Pore Structure Of Tight Sandstone Research Articles

Influence of coupled dissolution-precipitation processes on the pore structure, characteristics, and evolution of tight sandstone: A case study in the upper Paleozoic reservoir of Bohai Bay Basin, eastern China

Due to the limitations of pore throat characterization techniques, in-depth studies on the evolution and influencing variables of tight sandstone reservoirs remain lacking, despite the importance of pore structure in evaluating their storage and seepage capacities. In this study, the effect of coupled dissolution–precipitation on the heterogeneity of the pore structure of tight sandstone was investigated using advanced techniques in conjunction with traditional geochemical analyses. The primary reasons for the variability in tight sandstone pore structure were different diagenetic processes and tectonic evolution. In tight sandstones, numerous burial phases led to a significant loss of porosity. However, tight sandstones that have been successively leached by meteoric water and deep burial organic acid have improved secondary pore space due to Yanshan tectonic uplift and Himalayan tectonic burial. The redistribution or blockage of pore space can occur as a result of dissolved material migrating and re-precipitating in tight sandstones that have been deeply buried and altered by organic acidic fluids. Furthermore, nuclear magnetic resonance cryoporometry and nuclear magnetic resonance experiments were used to provide precise and accurate quantitative analysis for discussing the evolution of pore structure by complex dissolution–precipitation process within the pore scale. The formation of good pore structure was mainly dependent on the increase in pore volume generated by dissolution in the submicron and micron range (less than 10 μm). Due to interfacial energy effects, authigenic minerals precipitated preferentially in macropores, resulting in a preponderance of microporosity and small changes in porosity but at least an order of magnitude reduction in permeability. This study provides a reliable reference for genesis analysis and evaluation of tight sandstones as well as a new perspective for understanding the formation and evolution of complex pore structures.

Pore structure of tight sandstones with differing permeability: The He 8 Member of the Middle Permian Lower Shihezi Formation, Gaoqiao area, Ordos Basin

AbstractTight sandstone has strong pore heterogeneity and complex pore structure, and the pore structure of tight sandstone varies with different permeability. To study the differences in the pore structure of tight sandstone with different permeability, this study investigated the tight sandstone of the He 8 Member in the Gaoqiao area of the Ordos Basin. Factors influencing pore formation are analyzed through experiments utilizing methods, such as distinguishing casting thin sections, scanning electron microscopy, high‐pressure mercury intrusion, and low‐temperature nitrogen adsorption. The results indicate that in this area, the pores of the tight sandstone are primarily dissolution and intercrystalline pores, with occasional intergranular pores and microcracks. Type Ⅱ samples (permeability > 0.2 × 10−3 μm2) are primarily composed of dissolution and intercrystalline pores, with a few visible intergranular and microcracks. In contrast, Type Ⅰ samples (permeability < 0.2 × 10−3 μm2) mainly consist of micropores and intercrystalline pores, where microcracks are observed locally. Pore size research demonstrates that Type Ⅰ samples have pore size under 1000 nm, with peaks primarily at 4–5 nm. There are also peaks between 10–1000 nm, but without a consistent pattern. For Type Ⅱ samples with sizes smaller than 1000 nm, pore distribution is evident. Peak values when the pore size exceeds 1000 nm. Type Ⅰ samples have fewer micron‐level pores, while Type Ⅱ samples are primarily dissolution pores. The nanopores of Type Ⅰ samples consist of flat pores with good connectivity, whereas those of Type Ⅱ samples are mainly blind pores with poor connectivity. Type Ⅰ samples have larger pore fractal dimensions, more intricate pore morphology, and rougher and irregular pore throat surfaces compared to Type II samples. Hence, the sedimentary environment, diagenesis, and mineral composition affect the pore distribution in tight sandstone. The research findings highlight the variations in the pore structure of tight sandstone with varying permeability, providing crucial guidance for classifying and assessing pore structure in tight sandstone reservoirs.

Insights into the Effect of Micropore Structure and Mineralogy on Adsorbed Tight Oil in the Huaqing Area, Ordos Basin, China

There is great potential for tight oil as an alternative to conventional fossil fuels throughout the world. Adsorption, mineralogy, and pore structure of tight sandstone are very important factors for the efficient development of adsorbed tight oil from tight sandstone oil reservoirs. In this study, the occurrence morphology of tight oil adsorbed to varying pore structures was examined, as well as the relationship between adsorption content and occurrence morphology. X-ray energy dispersive spectra, environmental scanning electron microscope observations, and thin fluorescence sections were used to describe the occurrence morphology of adsorbed tight oil in tight sandstone reservoirs. It was found that adsorbed tight oil can be classified into five types: emulsion, cluster, throat, thin-film, and isolation, with average contents of 34.9%, 14.5%, 3.1%, 42.7%, and 4.7%, respectively. In addition, a Soxhlet extraction was developed with four pore structures to describe the geological effects of adsorbed tight oil in reservoirs. The adsorption content of tight oil ranges from 4.015 mg/g to 16.987 mg/g, with an average of 10.115 mg/g. The order of adsorption content for different occurrence morphologies is as follows: emulsion form > throat form > cluster form > isolation form > thin-film form. It is the size and connectivity of the porous throat that determines the desorption efficiency of tight oil. An important outcome of this study can be the introduction of rational development plans and the improvement of oil recovery efficiency for tight oil reservoirs.

Pore Structure and Fractal Characteristics of Tight Sandstone: A Case Study for Huagang Formation in the Xihu Sag, East China Sea Basin, China

Various experiments, including routine petrophysical measurements, thin section and scanning electronic microscope (SEM), high-pressure mercury intrusion (HPMI), and nuclear magnetic resonance (NMR), were performed to characterize the microscopic pore structure of tight sandstone in the Huagang Formation (E3h), Xihu Sag, East China Sea Basin, China. Specifically, NMR was used to investigate the dynamic variation of fractal dimensions during centrifugation, and the comparison of HPMI and NMR were used to clarify the difference of fractal dimensions. The results showed that there were four types of pores observed in thin section and SEM images: primary intergranular pores, intergranular dissolution pores, intragranular dissolution pores, and micropores within clay aggregates. The geometric shape and pore size of different pore types showed huge differences, indicating the formation of complex and diverse pore structures in the E3h formation. The flow capability of the reservoir was dominated by large pores, while the storage capacity was determined by small pores. The dynamic variation of fractal dimensions calculated by NMR data showed the water residing in the pore structure with low fractal dimensions was removed preferentially, and the pore structure of the resided water was always more complicated than the pore structure of removed water, which indicated the flow capability of the reservoir was affected by the complexity of the pore structure. Based on the comparison of the fractal dimension data from HPMI and NMR, it was found that the variation trends of the fractal dimensions were consistent, as the radius of the pore throat increased, the fractal dimensions increased, and the pore structure became more complicated. Both fractal dimensions of macropores (Dmac) and movable-fluid pores (Dmov) can reflect the flow capability of reservoir effectively, but the correlations between fractal dimensions from HMPI and NMR were poor, which could be due to the different working mechanism in these methods.

Effect of CO2-Brine-Rock Interactions on the Pore Structure of the Tight Sandstone during CO2 Flooding: A Case Study of Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, China.

CO2 flooding is an effective technique to enhance oil recovery from tight sandstone reservoirs. The CO2-rich fluid reacts with the in situ minerals and results in the corrosion and precipitation of minerals in the sandstone, affecting the connectivity and morphological features of pores. However, the diagenesis of the tight sandstone is complex, and the structural modification behavior and mechanism in different lithofacies of tight sandstone during CO2 flooding are still unclear. This study combined CO2 flooding, thin-section casting, scanning electron microscopy, nuclear magnetic resonance, X-ray diffraction, and fractal analysis methods to investigate the changes in the pore structure of tight sandstone after CO2 flooding. The results show that the cement of the tight sandstone is complex and diverse. The tight sandstone can be divided into two types of lithofacies: clay cementation (CL) and ferrocalcite cementation (CA). The mineralogical alterations occur differently in each lithofacies of tight sandstone. Alterations in the cement minerals affect the pores morphology of tight sandstone depending on their mineralogical structure and texture, and the CO2 flooding mainly changes the micromorphology and heterogeneity of large pores. Clay minerals dominate the cement in the CL lithofacies of tight sandstone. The dissolution mainly occurs in small pores because the precipitation of new minerals and exfoliation of skeleton particles partially block large pores and transform them into small pores. Thus, the number of small pores of CL lithofacies increases while the number of large pores decreases. Such a phenomenon hinders the increase in porosity and permeability. On the other side, in the CA lithofacies of tight sandstone, the cement is dominated by ferrocalcite, and the dissolution of ferrocalcite is the primary mechanism of mineralogical alteration. As the ferrocalcite dissolution expands, it creates new flow paths, improving the connectivity of pores. The petrophysical properties of CA lithofacies improve significantly after CO2 flooding. There is a crucial need to study the changes in diagenetic characteristics of tight sandstone due to CO2 flooding. Such type of study provides insights related to the improvement and evaluation of the development of tight sandstone reservoirs during CO2 flooding.

Digital characterization and fractal quantification of the pore structures of tight sandstone at multiple scales

Pore structures determine reservoir storage capacity, control rock transportation characteristics and represent microscopic properties of the rock. Therefore, the characterization and quantification of the pore structures of tight oil and gas reservoir are of primary importance for quality evaluation and the successful production of these unconventional resources. In this study, we obtained X-CT images with two resolutions of the same tight sandstone and studied the pore structures and heterogeneity of tight sandstone using digital rock technology combined with fractal theory. In traditional Euclidean space, digital image analysis shows that the pore structure of tight sandstone is mainly flat, isolated pores that occupy a large number ratio in high-resolution images and a large volume ratio in low-resolution images. Most seepage channels are mainly composed of large pores. The porosity analysis of 2D and 3D suggests that the representative elementary volume of low-resolution digital rock is 300 voxels, and the axial heterogeneity of tight sandstone is stronger than the bulk heterogeneity. In non-Euclidean space, fractal characterization parameters indicate that the fractal dimension (FD) of low-resolution digital rock is 2.6548, that of high-resolution digital rock is 2.6194, and the FD of tight sandstone is insensitive to imaging resolution. The lacunarity of high-resolution digital rock is obviously larger than that of low-resolution digital rock, which suggests that lacunarity can be used to analyze the heterogeneous structures with similar FD of tight sandstone precisely.

Seismic rock physics inversion with varying pore aspect ratio in tight sandstone reservoirs

Reservoir parameters, such as porosity, saturation, and clay volume, are directly related to the hydrocarbon-bearing properties of potential reservoirs, of which estimation is one of the ultimate goals of data processing for seismic exploration. However, the complex pore structure of tight sandstone presents difficulty in evaluating reservoir properties. This study proposes a novel reservoir parameter inversion method intended for tight sandstone reservoirs. The method builds the forward operator by combining the rock physics model and the seismic reflectivity equation, enabling the direct inversion of reservoir parameters from observed seismic data. In particular, the (sand- and clay-related) pore aspect ratios of rock frame are treated as internal variables, which are iteratively updated during the inversion process. The varying aspect ratios address the complex pore structure, which facilitates the improved accuracy in rock physics modeling of tight sandstone reservoirs. Besides, the Bayesian inversion constrained by the prior Gaussian mixture model considers the complex prior distributions of reservoir parameters in different lithofacies, which stabilizes the inversion process and achieves the optimal solution. The method is tested by the synthetic data which exhibits less uncertainty. The application to the field data from tight sandstone gas reservoirs in southwestern China demonstrates the method has the good capability of indicating the gas-bearing areas. • Seismic rock physics inversion method is proposed for tight sandstone reservoirs. • Reservoir property parameters can be directly inverted from seismic data. • Varying pore aspect ratio describes the complex pore structures in the inversion. • The estimated reservoir parameters can be good indicators for gas-bearing areas.

Research on the Characteristics of Microscopic Damage to Different Pore Types by Slippery Water Fracturing Fluid in the Chang Member Tight Sandstone Reservoir, Ordos Basin, NW China

This paper examines the damage caused by slickwater fracturing fluid to the microscopic pore structure of tight sandstone in the Chang 7 member in the Ordos Basin. A submicron CT in situ displacement system was used to analyze and graphically represent changes in pores in core samples following fracturing fluid damage. The results show the following: (1) the damage caused by slickwater fracturing fluid to tight sandstone fractures mainly occurs in the early stage of fluid incursion. The damage is characterized by a decrease in the effective pore volume, increase in the number of pores, and insignificant subsequent damage. The main causes of pore damage by slickwater fracturing fluid are retention of slickwater in the liquid phase and hydration swelling of clay minerals in the pores. (2) After the high pressure intrusion of slickwater fluid, the pore size of large-size intergranular pores increases, and there is no obvious damage after water flooding. However, fractures and small dissolution pores in the cores are the main areas of fluid retention after fracturing fluid invasion due to their small flowing radius and complex structure. These are the locations where damage mostly occurs.

Analysis of pore throat characteristics of tight sandstone reservoirs

Abstract The characterization of pore throat structure in tight reservoirs is the basis for the effective development of tight oil. In order to effectively characterize the pore -throat structure of tight sandstone in E Basin, China, this study used high-pressure mercury intrusion (HPMI) testing technology and thin section (TS) technology to jointly explore the characteristics of tight oil pore throat structure. The results of the TS test show that there are many types of pores in the tight sandstone, mainly the primary intergranular pores, dissolved pores, and microfractures. Based on the pore throat parameters obtained by HPMI experiments, the pore throat radius of tight sandstone is between 0.0035 and 2.6158 µm. There are two peaks in the pore throat distribution curve, indicating that the tight sandstone contains at least two types of pores. This is consistent with the results of the TS experiments. In addition, based on the fractal theory and obtained capillary pressure curve by HPMI experiments, the fractal characteristics of tight sandstone pore throat are quantitatively characterized. The results show that the tight sandstones in E Basin have piecewise fractal (multifractal) features. The segmentation fractal feature occurs at a pore throat radius of approximately 0.06 µm. Therefore, according to the fractal characteristics, the tight sandstone pore throat of the study block is divided into macropores (pore throat radius > 0.06 µm) and micropores (pore throat radius < 0.06 µm). The fractal dimension D L of the macropores is larger than the fractal dimension D S of the micropores, indicating that the surface of the macropores is rough and the pores are irregular. This study cannot only provide certain support for characterizing the size of tight oil pore throat, but also plays an inspiring role in understanding the tight pore structure of tight sandstone.

Investigation of the Pore Structure of Tight Sandstone Based on Multifractal Analysis from NMR Measurement: A Case from the Lower Permian Taiyuan Formation in the Southern North China Basin

Understanding the pore structure can help us acquire a deep insight into the fluid transport properties and storage capacity of tight sandstone reservoirs. In this work, a series of methods, including X-ray diffraction (XRD) analysis, casting thin sections, scanning electron microscope (SEM), nuclear magnetic resonance (NMR) experiment and multifractal theory were employed to investigate the pore structure and multifractal characteristics of tight sandstones from the Taiyuan Formation in the southern North China Basin. The relationships between petrophysical properties, pore structure, mineral compositions and NMR multifractal parameters were also discussed. Results show that the tight sandstones are characterized by complex and heterogenous pore structure, with apparent multifractal features. The main pore types include clay-dominated micropores and inter- and intragranular dissolution pores. Multifractal parameters of sandstone samples were acquired by NMR and applied to quantitatively describe the pore heterogeneity in higher and lower probability density regions (with respect to small and large pore-scale pore system, respectively). The multifractal parameter (D−10) of lower probability density areas has better correlation with the petrophysical parameters, which is more suitable for evaluating the reservoir properties of tight sandstone. However, the multifractal parameter (D10) of higher probability density areas is more conducive to characterize the pore structure of tight sandstone. Additionally, the mineral compositions of sandstone have a complex effect on multifractal characteristics of pores in different probability density areas. The D10 increases with the decrease of quartz content and increase in clay mineral content, whereas D−10 decreases with the increase in clay minerals and decrease of authigenic quartz content and feldspar content.

Reconstruction and Analysis of Tight Sandstone Digital Rock Combined with X-Ray CT Scanning and Multiple-Point Geostatistics Algorithm

Unconventional rocks such as tight sandstone and shale usually develop multiscale complex pore structures, with dimensions ranging from nanometers to millimeters, and the full range can be difficult to characterize for natural samples. In this paper, we developed a new hybrid digital rock construction approach to mimic the pore space of tight sandstone by combining X-ray CT scanning and multiple-point geostatistics algorithm (MPGA). First, a three-dimensional macropore digital rock describing the macroscopic pore structure of tight sandstone was constructed by micro-CT scanning. Then, high-resolution scanning electron microscopy (SEM) was performed on the tight sandstone sample, and the three-dimensional micropore digital rock was reconstructed by MPGA. Finally, the macropore digital rock and the micropore digital rock were superimposed into the full-pore digital rock. In addition, the nuclear magnetic resonance (NMR) response of digital rocks is simulated using a random walk method, and seepage simulation was performed by the lattice Boltzmann method (LBM). The results show that the full-pore digital rock has the same anisotropy and good connectivity as the actual rock. The porosity, NMR response, and permeability are in good agreement with the experimental values.

Study on seepage characteristics of tight sandstone reservoir

Seepage characteristics of reservoir rocks are important factors affecting the effect of tight oil and gas production. Therefore the pore structure of tight sandstone from Dong 6 well in 4 block of the central Dzungaria Basin were studied by mercury injection experiment, and the seepage characteristics of tight sandstone were comprehensively studied by stress sensitivity experiment. The results show that: the pore throat distribution of the tight sandstone in the study area is not uniform, but the sorting property is good. The pore throat which promotes the permeability is mainly distributed near the threshold pressure of mercury injection experiment. The stress sensitivity of tight sandstone is affected by stress state and pore structure. Stress sensitivity is negatively correlated with confining pressure, and permeability loss is negatively correlated with pore throat radius.

Detailed characterization of micronano pore structure of tight sandstone reservoir space in three dimensional space: a case study of the Gao 3 and Gao 4 members of Gaotaizi reservoir in the Qijia area of the Songliao basin

Tight sandstone samples from the Gao 3 and Gao 4 members of Gaotaizi reservoir in Qijia area of the northern Songliao Basin were targeted as the main research objects in this paper. Through micro/nano CT technology and auto skeleton module and connectivity module in avizo software, three-dimensional models for the micro pore structure of Gaotaizi tight sandstone in the research area were constructed. The experimental results demonstrate that types of reservoir space include intergranular dissolution pores, intragranular dissolution pores, and micro-fractures, indicating remarkable microscopic heterogeneity with characteristics of a continuous distribution in a micronano scale. Under the micro scale, different physical property and sedimentary microfacies of tight reservoir show various pore structure characteristics in three-dimensional space, with the primary front of pore throats mainly distributed in 0.6~1.5 μm. Under the nanoscale, micro heterogeneity is remarkable, with the primary front of pore throats distributed in 40~100 nm. In addition, the nanopore distribution pattern in three-dimensional space mainly includes the nanopores in the granules and the nanofractures, of which the latter type plays a crucial role in connecting nanopores and migration of nano oil and gas migration; at the same time, this paper verify the digital core experimental results by using MICP technology. With the further development of big data technology and deep learning technology, three-dimensional digital core technology based on micro/nano CT technology will become an important technology applied in the exploration and development of unconventional oil and gas.

Sensitive parameters of NMR T2 spectrum and their application to pore structure characterization and evaluation in logging profile: A case study from Chang 7 in the Yanchang Formation, Heshui area, Ordos Basin, NW China

Abstract The evaluation of the pore structure of tight sandstone reservoirs has a significant influence on the effective exploration and development of tight sandstone oil. Laboratory measurements including scanning electron microscopy (SEM), mercury-injection capillary pressure (MICP), gas adsorption, nuclear magnetic resonance (NMR), and Nano-CT can provide detailed pore structure data. However, only NMR results can be used to evaluate the pore structure in a logging profile due to NMR logging. In this study, the relationships between NMR T2 distribution, pore structure, and pore size of tight sandstone in Chang 7 of the Yanchang Formation, in the Heshui area of the Ordos Basin, are analyzed based on NMR principles. The characteristics of the NMR T2 distribution of different rock samples are analyzed; the sensitive parameters of the NMR T2 spectrum are proposed. These parameters are then used to classify pore structure types in tight sandstone reservoirs and divide the active layers in the logging profile of the study area. The results indicate that different pore size structures have different distributions, and that the NMR T2 spectrum can highlight the difference in the pore structure type and pore size distribution using sensitive parameters such as T2P2 (the value of T2 corresponding to the last peak of the bimodal NMR T2 spectrum) and TDM (the mean value of T2 relaxation time obtained by weighted average method). As the pore structure of the rock samples in the study area worsened, their porosity and permeability worsened, T2P2 and TDM decreased, and the displacement pressure and NMR irreducible water saturation increased. The results of classifying the pore structure types obtained in the logging profile are helpful in evaluating the effectiveness of the reservoir and in the broad application of NMR logging in terms of understanding pore structure.

Pore structure and multi-fractal analysis of tight sandstone using MIP, NMR and NMRC methods: A case study from the Kuqa depression, China

The pore structure and fractal characteristic of tight gas sandstone from Cretaceous Bashijiqike Formation in Kuqa depression were investigated, using thin section, scanning electronic microscope (SEM), mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR) and NMR cryoporometry (NMRC) measurements. The pores of tight sandstone samples mainly contain residual primary intergranular pores, intergranular and intragranular dissolved pores, micropores related to clay aggregates and microfractures. The MIP, NMR and NMRC analyses show that the pore structure of tight sandstone is complex and heterogeneous at different pore scales, displaying as a multi-fractal feature. Fractal dimension Dg-s (with respect to small pore throats) from MIP and Dn-l (long T2 relaxation time, representing large pores and fractures or movable-fluid pores) are negatively correlated with porosity, permeability, median pore throat radius and skewness, and have inverse relationships with bound water saturation (Sbnd). Fractal dimension Dg-b (>3.0, with respect to larger pore throats) and Dn-s (<2.0, short T2 relaxation time, representing small pores or bound-water pores) from NMR don't present clear relationships with most of pore structure parameters. Fractal dimension Dc-1 and Dc-2 from NMRC show more complex or inconspicuous relationships with pore structure parameters, in the pore diameter size <500 nm. Hence, Dg-s from MIP and Dn-l from NMR can be considered as effective parameters for pore structure and fluid properties characterization of tight sandstone. Fractal dimension Dg-b (>3.0) and Dn-s (<2.0) have no practical significance to characterize the pore structure of tight sandstone. The formation of residual primary intergranular pores and intergranular dissolution pores can reduce the fractal dimension which could be an important sign of effective reservoirs in the study area.

Fractal study on pore structure of tight sandstone based on full-scale map

The pore characteristics of tight sandstones are of great significance for studying the size distribution, surface roughness and seepage mechanism. In order to evaluate the pore size distribution and pore comprehensive fractal characteristics of tight sandstone in Y Basin, the pore structure of tight sandstone is characterised by low temperature nitrogen adsorption (LTNA) and high pressure mercury intrusion (HPMI), and fractals of different scales are fitted and calculated. We calculate the pore comprehensive fractal dimension of tight sandstone based on the full-scale map and discuss the difference in pore roughness from the shale in the Qinshui Basin. Finally, we also studied the relationship between comprehensive fractal dimension and permeability and porosity. Our results suggest that the pore radius of tight sandstones is distributed between 2 nm to 1,000 nm, and the surface of the macropores (> 100 nm) is rougher and more complex than the small pores (< 100 nm). The pore structure of the tight sandstone in the Y Basin is generally simpler than that of the shale in the Qinshui Basin. Besides, there is no significant correlation between the comprehensive fractal dimension and the permeability and porosity, suggesting the fractal dimension is not affected by porosity and permeability. [Received: September 11, 2018; Accepted: December 17, 2018]

Fractal study on pore structure of tight sandstone based on full-scale map

Fractal study on pore structure of tight sandstone based on full-scale map

Evaluation of the pore structure of tight sandstone reservoirs based on multifractal analysis: a case study from the Kepingtage Formation in the Shuntuoguole uplift, Tarim Basin, NW China

Both the microscopic pores and the pore structure of a reservoir affect its properties, including permeability and the distribution of oil and gas. Due to the stronger pore structure heterogeneity in tight sandstone reservoirs, it is difficult to characterize the pore structure of tight sandstone with traditional Euclidean geometry. In this study, based on the comprehensive utilization of reservoir data, such as petrophysical property data, mercury injection capillary pressure data and thin sections images, image processing and multifractal analyses are used to classify and describe the pore structure of tight sandstone in the Kepingtage Formation in the Shuntuoguole low uplift, Tarim Basin. The relationships among pore structure parameters, physical properties, and multifractal feature parameters are discussed in detail. Finally, a pore structure classification method using multifractal parameters based on K-means clustering analysis is proposed and applied to tight sandstone. The results show that the pore structure of the Kepingtage Formation can be divided into four types with distinct multifractal characteristics. The multifractal parameters correlate well with the porosity and permeability. The multifractal parameters αmin, αmax, Dmin, and Dmax are positively correlated with the average pore throat radius (rt) but are negatively correlated with displacement pressure (Pd). The parameters αmin, αmax, Dmin, and Dmax are the most sensitive indicators of pore structure and can serve as good indicators of the pore structure type. This pore structure classification method based on multifractal analysis and K-means clustering can be effectively used to classify the pore structure of tight sandstones. This study provides an effective means of pore structure classification and heterogeneity characterization based on thin sections, which is especially valuable in regions without high-pressure mercury injection data.

Applying octamethylcyclotetrasiloxane as a probe liquid for characterizing the pore size distribution of oil-bearing tight sandstones by nuclear magnetic resonance cryoporometry

The characterization of tight sandstone pore structure is extremely important to the study of tight sandstone oil and gas. There are many techniques for the quantitative analysis of pore size distribution (PSD) in tight sandstone. Nuclear magnetic resonance cryoporometry (NMRC) is a relative new method that can be applied to obtain PSD of porous materials. However, the choice of the NMRC probe liquid is critical to the test result. In this study, we used a potential probe liquid named octamethylcyclotetrasiloxane (OMCTS) for NMRC. OMCTS, exhibiting excellent properties, such as amphiphilic wetting, a large KGT, and superior NMR behavior, has obvious advantages over water as a probe liquid for the PSD measurement of tight sandstone by NMRC, especially for oil-bearing tight sandstone. The applicability of this probe liquid for measuring nanometer pores is demonstrated through the accurate measurement of standard sample. The PSDs of four tight sandstone samples with different wettability from the Upper Triassic Yanchang Formation in Ordos Basin are measured by OMCTS. Mercury intrusion capillary pressure (MICP) is also performed on all samples for comparison. The experimental results show that OMCTS can measure pores in tight sandstone from 4 to 1400 nm. OMCTS can, as NMRC probe liquid, not only measure the PSD of hydrophilic tight sandstone, but also oil-wet tight sandstone. Furthermore, OMCTS does not damage the pore structure of tight sandstone. We think the NMRC with OMCTS as probe liquid will be proved to be a useful complement to current methods.

Reservoir Characterization of Tight Sandstones Using Nuclear Magnetic Resonance and Incremental Pressure Mercury Injection Experiments: Implication for Tight Sand Gas Reservoir Quality

A series of experiments including porosity and permeability measurements, thin section and scanning electron microscopy (SEM) observations, incremental pressure mercury injection (IPMI), and nuclear magnetic resonance (NMR) were conducted to systematically characterize the pore structure of tight sandstone from the Lower Shihezi Formation of Permian (P2x) in the northeastern Ordos Basin, China. The influences of pore types, pore size distribution, and fractal characteristics on reservoir quality of tight sandstones are also investigated. Results show that the studied tight sandstones generally possess poor quality and complex pore structure. The porosity and permeability range from 4.08% to 17.56% (average 9.22%) and from 0.05 to 16.66 mD (average 2.49 mD), respectively. Five pore types were observed in thin section and SEM images: primary intergranular pores, intergranular dissolution pores, intragranular dissolution pores, micropores within clay aggregates, and microfractures. The pore throats are mainly hairy/fibrous, inhibiting the connectivity between pores. Three types of pore structures were identified in the mercury-injection curves and pore size distribution curves from the IPMI experiment and in the T2 relaxation time spectrum obtained by NMR. Both experiments yielded consistent classifications, and their combination was necessary to analyze the pore structure effectively. In general, permeability and porosity are positively related and depend on pore types. Large numbers of small pores confer high storage capacity, whereas small numbers of larger pores improve the flow capability. In the high porosity–permeability zone, larger pores also determine the storage capacity. The P2x tight sandstone is fractal, and macropores are more heterogeneous while micropores are more homogeneous. The fractal dimensions of macropores are good indicators of the reservoir quality of the P2x tight sandstone as larger fractal dimension values of macropores reflect poor reservoir quality.