Rock Pore Structure Research Articles

Pore structure analysis and classification of pyroclastic reservoirs in the Dehui fault depression based on experimental and well-logging data

The classification of pyroclastic reservoirs based on their pore structure has attracted the attention of many researchers in recent years. In this study, through thin section observations and scanning electron microscopy (SEM), mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR) and other relevant analyses, the reservoir in the third member of the Huoshiling Formation in the Dehui fault depression is classified into three separate types. The results show that a large number of devitrification corrosion pores are developed in the type-I reservoir, while the type-II reservoir contains matrix and dissolution pores. Moreover, the type-III reservoir only contained matrix-pores. Considering the complexity of the pore structure of the pyroclastic reservoir rocks, we established a relationship between the pseudo-capillary pressure curves and the NMR logging for the micro- and macro-pores in each reservoir type. Through the processing and analysis of key wells, it was found that the threshold pressure and median pressure obtained via NMR logging correspond well with the experimental core data. This enabled us to classify the reservoir type in several other wells and delineate the prolific regions in the study area that contain type-I and type-II reservoirs. The pseudo-capillary pressure curve can be used to continuously evaluate the pyroclastic reservoir pore structure longitudinally, which has very important practical significance in production applications.

Experimental Study on the Dissolution Characteristics and Microstructure of Carbonate Rocks under the Action of Thermal-Hydraulic-Chemical Coupling.

Microdamage in a rock induces a change in the rock's internal structure, affecting the stability and strength of the rock mass. To determine the influence of dissolution on the pore structure of rocks, the latest continuous flow microreaction technology was used, and a rock hydrodynamic pressure dissolution test device simulating multifactor coupling conditions was independently developed. The micromorphology characteristics of carbonate rock samples before and after dissolution were explored using computed tomography (CT) scanning. To conduct the dissolution test on 64 rock samples under 16 groups of working conditions, 4 rock samples under 4 groups were scanned by CT under working conditions, twice before and after corrosion. Subsequently, the changes in the dissolution effect and pore structure before and after dissolution were quantitatively compared and analyzed. The results show that the dissolution results were directly proportional to the flow rate, temperature, dissolution time, and hydrodynamic pressure. However, the dissolution results were inversely proportional to the pH value. The characterization of the pore structure changes before and after sample erosion is challenging. After erosion, the porosity, pore volume, and aperture of rock samples increased; however, the number of pores decreased. Under acidic conditions near the surface, carbonate rock microstructure changes can directly reflect structural failure characteristics. Consequently, heterogeneity, the presence of unstable minerals, and a large initial pore size result in the formation of large pores and a new pore system. This research provides the foundation and assistance for predicting the dissolution effect and evolution law of dissolved pores in carbonate rocks under multifactor coupling, offering a crucial guide for engineering design and construction in karst areas.

Bohemian sandstone for restoration of cultural heritage sites: 3D microstructure and mass transport properties

We characterised the microstructure of sandstone from the Msene locality (Czech Republic) by combining X-ray computed micro-tomography, back-scattered electron imaging, chemical composition analysis and textural analysis and gas permeation. Both, 2D and 3D images were commonly processed by linking an anisotropic non-linear diffusion filter and a segmentation method based on power watershed. This approach guaranteed binarised outputs that were almost the same in wide ranges of spatial-filter and power-watershed parameters, i.e., subjective choices of the parameters played the negligible role. The rock pore structure was found to be statistically homogeneous and almost isotropic with perfectly connected pore space. We also partitioned the void and solid phases into either grains or pores and throats, which enabled us to study characteristic sizes and connectivity of partitioned regions. By comparing pore and throat sizes, we demonstrated the significant convergent-divergent nature of the pore space because the throat size covered at most one half of the total surface area per pore (cavity). In addition, we calculated coordination numbers for all pores (cavities) to verify almost perfect connectivity of internal pores with those at the external surface. Effective (macroscopic) transport properties of the reconstructed pore space were simulated and the results were validated by experimentally observing steady state flow of inert gas. These findings appear to be a favorable starting point for future investigation of consolidation procedures. The well-connected pore structure with minimum occurrence of dead-end pores suggests that the consolidation agent is very likely to flood the whole pore space.

Evidence of hydrothermal fluid migration pathways in pores and microfractures based on nuclear magnetic resonance and electron microscopic analyses from the Southwest Indian Ridge

The formation of massive sulfide deposits on the seafloor is closely related to sub-seafloor hydrothermal circulation. Hydrothermal circulation depends on the connected pores and fractures. Most previous studies have focused on hydrothermal circulation at the macroscopic scale; however, the transport pathways of hydrothermal fluids have rarely been studied at the microscopic scale. This study showed that multiple measurement techniques could characterize the pores of rocks and the mineral fraction surrounding these pores. Nuclear magnetic resonance (NMR) was used to measure the porosity of fresh basalt, altered basalt, massive sulfide, and sulfide chimneys obtained from the Southwest Indian Ridge. Meanwhile, the distribution of pore radius and pore throats in these samples was obtained using the NMR transverse relaxation time distribution. The scanning electron microscopy results and polarizing light microscope images showed that the pores of altered basalts are more developed than those of fresh basalts, which is consistent with the NMR measurement results that altered basalts have a large pore size distribution. In addition, metallic minerals such as pyrite were found to be precipitated in these pores, and microfractures were observed in altered basalts. This demonstrates that the pores and microfractures in these altered basalts provide pathways for fluid transport and are enriched with metal minerals. Altered basalt is closely related to the formation of massive sulfide deposits and is an important ore-prospecting indicator. Therefore, the study of rock pore structures and components can provide a better understanding of hydrothermal circulation and contribute to the exploration of mineral resources.

Dependence of electrical conduction on pore structure in reservoir rocks from the Beibuwan and Pearl River Mouth Basins: A theoretical and experimental study

The electrical conduction of reservoir rocks is crucial for estimating hydrocarbon and water saturation. The dependence of the electrical formation factor as a key parameter of electrical conduction on pore structure is analyzed through theoretical development, petrophysical experiments, error analysis, core-scale displacement experiments, and pore-scale numerical simulations. Through the tortuous fractal capillary bundle pore model, the electric formation factor is theoretically a function of three pore structure parameters: porosity, tortuosity fractal dimension, and pore fractal dimension. Through rock-electrical experiments from 46 reservoir rocks from the Beibuwan and Pearl River Mouth Basins, Thanh’s model can give a satisfactory performance with a suitable value for the ratio of minimum to maximum pore radius. Porosity-based formation factor models, including classical Archie’s law, have an unacceptable error at high formation factor. Our formation factor model provides better predictions with an error factor of ±10. The Archie’s cementation exponent can be expressed as the average tortuous degree of electrical conduction paths. The dependences of hydraulic and electrical conductions on pore structure are distinctly different. Hydraulic conductance has obvious pore-size dependence, the dominant flow channel, and threshold pressure, which are affected by displacement order. However, the case of electrical conduction is the opposite, in which electrical conduction has no dominant channel, and does not reflect the pore-size information under the same porosity.

Equivalent numerical method and sensitivity analysis of microparameters for micropore compression stage of cemented tailings backfill

The discrete element method has been widely used to study the mechanical properties of rock and cemented tailings backfill (CTB). The high matching degree of mechanical properties between numerical simulation and experiment is the precondition of accurate numerical simulation research. However, as the discrete element method’s particle system cannot characterize pore structure of rock and CTB, it cannot simulate the micropore compression stage (MPCS) of laboratory test. Therefore, the traditional discrete element method cannot accurately simulate the whole process of stress − strain curve of rock and CTB. To solve this problem, in this study, the variation law of the porosity and pore radius of CTB samples’ MPCS were first explored using nuclear magnetic resonance and computed tomography scanning tests. Based on the findings, the equivalent cell model of CTB is proposed. The relative deformation of cells can equivalently characterize the compressive deformation of pores in CTBs. Thus, the initial effective modulus, E0, the progressive coefficient of the effective modulus, k, and cycle steps, b, were introduced to improve the soft-bond model (SBM) in a particle flow code (PFC). This improvement changes the contact structure between particles and makes the contact deformation between particles more consistent with the deformation behavior of micropores in laboratory test. A numerical method for the nonlinear deformation of MPCS is established in PFC. Finally, the parameter sensitivity of the softening factor, softening tensile strength factor, friction coefficient, E0, k, and b in the improved SBM was analyzed. The variation law of samples’ macromechanical parameters with different microparameters is obtained. The results provide a basis for a more accurate study of the mechanical behavior of rock and CTB based on the discrete element method.

Numerical Simulation Study of the Effect of Fine View Pore Structure on Rock Burst

With the gradual shift of coal mining to deeper levels in recent years, rock burst has become one of the primary dynamic hazards faced in deep mining. It has been shown that the pore structure in rocks affects the mechanical properties, but the relationship with the rock burst phenomenon still needs to be clarified. In this paper, we investigated the causes and effects of pore structure on impact mechanical properties using RFPA2D numerical simulation software, established several numerical models with different porosities and pore diameters, and analyzed the stress-strain curves, the relationships between porosity and pore diameter and each the bursting liability indices of the coal rock body were elaborated, and the fitting equations in the range of porosity (0%~10%) and pore diameter (0.25~2.0 mm) were obtained. The results showed that the increase in porosity and pore diameter effectively attenuated the bursting ability of coal rocks, which has some reference significance for the study of early warning and prevention of rock burst phenomenon.

Study on Pore Structure Evolution Characteristics of Weakly Cemented Sandstone under Freeze–Thaw Based on NMR

Taking saturated, weakly cemented sandstone as the research object, nuclear magnetic resonance (NMR) tests were performed before and after six freeze–thaw cycles without water replenishment in order to study and reveal the evolution characteristics of the pore structure of weakly cemented sandstone under a freeze–thaw cycle. The evolution of pore structure under repeated freeze–thaw cycles was studied using T2 fractal theory and spectral peak analysis. The results show that the evolution of the pore structure of weakly cemented sandstone can be divided into three stages during the freeze–thaw cycle. In stage 1, the rock skeleton can still significantly restrict frost heave, and the effect of rock pore expansion occurs only on the primary pore scale, primarily in the transformation between adjacent scales. In stage 2, as the restraint effect of the skeleton on frost heave decreases, small-scale secondary pores are gradually produced, pore expansion occurs step by step, and its connectivity is gradually enhanced. In stage 3, as rock pore connectivity improves, the effect of pore internal pressure growth in the freezing process caused by water migration is weakened, making it impossible to break through the skeleton constraint. Thus, it becomes difficult for freezing and thawing to have an obvious expansion effect on the rock pore structure. The strength of the freeze–thaw cycle degradation effect is determined by the effect of the rock skeleton strength under the freeze–thaw cycles and the connectivity of small-scale pores in the rock. The lower the strength of the rock skeleton, the worse the connectivity of pores, and the more obvious the freeze–thaw degradation effect, and vice versa.

Evaluating the Pore Structure of Low Permeability Glutenite Reservoir by 3D Digital Core Technology

In order to analyze the micro pore structure characteristics of low permeability glutenite reservoirs more intuitively and accurately, CT scanning imaging technology is used to evaluate the development characteristics of pores, fractures and high-density interstitial materials in the three-dimensional core space of samples, and the rock pore structure is analyzed from the perspective of image gray, homogeneity and pore throat connectivity.

Multi-scale Pore Structure Characterization of Lacustrine Fine-Grained Mixed Sedimentary Rocks and Its Controlling Factors: A Case Study of Lucaogou Formation in Jimusar Sag

The microscopic pore structure of fine-grained mixed sedimentary rocks (FMSRs) is of great significance for the occurrence mechanism and sweet spot optimization of mixed shale oil. At present, there is no clear understanding of the microscopic pore system of FMSRs and their coupling relationship with rock composition, which restricts the fine understanding of the reservoir forming mechanism of this type of reservoir. Taking the FMSR samples of the Permian Lucaogou Formation in the Jimusar Sag, the southeastern margin of the Junggar Basin, China, as an example, this paper quantitatively characterizes the full-scale pore structure of lacustrine FMSRs by casting thin sections, scanning electron microscopy, low-temperature nitrogen adsorption (LTNA), high-pressure mercury injection, and nuclear magnetic resonance (NMR); analyzes the coupling relationship between the full-scale pore structure and rock composition; and then summarizes the controlling factors. The results showed that the full-scale pore size distribution (PSD) can be revealed by combining the LTNA and NMR methods. The PSDs of FMSRs are evaluated, mostly corresponding to small pores (30–200 nm) and mesopores (200–1500 nm). The sediment grain size, mineral composition, sedimentary environment, and diagenesis jointly controlled the development of FMSR reservoirs. The development of coarser grain components (such as fine sand and arenaceous dolomite), high content of feldspar, low content of clay and calcite, and appropriate content of dolomite (less than 20%) were conducive to high-quality reservoirs. A near-provenance sedimentary pore throat is better. Siltstones, dolomitic siltstones, and dolarenites were mainly developed in shoreland sandy bar, distal sand bar, and dolomitic mud flat microfacies; type I pore throats with residual intergranular pores and dissolved pores are the most favorable for the Lucaogou Formation.

Effect of long-term infiltration on porosity-permeability evolution in carbonate rocks: An online NMR coupling penetration test

Investigating the porosity–permeability evolution in carbonate rocks during long-term infiltration is a challenging hydrodynamic problem in rock engineering (hydroelectric engineering, CO2 geological storage, and oil/gas development). In general, we consider the evolution of porosity and permeability to be positively correlated. However, the complex pore structures in carbonate rocks include multiscale pore-fracture structures, such as intergranular pores, dissolution pores, and even fractures. Although the permeability levels of carbonate rocks are similar, the evolution of the pore structure is highly variable. Here, we use the “rock seepage coupled nuclear magnetic resonance (NMR) online analysis and imaging system” platform to conduct a long-term penetration test in carbonate rocks under the coupling of stress and reactive fluid. The variation in the multiscale pore volume was quantitatively characterized by the inversion of the T2 spectrum. Subsequently, in combination with permeability measurement, the permeability-porosity evolution during long-term infiltration can be roughly divided into three stages. The experimental results show that the evolution of permeability exhibits noticeable stress sensitivity at the initial stage of the test (0–156 h). Since the compression of the pore volume is the dominant mechanism, the permeability decreases with porosity at this stage. A negative correlation between the evolution of porosity and permeability was observed during the subsequent stage of the test (156–400 h). This phenomenon may be attributed to mineral dissolution at this stage, causing the cementation to be exfoliated from the pore surface and to re-precipitate at the pore throat, locally blocking the seepage channels. As the test continued (400–720 h), mineral dissolution transitioned from micropores to mesopores and macropores, with a synergistic increase in porosity and permeability. These results revealed the competing mechanism of stress compression and mineral dissolution during long-term infiltration, providing an accurate prediction of the seepage properties in carbonate rocks.

Investigation of Stress Sensitivity of Shale Nanopores via a Nuclear Magnetic Resonance Method

Nuclear magnetic resonance (NMR) is widely used to characterize the pore structure of rock. The nanoscale pores and fractures are well developed in a shale gas reservoir. The closure of nanopores caused by the increase in effective stress during the gas production process could induce stress sensitivity in shale nanopores, which has a great impact on the single-well productivity in the middle–late development stage. In this paper, shale samples from the Longmaxi Formation were taken to investigate the nanopore stress sensitivity via an NMR method. Samples with different degrees of pore and fracture development were selected and NMR experiments under different effective stress conditions were carried out. The results show that: (1) As the effective stress increases, the pore space in shale is continuously compressed, and the cumulative pore volume of shale decreases; (2) There is a more pronounced decrease in the cumulative pore volume of samples containing larger pores with the increase in effective stress. However, there are obvious differences in the pore volume changes in different pore sizes; (3) The transformation of nanopores of different sizes occurs in the process of effective stress loading. When the effective stress is small, the pores with diameters larger than 50 nm are mainly transformed to those with diameters of 10–50 nm. When the effective stress increases to a certain extent, the pores with diameters of 10–50 nm are mainly transformed to those with diameters of 0–10 nm; (4) There are significant differences in the compressibility of nanopores of different sizes. Larger nanopores generally have a higher compression coefficient and a stronger stress sensitivity. In the process of effective stress loading, the compression coefficient of pores with diameters between 10 and 50 nm changes relatively slowly, which can well-maintain the pore shape and quantity. Based on the variation in porosity ratio with effective stress, a new method of dividing shale nanopores is proposed; those with diameters smaller than 10 nm, those with diameters of 10–50 nm, and those with diameters larger than 50 nm.

A Minireview of the Influence of CO2 Injection on the Pore Structure of Reservoir Rocks: Advances and Outlook

The recoverable hydrocarbon reserves of conventional oil and gas resources are very limited in China. As important alternative resources, unconventional oil and gas have become a research hotspot. Though tight reservoirs have great potential to alleviate the increasing demand, issues during the development process, such as the rapid pressure depletion, fast decline in production, low productivity, and difficulties in water injection, are usually encountered due to poor physical properties like small pore throats and strong heterogeneity of the pore structure. The CO2 flooding technique could effectively replace crude oil from micro-nanopores, which is considered as a promising way to enhance the development performance of tight oil. However, precipitation and dissolution phenomena usually occur along with the CO2 injection process into reservoirs, affecting the pore structure evolution and oil displacement efficiency. In addition, artificial and natural fractures will even make this process more complicated. This paper presents the commonly used experimental approaches for CO2 injection into tight reservoirs and summarizes the main methods for investigating the influence of CO2 injection on the pore structure of reservoir rocks. Based on this, we highlighted that more attention should be paid to the influence of fractures and their dynamic changes on the evolution of pore structure during CO2 injection and the study of the solid–liquid interactions. To establish a method that could quantitatively evaluate the full-scale evolution of pore throats after CO2 injection is necessary. Meanwhile, the interaction strength of precipitation and dissolution and their effects on pore structure also remain open. Finally, a rigorous framework that could reveal the evolution mechanism and characterize the multiscale pore structure involving multiple influencing factors is urgently warranted.

Permeability Characterization and Its Correlation with Pore Microstructure of Stress-Sensitive Tight Sandstone: Take Chang 6 in Ordos Basin for Example

Tight reservoirs are sensitive to stress changes during fracturing and oil and gas production. Facing different production modes, the variation characteristics of rock permeability and pore structure need to be further clarified. In this study, using a self-built high temperature and pressure physical simulation device and NMR equipment, the influence of the stress loading method, cyclic loading, and loading rate on rock permeability and pore characteristics were analyzed, and the relationship between them was clarified. The permeability sensitivity under variable confining pressure (63.3%) was greater than that of variable flow pressure (46.4%). The damage rate decreased with repeated loading (63.3%-35.8%) and increased loading rate (53.1%-42.3%). As for the pore features, when the net stress increased, the volume variation range of micropores was greater than that of mesopores. The damage rate of permeability (63.3%) was obviously larger than that of pore volume (10.4%). The slope of the fitted curve of permeability and pore volume decreased evidently with loading times. The structure deformation of rock skeleton and the migration of cement had a great influence on permeability in the first loading. Later, it was mainly the bulk deformation of rock particles, the particles’ contact surface increasing and the seepage space shrinking slowly. Eventually, the permeability remained stable due to the limited pore compression. This study can provide a reference for designing reasonable production parameters and reducing formation damage.

Pore morphology effect on elastic and fluid flow properties in Bakken formation using rock physics modeling

Unconventional geo-resources are critical due to their important contributions to energy production. In this energy transition and sustainability era, there is an increased focus on CO2-enhanced oil recovery (CO2-EOR) and geological CO2 storage (GCS) in unconventional hydrocarbon reservoirs, and the extraction of hot fluid for energy through enhanced geothermal systems. However, these energy solutions can only be achieved through efficient stimulation to develop a complex fracture network and pore structure in the host rocks to extract heat and hydrocarbon, or for CO2 storage. Using Bakken formation well data and rock physics models, this study aimed to identify the post-depositional effect of pore structure on seismic velocity, elastic moduli, and formation fluid; and further predict the best lithofacies interval for well landing, and the implications for fluid (gas, oil, and water) recovery in naturally- and often systematically-fractured geosystems. The KT and DEM models' predictions show distinct formation intervals exhibiting needle-like pores and having higher seismic velocities (Vp and Vs) and elastic moduli (K and µ), relative to other formation intervals that exhibit moldic pores. At the same fluid concentration, the needle-like pores (small aspect ratios) have a higher impact on elastic moduli, Vp, and Vs than on the moldic spherical pores with all other parameters held constant. Vp is affected more than Vs by the properties of the saturating fluid (gas, oil, or water) with Vp being greater in Bakken formation when it is water-saturated than when it is gas-saturated. Vs exhibit the reverse behavior, with Vs greater in the gas-saturated case than in the water-saturated case. Further, analyses suggest that the middle Bakken formation will have a higher susceptibility to fracturing and faulting, and hence will achieve greater fluid (oil and water) recovery. Our findings in this study provide insights that are relevant for fluid production and geo-storage in unconventional reservoirs.Article highlightsIntegrated well log data and rock physics models.Investigated the effect of changes in pore structure on elastic properties and fluid flow in shale.Increase in porosity causes a reduction in elastic moduli and seismic velocities.Vp is more affected by pore geometry than Vs depending on density and properties of saturating fluid.Lithofacies with needle−like pores are more susceptible to fracturing than lithofacies with intragranular pores.

Study on the dense structure and properties of cement-stabilized coral aggregates

The performance of cement stabilized coral aggregates semi-rigid base of pavement with coral reef limestone fragments and biological debris as the main materials is essential in the current reef engineering constructions. The performance of inorganic mixtures such as cement-stabilized coral aggregates mainly depends on the packing state of aggregates and the interfacial bonding among each component. Via digital image analysis theory, a simulation model of the dense packing of particles method was established, considering the influence of arbitrary particles shape distribution, upper limit of particles size, and aggregates gradation. The influencing mechanism of different factors on the packing state was revealed. It was found that the dense packing of coral aggregates is a strong suspension dense structure which did not form the skeleton structure. The strengthening mechanism of the coral particle-coated cement slurry was studied through the experimental analysis of the pore structure of coral rocks and debris particles and the crushing strength of particle-coated cement slurry, combined with the microhardness test of the cement- coral particle interface and the analysis of hydration products. The physicochemical excitation mechanism of hydration products densification，“physical interlocking” and “micro pump” effect at the interface of the cement-coral particle was revealed. Furthermore, based on the macroscopic mechanical properties of cement stabilized coral aggregates, the optimal design ratio of coral aggregates with upper limit particle size of 53mm, fine aggregates content of 45% and cement content of 8% was proposed. The findings of this study can provide a reference for the design and application of cement stabilized coral aggregates semi-rigid pavement base.

Fractal Dimension of Digital 3D Rock Models with Different Pore Structures

The macroscopic physical properties of rocks are profoundly determined by their microstructure, and the research of accurately characterizing rock pore structure has been extensively carried out in the fields of petroleum engineering and geoscience. Fractal geometry is an effective means of quantitatively estimating the pore structure properties of porous media. In this study, the evolution law of the fractal dimension and the quantitative relationship between the fractal dimension and porosity were investigated based on the digital 3D rock models. First, three kinds of models with gradually changing pore structures, namely sedimentation, compaction, and cementation, were systematically reconstructed by the process-based approach. Then, the fractal dimensions of the skeleton, pore, and surface of the models were computed and analyzed. Finally, the relationships among the fractal dimension, porosity, and complexity were explored qualitatively. These works reveal the changing laws of three types of fractal dimensions for different pore structure models. The pore structure differences in sedimentation model can only be distinguished by the surface fractal dimension, while both pore and surface fractal dimensions are available parameters for characterizing different pore structures in compaction and cementation models. The quantitative relations between box-counting fractal dimension and porosity were established, which can be expressed by combining linear and logarithmic formulas. The comparison of fractal dimensions of compaction and cementation models proves that fractal dimensions can distinguish the subtle pore structure differences in digital 3D rock models. Understanding the evolution law between the fractal dimension and pore structure parameters provides more references for classifying and evaluating rock pore structure features using fractal dimensions.

Evaluating the pore structure of low permeability glutenite reservoir by 3D digital core technology

In order to analyze the micro pore structure characteristics of low-permeability glutenite reservoirs more intuitively and accurately, µ-CT scanning imaging technology is used to evaluate the development characteristics of pores, fractures and high-density interstitial materials in the three-dimensional core space of samples, and the rock pore structure is analyzed from the perspective of image gray, homogeneity and pore throat connectivity. According to the maximum sphere algorithm, a digital core pore network model is extracted and established, and the probability distribution characteristics of pore radius, throat radius, pore throat ratio and core pore throat coordination number of different rock samples are studied. The results show that the reservoir space of the samples in the study area is dominated by dissolution pores and micro fractures, with pore radius ranging from 0.1 to 10 μm, throat radius from 0.1 to 7 μm, pore throat ratio from 0 to 4, and coordination number from 0 to 7. The overall pore throat ratio, throat radius and poor throat connectivity are the main reasons for the low core permeability in the study area. It is found that the 3D digital core technology can more accurately evaluate the internal heterogeneity of rocks, and the calculated pore structure parameters include both connected pore information and isolated pore information. Compared with conventional mercury injection and nuclear magnetic resonance, the 3D digital core technology has obvious advantages in the evaluation of rock micro pore structure.

Effect of high-multiple water injection on rock pore structure and oil displacement efficiency

Experimental methods, including mercury pressure, nuclear magnetic resonance (NMR) and core (water-oil) displacement, are used to examine the effects of high-multiple water injection (i.e. water injection with high injected pore volume) on rock properties, pore structure and oil displacement efficiency of an oilfield in the western South China Sea. The results show an increase in the permeability of rocks along with particle migration, an increase in the pore volume and the average pore throat radius, and enhanced heterogeneity after high-multiple water injection. Compared with normal water injection methods, a high-multiple water injection is more effective in improving the oil displacement efficiency. The degree of recovery increases faster in the early stage due to the expansion of the swept area, and the transition from oil-wet to water-wet. The degree of recovery increases less in the late stage due to various factors, including the enhancement of heterogeneity in the rocks. Considering both the economic aspect and the production limit of water flooding, it is recommended to adopt other technologies to further enhance oil recovery after 300 PV water injection.

A model of unfrozen water content in rock during freezing and thawing with experimental validation by nuclear magnetic resonance

The unfrozen water content of rock during freezing and thawing has an important influence on its physical and mechanical properties. This study presented a model for calculating the unfrozen water content of rock during freezing and thawing process, considering the influence of unfrozen water film and rock pore structure, which can reflect the hysteresis and super-cooling effects. The pore size distribution curves of red sandstone and its unfrozen water content under different temperatures during the freezing and thawing process were measured using nuclear magnetic resonance (NMR) to validate the proposed model. Comparison between the experimental and calculated results indicated that the theoretical model accurately reflected the water content change law of red sandstone during the freezing and thawing process. Furthermore, the influences of Hamaker constant and surface relaxation parameter on the model results were examined. The results showed that the appropriate magnitude order of Hamaker constant for the red sandstone was 10 −19 J to 10 −18 J; and when the relaxation parameter of the rock surface was within 25–30 μm/ms, the calculated unfrozen water content using the proposed model was consistent with the experimental value. • Theoretically derived formula for unfrozen water film calculation. • Considering the super-cooling effect and hysteresis effect during freezing-thawing. • The calculation model of unfrozen water content was proposed and verified by experiments.