequestration in coal

A comprehensive characterization of pore-fracture alteration of coal after exposure to CO2 is critical to CO2 sequestration in deep coal seams. In order to investigate the effect of supercritical CO2 (ScCO2) on pore-fracture structure of coal, three coal samples with different ranks (lignite, bituminous and anthracite) were saturated with ScCO2 for 14 days at 50 ℃ temperature and 10 MPa pressure. Multiscale characterization techniques, i.e. low-temperature N2 adsorption, mercury intrusion porosimetry (MIP) and X-ray CT scanning, were adopted to capture the changes of pore-fracture characteristics from nanometre to millimetre. Results show mesopores and macropores were well-developed in lignite due to its low maturity, whereas fractures were well developed in bituminous and anthracite. Lignite was relatively non-reactive with slight changes in mesopores and macropores as determined by MIP and N2 adsorption. An increase of macropore volume in lignite was observed from CT scanning, which possibly resulted from the shrinkage of coal matrix and collapse of pores caused by the drying effect of ScCO2. However, the induced macroporosity was not well-connected. The mesopores, macropores and fractures increased after ScCO2 treatment in bituminous and anthracite. Swelling-induced cracking, mineral and maceral dissolutions were the main causes for porosity increase in bituminous and anthracite and the induced fracture networks were well-connected. The fractal dimension of pore in three coals reduced after ScCO2 treatment, which indicates the surface of pore surface became smooth and homogeneous. Overall, ScCO2 has great potential of increasing coal porosity and enhancing coal permeability under unconstrained conditions, although the effect is rank dependent, which provides additional pathways for carbon storage and methane recovery.

Study on seepage and deformation characteristics of coal microstructure by 3D reconstruction of CT images at high temperatures

To study the seepage and deformation characteristics of coal at high temperatures, coal samples from six different regions were selected and subjected to computed tomography (CT) scanning studies. In conjunction with ANSYS software, 3D reconstruction of CT images was used for the establishment of fluid-solid conjugate heat transfer model and coal thermal deformation model based on the microstructures of coal. In addition, the structure of coal was studied in 2D and 3D perspectives, followed by the analysis of seepage and deformation characteristics of coal at high temperatures. The results of this study indicated that porosity positively correlated with the fractal dimension, and the connectivity and seepage performances were roughly identical from 2D and 3D perspectives. As the porosity increased, the fractal dimension of coal samples became larger and the pore-fracture structures became more complex. As a result, the permeability of coal samples decreased. In the meantime, fluid was fully heated, generating high-temperature water at outlet. However, when the porosity was low, the outlet temperature was very high. The average deformation of coal skeleton with different pore-fracture structures at high temperatures showed a trend of initial increase and subsequent decrease with the increase of porosity and fractal dimension. The maximum deformation of coal skeleton positively correlated with connectivity but negatively correlated with the fractal dimension.

Petrophysics characteristics of coalbed methane reservoir: A comprehensive review

Petrophysics of coals directly affects the development of coalbed methane (CBM). Based on the analysis of the representative academic works at home and abroad, the recent progress on petrophysics characteristics was reviewed from the aspects of the scale-span pore-fracture structure, permeability, reservoir heterogeneity, and its controlling factors. The results showed that the characterization of pore-fracture has gone through three stages: qualitative and semiquantitative evaluation of pore-fracture by various techniques, quantitatively refined characterization of pore-fracture by integrating multiple methods including nuclear magnetic resonance analysis, liquid nitrogen, and mercury intrusion, and advanced quantitative characterization methods of pore-fracture by high-precision experimental instruments (focused-ion beam-scanning electron microscopy, small-angle neutron scattering and computed tomography scanner) and testing methods (µ-CT scanning and X-ray diffraction). The effects of acoustic field can promote the diffusion of CBM and generally increase the permeability of coal reservoirs by more than 10%. For the controlling factors of reservoir petrophysics, tectonic stress is the most crucial factor in determining permeability, while the heterogeneity of CBM reservoirs increases with the enhancement of the tectonic deformation and stress field. The study on lithology heterogeneity of deep and high-dip coal measures, the spatial storage-seepage characteristics with deep CBM reservoirs, and the optimizing production between coal measures should be the leading research directions.

Effects of pore-fracture structure of ductile tectonically deformed coals on their permeability: An experimental study based on raw coal cores

The distribution area of ductile tectonically deformed coals (TDCs) usually coincides with the forbidden zones for coalbed methane (CBM) development. Permeability of coal reservoirs, an important indicator to evaluate the energy potential of CBM, is directly affected by the pore-fracture structures. To study the controlling mechanism of pore-fracture structures on permeability, we investigated the micropore - micro-fracture structures of various ductile TDCs by adopting mercury intrusion porosimetry (MIP) and low-pressure gas adsorption. The permeability and the pressure sensitivity of the standard TDC cores were tested by the pressure pulse decay method. The results showed that the total pore volume of ductile TDCs, especially the mesopores and macropores, significantly increased from that of primary structure coals (PSCs), while the volume of transitional pores only slightly increased in strongly crumpled and mylonitized coals, and thus, the dominant pores in coals gradually changed from micro-fractures to mesopores and macropores. The pore-fracture structure of ductile TDCs showed a poorer connectivity and a higher roughness than PSCs. The permeability of typical ductile TDCs obviously decreased, the permeability heterogeneity significantly increased, and the pressure sensitivity of permeability generally showed a pattern of “weakly crumpled - > PSCs > strongly crumpled - and mylonitized coals”. Correlation analysis showed that the weakly crumpled coals that contain well-developed and well-connected micro-fractures and fewer <10 μm coal particles usually display high permeability. In strongly crumpled – and mylonitized coals, the connected pore-fracture structures were damaged and many pore throats and flask-shaped pores formed, which is the main reason for the reduced permeability and the high permeability damage rate.

Description of Shale Pore Fracture Structure Based on Multi-fractal Theory

In order to describe the shale-fissure's structure and distribution accurately, this paper combines with complex features of shale rock mass' multilevel micro-scale fissure network, putting forward that utilizing the multi-fractal method to describe the distribution of fissures with different dimensions. By choosing some oil field's block shale samples, obtain the fissure distribution characteristics by CT scanning and calculate with the multi-fractal theory, the results show that the shale rock mass' fissure distribute in a multiple-fractal regularity. The study results lay a solid foundation of subsequent press shale fissure network's formation and description.

Evaluation of coal petrophysics incorporating fractal characteristics by mercury intrusion porosimetry and low-field NMR

Mercury intrusion porosimetry (MIP) and low-field nuclear magnetic resonance (NMR) were combined to investigate pore-fracture structure and fractal characteristics of coals (0.93% < Ro,m < 2.77%), and their impacts on coal permeability were assessed using two newly-proposed models. The results indicate that coals with type I mercury intrusion/extrusion curve are beneficial to gas flow due to well-developed micro-fractures (73.9–86.74%) and high pore-fracture connectivity, whereas coals with type II and III curves are less conducive to gas flow because of non-uniform pore-fracture structure and poor connectivity. Based on NMR analysis, pore-fracture structure of low-rank bituminous coals (0.9% < Ro,m < 1.2%) presents irregular three T2 peaks, whereas two and irregular three T2 peaks simultaneously exist in other coals (1.2% < Ro,m < 2.8%). The variation of coal composition and gas generation process may have complex effects on pore-fracture structure during the coalification process. Moreover, MIP fractal dimension ranges from 2.265 to 2.873, whereas NMR fractal dimension varies from 2.744 to 2.976. Due to different sample morphology and fractal estimation methods, most of MIP fractal dimension is larger than NMR fractal dimension. A modified Kozeny-Carman equation was used to calculate MIP permeability, ranging from 3.62 × 10−4 to 1.229 mD. The effect of mesopores on MIP permeability may be related to the gas flow pathway and interlinkage mechanism of adsorption pores and fractures. The NMR permeability was estimated by a movable porosity-permeability model (kN=0.0045∗eϕNM0.6851+0.037), ranging from 0.043 to 2.767 mD. The NMR permeability should be related to movable fluid space and pore-fracture connectivity, and micro-fractures can largely contribute to free fluid volume.

The characterization of bituminous coal microstructure and permeability by liquid nitrogen fracturing based on μCT technology

Studies of the microstructure and permeability evolution behavior of coal under conditions of liquid nitrogen (LN2) fracturing are helpful to clarify the mechanism of fracturing. The evolution of the 3D microstructure of the bituminous coal before and after LN2 fracturing was studied, using a high-resolution 3D X-ray microanalyzer and ‘Dragonfly’ software. The pore-fracture structure was preliminarily segmented using a digital terrain model (DTM) threshold segmentation and then was optimized using a ‘deep learning’ model. The box-counting method and a pore network model (PNM) were used to analyze the fractal features, spatial size distribution, and connectivity of pore-fractures. The results of the fractal analysis showed that the 2D fractal dimension of pore-fracture in the horizontal and vertical directions increased significantly after LN2 fracturing. The PNM analyses showed that the number and size of equivalent pores and throats after fracturing were significantly higher than prior to fracture. The relatively large fractures play a decisive role in the coal permeability, although they account for a small proportion of the total number of fractures. The results of the permeability tests showed that LN2 fracturing can effectively destroy the original structure of the coal and improve its permeability. At a constant confining pressure, the permeability and permeability increment increased exponentially with increasing gas pressure. The permeability increment curve can be divided into two stages: micro-fracture seepage and large-fracture seepage. During the first phase, the incremental change in permeability is small. During the second stage, the permeability increment increases sharply due to the re-opening of micro-fractures and the formation of large fractures.

Experimental Study on Pore-fracture Evolution Law in the Thermal Damage Process of Coal

ABSTRACT High temperature causes thermal damage to coal, alters its pore-fracture structure, and then influences its permeability and spontaneous combustion characteristics. In this paper, to study the evolution law of coal pore-fracture in the thermal damage process and the influence mechanism of thermal damage on pore-fracture development, the microfracture evolution characteristics and pore morphology of coal samples treated under different temperatures were qualitatively studied by susing a scanning electron microscope (SEM). Besides, the evolution of coal pore structure as a function of temperatures in the heating process was quantitatively studied based on Nuclear magnetic resonance (NMR) technique. In addition, the pore structure of coal samples was quantitatively characterized in combination with the fractal theory, and the variation of fractal dimension of coal sample pore structure with heat treatment temperature was obtained. The results show that after heat treatment, the development of fractures of coal was promoted, and the connectivity between the fractures was effectively enhanced. Moreover, the higher the temperature is, the more favorable it is for the development, expansion, and connectivity of fractures and for the formation of microfracture networks in coal. The high-temperature thermal damage changes the pore structure of coal, increases the pore volume and porosity of coal and enhances the pore connectivity. Moreover, after high-temperature heat treatment, the pore structure of DL coal with low metamorphism changed more significantly than that of XG coal. The porosity of DL coal and XG coal increased with the rising temperature, while the pore fractal dimension DmNMR, DMNMR, and DNMR of the two kinds coal samples show the opposite trend. Furthermore, the variation amplitude of DmNMR is significantly larger than that of DMNMR, and the variation of three fractal dimensions of DL coal is larger than that of XG coal. The research results can support a reference for coal fire prevention.

Full-scale pores and micro-fractures characterization using FE-SEM, gas adsorption, nano-CT and micro-CT: A case study of the Silurian Longmaxi Formation shale in the Fuling area, Sichuan Basin, China

Pore-fracture structure of shales is important to shale gas resource potential evaluation, exploration and development. However, investigations about the pore-fracture structure of shales were mainly focused on nano-submicron pores, while the contribution of micrometer-scaled pores and micro-fractures to pore system of shale reservoirs lack sufficient attention. Here, using a combination of field-emission scanning electronic microscopy (FE-SEM), gas adsorption (N2 and CO2) and CT scanning (Nano-CT and Micro-CT), the Longmaxi shale cored from JYA well, in the Fuling area, was selected to comprehensively evaluate the characteristics of the pore-fracture structure at different scales. The results show that there are diverse types of pores in the organic-rich shale, mainly consisted of organic pores, inorganic pores (interP pores and intraP pores) and micro-fractures. Many pores are ink bottle-shaped characterized by narrow necks and wide bodies, while some others are slit-shaped. Pore size range from 0.305 nm to 98.5 μm. Pores with diameter of 0.305–2 nm (micro-pores), 2–50 nm (meso-pores), 50 nm-2 μm (macro-pores) and 2–98.5 μm (micro-fractures) account for 33.59%, 36.28%, 14.04% and 16.09% of the total pore volume, respectively. Moreover, shale permeability and specific surface area were calculated to be 0.005 mD and 39.44 m2/g, respectively, based on the full-scaled pore splicing. About 92% of specific surface area is contributed by pores with diameter of 0.305–5 nm. Micro-fractures connect with each other and form net-shaped structure with great connectivity from observational results. The micro-fractures with size of 20–98.5 μm provide the main permeability (about 90%). Micro-pores and small meso-pores in shale reservoirs provide large adsorption space for adsorbed gas, while micro-fractures enhance the seepage capability of shale gas, which are favorable for shale gas accumulation.

Comparative analysis of shale reservoir characteristics in the Wufeng-Longmaxi (O3w-S1l) and Niutitang (Є1n) Formations: A case study of wells JY1 and TX1 in the southeastern Sichuan Basin and its neighboring areas, southwestern China

A systematic comparative analysis of shale reservoir characteristics of the Wufeng-Longmaxi (O3 w-S1 l) and Niutitang (Є1 n) Formations in southeastern Sichuan Basin and its neighboring areas was conducted with respect to mineralogy, organic geochemistry, pore structure, methane sorption, brittleness, and fractures. Results indicate that (1) organic matter (OM)-hosted pores that are hundreds of nanometers to micrometers in size in the Longmaxi shale are well-developed in migrated OM rather than in the in situ OM, and they are the dominant reservoir spaces. Furthermore, the total organic carbon (TOC), brittleness, organic pores, and bedding fractures have good synergistic development relationships. However, there are fewer OM-hosted pores in the Niutitang shale; they are smaller in size, usually less than 30 nm, and have a more complicated pore structure. The intergranular pores in cataclastic organic-inorganic mineral fragments are the dominant reservoir spaces in the Niutitang shale and are coupled with stronger methane sorption and desorption capacities. (2) The piecewise correlation between TOC and brittleness indicates the significant differences in pore and fracture characteristics. When the TOC [Formula: see text], the TOC, brittleness, organic/inorganic pores, and fractures synergistically develop; when the TOC [Formula: see text], even though the increase in ductility reduces the number of fractures, the lower cohesive strength, internal friction angle, and weaker surfaces of interlayer fractures and cataclastic minerals promote the development of slip fractures, which significantly improves the fracture effectiveness and reservoir spaces for free and absorbed shale gas. (3) The Longmaxi, Wufeng, and Niutitang shales formed and evolved in different evolutionary stages. With the evolution of hydrocarbon generation, diagenesis, tectonic deformation, and pressure, the size and proportion of OM-hosted pores gradually decrease. At the same time, the complexity of the pore-fracture structure, the methane adsorption/desorption capacity, and the proportion of inorganic pores and fractures increase.

Effect of coalification jumps on petrophysical properties of various metamorphic coals from different coalfields in China

The effect of coalification jumps on petrophysical properties, and the evolution of porosity and permeability of metamorphic coals are poorly understood, which significantly influences coalbed methane extraction. We estimated pore throat diameter, specific surface area, connectivity, moveable fluid space, heterogeneity, porosity and permeability in a series of 41 coal samples (maximum reflectance of vitrinite in 0.34–4.24%) over six coalification jumps, by processing low temperature nitrogen adsorption, mercury intrusion porosimetry and nuclear magnetic resonance (NMR) measurements. Each coalification jump generally leads to abrupt change of petrophysical properties from dehydration to graphitization. Connectivity parameters (efficiency mercury withdrawal and the ratio of movable fluid to bounded fluid) and fractal dimensions (DNA1, DNA2, DMIP, DNMRS and DNMRM) present binomial function with vitrinite reflectance. Generations of thermogenic gas and fractures growth are attributed to increasing pore-fracture connectivity in bituminization and debituminization. Fractures begin healing and compaction, as well as reduced connectivity, at the fourth jump in graphitization. Heterogeneous pore-structures (high DMIP and DNMRM) usually have low connectivity. Moveable fluid space and its porosity from NMR are negatively correlated with the increase in coal rank. The evolution and origin of porosity and permeability (<1 mD) during coalification is proposed. Coal permeability (>1 mD) has no relation with coal rank and is related to fracture characteristics. Unlike the origin of porosity, which intrinsically inherits from progressive coalification, the origin of permeability is attributed to both progressive coalification and tectonic stresses. This study reveals the complex pore-fracture structures variation and the effect of stages in coal maturation on petrophysical properties of coalbed methane reservoirs.