Low-temperature Nitrogen Adsorption Experiments Research Articles

Reservoir Characteristics of the Main Coal Seams in the Longtan Formation, Guxu Coal Mining Area, Sichuan

To facilitate the efficient exploration and development of coalbed methane of the Longtan Formation in the Guxu coal mining area, Sichuan, it is essential to evaluate the reservoir characteristics of the main production layers. In this study, scanning electron microscopy, low-temperature liquid nitrogen adsorption experiments, high-pressure mercury pressure experiments, and isothermal adsorption experiments were conducted to investigate the reservoir characteristics of favorable coalbed methane reservoirs. The results indicate that the main coal seams in the Longtan Formation are medium-to-high ash content anthracite with a well-developed pore–fracture system. The pore size distribution exhibits both unimodal and bimodal types, while the pore morphology includes impermeable pores closed at one end, open permeable pores, and ink bottle-shaped pores. It shows that the middle part of Longtan Formation acts as an enrichment zone for coalbed methane, which is characterized by a stronger adsorption capacity, high Langmuir volume, high gas content, and high gas saturation distribution. The pH value, mineralization degree, and hydrogen–oxygen isotope of the produced water in the main coal seams indicate that the enrichment zone is typically located in a stagnant flow zone with a reducing environment and favorable storage conditions.

Reservoir characteristics and formation model of Upper Carboniferous bauxite series in eastern Ordos Basin, NW China

Through core observation, thin section identification, X-ray diffraction analysis, scanning electron microscopy, and low-temperature nitrogen adsorption and isothermal adsorption experiments, the lithology and pore characteristics of the Upper Carboniferous bauxite series in eastern Ordos Basin were analyzed to reveal the formation and evolution process of the bauxite reservoirs. A petrological nomenclature and classification scheme for bauxitic rocks based on three units (aluminum hydroxides, iron minerals and clay minerals) is proposed. It is found that bauxitic mudstone is in the form of dense massive and clastic structures, while the (clayey) bauxite is of dense massive, pisolite, oolite, porous soil and clastic structures. Both bauxitic mudstone and bauxite reservoirs develop dissolution pores, intercrystalline pores, and microfractures as the dominant gas storage space, with the porosity less than 10% and mesopores in dominance. The bauxite series in the North China Craton can be divided into five sections, i.e., ferrilite (Shanxi-style iron ore, section A), bauxitic mudstone (section B), bauxite (section C), bauxite mudstone (debris-containing, section D) and dark mudstone-coal section (section E). The burrow/funnel filling, lenticular, layered/massive bauxite deposits occur separately in the karst platforms, gentle slopes and low-lying areas. The karst platforms and gentle slopes are conducive to surface water leaching, with strong karstification, well-developed pores, large reservoir thickness and good physical properties, but poor strata continuity. The low-lying areas have poor physical properties but relatively continuous and stable reservoirs. The gas enrichment in bauxites is jointly controlled by source rock, reservoir rock and fractures. This recognition provides geological basis for the exploration and development of natural gas in the Upper Carboniferous in the study area and similar bauxite systems.

Microscopic mechanism of carbon oxides formation during long-flame coal oxidation at molecular scale

Precursors of carbon oxides in coals are considered to be related to aldehyde group(–CHO) and carboxyl group (–COOH), but the way of decomposition of precursors to produce carbon oxides is not clear. Through theoretical analysis, experimental research and quantum chemical calculation at molecular scale, this paper explores the decomposition mode and products of carbon oxide precursors during coal spontaneous combustion (CSC). A programmed heating device was used to investigate the change rule of gas product concentration during the constant temperature oxidation of coals, and the microstructure of the coals was characterized by low-temperature nitrogen adsorption experiments (BET), X-ray Diffraction (XRD) and X-ray photoelectron spectrometer (XPS). The results of the study show that during coal oxidation, the inhomogeneous energy exchange between the coals and the external heat source results in the change of gas product concentration with time following two rules. BET experiments reveal that oxidation leads to the evolution of fine mesopores to macropores and an increase in the number of pores with diameters larger than 20 nm in the coals, which facilitates the transport of oxygen molecules through the pore structure. Within 200 °C, the microcrystalline structure of coals is basically unchanged, and the active structure C–C/C–H on the side chain is converted to –CHO and –COOH and then decomposed to produce carbon oxides. Quantum chemical calculation shows that the reaction energy barrier of carbonyl group and carboxyl group decomposed step by step to form carbon oxides is much smaller than that of direct decomposition, and carbon oxides and alkyl radicals can be produced even at room temperature by step decomposition. The research results provide theoretical guidance for the formation mechanism of carbon oxides during low temperature oxidation of coals.

Evolution characteristics of pore structure of coal under freeze-thaw cycles combined with scanning electron microscope instrument and nitrogen adsorption experiment

ABSTRACT In this paper, anthracite in Shanxi region is taken as a research object, and the pore structure of coal under different freeze-thaw times is characterized and analyzed by scanning electron microscope and nitrogen adsorption experiment. The SEM experimental results show that the micron-scale cracks on the surface of anthracite gradually connect after freeze-thaw, and the secondary crack channels are formed around the pores and air holes, so that the pores also begin to connect. Further, the quantitative information of the picture was extracted, and it was found that the surface porosity and fractal dimension of the coal sample changed significantly after three freeze-thaw cycles, which increased by 163.3% and 15.9% respectively, and increased with the increase of freeze-thaw cycles. The results of low-temperature nitrogen adsorption experiments show that the coal samples after freeze-thaw are mainly crack, and the pore structure changes from simple to complex. In addition, the maximum nitrogen adsorption capacity, specific surface area and pore volume increased by 125.7%, 69.3% and 72.6%, respectively. With the increase of freeze-thaw times, the growth rates of maximum nitrogen adsorption capacity, specific surface area and pore volume are significantly different. The results of joint characterization show that the number of pores and cracks increases to different degrees after freezing and thawing. Comparing the increase in the degree of fracturing (163.3%) with the increase in the total pore volume (30.68%), we find that the degree of change of the crack after thawing is higher than that of the pore. In this paper, the damage effect of coal samples after freezing and thawing with liquid nitrogen is investigated from an experimental point of view, which accelerates the great leap of cyclic cryogenic fracturing technology from basic theory to technical practice.

Microstructure Study of High-Rank Coal in an Alkaline Solution at the Chengzhuang Mine.

To study the adsorption performance of coal bodies after alkaline solution erosion and the microscopic mechanism of alkali erosion on coal bodies, isothermal adsorption experiments at different pH values and with different numbers of soaking days were conducted on high-order coal bodies from the Chengzhuang mine. The results showed that the adsorption capacity of the coal bodies after alkali leaching was improved compared to that of the original coal, all of which was in accordance with the Langmuir equation. The unit adsorption capacity of coal samples increased gradually with an increase in the number of soaking days and solution pH, reaching the maximum at pH 13 and eight soaking days. The adsorption constant a of the coal sample was positively correlated with the pH, and the number of soaking days was a power exponential function; the adsorption constant b increased gradually with an increase in the pH of the solution and increased first and then decreased with an increase in the number of soaking days. The change in the adsorption of coal samples occurs because the alkaline solution reacts with the minerals in the coal as well as the mineral ions, and the resulting complex gels and precipitates block the pore channels of the coal body, which in turn inhibits the adsorption of gases. The presence of Na, Mg, Al, Si, Ca, Fe, and other elemental compounds detected in the generated sediments verified the mechanism of alkaline solution erosion. The changes in the microscopic pore structure of the coal body were quantified by low-temperature liquid nitrogen adsorption experiments. The small and medium pore volumes of the coal samples reached the maximum values at pH 13 and with eight soaking days, which is in agreement with the conclusion of optimal alkali modification.

Effect of water immersion on pore structure of bituminous coal with different metamorphic degrees

It is important to understand the disaster-causing mechanism of dried water-immersed coal by investigating the influence of water immersion on the coal pore structure. In this study, low-temperature nitrogen adsorption experiments of eight bituminous coals with different metamorphic degrees and their dried water-immersed coals were carried out to analyze the adsorption/desorption mechanism and investigate the effects of water immersion on the pore shape, pore-structure parameters, and fractal dimension of coal. The results showed that adsorption of nitrogen on coal mainly includes micropore filling, monolayer adsorption, multilayer adsorption, and capillary condensation. The pores of coal were a continuous system from nanometer pores to relatively unlimited size pores. Water immersion did not significantly affect the pore shape of the coal, decreased the specific surface area and adsorption capacity, and increased the total pore volume and average pore diameter. It had the greatest effect on the average pore size of moderately metamorphosed coals, which may accelerate the oxidation process of coal and aggravate gas emission. The fractal results showed that water immersion reduced the surface fractal dimension of higher and lower metamorphic coal, while it increased that of moderately metamorphic coal, and it had the opposite effect on the structural fractal dimension.

Effect of new modified materials on the microscopic pore structure and hydration characteristics of sealing materials in coal seam boreholes

To improve the efficiency of coal mine gas extraction, it is necessary to optimize the performance of sealing materials. In this paper, fly ash, nano-silica, and triethanolamine was selected to optimize the pore structure of the sealing materials. The qualitative and quantitative analysis of the seal material was performed by low-temperature liquid nitrogen adsorption experiments, scanning electron microscopy, and combined with fractal theory. The results show that the pores of common sealing materials are mainly mesoporous, accounting for 52.19%, and the volume of micropores is the smallest, accounting for 9.88%. The single doping of fly ash made the pore volume of the material decrease by 9.5%, and the percentage of mesopores increased by 1.3%, but the percentage of micropores decreased by 0.9%. Co-doping with nano-silica and fly ash increased the microporous percentage of the material by 0.77% and the peak stress by 17.43%. The synergistic effect of fly ash, nano-silica, and triethanolamine was most effective in optimizing the pore structure of the sealing material. The pore volume was reduced to 0.08198 cm3/g, and the fractal dimensions D1 and D2 were increased by 2.88% and 2.48%, respectively. The microscopic morphology of the material was also flatter and denser, and the hydration of fly ash was also more complete.

Study on the Variation Laws and Fractal Characteristics of Acoustic Emission during Coal Spontaneous Combustion

Acoustic emission (AE) technology has the advantage of online localization to study the change laws of AE in the process of coal spontaneous combustion and to reveal the generation mechanisms of AE signal during the process of heating and rupture of coal body from a microscopic perspective. This paper first constructs a large-scale coal spontaneous combustion AE test system and conducts experimental tests on the AE signal in the process of coal spontaneous combustion. The results show that with the increase of temperature in the process of coal spontaneous combustion, the AE signal shows a trend of increasing fluctuations. Low-temperature nitrogen adsorption experiments studied the pore structure of coal spontaneous combustion, and the results showed a correspondence between the development of pores and the temperature of coal spontaneous combustion. Further, through the analysis of the evolution of the pore structure of coal through Fourier transform and fractal theory, it is found that the high-frequency main frequency AE signal and average frequency are continuously enhanced with the increase of temperature. The fractal dimension of the pore structure and the fractal dimension of the AE count of the coal body first rise and then decline. The mechanism of coal spontaneous combustion AE of coal is revealed, and the pore development caused by thermal stress when coal heats up is the main source of AE signal generation. The research in this paper is of great significance for applying AE technology to detect the position of coal spontaneous combustion.

Diagenetic Facies Controls on Differential Reservoir-Forming Patterns of Mixed Shale Oil Sequences in the Saline Lacustrine Basin

The Permian Lucaogou Formation has developed mixed shale reservoirs, but there are few studies on the diagenetic facies, and the control effect of differential diagenesis on the reservoir capacity of shale oil reservoirs in this area is not clear. Therefore, shale samples of the Lucaogou Formation were systematically selected in this study, and through cast thin sections, field emission scanning electron microscopy, XRD mineral analysis, low-temperature nitrogen adsorption and high-pressure mercury injection experiments, the reservoir capacity of the shale oil reservoirs was evaluated from the perspective of diagenetic evolution. The results show that the shale oil reservoir of the Lucaogou Formation in Jimsar Sag is in the middle diagenetic stage A. The diagenetic evolution sequence is compaction—chlorite cementation—silica cementation—first-stage carbonate cementation—first-stage dissolution of authentic albite—illite/smectite mixed layer cementation—second-stage carbonate cementation—second-stage dissolution. The shale reservoirs are divided into five diagenetic facies: tuffaceous–feldspar dissolution facies, mixed cementation dissolution facies, chlorite thin-membrane facies, carbonate cementation facies and mixed cementation compact facies. Among them, the former two diagenetic facies have strong dissolution and weak cementation and are high-quality diagenetic facies, mainly characterized by large pore volume and good pore connectivity, with relatively low D2 values defined as the fractal dimension of mesopores. On the basis of the above research, three different control models of Lucaogou Formation shale oil reservoirs are proposed, including dissolution to increase pores, chlorite cementation to preserve pores, and strong compaction cementation to reduce pores. The quality of reservoirs developed in this model is successively high, medium, and low. This work can provide guidance for the fine characterization and grading evaluation of mixed shale oil reservoirs in saline lake basins and has important theoretical and practical significance for the prediction of shale oil “sweet spot” distribution.

Characteristics of Coal Porosity Changes before and after Triaxial Compression Shear Deformation under Different Confining Pressures.

It is important to explore the changes in coal pores in response to triaxial compression and shear deformation for coal mine gas drainage and efficient coalbed methane mining. To study the variation in coal pores depending on stress, first, a mechanical analysis was carried out, and then the characteristics of coal samples before and after triaxial compression were quantitatively analyzed combined with low-temperature nitrogen adsorption experiments. The compressive strength of the coal samples with a high elastic modulus is significantly greater than that of coal samples with a low elastic modulus. Sihe coal samples with a larger elastic modulus experienced higher peak stress and strain during compression than those from the Chengzhuang Mine with a smaller elastic modulus. With the exception of the coal sample from the Chengzhuang Mine with a confining pressure of 15 MPa, the peak strength and axial strain of the coal samples gradually increased with an increase in confining pressure. The larger the elastic modulus, the greater the axial strain. After triaxial compression, pores with diameters ranging from 2 to 5 nm exhibited a significant change. After the compression of coal with a high elastic modulus, the pore volume and pore specific surface area decreased with the increase in confining pressure, by 60.7 and 59.7%, respectively (compared with raw coal). The complex pore structure consisting of mesopores and macropores (>11 nm) became simpler. The volume and specific surface area of the pores of the coal samples with a low elastic modulus first increased, then decreased, and then increased again with the increase in confining pressure, and after compression, the roughness and complexity of macropores of coal samples are greater than those of micropores. The changes induced in the coal samples of the two mining areas in response to compression differ, which are related to the mechanical properties of the coal bodies.

Study on Pore Structure Change Characteristics of Water-Immersed and Air-Dried Coal Based on SEM-BET

ABSTRACT The pore structure of coal affects spontaneous combustion to a certain extent. The pore structure of the coal body will be altered after prolonged waterlogging and is highly susceptible to spontaneous combustion in the presence of elevated ground temperatures. To study the pore structure changes in the coal body after immersion and drying. The coal samples from the Jinjitan coal mine in Yulin, Shaanxi Province, the Tangkou coal mine in Jining, Shandong Province, and the Gaojiapu coal mine in Shaanxi Province were scanned by electron microscopy and low-temperature nitrogen adsorption experiments. This study indicates that after water immersion, the fracture pores of the coal body are developed, the morphology changes, and the percentage of pore fracture area increases significantly. With the increase in immersion time, the pore types of Coal samples A and C did not change, being delicate necked bottle-shaped pores and open, permeable pores, respectively. Coal sample B pore type developed from impermeable pores to open pores. The cumulative pore volume of coals from all three areas increased for the same pore size. The contribution of micropores to the BET specific surface area and total BJH pore volume was more significant than the original coal samples. The specific surface area of A90, B30, and C60 was larger than that of the other coal samples for the soaking time, so it can be assumed that A90, B30, and C60 have a higher propensity for spontaneous combustion. Underwater immersion, the pore type of coal samples from coal mines in Northwest China did not change, but the pore types of coal samples from coal mines in East China changed. Therefore, the pore structure of gas coal was easily changed underwater dissolution.

Effect of Nanoparticle Adsorption on the Pore Structure of a Coalbed Methane Reservoir: A Laboratory Experimental Study.

Coalbed methane (CBM) is an important unconventional energy resource, and its micropore structure has a vital impact on its exploitation. Based on the nuclear magnetic resonance (NMR) experiment, the low-temperature liquid nitrogen adsorption experiment, and the contact angle experiment, in this paper, we investigated the influence of nanofluids on the micropore structure of a CBM reservoir from many aspects. The influence of different adsorption mechanisms of TiO2 nanoparticles on the surface wettability of rock samples was analyzed. The influence of nanoparticle adsorption on the drainage and distribution of liquid in the rock sample was discussed in depth. In addition, the effects of nanofluid treatment on the micropore structure were investigated by comparing the data of low-temperature liquid nitrogen adsorption experiments, including the pore diameter, pore volume, and specific surface area (SSA). The experimental results show that the treatment of nanofluids helps to open the micropores and greatly increases the SSA, pore diameter, and pore volume of the sample. The maximum increase percentages of SSA, pore volume, and pore diameter are 228.12, 80.65, and 18.89%, respectively. It is found that the adsorption of particles is conducive to enhancing the water wettability of the pore throat surface and reducing the damage to water locks.

Characteristics of methane adsorption/desorption heat and energy with respect to coal rank

Changes of heat and energy during methane adsorption and desorption are essential factors affecting the production of coalbed methane. To characterize adsorption and desorption behaviors, low-temperature nitrogen adsorption experiments and methane isothermal adsorption/desorption experiments were carried out on coals with different rank. The Brunauer–Emmett–Teller (BET) specific surface area (SSA) of the coal samples studied range from 0.29 m2 g−1 to 2.31 m2 g−1, and Barrett–Joyner–Halenda (BJH) pore volume ranges from 0.95 × 10−3 cm3 g−1 to 6.14 × 10−3 cm3 g−1. It is observed that the BET SSA and BJH pore volume decrease first and then increase with the increase of coal rank. The isometric heat in the adsorption process is less than that in the desorption process. There is an energy difference between adsorption and desorption, which makes the sorption process irreversible. The larger the Langmuir volume, the higher the adsorption capacity, making the external energy required for the adsorption process smaller and the limit isosteric heat more minor. At different temperatures, the Gibbs free energy of samples BD-1, LL-1 and HC-1 changes between 2.26–2.98 kJ mol−1, 2.05–3.24 kJ mol−1 and 1.91–3.08 kJ mol−1, respectively, indicating that the process of methane desorption on coal is a spontaneous reaction. The cumulative reduction of surface free energy (Δγ) and the reduction of surface free energy at each pressure point (Δγp) show that the adsorption capacity of the low- and medium-rank coal are more easily affected by temperature, while that of the high-rank coal is less affected by temperature. Moreover, the smaller the BET SSA, the greater Δγ and Δγp.

Influence of active water on gas sorption and pore structure of coal

Active water fracturing is one of the commonly used technologies in area for enhanced coalbed methane (CBM) extraction. However, after the fracturing process, active water residues may block the pore of coal and ultimately reduce the production of CBM. To address this limitation, pore structures of Dongtan (DT) and Houwenjialiang (HW) coal samples were subjected to active water fracturing effect using active water with different concentrations of PAM prepared using polyacrylamide (PAM). Methane adsorption experiment was conducted to probe the effect of active water on the adsorption of methane in coal samples. Additionally, low-temperature nitrogen-adsorption experiments (LT-NAT) and nuclear magnetic resonance (NMR) were probed to characterize the discrepancies in the pore structure and chemical composition surface functional groups of the two coal-sample types. The results showed that considerable discrepancies existed in the pore structures of the two coal-sample types; however, their surface functional groups were similar. The active water treatment had contrasting effects on the adsorption behavior of methane features of the two coal-sample types, that is, as concentration of PAM in the active water increased, the methane adsorption of the treated HW coal sample increased whereas that of the treated DT coal sample gradually decreased. The PAM molecules in the active water residue could adsorb methane, and blockage of pores by the residue will reduce the amount of methane adsorption. The combined effect of the two dominated the influence of active water fracturing on methane sorption. Further optimizing the active water formulation to improve the flowback effect can be a very useful channel of reducing damage to the coal seam.

Effects of pore-throat structures on the fluid mobility in chang 7 tight sandstone reservoirs of longdong area, Ordos Basin

Pore structure and fluid mobility are important factors affecting reservoir quality evaluation and effective hydrocarbon resource assessment in tight sandstone reservoirs. In this research, 14 tight sandstone samples from the Chang 7 member of the Upper Triassic Yanchang Formation in the Longdong area of the Ordos Basin were analyzed through high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption (LTNA) and nuclear magnetic resonance (NMR) experiments to clarify the pore-throat structure characteristics of tight sandstone reservoirs and their influence on the occurrence characteristics and mobility of reservoir fluids. The results suggest that the different distributions and combinations of pore and throat types and sizes resulted in three pore-throat structures and four pore-throat spaces in the study area, corresponding to different occurrence characteristics of movable fluid. Among them, the type I pore-throat structure is characterized by large pore throats, high pore-throat connectivity and weak heterogeneity, with the highest corresponding average movable fluid saturation (MFS) and movable fluid porosity (MFP). The pore-throat size distribution (PSD) of type II pore-throat structure has bimodal characteristics with a low pore-throat sorting degree but a good pores and throats configuration relationship with the higher average MFS and MFP. The type III pore-throat structure has developed intercrystalline pores with a smaller average pore-throat radius, the highest fractal dimension and the lowest fluid mobility. Micropores and mesopores, as the main pore-throat spaces in the three types of pore-throat structures, are the main contributors to the petrophysical properties and the main occurrence place for movable fluids. Nanopores with strong heterogeneity and macropores with high permeability both occupy small pore-throat spaces and have little influence on reservoir fluid mobility. It is thus cleat that the pore-throat structure of tight sandstone reservoirs is the main factor affecting reservoir fluid mobility. More specifically, a pore-throat structure with a large pore-throat radius, good pore-throat connectivity and weak heterogeneity is favorable to the occurrence and seepage of movable fluids. Moreover, reservoir fluid mobility is comprehensively influenced by reservoir petrophysical properties and various clay minerals. Lower clay mineral contents and higher reservoir quality are conducive to the high flowability of tight sandstone reservoir fluids.

Experimental study of effect of slickwater fracturing on coal pore structure and methane adsorption

Driven by high pressure, slickwater may intrude into the pores of coal seams, causing changes in the pore structure, and ultimately affecting the flow of Coalbed Methane (CBM). In this study, slickwater prepared with different concentrations of polyacrylamide (PAM) is used to soak coal samples from Inner Mongolia under high pressure to explore the effect of slickwater fracturing on coal seam pores. To study the evolution characteristics of the pore structure of coal samples treated with slickwater, low temperature nitrogen adsorption experiments and methane adsorption experiments are combined. The experimental results show that under the action of external pressure, slickwater invades the pore structure of coal, resulting in a significant decrease in pore volume and specific surface area. Furthermore, with the increase of the pressure and viscosity of the slickwater, the slickwater residue blocks the micropores in the coal pore structure more severely. The damage to methane adsorption by residue is more serious than that to nitrogen adsorption, reflecting that more residue remains in the micropores of coal samples. The development of a gel breaker suitable for slickwater can promote the degradation of PAM polymer molecules and the reduction of residual liquid viscosity after fracturing, and improve the flowback effect. This may be an effective way to reduce reservoir damage.

Insights into the pore structure and pore development pattern of subaqueous volcanic rocks in the Santanghu Basin, western China

Pore structure plays an important role in controlling the oil migration and accumulation mechanism of subaqueous volcanic rocks (SVRs). The Santanghu Basin in western China is a unique region for providing insight into the pore structure characteristics of SVRs; however, little attention has been given to them. Thin-section analysis, scanning electron microscopy, low-temperature nitrogen adsorption (LTNA), pressure-controlled porosimetry, and nuclear magnetic resonance (NMR) experiments are performed on SVRs in the Santanghu Basin to reveal their pore structure characteristics and pore development patterns. The results show that the SVRs are dominated by ink bottle-shaped pores and have the characteristics of rather low matrix permeabilities (the Swanson parameters are mainly <4.0%/MPa), fine pore throats (rapex mainly <0.5 μm) and high capillary pressures (P50 > 14.0 MPa). An effective method, namely, integrating NMR and LTNA experiments, is proposed to estimate the full-range pore size distribution (PSD) of the SVRs. In the full-range PSD, three peaks are identified at pore sizes of <1 μm, 1–10 μm and >10 μm, mainly corresponding to intercrystalline pores (IPs), dissolved pores (DPs), and residual vesicles (RVs), respectively. The storage space of SVRs is dominated by IPs, while the contents of DPs and RVs are the key to SVRs becoming effective reservoirs. The content of RVs is mainly controlled by the depth of the water body during deposition and the distance from the volcanic strata boundary. The higher the content of K and Na in the original composition and the closer the distance to the source rock is, the higher the proportion of DPs. In accordance with the original composition and the depth of the water body during deposition, the SVRs are divided into four types. Moreover, we propose the pore development patterns of the four types of volcanic rocks. RVs mainly exist in type I and type II SVRs, while DPs are important storage spaces in type I and type III SVRs. Finally, type IV SVRs only develop IPs and a few nanoscale DPs.

Study on the Spontaneous Imbibition Characteristics of the Deep Longmaxi Formation Shales of the Southern Sichuan Basin, China

Deep shale gas reservoirs are a significant alternative type of shale gas reservoir in China. The productivity of deep shale gas wells is lower than that of shallow shale, and the imbibition characteristics of deep shale have a significant effect on the retention and backflow of fracturing fluid and the productivity of shale gas wells. In this study, the pore structure characteristics of organic-rich deep shale in the Lower Silurian Longmaxi Formation of Weiyuan-Luzhou play were analyzed by low-temperature nitrogen adsorption experiments, and then the imbibition characteristics and factors influencing deep shale were extensively investigated by spontaneous imbibition and nuclear magnetic resonance experiments. The results show that mainly micropores and mesopores are growing in the deep organic-rich shale of the Longmaxi Formation. The spontaneous imbibition curve of deep shale can be divided into an initial spontaneous imbibition stage, an intermediate transition stage, and a later diffusion stage, and the imbibition capacity coefficient of deep shale is lower than that of shallow shale. The transverse relaxation time (T2) spectrum distributions suggest that clay hydration and swelling produce new pores and microcracks, but then some pores and microfractures close. Deep shale reservoirs have an optimal hydration time when their physical properties are optimal. The increasing pore volume and the decreasing TOC content can enhance the imbibition capacity of shale. An inorganic salt solution, especially a KCl solution, has an inhibitory effect on the imbibition of shale. Higher salinity will result in a stronger inhibitory effect. It is crucial to determine the optimal amount of fracturing fluid and soaking time, and fracturing fluid with a high K+ content can be injected into the Longmaxi Formation deep shale to suppress hydration. These results provide theoretical guiding significance for comprehending the spontaneous imbibition and pore structure evolution characteristics of deep shale and enhancing methane production in deep shale gas reservoirs.

Evolution of pore and fracture of coal under heating–freezing effects: An experimental study

To explore the evolution of pores and fractures of coal under heating–freezing effects, a tube furnace was used to heat samples. The coal sample was heated to 100 °C and 300 °C in air, and liquid nitrogen was used to freeze the heated coal sample. Low-temperature nitrogen adsorption experiments were used to analyse the damage degree of the coal pore structure quantitatively, and the surface characteristics of the coal sample were qualitatively analysed using a scanning electron microscope. Experimental studies have shown that liquid nitrogen injection increases the pore volume of coal. By injecting liquid nitrogen into the heat-treated coal sample, the pore volume of the macropores of the water-saturated coal sample increased by 19.61%. When the heat treatment temperature reached 300 °C, the macroporosity of the dry coal sample increased by 38.75%. In addition, the fractal dimension of the pores increased after liquid nitrogen treatment, indicating that the freezing of liquid nitrogen caused the surface of the coal sample to become rough. The development of pores and fractures caused by liquid nitrogen injection results from the low-temperature fracturing characteristics of liquid nitrogen and the volume expansion caused by the freezing of pore water.

Mechanisms of shale hydration and water block removal

Shale samples of Silurian Longmaxi Formation in the Changning area of the Sichuan Basin, SW China, were selected to carry out scanning electron microscopy, CT imaging, high-pressure mercury injection, low-temperature nitrogen adsorption and imbibition experiments to compare the hydration characteristics of montmorillonite and illite, analyze the main factors affecting the water block removal of shale, and reveal the mechanisms of pore structure evolution during shale hydration. The hydration characteristics of shale are closely related to the composition of clay minerals, the shale with high illite content is not susceptible to hydration and thus has limited room for pore structure improvement; the shale with high montmorillonite is susceptible to hydration expansion and thus has higher potential of pore structure improvement by stimulation; the shale with high illite content has stronger imbibition in the initial stage, but insufficient diffusion ability, and thus is likely to have water block; the shale with high montmorillonite content has weaker imbibition in the initial stage but better water diffusion, so water blocking in this kind of shale can be removed to some degree; the shale reservoir has an optimal hydration time, when it is best in physical properties, but hydration time too long would cause damage to the reservoir, and the shale with high illite content has a shorter optimal hydration time; inorganic cations can inhibit the hydration of clay minerals and have stronger inhibition to illite expansion, especially K + ; for the reservoir with high content of montmorillonite, the cation content of fracturing fluid can be lowered to promote the shale hydration; fracturing fluid with high K + content can be injected into reservoirs with high illite content to suppress hydration.