Intercrystalline Micropores Research Articles

Pore structure characterization and reservoir quality prediction in deep and ultra-deep tight sandstones by integrating image and NMR logs

Pore structure and fracture determine reservoir quality in deep and ultra-deep tight sandstones, however, the limited availability of core data makes it challenging to describe the complexity of pore-throat structure and characterize the wide range of fractures. To fill this gap, a comprehensive analysis was conducted to evaluate pore structure and fractures from multiple perspectives. Integration of core, thin sections, scanning electron microscopy (SEM), high pressure mercury injection (HPMI) and nuclear magnetic resonance (NMR) tests are used to evaluate pore structure and characterize fracture of deep and ultra-deep tight sandstones in the Cretaceous Bashijiqike Formation in the Kuqa Depression. Pore structure and reservoir quality types can be divided into dominantly three types according to characteristics of NMR transverse relaxation time (T2) distribution and HPMI curves. Correlation analysis among HPMI and NMR parameters with petrophysical parameters indicate that maximum pore throat size (rmax), logarithmic mean of T2 (T2gm), irreducible water saturation (Swi), and reservoir quality index (RQI) are the most sensitive parameters for pore structure characterization. The Type Ⅰ pore structure is composed of intergranular and intragranular pores, and NMR T2 spectrum shows the highest magnitude, with the calculated mobile fluid saturation content also being the highest. The Type II and III pore structures are composed of intergranular pores, intragranular dissolved pores and intercrystalline micropores in clay minerals. T2gm of Type II and III are smaller, with the calculated mobile fluid saturation content being the lower.The fracture features and parameters including fracture density, porosity, aperture, and length are analyzed using image logs and core. The pore structure characteristics are quantificationally and qualitatively analyzed by NMR logs. Integration of pore structure and fractures by image logs and NMR logs are used for prediction of high quality reservoir and hydrocarbon productivity. High hydrocarbon productivity is obtained in well intervals with abundant natural fractures, and favorable pore structure will also contribute to high matrix porosity. Therefore, NMR logs and image logs are crucial for evaluating reservoir quality and predicting productivity when calibrated with core and analysis data. The comprehensive analysis integrating core data with geophysical well logs provide important insights for reservoir quality prediction of deep and ultra-deep tight sandstones.

Reactive transport modeling constraints on the complex genesis of a lacustrine dolomite reservoir: A case from the Eocene Qaidam Basin, China

Reactive transport modeling (RTM) is an emerging method used to address geological issues in diagenesis research. However, the extrapolation of RTM results to practical reservoir prediction is not sufficiently understood. This paper presents a case study of the Eocene Qaidam Basin that combines RTM results with petrological and mineralogical evidence. The results show that the Eocene Xiaganchaigou Formation is characterized by mixed siliciclastic-carbonate-evaporite sedimentation in a semiclosed saline lacustrine environment. Periodic evaporation and salinization processes during the syngenetic-penecontemporaneous stage gave rise to the replacive genesis of dolomites and the cyclic enrichment of dolomite in the middle-upper parts of the meter-scale depositional sequences. The successive change in mineral paragenesis from terrigenous clastics to carbonates to evaporites was reconstructed using RTM simulations. Parametric uncertainty analyses further suggest that the evaporation intensity (brine salinity) and particle size of sediments (reactive surface area) were important rate-determining factors in the dolomitization, as shown by the relatively higher reaction rates under conditions of higher brine salinity and fine-grained sediments. Combining the simulation results with measured mineralogical and reservoir physical property data indicates that the preservation of original intergranular pores and the generation of porosity via replacive dolomitization were the major formation mechanisms of the distinctive lacustrine dolomite reservoirs (widespread submicron intercrystalline micropores) in the Eocene Qaidam Basin. The results confirm that RTM can be effectively used in geological studies, can provide a better general understanding of the dolomitizing fluid-rock interactions, and can shed light on the spatiotemporal evolution of mineralogy and porosity during dolomitization and the formation of lacustrine dolomite reservoirs.

Evolution characteristics and exploration targets of Permian clastic rock reservoirs in Bohai Bay Basin, East China

Based on core observation, thin section examination, fluid inclusions analysis, carbon and oxygen isotopic composition analysis, and other approaches, the structural and burial evolution histories were investigated, and the diagenetic evolution process and genetic/development models were systematically discussed of the Upper Paleozoic Permian clastic rock reservoirs in the Bohai Bay Basin, East China. The Bohai Bay Basin underwent three stages of burial and two stages of uplifting in the Upper Paleozoic. Consequently, three stages of acid dissolution generated by the thermal evolution of kerogen, and two stages of meteoric freshwater leaching occurred. Dissolution in deeply buried, nearly closed diagenetic system was associated with the precipitation of authigenic clay and quartz, leading to a limited increase in storage space. Different structural uplifting–subsidence processes of tectonic zones resulted in varying diagenetic–reservoir-forming processes of the Permian clastic reservoirs. Three genetic models of reservoirs are recognized. The Model I reservoirs with pores formed in shallow strata and buried in shallow to medium strata underwent two stages of exposure to long-term open environment and two stages of meteoric freshwater leaching to enhance pores near the surface, and were shallowly buried in the late stage, exhibiting the dominance of secondary pores and the best physical properties. The Model II reservoirs with pores formed in shallow strata and preserved due to modification after deep burial experienced an early exposure-open to late burial-closed environment, where pore types were modified due to dissolution, exhibiting the dominance of numerous secondary solution pores in feldspar and the physical properties inferior to Model I. The Model III reservoirs with pores formed after being regulated after multiple periods of burial and dissolution experienced a dissolution of acidic fluids of organic origin under a near-closed to closed environment, exhibiting the dominance of intercrystalline micropores in kaolinite and the poorest physical properties. The target reservoirs lied in the waterflood area in the geological period of meteoric freshwater leaching, and are now the Model II deep reservoirs in the slope zone–depression zone. They are determined as favorable options for subsequent exploration.

Characteristics and Controlling Factors of Crystalline Dolostone Reservoirs of the Sanshanzi Formation, Central-Eastern Ordos Basin, Northwestern China

In recent years, Ordos Basin has been the largest petroleum field in China and represents significant potential for gas exploration in the Lower Paleozoic carbonate reservoirs. Taking the central-eastern Ordos Basin as an example, this work studies the reservoir characteristics, origin, and controlling factors of the crystalline dolomites of the Sanshanzi Formation by means of petrological and geochemical studies. The Sanshanzi Formation dolostones comprise three kinds: crystalline dolostone with grain-shadow (Rd1), very fine–fine crystalline dolostone (Rd2), and medium-coarse crystalline dolostone (Rd3). The pore spaces include intercrystalline pores and intercrystalline solution pores, residual intergranular pores, vugs, and fractures. The medium-coarse crystalline dolostone is the best reservoir. The average porosity is 4.61%, and the average permeability is 0.91 mD. The sedimentary environment in the upper Cambrian was a large area of tidal flats. Under the influence of penecontemporaneous meteoric water leaching, dissolution developed at the top of the dolomite flat–lagoon depositional cycle, with vugs developing in the very fine–fine crystalline dolostone. Penecontemporaneous dolomitization enhanced the compaction resistance and effectively preserved the pore space of the reservoir. In addition, recrystallization improved the crystalline texture, turning the intergranular pores into residual intergranular pores and intercrystalline micropores into intercrystalline pores. In this case, the very fine–fine crystalline dolostone was transformed into medium-coarse crystalline dolostone. The results of this study provide a case for the in-depth understanding of the development and preservation mechanism of ancient dolomite reservoirs and also point to the research pathways for oil and gas exploration in the Cambrian in the Ordos Basin in the future.

Full Scale of Pore-Throat Size Distribution and Its Control on Petrophysical Properties of the Shanxi Formation Tight Sandstone Reservoir in the North Ordos Basin, China

Abstract Pore-throat size distribution is a key factor controlling the storage capacity and percolation potential of the tight sandstone reservoirs. However, the complexity and strong heterogeneity make it difficult to investigate the pore structure of tight sandstone reservoirs by using conventional methods. In this study, integrated methods of casting thin section, scanning electron microscopy, high-pressure mercury intrusion (HPMI), and constant-pressure mercury intrusion (CPMI) were conducted to study the pore-throat size distribution and its effect on petrophysical properties of the Shanxi Formation tight sandstones in the northern Ordos Basin (China). Results show that pore types of the Shanxi tight sandstone reservoirs include intergranular pores, dissolution pores, intercrystalline micropores, and microfracture, while the throats are dominated by sheet-like and tube-shaped throats. The HPMI-derived pore-throat size ranges from 0.006 to 10 μm, and the pore-throats with a radius larger than 10 μm were less frequent. The pore body size obtained from CPMI shows similar characteristics with radii ranging from 100 to 525 μm, while the throat size varies greatly with radii ranging from 0.5 to 11.5 µm, resulting in a wide range of pore-throat radius ratio. The full range of pore size distribution curves obtained from the combination of HPMI and CPMI displays multimodal with radii ranging from 0.006 to 525 µm. Permeability of the tight sandstone reservoirs is primarily controlled by relatively larger pore throats with small proportions, and the permeability decreases as the proportions of smaller pore-throats increase. The pervading nanopores in the tight gas sandstone reservoirs contribute little to the permeability but play an important role in the reservoir storage capacity. A new empirical equation obtained by multiple regression indicates that r15 (pore-throat size corresponding to 15% mercury saturation) is the best permeability estimator for tight gas sandstone reservoirs, which yields the highest correlation coefficient of 0.9629 with permeability and porosity.

Pore system characterization in diagenetically complex Mississippian-aged carbonate reservoirs (Kansas, USA)

Advanced petrophysical methods and detailed statistical analysis help to quantitatively and qualitatively characterize pore structure variations in carbonate rocks. Using a core from the Mississippian limestone play in south-central Kansas, we have investigated geologic constraints on reservoir properties using observations of the variability in pore architecture and measured petrophysical properties, i.e., porosity, permeability, and nuclear magnetic resonance (NMR) response. The sample set includes facies that retain their original depositional texture and rocks that have been subject to dolomitization, silicification, and dissolution to varying extents. The dominant macropore types include intercrystalline, moldic, and dissolution-enhanced vuggy porosity, whereas the dominant microporosity types are micritic and dolomitic intercrystalline micropores and nano-intercrystalline within silicified samples. The porosity-permeability relationships and [Formula: see text] modal distribution of the carbonates that we investigated correlate with dominant pore types depending on the extent and type of diagenetic modification. The fractal nature of pore systems in dolograinstones is not apparent in partially dolomitized samples, and pore complexity significantly decreases with increasing dolomitization. [Formula: see text] and bound fluid volume estimates indicate that independent of geologic origin, small and intricate pore systems with microporosity are likely to host higher amounts of capillary-bound fluids. This relationship holds, especially among samples with nano-intercrystalline porosity. Correlation coefficients from predicting NMR-derived parameters with porosity improved from 0.1 to 0.78 by including pore architecture data and pore types in the multiple linear regression model. The results and observations presented here improve the current understanding and predictability of petrophysical parameters in carbonate rocks with complex pore systems.

Classification and Evaluation of Volcanic Rock Reservoirs Based on the Constraints of Energy Storage Coefficient

The current classification and evaluation methods of volcanic rock reservoirs have low accuracy and cannot effectively provide guidance for the selection of volcanic rock gas reservoirs, which have efficient properties for gas production. In this research, we have analyzed the lithology, lithofacies, reservoir space type, pore combination mode, and reservoir microscopic characteristics of volcanic reservoirs using the energy storage coefficient as a constraint. Then, the method of reservoir classification was proposed. The results showed the following: 1) The energy storage coefficient can better characterize the single-layer productivity of gas wells. The volcanic rock reservoirs in the Wangfu gas field can be subdivided into three categories by considering the energy storage coefficient. 2) Type I reservoirs mainly develop structural fractures–matrix dissolution pores, structural fractures–intercrystalline micropores, and matrix dissolution pores. Type II reservoirs mainly develop matrix dissolution pores and residual intergranular pores, and Type III reservoirs are dominated by structural fractures. From Type I to Type III reservoirs, the skewness of the mercury intrusion curve and the sorting coefficient deteriorated, and the physical properties of the reservoir and the maximum mercury saturation and other parameters also decreased, whereas the displacement pressure and the median saturation pressure increased. 3) The characteristics of conventional and special logging curves of typical reservoirs were comprehensively analyzed, and the combination of sensitivity parameters reflecting gas-bearing properties and logging curves was optimized. Furthermore, a reservoir classification chart was established, and the results enabled to confirm the choice of the reservoir and demonstrated that the standard classification has high accuracy. 4) The diagenesis processes such as weathering, leaching, and dissolution improved the physical properties of the reservoir. The research results can effectively provide guidance for the evaluation of the “sweet spot” of volcanic rock reservoirs.

Effect of authigenic clay minerals and carbonate cements on quality of tight sandstone reservoirs: Insight from Triassic tight sandstones in the Huaqing area, Ordos Basin, Northern China

• Pore and throat structure can be characterized separately by XCT scanning. • Chlorite and illite produced most of the intercrystalline pores in tight sandstone . • Chlorite coatings are effective in preserving the large primary pore-throat. • The densification was completed by cementation instead of compaction. • Tight sandstone with high chlorite is the best quality reservoir in the Ordos Basin. As an important part of tight sandstone, authigenic clay minerals and carbonate cements have a strong influence on the densify of tight sandstone, but the details of these effects on pores and throats need be further clarified. The Ordos Basin is the second-largest sedimentary basin in China with the highest annual oil output and is characterized by tight sandstone. In this paper, the thin section, HPMI, XCT scanning and fluid inclusion homogenization temperature were combined to study the densification process of the tight sandstone in the Ordos Basin. The results show that early chlorite coating can effectively reduce further damage to the pore-throat space. But the pore-lining chlorite and illite led to reduce of the radius of intercrystalline micropores to <1 μm. After mechanical compaction, authigenic illite and the late ferrocalcite and ferrodolomite cements continued to grow and eventually complete the densification. Authigenic clay minerals and carbonate cements played contrasting role on evolving of pores and throats: tight sandstone with high chlorite showed obvious “right peak” PSD and TSD in XCT scanning with the largest number of throats, formed the best-quality reservoir; tight sandstone with high carbonate cement suggesting flat “right peak” TSD and increased the average length of throats; tight sandstone with high illite showed flat and “left shift” PSD and obvious “bimodal” TSD with the fewest number of throats, and formed the worst-quality reservoir.

The Characteristics and Main Controlling Factors of the Upper Paleozoic Sandstone Reservoirs in Dongpu Depression, Bohai Bay Basin

Based on the existing core and logging data, as well as previous research achievements, various experimental data were obtained by means of thin section, casting thin section, scanning electron microscopy, X-ray diffraction analyses, and cathodoluminescence. These data were used for the purposes of examining the features, diagenetic characteristics, and diagenetic evolution sequences of the Upper Paleozoic reservoirs of the Dongpu Depression, and analyzing the main controlling factors. The research results showed that the reservoir lithology was mainly subarkose, sublitharenite, feldspathic litharenite, and lithic arkose. The reservoir was generally characterized by ultra-low porosity and ultra-low permeability, with low porosity and low permeability in some areas. The pore types mainly include primary intergranular residual pores, dissolution pores, intercrystalline micropores, and microfractures. The pore throats are small and the throat channels are poorly sorted. The diagenetic processes mainly include compaction, pressure dissolution, cementation, metasomatism, and dissolution features of the research area. The diagenetic stages were determined to be mainly the middle diagenetic stages (Stage A and Stage B). The delta distributary channel and distributary bay of the Upper and Lower Shihezi Formation, along with the river channel sub-facies of the Shiqianfeng Formation, were considered to be the strata with good reservoir conditions. Moreover, the sand grains are dominated by medium-to-good sorting properties, sub-roundness and sub-angular, which provided a good spatial basis for the development of the primary pores. The calcite and kaolinite in the interstitial materials were corroded, which played a constructive role in the generation of the secondary pores. The results of the compaction, pressure dissolution, and cementation analyses indicate destructive diagenesis, which potentially reduced the porosity and permeability values. However, the dissolution, metasomatism, and chlorite lining cementation indicate improved diagenesis, which potentially increased the porosity and permeability values.

The rock-fabric/petrophysical characteristics and classification of the micropores hosted between the calcite and dolomite crystals

For the substantial hydrocarbon potential and academic significance, focus of interest has been directed towards the microporous carbonate. Compared with the documented shallow burial microporous carbonate, carbonate with a depth deeper than 6000 m present, of course, different characteristics due to the long-term influences of the strong cementation and compaction. With the deepening of exploration, it is therefore essential to improve the understanding about the deep burial microporous carbonate. In this paper, an integrated petrographic and petrophysical approach, which combines thin section, high-resolution field emission scanning electron microscopy (FESEM) observations, and nuclear magnetic resonance (NMR) measurements, is chosen to study the deep to ultradeep carbonate in the Shunnan area, Tarim Basin, NW China.Results show that all the studied carbonate samples consist entirely of micropores with a radius less than 10 μm, belonging to the strictly microporous carbonate. These micropores exist between the calcite and dolomite microcrystals. To reliably calculate the permeability based on NMR data, a newly introduced factor P aiming to adjust the T2 cut-off value derived from the relaxation curve is applied to enhance the Coates model. Values of P are found to correlate well with the FFI (free fluid index) of the bioclastic and bioclasts-free detrital limestones through different quantitative relationships, respectively. Permeabilities calculated from the modified Coates model are in good agreements with those measured by the pulse-decay method (R2 = 0.93, 0.67). Considering the existing classifications of pore systems are not applicable to the peculiar intercrystalline micropores within the deep burial microporous carbonate, a noval classification scheme is thus explored by integrating the rock-fabric and petrophysical properties. Micropores hosted within the subhedral to anhedral microcrystals with distinct boundaries are the main reserving space of the microbial lime-/dolostone, which lead to relatively higher porosities (averaging 3.41%) than the detrital limestone (1.99%) and dolostone (2.47%), and an identical right-skewed trimodal NMR T2 distribution. Micropores related with the coalescent polyhedral to anhedral microcrystals, that are always glued together to form fusing boundaries, are mainly located within the micrite matrix and allochem of the detrital limestone. Certain relationships among porosity, permeability and FFI/BVI also exist for the bioclastic and bioclasts-free detrital limestones, respectively. Micropores within the dolostone exist between the rhombic dolomite crystals. This proposed classification of micropores comprehensively reflects the depositional, diagenetic textures and physical properties of the deep burial microporous carbonate. The particularity of microbial lime-/dolostone is highlighted.

Occurrence, composition, and origin of analcime in sedimentary rocks of non-marine petroliferous basins in China

Abstract Natural analcime, a highly brittle aluminosilicate mineral that dissolves easily during diagenesis, occurs in the sedimentary rocks of eight main non-marine petroliferous basins in China. This mineral has been well documented for the past 30 years and has proved to be significant for the formation of high-quality hydrocarbon reservoirs. The present study fully reviews analcime occurrences, chemical compositions, Si/Al ratios, origin and controlling factors, and influence on the hydrocarbon reservoir quality in Permian to Paleogene sedimentary rocks. Previous research has shown that analcime is commonly distributed within argillaceous laminae (l-type); occurs as intergranular cement (c-type), replacement (r-type), and crack filling (f-type) forms; or is closely intergrown with other authigenic/exhalative minerals (a-type). Analcime in lacustrine basins is slightly Al-rich and Na-poor, and its Si/Al ratios range from 1.95 to 2.80. The geochemical compositions of the analcime- and ankerite-bearing exhalites are characterised by enrichment in light rare earth elements and 13C and depletion in heavy rare earth elements and 18O, with negative Eu and positive Ce anomalies. Diagenetic analcime forms by the alteration of volcanic glass, clays, or zeolite precursors in saline-alkaline lakes or during buried diagenetic stages in sedimentary rocks. In addition, it coexists with other authigenic zeolite, carbonate, and clay minerals as well as albite. Hydrothermal analcime presumably precipitates from hydrothermal fluids and is not dependent on a local precursor mineral or volcanic glass; this type is commonly related to exhalative mineralisation of hydrothermal springs. The formation of natural analcime is controlled by various factors such as the composition of the parent material, temperature prevalent during crystallisation, chemistry of the fluid phase, and thermodynamic characteristics of the geological environment. As zeolites are formed in alkaline fluid phases, they become unstable in acidic pore water. Secondary pores created by analcime mineral dissolution, and intercrystalline micropores among analcime minerals, are highly significant in hydrocarbon reservoirs. Analcime-bearing mudstone, siltstone, dolostone, and exhalite are favourable lithofacies for unconventional hydrocarbon accumulation owing to its high brittleness and selective dissolution of analcime and other associated minerals.

Characterization of the pore-throat size of tight oil reservoirs and its control on reservoir physical properties: A case study of the Triassic tight sandstone of the sediment gravity flow in the Ordos Basin, China

Tight oil is a significant component of unconventional oil and gas resources. The pore structures of tight oil reservoirs determine the seepage law, which is a significant factor in evaluating reservoir quality. However, because of the strong heterogeneity of the pore structure in tight oil reservoirs, it is difficult to characterize the pore structure by conventional methods. Therefore, on the basis of conventional reservoirs research methods, combined with high-pressure mercury intrusion (HPMI) and constant-pressure mercury intrusion (CPMI), the pore-throat size distribution of tight sandstone reservoirs and its control on reservoir physical properties were studied. The main pore types in the study area were residual intergranular pores, secondary intragranular pores, intercrystalline micropores and microfractures, and throat types were sheet-like and tube throats. The pore-throat size distribution characterized by HPMI ranges from 0.0025 to 2.79 μm, and pore-throats larger than 10 μm occurred less frequently. The pore size distribution characterized by CPMI was between 90 and 350 μm, and the peak value distribution of the pore radius of the different samples was relatively concentrated, while the throat size distribution was between 0.12 and 6 μm, indicating that the throat radius of the different samples showed a strong heterogeneity, which also led to a strong heterogeneity of pore-throat radius ratio. Although HPMI and CPMI had their own limitations in characterizing the pore-throat size distribution, their combination could effectively characterize the whole pore-throat size distribution of tight reservoirs, which ranged from 0.0025 to 350 μm. The permeability of the tight sandstone reservoirs is primarily determined by the relatively wider pore-throats with small proportions, and the permeability decreased as the proportion of narrower pore-throats increased. The nanoscale pore throats (radius < 0.1 μm) were widely developed in the tight reservoirs, which had little influence on permeability, but these pore throats could be used as oil and gas storage spaces and played an important role in improving reservoir porosity.

Pore size distribution and reservoir characterization: evaluation for the Eocene beach-bar sequence, Dongying Depression, China

Pore size distribution and associated heterogeneities of thinly bedded beach-bar sandstone reservoir from upper fourth member of Eocene Shahejie Formation (Es4s) are characterized with the help of mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) transversal relaxation time (T2) data. Permeability and porosity are two represented important characteristics of rocks that control the movement and storage of fluids. The aim of this paper is to establish relationships between pore throat sizes and reservoir quality. The results derived from thin-section petrography, scanning electron microscopy (SEM), MICP, NMR T2 relaxation time, and 3D micro-CT (μ-CT) are compared to characterize pore space dimensions and types comprehensively. Average pore throat size from MICP ranges between 0.47 μm and 2.83 μm while maximum pore throat size ranges between 2.48 μm and 7.36 μm. The combination of pore size distribution obtained from MICP and NMR seems appropriate to cover the range of pore size from beach-bar sand and overcome the individual method limits. Afterwards, digital 3D μ-CT tomographic images are used to characterize and visualize pore space and pore network model to compare with the experimental data. MICP and NMR experiment showed generally bimodal (meso and micro) pore size distribution. Usually mesopore corresponding to intergranular pores is dominant, while heavily cemented sandstones show large amounts of intercrystalline micropores. Complex and heterogeneous beach-bar sandstone reservoir requires comprehensive study program to evaluate the reservoir.

The diagenetic controls of the reservoir heterogeneity in the tight sand gas reservoirs of the Zizhou Area in China's east Ordos Basin: Implications for reservoir quality predictions

Abstract The Shan 23 sandstone formations are important exploratory targets for tight gas resources in the Zizhou Area of the Ordos Basin. In this study, we investigated the effects of diagenesis on the heterogeneity of tight sandstone deposits in similar sedimentary facies and identified the main controlling factors of reservoir quality. A total of 169 samples were collected from 50 wells for a series of analyses. The heterogeneity of the sandstones is mainly controlled by cement and pore-throat characteristics, which showed regional differences in the study area consistent with the regional variations in the detrital components. These results indicate that the diagenesis influenced by the sediment composition is the main controlling factor of the reservoir heterogeneity. The compaction dominated by ductile grains is the major cause of the primary porosity losses in the sandstones, followed by the quartz and clay (kaolinite and illite) cementation. Residual intergranular pores rarely occur in the sandstones with a ductile lithic grain content greater than 2%. Altered kaolinite and recrystallized illite commonly occur in the sandstones with high ductile grain content. Residual intergranular pores, quartz overgrowths, and authigenic kaolinite are the most common phases in the sandstones rich in detrital quartz. The dissolution formed secondary pores and provided a minor contribution to reservoirs permeability. Secondary pores predominantly occur in the quartz arenite due to the preservation of the primary pores. The main controlling factor for the quality of the sandstones is the pore type, which has pronounced effects on both the pore structure and the pore-throat size distribution. The residual intergranular pores preserved in sandstones will lead to larger radii and better sorting characteristics of the pore-throats, which subsequently contribute to the higher permeability. Therefore, the sandstones with the higher content of primary intergranular pores are considered high-quality reservoirs. They are characterized by quartz richness and mainly located in the Zizhou-Mizhi Area. The inter-crystalline micropores occur in the clay mineral, resulting in small-sized and poor-sorted pore-throats. They give subordinate contributions to the reservoir quality and are the key factor of the extremely low permeability in sandstones. The dry layers of the study area are mainly composed of sandstones dominated by inter-crystalline micropores. These sandstones are characterized by the high content of ductile grains and are mainly located in the Qingjian Area.

Pore‐Throat Combination Types and Gas‐Water Relative Permeability Responses of Tight Gas Sandstone Reservoirs in the Zizhou Area of East Ordos Basin, China

With the aim of better understanding the tight gas reservoirs in the Zizhou area of east Ordos Basin, a total of 222 samples were collected from 50 wells for a series of experiments. In this study, three pore‐throat combination types in sandstones were revealed and confirmed to play a controlling role in the distribution of throat size and the characteristics of gas‐water relative permeability. The type‐I sandstones are dominated by intercrystalline micropores connected by cluster throats, of which the distribution curves of throat size are narrow and have a strong single peak (peak ratio &gt;30%). The pores in the type‐II sandstones dominantly consist of secondary dissolution pores and intercrystalline micropores, and throats mainly occur as slice‐shaped throats along cleavages between rigid grain margins and cluster throats in clay cement. The distribution curves of throat size for the type‐II sandstones show a bimodal distribution with a substantial low‐value region between the peaks (peak ratio &lt;15%). Primary intergranular pores and secondary intergranular pores are mainly found in type‐III samples, which are connected by various throats. The throat size distribution curves of type‐III sandstones show a nearly normal distribution with low kurtosis (peak ratio &lt;10%), and the micro‐scale throat radii (&gt;0.5 (μm) constitute a large proportion. From type‐I to type‐III sandstones, the irreducible water saturation (Swo) decreased; furthermore, the slope of the curves of Krw/Krg in two‐phase saturation zone decreased and the two‐phase saturation zone increased, indicating that the gas relative flow ability increased. Variations of the permeability exist in sandstones with different pore‐throat combination types, which indicate the type‐III sandstones are better reservoirs, followed by type‐II sandstones and type‐I sandstones. As an important factor affecting the reservoir quality, the pore‐throat combination type in sandstones is the cumulative expression of lithology and diagenetic modifications with strong heterogeneity.

Reservoir properties and controlling factors of contact metamorphic zones of the diabase in the northern slope of the Gaoyou Sag, Subei Basin, eastern China

Hydrocarbons reserved in the contact metamorphic zones of magmatic rocks are found in rift basins as a new type of unconventional resources. However, their reservoir characteristics are relatively underexplored. On the basis of core samples, thin sections, and analysis data available, this study investigates the reservoir properties of contact metamorphic zones of the diabase from the northern slope of the Gaoyou Sag, Subei Basin in eastern China. The factors that control the reservoir quality are also investigated. In the studied area, diabase intrusion exerts significant influences on contact sedimentary rocks (including contact mudstones and sandstones) as hydrocarbon reservoirs. After contact metamorphism, the impermeable mudstones are altered into permeable metamudstones, including intermediate-metamorphosed hornfelses and low degree-metamorphosed slates. Various types of reservoir spaces, including structural fractures, hydrothermal microfissures, contraction microfissures, cleavage microfissures, intercrystalline micropores, and dissolved micropores, are created in metamudstones. Among them, numerous fractures and microfissures are the main factors of reservoir quality enhancement. Contact sandstones are altered into metasandstones, whose reservoir space types are similar to those of unaffected sandstones. The reservoir spaces mainly include primary intergranular pores, dissolved intergranular pores, dissolved intragranular pores, moldic pores, ultralarge pores, and dissolved micropores.The formation of reservoir spaces in contact metamudstones is mainly controlled by diabase intrusion, and it is not influenced by burial compaction. The controlling mechanisms of this formation include physical fracturing, hydrothermal activity, dissolution, and cooling contraction. The reservoir quality of contact metasandstones is primarily controlled by lithofacies and diagenesis before intrusion. However, later-stage diabase intrusion also exerts considerable control on reservoir quality by causing physical pressure, dissolution, and cementation. Although dissolution increases porosity moderately, it insufficiently compensates for the porosity reduction caused by physical pressure and cementation. Thus, the reservoir quality of contact metasandstones is generally decreased by diabase intrusion. A comprehensive reservoir model of contact metamorphic rocks is proposed based on the preceding analysis. This model serves to predict the distribution of high-quality reservoir in contact metamorphic rocks in the studied area. It can also be applicable to other formations of similar intrusive background.

Pore structure characterization of supported polycrystalline zeolite membranes by positron annihilation spectroscopy

It is important to characterize the pore structure of supported polycrystalline zeolite membranes but it has remained a major challenge in studying microporous inorganic membranes. This paper reports the use of positron annihilation spectroscopy including positron annihilation lifetime spectroscopy and Doppler broadening energy spectroscopy to non-destructively characterize the pore structure of four MFI zeolite membranes of different microstructures on alumina supports. Positron annihilation lifetime spectroscopy analysis reveals a bimodal pore structure consisting of intracrystalline zeolitic micropores of around 0.6nm in diameter and irregular intercrystalline micropores of 1.4−1.8nm in size for the four MFI zeolite membranes studied. Distributions of the micropores along the membrane thickness direction can be inferred from Doppler broadening energy spectroscopy results, illustrating development of intercrystalline gaps during the growth of the zeolite layer. The amount and size of the intercrystalline micropores of the zeolite membranes vary with the synthesis method, and are the smallest for the randomly oriented MFI zeolite membrane synthesized without template and the largest for the c-oriented MFI zeolite membrane synthesized with the template. The c-oriented membrane has an asymmetrical distribution of intercrystalline pores along the film growth direction as compared to the uniform distribution of the bimodal structure for the other three membranes. The pore structure data obtained by positron annihilation spectroscopy are consistent with the xylene isomer separation performance of these membranes.

Complex relationship between porosity and permeability of carbonate reservoirs and its controlling factors: A case study of platform facies in Pre-Caspian Basin

Abstract Based on a large amount of core analysis data in eastern Pre-Caspian Basin, the relationship between permeability and porosity and its influencing factors are studied. The sedimentary environments of the Carboniferous System in eastern Pre-Caspian Basin include open platform, restricted platform and evaporate platform. For dolomite reservoirs there are three main combination patterns of pores, namely, inter-crystalline and solution pores, inter-crystalline micro-pores, inter-crystalline and intra-crystalline pores, among which the first combination are highest in porosity and permeability. For limestone reservoirs, the main combination patterns of pores are inter-particle pores, inter-particle and intra-particle pores and moldic pores. Lacking connecting pore throats, moldic pores are poor in permeability. In the dolomite reservoirs, fractures and vugs are well-developed and increase the permeability significantly. Under similar porosity, the permeability is obviously determined by the size of pore throat and the percentage of middle to large size throats in different types of reservoirs. In limestone reservoirs, fractures are principally micro-fractures, the percentage of connecting pores to total pores is positively correlated with permeability; while in dolomite reservoirs, there is not such an obvious relationship between the percentage of connected pores to total pores and permeability. Sedimentary environment, controlling the distribution features of reservoir types, is the cause of the complex relationship between permeability and porosity.

Investigation of the effects of ion and water interaction on structure and chemistry of silicalite MFI type zeolite for its potential use as a seawater desalination membrane

In this work we study the structural response of silicalite based MFI type zeolite powder to the presence of water and cations commonly found in sea salts including Na+, K+, Ca2+, and Mg2+. MFI zeolite powder was exposed to different solutions (3.8 wt% TDS (total dissolved solids) seawater (SW), or 1 M NaCl, or 0.05 M KCl), and their effects on the structure of the zeolite powder were characterized. The results obtained from ion adsorption testing and synchrotron X-ray powder diffraction showed that the zeolite powder interacted with water, monovalent and divalent cations, and these interactions altered the crystal dimensions of the zeolite. Most exposures to ions and water led to increases in crystal volumes of <0.1%, but exposure to KCl solution was observed to make the biggest contribution to crystal expansion, at 0.59%, particularly in the a- and c-directions as determined by Rietveld refinement. Upon closer inspection of SW exposed silicalite, a rapid exchange of Na+ and K+ was observed within 1 h, but after 8 h, these monovalent cations were gradually exchanged with divalent ions. Ca2+ was especially observed to adsorb into the zeolite at 8 h. N2 adsorption and positron annihilation lifetime spectroscopy (PALS) measurements were made on SW exposed zeolites. PALS showed that the silicalite powder has mesopores (2.6 nm) and micropores (1.0 nm), which are both attributed to intercrystalline spaces. The intrinsic intracrystalline zeolite pore size was measured as 0.33 nm. Both N2 adsorption and PALS showed that the relative amount of micropores to total pores decreased with seawater exposure which was attributed to ion adsorption and ion exchange. The mesopore size and number increase due to exposure, and although mesopores represent defects in crystalline zeolites, their expansion on exposure to seawater is important for membrane stability and diffusion properties and warrants further investigation. The intrinsic zeolite pore size was smaller in zeolites exposed to seawater (∼0.29 nm), and the changes observed by PALS lead us to conclude that most of the ion activity observed is attributed to the intercrystalline micropores and mesopores. No other changes were observable by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) due to SW exposure. Results indicate that the interaction between silicalite and the water and ions in SW causes a change in structure and porosity expected to affect diffusion properties of these materials when used as membranes for desalination.

Gas permeation through DDR‐type zeolite membranes at high temperatures

AbstractDDR‐type zeolite membranes were prepared by the secondary growth method on porous α‐alumina disk, followed by on‐stream counter diffusion chemical vapor deposition modification to eliminate the intercrystalline micropores. Single gas permeation of He, H2, CO2, and CO through this zeolite membrane before and after CVD modification was measured in 25–500°C. Intracrystalline diffusivities for these four gases in DDR‐type zeolite were obtained from the permeation data above 300°C to examine the effects of the size and molecular weight of permeating gases on diffusion and permeation rate for this zeolite membrane. For the unmodified DDR‐type zeolite membrane with presence of a small amount intercrystalline micropores the diffusivity (or permeance) with a low activation energy depends on both the size and molecular weight of permeating gases. For the CVD‐modified DDR‐type zeolite membrane with intercrystalline micropores eliminated, the activation energy for diffusion and diffusivity increases with increasing molecular size of the permeating gases. © 2008 American Institute of Chemical Engineers AIChE J, 2008.