Deformation Of Pore Structure Research Articles

How pore structure evolves in shale gas extraction: A new fractal model

The reservoir can be divided into four components with different properties: inorganic matrix, hydraulic fracture, natural fracture and kerogen according to the internal structure characteristics and the natural gas migration mode after hydraulic fracturing. Previous studies usually focus on the relationship between reservoir porosity and permeability, but ignore the influence of reservoir microstructure on gas production. The microstructure of different reservoir components can be described by corresponding fractal dimensions. The structure of fractures can be characterized by fractal dimension of fractures and the fractal dimension of pores can characterize the pore structure of inorganic matrix. Considering the complexity of mass transfer between components and structural deformation, in order to describe the evolution of reservoir microstructure and the influence of various factors on gas production, a fully coupled multi-scale fractal model is established. To verify that the model is reliable, the simulation results contrast with the conclusions of field tests. The effects of adsorption parameters, initial diffusion coefficient and compressibility coefficient on gas production and pore structure evolution were studied. The results show that:(1) when the adsorption constant decreases, the variation of the fractal dimension of pore structure becomes more obvious, and the shale reservoir's gas production and pore structure are most sensitive to the volume constant; (2) the deformation of pore structure increases with the increase of effective diffusion coefficient; (3) the change of compressibility of a single medium only has a major impact on the evolution of its own structure.

Digital Rock Mechanical Properties by Simulation of True Triaxial Test: Impact of Microscale Factors

Complex fractures and pore structures in the rock strongly influence the mechanical properties, and the process from compression to failure is complicated. Under the action of rock stress, pore structure deformation and fractures close or propagate, easily leading to deterioration in the rock mechanical properties until rock failure. Thus, the effects of microscale factors are critical in mechanical properties such as rock strength, elastic modulus, and stress–strain state under the triaxial stress state. It is difficult for physical and mechanical experiments to obtain the qualitative rules of regular structures, but numerical simulation can make up for this defect. In this work, the accuracy of the model was proven through a comparison with previous experimental results. The true triaxial numerical simulation experiments were conducted on representative rocks and natural pore structures. These simulated results revealed that the pore and throat parameters will change abruptly when the particle model volumetric strain is between 0.0108 and 0.0157. When the fracture angle is between 45° and 75°, the fracture has a great influence on the peak stress. The angle between the natural fracture and the fracturing direction should be less than 45° as much as possible. Clay affects the rock strength by influencing the force chains formed by the rock skeleton. Fracturing is easier when the structural clay content is higher than 25%. It is easier to fracture in a direction parallel to the laminated clay when the clay content is below 27%. This work indicates the effects of rock particles, fractures, and clay on the mechanical parameters, providing key fundamental data for further quantifying the fracturing patterns.

Study on three-dimensional immiscible water–Oil two-phase displacement and trapping in deformed pore structures subjected to varying geostress via in situ computed tomography scanning and additively printed models

Accurate understanding and quantitative characterization of the three-dimensional (3D) immiscible water–oil two-phase displacement process and of the factors that affect this process in porous reservoir rocks are prerequisites for the enhancement of petroleum recovery efficiency. To meet these prerequisites, it is crucial to directly visualize and quantify the pore scale physics and dynamic evolution of the water–oil two-phase displacement behavior. However, engineering activities, such as borehole drilling, reservoir fracturing, and oil recovery, can cause geostress redistribution in petroleum reservoirs and drastically change the 3D pore structures of reservoir rocks. This makes it difficult to apply conventional experimental techniques and analytical models to directly reveal and evaluate the 3D immiscible fluid displacement in reservoir rocks during pore structure deformation. In this study, we used X-ray computed tomography, integrated with triaxial loading techniques, to capture in situ the immiscible water–oil displacement and oil trapping inside 3D pore spaces during the deformation induced by various geostresses. An additive manufacturing or 3D printing (3DP) technology was applied to replicate the transparent models of actual porous rocks, facilitating the representation and quantification of pore space deformation and the dynamic process of water–oil displacement and oil trapping. Pore scale displacement behaviors (including water sweeping, fingering effects, preferential flow paths, and oil trapping) and their evolution and pore structure deformation with varying geostress were directly visualized and quantified. The relationships between the characteristics of the deformed structures, water–oil displacement efficiency, and effective geostress changes were formulated. The results indicate that stress-induced pore structure deformation has an evident impact on the 3D water–oil displacement behavior and efficiency. A comparison of the 2D and 3D water-oil displacement behaviors indicates that the 3D model predicts a more realistic displacement performance, whereas the residual oil saturation is overestimated in a 2D pore system owing to the truncation of pore connectivity in the third dimension. This study provides a method for directly visualizing and quantifying the effects of geostress-induced pore deformation on the 3D water–oil displacement in porous reservoirs; this method can help draft a strategy to enhance petroleum recovery.

Pore characteristics and pore structure deformation evolution of ductile deformed shales in the Wufeng-Longmaxi Formation, southern China

Marine shales in southern China experiences multiple complex tectonic movements, which highly affects shale pore characteristics and pore evolution. The effects of tectonic deformation on nanopore structure and petrophysical properties of organic shales remain unclear. Three ductile deformed shale samples were collected and analyzed through an integrated experiment procedure for a comparative research against undeformed shale samples. Field emission-Scanning electron microscopy (FE-SEM), low pressure N2 and CO2 adsorption and mercury injection porosimetry (MIP) analyses were performed to characterize pore structure of the shales. The results reveal that the mineral compositions of these shale samples are controlled by sedimentary environment and source and exhibits no obvious relationship with tectonic deformation. With increasing deformation degree, the micropore proportion gradually decreases (from 17% to 6%), and the macropore proportion increases from 16% to more than 20%. Micropore volumes and proportion in the fold core samples are the smallest (4.2 × 10−3 cm3/g and 6.3%) and account for only one-third of the undeformed shale values, while the meso- and macropore values are larger-1.2 times those of the undeformed shale values. The evolution characteristics and deformation modes of organic matter (OM), interparticle (interP) and intraparticle (intraP) pores and microfractures under ductile deformation were examined. All pore types become interconnected during deformation and form microfractures ranging from a few nanometers to tens of microns, which is the main reason for the enhanced shale connectivity caused by deformation. Under the same porosity conditions, the deformed shale permeability is much higher than the undeformed shale permeability, and stronger deformation corresponds to higher permeability. The fold core permeability is generally higher than the limb permeability and exceeds 30 times. The different ductile deformation positions were divided into five levels, and the corresponding reservoir quality was evaluated. This study is of great significance to better understand the pore structure evolution process for shale exploration and development under tectonic deformation.

An Alternative Parameterization of Relative Permeability and Capillary Pressure Curves

SummaryThis paper presents a theoretically rigorous correlation of the performance of capillary pressure and relative permeability of naturally fractured sandstone and carbonate reservoirs involving saturation shocks and loading/unloading hysteresis under various stress and thermal conditions. The proposed modeling approach accounts for the combined effects of the porous-rock alteration by various processes, including deformation, transformation, deterioration, and collapse of pore structure, under prevailing temperature and stress conditions during loading and unloading processes, and their effect on the capillary pressure and relative permeability of naturally fractured reservoirs. A saturation shock causing a slope discontinuity in the capillary pressure and relative permeability is shown to occur during saturation change in some sandstone- and carbonate-reservoir formations at a critical saturation condition. This phenomenon can be triggered by alteration of fluid-percolation pathways as a result of the transition from open to closed natural or induced fractures and the deformation of pore structure. The effect of the saturation shock and loading/unloading hysteresis on the capillary pressure and relative permeability of reservoir-rock formations is formulated by means of a phenomenological kinetics model and its applicability is demonstrated by analyzing and correlating the available experimental data.In this paper, the proven comprehensive model developed from a kinetics equation is shown to lead to a theoretically meaningful, universal, and practical constitutive equation in the form of a modified power law. This kinetics equation expresses the probability of dependence of a petrophysical property of porous rocks on a variable, such as saturation for capillary pressure and relative permeability, based on the value of the property relative to its low- and high-end limit values. The applicability of the modified power-law equation is validated by means of the experimental data of the capillary pressure and relative permeability gathered by testing of representative samples from various sandstone and carbonate reservoirs. The phenomenological parameters of the core samples obtained from sandstone and carbonate reservoirs are determined for best match of the experimental data with the modified power-law equation. The value of the critical fluid saturation is determined by the observance of a slope discontinuity occurring in the measured experimental data of the variation of the capillary pressure and relative permeability with saturation. The scenarios presented in this study indicate that loading/unloading hysteresis and saturation shock have significant effects on the stress- and temperature-dependent capillary pressure and relative permeability of the porous reservoir-rock formations. The data-inferred physics-based model presented in this paper is proved to describe the stress- and temperature-dependent capillary pressure and relative permeability of sandstone and carbonate rocks with high accuracy while meeting the endpoint-limit conditions satisfactorily.

Possible pore structure deformation effects on the shale gas enrichment: An example from the Lower Cambrian shales of the Eastern Upper Yangtze Platform, South China

Shale gas reservoir performance and canister desorption experiments of the Lower Cambrian organic-rich shales in the eastern Upper Yangtze Platform reveal a significant difference in shale gas content between the Dabashan arc-like fold-thrust belt in northeastern Chongqing (Deformed Zone) and the slightly folded area in southeastern Chong (Non-deformed Zone). Integrated pore characterization methods including scanning electron microscopy (SEM), low-temperature N2 and CO2 adsorption, and mercury injection capillary pressure (MICP) analyses were comparatively conducted in both areas in order to examine shale gas reservoir pore size variations and thus the possible microscopic pore structure controls on shale gas enrichment.The Lower Cambrian shales in both Deformed Zone (DZ) and Non-deformed Zone (NDZ) were deposited in the deep-water shelf and show similar organic matter richness and thermal maturity. The majority of organic matter (OM)-hosted pores in DZ samples are in nanoscale size range with the dominance of micro-fractures within the OM or at the interface of OM and minerals. In contrast, OM-hosted meso-(2–50 nm) to macropores (>50 nm) are the dominant pore types in the NDZ samples. OM-hosted micropores (<2 nm) are abundant in both zones. Helium ion microscopy observations further confirm the presence of OM-hosted micropores in the studied samples. Mineral-hosted pores in carbonate minerals are abundant in both zones, while dissolution rims around carbonate minerals are more abundant in NDZ samples.The Dabashan arc-like fold-thrust belt took place by the end of the Late Triassic, while the Lower Cambrian shales have reached thermal maturity peak. OM-hosted meso-(2–50 nm) to macropores (>50 nm) in DZ samples are most probably collapsed during structural deformation related to tectonic compression, while micropores due to their smaller size survived the tectonic stress. The OM-hosted micropores are the main storage space for adsorbed gas in the DZ area. The dominance of micro-pores in DZ and lack of connection between those pores and matrix pores led to higher gas content in DZ samples. On the contrary, the well-connected OM-hosted pore network in NDZ allows easier gas flow in the rock matrix that eventually led to significant gas leakage during uplift and exhumation and lower gas content in this zone. The results of this study suggest that structural deformation can potentially change the pore structure of shales and thus shale gas content which has major significance for shale gas exploration and development in south China where had experienced complex tectonic movements.

Interpreting the pH-dependent mechanism of simazine sorption to Miscanthus biochar produced at different pyrolysis temperatures for its application to soil

Biochar has received considerable attention as an eco-friendly bio-sorbent; however, multifarious characteristics caused by pyrolysis and feedstock pose difficulties in its application. We characterized the pH-dependent sorption of the pesticide simazine on Miscanthus biochar produced at two pyrolysis temperatures (400 and 700 °C; hereafter B-400 and B-700). The specific surface-area (SSA) of the micro- and nanopores, elemental composition, surface acidity and infrared spectra were determined. The SSA was greater in B-700 than in B-400, and the former had greater SSA in micro-pores and lower SSA in nanopores than the latter. During pyrolysis, the single-bond structures of the feedstock were converted to aromatic structures, and further pyrolysis led to ligneous structures. Alterations in pore structure and concave-up Scatchard plot corroborated the presence of two sorption mechanisms: electrostatic attractions (Ses) and hydrophobic attractions (Shp). Decreases in maximum sorption in the qmax-L with increasing pH was due to decreased Ses via deprotonation of carboxylic groups on biochar, while those in the qmax-H with increasing pyrolysis temperature were due to decreased Shp, resulting from pore structure deformation. We believe that our approach, which addresses the pH-dependence of charge density of sorbate and sorbent, could contribute to a better understanding of the behavior of simazine.

Experimental investigation of shale gas production with different pressure depletion schemes

A matrix pressure depletion scheme can result in pore structure deformation and permeability reduction in shale, which affects the process of gas production in shale. Shale gas production tests were carried out with four core samples and with two crushed samples to investigate the effects of three different pressure depletion schemes on gas production rate and on ultimate gas recovery. The three pressure depletion schemes tested include constant production pressure, linear pressure decline, and step-wise pressure decline. Results of the gas production tests show that, for shale core samples, a production pressure depletion scheme affects not only gas production rate but also ultimate gas recovery. Pressure sensitivity tests of the four shale core samples were also performed, and the results show that permeability reduction of shale cores due to rapid pressure depletion caused the decrease in ultimate gas recovery. The gas production tests with crushed shale samples which were under zero effective stress show that a pressure depletion scheme only affects the gas production rate; that is, the same ultimate gas recovery was obtained for different pressure depletion schemes. The experimental results also show that the linear pressure decline and step-wise pressure decline depletion schemes delay the permeability reduction of the shale matrix, thereby resulting in a greater ultimate gas recovery than the constant production pressure scheme in which the production pressure was dropped immediately to an end value. In a step-wise pressure decline depletion scheme, in which gas diffusion and desorption were allowed to reach equilibrium at each pressure step, the ultimate gas recovery increases with a decrease in the size of the pressure step. However, in a linear pressure decline depletion scheme, the maximum ultimate gas recovery is obtained with that optimum pressure decline rate which causes the best match between the permeability reduction rate and the gas diffusion and desorption rates.

Mechanical properties and pore structure deformation behaviour of biomedical porous titanium

Porous titanium has been shown to exhibit desirable properties as biomedical materials. In view of the load-bearing situation, the mechanical properties and pore structure deformation behaviour of porous titanium were studied. Porous titanium with porosities varying from 36%–66% and average pore size of 230 μm was fabricated by powder sintering. Microstructural features were characterized using scanning electron microscopy. Uniaxial compression tests were used to probe the mechanical response in terms of elastic modulus and compressive strength. The mechanical properties of porous titanium were found to be close to the those of human bone, with stiffness values ranging from 1.86 to 14.7 GPa and compressive strength values of 85.16–461.94 MPa. The relationships between mechanical properties and relative densities were established, and the increase in relative density showed significant effects on mechanical properties and deformations of porous titanium. In a lower relative density, the microscopic deformation mechanism of porous titanium was yielding, bending and buckling of cell walls, while the deformation of yielding and bending of cell walls was observed in the porous titanium with higher relative density.

Sustainable activity of hydrothermally synthesized mesoporous silicates in acetic acid esterification

A hydrothermal method was applied in the synthesis of mesoporous silicates containing silicotungstic acid (STA). The synthesis procedures were developed by modification of procedures previously applied in the synthesis of MCM-41 and SBA-15. The synthesized catalysts were named MCM-41-S and SBA-15-S based on MCM-41 and SBA-15. Their activities were investigated in ethyl acetate production, which was selected as the model reaction. The results indicated that the activity of SBA-15-S catalysts increased with increasing STA in the catalyst. Reaction experiments were also conducted in identical conditions in the presence of catalysts recovered after the reaction. The results indicated that sustainable activity could be achieved in the presence of these spent catalysts with activity losses around 10%. Evaluation of characterization analyses conducted with fresh and spent catalysts showed that the activity loss of SBA-15-S catalysts had merely been due to STA leaching, whereas deformation of pore structure had occurred in MCM-41-S catalysts.

Nondestructive observation of pore structure deformation behavior of functionally graded aluminum foam by X-ray computed tomography

Cellular materials are expected to be used for the components of automobiles and as railway and building materials owing to their lightweight structures and desirable mechanical properties. The newest class of cellular materials consists of functionally graded aluminum (FG Al) foams, in which the properties vary with the position. In this study, closed-cell FG Al foam with a varying pore structure was fabricated and its deformation behavior during the compression tests was nondestructively observed by X-ray computed tomography (X-ray CT). The fabricated FG Al foam had different pore structures (different densities) in the same Al foam. The FG Al foam initially started to deform only in the low-density region, which was followed by deformation in the high-density region. The deformation of the FG Al foam indicated its novel mechanical properties, such as multiple plateau stresses, corresponding to the pore structures of the regions where deformation occurred. Consequently, it was shown that the FG Al foam with a varying pore structure and a seamless bonding interface has the potential to be deformed at a controlled and desired location and with a desired plateau stress.