Formation and Distribution of Different Pore Types in the Lacustrine Calcareous Shale: Insights from XRD, FE-SEM, and Low-Pressure Nitrogen Adsorption Analyses (original) (raw)
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Pore Characteristics Analysis of Shale from Sichuan Basin, China
International journal of georesources and environment
Pore characteristics are significant for shale gas exploration and production. In this paper, the method of field emission scanning electron Microscopes (FE-SEM) was applied to qualitatively describe minerals and pore structures of shale samples. Low pressure nitrogen adsorption-desorption and carbon dioxide adsorption were applied to analyse mesopores and micro-pores respectively. Inter-particle pores are always associated with rigid mineral grains and intra-particle pores are mainly located in unstable minerals. The BET (Brunauer-Emmett-Teller) surface area of Longmaxi Formation (LMX) is 5.47m 2 /gr and 16.33m 2 /gr of Wufeng Formation (WF). N2 and CO2 adsorption shows that the diameter of micropores in the LMX and WF formation is approximately 1nm. Most meso-pores in WF formations range from 2nm-20nm, while meso-pores existing in LMX formations range from 2nm-30nm.
International Journal of Coal Geology, 2016
Silurian shale in Sichuan Basin is currently the most important target zone for shale gas exploration and development in China. Pore structure characteristics of Lower Silurian Longmaxi shales from southern Sichuan Basin were investigated. The combination of field emission scanning electron microscope (FE-SEM) and argon ion beam milling was utilized to describe the nanometer-to micrometer-scale (> 1.2 nm) pore systems. The shales were characterized by organic geochemical and mineralogical analyses. Total porosity, pore size distribution (PSD), specific surface area, and gas content were determined. Controls of organic matter richness, thermal maturity, and mineralogy on porosity were examined. The contribution of individual mineral components to total porosity was analysed quantitatively. Total gas contents of the shales determined from canister desorption data were compared with theoretical (sorptive and volumetric) gas storage capacities. The total organic carbon (TOC) content of the shale samples ranges between 0.1 to 8.0 wt. % and helium porosity varies between 0.7 and 5.7%. Maturity in terms of equivalent vitrinite reflectance of bitumen (R eqv) ranges from 1.8 to 3.2 %. TOC content is a strong control for the pore system of these shales, and shows a positive correlation with porosity. Porosity increases with increasing thermal maturity when R eqv is less than 2.5%, but decreases for higher thermal maturity samples. FE-SEM reveals four pore types related to the rock matrix that are classified as follows: organic matter (OM)-hosted pores, pores in clay minerals, pores of framework minerals, and intragranular pores in microfossils. Pores in clay minerals are always associated with the framework of clay flakes, and develop around rigid mineral grains because the pressure shadows of mineral grains prevent pores from collapsing. Pores of framework minerals are probably related to dissolution by acidic fluids, and the dissolution-related pores promote porosity of shales. A unimodal PSD exists in the micropore range of TOC-rich samples, while the PSD of carbonate-rich samples are bimodal. A PSD maximum in the micropore range is attributed by OM and
Marine and Petroleum Geology, 2009
The effect of shale composition and fabric upon pore structure and CH 4 sorption is investigated for potential shale gas reservoirs in the Western Canadian Sedimentary Basin (WCSB). Devonian-Mississippian (D-M) and Jurassic shales have complex, heterogeneous pore volume distributions as identified by low pressure CO 2 and N 2 sorption, and high pressure Hg porosimetry. Thermally mature D-M shales (1.6-2.5% VRo) have Dubinin-Radushkevich (D-R) CO 2 micropore volumes ranging between 0.3 and 1.2 cc/100 g and N 2 BET surface areas of 5-31 m 2 /g. Jurassic shales, which are invariably of lower thermal maturity ranging from 0.9 to 1.3% VRo, than D-M shales have smaller D-R CO 2 micropore volumes and N 2 BET surface areas, typically in the range of 0.23-0.63 cc/100 g (CO 2) and 1-9 m 2 /g (N 2). High pressure CH 4 isotherms on dried and moisture equilibrated shales show a general increase of gas sorption with total organic carbon (TOC) content. Methane sorption in D-M shales increases with increasing TOC and micropore volume, indicating that microporosity associated with the organic fraction is a primary control upon CH 4 sorption. Sorption capacities for Jurassic shales, however, can be in part unrelated to micropore volume. The large sorbed gas capacities of organic-rich Jurassic shales, independent of surface area, imply a portion of CH 4 is stored by solution in matrix bituminite. Solute CH 4 is not an important contributor to gas storage in D-M shales. Structural transformation of D-M organic matter has occurred during thermal diagenesis creating and/or opening up microporosity onto which gas can sorb. As such, D-M shales sorb more CH 4 per weight percent (wt%) TOC than Jurassic shales. Inorganic material influences modal pore size, total porosity and sorption characteristics of shales. Clay minerals are capable of sorbing gas to their internal structure, the amount of which is dependent on claytype. Illite and montmorillonite have CO 2 micropore volumes of 0.78 and 0.79 cc/100 g, N 2 BET surface areas of 25 and 30 m 2 /g, and sorb 2.9 and 2.1 cc/g of CH 4 , respectively (dry basis)-a reflection of microporosity between irregular surfaces of clay platelets, and possibly related to the size of the clay crystals themselves. Mercury porosimetry analyses show that total porosities are larger in clay-rich shales compared to silica-rich shales due to open porosity associated with the aluminosilicate fraction. Clay-rich sediments (low Si/Al ratios) have unimodal pore size distributions <10 nm and average total porosities of 5.6%. Siliceous/quartz-rich shales (high Si/Al) exhibit no micro-or mesopores using Hg analyses and total porosities average 1%, analogous to chert.
Study on pore structure characteristics of marine and continental shale in China
Shale gas is a key natural gas resource and has been a certain success in China. The Weiyuan marine shale (1#), Jiaoshiba marine shale (2#), Yaoqu tuff (4#) and Yaoqu continental shale (5# and 6#) were selected and subjected to thin section analyses, ice field emission scanning electron microscope (FE-SEM), mercury intrusion capillary pressure (MICP) and gas adsorption tests to study the pore structure characteristics of typical marine and continental shale. Results of pore structure characteristics and controlling factors, puts forward a new standard of pore size naming for hydraulic fracturing technology. Pore distribution uniformity coefficient h u is proposed to describe the continuity of different pore size distribution of shale. The test results of N 2 adsorption and CO 2 adsorption can be unified to attain continuous distribution of nanometer pore by using density function theory (DFT) model. The pore development degree of the samples from high to low are from 2#, 5#, 1#, 6# to 4# in the pore range of 2 e100 nm while for the pore range of 10e100 mm the pore development degree from high to low are 2#, 1#, 4#, 6# to 5# in that order. Micro-nano pore of sample 2# and 5# are more developed compared to the other samples. Also gas storage capacity of sample 2# and 5# are stronger. The results can thus be used to guide the shale gas development, optimize shale location and increase production of shale gas.
Impact of Shale Properties on Pore Structure and Storage Characteristics
All Days, 2008
Characterising the pore structure of gas shales is of critical importance to establish the original gas in place and flow characteristics of the rock matrix. Methods of measuring pore volume, pore size distribution, and sorptive capacity of shales, inherited from the coalbed methane and conventional reservoir rock analyses, although widely applied, are of limited value in characterising many shales Helium which is routinely used to measure shale skeletal and grain density, permeability and diffusivity, has greater access to the fine pore structure of shale than larger molecules such as methane. Utilizing gases other than He to measure porosity or flux requires corrections for sorption to be incorporated in the analyses. Since the permeability of shales vary by several orders of magnitude with effective stress, methods that do not consider effective stress such as crushed permeability, permeability from Hg porosimetry, and from desorption are of limited utility and may be at best ins...
Specific surface area and pore size distribution in gas shales of Raniganj Basin, India
Journal of Petroleum Exploration and Production Technology, 2018
Understanding of multiscale transports of shale gas is important for shale gas exploration and exploitation. Traditional porosity determining approaches normally underrate the shale gas transport capacity as these techniques do not include adsorb gas in nanometer-sized slit pores. Silty shale, carbonaceous shale, claystone and ironstone shale unit of Barren Measures Formation was examined to understand the pore system at various scales. The pores are intergranular, intragranular, interlayer, dissolved pore and fracture pores where gas molecules are present as free state and/or adsorbed gas in the internal structure of the pores and at the edge of their structures. Here, we used the Brunauer-Emmett-Teller technique with scanning electron microscopy for considering the adsorption mechanism to understand the gas transport in micro and nano pores in shales. The adsorption parameters between organic wall and grain surface were observed to be controlled by clay mineralogy. SEM, X-ray diffraction and BET manifest significant information about role of clays, organic matter and mineral composition in development of pore network, which also governs the gas storage and transport properties. A large portion of pores in Barren Measures shales ranges between 20 and 55 nm and the pore size diameter ranges from 5.49 to 29.75 nm.
Lithofacies and reservoir characteristics of shale
The increasing enthusiasm for continental shale gas exploration in China has made the Shahezi shale in the Changling Fault Depression an important research target due to its impressive demonstrated gas capacity. In this work, geochemical and petrologic analyses, field emission-scanning electron microscopy (FE-SEM) observation, low-pressure adsorption isotherms and mercury intrusion porosimetry (MIP) were conducted on 19 core samples to comprehensively analyze the reservoir characteristics of different lithofacies. The results show that the Shahezi shale has the characteristics of low content of organic matter (OM), high R o , high content of clay minerals and small fractions of calcite. The kerogen is type III, dominated by vitrinite and inertinite. Clay minerals are dominated by the mixed illite-smectite, followed by illite and a small amount of chlorite. The Shahezi shale develops three kind of stratification structures according to the thickness of laminae: laminated structures, bedded structures, and massive structures. On the basis of TOC, mineral composition and petrologic texture, eight types of lithofacies were recognized. The organic pores show strong heterogeneity and are poorly developed, while clay-related pores are ubiquitous. The Shahezi shale has a high pore volume (PV) (0.005-0.031 ml/g, averaging 0.0173 ml/g) and specific surface area (SSA) (2.57-27.48 m 2 /g, averaging 16.61 m 2 /g), indicating an excellent storage capacity. Low-pressure CO 2 and N 2 isotherms and MIP were utilized to construct the whole-range pore size distribution (PSD). Based on the whole-range PSDs, mesopores were observed to contribute most to the PV, followed by macropores. Micropores and mesopores account for more than 99% of SSA. The reservoir capacity of different lithofacies is following the order in terms of PV and SSA, from high to low: organic-medium massive mixed shale (OMMMS), organic-medium massive argillaceous shale (OMMAS), organic-rich laminated argillaceous shale (ORLAS), organic-medium laminated argillaceous shale (OMLAS), organic-rich bedded mixed shale (ORBMS), organic-rich bedded siliceous shale (ORBSS), organicmedium bedded argillaceous shale (OMBAS), organic-poor shale (OPS). The main controlling factors of pore structure for the Shahezi shale are clay minerals rather than OM, which is similar to the continental Chang 7th shale but contrary to the Longmaxi shale. However, the Shahezi shale is mainly of low TOC and strong heterogeneity pores development result from OM macerals, while the Chang 7th shale is mainly of low maturity. R o play an important role in promoting pore development. Only high content of calcite can greatly improve the pore space due to its solubility. This work contributes to the theory of continental shales and how to identify continental high-quality shale reservoirs.
Comparison of methods for the determination of the pore system of a potential German gas shale
Filling the gaps - from microscopic pore structures to transport properties in shales, 2016
The aim of the present study was to investigate the porosity of the Posidonia shale as a potential gas shale and to compare the results with data published on gas shales known to be productive. The characterization of the porosity of clays and shales still poses an analytical challenge, however. Different methods were investigated based on a comparison of four different Posidonia shales, with different degrees of maturity. Both direct microscopical methods as well as indirect methods based on gas adsorption or Hg intrusion were applied. Most of the pores in clays and shales were too small to be detected by any of the existing direct methods. About 80% of the pores were ,30 nm wide. The Posidonia shales, as is the case with most shales, are dominated by mesoporosity (ranging from 20 to 50 mm 3 /g). The mesopore peak representing the average pore diameters could be resolved by Hg intrusion and was found to decrease with increasing maturity which may be explained by increased compaction and/or temperature. This relation, if not applicable to the entire Posidonia shale, may be restricted to a single sedimentary system (e.g. a basin or a sequence). The results of the indirect methods (except for CO 2 microprosity) were comparable and this might be explained by the low macroporosity. The most important question concerning shale-gas production and porosity relates to the pore diameter needed to allow gas migration. Natural gas in micropores may be bound too strongly to liberate it without low pressure/vacuum. For gas production either meso-or macropores may be important. Notably, all methods used for characterization of porosity were performed on dry samples. A nm-scale connectivity determined at an illite-smectite interface pore is not supposed to provide a gas pathway in the water-saturated state. Prediction of the gas-production potential from porosity measurements has not been possible to date, therefore. Taking the organic carbon content, vitrinite reflectance, and comparison of the porosity with that of North American gas shales into account, however, indicates potential for the Posidonia shale which still has to be proven by shale-gas production tests.
The analysis of pore space parameters of shale gas formations rocks within the range of 50 to 2 nm
2015
Modern measuring techniques enable the characterization of porous materials in the range up to tens of angstroms, including imaging techniques of such small objects. The paper focuses on the methodology of pore space characterization for shale gas rocks. It presents certain aspects of pore space studies by means of porosimetric analyses, using adsorption isotherm analysis, presenting the methods for choosing physical and analytical models allowing the determination of the pore space parameters for those samples. It refers to detailed applications of selected computational algorithms, adequate for determining parameters of the rocks’ pore space developed within the range of mesoand micropores, i.e. below 50 nm (in accordance with the IUPAC convention).