Sandstone Samples Research Articles

Experimental study on the influencing factors and pore production law of CO2 huff-and-puff in tight sandstone reservoirs

In order to determine the influence of different factors on the CO2 huff-and-puff displacement effect, a CO2 huff-and-puff experiment was carried out with Chang 6 tight sandstone samples in Ordos Basin as the research object. Combined with nuclear magnetic resonance technology, the influences of injection pressure, cycle numbers and soaking time on the CO2 huff-and-puff effect were evaluated, and the optimal CO2 huff-and-puff parameters were optimized. The microscopic degree of crude oil production in different scale pores was quantitatively characterized. The results show that the injection pressure and the cycle numbers have a significant influence on the effect of CO2 huff-and-puff. With the increase of injection pressure, huff-and-puff cycle numbers and soaking time, the recovery efficiency increases, but the growth range decreases. When the injection pressure is increased from 6 to 12MPa, the degree of pore mobilization in the oil in macro and medium pores (≥ 10ms) increases by 13.0%-22.63%. The recovery efficiency of a single round gradually decreased with the increase of huff-and-puff rounds. The first cycle of CO2 huff-and-puff was the main contribution of crude oil recovery. However, the production effect of micro and small pores (< 10ms) was significantly improved after multiple cycle numbers of CO2 huff-and-puff. When the soaking time increases from 6 to 24h, the recovery efficiency increases by 11.47%-14.93%. After that, the influence of prolonged soaking time on the porosity production degree of cores with different permeability decreases. It is found that medium pores and macro pores are the main contributors to pore mobilization during multiple cycles of CO2 huff-and-puff.

Quantitative characterization of the multiscale mechanical properties of low-permeability sandstone roofs of coal seams based on nanoindentation and triaxial tests and its implications for CO2 geological sequestration

Microstructural heterogeneity of low-permeability sandstone roofs of deep unmineable coal seams due to diagenesis significantly affects rock mechanical behavior, greatly impacting the sealing potential of in situ CO2 sequestration and the structural stability of the geological formation. However, little is known about how the microstructure of different mineral groups influences the multiscale mechanical behavior of deep sandstone. This study proposes a new method for quantitatively characterizing the multiscale mechanical properties of low-permeability sandstone and shows the mechanisms responsible for mechanical failure at the micro-, meso-, and macroscale. Triaxial compression tests and targeted nanoindentation tests were conducted to assess the micro- and macroscale mechanical properties of different types of sandstone. The micro- and macroscale experiments were coupled with numerical simulations of compression using a unified cohesive model based on Voronoi polygons to clarify the multiscale mechanical behavior. The results indicate that quartz, the primary mineral component of the sandstones examined, exhibits the strongest micromechanical properties, followed by feldspar, calcite, and clay minerals. Compared to polycrystalline quartz, monocrystalline quartz has a more stable microstructure and is mechanically stronger. The macro-mechanical properties of tight sandstone samples are weakened by increased microstructural inhomogeneity and larger grain size. This leads to a higher likelihood of splitting damage, characterized by a high degree of discrete and weak stress sensitivity. The major conclusion is that the positive rhythm lithofacies of medium-grained sandstone to siltstone are the most favorable for efficient CO2 sequestration in deep unmineable coal seams.

A new procedure for volcanic glass hydrogen isotope studies of tuffaceous sediments and its application in the North American Cordillera

Hydrated volcanic glass δD (δDg) values of pure tuffs have been used to reconstruct paleoprecipitation δD in paleoelevation and paleoclimate studies. These hydrated glasses in tuffaceous sediments hold the potential for providing extended and continuous paleoprecipitation δDw records due to their abundance in the geologic record. However, glasses in such materials may be recycled and subjected to surface alteration, and the δDw values of the water that hydrated the glasses can vary across different depositional environments. Here we first develop a method to reduce the influence of recycled glass and surface alteration, then apply the method to the Eocene−Oligocene tuffaceous sediments in the Sage Creek basin, southwestern Montana, USA, to examine the variation of δDg in different depositional environments and test a previous interpretation of early Oligocene surface uplift in southwestern Montana. Our integration of microscopic observations of glass morphology and lab experiments of hydrofluoric acid (HF) abrasion of glasses for various durations (30 s to 30 min) shows that 10% HF abrasion for at least 5 min removes most of the recycled glass in tuffaceous sediments and does not change the δDg values and hydration water contents. Our data also show that pumice and cuspate glasses have higher δDg and water content than blocky glasses, which explains some of the δDg variations. The extended HF abrasion method was applied to 91 tuffaceous sandstone samples from floodplain and alluvial/fluvial channel facies in the Renova Formation in the Sage Creek basin. The results show that the δDg values exhibit greater variations in trunk and avulsion channels and floodplain facies than in paleosols. These variations are likely due to the mixing of waters derived from diverse elevations and sources, with differing evaporation degrees and glass types. This study underscores the significant potential of utilizing hydrated volcanic glass δDg from tuffaceous sediments for paleoclimate and paleoelevation studies. Nonetheless, it is important to assess the influence of depositional environment when interpreting such data. Paleosols are more suitable than other facies for reconstructing basinal precipitation δDw, but it is crucial to evaluate the effect of evaporation on paleosol δDg before interpretation. Additionally, the study reveals an absence of the anticipated early Oligocene negative δDg shift, calling for further investigation of this hypothesis.

Quantification of Rock’s Wettability Index Using Dielectric Measurements

Summary Wettability is a crucial parameter that governs several petrophysical attributes of oil- and gas-bearing rocks. However, the traditional methods to measure the wettability index are restricted to laboratory techniques, which makes that measurement expensive and time-consuming. Due to its sensitivity to fluid-solid interaction and the large contrast between the dielectric constant of oil and water, earlier studies investigated the use of dielectric measurements for wettability evaluation. Nevertheless, these studies mainly focused on the qualitative assessment of the wettability using the dielectric measurements and did not yield a practical and easy-to-implement dielectric-based wettability index correlation. Therefore, the objective of this study is to explore the response of the dielectric dispersion to wettability changes at two water saturation endpoints, full water saturation (Sw=1) and irreducible water saturation (Swirr), and to develop a correlation for obtaining the wettability index from dielectric measurements. A pair (master and sister) of Berea sandstone (BS) and Fontainebleau (FB) sandstone and Indiana limestone (IL) samples were used in the study. The wettability of the sister samples was altered (to make them less water-wet) without impacting porosity before measuring their wettability index. The widely recognized US Bureau of Mines (USBM) wettability index (WIUSBM) served as the benchmark for evaluating the newly developed dielectric wettability index (WIdielectric). Among all measured parameters, imaginary permittivity showed the most consistent and conclusive results, displaying (at frequencies below 200 MHz) a significant drop due to altering the wettability toward more oil-wet. This drop can be attributed to reduced electrical interactions between water and the grain surface due to wettability alteration. A strong linear relationship was also observed between the wettability index of each sample and the drop in its imaginary permittivity as saturation was changed from Sw=1 to Swirr. Therefore, a WIdielectric correlation was developed by incorporating the imaginary permittivity measured at both Sw=1 and Swirr. The developed correlation was able to predict the WIUSBM of the tested samples with R2 of 0.97 and a root mean square error of 0.066. Overall, this study offers an in-depth analysis of the dielectric response to various wettability conditions across different saturation levels, which has facilitated the formulation of an equation for determining the wettability index through dielectric measurements. The findings of this research lay the groundwork for broadening the application of dielectric measurements to additional petrophysical analyses and for more accurate characterization of the fluid-rock and fluid-fluid interactions.

Macro-micro mechanical behavior and crack propagation mechanism of sandstone samples containing intermittent cracks

Macro-micro mechanical behavior and crack propagation mechanism of sandstone samples containing intermittent cracks

Hydrocarbons, Hydrogen, and Organic Acids Generation by Ball Milling and Batch Incubation of Sedimentary Rocks

Pulverized rock samples are widely used in laboratory experiments, e.g., to assess microbial or abiotic processes in batch incubation tests. However, it is unclear if ball-milled samples accurately reflect in-situ conditions and if observed processes are affected by by-products artificially generated during the sample preparation procedure. As such, this study examined the effects of dry ball milling on the release of gases, which include C1-C4 hydrocarbons, carbon dioxide (CO2), hydrogen (H2), and low molecular weight organic acids (LMWOAs) from different sedimentary rocks. The experiments involved pulverization using a gas-tight zirconium oxide planetary ball mill followed by wet and dry batch incubation and thermal desorption up to 200 o C. During milling, all sedimentary rocks, except a low organic carbon sandstone sample, yielded methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), H2, CO2, and unsaturated hydrocarbons, e.g., ethene (C2H4). Sandstones only yielded H2, CH4, and CO2. Stable carbon isotope signatures of these products are similar to thermogenic gases. The gases were also detected in subsequent wet incubation experiments (after gases from milling had been removed). Additionally, formate, acetate, and citrate, were detected in all samples except for sandstone. Pyruvate, malate, and succinate were also detected in some samples. Thermal desorption products of powdered limestone, shale, and pyrite concretion samples included organic acids, such as acetate and formate, which were higher in milled samples than in crushed particles (∼1 mm). The original geological thermal maturity of the studied sedimentary rock samples was low (below “oil window”) and, thus, considerably below “gas window”, suggesting that most of the detected gases were generated during ball milling. H2 generated during ball milling may be derived from the reduction of water from fluid inclusion or phyllosilicates, involving the formation of reactive mineral surfaces or radicals. Notably, the concentrations of gaseous hydrocarbons derived from milling are relatively high and comparable to “wet” gases. At the same time, these data indicate that substantial amounts of gases and LWMOAs might be artificially generated during laboratory batch experiments with milled samples. Hence, ball milled rock samples must be used with caution if assessing (bio-)geochemical processes for natural environments.

Sedimentary facies and grain size distribution; Insights into Ogwashi-Asaba Formation, Niger Delta Basin

The Niger Delta is renowned for being one of the most productive hydrocarbon provinces globally. This productivity is primarily attributed to its complex geological history, which is marked by intricate stratigraphy and various sedimentary facies. These sedimentary facies, which are distinct bodies of sediment with specific characteristics, provide essential information about past environments and sedimentary processes, about the complex stratigraphy and diverse sedimentary facies. Stratigraphic sequences in the Niger Delta include alternating layers of sandstones, shales, and siltstones. Sediment samples were collected with cores too, from the Ogwashi-Asaba formation, and a detailed field analysis was documented. Grain size, petrographic, and geochemical analysis were carried out in the lab, from the samples collected. The analyzed parameters suggest that the sandstone samples from the Ogwashi-Asaba Formation are predominantly fluviatile. Graphic Mean (Mz) ranges from 1.4 to 2.1 phi. Grain Size Composition 2-20% very fine sand, 6-66% fine sand, 45-87% medium to coarse sand, and 70-100% very coarse sand. Sorting (σ1) ranges from 1.1 to 2.1 phi, with 64.71% of sediments being very poorly sorted and 35.29% poorly sorted. Graphic Skewness (Sk) ranges from -0.6 to +0.5, indicating varying energy conditions. Graphic Kurtosis (KG): Ranges from 0.8 to 1.9, with curves varying from leptokurtic to platykurtic. These sedimentary processes shaping the Ogwashi-Asaba formation creates a diverse stratigraphy and a variety of sedimentary facies. Each provides valuable insights into past environmental conditions and sedimentary dynamics. A comprehensive understanding of these processes enhances the potential for hydrocarbon exploration and the reservoir characterization of the province.

Локальное изменение напряженного состояния в образцах горных пород перед разрушением по данным коэффициента Лоде–Надаи

In this paper, the relationship of the Lode–Nadai coefficient, which determines the shape of the stress ellipsoid, with the nature of brittle fracture in sandstone, pyrophyllite and dolomite samples for a rigid loading machine is investigated. The measurement of the inhomogeneous deformation field in the samples and the observation of the internal fracture process were performed using strain gauges and acoustic sensors. It is shown that the formation of the main crack is preceded by the evolution of local deformations in the sample, recorded by the strain gauge method. Special processing of strain gauge data, taking into account the free lateral surface of the samples, made it possible to calculate the principle deformations and three invariants of the strain tensor: maximum shear deformation, volume change deformation and the Lode–Nadai coefficient. It has been established that at the final stage of deformation, when a main crack is formed in local sections of the sample, the axes of the principal deformations are reindexed, which is expressed in the achievement of the maximum values of +1 and -1 by the Lode–Nadai coefficient. A comparison with field observations was made.

Removal of MTBE and BTEX Pollutants from Contaminated Water Using Colloidal Activated Carbon (CAC).

Methyl tertiary-butyl ether (MTBE) and BTEX (benzene, toluene, ethylbenzene, and xylenes) are common groundwater contaminants that pose significant health risks. This study investigated the efficiency of a colloidal activated carbon (CAC) material in removing MTBE and BTEX from contaminated water using batch and continuous core flooding systems. In the batch system, a mixture of sand and carbonate was coated with 1-3 g of CAC for the removal of contaminants. The core flooding system was packed with similar materials and mixed with 10 g of CAC. X-ray diffraction (XRD) revealed that the carbonate consists of more than 97% calcite, with traces of clay and quartz minerals. On the other hand, the sandstone sample showed around 89% quartz, 8% clay minerals, and traces of feldspar. The CAC has a surface area of 1050 m2/g, mean particle size of 31.8 μm, density of 1.0976 (g/cm3), viscosity of 24.4 mPa·s, and a negative surface charge. Spiked water samples with 2000 ppb MTBE and BTEX concentrations ranging between 1000 and 200 ppb were injected through the system at rates of 0.5 and 1.0 mL/min. The results showed that the type of packing materials and flow rate have a significant impact on contaminant removal. For example, the removal efficiency was higher in sandstone due to uniform particle shapes facilitating better water distribution and CAC accessibility within the pore medium. A lower injection rate (0.5 mL/min) resulted in higher removal efficiencies due to increased contact time between contaminants and CAC. At 1 mL/min, the maximum removal of 94% and 65% for MTBE and benzene was achieved in carbonate, while 99% and 96% were achieved in sandstone, respectively. At 0.5 mL/min, complete MTBE removal and over 95% benzene removal were achieved in both materials. Overall, the CAC demonstrated excellent MTBE and BTEX removal capabilities, exceeding 95%, offering a promising approach for in situ groundwater remediation.

Study on Splitting Intensity and Acoustic Emission Characteristics of Brazilian Deep Gray Sandstone at Different Scales

In order to determine the influence of size on the mechanical properties of sandstone and acoustic emission damage characteristics, the Brazilian splitting test of 6 sandstone samples with diameters of 25mm, 37mm, 50mm, 69mm, 80mm and 100mm was carried out by using the rock weak surface direct shear instrument and acoustic emission monitoring system The effects of size change on the splitting strength, deformation, acoustic emission activity and damage characteristics of rock samples are discussed. The results show that the larger the specimen size is, the more original defects are contained in the specimen, which has a great influence on the development trend of stress-strain curve, peak strength and peak strain during loading. With the increase of sample size, the peak strength and peak strain become smaller and smaller, and the ratio of peak compressive strain to peak tensile strain remains in the range of 2~4 times. According to the development characteristics of acoustic emission signals, the whole process of rock damage is divided into four stages: micro-crack compaction stage, new crack generation and expansion stage, crack rapid expansion stage and failure stage. The diameter sample of 69mm~100mm showed almost no AE signal at the beginning of test loading, and the activity of acoustic emission signal is intense at the moment of failure.The larger the specimen size, the larger the stress value and b value corresponding to the fracture entering the instability expansion stage. Tensile microcracks are the main types of cracks in the splitting test, and with the increase of the size, the shear microcracks gradually decrease, and the tensile microcracks increase.

Gas Charging Characteristics and Controlling Factors in Tight Sandstone Reservoir of Xujiahe Formation, Sichuan Basin

The tight reservoirs in the Sichuan Basin generally contain water and have complex gas–water relationships. The dynamic changes and main controlling factors of natural gas injection are unclear, which has had a serious impact on the exploration and development of tight sandstone gas. This article selects samples from Yongqian and Qiulin gas fields to characterize the reservoir characteristics of the tight sandstone samples in the Xu-3 section. Nuclear magnetic resonance technology is applied to plan gas–water injection simulation experiments, and the dynamic changes in pore water and gas content during the natural gas injection of tight reservoir rock samples are characterized. The main controlling factors are analyzed based on the theory of nuclear magnetic resonance singlet and multifractal models. The results showed that material composition, pore type, structural characteristics, and physical properties cooperatively control the charging characteristics of natural gas. There was no significant difference in mineral content among the tight sandstone samples, and the pore morphology types were mainly parallel plate-like pores and fracture-type pores. There were significant differences in the pore structure characteristics of the samples with varying burial depths. The heterogeneity of gas-bearing pores is negatively related to the buried depth of tight sandstone, is a coupling relationship with quartz and feldspar content, and is negatively correlated with pore permeability. The stronger the sample heterogeneity, the more unfavorable it is for natural gas migration and accumulation.

The Effect Of Co2-Brine-Rock Interaction Towards Sand Onset Modeling In Dolomite-Rich Sandstone: A Case Study In Air Benakat Formation, South Sumatera, Indonesia

Carbon Capture Utilization Storage (CCUS) into geological storage (e.g., Enhanced Oil or Gas Recovery) provides a solution to reduce CO­2 emissions. However, it still remains a potential operational problem, such as sand problem phenomena in producer wells. This study observes the phenomenon of sand problems in production wells possibly triggered by CO2-brine-rock interactions on CO2 injection in rich dolomite sandstone reservoir. This research performs several experimental works (i.e., time-lapse dry mass measurements, X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), and elastic wave measurements) by using CO2-brine-rock batch experimental setup as well as geochemical simulation to observe mineral dissolution, pore structures alteration as well as rock physics alteration due to CO2-brine-rock interactions. We used an outcrop sample of dolomite-rich sandstone from the Air Benakat Formation, South Sumatera, Indonesia. Our experimental and simulation works show that dolomite dissolution (dolomite reduction of ~4% after 14 soaking days), secondary porosity development (11% of visible porosity improvement), as well as rock strength reduction, occur indirectly (shown by elastic wave velocity, i.e. and reduction of ~3.8% and ~4.4%, respectively) due to CO2-brine-rock interactions. Subsequently, the results of elastic wave velocity measurements were then used to modify a considerable sand onset prediction (sand-free envelope) model. The modified model showed that the production well was more prone to sand problems due to CO2-brine-rock interactions. Thus, it is concluded that the sand onset prediction model with considering CO2-brine-rock interactions could help to design a better sand management strategy in producer wells.

Experimental Study: Stress Path Coefficient in Unconsolidated Sands: Effects of Re-Pressurization and Depletion Hysteresis

Accurate estimation of in-situ stresses is a critical parameter for geo-mechanical modelling. In-situ stresses are estimated in the field from logs and frac tests. Laboratory tests are performed with cored material to estimate horizontal stress changes under defined boundary conditions to complement field data. Horizontal stress path coefficient is used to estimate a change in in-situ stresses as the reservoir undergoes depletion or injection. Uniaxial Strain boundary conditions are representative of far field stress state. The laboratory data provides the change in horizontal stress with a change in pore pressure. It is used to complement the field data acquisition of absolute stress values to predict the value of total stresses. This experimental study provides a novel method of simulating geological compaction for fabricating representative samples from unconsolidated sands. It investigates the variability of horizontal stress path coefficient as a function of changing pore pressure (depressurization and re-pressurization) in unconsolidated sandstone reservoirs. Synthetic sandstones samples were made from sand packs by consolidating them under an isostatic stress path at ambient pore pressure. After getting to initial reservoir conditions, a series of pore pressure depletion and injection tests with varying magnitudes (injection and depletion) were performed to study the effects of stress path direction and associated hysteresis. The magnitude of the stress path coefficient under depletion is lower than that under injection for the first load-unload cycle. In subsequent load-unload cycles, the stress path coefficient values remain constant until the sample is depleted to a new level of pore pressure. A Modified Cam Clay model is fit to the data to map the expansion of the yield surface and quantify the model parameters. Application of this research includes accurate prediction of changes in-situ stresses during depletion and injection stress paths for simulating unconsolidated reservoirs behavior under fluid injection or further depletion.

Mechanical Properties and Damage Constitutive Model of Saturated Sandstone Under Freeze-Thaw Action.

In order to investigate the impact of freeze-thaw damage on sandstone under the coupling of ground stress and pore water pressure, three types of porous sandstone were subjected to freezing at different negative temperatures (-5 °C, -10 °C, -15 °C, and -20 °C). Subsequently, hydraulic coupling triaxial compression tests were conducted on the frozen and thawed sandstone. We analyzed the effects of porosity and freezing temperature on the mechanical properties of sandstone under hydraulic coupling and performed nuclear magnetic resonance tests on sandstone samples before and after freezing and thawing. The evolution of the pore structure in sandstone at various freezing and thawing stages was studied, and a statistical damage constitutive model was established to validate the test results. The results indicate that the stress-strain curves of sandstone samples under triaxial compression after a freeze-thaw cycle exhibit minimal changes compared to those without freezing at normal temperature. The peak deviator stress shows a decreasing trend with decreasing freezing temperature, particularly between -5 °C and -10 °C, and then gradually stabilizes. The elastic modulus of sandstone with different porosity decreases with the decrease in freezing temperature, and the decrease is more obvious in the range of -5 °C~-10 °C, decreasing by 2.33%, 6.11%, and 10.5%, respectively. Below -10 °C, the elastic modulus becomes similar to that at -10 °C, and the change tends to stabilize. The nuclear magnetic porosity of sandstone samples significantly increases after freezing and thawing. The smaller the initial porosity, the greater the rate of change in nuclear magnetic porosity after a freeze-thaw cycle. The effects of freeze-thaw damage on the T2 distribution of sandstone with different porosity levels vary. We established a statistical damage constitutive model considering the combined effects of freeze-thaw damage, ground stress, and pore water pressure. The compaction coefficient K was introduced into the constitutive model for optimization. The change trend of the theoretical curve closely aligns with that of the test curve, better characterizing the stress-strain relationship of sandstone under complex pressure environments. The research findings can provide a scientific basis for wellbore wall design and subsequent maintenance in complex environments.

Experimental investigation on the effects of deep eutectic solvents (DES) on the wettability of sandstone samples

Experimental investigation on the effects of deep eutectic solvents (DES) on the wettability of sandstone samples

Investigation of geometric morphology and fluid flow characteristics of natural fracture in tight sandstone under non-loading and true triaxial cyclic loading

Understanding the flow characteristics of rock fractures under stress is critical for many geological engineering applications. In this study, flow experiments are conducted on tight sandstone samples with a single natural fracture under true triaxial cyclic loading using the geotechnical consulting and testing systems. The geometric morphology of the fracture is scanned before and after loading using a profilometer. An improved cubic law is developed by including correction factors for stationary roughness, surface tortuosity, and hydraulic tortuosity. The evolution of fracture permeability during cyclic loading of each principal stress is measured using the steady-state method. The results show that (1) the surface tortuosity of the natural fracture correlates as a binary quadratic function with its fractal dimension and joint roughness coefficient. (2) The improved cubic law model has higher accuracy in predicting the permeability of the opening natural fracture than other commonly used modified cubic law models. (3) The principal stresses exhibit an anisotropic influence on fracture permeability. During cyclic loading of principal stress parallel to the fracture, the changes in fracture permeability are neglectable. (4) During cyclic loading of principal stress perpendicular to the fracture, the fracture permeability decreases significantly in the first loading cycle, exhibiting a hysteresis effect. In subsequent cycles, the changes in fracture permeability are nearly reversible, indicating the stress-memory effect of the natural fracture. This study provides direct evidence for the hysteresis and stress-memory effects in the permeability evolution of fractured rock during true triaxial cyclic loading.

Applying and validating Coulomb rate-and-state seismicity models in acoustic emission experiments

A central goal of laboratory seismology is to infer large-scale seismic processes from small-scale experiments, with acoustic emissions (AE) being a common observable. These signals, indicative of microfracturing, slip localization, and damage evolution, are often paralleled with earthquakes to understand seismic behaviors. This study challenges traditional perspectives by applying Coulomb rate-and-state seismicity theory, originally developed for earthquake clustering, to AE experiments. This theory maps stressing history to seismicity rates using rate-and-state friction, however, its validity under controlled experimental conditions remains an open question. We conducted four experiments on a sawcut sample of red felser sandstone, representing a fault under variable stress conditions. Adjustments in loading rates and initial conditions revealed that, while a single free parameter A—related to the direct effect—should suffice, a rescaling of the model by 1.5 to 2.2 was necessary for fitting the data. Differences in values across experiments appeared mostly non-systematic, and partial data usage did not yield consistently systematic parameter migrations. These findings suggest that fault microstructure may complexly alter parameter values during loading beyond what is accounted for in the Coulomb rate-and-state theory. Nonetheless, with the introduction of the scaling parameter, the Coulomb rate-and-state theory effectively captures the fundamental aspects of AE responses to complex controlled loading histories.

Frost heave cracking and uniaxial compression failure behavior of sandstone samples containing a flaw filled with water

The frost heave failure mechanism of fractured rock mass is a complicated problem faced by engineering construction in cold area. In this study, low temperature frost heave test and post-freezing uniaxial compression test on sandstone samples with a water-filled flaw were carried out. The frost heave cracking mode, frost heave pressure, frost heave cracking criterion and the effect of frost heave-induced cracks on post-freezing uniaxial compression failure behavior under different flaw inclination angles (0 ~ 90°) and freezing temperatures (–10~–40 °C) were investigated. Frost heave cracks initiate and extend along the coplanar direction of flaws. The frost heave damage degree decreases with the increase of flaw inclination angle. As the freezing temperature drops, the flaw cracking length first increases greatly, and then decreases, reaching the maximum at − 15 °C. The evolution of frost heave pressure can be divided into six stages: pre-freeze stage, rapid increase stage, rapid decrease stage, stable stage, slight rebound stage and dissipation stage. The peak frost heave pressure first increases and then decreases with the increase of flaw inclination angle, and reaches the maximum at 60°, and increases as the freezing temperature drops. Formulas for calculating the critical stress intensity factor KIC for flaw frost heave cracking are provided, considering flaw inclination angle or freezing temperature. When the flaw is within a certain range of inclination angles (45 ~ 75°), cracks under uniaxial compression will extend from the frost heave-induced cracks.

Thermodynamically consistent interfacial curvatures in real pore geometries: Implications for pore-scale modeling of two-phase displacement processes

In conventional Pore-network Modeling (PNM) approaches, fluid flow and transport are solved in a network of pore elements with idealized geometries. Such simplification can lead to inaccurate predictions when the original pore space features complex geometries. To overcome this limitation, this study introduces a novel workflow that integrates four key components: (i) an enhanced pore network extraction (PNE) platform capable of identifying and extracting pore cross-sections from high-resolution micro-computed tomography (micro-CT) images of the pore space, (ii) a computationally-efficient semi-analytical model that can faithfully predict capillary entry pressure and the corresponding fluid configuration of piston-like displacements using real two-dimensional cross-sections of pores, (iii) a PNM approach for two-phase flow modeling that utilizes the capillary entry pressure of pores predicted by the semi-analytical model, and (iv) an Artificial Intelligence (AI)-driven model that paves the way for future advancements in efficiently predicating fluid displacement properties in intricate pore structures. To validate this new workflow, we constructed various pore networks containing real pore and throat cross-sections over a diverse group of sandstone and carbonate rock samples. Subsequently, we simulate capillary pressure curves of Mercury Intrusion Capillary Pressure (MICP) and oil–water primary drainage displacements in Bentheimer and Berea sandstones, respectively, using both the conventional and enhanced PNM approaches. The latter demonstrated improved prediction accuracy compared to conventional methods. Next, primary drainage simulations are conducted for two carbonates, and the resulting capillary pressure curves from both PNM approaches are compared. In addition, we conduct an in-depth analysis of fourteen geometric features of the pore space, identifying key factors of hydraulic radius, circumradius, sphericity, and area, that significantly impact capillary entry pressure of pores. After that, we construct an Artificial Neural Network (ANN) to predict the capillary entry pressure of pores using their critical geometric features. This AI model, trained using data derived from the semi-analytical model, exhibits excellent predictive accuracy (with a R2 of 0.995 for the test data set) in estimating capillary entry pressure of pores. Overall, our newly proposed, integrated workflow represents a significant step forward in the field of digital rock technology (DRT), offering an accurate and efficient method for modeling fluid flow in rock samples with complex pore geometries.

U-Pb ages and Hf isotopic composition of detrital zircon in western Algarve Triassic sandstone (SW Iberia): Implications for crustal evolution, provenance, and paleogeography

The Triassic paleogeographic configuration before the Central Atlantic Ocean's opening and the Pangea's breakup establishes that Iberia was close to North America and North Africa. We present UPb and Hf isotope data of Triassic siliciclastic rocks of the Western Algarve basin (Southwest Iberia, Portugal) to draw inferences on their provenance to define the crustal evolution of the potential sources. Detrital zircon age populations of five samples of Triassic sandstones (68–80 % Neoproterozoic; 5–23 % Paleoproterozoic; 5–14 % Mesoproterozoic; <5 % Archaen and < 4 % Paleozoic) differ significantly from those of the underlying Carboniferous turbidites of the South Portuguese Zone suggesting provenance from outside present-day SW Iberia. Hf isotope signatures of the zircon grains from the western Algarve Triassic sandstones unravel a complex crustal evolution characterized by successive cycles of input of juvenile magmas and magmas derived from the reworking of older crust. Archean detrital zircon grains with positive εHf(t) values and model ages of 3.1–3 Ga indicate juvenile crust formation (Leonian event), while a grain dated at ca. 2.6 Ga showing negative εHf(t) values and model ages of 3.1 Ga suggests that the Archean crust became reworked later (Liberian event). Zircon grains dated between 2.4 and 2.5 Ga having positive to slightly negative εHf(t) values and model ages of 3.3–2.6 Ga model ages correspond to magmatism older than the Eburnean event known in the Reguibat shield, involving a juvenile component mixed with variable proportions of reworked Archean crust. Younger Paleoproterozoic grains (ca. 2.10 to 2.05 Ga; Eburnean-Birimian event) having positive εHf(t) values and model ages of 2.3–2.1 Ga and negative εHf(t) values and model ages of 3.6–2.6 Ga, also indicate interaction of juvenile sources with mixing of old crust. Mesoproterozoic grains show positive εHf(t) values and model ages of 2.3–2.1 Ga, suggesting juvenile crust formation and reworking of the Eburnean crust. Neoproterozoic (ca. 695–555 Ma) detrital zircon with positive εHf(t) values and model ages of 0.76–0.58 Ga point to juvenile crust formation (i.e. Cadomian/Pan-African and Avalonian arcs). They also have negative εHf(t) values in the range − 21.6 to −11.7 and model ages of 3.2–2.1 Ga, suggesting reworking of older crust. In summary, the εHf(t) values and UPb age pattern of the zircon grains from the Triassic siliciclastic rocks are similar to NW Africa and North America potential sources. Still, there is a notable gap in juvenile crust production during the Siderian in the peri-Eastern Laurentia, Avalonia, and Ganderia potential sources unknown in NW Africa. The lack of ca. 525–380 Ma and ca. 380–330 Ma detrital zircon in the western Algarve Triassic sandstones makes it challenging to admit an Atlantic margin of North America and SW Iberia provenance.