Small Pore Spaces Research Articles

Dynamic wetting mechanism of coal under different CO2 pressure: A new analytical basis through T2 cutoff

The technology of injecting carbon dioxide into coal seams to reduce greenhouse gas emissions and has brought attraction on the wettability of coal. As a fast and non-destructive method, nuclear magnetic resonance (NMR) is widely used in the evaluation of coal wettability, but the inconsistency of analytical basis makes the quantitative evaluation of coal wettability changes uncertain. In this study, based on NMR combined with centrifugal experiments, a novel analytical basis using irreducible water defined by T2 cutoff value was proposed to quantitatively analyze the dynamic wetting mechanism under different CO2 pressures. The results show that when T2 < 72.32 ms, the water in coal is irreducible water; when T2 > 72.32 ms, the water in coal is movable water. CO2 pressure hindered free water conversion to adsorbed and capillary water, but impacted adsorbed water more. Increased CO2 pressure caused the wetting porosity of coal to decrease from 1.62 % to 1.08 %, and coal gradually changed from water-wet to CO2-wet. The increase in CO2 pressure enlarged the CO2 aggregations in the seepage channel, thus weakening the wetting rate of water to coal. CO2 preferentially occupied small pores, which will hinder the migration of water into small pore spaces. Molecular diffusion controls water to achieve wetting equilibrium more than gravity/pressure difference and capillary force. As CO2 pressure increases, the wetting equilibrium time decreases.

Mineral effects on methane hydrate formation and distribution in sand sediments

Methane hydrate is a promising energy resource widely occurring in the world, but the formation and distribution of methane hydrate in marine sediments with complex minerals remains unclear. Due to the small pore space and fine-grained size of clay minerals, the contact relationship between methane hydrate and clay mineral surface under excess water has not been characterized fully, which is required to investigate the mineral effect on methane hydrate formation. In this study, cryo-SEM was applied to observe the distribution of methane hydrate synthesized in pure quartz sands and the quartz sands mixed with montmorillonite or illite separately. According to the results of the research, methane hydrate dispersedly forms in the pore space away from and on the surface of quartz under a local excess water condition. In addition, methane hydrate occurs away from montmorillonite in pores with excess water and contacts the edge of montmorillonite under a local excess-gas condition. Moreover, methane hydrate was only observed to form away from illite aggregates in the pore with residual water. Montmorillonite and illite influence the preferable distribution of methane hydrate to occur away from mineral surface in excess water condition. In the sediments composed of different minerals, methane hydrate forms stochastically in the pore space and grows up individually. As a result, the findings of this study significantly expand the understanding of the formation and distribution of methane hydrate in marine sediments and the prediction of physical properties of hydrate-bearing sediments, and provide insights into natural gas hydrate exploration and production.

Damage Factors and Protection of Low Permeability Reservoir

With the development of science and technology and the progress of society, people's production level has been significantly improved. In this context, people have also put forward higher requirements for the application and development of technology and processes in various production fields. The relevant construction technology of oilfield enterprises is no exception. Low permeability reservoir occupies a large proportion in China's reservoir resources. However, due to the small pore space and large fluid flow resistance of low permeability reservoir, it is more prone to reservoir damage in the development process, and even causes the reservoir to fail to produce in serious cases. Therefore, research on the potential damage factors and damage mechanism of low permeability reservoir and propose corresponding protection strategies are conducive to improving the development effect of low permeability reservoir. The potential damage factors of low permeability reservoir are systematically analyzed, which is of great significance to guide the efficient exploitation of low permeability reservoir.

Fine quantitative characterization of high-H2S gas reservoirs under the influence of liquid sulfur deposition and adsorption

In order to clarify the influence of liquid sulfur deposition and adsorption to high-H2S gas reservoirs, three types of natural cores with typical carbonate pore structures were selected for high-temperature and high-pressure core displacement experiments. Fine quantitative characterization of the cores in three steady states (original, after sulfur injection, and after gas flooding) was carried out using the nuclear magnetic resonance (NMR) transverse relaxation time spectrum and imaging, X-ray computer tomography (CT) of full-diameter cores, basic physical property testing, and field emission scanning electron microscopy imaging. The loss of pore volume caused by sulfur deposition and adsorption mainly comes from the medium and large pores with sizes bigger than 1 000 μm. Liquid sulfur has a stronger adsorption and deposition ability in smaller pore spaces, and causes greater damage to reservoirs with poor original pore structures. The pore structure of the three types of carbonate reservoirs shows multiple fractal characteristics. The worse the pore structure, the greater the change of internal pore distribution caused by liquid sulfur deposition and adsorption, and the stronger the heterogeneity. Liquid sulfur deposition and adsorption change the pore size distribution, pore connectivity, and heterogeneity of the rock, which further changes the physical properties of the reservoir. After sulfur injection and gas flooding, the permeability of Type I reservoirs with good physical properties decreased by 16%, and that of Types II and III reservoirs with poor physical properties decreased by 90% or more, suggesting an extremely high damage. This indicates that the worse the initial physical properties, the greater the damage of liquid sulfur deposition and adsorption. Liquid sulfur is adsorbed and deposited in different types of pore space in the forms of flocculence, cobweb, or retinitis, causing different changes in the pore structure and physical property of the reservoir.

Adsorption enhances CH4 transport-driven hydrate formation in size-varied ZIF-8: Micro-mechanism for CH4 storage by adsorption-hydration hybrid technology

The adsorption-hydration hybrid technology holds significant potential for future CH4 storage applications. However, the transport of CH4/H2O and the mechanisms governing hydrate formation within metal-organic framework (MOF) systems remain unclear. In this study, the mechanism of CH4 adsorption, transport, and hydrate formation within ZIF-8 pores of different sizes was delineated utilizing the magnetic resonance imaging (MRI) technique. Primarily, CH4 undergoes physisorption within the inner cavity of ZIF-8, facilitating diffusion through the pores, which preferentially accumulate in larger pore spaces. Due to favorable interactions between CH4 and pre-absorbed water on the ZIF-8 surface, ZIF-8–01 and ZIF-8–02 preferentially facilitate substantial hydrate formation within larger pore spaces. The continuous hydrate growth depletes CH4 in the pore space, enabling two-way migration of adsorbed CH4 to support ongoing hydrate formation. This limited availability of adsorption sites within ZIF-8 induces the diffusion of surrounding CH4 into smaller pore spaces, facilitating the conversion of bound water. The size of the particles considerably influences the transport of physically adsorbed CH4 and impacts hydrate formation. Similar patterns are observed in the adsorption mass transfer coefficients of ZIF-8–01 and ZIF-8–02, which range from 0.0038 to 0.0044 and 0.0031 to 0.0124, respectively. The presence of more adsorption vacancies in the early stage leads to rapid diffusion adsorption of CH4 owing to strong adsorption effects. As adsorption within ZIF-8 pore cavities approaches saturation, increased water transport elevates internal gas-liquid two-phase resistance, slowing the kinetics of late-stage CH4 adsorption. At 6 MPa, CH4 with a larger particle size preferentially overcomes the gas-liquid energy barrier, expediting transport and promoting hydrate formation. Furthermore, the optimal particle sizes of ZIF-8 substantially influence physisorption and significantly improve the kinetics of hydrate formation.

A microscopic interpretation of hysteresis in the water retention curve of sand

The hysteresis observed in a water retention curve was experimentally studied through the use of X-ray micro-computed tomography (μCT). The CT images were acquired during the main drying and wetting processes of a water retention test for uniform sand. The porosity and degree of saturation (Sr) of pore-scale local subsets were calculated using images segmented into soil, water and air phases. The simultaneous distribution of the local porosity and Sr was also analysed to investigate the relationship between the pore size and retained water volume. At similar suction levels, only small pore spaces retained water during wetting, while relatively large pore spaces retained water during drying, demonstrating the ink-bottle effect. At similar saturation levels, the simultaneous distribution was independent of the wetting and drying processes, indicating no significant difference in the water retention state between the two processes. Another image analysis was performed to calculate the principal curvatures of the air–water interface. At similar saturation levels, the calculated curvature of the drying process was greater than that of the wetting process, indicating contact angle hysteresis. The contact angle between the soil particles and water is different at the microscopic level, whereas the water retention states of the two processes are similar.

Investigation of the transport and metabolic patterns of oil-displacing bacterium FY-07-G in the microcosm model using X-CT technology.

Microbially Enhanced Oil Recovery (MEOR) is dedicated to enhancing oil recovery by harnessing microbial metabolic activities and their byproducts within reservoir rocks and fluids. Therefore, the investigation of microbial mobility and their extensive distribution within crude oil is of paramount importance in MEOR. While microscale models have been valuable for studying bacterial strain behavior in reservoirs, they are typically limited to 2D representations of porous media, making them inadequate for simulating actual reservoir conditions. Consequently, there is a critical need for 3D models and dependable visualization methods to observe bacterial transport and metabolism within these complex reservoir environments. Bacterial cellulose (bc) is a water-insoluble polysaccharide produced by bacteria that exhibits biocompatibility and biodegradability. It holds significant potential for applications in the field of MEOR as an effective means for selective plugging and spill prevention during oil displacement processes. Conditionally cellulose-producing strain, FY-07-G, with green fluorescent labeling, was engineered for enhanced oil recovery. 3D micro-visualization model was constructed to directly observe the metabolic activities of the target bacterial strain within porous media and to assess the plugging interactions between cellulose and the medium. Additionally, X-ray computed tomography (X-CT) technology was employed for a comprehensive analysis of the transport patterns of the target strain in oil reservoirs with varying permeabilities. The results indicated that FY-07-G, as a microorganism employing biopolymer-based plugging principles to enhance oil recovery, selectively targets and seals regions characterized by lower permeability and smaller pore spaces. This work provided valuable insights into the transport and metabolic behavior of MEOR strains and tackled the limitation of 2D models in faithfully replicating oil reservoir conditions, offering essential theoretical guidance and insights for the further application of oil-displacing bacterial strains in MEOR processes.

Study on the Mechanism of Water Blocking in Tight Sandstone Gas Reservoirs Based on Centrifugal and Nuclear Magnetic Resonance Methods

Tight sandstone gas reservoirs are characterized by deep burial, high pressure, tight matrix, fracture development, and the prevalence of edge and bottom water. Because of the small pore throats, the phenomenon of capillary force is evident. In addition, the low permeability of the reservoir and the difference in fluid properties make the gas reservoir undergo severe water-blocking damage. In this paper, centrifugal and nuclear magnetic resonance methods are used. The relationship between pore throat characteristics, fluid distribution, and gas reservoir water-blocking mechanism is studied and analyzed. The experimental results show that fracture formation increases the porosity of the small pores and expands the pore size distribution. It is conducive to the displacement of the bound water in the small pore space and the reduction in the bound water saturation. When increasing the same displacement pressure, the core porosity increases. More residual water in the tiny pores is converted to moveable water, thereby reducing the capillary resistance. The high-angle penetration fractures and complex seam networks are created by fractures. They connect the pores to form a fracture network structure, which is conducive to the communication of seepage channels. The increase in porosity and the creation of a complex fracture network make the water inrush along the fractures more even in the process of mining. This slows the advance speed of the water invasion front and reduces the damage of water blocking. The results enhance the understanding of the water invasion mechanism of edge and bottom water so as to improve the recovery factors of tight sandstone gas reservoirs.

Distribution Characteristics of Micro Remaining Oil of Class III Reservoirs after Fracture Flooding in Daqing Oilfield

The class III reservoir in the Daqing Oilfield has poor sand body development, poor reservoir physical properties, and poor effects of measures. Its water drive recovery degree is low and the remaining reserves are large. It is the key target oil layer of the Daqing Oilfield. Due to the sedimentary characteristics and reservoir physical properties of class III reservoirs, conventional EOR technology (chemical flooding) and conventional stimulation and injection measures (fracturing) have poor potential tapping effects on class III reservoirs. According to the special reservoir conditions and development characteristics of the class III reservoir in the Daqing Oilfield, fracture-flooding technology is innovatively proposed, which greatly improves the recovery of remaining oil in class III reservoirs. The rapid injection of hydraulic surface activators into the formation and displacement of the remaining oil in class III reservoirs through rock core flooding experiments were simulated in this paper. The nuclear magnetic resonance (NMR), confocal scanning laser, and computed tomography (CT)-scanning technologies were applied to study the remaining oil distribution after fracture flooding. The results show that: (1) After fracture flooding, the peak value of the T2 spectrum curve of NMR shifts to the left and the degree of middle and small pore space production increases obviously. (2) Confocal scanning laser study shows that the remaining oil in thin membranous and clustered forms on pore surfaces is highly utilized. (3) CT scan study shows that the remaining oil in membranous and clustered forms is effectively utilized after fracture flooding. In summary, fracture-flooding technology can improve the washing efficiency and sweep volume of class III reservoirs, thus enhancing the recovery efficiency of class III reservoirs.

Tree Bark: A Surprising and Diverse Reservoir for Water

Bark is the outside layer of wood that all trees have. Bark protects trees from harsh environmental conditions including weather, pests, disease, and damage from hungry animals. Just like leaves, bark is different across species. Some trees have thick, rough bark while others have thin, smooth bark. When it rains, bark acts like a sponge and absorbs water. Some trees have bark with large pore spaces that make it easy to absorb rain water quickly. Other trees have bark with smaller pore spaces, which absorb water slowly. Each tree species has a maximum storage capacity of water that can be held in the bark. In fact, some mature trees can store more than 100 L of water in their bark—that is about as much water as you would use in a 10-min shower! In this way, bark influences the water cycle of individual trees and entire forests.

Plant–environment microscopy tracks interactions of Bacillus subtilis with plant roots across the entire rhizosphere

Our understanding of plant-microbe interactions in soil is limited by the difficulty of observing processes at the microscopic scale throughout plants' large volume of influence. Here, we present the development of three-dimensional live microscopy for resolving plant-microbe interactions across the environment of an entire seedling growing in a transparent soil in tailor-made mesocosms, maintaining physical conditions for the culture of both plants and microorganisms. A tailor-made, dual-illumination light sheet system acquired photons scattered from the plant while fluorescence emissions were simultaneously captured from transparent soil particles and labeled microorganisms, allowing the generation of quantitative data on samples ∼3,600 mm3 in size, with as good as 5 µm resolution at a rate of up to one scan every 30 min. The system tracked the movement of Bacillus subtilis populations in the rhizosphere of lettuce plants in real time, revealing previously unseen patterns of activity. Motile bacteria favored small pore spaces over the surface of soil particles, colonizing the root in a pulsatile manner. Migrations appeared to be directed toward the root cap, the point of "first contact," before the subsequent colonization of mature epidermis cells. Our findings show that microscopes dedicated to live environmental studies present an invaluable tool to understand plant-microbe interactions.

Experimental study on residue oil distribution after the supercritical CO2 huff-n-puff process in low permeability cores with Nuclear Magnetic Resonance (NMR)

In this paper, two sandstone cores with permeabilities of 1.7mD and 2.6mD were employed and the ScCO2 huff-n-puff processes were carried out to check the oil recovery efficiency as well as the oil mobilization laws under the conditions of 50 °C and 25 MPa. Both cores were cut half after the completion of the process and the residue oil distribution in the entire core as well as the front half and back half of the core were scrutinized with help of the Nuclear Magnetic Resonance (NMR) measurements. It is found longer soaking time of 5 days leads to higher oil recovery rate of 17.2% in comparison with 15.2% after 1 day’s operation time. NMR measurement results show that more oil in the large pore spaces of the core is produced, while the oil in the small pore spaces is rarely mobilized after the ScCO2 huff-n-puff process. NMR results further show that the oil saturation in the large pores of the front part is obviously lower than that of the back part of the core, which indicates that a round of huff and puff operation mainly produced the oil in the large pore spaces from the front part of the core, while leave a considerable part of the oil undeveloped in the rear part of the core.

Evaluation and X-Ray tomography analysis of super-absorbent polymer for water management in high salinity mature reservoirs

The oil reservoirs in the Perm Krai region in Russia are characterized by high formation water salinity (200–230 g/l) and low reservoir temperature (25–30 °C). Currently, about 75% of their remaining reserves have a water cut of more than 50%. Super absorption polymers, also called preformed particle gels (PPGs), have been widely applied to control water production in oilfields; however, no work has been carried out in this area. Based on the Perm Krai reservoir characteristics, we synthesized a new PPG using a free radical polymerization method through a thermal imidization process. The swelling kinetics and equilibrium of the acquired PPG have been evaluated under different types and concentrations of brines. Core-flooding experiments were conducted using formation sandstone cores to compare the plugging capacity of the new PPG with a traditional polyacrylamide solution. The change in the pore space of the cores was analyzed using X-ray tomography to explore the conformance control mechanisms resulted from PPG treatment. The results show that the PPG can be used in the target oil reservoirs and can plug the pore space of the core six times more than traditional polyacrylamide, indicating that the PPG can efficiently plug large channels and redistribute the water flow to a small pore space.

Pore‐Scale Study of Water Adsorption and Subsequent Methane Transport in Clay in the Presence of Wettability Heterogeneity

AbstractClay minerals are abundant in shale, characterized by a lamellar structure and dimensions smaller than a micron, giving rise to nanometer‐scale pore sizes and large specific surface area. They are commonly associated with water. However, the spatial distribution of the unsaturated water in clay is not very well understood, which significantly affects the subsequent shale gas flow capacity. Wettability heterogeneity in the presence of hydrocarbons further complicates the water distribution in clay. In this study, we use a 3‐D lattice Boltzmann model to study water adsorption and condensation in a reconstructed clay microstructure based on broad‐ion‐beam scanning electron microscopy images. Three wettability conditions are considered including a water‐wet case, a water‐repellent case, and a mixed‐wet case. The spatial distribution of the condensed liquid water phase is visualized, and the connectivity of the remaining open pore spaces is characterized. For the water‐wet case, water preferentially condenses in the small and narrow pore spaces, while the water‐repellent case reports the opposite trend. The mixed‐wet case results in mixed behavior, and connectivity of the pore space is bound by the former two cases. We then study the subsequent methane flow capacity with the presence of water for these three cases, and the calculated methane relative permeability curves show a percolation threshold for the water‐wet case, which is in good agreement with available experimental data in the literature measured on geosynthetic clay liners.

Physicochemical properties, sensory quality, and coagulation behavior of camel versus bovine milk soft unripened cheeses

This study investigated the coagulation properties of camel milk (CM) and bovine milk (BM) and the resulting soft unripened cheeses obtained by treatment with chymosin (1000 IMCU/ML milk) or citric acid or acetic acid (30% acid/L milk). The cheeses were evaluated for yield, moisture, microstructure, texture profile, rheology, and sensory quality. CM cheeses were significantly lower in hardness and rheological properties than BM cheeses (p < .05). Photo images and scanning electron microscopy revealed the characterization of CM cheeses by a smooth and continuous casein network, thinner aggregate strands, and smaller pore spaces, while BM cheeses showed large pore spaces with irregular aggregates. Panelists evaluated the CM and BM cheeses as comparable for some attributes and preferred the cheeses prepared using citric acid. In conclusion, soft cheese with weak structures can be prepared from CM. The coagulation time for CM was longer than that for BM and the specific coagulation behavior of CM was attributed to differences between the composition of the casein fraction in CM and that in BM.

Characteristics and formation process of contractional deformation bands in oil-bearing sandstones in the hinge of a fold: A case study of the Youshashan anticline, western Qaidam Basin, China

The deformation bands in oil-bearing sandstones in a thrust fault-related fold were studied. The outcrop is exposed in the hinge of the Youshashan anticline. The deformation bands are compactional shear bands (CSBs), which are characterized by low net shear displacement (S) and compaction (C) ratios and by conjugate networks. The kinematics of the deformation bands are the result of the combined effects of shear and compaction, in which shear played a dominant role and was accompanied by additional compaction. CSBs are identified in two sets, including D1 CSBs that were cut by D2 CSBs. The CSBs formed under a contractional regime due to regional shortening and are oriented approximately in the NE-SW direction. The time of formation of the D1 CSBs (NNE-SSW-striking and E-W-striking) is consistent with or slightly later than the formation of the Youshashan anticline. At the beginning of the anticline formation, the D1 CSBs formed under a strike-slip regime due to the deeper burial depths (approximately 2.5 km). With the evolution of the anticline, the burial depths gradually become shallower. The D2 CSBs formed a NW-SE-striking direction under a thrust-fault regime. The porosities of the contractional D1 and D2 CSBs that developed in oil-bearing sandstones are 14–19.7% and 11–18% lower than those of the host rock, respectively. The permeability reduction for the D1 and D2 CSBs are between 1 and 2 orders of magnitude. Our findings suggest that the stress state changed during the formation uplift in the contractional regime, from a strike-slip stress state to a thrust-fault stress state, forming CSBs with low S/C ratios. The CSBs have low porosity and small pore space for hydrocarbon storage, and thus, the oil-bearing properties of the CSBs are less favorable than those of the host rock. Nevertheless, the oil saturations in the CSBs are 15.7–47.9% higher than those in the unformed oil-bearing sandstones.

Infiltration into Frozen Silty Clay Loam Soil with Different Soil Water Contents in the Red River of the North Basin in the USA

Predicting surface runoff and flooding in seasonally frozen areas such as the Red River of the North Basin (RRB) in USA is a challenging task. It depends on the knowledge of the complex process of infiltration in frozen soil, such as phase changes of water, ice content and distribution in the infiltration zone (the top 0–30 cm of the soil profile), soil pore size distribution, soil temperature and freeze–thaw cycles. In this study, the infiltration rates into frozen soil (Colvin silty clay loam according to the United States Department of Agriculture (USDA) Classification, and Chernozem according to Food and Agriculture Organization of the United Nations (FAO) international soil Classification) were measured at three different initial water contents: permanent wilting point (PWP), θpwp; field capacity (FC), θfc; and between FC and PWP, θmid. Laboratory infiltration experiments were conducted using a Cornell sprinkle infiltrometer with three replications for each initial water content. Volumetric soil water content (θv) and soil temperature at three depths were also continuously monitored using sensors. The average infiltration rates were 0.66, 0.38, and 0.59 cm/min for three initial water contents (θpwp, θmid, and θfc, respectively). Initial infiltration into frozen soil occurred quickly in the soil with θpwp because the soil was dry. Melted ice water contributed to the total soil water content over time, so it made the initial infiltration comparatively slower in the soil with θmid. Initial infiltration was also slower in the soil with θfc because the wet soil had very small pore space, so the soil rapidly reached its saturation after the infiltration started. The Horton infiltration equation was fitted with the observed infiltration rates for the soils with three initial water contents, and the goodness of fit was evaluated by using the coefficient of determination (R2) and the root-mean-square error (RMSE). The final infiltration rates from the fitted Horton equations were 0.060, 0.010, and 0.027 cm/min for the initial water contents (θpwp, θmid, and θfc, respectively). The soil water content along the soil profile changed with the amount of infiltrating water over time. However, the initial soil water content and melt water from ice resulting from soil temperature rise regulated the change in soil water content. The amount of ice melt water contribution to soil water content change varied among the soils with different initial water contents (θpwp, θmid, and θfc, respectively). The θv changed gradually in the θpwp soil, rapidly at 0 °C in the θmid soil, and less in the θfc soil. The change in pore distribution due to freeze–thaw cycles and soil packing altered the soil hydraulic properties and the infiltration into the soil. This study can provide critical information for flood forecasting model and subsurface drainage design in the RRB.

Bacterial Treatment of Remoulded Fine-Grained Cohesive Soils

The main objective of this research is to study the effect of microbial-induced calcite precipitation on the geotechnical properties of fine-grained soils. For this purpose, a high plastic cohesive soil retrieved from a special zone of Tabriz city, located in east Azerbaijan province in Iran, and two cultured Pseudomonas and Bacillus bacteria with various concentrations were added to the soil samples, and then compaction and consolidated undrained triaxial tests were performed on them. The results of compaction tests indicated that bacterial treatment did not affect meaningfully maximum dry density of soil, while it reduced the optimum moisture of soil from 30.46% up to 23.83%. Moreover, bacteria inclusion increased shear strength of soil about 27.0% and 35.0%, respectively, for consolidation pressures of 200 kPa and 300 kPa. SEM analysis revealed that after treatment, the soil particles arrange in an aggregative structure with a number of small pore spaces and form a few large pore spaces. The formation of the new structure in treated soil has led to strength improvement. In general, calcite participation led to an increase in shear-induced pore water pressure, which is due to formation of calcite crystals between clay particles. Obtained results revealed potential ability of these bacteria in treatment of fine-grained soils, but it needs more researches in this field.

Estimating the seepage effect of SC-CO2 and water fracturing with a steady-state flow model considering capillary and viscous forces at the pore scale

Supercritical carbon dioxide (SC-CO2) fracturing is a promising technology for unconventional energy development and carbon capture and storage. Experimental studies have shown that SC-CO2 fracturing can form complex fracture networks and reduce crack initiation pressure, which are different results from those when fracturing with aqueous fluids. The complex fracture networks that form from SC-CO2 fracturing may be the result of strong seepage effects (i.e., low capillary and viscous forces). To understand the different injection behaviors induced by SC-CO2 and aqueous fluids in low-permeability rocks, this study develops a new two-phase steady-state model based on the pore-scale network method. Although other models consider the viscous force, our model implements the capillary and viscous forces to reproduce the seepage effect. Because of the capillary force, the flow model is nonlinear and solved by iteratively solving matrix equations until a conservation of volumetric flux is satisfied. Simulation results show that the capillary force in a two-phase flow system in small pore spaces has consequential effects on its pressure distribution. Such factors lead to discontinuous pressure drops. This study shows that the seepage effect of SC-CO2 is stronger than that of aqueous fluids and can largely avoid the capillary blockage that arises in oil-water systems.

MicroCT-based bulk density measurement method for soils

High-resolution micro-computed tomography (microCT) is a method growing rapidly in popularity and has been applied to various soil studies with great success, especially for 3D characterisation of pore spaces or mineral distributions. However, microCT is not typically used for soil bulk density measurements, probably due to relatively simple and fast alternatives. Besides the complex process of image analysis from microCT scans, the method is also limited in resolution, which can result in incorrect total porosity estimation. This is especially true for granular materials, such as soils with small pore spaces between particles. In this work we demonstrate a different, yet very simple methodology for microCT adapted to overcome these limitations by using only volumetric measurements of the samples, and not segmentation of pore spaces or density calibrations. This method allows accurate bulk density determination for soil clods and cores. The method is faster than tradition methods, and it allows for additional analyses, such as surface area, macro-porosity, connected pore network and macro-particle shape analysis. The method is tested and directly compared for the same samples to the traditional waxing Archimedes method, with good correlation. The microCT scans of waxed samples also indicate sources of possible error in the waxing Archimedes method by visualising trapped air and wax penetration into open pore spaces. The method is then applied to cores and local bulk density measurements, and their variability down the cores is demonstrated, which can be very useful in complex soil profiles. The method is robust in varying resolution and image blur as it makes use only of volumetric measurements of the entire sample, not image grey-value calibration or segmentation of pore spaces.