To more finely describe the physical basis of coalbed methane (CBM) accumulation and flow mechanisms, two different rank coals (Ro,m of 0.4% and 0.85%) from the southern Junggar Basin of northwest ...China were systematically investigated to characterize pore-fracture spaces and their nanoscale connectivity in three dimensions (3D) using focused ion beam scanning electron microscopy (FIB-SEM) tomography. We reconstructed the 3D pore morphology of two different rank coals, quantify the pore size/volumes distribution and developed a novel pore network extraction (PNE) model. Samples of subbituminous coal (SC) and high-volatile bituminous coal (HBC) presented significant pore spaces for CBM adsorption, with total voxel numbers of 28,944 and 83,866, respectively; both samples predominantly consisted of closed-pore diameters of 10–50nm. The numbers, volumes, and areas of pores (<100nm) were less in SC than in HBC, revealing that the SC adsorption capability was weaker than HBC. The porosities acquired by FIB-SEM were 26.06 and 42.85% for the samples SC and HBC, respectively, which could indicate the storage capability of the SC reservoir was lower than that of the HBC reservoir. The SC and HBC samples revealed a significant connectivity for CBM seepage, with a connected pores proportion of ~100% in pores >300nm. The SC sample allowed pore space connections throughout, whereas the HBC sample partially displayed pore space connections in a 3D pore network. It was found that a throat size of 400–500nm could possibly dominate coal permeability in the SC sample; the numbers of throats (fractures) decreased with rising throat size. These results show that the gas accumulation capacity of the SC reservoir are lower than that of the HBC, whereas the fluid flow capability of the SC reservoir is higher than that of the HBC reservoir. These findings may be scientifically significant for lucubrating nanoscale pores to affect CBM storage and seepage. This paper comments on the reduction of the gas sorption capability and mass transport rate in CBM development, and we provide recommendations for relevant future work.
•FIB-SEM tomography improved the resolution to recognize a fully connected pore network.•The coal closed/connected pores were distinguished and quantified using FIB-SEM.•Tens of thousands of pore size/volumes distributions were achieved on two Chinese coals.•A conceptual model of the coal pore-fracture space connectivity was displayed.
One of the biggest challenges in evaluating the transport and storage properties of shales is understanding how the pore structure and network evolve in the same stratigraphic shale during ...catagenesis. Using a combination of CO2 adsorption, mercury intrusion porosimetry, as well as scanning electron microscopy, we have described the evolution and the abrupt changes (jump) of pore characteristics in lacustrine shale at oil and gas maturation. With an increase maturity, our sample set exhibited abrupt changes in porosity-related characteristics. Porosity and surface areas presented a decline and then an increase, and a jump occurred due to hydrocarbon generation, with the minimum value in late maturity (i.e. vitrinite reflectance of ~1.3%). These changes were accompanied by corresponding changes in pore-size distributions, surface area percentage, and multi-scale pores volumes. The trends in porosity variation were related to different organic matter and mineral contents in our samples, to some extent, with the degree of maturation being the dominant influence. We found that inorganic pores were the main contributor to the porosity of less mature shales, while porosity in gas-mature shales was dominated by organic matter pores. In this work, we showed that organic matter transformation was a centric component of pore evolution. Our findings may fill significant gaps in understanding and predicting pore development, with respect to the maturity of lacustrine shales.
•Pore volume and surface area in lacustrine shales show jumps with maturity.•The porosity of oil- and gas- mature shales is governed by mineral interparticle pores and OM-hosted pores, respectively.•From oil-window to gas-window, porosity-related characteristics present a decline and then an abrupt increase.
Resolving pore attributes and connectivity of oil-bearing sandstone formations is key to the precise estimation of the oil storage and flowing capacity of the reservoirs. However, very few studies ...have quantitatively characterized the individual porosities of interconnected pores and inaccessible pores. This study addresses these research gaps by studying Zhuhai sandstone from the western Pearl River Mouth Basin, China, with very low to medium permeability (0.055–346 mD). Nuclear magnetic resonance (NMR) centrifugation distinguished the interconnected and inaccessible pores. NMR T2 parameters (T2 cutoff and T2g) had a distinct impact on the connectivity index and irreducible water saturation. This study successfully developed an adaptable model that can efficiently estimate permeability in heterogeneous porous media, where no samples are taken, using NMR log. The permeabilities calculated by Schlumberger Doll Research, Coates-cutoff, Coates-BVI, and SDR-reg models were compared with those measured using a gas permeameter. Permeabilities were best calculated using flow-accessible porosity (i.e., Coates-BVI model, 95% matching) rather than other models. The connectivity (0.13–0.84) and movable fluid porosity (1.46–19.32%) were the primary factors that control permeability, and the fracture/crack attributes determined dual porosity. The study revealed that illite in clay minerals exerts a positive control on permeability by sheltering the interconnected primary porosity.
•Moveable water spaces in sandstones are probed via low-field NMR.•The influence of pore connectivity on gas transport capacity is established.•A new Coates-cutoff-based method accurately estimates permeability using NMR logging.
Early Silurian black shales are widely deposited over the entire Yangtze platform and their origin are highly controversial. In order to identify the controlling factors of these black shales ...formation, the mineralogy and geochemistry of the Early Silurian organic-rich shales from the Yangtze Platform, South China, were investigated by using an optical microscopy, X-ray diffraction analysis, X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry and field emission scanning electron microscopy. These black shales are characterized by high total organic carbon (0.37%–10.47%, avg. 3.86%) and total sulphur (0.02%–1.68%, avg. 0.67%) content as well as enrichment of some trace elements, including V (70.66 ppm–1740.52 ppm, avg. 312.75 ppm), Cr (68.72 ppm–301.94 ppm, avg. 117.97 ppm), U (3.90 ppm–52.49 ppm, avg. 16.58 ppm), Ba (316.69 ppm–2985.82 ppm, avg. 1523.38 ppm) and rare earth elements (94.26 ppm–340.50 ppm, avg. 202.46 ppm). The minerals mainly consist of quartz and clay mineral with average content of 54.8% and 29.3%, respectively. K-feldspar, dolomite, calcite and pyrite are next in abundance, followed by trace amounts of apatite, barite and Cr-bearing minerals. The dissolution of biogenic silica and recrystallization during burial is an important source of quartz cement. In addition to biological sources, terrestrial, volcanic and authigenic sources are responsible for these mineralogical and geochemical compositions. The Al2O3/TiO2, Zr/Sc, Th/Sc, LREE/HREE and Eu anomalies indicate that the sediment-source region is derived from a mixture of felsic and intermediate rocks of the old upper continental crust. The Th/U and V/(V + Ni) ratios, Ce anomalies and authigenic U suggest that these black shales are deposited under an anoxic bottom water environment. The elevated concentrations of U-V-Ni-Cu-Zn-Cr-Ba and REE in these shales are predominated by the marine environment during deposition and influenced by biologic production in water columns and volcanic ash fall. High marine productivity resulting from volcanic nutrients promote the anoxic/sulfidic water column conditions of the restricted Yangtze basin, further resulting in organic matter enrichment. Elemental enrichment processes usually result in a valence change under anoxic environment, which is associated either with the clay minerals or direct precipitation from sea water. Understanding the coenrichments of organic carbon and trace elements in these black shales is important for studying the formation mechanism and resource potential of hydrocarbon and critical elements.
•Longmaxi black shales are characterized by the enrichment of trace elements.•Longmaxi black shales mainly consist of quartz and clay minerals.•Longmaxi black shales deposited under an anoxic bottom water environment.•Terrestrial, biological and volcanic sources controlled geochemical compositions.
Pore compressibility is an important physical property of coal, which controls geomechanical behavior and hydrocarbon reserve evaluation of coal seams. However, the representation of pore ...compressibility is usually oversimplified in coalbed methane (CBM) reservoir analysis. This study proposes the adoption of the dual compressibility concept, where both accessible porosity (connected pore) and the inaccessible portion of the coal compressibility contribute to the storage and transport capacities of coal reservoirs, provided the availability of mercury porosimetry data. We characterize dual compressibility as a function of confining pressure and identified the effect of factors, such as coal rank, moisture content, and porosity. It was found that pore compressibility reduces decreased one order of magnitude, from 10
−3
–10
−2
to 10
−4
–10
−3
MPa
−1
, with confining pressure increasing up to 413.3 MPa. The calculated compressibility of accessible pores is one-to-two orders of magnitude higher than the inaccessible portion of the coal. Using a compressibility correction on pore volume connectivity and permeability estimates showed a 23% reduction in those estimates. Results suggest that high pore compressibility affects storage and transport capacity more significantly than previously regarded. We propose a new model to estimate coal permeability as a function of confining pressure and accessible pore compressibility. The model uses integrated intrusion correction and pore compressibility values from the Katz–Thompson method. Pore volume and permeability response with pressure were evaluated according to coal rank, macro-lithotype, and seam. This work provides an improved methodology to estimate hydrocarbon reserves and CBM reservoir performance through more accurate pore volume and permeability evaluations from mercury porosimetry data.
The Upper Cretaceous Qingshankou (K
2
n) and Nenjiang (K
2
q) shale oil from the Changling Sag (CLS) of the Southern Songliao Basin located in NE China was studied to determine shale reservoir ...properties and evaluate oil potential. Multiple methods, including optical microscope, scanning electron microscope and energy dispersive spectrum analysis (SEM-EDS), nitrogen adsorption/desorption analysis, and X-ray diffraction, were conducted to determine the shale reservoirs. The mineral analyses present in this shale consist of blende, albite, chlorite, pyrite, and calcite, whose textures are crystals, schistose, particle, or clusters. The mineral results show a significant percentage of brittle minerals (40.5–61.7%), and clay minerals are dominated by illite and montmorillonite. The mineralogical brittleness indexes of the Upper Cretaceous K
2
n and K
2
q shales are 0.15–0.67 and 0.18–0.42, respectively. The K
2
q shale has a relatively low brittle mineral content, which may not be favorable for shale oil seepage. Higher brittleness in K
2
n shales is more favorable for hydraulic fracturing with larger width and smaller initial stress. Although the reservoir has an ultra-low permeability, they have a relatively good porosity. The organic matter is residual in bioclastic pyritization, calcareous bioclastic grain pores, animal skeletal grain, and algous cystocarp. Moreover, the K
2
n and K
2
q shales contain a substantial number of pores, micro-fractures, and oil. The mineral intragranular pores, dissolution pores, organic pores, and micro-fractures are significantly developed. Hysteresis loop is acquired in open pores, which have mainly inkbottle-shaped pores and few parallel-plate pores or cylindrical pores. Our findings may fill significant gaps in understanding and predicting pore systems and mineralogical brittleness indexes, with respect to the lacustrine shale oil in China.
The injection of CO2 into shale reservoirs potentially increases rates and masses of CH4 recovery and simultaneously contributes to the sequestration of CO2. At typical reservoir conditions ...(T≥31.08 °C, P≥7.38 MPa) the CO2 will be supercritical. We compile, analyze, and supplement experimental data of shales from several basins across China, and use X-ray diffraction, scanning electron microscopy and low-pressure gas adsorption to characterize variations in shale pore structure before and after supercritical CO2 (ScCO2) treatment, and supplement these with CH4/CO2 adsorption experiments to characterize changes in shale adsorption capacity. The results show that clay and carbonate contents significantly decrease, and the relative content of quartz is increased after ScCO2 treatment. Pore structure changes significantly after ScCO2 treatment, with the majority of the shales showing a decrease in total specific surface area and total pore volume and an increase in average pore size — indicating the transformation of some micropores and smaller mesopores into mesopores and macropores. After ScCO2 treatment, the experimentally derived absolute adsorption volumes of both CH4 and CO2 decrease, and the volumes of both CH4 and CO2 fitting a Langmuir isotherm decrease with an increase in treatment pressure and increase with an increase in temperature. The adsorption selectivity factors αCO2/CH4 all remain greater than 1 with αCO2/CH4 primarily controlled by the pore structure. The fractal dimension is positively correlated with Langmuir volume and negatively correlated with Langmuir pressure while the fractal dimensions are negatively correlated with αCO2/CH4. The selectivity factor αCO2/CH4 decreases rapidly above a fractal dimension threshold (D1>2.65, D2>2.80). This paper further reveals critical interactions between ScCO2 and shale and defines controls on and of pore structure and adsorption capacity to speculate on physical and chemical storage mechanisms of CO2 in shale reservoirs. This provides several theoretical bases for shale gas recovery and the sequestration of CO2.
•Total specific surface area and total pore volume decrease with ScCO2 exposure.•The control of Fractal dimension by pore structure is influenced by ScCO2 exposure.•The adsorption capacity of CO2 and CH4 in the shale is reduced by ScCO2 exposure.•Fractal dimension has a nonlinear negative correlation with the adsorption selectivity factors.
•The low-field NMR fractal dimensions were measured.•Effect of the DNMRA on CBM adsorption capability was performed.•The permeability and DNMRM model was obtained.•Characteristics of T2 distribution ...and permeability were analyzed.
To describe a low-field nuclear magnetic resonance (NMR) method for quantifying pore–fracture fractal dimensions and their influence on effective porosity and permeability, we performed modeling comparisons between fractal analysis and pore–fracture physical properties in low-rank coals. The adsorption space fractal (DNMRA), seepage space fractal (DNMRS) and moveable fluid space fractal (DNMRM) were calculated to be 1.62–1.91, 2.77–2.98 and 1.56–2.75, respectively. The DNMRA generally increased with increasing Langmuir volume (VL, 9.54–31.06m3/t), Langmuir pressure (PL, 0.58–8.13MPa), the Brunauer–Emmett–Teller (BET) surface area and its fractal dimension. Higher DNMRA indicated the significant coalbed methane (CBM) adsorption capability. Both the DNMRS and DNMRM decreased with increasing areas of T2>2.5ms distribution (ST and SCT) and sorting coefficient. These phenomena showed that the NMR fractal method could reflect the coal pore–fracture heterogeneity and had significant influence on seepage space content. The correlations of moveable fluid porosity and permeability with DNMRM can be found by performing the models of y=ax+b (a<0), so coals with high DNMRM occur to have low flow capability. Furthermore, the pore–fracture porosity and permeability have positive correlations with ST and SCT, which result from the connection between pores and fractures. These results also show that fractal analysis calculated with T2 can be developed to appraise the physical properties of low-rank coals and supply some reference for a relatively full identification of porous media. We advise that low-field NMR can be employed as a lossless analytic method to quantify moveable fluid space fractal theory.
Reservoir sensitivity is a parameter that is used to evaluate the degree of change in reservoir permeability under the influence of external fluids. Accurate evaluation of reservoir sensitivity is ...conducive to the optimization of fluid parameters during exploration and development. Taking the Wenchang Formation and Enping Formation of the Paleogene in the Huilu area of the Pearl River Mouth Basin as the research object, reservoir sensitivity experiments were carried out. Combined with the corresponding experimental results obtained using methods such as thin section identification, scanning electron microscopy (SEM), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and screening analysis, based on mineral sensitization and pore structure sensitization, qualitative and quantitative evaluations of reservoir sensitivity were carried out, and factors affecting sensitivity and sensitization mechanisms were analyzed. This work shows the following: (1) The sandstone reservoirs in the two areas have the same clay type, but the total clay content of the Wenchang Formation is greater than that of the Enping Formation. The porosity of the Wenchang Formation is less developed than the Enping Formation. (2) The Wenchang Formation has weak or moderately weak water sensitivity and moderately weak or moderately strong flow velocity sensitivity. The water sensitivity of the Enping Group samples is moderately weak or moderately strong, the flow rate sensitivity is moderately weak, the alkali sensitivity is weak, the acid sensitivity is moderately weak, and the salinity sensitivity is moderately weak or moderately strong. (3) The sensitivity of the Wenchang Formation is mainly affected by the content of clay minerals. The sensitivity of the Enping Formation is also affected by the clay content and type. Although the clay content is not high, the permeability is more susceptible to sensitivity due to the pore structure and debris particle distribution characteristics. These conclusions are beneficial for the selection of fluid parameters and efficient reservoir development.
The Lower Cretaceous Yingcheng Formation continental organic-rich shale from Lishu Depression （LSD） of the southern Songliao Basin located in northeast China has significant shale gas potential ...petrology.
