Microscopic characteristics of tight sandstone reservoirs and their effects on the imbibition efficiency of fracturing fluids:A case study of the Linxing area,Ordos Basin

The Linxing area within the Ordos Basin exhibits pronounced reservoir heterogeneity and intricate micro-pore structures,rendering it susceptible to water-blocking damage during imbibition extraction.This study delved into the traits of tight sandstone reservoirs in the 8th member of the Shihezi Formation(also referred to as the He 8 Member)in the study area,as well as their effects on fracturing fluid imbibition.Utilizing experimental techniques such as nuclear magnetic resonance(NMR),high-pressure mercury intrusion(HPMI),and gas adsorption,this study elucidated the reservoir characteristics and examined the factors affecting the imbibition through imbibition experiments.The findings reveal that:①The reservoir,with average porosity of 8.40%and average permeability of 0.642×10^(-3)μm^(2),consists principally of quartz,feldspar,and lithic fragments,with feldspathic litharenite serving as the primary rock type and illite as the chief clay mineral;②Nano-scale micro-pores and throats dominate the reservoir,with dissolution pores and intercrystalline pores serving as predominant pore types,exhibiting relatively high pore connectivity;③Imbibition efficiency is influenced by petrophysical properties,clay mineral content,and microscopic pore structure.Due to the heterogeneity of the tight sandstone reservoir,microscopic factors have a more significant impact on the imbibition efficiency of fracturing fluids;④A comparative analysis shows that average pore size correlates most strongly with imbibition efficiency,followed by petrophysical properties and clay mineral content.In contrast,the pore type has minimal impact.Micropores are vital in the imbibition process,while meso-pores and macro-pores offer primary spaces for imbibition.This study offers theoretical insights and guidance for enhancing the post-fracturing production of tight sandstone reservoirs by examining the effects of these factors on the imbibition efficiency of fracturing fluids in tight sandstones.

Study on Physical Properties and Related Spectral Characteristics of Composited Soil with Different Ratio of Feldspathic Sandstone and Sand

[Objective] To study the soil texture, water storage and related spectral characteristics of composited soil. [Method] 3 different ratios of composited soils was designed by using feldspathic sandstone and sand by 3 different ratios, 1：1（C1）, 1：2（C2） and 1：5（C3）, and the CK used loess soil and sand by 1：2, then the soil texture, water storage, soil and winter wheat spectral characteristic were measured and analyzed under those treatments in 2010-2013. [Result] （1） With the increasing of the plant- ing year, treatment C1, C2 and C3 all showed decreasing in the proportions of sand and increasing of silt and the CK showed decreasing of clay and increasing of sand, along 1-30 cm soil depth. Treatment C2 showed the proportion of sand, silt and clay were 76.69%, 18.72% and 4.70%, respectively. （2） The water contents of all the treatments were significant different from other in 0-60 cm soil depth, and showed increasing trend with the increasing of sand proportion. Treatment C2 had higher average water content during the 3 years than treatment CK, C1 and C3 by 21.34%, 11.59% and 3.91% in the same soil depth, respectively. （3） In 2013, the spectral reflection curve characters were similar for all treatment of winter wheat at the jointing stage and filling stage along the full-wave band （350-2 500 nm）, and the reflectance was higher in the jointing stage than the filling stage; the reflection peak was found around 550 nm, a part of green light wave band. Treatment C2 showed the highest canopy that all treatments had similar spectral curves, and the relative reflectance of soil increased during 350-1 750 nm wave length along with the increase of the sand proportion; for treatment CK, C1, C2 and C3, the soil spectral reflectance （y） and wave length （x） appeared highly correlated relationships, they were y=0.18 71 In（x）-0.979 4, y=0.158 7 In（x）-0.801 2, y=0.177 1 In（x）-0.910 8 and y=0.184 5 In（x）-0.944 5, respectively. [Conclusion] Synthesizing the soil physical properties and related spectral character indices, treatment C2 generated the best combination ratio of feldspathic sandstone and sand.

Geological and Geochemical Characteristics and Exploration Prospect of Coal-Derived Tight Sandstone Gas in China： Case Study of the Ordos, Sichuan, and Tarim Basins

This work extensively investigated global tight sandstone gas, and geologically and geochemically analyzed the tight sandstone gas in China＇s Ordos, Sichuan, and Tarim basins. We compared typical tight sandstone gas in China with that in North America. We proposed six conditions for the formation of China＇s tight sandstone gas, and illustrated the geological characteristics of tight sandstone gas. In China, gas-bearing tight sandstones were mainly deposited in continental lake deltas and marine-terrigenous facies basin environments, associated with coal-measure strata, and were mostly buried deeper than 2000 in under a formation pressure of 20-30 MPa, with pressure coefficients varying from overpressure to negative pressure. In other countries, tight gas bearing sandstones were dominantly deposited in marine to marine-terrigenous facies environments, occurred in coal-measure strata, and were mostly buried shallower than 2000 m in low-pressure systems. We systematically analyzed tight sandstone gas in the Ordos, Sichuan, and Tarim basins in terms of chemical compositions, geochemical characteristics of carbon isotopes, origins, and sources. Tight sandstone gas in China usually has a hydrocarbon content of 〉95%, with CH4 content 〉90%, and a generally higher dry coefficient. In the three above-mentioned large tight sandstone gas regions,δ13C1 and δJ3C2 mainly ranges from -42%o to -28%o and from -28%o to -21%o, respectively. Type III coal-measure source rocks that closely coexist with tight reservoirs are developed extensively in these gas regions. The organic petrology of source rocks and the carbon isotope compositions of gas indicate that tight sandstone gas in China is dominantly coal-derived gas generated by coal-measure strata. Our analysis of carbon isotope series shows that local isotope reversals are mainly caused by the mixing of gases of different maturities and that were generated at different stages. With increasing maturity, the reversal tendency becomes more apparent. Moreover, natural gas with medium-low maturity （e.g., Xujiahe Formation natural gas in the Sichuan Basin） presents an apparent reversal at a low-maturity stage, a normal series at a medium -maturity stage, and a reversal tendency again at a high-maturity stage. Finally, we proposed four conditions for preferred tight sandstone gas ＂sweep spots,＂ and illustrated the recoverable reserves, proven reserves, production, and exploration prospects of tight sandstone gas. The geological and geochemical characteristics, origins, sources, and exploration potential of tight sandstone gas in China from our research will be instructive for the future evaluation, prediction, and exploration of tight sandstone gas in China and abroad.

Hydrocarbon accumulation characteristics of beachbar sandstones in the southern slope of the Dongying Sag, Jiyang Depression, Bohai Bay Basin, China

A number of beach-bar sandstone reservoir beds are developed in the upper fourth member of the Eocene Shahejie Formation （Es4s） on the southern slope of the Dongying Sag.Based on the analysis of seismic and logging data,with characterization and petrographic studies of core and cutting samples,this paper analyzes the hydrocarbon accumulation characteristics in two typical blocks of the Boxing and Wangjiagang oilfields,especially reservoir bed heterogeneity and migration conditions that influence oil and gas distribution,calculates the index of reservoir bed quality （IRQ） with a mathematical method,and discusses the relationship between driving force and resistance of hydrocarbon accumulation.Taking into account the characteristics of thin interbeds in beach-bar sandstones,an experimental model simulated the characteristics of hydrocarbon migration and accumulation in thin interbedded sandstones with reservoir bed heterogeneity.The results showed that hydrocarbon distribution and properties were extremely non-uniform.Reservoir bed and migration conditions controlled hydrocarbon accumulation in beach-bar sandstones.IRQ is above 0.4 in the main hydrocarbon region.Sand body distribution,structural configuration and fault systems controlled the direction of regional migration and location of hydrocarbon accumulation.Simulation experiments indicated that the change of driving force for hydrocarbon migration affected selective accumulation mechanisms.Hydrocarbon moved vertically along fault zones to the reservoir and resulted in the distribution of hydrocarbon in the reservoir.Two kinds of hydrocarbon accumulation models exist in the study area.One is a hydrocarbon accumulation model controlled by reservoir bed heterogeneity and the second is a hydrocarbon accumulation model controlled by a complex migration system with faults connecting sandbodies.Finally,different exploration strategies should be adopted for the detailed exploration for beach-bar sandstone reservoirs according to different geological backgrounds.

Characteristics and mechanism of low permeability beach-bar sandstone reservoir of Es4 in Dongying sag

Core and cast sections observation and description,and logging,scanning electron microscope and core lab analysis data etc. were applied to the present research of the characteristics and mechanism of low permeability beach-bar sandstone reservoir of Es4 in Dongying sag. The results indicated the reservoir has the characteristics of middle-low pores,low-permeability,low compositional and structural maturity,and thin throat. The low-permeability is mainly due to sedimentation (fine particles and argillaceous inter beds) and diagenesis (compaction,cementation,and dissolution). The cementation reduced the physical property of the reservoir mainly by carbonate cementation,quartz autogeny and enragement,and autogeny clay. Clay minerals usually jam the pores by filling holes,close-fitting the wall of hole,bridging,wrapping grains,and separate attaching the pores and so on. The dissolution is insufficient so as not to improve the porosity and permeability of the reservoir obviously. So it is also an important factor of forming low-permeability reservoir.

Numerical investigation on the fatigue failure characteristics of waterbearing sandstone under cyclic loading

The strength of sandstone decreases significantly with higher water content attributing to softening effects.This scenario can pose a severe threat to the stability of reservoirs of pumped storage power stations developed from abandoned mines,especially when subjected to the cyclic loading condition caused by the repeated drainage and storage of water(fatigue damage).Based on this,it is essential to focus on the fatigue failure characteristics.In this study,the mineral composition of the used sandstone of Ruineng coal mine in Shanxi Province,China,was first tested to elucidate the rock softening mechanism after absorbing water.Next,a numerical model for replicating the mechanical behavior of water-bearing sandstone was established using twodimensional particle flow code(PFC2D)with a novel contact model.Then,16 uniaxial cyclic loading simulations with distinct loading parameters related to reservoir conditions(loading frequency,amplitude level,and maximum stress level)and different water contents were conducted.The numerical results show that all these three loading parameters affect the failure characteristics of sandstone,including irreversible strain,damage evolution,strain behavior,and fatigue life.The influence degree of these three parameters on failure behavior increases in the order of maximum stress level,loading frequency,and amplitude level.However,for the samples with different water contents,their failure characteristics are similar under the same loading conditions.Furthermore,the failure mode is almost unaffected by the loading parameters,while the water content plays a significant role and causing the transformation from the tensile splitting with low water content to the shear failure with higher water content.

Determination of microscopic waterflooding characteristics and influence factors in ultra-low permeability sandstone reservoir

Actual sandstone micromodel was used in this work to conduct the microscopic waterflooding experiment of ultra-low sandstone reservoir,since the inside seepage characteristics of microscopic waterflooding process of Chang 8 ultra-low permeability sandstone reservoir of Upper Triassic Yanchang formation in Huaqing region of the Ordos Basin,China is difficult to observe directly.Combined with physical property,casting thin sections,constant-rate mercury injection capillary pressure and nuclear magnetic resonance,the influence of reservoir property on the waterflooding characteristics in pores were analyzed and evaluated.Seepage paths of waterflooding characteristics were divided into four types:homogeneous seepage,reticular-homogeneous seepage,finger-reticular seepage and finger-like seepage,the waterflooding efficiency of which decreases in turn.More than 70%of residual oil occurs as flowing-around seepage and oil film.Physical property,pore structure and movable fluid characteristics are all controlled by digenesis and their impacts on waterflooding efficiency are in accordance.Generally,the pore throat radius size and distribution and movable fluid percentage are closely related to waterflooding law.

Investigating microscopic seepage characteristics and fracture effectiveness of tight sandstones:a digital core approach

Microscopic seepage characteristics are critical for the evaluation of tight sandstone reservoirs.In this study,a digital core approach integrating microscopic seepage simulation and CT scanning was developed to characterize microscopic seepage and fracture effectiveness(the ratio of micro-fractures that contributes to fluid flow)of tight sandstones.Numerical simulations were carried out for characterizations of tight sandstones.The results show that the axial permeability of the investigated cylindrical tight sandstone from Junggar Basin in China is 0.460μm~2,while the radial permeability is 0.3723μm~2,and the axial and radial effective fracture ratios are 0.4387 and 0.4806,respectively,indicating that cracks are not fully developed and the connectivity between micro-cracks is poor.Directional permeability that is difficult to measure by laboratory experiments can be obtained readily using the proposed method in this paper.The results provide important information for improving the exploration and development of tight sandstone reservoirs.

Heavy Minerals and Geochemical Characteristics of Sandstones as Indices of Provenance and Source Area Tectonics of the Ogwashi-Asaba Formation, Niger Delta Basin

Heavy mineral petrographic and geochemical compositions (major and trace/rare earth elements)?of sandstones obtained from the Oligocene-Miocene Ogwashi-Asaba Formation, Niger Delta were studied to determine their provenance, source area weathering conditions and tectonic setting. The heavy mineral suite (opaque minerals, zircon, tourmaline, and rutile) revealed that the sandstones are mineralogically mature and implied rapid disintegration and chemical decomposition of sediments mostly of recycled orogen. The sandstones were geochemically classified as Fe-sand and partly quartz arenitic. Chemical Index of Alteration and Chemical Index of Weathering values of 89.92% and 91.87% respectively suggest that the source region was predominantly felsic and was subjected to intense chemical weathering probably under tropical palaeoclimatic conditions with abundant rainfall that enhanced sediment recycling. Major element concentration discriminant plots also indicated that the sediments were derived from mixed sources (granitic, gneissic or recycled orogen) under passive margin setting. Chondrite normalized plot of the rare earth element pattern is marked by light rare earth element enrichment and negative Eu anomalies, interpreted to mean that provenance was mainly continental crustal rocks. Trace elemental ratios that are provenance diagnostic (La/Sc, Th/Sc, Cr/Th, La/Co, Th/Co, Th/Cr, Eu/Eu*, and Eu*) all point to sediments derived from felsic source and upper continental crust. The mixed provenance of the sandstones can be traced to the southwestern and southeastern Basement Complex (consisting of granites, gneisses, etc.) and sediments derived from the adjacent sedimentary basins (Anambra and Benue Trough).

Geological characteristics and models of fault-foldfracture body in deep tight sandstone of the second member of Upper Triassic Xujiahe Formation in Xinchang structural belt of Sichuan Basin,SW China

In the second member of the Upper Triassic Xujiahe Formation(T_(3)x_(2))in the Xinchang area,western Sichuan Basin,only a low percent of reserves has been recovered,and the geological model of gas reservoir sweet spot remains unclear.Based on a large number of core,field outcrop,test and logging-seismic data,the T_(3)x_(2) gas reservoir in the Xinchang area is examined.The concept of fault-fold-fracture body(FFFB)is proposed,and its types are recognized.The main factors controlling fracture development are identified,and the geological models of FFFB are established.FFFB refers to faults,folds and associated fractures reservoirs.According to the characteristics and genesis,FFFBs can be divided into three types:fault-fracture body,fold-fracture body,and fault-fold body.In the hanging wall of the fault,the closer to the fault,the more developed the effective fractures;the greater the fold amplitude and the closer to the fold hinge plane,the more developed the effective fractures.Two types of geological models of FFFB are established:fault-fold fracture,and matrix storage and permeability.The former can be divided into two subtypes:network fracture,and single structural fracture,and the later can be divided into three subtypes:bedding fracture,low permeability pore,and extremely low permeability pore.The process for evaluating favorable FFFB zones was formed to define favorable development targets and support the well deployment for purpose of high production.The study results provide a reference for the exploration and development of deep tight sandstone oil and gas reservoirs in China.

Geochemical characteristics of Dongsheng sandstone-type uranium deposit, Ordos Basin

Generally, sandstone-type uranium deposits can be divided into three zones according to their redox conditions: oxidized zone, ore zone and reduced zone. The Dongsheng uranium deposit belongs to this type. In order to study its geochemical characteristics, 11 samples were taken from the three zones of the Dongsheng uranium deposit. Five samples of them were collected from the oxidized zone, four samples from the ore zone and two samples from the reduced zone. These samples were analyzed using organic and inorganic geochemical methods. The results of GC traces and ICP-MASS indicate that the three zones show different organic and inorganic geochemical characteristics.

Sedimentary facies and textural characteristics of Cretaceous sandstones in the southern Bida Basin,Nigeria:Implication for reservoir potential and depositional environment

A study of sedimentary facies,mineral and textural characteristics of sections of conglomerates/pebbles and sandstone facies found within the Lokoja and Patti Formations in southern Bida Basin,Nigeria,is carried out to evaluate the paleo-environment and reservoir quality of the sandstones of the formations through field observations and textural and mineralogical(using X-Ray Diffractogram)analyses.Nine lithofacies are identified and grouped into three main facies associations.These sedimentary facies suggest deposition in both foreshore and estuaries.The mineral analysis(XRD)shows that the quartz content in sandstone facies of the Lokoja Formation ranges between 49%and 67%,and that of the Patti Sandstone between 43%and 56%,indicating a quartz dominance.The granulometric studies reveal that the Lokoja Formation comprises coarse-to medium-grained(-0.63 to 1.30φ)sandstone with moderatepoor sorting of 0.72e1.824.On the other hand,medium to fine-grained sandstones dominate the Patti Formation,which is averagely poorly sorted(1.29-1.54φ).The sandstones are coarsely skewed to finely skewed,with the kurtosis ranging from very platykurtic to leptokurtic.Morphological results of the pebbles from both Lokoja and Patti Formations indicate that the study areas are mainly characterized by the interplay of both wave and fluvial processes,which suggests that the environment of deposition of sandstone facies is predominantly of the fluvial to the shallow marine environment with the indication of tidal influence.Integrating sedimentary facies and textural results suggests subtidal sand ridge/foreshore,estuarine mudflat,and estuarine delta environments with fluvial influences.Evidence from the textural and mineral characteristics indicate that the sandstone facies of the Lokoja and Patti Formations are fairly matured and can act as potential hydrocarbon reservoirs in the formations.The facies,XRD mineral,and textural results have revealed potential hydrocarbon reservoir areas and paleo-depositional features of the Lokoja and Patti Formations.

Imbibition characteristics of sandstone cores with different permeabilities in nanofluids

The core imbibition and shifting nuclear magnetic resonance(NMR) imaging experiment has loss of surface oil phase and air adsorption, which will affect the accuracy of the experiment result. To solve this issue, a modified experiment method, in-situ imbibition NMR method has been worked out. This method was used to carry out sandstone core imbibition experiment in nanofluid, and the oil migration images in the entire process were recorded. In combination with physical properties of the sandstone cores and the variations of the driving force during the imbibition process, imbibition characteristics of the sandstone cores with different permeabilities in nanofluid were analyzed. The results show that: the nanofluid can greatly reduce the interfacial tension of oil phase and improve the efficiency of imbibition and oil discharge, the higher the concentration, the lower the interfacial tension and the higher the efficiency of imbibition and oil discharge would be, but when the concentration reaches a certain value, the increase in imbibition and oil discharge efficiency slows down;the rise of temperature can reduce the oil viscosity resistance and interfacial tension, and hence enhance the imbibition and oil discharge rate;when the sandstone core is higher in permeability, the bottom crude oil would migrate upward and discharge during the imbibition, the higher the permeability of the sandstone core, the more obvious this phenomenon would be, and the phenomenon is shown as top oil discharge characteristic;when the sandstone core is low in permeability, the crude oil in the outer layer of the sandstone core would discharge first during the imbibition, then crude oil in the inside of the core would disperse outside and discharge, which is shown as oil discharge characteristic around the core;but under long time effect of nanofluid, the core would become more and more water-wet and reduce in the oil-water interfacial tension, so would have top oil discharge characteristic in the later stage of imbibition.

Textural Characteristics and Depositional Environment of Ngrayong Sandstone (Middle Miocene) from Rembang Area, Northeast Java, Indonesia

Granulometric analysis of Ngrayong sandstone samples from Rembang area of NE Java (Indonesia) has been carried out to examine the textural characteristics and elucidate depositional environment. The result data from the grain size analysis indicates that the Ngrayong sediments are classified as sand or muddy sands. They are very fine- to medium-grained (phi values: 1.67 to 3.35), moderately- to well-sorted (standard deviation: 0.19 to 0.57), strongly fine-skewed to strongly coarse-skewed (skewness values: 4.82 to −5.97), and very platykurtic to very leptokurtic (kurtosis values: 0.36 to 2.41). Sandstone samples show unimodal grain size distribution. The sediments are interpreted to be transported in all three modes-traction, saltation and suspension, however, suspension and rolling are the major processes during transportation. Discriminant functions indicate diversity in the depositional environment for the sandstones. However, shallow marine is regarded as the dominant depositional environment. The preponderance of fine-grained sediments and lack of coarse sands suggest low to moderate energy conditions during deposition of Ngrayong sediments.

Distribution patterns of tight sandstone gas and shale gas

Based on an elaboration of the resource potential and annual production of tight sandstone gas and shale gas in the United States and China,this paper reviews the researches on the distribution of tight sandstone gas and shale gas reservoirs,and analyzes the distribution characteristics and genetic types of tight sandstone gas reservoirs.In the United States,the proportion of tight sandstone gas in the total gas production declined from 20%-35%in 2008 to about 8%in 2023,and the shale gas production was 8310×10^(8)m^(3)in 2023,about 80%of the total gas production,in contrast to the range of 5%-17%during 2000-2008.In China,the proportion of tight sandstone gas in the total gas production increased from 16%in 2010 to 28%or higher in 2023.China began to produce shale gas in 2012,with the production reaching 250×10^(8)m^(3)in 2023,about 11%of the total gas production of the country.The distribution of shale gas reservoirs is continuous.According to the fault presence,fault displacement and gas layer thickness,the continuous shale gas reservoirs can be divided into two types:continuity and intermittency.Most previous studies believed that both tight sandstone gas reservoirs and shale gas reservoirs are continuous,but this paper holds that the distribution of tight sandstone gas reservoirs is not continuous.According to the trap types,tight sandstone gas reservoirs can be divided into lithologic,anticlinal,and synclinal reservoirs.The tight sandstone gas is coal-derived in typical basins in China and Egypt,but oil-type gas in typical basins in the United States and Oman.

Characteristic strength and acoustic emission properties of weakly cemented sandstone at different depths under uniaxial compression

As coal mining is extended from shallow to deep areas along the western coalfield,it is of great significance to study weakly cemented sandstone at different depths for underground mining engineering.Sandstones from depths of 101.5,203.2,317.3,406.9,509.9 and 589.8 m at the Buertai Coal Mine were collected.The characteristic strength,acoustic emission(AE),and energy evolution of sandstone during uniaxial compression tests were analyzed.The results show that the intermediate frequency(125-275 kHz)of shallow rock mainly occurs in the postpeak stage,while deep rock occurs in the prepeak stage.The initiation strength and damage strength of the sandstone at different depths range from 0.23 to 0.50 and 0.63 to 0.84 of peak strength(σ_(c)),respectively,decrease exponentially and are a power function with depth.The precursor strength ranges from 0.88σ_(c)to 0.99σ_(c),increases with depth before reaching a depth of 300 m,and tends to stabilize after 300 m.The ratio of the initiation strength to the damage strength(k)ranges from 0.25 to 0.62 and decreases exponentially with depth.The failure modes of sandstone at different depths are tension-dominated mixed tensile-shear failure.Shear failure mainly occurs at the unstable crack propagation stage.The count of the shear failure bands before the peak strength increases gradually,and increases first and then decreases after the peak strength with burial depth.The cumulative input energy,released elastic energy and dissipated energy increase with depth.The elastic release rate ranges from 0.46×10^(-3)to 198.57×10^(-3)J/(cm^(3)s)and increases exponentially with depth.

Pore-Throat Combination Types and Gas-Water Relative Permeability Responses of Tight Gas Sandstone Reservoirs in the Zizhou Area of East Ordos Basin, China

With the aim of better understanding the tight gas reservoirs in the Zizhou area of east Ordos Basin,a total of 222 samples were collected from 50 wells for a series of experiments.In this study,three pore-throat combination types in sandstones were revealed and confirmed to play a controlling role in the distribution of throat size and the characteristics of gas-water relative permeability.The type-I sandstones are dominated by intercrystalline micropores connected by cluster throats,of which the distribution curves of throat size are narrow and have a strong single peak(peak ratio>30%).The pores in the type-II sandstones dominantly consist of secondary dissolution pores and intercrystalline micropores,and throats mainly occur as slice-shaped throats along cleavages between rigid grain margins and cluster throats in clay cement.The distribution curves of throat size for the type-II sandstones show a bimodal distribution with a substantial low-value region between the peaks(peak ratio<15%).Primary intergranular pores and secondary intergranular pores are mainly found in type-III samples,which are connected by various throats.The throat size distribution curves of type-III sandstones show a nearly normal distribution with low kurtosis(peak ratio<10%),and the micro-scale throat radii(>0.5μm)constitute a large proportion.From type-I to type-III sandstones,the irreducible water saturation(Swo)decreased;furthermore,the slope of the curves of Krw/Krg in two-phase saturation zone decreased and the two-phase saturation zone increased,indicating that the gas relative flow ability increased.Variations of the permeability exist in sandstones with different porethroat combination types,which indicate the type-III sandstones are better reservoirs,followed by type-II sandstones and type-I sandstones.As an important factor affecting the reservoir quality,the pore-throat combination type in sandstones is the cumulative expression of lithology and diagenetic modifications with strong heterogeneity.

Strength and energy exchange of deep sandstone under high hydraulic conditions

To investigate the influence of confining pressure and pore water pressure on strength characteristics,energy storage state and energy release intensity at peak failure of deep sandstone,a series of triaxial compression tests under hydraulic coupling conditions are carried out.By analyzing the process of rock deformation and failure,the stress thresholds of the rock are obtained.The change trend of total energy density,elastic energy density and dissipated energy density of deep sandstone in the pre-peak stage is obtained by the graphical integration method.By comparing the dynamic energy storage level of rocks under different confining pressures,the influence of pore water pressure on the energy dissipation at stress thresholds of crack closure stress,crack initiation stress,crack damage stress and peak stress is analyzed.Based on the ratio of pre-peak total energy density to post-peak total energy density,the interaction mechanism of confining pressure and pore water pressure for the rock burst proneness of deep sandstone is studied.The experimental results show that the peak stress of sandstone increases with the increase of confining pressure,while the existence of pore water pressure can weaken the peak stress of sandstone.In the stress stage from crack closure stress to peak stress,the dynamic energy storage level of rock presents a trend of the inverse“check mark”.Meanwhile,the larger the confining pressure,the higher the energy storage level of rock.However,the pore water pressure increases the degree of energy dissipation of rock and reduces the energy storage capacity of rock,and the degree of dissipation is linear with pore water pressure.The increase of confining pressure aggravates the instability and failure of deep sandstone,while pore water pressure has the opposite effect.The research results will provide necessary data support for the stability analysis of rock mass excavation in sandstone stratum under high stress and high pore water pressure.

Effect of cyclic wetting-drying on tensile mechanical behavior and microstructure of clay-bearing sandstone

The understanding of the weakening mechanism of tensile strength of rock subjected to cyclic wetting-drying is critical for rock engineering.Tensile strength tests were conducted on a total of 35 sandstone specimens with different wetting-drying cycles.The crack propagation process and acoustic emission characteristics of the tested samples were obtained through a high-speed camera and acoustic emission system.The results indicate that the tensile strength is observably reduced after cyclic wetting-drying,and the extent of the reduction is not only related to the number of wettingdrying cycle,but also closely related to the clay mineral content of the sample.In addition,as the cycles of wetting-drying increase,the effect of each single cycle on tensile strength get reduced until it becomes constant.Moreover,the crack initiation and penetration time is prolonged as the number of wetting-drying cycle increases,which indicates that cyclic wetting-drying weakens the rock stiffness and enhances the ductility of sandstone.Meanwhile,the acoustic emission characteristics of the tested samples further confirmed the ductile behaviour of the sandstone samples with increasing wetting-drying cycle.Furthermore,through the analysis of the microstructure and mineral composition of the samples with different wetting-drying cycles,it is concluded that the main weakening mechanisms of sandstones containing clay minerals are frictional reduction,chemical and corrosive deterioration.