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Zero echo time (ZTE) imaging at 9.4T was used to assess local water saturation level in the tight sandstone rocks. The results were compared with the industry standard porosity estimation basing on T2 relaxation analysis at 0.05T. A linear dependence between the two was achieved. This suggests the possibility to use 3D ZTE method for assessment of local amount of water in rocks. The method can be applicable in investigation of water saturation processes in tight rocks, where imaging methods based on spin echo like RARE failed due to short T2, while single point imaging (SPI) is impractical due to long acquisition time., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Comprehending the effect of freeze-thaw cycles on the damage and degradation of coal-bearing sandstones is crucial for the end-wall slope stability of open-pit mines in cold areas. In this study, freeze-thaw cycle tests on water-saturated coal-bearing sandstone samples under different freezing temperatures and different freeze-thaw cycles were conducted by a fully automatic low-temperature freeze-thaw testing system, and the effects of freeze-thaw cycle parameters on P-wave velocity and porosity of sandstone samples were obtained. With the assistance of CT scanning imaging technology, the microscopic damage and deterioration mechanism of sandstone samples under freeze-thaw cycles was further revealed, and a characterization method for the damage and deterioration of sandstone samples under freeze-thaw cycles was established, and damage and degradation effects of freeze-thaw cycles on the sandstone samples were predicted. The research results suggest that as the freezing temperature decreases and the number of freeze-thaw cycles increases, the P-wave velocity of the sandstone sample decreases, while the volume of the sandstone sample increases. The relative change rate of P-wave velocity and porosity increment of the sample are positively correlated with freezing temperature, and negatively correlated with the number of freeze-thaw cycles. The CT scan results show that with the decrease of the freezing temperature and the increase of the number of freeze-thaw cycles, the number and geometric size of pores on the sample cross section increase significantly. Additionally, the evolution equation of freeze-thaw damage factors was established with freezing temperature and number of freeze-thaw cycles as parameters, and the internal mechanism and physical characterization of freeze-thaw damage degradation of coal measure sandstone were revealed. This research provides a reference for the safety and stability evaluation and technology research and development of related rock engineering in cold areas. [ABSTRACT FROM AUTHOR]

The migration system, as the primary medium linking source rocks and traps, plays a vital role in studying hydrocarbon migration, accumulation, and reservoir formation. This study focuses on Late Paleozoic source rock (mudstone and coal rock) and sandstone samples from the Ordos Basin. By analyzing permeability, porosity, and their ratios under various conditions, this study evaluates the quality of hydrocarbon migration pathways across different lithologic strata, identifies optimal migration routes, and offers new insights for identifying favorable hydrocarbon exploration areas in the Late Paleozoic of the Ordos Basin. The findings indicate that the permeability ratio between parallel and vertical bedding planes in source rock and sandstone samples ranges from 1 to 4. Post-fracturing, permeability increases by over twofold. On average, sandstone permeability is approximately 0.1 × 10⁻3 μm2, while source rock permeability is about 0.03 × 10⁻3 μm2. Key conclusions include that without fracture development, permeability, and porosity parallel to bedding planes outperform those perpendicular to bedding planes, with sandstone showing better properties than source rocks. When fractures are present, permeability and porosity along the fracture direction are highest, followed by sandstone, with source rocks showing the lowest values. These results advance the theoretical understanding of hydrocarbon migration systems and provide significant guidance for hydrocarbon reservoir exploration and development. [ABSTRACT FROM AUTHOR]

The Adigrat Sandstone Formation, representing the siliciclastic assemblage of the Lower Mesozoic succession, underlies the Gohatsion Formation in the Blue Nile Basin. Despite its stratigraphic and geological significance, the impact of diagenesis on the porosity value of the Adigrat sandstone remains poorly understood. By analyzing a detailed field description of two stratigraphic logs, along with the associated thin sections and additional correlated well data, the environment of deposition has been interpreted. The petrographic analysis was carried out to 30 sandstone samples collected from five selected outcrops, and complemented by the two stratigraphic columns. The main diagenetic processes affecting the Adigrat sandstone porosity were compaction, cementation, mineral dissolution, replacement, authigenesis, and recrystallization. The framework grain and cement relationship suggests an early quartz cement precipitation, followed by partial or intense calcite and hematite development in some samples as the second cementation phase. Feldspar alteration to lath-shaped kaolinite clusters causes kaolinite to act as a pore-lining and pore-filling cement, thereby reducing porosity. Conversely, the fracture and dissolution of some samples enhanced the fluid storage capacity. The estimated existing optical porosity (EOP) varies between 1 and 8%, with a mean value of 5%, of which 70% of the samples possess catenary and cul-de-sac porosities. Based on petrographic analysis, the sandstone is mineralogically categorized as sub-mature to mature. These findings significantly contribute to understanding the diagenetic evolution of the Adigrat Sandstone Formation, providing valuable insights for reservoir characterization and exploration strategies in the Blue Nile Basin (BNB). [ABSTRACT FROM AUTHOR]

The stability of the surrounding rock under high temperature is pivotal to the efficient and safe production of high-temperature fluidized mining engineering. To investigate the stability of rocks under high temperature, this paper takes the roof sandstone of Tongxin coal mine, examining changes in its physical parameters such as mass, dimensions, wave velocity, porosity, and permeability after treatment at various temperatures (20–700°C). The results showed that parameters like mass and wave velocity decreased with increasing temperature, while dimensions, porosity, fracture density, and permeability increased. The patterns of change in these physical properties with temperature exhibit a high degree of consistency. Additionally, composition analysis and thermal analysis were conducted to understand the physical and chemical changes occurred in sandstone. Scanning electron microscopy was used to observe microstructural changes in the sandstone. After analysis, the evolution of the internal structure of Tongxin sandstone with heat treatment is categorized into three stages. (1) stable change Stage (20–450°C, 650–700°C): Dominated by dehydration and thermal stress, where pore and fracture structures develop slowly; (2) rapid change stage (450–550°C, 600–650°C): Dominated by the kaolinite dehydroxylation, leading to increased porosity but decreased average pore size; (3) intense change stage (550–600°C): Dominated by the quartz phase transitions, where the thermal stress generated by quartz phase transitions causes dramatic alterations in the internal structure of the sandstone. Furthermore, a correlation model between wave velocity and permeability of sandstone at high temperatures was established based on the interrelationship of these physical properties, providing a new method for real-time monitoring of permeability under high-temperature conditions. [ABSTRACT FROM AUTHOR]

As global CO2 levels increase, the storage of CO2 in saline aquifers is considered a large-scale and cost-effective method for mitigating CO2 emissions. To examine the impact of cyclic CO2 injection on sandstone formations within various saline aquifers, five cyclic injections of differing durations were performed, and the effects of the frequency and duration of CO2 injection cycles on sandstone pore structure were monitored using nuclear magnetic resonance. Additionally, ICP-OES was employed to assess changes in the total dissolved solids and metal ion concentrations in the saline water layer post-injection. The findings reveal that sandstone samples experience structural degradation as a result of cyclic injection. Initially, the proportion of macropores in sandstone samples increases with the number of injections but subsequently decreases. SO42– ions exert a more significant influence on the sandstone pore structure compared to Cl−ions. The concentrations of Ca2+ and Mg2+ ions in the solution initially rise, followed by a subsequent decline. By integrating the principles of pore structure and considering the formation of crystals through the combination of metal cations and anions, the mechanism underlying the changes in sandstone pore structure due to cyclic injection is elucidated. This study explores the effects of CO2 cyclic injection on reservoir sandstone and evaluates how different saline aquifer types affect pore structure deterioration. The research offers valuable reference points and a foundation for laboratory investigations of underground CO2 storage in saline aquifers. [ABSTRACT FROM AUTHOR]

The estimation of permeability is essential as it is closely related to the production rate in hydrocarbon reservoirs. In general, permeability increases with reservoir porosity. However, recent studies have shown that porosity is not a particularly reliable indicator of permeability and the effect of pore structure (pore aspect ratio and distribution) should be considered. In this study, four samples of tight sandstones were collected from the Q area of the Ordos Basin, China, including two samples with the same porosity but different permeabilities, and two samples with the same permeability but different porosities. X-ray-diffraction and thin-section analyses and experiments, under variable pressure for ultrasonic waves and permeability, were carried out. Based on rock-physics modelling, the volume fraction, aspect ratio, and radius of pores/cracks are inverted and estimated with the P- and S-wave velocities and permeability as constraints. The results show good agreement between model predictions and measurements. A comparative analysis indicates that the pore/crack density, the distribution of the aspect ratio, and the radius jointly affect the permeability. [ABSTRACT FROM AUTHOR]

The Longdong region is a recently discovered area for exploring natural gas in the Upper Paleozoic era within the Ordos Basin. The primary layer that produces gas in this area is the Shan1 member of the Permian Shanxi Formation. The research focused on analyzing the petrological characteristics, physical properties, pore structure, and diagenesis characteristics of the reservoir in the 13th member of the Permian Shanxi Formation in the Longdong area of the southwest Ordos Basin. The research was conducted by combining core observation, cast-thin members, physical properties, and other relevant data. Additionally, the study also discussed the primary factors that influence reservoir performance. The analysis indicates that the dominant sandstone types found in the 13th member of the mountain reservoir are primarily quartz sandstone and lithic quartz sandstone. These sandstone types originate from high-energy environments and are located far from the source. The predominant types of pores are intragranular pores and cutting-karst pores, with the pore size primarily falling within the microporous to mesoporous range. The sedimentary facies and diagenesis exert control over the physical qualities of the reservoir. Specifically, the sedimentary facies determines the fundamental physical conditions of the reservoir, while the structure of the sand body significantly influences its physical properties. Compaction is the primary cause of the increased density of the layer's physical properties. The presence of illite is the main factor contributing to the densification of the reservoir through cementation. The limited extent of reservoir dissolution has a minor effect on enhancing reservoir quality. [ABSTRACT FROM AUTHOR]

Porosity, tortuosity, specific surface area (SSA), and permeability are four key parameters of reactive transport modeling in sandstone, which are important for understanding solute transport and geochemical reaction processes in sandstone aquifers. These four parameters reflect the characteristics of pore structure of sandstone from different perspectives, and the traditional empirical formulas cannot make accurate predictions of them due to their complexity and heterogeneity. In this paper, eleven types of sandstone CT images were firstly segmented into numerous subsample images, the porosity, tortuosity, SSA, and permeability of the subsamples were calculated, and the dataset was established. The 3D convolutional neural network (CNN) models were subsequently established and trained to predict the key reactive transport parameters based on subsample CT images of sandstones. The results demonstrated that the 3D CNN model with multiple outputs exhibited excellent prediction ability for the four parameters compared to the traditional empirical formulas. In particular, for the prediction of tortuosity and permeability, the 3D CNN model with multiple outputs even showed slightly better prediction ability than its single-output variant model. Additionally, it demonstrated good generalization performance on sandstone CT images not included in the training dataset. The study showed that the 3D CNN model with multiple outputs has the advantages of simplifying operation and saving computational resources, which has the prospect of popularization and application. [ABSTRACT FROM AUTHOR]

This study focuses on evaluating the quality of the Aouinet Wanin F3B sandstone as a potential hydrocarbon reservoir in Well A37, NC 169A, Wafa Field, Ghadamis Basin, northwest Libya. Capillary pressure data, a key indicator of pore throat size distribution and fluid percolation capability, is crucial for reservoir characterization. However, due to the high costs, time constraints, and environmental concerns associated with mercury injection capillary pressure testing, this study introduces an alternative approach. We utilize routine core analysis data specifically porosity and permeability to model synthetic drainage capillary pressure curves based on Pittman's modified equations. Our results reveal three distinct rock types represent the reservoir intervals, categorized into mega-, macro-, and micro-pores. The uppermost zone of mega- and macro-pores demonstrates excellent to good reservoir qualities. The log–log plot of pore throat radius versus permeability using Pittman's R50 equation yielded a 1 mD permeability cutoff, aligning with common reservoir benchmarks, while the Winland R35 equation produced a cutoff of 0.4 mD, slightly outside the acceptable range which is between 0.5 and 1 mD. These findings offer a cost-effective and reliable alternative for reservoir quality assessment. [ABSTRACT FROM AUTHOR]

Cambrian sandstones comprise a large and saline-only aquifer that can be utilized for CO2 geological storage in the Baltic basin, including Lithuania. The two prospective storage sites with the most potential are located in west Lithuania. Despite the larger area of the Gargždai elevation (233 km2), the Syderiai uplift (62 km2) is characterized by the largest storage volume. The most significant difference between the studied structures is primarily related to the much higher reservoir quality of the Cambrian sandstones at the Syderiai site. The sandstones' average porosity is 16% and their permeability measures 310 mD, while the Gargždai site is characterized by poor reservoir quality (average porosity of 7% and permeability as low as 10 mD in the sandstone). The main controlling parameter for the sandstones is authigenic quartz cementation. The reservoir type is classified as the porous sandstone type for the Syderiai site and as the fractured reservoir type for the Gargždai site. The storage volumes of CO2 of the sites were assessed as 56.7 Mt and 31.3 Mt, respectively. The present study determined that the Syderiai uplift was the prospective site with the most potential for the geological storage trapping of CO2, owing to its high reservoir quality, while the Gargždai elevation is characterized as a potential alternative for CO2 storage combined with EOR technology for oil exploitation, despite its poor reservoir quality. [ABSTRACT FROM AUTHOR]

To investigate the erosive damage effect of the acidic solution on sandstone, experiments were conducted to examine the macro‐ and micro‐characteristics of sandstone samples under different acid etching times. Microscopic morphology changes, pore structure characteristics, mineral composition changes, and mechanical response characteristics were obtained before and after acid etching. Finally, a comprehensive evaluation index for acid etching damage was proposed. The results show that: (1) Acidic solution will erode the sandstone skeleton and cause the sandstone to collapse. The etched surface appears "flocculent" and yellow sediment is produced. (2) The pores in sandstone samples are mainly micro‐ and mesopores, and the total porosity increases exponentially with the duration of acid etching. The volume fraction of micropores can reach up to 81.5%. (3) The acid etching process of sandstone samples includes physical diffusion and chemical dissolution, which can be divided into four stages and three regions from the outside to the inside. After acid etching, the uniaxial compression failure mode of the sample changes from shear to mixed shear‐tensile failure. (4) The comprehensive evaluation index based on three failure modes generated during the loading process shows good consistency with the overall changes of characteristics parameters such as elastic modulus, peak stress, and peak strain. The research findings of this paper can provide theoretical support for the assessment of rock mass stability and disaster prevention in acidic environments. [ABSTRACT FROM AUTHOR]

Water flooding is an important way to improve recovery in low‐permeability sandstone oil reservoirs. How to decouple the water flooding process using dynamic and static information is a hot topic. In this paper, taking the Paleocene low‐permeability oil sandstone, BY area, eastern Nanxiang Basin as an example, the microscopic water flooding process in the low‐permeability sandstone matrix was systematically investigated, and the characteristics of water channeling under the conditions of fracture existence were analyzed using the dynamic and static monitoring data. The results show that the target layer mainly develops frequently thin stacked composite sand bodies. Under the combined influence of matrix and fracture seepage, the low‐permeability sandstone developed by water flooding shows that there is a single direction of efficiency. The direction of advantageous water advancement is 45° north–east, and the speed of water flooding advancement is 2.57 m/day. Microscopic water‐drive oil experiments show that bound water is mainly distributed in the original low‐permeability sandstone as a membrane in the pore wall and as short rods in the throat. Differences in pore structure and petrophysical properties affect the residual oil content and degree of oil recovery. For sandstones with good petrophysical properties, mild water flooding can improve crude oil recovery. The increase in oil production is mainly concentrated in the early stage of water flooding development, and the increase in oil recovery degree is not significant with the increase in injection multiples in the middle and late stages. However, for sandstones with relatively poor petrophysical properties, water flooding is more effective in the early and late stages than in the middle stages. Therefore, it is necessary to adjust the water flooding measures according to the differences in the petrophysical properties of the sand body. Local tectonics and natural fracture strikes are important factors affecting the direction of the expansion of water flooding fractures. Overall, the prevention of water channeling in low‐permeability sandstones has to take into account the complex coupling between water flooding fractures, natural fractures, and hydraulic fractures. [ABSTRACT FROM AUTHOR]

Tight reservoirs, with their nanoscale pore structures and limited permeability, present significant challenges for oil recovery. Composite fracturing fluids that combine both fracturing and oil recovery capabilities show great potential to address these challenges. This study investigates the performance of a slickwater-based fracturing fluid, combined with a high-efficiency biological oil displacement agent (HE-BIO), which offers both production enhancement and environmental compatibility. Key experiments included tests on single-phase flow, core damage assessments, interfacial tension measurements, and oil recovery evaluations. The results showed that (1) the slickwater fracturing fluid effectively penetrates the rock matrix, enhancing oil recovery while minimizing environmental impact; (2) it causes substantially less damage to the reservoir compared to traditional guar gum fracturing fluid, especially in cores with little higher initial permeability; and that (3) oil recovery improves as HE-BIO concentration increases from 0.5% to 2.5%, with 2.0% as the optimal concentration for maximizing recovery rates. These findings provide a foundation for optimizing fracturing oil displacement fluids in tight sandstone reservoirs, highlighting the potential of the integrated fracturing fluid to enhance sustainable oil recovery. [ABSTRACT FROM AUTHOR]

This study aims at simulating rock erosion by slake durability testing under wet and dry conditions. Phra Wihan sandstone and conglomeratic and bedded sandstones from Phu Phan formation are used as rock specimens. The test parameters are modified from the standard to accelerate the erosion process, where 2,000 drum revolutions are used instead of 200 revolutions for up to 80 test cycles (80 days). Results indicate that fragment roundness and sphericity increase with test cycles. Bedding planes reduce the roundness of bedded sandstone as the fragments become smaller. Phra Wihan sandstone is physically insensitive to water. The water-sensitive and soft Phu Phan sandstone, however, shows notable increases of porosity and reduction of density under both wet and dry conditions. Scrubbing and colliding processes mainly reduce the fragment sizes, under dry condition. Under submerging condition, even though fragment weight is decreased by its buoyancy force, intergranular bonding of the two Phu Phan sandstones is weakened by water penetration, leading to higher percentage of passing materials and lower energy required to disintegrate the rocks than under dry condition. Water insensitive Phra Wihan sandstone erodes more quickly under dry condition than under wet condition. Even though it requires longer period to erode under water submersion, due to buoyancy force, it consumes less energy than those under dry condition to reach the same fragment sizes. Larger sandstone fragments use energy more efficiently to reduce their size than the smaller ones. [ABSTRACT FROM AUTHOR]

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Due to excavation disturbances and the coupled hydro-mechanical effects, deep rock masses experience nonlinear large deformations in the surrounding rock, necessitating an urgent exploration of the rock damage and failure mechanisms from the perspectives of hydro-mechanical coupling and mechanical properties. Therefore, this study conducted uniaxial cyclic loading-unloading tests on sandstone samples with different water contents (0%, 0.26%, 0.52%, 0.78%, and 1.04%) to investigate the microstructural evolution, energy evolution laws, and failure characteristics under varying water contents and cyclic loading conditions. The main conclusions are as follows:(1) Concerning micro-pore structures, as the water content increases, the porosity and maximum pore size of the sandstone first decrease and then increase. At 0% water content, the porosity is 4.82% and the maximum pore size is 31.94 µm. At 0.26% water content, both porosity and maximum pore size decrease to 3.03% and 16.15 µm, respectively. When the water content reaches 1.04%, the porosity and maximum pore size increase to 14.34% and 45.99 µm, respectively. (2) Regarding energy evolution laws, the energy evolution of the specimens during cyclic loading-unloading mainly converts to elastic energy, showing a step-wise increase in energy. Further analysis reveals that the water content has a significant impact on the dissipation energy coefficient of the sandstone. At lower stress levels (<0.4σmax), the water content has a negligible effect, while at higher stress levels (>0.85σmax), an increase in water content leads to increased fluctuations in the dissipation energy coefficient. (3) In terms of failure characteristics, with increasing water content, the failure mode of the specimens shifts from primary crack failure to microcrack failure, corresponding to the energy evolution during cyclic loading-unloading processes. [ABSTRACT FROM AUTHOR]

Tight sandstone reservoirs have extremely low porosity and permeability. Bedding-parallel fractures (BPFs) contribute prominently to the storage and seepage capability. However, the distribution of BPFs is remarkably heterogeneous, impeding the prediction and modeling of sweet spots. BPFs are controlled fundamentally by laminations, which are widely distributed in lacustrine tight reservoirs and provide most weakness planes. Based on core and thin section data, BPFs of the upper Triassic Chang 7 tight oil reservoir are characterized microscopically. The lamination combination unit, which is defined by distinctive lamination assemblage and relatively stable lamination thickness and space, is utilized as a homogeneous unit to measure the density of lamination and related BPFs. The influence of laminations on BPFs is discussed further. Results show that most bedding-parallel fractures are unfilled, with apertures generally <40 μιτι, mainly <10 μιτι. Larger apertures correlate with low filling degrees. The distribution of BPFs is intricately controlled by lamination type, density, and thickness. (1) BPFs tend to develop along different types by a priority sequence which reflects their mechanical strength. The development degree of BPFs also depends on the mechanical contrast with adjacent laminations; (2) When controlled by a single type of lamination, the density of BPFs increases with lamination density under a turning point and then decreases; (3) BPFs prefer to develop along the thinner lamination and are usually inside it, while controlled by thick lamination, BPFs tend to extend along the edge. The change in the thickness of laminations leads to a change in the development position of BPFs, indicating that the position of the weak plane controls the development position of BPFs; (4) When multiple types of lamination coexist, the type and thickness of laminations jointly influence the development of BPFs. Plastic thin laminations are conducive to the development of BPFs, while brittle thick laminations are not conducive. When the thickness of the plastic lamination is close to or less than that of the brittle, the influence of lamination type dominates BPFs, while the thickness of the plastic laminations is much larger than the brittle, the influence of lamination thickness will dominate. [ABSTRACT FROM AUTHOR]

Fractures play a crucial role in tight sandstone gas reservoirs with low permeability and low effective porosity. If open, they not only significantly increase the permeability of the reservoir but also serve as channels connecting the storage space. Among numerous fracture identification methods, seismic data provide unique advantages for fracture identification owing to the provision of three-dimensional information between wells. How to accurately identify the development of fractures in geological bodies between wells using seismic data is a major challenge. In this study, a tight sandstone gas reservoir in the Kuqa Basin (China) was used as an example for identifying reservoir fractures using deep-learning-based method. First, a feasibility analysis is necessary. Intersection analysis between the fracture density and seismic attributes (the characteristics of frequency, amplitude, phase, and other aspects of seismic signals) indicates that there is a correlation between the two when the fracture density exceeds a certain degree. The development of fractures is closely related to the lithology and structure, indirectly affecting differences in seismic attributes. This indicates that the use of seismic attributes for fracture identification is feasible and reasonable. Subsequently, the effective fracture density data obtained from imaging logging were used as label data, and the optimized seismic attribute near the well data were used as feature data to construct a fracture identification sample dataset. Based on a feedforward neural network algorithm combined with natural fracture density and effectiveness control factor constraints, a trained identification model was obtained. The identification model was applied to seismic multi-attribute data for the entire work area. Finally, the accuracy of the results from the training, testing, and validation datasets were used to determine the effectiveness of the method. The relationship between the fracture identification results and the location of the fractures in the target reservoir was used to determine the reasonableness of the results. The results indicate that there is a certain relationship between multiple seismic attributes and fracture development, which can be established using deep learning models. Furthermore, the deep-learning-based seismic data fracture identification method can effectively identify fractures in the three-dimensional space of reservoirs. [ABSTRACT FROM AUTHOR]

In low-pressure gas reservoirs, water-based fracture fluid is difficult to flowback, which is unfavorable for several tight sandstone gas reservoirs in the Sichuan Basin with low pressure and high permeability geological characteristics. Supercritical CO2 possesses a number of remarkable physical and chemical features, including a density near to water, a viscosity close to gas, and high diffusion. Supercritical CO2 fracturing is a new type of non-aqueous fracturing method that is favorable to fracturing flowback in low-pressure tight sandstone and has a wide range of applications. To discuss on whether supercritical CO2 fracturing with low pressure tight sandstone is feasible. Tight sandstone cores from the Jinqiu gas field in the Sichuan Basin were used to study the influence of supercritical CO2 on the physical properties of sandstone reservoirs. Supercritical CO2 was used to interact with tight sandstone samples, and then the changes in porosity, permeability, and rock microstructure of tight sandstone were observed under various time, pressure, and temperature conditions. After the interaction between tight sandstone and supercritical CO2, new dissolution pores will appear, or the original pores will be increased, and the width of some natural fractures will also be increased, and the porosity will increase by 1.09%e8.85%, and the permeability will increase by 2.34%e21.26%, quantifying the influence of supercritical CO2 on physical properties of tight sandstone, and further improving the interaction mechanism between supercritical CO2 and tight sandstone. This study improves in the understanding of the tight sandstone-supercritical CO2 interaction mechanism, as well as providing an experimental foundation and technological guarantee for field testing and use of supercritical CO2 in low-pressure tight sandstone gas reservoirs. [ABSTRACT FROM AUTHOR]

To improve the signal-to-noise ratio (SNR) of nuclear magnetic resonance (NMR) echo data with SNR below 8 obtained from tight sandstone reservoirs and improve the accuracy of the porosity obtained from T 2 inversion, a novel de-noising method is developed to improve the processing of NMR echo data. The algorithm is based on variational mode decomposition wavelet threshold (VMD-WT). It was found that as the decomposition mode number increases, the center frequency difference between the first mode and the second mode gradually decreases. A method to determine a reasonable mode decomposition number based on the center frequency difference of the modes is proposed. The effectiveness of the algorithm is verified by the application in the numerical simulation experiments and actual NMR echo data, respectively. The results indicated that, compared with wavelet threshold de-noising method, VMD-WT de-noising method is more suitable for the de-noising of NMR echo data with SNR below 8 obtained from tight sandstone reservoirs. [ABSTRACT FROM AUTHOR]

For reservoirs of oil wells with no cored data, predicting porosity from wireline logs and core samples is an effective approach. Integration of conventional well logs and core samples to predict porosity with high accuracy is particularly challenging owing to complex logging responses of tight sandstone. Therefore, a novel prediction workflow based on a linear interpolation algorithm (LIA) is described to estimate porosity from well logs in the present study. Based on core reposition, porosity correction under overburden pressure, core–log data matching and calculation of shale content, two multi-regression formulae to estimate porosity values are obtained using the nearest neighbour algorithm and LIA respectively. The formulae are applied to the tight sandstone in the Chang 9 member of the Yanchang Formation in the NE Wuqi Oilfield, Ordos Basin. The comparison results indicate that the porosity predicted from the formula obtained by the LIA is in better agreement with the measured porosity, showing a better prediction effect. The application example demonstrates that the LIA formula is applicable for core porosity prediction in the study region. This approach can also be applied for porosity prediction in other oil regions that have similar geological background. [ABSTRACT FROM AUTHOR]

Solitary boulders emerge in the thick-bed red sandstone in the Chishui red bed area due to weathering, which can potentially induce engineering disasters. This paper measures the porosity, pore size, pore throat distribution, T2 spectrum distribution, chemical oxide content, and rebound value of the red sandstone boulder at different depths, and explores their variation trends, rules, and relationships. The research results show that: (1) the deterioration of red sandstone pores caused by weathering is reflected in the increase of porosity and pore size. The pore size of the red sandstone in the slightly weathered zone is dominated by micro and small pores, accounting for > 75.11%. There are mainly three peaks on the T2 spectrum curve of red sandstone. With the enhancement of weathering, the T2 spectrum curve shifts to the right, and the peak area gradually increases as a whole. (2) The element enrichment intensity of red sandstone is in the order of Fe > Ti > Si > Al, and the loss intensity is in the order of Ca > Mg > Na. The growth rate of rock rebound value decreases continuously with the increase of rock depth, and the rebound value eventually tends to be stable. (3) The chemical alteration index CIA can be indicative of the deterioration degree of physical properties (porosity etc.) of red sandstone, and the division of slightly and moderately weathered zones. (4) The rock rebound value shows that the mechanical properties of red sandstone decrease with the increase of weathering degree, which is closely related to the damage of rock pore structure. [ABSTRACT FROM AUTHOR]

The Wenchang Formation in the Lufeng Sag is a typical low-permeability sandstone reservoir. The complex pore structure and reservoir heterogeneity are the keys to restricting the increase in reserves and production in the study area. The pore throat structure of sandstone reservoir in the study area was characterized by experimental methods such as casing thin section observation, high-pressure mercury intrusion and constant velocity mercury induction. The pore throat types of reservoir are divided based on fractal curve characteristics and pore throat structure parameters, and the pore evolution patterns of different types of reservoir and their impact on physical properties are discussed based on the physical properties of the reservoir and the properties of interstitial materials. The findings delineate the classification of reservoris within the study area into three distinct categories: type I reservoirs are identified as those of exceptional quality, possessing optimal physical attributes with an average discharge pressure of 0. 1 MPa; type II reservoir are acclaimed as superior, exhibit discharge pressures predominantly within the range of 0. 5 to 1. 0 MPa; whereas type III reservoirs are regarded as the least favorable, characteristically have discharge pressures exceeding 1. 0 MPa. The observed low porosity and reduced permeability of the sandstone reservoir in this area emerge from a synthesis of variables including the type of fill material, degree of solubility, and the properties of alteration by-products. Predominantly, the extensive infusion of tuffaceous material within the primary pore spaces, compounded by a modest extent of dissolution and erosion, is the principal contributor to the diminished porosity of these reservoirs. Concurrency, the by-products of these processes tend to obstruct the pore throats, thus playing a pivotal role in the genesis of the low-permeability attributes of the reservoir under study. The results provide guidance for the division and effective development of low-permeability reservoir. [ABSTRACT FROM AUTHOR]

Tight sandstone reservoirs have the characteristics of strong heterogeneity, complex pore throat relationship and poor correction between porosity and permeability. Taking the tight sandstone reservoirs of Xujiahe Formation in the Central Sichuan Basin as the research object, the full-scale capillary pressure curve was established by using high-pressure mercury injection and nuclear magnetic resonance method to characterize the full pore size distribution, and the relationship between different pore throat structure characteristic parameters and reservoir macroscopic physical properties was clarified, and the pore throat structure of tight sandstone reservoir was classified and evaluated. The results show that the NMR pore size distribution calibrated by large pores (≥50 nm) in the high-pressure mercury intrusion experiment can accurately characterize the full pore size distribution. The kurtosis parameters calculated by the full aperture distribution spectrum have a good correlation with porosity, which can be used to evaluate the macroscopic reservoir capacity of the reservoir. The correlation factor calculated by the pseudo capillary pressure curve has a good correction with the fractal dimension parameters and permeability, which can evaluate the macroscopic seepage capacity of the reservoir. The pore throat structure of tight sandstone reservoirs in the study area is divided into three categories, from class I to class III. The reservoir space type changes from the dominant residual intergranular pores to the dominant intragranular dissolved pores. The kurtosis gradually becomes smaller, the fractal dimension becomes larger, and the reservoir capacity and seepage capacity decrease. The results provide a bass for improving the exploration and development efficiency of tight sandstone gas reservoirs. [ABSTRACT FROM AUTHOR]

Taking the Jurassic Qigu Formation in Yongjin Area of Junggar Basin as an example, based on the data of core, thin section, reservoir physical properties and logging data, the quantitative study of heterogeneity parameters was carried out to analyze the heterogeneity characteristics of deep tight sandstone reservoirs and clarify the controlling factors of reservoir heterogeneity. The results show that the Qigu Formation in the study area is divided into 1-6 sub-layers from bottom to top. The grain size rhythm and permeability rhythm are mainly positive rhythm. In terms of intra-layer heterogeneity, the heterogeneity of the three sub-layers is the strongest. In terms of interlayer heterogeneity, the sand bodies of 1-3 layers have a large distribution area and good connectivity, and the sand bodies of 3 layers are the most developed. In terms of plane heterogeneity, 1-4 layers have strong heterogeneity. The heterogeneity of small layer 3 is the strongest, followed by small layer 2, small layer 4 and small layer 1. Sedimentation affects sand body distribution and material composition, which is the basis of reservoir heterogeneity. Diagenesis can improve pore structure and type, which is the key factor affecting reservoir heterogeneity. The results have guiding significance for oil and gas exploration in Yongjin Area of Junggar Basin. [ABSTRACT FROM AUTHOR]

In order to understand pore evolution in relation to sedimentary facies, a study has been conducted on the Chang 6 reservoir in Western Ordos Basin. Tight reservoir space comprises primary intergranular pores with an average original porosity is 36.29%, and secondary pores related to feldspar dissolution. Reservoir porosity is 8.03–11.43% and permeability is 0.12 × 10−3–1.31 × 10−3 μm2. Cementation and mechanical compaction are the main reasons for the quality of the reservoir. Dissolution of feldspar grains improved the reservoir quality. The reservoir has undergone four evolutionary stages: ① Syn-sedimentary compaction which decreased the porosity by 21.27%, to an average remaining porosity was 15.12%. ② Early diagenetic cementation which decreased the porosity by 6.23% to an average residual porosity was 8.89%. ③ Early stage of mesodiagenesis: involving dissolution and formation of micro-fractures that led to an increase in porosity by 5.47% resulting in an average porosity of 14.36%. ④ Compaction and cementation during the late mesodiagenetic stage resulting in an average porosity of 10.87%. Quantitative calculations reveal an average reservoir porosity is 10.68% with an error of 1.75%. The results contribute to a better understanding of the main controlling factors and pore evolution characteristics of tight reservoir development. [ABSTRACT FROM AUTHOR]

Reducing CO2 emission from fossil fuels is crucial for the global aim of constraining greenhouse gas release into the atmosphere and the consequent adverse impact of the rising global temperature. One prominent approach for reducing the CO2 influx concerns capturing and storing CO2 in subterranean reservoirs. The properties of deep subsurface reservoirs that are appropriate for storing gasses require extensive scrutiny, including (i) the assessment of their reservoir characteristics, (ii) examinations of the nature of the caprock, and (iii) continuous monitoring of the movement of injected gas plumes. The sedimentary strata of Saskatchewan contains a number of reservoirs that are potentially good for Carbon Capture and Storage (CCS). The Late Ordovician Winnipeg Formation in Southern Saskatchewan constitutes a lower highly porous sandstone unit of the Black Island Member. Volumetric calculations of the Storage Space Capacity Potential of the sandstone indicate a subterraneous 974 km3 pore space. The porous sandstone unit is capped by a shale unit (Icebox Member). Thus, juxtaposition of these two lithologies makes the formation an excellent candidate for CCS. [ABSTRACT FROM AUTHOR]

Cataclastic bands in high-porosity sandstones significantly influence fluid flow, thus impacting the exploration and development of oil and gas. However, little experimental research has been conducted on the main factors controlling the formation, evolution, and physical properties of cataclastic bands. Moreover, it is difficult to use field surveys to discern variations and trends in the structural and physical properties of cataclastic bands formed during different deformation processes. In this study, we used a high-pressure and low-velocity ring-shear apparatus to analyze high-porosity, pure sandstone. Multiple sets of ring-shear experiments were carried out using the effective normal stress or shear displacement as a single variable. The experimental samples were analyzed based on physical property tests and thin sections. Our results indicate that the particles in the cataclastic bands generally have better roundness and are smaller (by at least two to three orders of magnitude) than the host rock. The porosity and permeability of the cataclastic bands are ~70% lower and two to three orders of magnitude lower than those of the host rock, respectively. The characteristics of the cataclastic bands are controlled by two main factors, namely, the effective normal stress and shear displacement. The effective normal stress controls the intensity of the cataclasis, and the shear displacement controls the physical properties of the grains and indirectly controls the evolutionary stage, which corresponds to the intensity of cataclasis. As the effective normal stress or shear displacement increases, the cataclasis in the cataclastic bands intensifies, and the grain size decreases; then, the decrease in the porosity gradually declines, and the permeability decrease and thickness increase and then plateau. The results of this study reveal the evolutionary mechanisms of the structural and physical properties of cataclastic bands in high-porosity sandstones and lay a theoretical foundation for determining the effect of these bands on fluid flow in oil and gas reservoirs. [ABSTRACT FROM AUTHOR]

In this study, the pore structure and oil-water phase seepage characteristics of sandstone reservoirs of the Yanchang Formation in the Ordos Basin were deeply analyzed. Through scanning electron microscopy observation, oil-water phase percolation experiments and nuclear magnetic resonance (NMR) experiments on 30 core samples, the pore structure was categorized into uniform macroporous, striated, microfractured and dense types. The experimental results show that the uniform macroporous type pore structure shows the strongest seepage capacity, and its ratio of liquid-measured to gas-measured permeability is 0.724 on average, while the dense type pore structure has the weakest seepage capacity, and the ratio is 0.091 on average; NMR T2 spectral analysis reveals that the cores with different types of pore structure show significant characteristic differences in the water-driven oil drive process, especially in the substitution of oil in the macroporous and micrometer pore In particular, the oil substitution in large pores and micron pores is remarkable, that is, the oil-water seepage process and the response of seepage and suction of cores with different types of pore structure are obviously different, which is a better way to study the characteristics of oil-water seepage and distribution of oil and water; the uniform macroporous type and the striptype pore structure are more effective in oil-water two-phase seepage, and the microfracture type and the dense type are relatively poor. This paper provides a basis for the exploitation of tight reservoirs in the study area, which is of great theoretical and practical significance for optimizing the oilfield development strategy and improving the efficiency of oil and gas exploitation. [ABSTRACT FROM AUTHOR]

Particle migration in oil and gas reservoirs is a common phenomenon in the process of oil and gas development, and is considered to be an important reason for the damage of reservoir permeability and the reduction of oil and gas productivity. The mechanism of this phenomenon includes the desorption, migration, and precipitation of particles, which eventually clogs the throat and causes reservoir damage. Therefore, it is necessary to accurately characterize the complex mechanism of particle migration and identify the main controlling factors of particle migration, which is very important for efficient oilfield development and plugging solution. First, the reservoir types are divided into three types and the pore structure models of different types of reservoirs are established. Then, computational fluid dynamics and discrete element coupling method numerical simulation and microscopic visualization of pore throat structure model were combined, to characterize the rules of particles and migration, and analyze the main controlling factors. Finally, a typical model of particle migration and clogging is established. The results show that particle size/throat and particle concentration are the key factors affecting particle plugging, and particle migration has the least effect on the permeability of Type I reservoir and the greatest damage to Type III reservoir. According to the mechanical and hydrodynamic behavior of particles in porous media, three mechanisms and six modes of particle plugging are proposed. [ABSTRACT FROM AUTHOR]

This study undertakes a detailed analysis of the pore systems in the mid-Qusaiba or intra-Qusaiba sandstones within the Tabuk basin, focusing on their potential as unconventional hydrocarbon reservoirs. The research leverages the value of outcrop studies to mirror the subsurface characteristics of analogs in the Ghawar and RubalKhali oil fields, emphasizing the significance of outcrop analyses in enhancing the understanding of subsurface conditions. By examining sedimentological and diagenetic processes, the study provides insights into the depositional environment. This study investigates various facies, depositional units, and diagenetic processes affecting the pore system of Intra-Qusaiba sandstone. We integrated field sedimentological sections and laboratory-based analyses for facies examination. Techniques such as SEM–EDS, permeameter–porosimeter, NMR, N2 adsorption isotherm, and micro-CT scanning were utilized to measure porosity, permeability, and the multi-scale pore size distribution. Three depositional units comprising eight facies were identified, interpreted as having been deposited in settings influenced by turbidity currents, spanning from slightly below to above the storm-wave base. The lower unit, characterized by turbiditic and finer grains, exhibits abundant dissolution pores resulting from k-feldspar and clay mineral dissolution, alongside smaller-scale pores associated with clay and mica cleavage planes. The middle unit, with turbiditic and coarser grains, is marked by relatively larger triangular intergranular pores between detrital quartz and feldspar grains and minor smaller pores within chlorite flakes, denoting optimal reservoir conditions. The third, bioturbated unit displays a highly heterogeneous pore system, with both negatively and positively skewed pore size distributions, and a lesser abundance of intergranular pores compared to the middle unit. Early carbonate and phosphate cementation in this unit adversely impacts the pore system. Digenetic alterations, including compaction, quartz overgrowth, and the dissolution of feldspar, play a critical role in shaping the pore structure. Compaction leads to pore destruction, especially in deeper burial stages, whereas feldspar dissolution can enhance porosity. The findings of this study offer insights into the pore systems of equivalent subsurface Intra-Qusaiba sandstone and other turbiditic-influenced successions. [ABSTRACT FROM AUTHOR]

Petrographic and petrophysical characterizations of the pre-salt Chela formation of the Nsiamfumu and Liawenda fields in the onshore Coastal Basin of the D.R. Congo it was performed. These characterizations, as part of the static modelling of the reservoirs, involved reading log measurements, thin-slice observations of rock samples taken from this formation, calculations and interpretation of logs, CPI (Computer Processed Interpretation) and DST (Drill Stem Testing) from well tests. The following observations were obtained: • Wells Lw-1: the formation is characterized by dolomitic limestone and micaceous black shale with a thickness of 34 m having heavy oil indices and an average porosity of 17% and a salinity greater than 300 gr/L; • Well Lw-2: the formation is characterized by islands of sand, coarse sandstone and a few quartz pebbles with a thickness of 10m showing evidence of light oil, gas and an average porosity of 24%; • Shaft Lw-3: the formation is dominated by micaceous shale and dolomite interbeds with a thickness of 14m showing little evidence of hydrocarbons, only salt water and residual oils; • Puits Ns-1: the formation is characterized by beige to grey dolomite, grey shale and white sand with a thickness of 6m showing evidence of light oil and an average porosity of 20%. [ABSTRACT FROM AUTHOR]

In the context of global efforts to transition toward renewable energy and reduce greenhouse gas emissions, geothermal energy is increasingly recognized as a viable and sustainable option. This paper presents a comprehensive assessment derived from a subset of a larger sample collection within the Dunántúli Group of the Pannonian Basin, Hungary, focusing on optimizing micro-computed tomography (µ-CT) resolution for analyzing pore structures in sandstone formations. By categorizing samples based on geological properties and selecting representatives from each group, the study integrates helium porosity and gas permeability measurements with µ-CT imaging at various resolutions (5 µm, 2 µm, and 1 µm). The findings reveal that µ-CT resolution significantly affects the discernibility and characterization of pore structures. Finer resolutions (2 µm and 1 µm) effectively uncovered interconnected pore networks in medium- to coarse-grained sandstones, suggesting favorable properties for geothermal applications. In contrast, fine-grained samples showed limitations in geothermal applicability at higher resolutions due to their compact nature and minimal pore connectivity, which could not be confidently imaged at 1 µm. Additionally, this study acknowledges the challenges in delineating the boundaries within the Dunántúli Group formations, which adds a layer of complexity to the characterization process. The research highlights the importance of aligning µ-CT findings with geological backgrounds and laboratory measurements for accurate pore structure interpretation in heterogeneous formations. By contributing vital petrophysical data for the Dunántúli Group and the Pannonian Basin, this study provides key insights for selecting appropriate µ-CT imaging resolutions to advance sustainable geothermal energy strategies in the region. The outcomes of this research form the basis for future studies aimed at developing experimental setups to investigate physical clogging and enhance geothermal exploitation methods, crucial for the sustainable development of geothermal resources in the Pannonian Basin. [ABSTRACT FROM AUTHOR]

To investigate the percolation mechanism of tight sandstone reservoirs, qualitative and quantitative methods are applied to analyze the pore structure of the Ordos Basin through scanning electron microscopy and high-pressure mercury injection. Based on gas–water relative permeability data and the visualization of real sandstone models, a gas–water displacement experiment was carried out to simulate the percolation mechanism and fluid distribution of different pore types. The results show that: ① The percolation of different pore types varies greatly, and the irreducible water saturation decreases when the reservoir properties improve. The relative permeability of gas at an irreducible water saturation increases, the width of the two-phase percolation area increases, and the percolation ability is enhanced as the reservoir properties improve. ② The gas–water displacement mode changes from fingering to uniform displacement with the improvement of pore type, which is beneficial to the formation of effective reservoirs filled with natural gas under the same hydrocarbon generation conditions. ③ The time required for the pressure relief of samples with good reservoir properties is short, and the recovery is higher with the improvement of pore type. ④ The residual water in samples with a poor pore type increases, which decreases the relative permeability of gas and recovery. A reasonable development pattern should be carried out, and the production pressure difference should be strictly controlled to maximize productivity. This study provides theoretical guidance for the development of tight sandstone reservoirs in the Ordos Basin. [ABSTRACT FROM AUTHOR]

This article presents the results of dynamic tests of sandstone samples differing in strength parameters and porosity, which were carried out with the use of the split Hopkinson pressure bar (SHPB). For this study, three types of sandstones were considered: two from the region of India (Kandla Grey and Apricot Pink) and one from Central Europe (Barwald). The strength parameters of the samples were identified in static tests (UCS, BTS tests), whereas the porosity was measured using computed tomography. The performed scanning allowed the volume of the pores and their distribution in the samples to be identified. Dynamic tests involved loading the cylindrical samples with a diameter of 23 m in the range of high strain rates, i.e., ε ˙   = 102 ÷ 103/s, using the SHPB (split Hopkinson pressure bar) method. Samples with three different values of slenderness were used for testing (L/D = 1, 0.75 and 0.5). Based on the dynamic characteristics of the samples, the maximum dynamic stresses, Dynamic Increase Factor (DIF) and the amount of energy absorbed by the samples were determined. The conducted research indicates a significant impact of material porosity on the amount of dissipated energy under conditions of high strain rates. The research indicates that the values of this parameter for Apricot Pink and Kandla Grey sandstones (slenderness L/D = ¾ and L/D = ½) are similar, although the uniaxial compressive strength (UCS) of Kandla Grey sandstone is approximately 60% higher than that of Apricot Pink sandstone. As a result of the sample destruction process, various forms of sample destruction were obtained. The performed grain analysis indicates a significant increase in the smallest fraction (<0.5 mm) in the case of the sandstone with the highest porosity (Apricot Pink—55% of mass outcome) in comparison to the sandstone with the lowest porosity (Kandla Grey—12% of mass outcome). [ABSTRACT FROM AUTHOR]

Brittleness is an important rock property that examines the mechanical behavior and failure properties of rocks in engineering applications. Although there is no standard method for measuring rock brittleness, it can be calculated indirectly as a function of rock mechanical properties based on the rock strength ratio. The aim of this study was to investigate the relationship between the Leeb hardness (HL) values and the brittleness properties of rocks using the simple statistical method. For this purpose, 18 sandstone rock samples were collected and tested in the laboratory to determine the HL and physico-mechanical properties of rocks. In addition, the brittleness properties of rocks were examined with S20 brittleness (BI8) test and five different empirical methods (BI4, BI5, BI11, BI13, and BI14) calculated from rock strength ratios. New models were proposed to predict the brittleness of the rocks, taking into account the HL. Based on the statistical analysis, the proposed models were in good agreement with those measured (for all of the data, R2: 0.76–0.77, for sandstone, R2: 0.78–0.81). [ABSTRACT FROM AUTHOR]

The seismic attributes of water‐rich sandstone contain much information about the rock's physical properties and seismic wave parameters. They are commonly used to predict the rock's physical properties (e.g. porosity). However, the seismic attributes of water‐rich sandstone are affected by porosity, water saturation and thickness. To eliminate the influence of thickness on the porosity prediction of water‐rich sandstone and improve the accuracy of the porosity prediction, the authors propose a Lambert W–R transform method to isolate the contribution of thickness and porosity from seismic attributes. First, a rock physical model is used to calculate the equivalent elastic parameters of water‐rich sandstones with different porosity values and water saturation levels. Second, the seismic attribute dataset of water‐rich sandstone is established by forward modelling the seismic response of the wedge‐shaped water‐rich sandstone model, and the selection of sensitive physical properties is completed. Then, the transformation parameters (ζAhRs and ηAhRs) are obtained by Lambert W–R transformation, which is exponentially related to instantaneous amplitude. ζAhRs and ηAhRs are sensitive to thickness and porosity, respectively. Finally, an interpretative template for porosity prediction of water‐rich sandstone is established by cross‐plot analysis (ζAhRs and ηAhRs) and verified by a practical case. The verification results show that the porosity predicted by the interpretation template is consistent with drilling fluid consumption. However, it is lower than the porosity of logging constrained P‐wave impedance inversion. [ABSTRACT FROM AUTHOR]

The correct selection of the value of p is a complex and iterative procedure that requires experience in the interpretation of the obtained interpolated maps. Inverse Distance Weighting is a method applied to the porosities of the K and L hydrocarbon reservoirs discovered in the Neogene (Lower Pontian) subsurface sandstones in northern Croatia (Pannonian Basin System). They represent small and large data samples. Also, a standard statistical analysis of the data was made, followed by a qualitative–quantitative analysis of the maps, based on the selection of different values for the power distance exponent (p-value) for the K and L reservoir maps. According to the qualitative analysis, for a small data set, the p-value could be set at 1 or 2, giving the optimal result, while for a large data set, a p value of 3 or 4 could be applied. For quantitative analysis, in the case of a small data set, p = 2 is recommended, resulting in a root mean square error value of 0.03458, a mean absolute error of 0.02013 and a median absolute deviation of 0.00546. In contrast, a p-value of 3 or 4 is selected as appropriate for a large data set, with root mean square errors of 0.02435 and 0.02437, mean square errors of 0.01582 and 0.01509 and median absolute deviations 0.00896 and 0.00444. Eventually for a small data set, it is recommended to use a p-value of 2, and for a large data set, a p-value of 3 or 4. [ABSTRACT FROM AUTHOR]

This study delves into the micro-occurrence states and enrichment mechanisms of residual oil, pivotal for advancing the production from tight sandstone reservoirs, particularly from the Chang 8 Member of the Upper Triassic Yanchang Formation, Ordos Basin. Through an analysis of 23 core samples, employing high-pressure mercury injection, field emission scanning electron microscopy, thin section, and X-ray diffraction techniques, distinct reservoir types were categorized. The utilization of environmental scanning electron microscope, multi-solvent continuous extraction, and an oil components separation system facilitated an intricate analysis of residual oil micro-occurrence states and their subsequent effects on porosity and permeability reduction across varying reservoir types. The findings accentuate the integral role of reservoir type in determining residual oil distribution within tight sandstone reservoirs. Favorable pore throat sorting and connectivity in specified reservoir types are identified as conducive to residual oil enrichment with a higher concentration of light components. In contrast, elevated carbonatite and clay content in other reservoir types leads to adsorption of heavy components, disrupting pore throat connectivity, and impeding crude oil filling. The varied interactions between oil and rock, oil–oil, and pore throat sealing significantly impact the distribution of oil components of residual oil, culminating in a notable reduction of porosity and permeability by 2.63% and 0.197 mD, with corresponding reduction rates of 27.19% and 46.69%, respectively. The insights derived from this study furnish a theoretical foundation for augmenting tight oil recovery and comprehending the enrichment mechanism of residual oil driven by the heterogeneity of tight sandstone reservoirs. [ABSTRACT FROM AUTHOR]

The sandstone reservoir in the Lule River group of Mahai East District is strong, densely dense, and the expansion of clay material is very easy to block the pores. Therefore, the new pressure cracks and cationic cluster stabilizers with double-season ammonium salt clay stabilizers are used. Experimental analysis is performed under the circumstances of the perimeter. Use nuclear magnetic real-time testing different fracture liquids to drive the pore structure structure changes before and after the sandstone, combined with differential theory to calculate the fractal dimensions of sandstone. As a result, under the condition of water full, as the perimeter pressure increases, the rocks are increased, and the rocks are increased, and the rocks are increased. The matrix occurs under the common action of the fence and the fracture fluid to prevent the inflation of clay minerals under the combined action of the fence and the fracture fluid, which causes a significant reduction in large holes and increased micropores and medium holes. Exploring different fracture liquids to reservoirs is of great significance to the development of dense sandstone reservoirs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper examines sandstones of varying particle sizes within the coal-bearing strata in the Fugu area of northern Shaanxi. Employing high-temperature heat treatment, the study performs tests at different temperatures (25 ~ 600 ℃) on rock samples under both oxygen and nitrogen environments. Utilizing nitrogen adsorption tests combined with fractal dimension analysis, we investigate the pore structure and fractal characteristics of sandstones with varying particle sizes in coal-bearing formations. The results indicate that the total pore volume (aperture range 0.35 ~ 500nm), specific surface area, and average pore diameter of sandstone increase with temperature. Moreover, smaller particles result in more pronounced changes in the sandstone's pore structure. At the same time, in comparison to the oxygen environment, sandstone treated in a nitrogen environment undergoes multiple occurrences of molecular layer nitrogen adsorption, leading to a higher degree of pore damage. In addition, the fractal dimension of sandstone pores decreases with rising temperature. Under high-temperature conditions, the pores undergo a transformation from the initial irregular crack structure to a homogenized pore structure. The weakening of non-uniformity contributes to the decrease in fractal dimension. [ABSTRACT FROM AUTHOR]

The upper member of the Lower Ganchaigou Formation (UMoLGF) is a highpotential hydrocarbon exploration area in the North margin of the Qaidam Basin (NMoQB). It represents a typical low-permeability sandstone reservoir. The current understanding of reservoir characteristics of the UMoLGF is poor, and the main controlling factors of high-quality reservoir development remaining unclear. This study, for the first time, integrated various factors to investigate the formation mechanism of high-quality reservoirs in the UMoLGF's low-permeability sandstone reservoirs. Results show three provenance systems developed in the study area: northwest, northeast, and east. The northwestern and northeastern areas share similar reservoir characteristics. The rock type is predominantly feldspar, with relatively poor particle sorting and rounding. Pore types are dominated by secondary dissolution pores. However, the northwestern area has more developed fractures and poorer pore structures than the northeastern. Meanwhile, in the eastern area, the rock fragment content was high, the rock type was mainly litharentie and lithic arkose, particles were well-sorted and well-rounded. Residual intergranular pores, with good structures, dominated the pore type. The UMoLGF has entered the eo-diagenesis B stage with minor progression into the meso-diagenesis A stage. Based on quantitive calculations, this study established porosity evolution models for the different study areas. The initial porosities in the northwestern, northeastern, and eastern areas were 30.8%, 30.4%, and 34.8%, respectively. Compaction and cementation are the major factors contributing to porosity reduction in the three areas, with the most significant impact in the northwestern area. Dissolution significantly improved the reservoir properties in the northwestern area, with little effect on the northeastern and eastern areas. The formation of high-quality reservoir in the UMoLGF was affected by provenance, diagenesis, and fractures, with the primary controlling factors varying by area. In the northwestern area, the formation of high-quality reservoirs benefited from strong dissolution and well-developed fractures. In the northeastern area, the high-quality reservoir was relied upon favorable provenance and dissolution. In the eastern area, provenance provided an excellent material basis for developing high-quality reservoirs, with dissolution and chlorite cementation further improving reservoir properties. This study provides a theoretical foundation for further exploration and development of UMoLGF and offers insights for exploring and developing similar low-permeability sandstone reservoirs. [ABSTRACT FROM AUTHOR]

Acidified CO2 fracturing is a viable method for increasing production in deep, tight sandstone reservoirs. However, the potential mechanism of changes in pore structure and mechanical properties of sandstone under acidified CO2 supercritical composite is not clear. Understanding this mechanism is important for the study of crack initiation and extension in tight sandstone reservoirs. This study utilizes sandstone samples from the Xujiahe Formation reservoir in Rongchang District as experimental specimens. The primary focus is to analyze the changes in pore structure and mechanical properties of these samples after acidification–supercritical CO2 composite action. Nuclear magnetic resonance (NMR) and uniaxial compression tests are employed as the main investigative techniques. The results show that there was a physicochemical synergy between the acidification–supercritical CO2 composite effect; the crack initial stress, damage stress, and peak stress of the sandstone after 16 MPa supercritical CO2 acidification treatment were reduced by 20%, 49.5%, and 49.8%, respectively; the crack volumetric strain accelerated and the sandstone evolved from brittle to ductile damage; and the larger pore space and microcracks of the sandstone increased significantly after the treatment, which can be attributed to the gradual dissolution of intergranular cement leading to the formation of new pores connected to the existing pore network. The change mechanism of sandstone after acidification–supercritical CO2 compound treatment is also proposed. [ABSTRACT FROM AUTHOR]

To investigate the characteristics of crack propagation and microscopic structure under water–rock coupling effect in deep surrounding rock, a triaxial mechanical experiment was conducted on sandstone strata in the Shandong Wanfu Coal Mine under the action of pressurized water absorption. The experimental results showed that the mass increment of the rock sample was exponentially related to the confining pressure, and the mathematical relationship curve of the mechanical strength variation of the rock sample under different confining pressures was obtained. Based on this, the strength softening formula of the rock sample with the absorption time under different confining pressures was obtained, and the softening process was divided into three stages: the relatively stable stage, the rapid softening stage and the deceleration softening stage. The T2 spectrum curve of the rock sample was normalized, and the pore distribution and strength softening curve under different confining pressures were constructed. The speed of water molecules entering pores in different softening stages was analyzed, and it was found that in the relatively stable stage, the speed was micro-pore > meso-pore > macro-pore, in the rapid softening stage, the speed was meso-pore > macro-pore > micro-pore, and in the deceleration softening stage, the speed was micro-pore ≈ meso-pore ≈ macro-pore. According to the law of microscopic pore structure changes under water–rock coupling, a pore structure model of sandstone under different confining pressures was constructed. Based on the crack propagation model and crack propagation rate formula, it was concluded that the increase in crack strain propagation rate was the result of both water saturation and permeation water pressure, and a distribution map of the crack strain propagation rate was established. By comparing the crack strain propagation rate, it was found that under the same characteristic stress, the axial strain propagation rate of the crack was greater than the radial strain propagation rate, and the radial strain propagation rate was greater than the volume strain propagation rate. Quantitative relationships between crack propagation in sandstone and water saturation under different stress conditions were obtained. [ABSTRACT FROM AUTHOR]