Unconnected Pores Research Articles

Connectivity of pores in shale reservoirs and its implications for the development of shale gas: A case study of the Lower Silurian Longmaxi Formation in the southern Sichuan Basin

The unconnected gas-bearing pores in shale gas reservoirs may be transformed into “potential recoverable pores” after large scale hydraulic fracturing. However, the mainstream pore classification methods of shale gas reservoirs, do not take the unconnected pores into account, which impacts the evaluation accuracy of reservoir pore effectiveness. To solve this problem, this paper took the shale of Lower Silurian Longmaxi Formation in the southern Sichuan Basin as the research object to carry out experiments on core porosity, centrifugal + gradual drying NMR after saturated with brine and NMR freeze–thaw using plunger samples and crushed samples to analyze the volume, main development location and main pore size distribution range of unconnected pores. Then, the pore systems were classified, and the lower limit of effective pore size of connected gas-bearing pores was determined. Finally, the total pore effectiveness of shale gas reservoirs was evaluated, and the influence of unconnected pores on the development of shale gas was discussed. And the following research results were obtained. First, there are a large number of unconnected pores in the shale gas reservoirs of the study area, accounting for 30.23%. Most of them are mainly developed in organic matters and a few are developed in clay minerals. The distribution of pore size ranges from 5 nm to 30 nm. Second, T2 cut-off value of clay bound water in the shale gas reservoirs of the study area is 0.26 ms, whose corresponding pore size is 5.35 nm, which is the lower limit of the effective pore size. Third, large-scale hydraulic fracturing can improve the unconnected pores with the size more than 5.35 nm, so effective development of shale gas will be realized. Fourth, after hydraulic fracturing stimulation, the unconnected pores can increase the storage space of fracturing fluid in the matrix, absorb the fracturing fluid in the fractures, replace the shale gas in the pores and promote the automatic alleviation of water lock in shale gas reservoirs, so the single-well shale gas production will be increased. In conclusion, fluid occurrence and pore system in shale pores can be quantitatively divided by means of centrifugal + gradual temperature drying method combined with NMR experiments, mobile water and capillary bound water can be determined by the high speed centrifugal + NMR experiments, and capillary bound water and clay bound water can be determined by the gradual drying + NMR experiments.

Development trend of marine shale gas reservoir evaluation and a suitable comprehensive evaluation system

At present, shale gas exploration and development in China is faced with some problems, such as the imperfect evaluation system of reservoir effectiveness and the limitations of reservoir evaluation system on efficient development of shale gas. In order to improve the content and the standard of reservoir evaluation, this paper analyzed the shortcomings and challenges in the static evaluation of shale gas reservoirs on the basis of existing reservoir evaluation, and established a method for evaluating shale gas reservoir effectiveness and a scheme for classifying pore systems. Then, the dynamic evaluation parameters after shale fracturing and their effects on drainage and production measures were discussed. In addition, the potential evaluation parameters of “automatic mitigating water blocking” were studied, and a comprehensive reservoir evaluation system of “static–dynamic” combination was established. And the following research results were obtained. First, new challenges to the shale gas reservoir evaluation are emerged as the lack of in-depth study on “reservoir effectiveness, dynamic evaluation parameter system after fracturing and drainage and production measures after fracturing”, which leads to the serious lag of existing shale gas reservoir evaluation system behind production. Second, the evaluation of reservoir effectiveness is mainly presented as the evaluation on the lower limit of effective porosity, and is embodied in the influence of clay bound water and unconnected pores on the development of shale gas. Third, the development of shale gas reservoir evaluation follows the trend of refining the static reservoir evaluation parameters, defining the potential evaluation indexes of “automatic mitigating water blocking” and establishing the reservoir comprehensive evaluation system of “static–dynamic” combination. Fourth, a post-frac dynamic evaluation system is determined for the potential evaluation indexes of “automatic mitigating water blocking” (e.g., wettability, water imbibition retention capacity, water imbibition expansion mode, expansion rate, and water imbibition cracking capacity). Fifth, a reservoir evaluation idea is put forward that “static evaluation of shale gas reservoir is the foundation and postfrac dynamic evaluation is the complement”, and a comprehensive reservoir evaluation system is established of “static–dynamic” combination suitable for the evaluation of marine shale gas reservoirs in China.

Study on failure mechanism of porous concrete based on acoustic emission and discrete element method

Porous concrete is an environmentally friendly material. To evaluate the damage and discuss the failure mechanism of porous concrete, a uniaxial compression test was carried out in this study. Based on acoustic emission (AE) technology, the changes in AE ringing counts and energy were recorded, and the b-value was analysed. In this research, the evolution of cracks and stress-strain curves of specimens under uniaxial compression were also analysed by the discrete element method (DEM). The results showed that the failure process of porous concrete could be divided into an initial stage, crack propagation stage and failure stage. The mechanical properties and failure characteristics of porous concrete are different from those of conventional concrete. The main failure mode of porous concrete under compression is the failure of the bonding layer between the aggregates, whereas the aggregate failure of conventional concrete rarely occurs. There are many connected or unconnected pores in porous concrete. During the failure process of porous concrete, cracks propagate into the pores until penetrating cracks occur. This paper provides a basis for further study on the microscopic mechanism of porous concrete failure.

Experimental study on the moving characteristics of fine grains in wide grading unconsolidated soil under heavy rainfall

The initiation mechanism of debris flow is regarded as the key step in understanding the debris-flow processes of occurrence, development and damage. Moreover, migration, accumulation and blocking effects of fine particles in soil will lead to soil failure and then develop into debris flow. Based on this hypothesis and considering the three factors of slope gradient, rainfall duration and rainfall intensity, 16 flume experiments were designed using the method of orthogonal design and completed in a laboratory. Particle composition changes in slope toe, volumetric water content, fine particle movement characteristics and soil failure mechanism were analyzed and understood as follows: the soil has complex, random and unstable structures, which causes remarkable pore characteristics of poor connectivity, non-uniformity and easy variation. The major factors that influence fine particle migration are rainfall intensity and slope. Rainfall intensity dominates particle movement, whereby high intensity rainfall induces a large number of mass movement and sharp fluctuation, causing more fine particles to accumulate at the steep slope toe. The slope toe plays an important role in water collection and fine particle accumulation. Both fine particle migration and coarse particle movement appears similar fluctuation. Fine particle migration is interrupted in unconnected pores, causing pore blockage and fine particle accumulation, which then leads to the formation of a weak layer and further soil failure or collapses. Fine particle movement also causes debris flow formation in two ways: movement on the soil surface and migration inside the soil. The results verify the hypothesis that the function of fine particle migration in soil failure process is conducive for further understanding the formation mechanism of soil failure and debris flow initiation.

Direct three-dimensional reconstruction of a nanoporous catalyst layer for a polymer electrolyte fuel cell

The direct three-dimensional reconstruction of a polymer electrolyte fuel cell cathode catalyst layer from focused ion beam/scanning electron microscope (FIB/SEM) images is presented. The carbon and pore distribution is shown and quantitatively analysed. A new catalyst layer sample (Fumapem-F950/HiSpec13100) is sliced with FIB and a series of SEM images is taken. The images are registered, segmented and a three-dimensional stack is reconstructed. The three-dimensional carbon and pore distribution is shown. Based on the reconstruction the pore size distribution is evaluated. The total porosity and the unconnected pores space is analysed. The fully segmented 2D images are provided as supplemental material to this paper for future analysis and modeling work.

Inverted Colloidal Crystal Scaffolds for Uniform Cartilage Regeneration

A uniform distribution of chondrocytes in 3D scaffolds is a critical challenge to cartilage regeneration. This study aims to resolve the problem by showing uniformly distributed chondrogenesis in chitin/chitosan matrix with pores of inverted colloidal crystal (ICC) structure. The results revealed that the effect of solvent on the regularity of colloidal crystal arrays was in the order of ethanol > ethylene glycol > acetone. When the concentration of chitin/chitosan gel was <2.5%, the porosity of ICC scaffolds with pure ethanol was approximately 84%. The highly porous freeform scaffolds produced random and unconnected pores, in general. The viability of bovine knee chondrocytes (BKCs) in ICC constructs was >92%. Over 4 weeks of cultivation, the percentage of biodegradation of ICC scaffolds with pure ethanol was approximately 34%. The order in the produced BKCs, glycosaminoglycans (GAGs), and collagen was freeform constructs > ICC constructs with pure ethanol > ICC constructs with 95% acetone. However, the spatial distribution of BKCs in ICC constructs was more uniform than that in freeform constructs. In addition, BKCs could secrete GAGs and type II collagen in the core of ICC constructs, indicating the maintenance of phenotypic chondrocytes and their metabolism. The ICC constructs with well-controlled pore regularity and unique topography can generate uniform tissue-engineered cartilage.

Approximate single wave equation derived from Biot’s porous media equations

Manipulation of Biot’s equations using low-frequency and long-wavelength approximations yields wave equation with a dissipation term that predicts plane wave attenuation coefficients proportional to the square of the frequency. Examination of Biot’s original derivation verifies that the wave speed in this equation is what results from Wood’s theory. The derivation predicts that the fluid displacement is a predictable fraction of that of the solid matrix, and this is consistent with the behavior in Brillouin’s model of an elastic spring loaded with masses of alternating magnitude. Biot’s assumption that the pores must be connected is argued to be unnecessary. Unconnected pores may be elongated with constrictions, and the sound field will cause some portions of the porelets to be compressed more than other portions, so that pressure differences across constrictions force viscous flow. A heuristic derivation shows that the wave attenuation coefficient is inversely proportional to viscosity and sound speed, and directly proportional to density, the number of porelets per unit volume, the average length of porelets, and the square of the cross-sectional area of the constrictions and frequency. The heuristic theory is supported with a detailed analysis of the energy dissipation within a dumbbell-shaped porelet within an extended isotropic elastic solid.

Reaction-induced fluid flow in synthetic quartz-bearing marbles

The reaction kinetics and fluid expulsion during the decarbonation reaction of calcite+quartz=wollastonite+CO2 in water-absent conditions were experimentally investigated using a Paterson-type gas apparatus. Starting materials consisted of synthetic calcite/quartz rock powders with variable fractions of quartz (10, 20, and 30 wt%) and grain sizes of 10 µm (calcite) and 10 and 30 µm (quartz). Prior to reaction, samples were HIPed at 700 °C and 300 MPa confining pressure and varying pore pressures. Initial porosity was low at 2.7–6.3%, depending on pore pressure during HIP and the amount and grain size of quartz particles. Samples were annealed at reaction temperatures of 900 and 950 °C at 150 and 300 MPa confining pressures, well within the wollastonite stability field. Run durations were between 10 min and 20 h. SEM micrographs of quenched samples show growth of wollastonite rims on quartz grains and CO2-filled pores between rims and calcite grains and along calcite grain boundaries. Measured widths of wollastonite rims vs. time indicate a parabolic growth law. The reaction is diffusion-controlled and reaction progress and CO2 production are continuous. Porosity increases rapidly at initial stages of the reaction and attains about 10–12% after a few hours. Permeability at high reaction temperatures is below the detection limit of 10−21 m2 and not affected by increased porosity. This makes persistent pore connectivity improbable, in agreement with observed fluid inclusion trails in form of unconnected pores in SEM micrographs. Release of CO2 from the sample was measured in a downstream reservoir. The most striking observation is that fluid release is not continuous but occurs episodic and in pulses. Ongoing continuous reaction produces increase in pore pressure, which is, once having attained a critical value (Pcrit), spontaneously released. Connectivity of the pore space is short-lived and transient. The resulting cycle includes pore pressure build-up, formation of a local crack network, pore pressure release and crack closure. Using existing models for plastic stretching and decrepitation of pores along with critical stress intensity factors for the calcite matrix and measured pore widths, it results that Pcrit is about 20 MPa. Patterns of fluid flow based on mineralogical and stable isotope evidence are commonly predicted using the simplifying assumption of a continuous and constant porosity and permeability during decarbonation of the rock. However, simple flow models, which assume constant pore pressure, constant fluid filled porosity, and constant permeability may not commonly apply. Properties are often transient and it is most likely that fluid flow in a specific reacting rock volume is a short-lived episodic process.

Comparison between Adsorption in Pores of a Simple Geometry and Realistic Models of Porous Materials

ABSTRACTWe study, by means of Grand Canonical Monte Carlo simulations, the effect of disorder in pore morphology and topology on adsorption of simple fluids at 77 K. We consider the case of microporous carbons and mesoporous silicas. For both systems, we found that morphological and topological disorder affects the adsorption isotherm. Consequently, we show that complex porous networks cannot be modeled as a collection of unconnected pores of simple geometry. In the case of microporous carbons, the isosteric heat of adsorption for disordered systems differs significantly from that obtained for an assembly of unconnected slit pores. In contrast, the isosteric heat of adsorption for the disordered mesoporous silica sample compares reasonably well with that observed for a silica material having regular cylindrical pores.

The Acoustic Properties of Expanded Clay Granulates

The purpose of this work is to characterise experimentally and theoretically the acoustic performance of mixes of loose expanded clay granulates. The surface acoustic impedance is obtained from measurements in the impedance tube and predicted using the Pade approximation approach. The random incidence absorption coefficient is measured using the ISO 345/85 method. Experimental data for the surface acoustic impedance are used to deduce the porosity, tortuosity and the statistical pore size distribution; these parameters are difficult to obtain directly, because of the relatively low density of the loose granulates and the fact that the granular base contains 5–30% of the closed, unconnected pores. Good agreement between the theory and the results is found; 50–100 mm layers of expanded clay granulates can be used efficiently for noise control over a broad frequency range.

On the Determination of Quantum Efficiencies in Heterogeneous Photocatalysis

A framework to determine the quantum efficiency η of a photoreaction in a porous layer of a photocatalyst is presented. The procedure relies on a model of the photoproduct diffusion in the porous structure of the photocatalyst. The model incorporates a position-dependent source term mirroring the light intensity profile in the layer and an effective diffusion coefficient Deff. It allows for a simultaneous determination of η as well as of Deff. The method is applied to the photosynthesis of CH4 from gaseous H2O and CO2 at the solid/gas interface of a porous layer of TiO2 (Degussa P25). A value of η = (8.79 ± 0.79) × 10-4 is found for the formation of CH4. The effective diffusion coefficient for the photoproduct CH4 is Deff = (5.64 ± 2.51) × 10-10 cm2 s-1. This value is much too low to be explained by classical Knudsen diffusion in the layers with an experimentally determined porosity φ = 0.58. We suggest that the TiO2 layers contain a large number of unconnected pores separated by walls of densely packed m...

HREM Studies on a New Zeolite AlPO4 #5

Catalytic studies demonstrate that subtle structural differences among the family of zeolites have a marked effect on catalytic properties; therefore, exact knowledge of the zeolite framework structure is desirable. The useful role of HREM in structural studies of zeolites has already been established. AlPO4#5 is a newly synthesized aluminophosphate “molecular sieve” with zeolite-like properties having potential applications as adsorbents, catalysts and catalyst supports. It differs from aluminosilicates mainly due to the substitution of Si with P in the framework. Non-connecting parallel channels spanned by 12-rings in the as-synthesized material contains tetrapropyl- ammonium hydroxide species (fig. 1). Its ideal chemical composition is TPAOH.12AlPO4 and removal of the TPAOH by calcination at 400-600°C produces a molecular sieve for which absorption studies are consistent with unconnected pores bounded by 12-rings. The structural details of AlPO4#5 have been reported recently on the basis of x-ray diffraction. As it has not been studied so far by high resolution electron microscopy, we have used a JEOL 200CX in an attempt to examine local details of its framework at atomic level, detail that x-ray diffraction fails to reveal due to averaging effects.

Field-Property Bounds for Porous Sintered Ceramics

Three sets of equations provide upper and lower values bound ing the experimentally obtained electrical and thermal conduc tivities as well as the magnetic permeabilities (=“field properties”) of porous ceramics. These equations are as I-order upper bound: φp=φM(1-P) (φ=effective field property of the porous material, φM=field property of the nonporous material, P=porosity), and as I-order lower bound: φr=φM (1-P)7. Closer bounds corresponding to isotropic porous materials are the II-order upper bound: φPII,=φM,[2(1-P)/ (2+P)], and the II-order lower bound: φPIII=φM(1-P)3. Even closer bounds, referring to isotropic porous materials containing closed, unconnected pores are the III-order upper bound: φPIII=φM(1-P)3/2, and the III-order lower bound: φPIII=φM(1-p)3.

Viscous flow during liquid phase sintering

1. Kinetic equations for sintering have been derived by substituting relationships for the linear rate of closing of a cavity in the deformation equation for viscous flow. It appears that the process is represented by a logarithmic dependence of shrinkage on time. 2. The retardation of the viscosity of flow is due to closing of porosity and compression of the trapped gases. 3. We have deduced that during the sintering of a component with closed and unconnected pores there is an actual cubic dependence of the shrinkage on the sintering time. 4. The derived relationships for sintering without pressure in the region of viscous flow were compared with experimental results for the isothermal sintering of two phase systems WC-Co, Cr3C2-Ni, Fe-Cu and TiC-NiCoCr. The results agree satisfactorily.