Pore Structure Of Sandstone Research Articles

Depositional and diagenetic processes in volcanic matrix‐rich sandstones from the Shanxi and Shihezi formations, Ordos Basin, China: Implication for volcano‐sedimentary systems

AbstractVolcanic eruptions can provide large amounts of sedimentary materials and expose fluvial valleys or lakes to catastrophic hyperpycnal flood events, but this process has not been documented in detail. The Ordos Basin Permian fluvio‐deltaic system reveals evidence for abundant volcanism linked to the tectonic evolution of orogenic belt around the basin and provides a ‘natural laboratory’ for investigating hyperpycnal flows associated with volcanic activity. This study analysed volcanic matrix‐rich sandstone (VMS) samples for petrology, mineralogy and geochemistry in order to address current and uncertain volcanogenic material provenance explanations and the lack of systematic investigations into the depositional and diagenetic processes of volcanic related sediments in the southwestern basin. By combining tectonic background surveys, detrital zircon geochronology and spatial distribution of volcanogenic materials, it was found that volcanogenic materials were not derived from the Yinshan‐Yanshan Orogenic Belt (YYOB) as previously thought, but instead evidence a southwestern origin from the North Qinling Orogenic Belt (NQOB). Volcanogenic materials retained in the provenance area during frequent volcanic eruptions were transported to the basin via fluvial systems shortly after the eruption. The associated sediments meet the criterion for hyperpycnites based on lithofacies associations, suggesting the occurrence of hyperpycnal flows during the deposition of VMS. During the subsequent burial stage, the VMS became three distinct types defined by their volcanic matrix content and have similar paragenetic sequences but different diagenetic intensities. The differing content of authigenic minerals in the three types was closely related to the sandstone pore structure characteristics and the evolution of volcanic‐matrix alteration materials. This article proposes a possible explanation for the previously unidentified tectonothermal events in the NQOB during the Permian, validating and reinforcing the theoretical work of hyperpycnal flow. This contribution provides new insights and understanding of the depositional and diagenetic processes of lacustrine basins with similar tectonic settings.

Reservoir Physical Properties and Micropore Nanocharacteristics of Cores in the Shihezi Formation, Sudong District, Sulige Gas Field, Ordos Basin.

Sulige gas field is a very complex large-scale gas field with low porosity, low permeability, low abundance, and strong heterogeneity. In this study, the nanopore structure of the reservoir in the study area was analyzed. The analysis of reservoir physical properties and core microscopic pore characteristics of Shihezi formation in Sudong District, Sulige gas field, Ordos Basin, was put forward. According to core observation, thin section identification, and grain size analysis, the lithology of gas reservoirs in Shan-1 member and He-8 member in the study area is mainly lithic sandstone, lithic quartz sandstone, and quartz sandstone. According to the analysis of physical data in the study area, the reservoirs in Shan-1 member and He-8 member have typical characteristics of low porosity and low permeability. According to that microscopic observation of sandstone thin section and cast thin section combined with the analysis of scan electron microscope, the results show that both Shan-1 member and He-8 member of Sudong area 2 in Sulige gas field are dominated by class I reservoirs and class II reservoirs. Especially, class I reservoirs are widely distributed, with the cumulative area of this type of reservoirs accounting for 70.12% of the total area in Shan section and 64.12% in He-8 section. Class I reservoirs are scattered in the study area, accounting for 0.27% of the total area in the mountain section and 0.36% in the He-8 section. Class IV reservoirs are distributed dispersedly and have a small distribution area. The cumulative area of reservoirs in mountain section accounts for 5.38% of the total area and that in He-8 section accounts for 5.46%. Therefore, Sudong area 2 in Sulige gas field is a typical low porosity and low permeability reservoir. On the basis of geological background, sedimentary microfacies, diagenesis, and pore structure characteristics of the study area, it is considered that the pore structure of sandstone in the study area is mainly controlled by sedimentary characteristics and diagenesis of sandstone.

Influence of Pore Geometry on Sandstone Pore Analysis Based on a Digital Core

A digital core obtained without damage and contact can provide intuitive and quantitative analysis data for sandstone pore structure analysis. However, pore analysis results based on digital cores are quite different from experimental results due to limitations of scanning resolution, quality, pore extraction method, and experimental errors. In this research, the influence of pore geometry on pore analysis is studied from pore angles and pore resolution. First, sharp angles are extracted by image processing, and the influence of angles on pore radius distribution is analyzed. Then, by up–down sampling, high- and low-resolution digital cores are reconstructed, and the effects of resolution on pore connectivity and throat structure are discussed.

Effects of the Sandstone Pore Structure on Spontaneous Imbibition: A Systematic Experimental Investigation Based on Fractal Analysis

Pore structure is one of the most fundamental properties in reservoir characterization and is closely related to spontaneous imbibition recovery. This work utilized tight sandstone reservoir rock samples in the Ordos Basin to probe the influencing factors of the pore structure and fractal characteristics on imbibition efficiency. A series of experiments including high-pressure mercury injection (HPMI) and low-pressure nitrogen gas adsorption (LP-N2GA) were performed to systematically characterize the pore structure of the tight sandstone rocks. Then, the effects of pore structures and fractal dimensions on the imbibition recovery factor were elaborately discussed. These factors were further ranked using the gray correlation analysis. Results indicated a relatively high imbibition recovery, which ranged from 64.91 to 89.88%. Mesopores were more conducive to gas production, accounting for up to 56.88%. The combination of HPMI and LP-N2GA data synergistically characterized pores in the entire range, from micro- to macropores of the subjected reservoir (0.02-10 μm). Sensitivity analyses showed that imbibition recovery correlated with the homogeneity coefficient, reservoir quality index, skew coefficient (Skp), sorting coefficient (Sp), micropore volume, displacement pressure, specific surface area, and Swanson parameter. Additionally, the gray correlation analysis further suggested that imbibition efficiency was highly related to the fractal dimension, and gas production recovery was mainly governed by the mesopore fractal dimension. This work provides insights into how the pore structure affects spontaneous imbibition efficiency in tight sandstone reservoirs from a fractal perspective.

Research on strength softening characteristics and microscopic mechanism of sandstone after absorbing water

The deformation of the surrounding rock in roadways occurs because of the coupling effect of water and rock, which decreases the strength of the surrounding rock. The microscopic pore structure in the rock mass develops through and develops into microcracks, which further the rock mass strength. Based on the rock mass strength reduce observation, this article selects Shandong Wanfu coal mine sandstone samples, based on XRD mineral composition and scanning electron microscopy testing, using nuclear magnetic resonance (NMR) and 2,000 kN rock single triaxial rock mechanics experimental equipment to perform NMR experiments and uniaxial compression tests under different water content. Quantitatively study the pore structure and strength change law of water to rock. The experimental results show that with the increase in water absorption time, the water absorption law of sandstone has three stages: accelerated water absorption, decelerated water absorption, and uniform water absorption. The microscopic pore structure of sandstone is gradually filled with water molecules. The water molecules first occupy the micropores, then the mesopores, and finally the macropores. After reaching the saturation state, the micropores account for 51.87%, the mesopores account for 42.76%, and the macropores account for 5.37%. Because sandstone contains hydrophilic iron dolomite and clay minerals, it dissolves and adsorbs hydrophilic minerals after encountering water. The micropores gradually decrease, the mesopores increase, and the macropores do not change. It can be seen from the mechanical strength of rock samples with various water contents that the dry state has the highest mechanical strength, with a strength of 171.58 MPa, a saturated state with the lowest mechanical strength, with a strength of 91.78 MPa, and a softening coefficient of 0.53. The surrounding rock strength is relatively unstable because the pore water pressure increases after the surrounding rock interacts with water. The increased pressure reduces the compressive stress between the particles, decreasing the strength of the surrounding rock.

Linking the mechanical properties of frozen sandstone to phase composition of pore water measured by LF-NMR at subzero temperatures

Water-bearing porous rocks can be greatly strengthened by freezing; its microscopic mechanisms lie in how subzero temperature modifies pore structure but have not yet been clearly defined. In this study, we link the mechanical properties of frozen sandstone to phase composition of pore water (i.e., relative amount of unfrozen water and ice). Both the strength (uniaxial compressive strength and tensile strength) and the phase composition of pore water (tested through the nuclear magnetic resonance (NMR) method) of frozen sandstone at nine subzero temperatures were measured. The results showed the following: (1) subzero temperature strengthened saturated sandstone significantly, both the strengths increased in a three-stage mode and each stage showed an approximately linear trend as temperature decreased from 0 to – 20 °C; and (2) change of phase composition with freezing displayed two distinct stages: the rapid decreasing stage of unfrozen water content (0 to – 4 °C), when bulk water and capillary water froze almost completely, and the slow decreasing stage (− 4 to – 20 °C), when adsorbed water froze partially. Based on direct observations on microscopic pore structure of sandstone and detailed discussion on the mechanical interactions between water, ice, and pore wall, phase composition may modify the mechanical properties of frozen rocks through the following effects: the supporting effect of ice under compression, the crack-filling effect of ice, the cementing effect of unfrozen water film under tension or under shearing load, and the frost damage effect. The former three are strengthening effects, while the latter one is weakening effect.

Pore structure characterization of tight sandstones via a novel integrated method: A case study of the Sulige gas field, Ordos Basin (Northern China)

• Pore structure can be characterized by combining multiple experimental techniques. • Full size PSDs of pore body and throat can be determined by combining NMR and CMI. • Complexity and irregularity pore structure can be depicted by fractal dimension. • Pore size can be divided into three categories to evaluate each category’s contribution. • A novel permeability model considering classified pore contributions is proposed. Pore structure characterization is a very crucial work to understand rock properties. In this work, a novel integrated method was proposed to characterize the pore structure of a real tight sandstone reservoir in the Sulige gas field. This integrated method requires a comprehensive analysis of experimental results based on cast thin section (CTS), nuclear magnetic resonance (NMR), constant-rate mercury intrusion (CMI) and X-ray computed tomography (XCT) techniques. Five main pore types were firstly identified from CTS images. Bimodal distribution curve of surface relaxivity rate, mercury injection curves of pore body and throat, 3D visual image of pore reconstruction were then generated from NMR, CMI and XCT measurements, respectively. Pore size distribution (PSD) was also obtained by and compared among NMR, CMI and XCT techniques. Full size PSDs of pore body and throat were ultimately determined using a combination analysis method of NMR and CMI. In addition, fractal dimension was calculated by NMR- and CMI-deduced formulas. Moreover, pores were classified into three categories and the contribution of different category pore to rock properties was qualitatively ascertained. At the end, a novel permeability prediction model in consideration of the contributions of three category pore was established. It was found that radii of pore body and throat ranged in 0.21–42.01 μm and 0.006–3.7 μm, respectively. Sandstone pore has multifractal characteristics and its fractal dimension ranged in 1.335–2.982. Overall, sandstone pore structure of the example reservoir was characterized by multiple pore types, nano-micrometer pore scale , irregular geometric shape, non-uniform distribution, poor connection status and bad development degree of pore space . The findings of this study can help for better understanding of pore structure characteristics of tight sandstones.

Numerical simulation of the relationship between resistivity and microscopic pore structure of sandstone

The microscopic pore structure of the sandstone rock layer determines the water richness and permeability of the rock layer. Mastering the relationship between the resistivity of the sandstone rock layer and the microscopic pore structure is an important way to evaluate the water richness of the water-bearing rock layer from qualitative analysis to quantitative calculation. Using the finite element method to study the basic single and double modal structures in the sandstone layer of constant current field, the matrix resistivity, pore throat water resistivity, pore shape and difference in single sand model, pore throat model and capillary model are studied. The relationship between the direction of the current and the resistivity of the rock layer. The results show that the resistivity of the sandstone rock layer changes with the change of the matrix resistivity and has a linear relationship. When the contrast ratio between the sand and the matrix resistivity is between 10−1 and 10, the resistivity of the rock layer changes linearly with the resistivity of the sand; when the contrast ratio is greater than 10, the influence of the resistivity of the matrix is smaller. And when the contrast is greater than 103, the resistivity of the rock layer tends to be constant; when the contrast is less than 10−2, the smaller the resistivity of the sand grains, the smaller the effect on the resistivity of the rock layer, and when the contrast is less than 10−3, the resistivity of the rock layer tends to constant. Since the water in the sandstone rock layer is the main conductive medium, when the water resistivity is fixed, the smaller the water saturation or the smaller the porosity, the smaller the pores through which water can flow and the greater the resistivity of the sandstone formation. When the microstructures of the horizontal and vertical rock layers are different, different transmitting and receiving electrode layouts have a greater impact on the resistivity of the rock layer, and the different microstructures lead to electrical anisotropy of the rock layer. The research results of this paper lay the foundation for the analysis of the relationship between the more complex sandstone rock model resistivity and micro-pore structure, and the quantitative calculation of the water richness of the sandstone rock layer.

Experimental investigation on the breakdown behaviours of sandstone due to liquid nitrogen fracturing

Liquid nitrogen (LN) fracturing is an efficient technology for the development of unconventional gas resources. Due to continuous injection of cryogenic LN, the reservoir temperature decreases and then cooled rock regions are gradually generated during fracturing process. To investigate the breakdown behaviours of sandstone induced by LN fracturing, laboratory simulation experiments were performed with a self-designed setup. The experimental results showed that rising initial rock temperature greatly reduced the breakdown pressure of sandstone, whereas hardly affected the fracture complexity. Under cryogenic conditions, both the compaction and cracking effects exerted influence on the pore structure of sandstone. Thus, the change in breakdown pressure depended on which effect played a domain role. Although the cryogenic conditions probably made the breakdown pressure of sandstone increase, the fracture complexity was expected to be significantly improved. In addition, the initial rock temperature increasing played a positive role in improving the fracture complexity under cryogenic conditions. For initial or heating sandstone samples, the fracture pattern mainly presented in the form of bi-wing fractures. However, for cryogenic samples, the fracture patterns contained both local fractures and complex fractures. The former was generated with lower breakdown pressure while the latter was generated with higher breakdown pressure. Additionally, the gas leak regions were observed in cryogenic samples during LN fracturing. Under cryogenic conditions, the sandstone used was favourable to generate more complex fractures and the permeability of rock matrix in unbroken region was proved to be improved due to LN fracturing. Collectively, fracturing with LN could improve the fracture complexity and the permeability of rock, which was very beneficial for well production.

Evolution of permeability and microscopic pore structure of sandstone and its weakening mechanism under coupled thermo-hydro-mechanical environment subjected to real-time high temperature

Thermal damage mechanisms and the characteristics of performance deterioration of a repository host rock is critical for evaluating the potential of an underground coal gasification project. Previously, many studies have primarily focused on the microstructural evolution of various rock types after high-temperature treatment only. However, relatively little is known regarding the coupled thermo-hydro-mechanical (THM) behavior of rock, which is frequently encountered in geo-engineering underground regions with high-temperatures. In this study, using a self-developed universal tester with the ability of THM coupling of high temperature and high pressure, permeability evolution in sandstone under real-time high temperature (20–700 °C) and triaxial stress (hydrostatic pressure = 25 MPa) were observed. The microphysical parameters of these specimens subjected to the THM environment were then measured, and the corresponding morphological evolution processes were also assessed using micro-computed tomography (MCT). Further, for the purpose of contrast, we have also determined the microphysical parameters for the sandstone after heat treatment only. The results show that the evolution of the microscopic structures within the sandstone under the coupled THM condition is vastly different from those subjected to heat treatment only. The experimental results in this study can provide theoretical guidance for the stability of surrounding rock of a combustion cavity during in-situ coal gasification.

Experiment Study on Topological Characteristics of Sandstone Coating by Micro CT

The pore structure is an important factor of tunnel coating failure, cracking and water leakage. Some investigations on the statistical law of pores and pore networks have been conducted, but little quantitative analysis is observed on topology structure of the pore network, and even the pore structure of sandstone is complex and cross-scale distributed. Therefore, it is of theoretical and engineering significance to quantitatively characterize the connectivity of the pore network in sandstone. This study proposes a new complex network theory to analyze the three-dimensional nature of pore network structure in sandstone. The topological network structure, such as clustering degree, average path length and the module, which cannot be analyzed by traditional coordination number and fractal dimension methods, is analyzed. Numerical simulation results show that a scale-free network model is more suitable for describing the sandstone pore network than random models. The pore network of sandstone has good uniformity. The connectivity of sandstone pore networks has great potential for permeability enhancement. Therefore, this new method provides a way to deeply understand the pore connectivity characteristics of sandstone and to explore the distribution of crack grids in the arch of tunnel coatings.

Evolution in sandstone pore structures with freeze-thaw cycling and interpretation of damage mechanisms in saturated porous rocks

Measuring the evolution of pore structures is the most straightforward approach to assessing frost damage in porous rocks. Systematic and fine measurement of pore structures is critical to the interpretation of frost damage mechanisms. We conducted experiments where sandstone samples were subjected to 50 freeze-thaw cycles and their T2 spectra measured after each cycle via the nuclear magnetic resonance (NMR) method. This allowed the evolution in pore size distribution, porosity (pore volume), permeability (pore connectivity), tortuosity (pore curvature), and pore size uniformity coefficient (pore non-uniformity) to be comprehensively estimated. The results show that: (1) variation in pore structures is mainly caused by the propagation and expansion of nanopores and micropores and generation of new nanopores. (2) The porosity of sandstone has a three-stage trend of increasing-stabilizing- increasing, while tortuosity exhibits an opposite trend of decreasing-stabilizing-decreasing with freeze-thaw cycling. (3) The permeability and surface roughness of sandstone increase with freeze-thaw cycling, while the pore size uniformity coefficient decreases linearly. Based on the above results and scanning electron microscopic observations of the pore structure of sandstone, we found that: (1) the pore structure of layered sandstone can be simplified as a main stem type. (2) In this pore structure, there are two main damage processes: (a) where water in secondary cracks migrates to stem cracks and freezes, causing propagation and expansion of stem cracks; and (b) where unfrozen water in secondary cracks freezes in-situ, leading to the propagation, expansion and generation of secondary cracks. The above two damage processes coexist and alternately play pivotal roles during repeated frost activity, resulting in multistage changes to the porosity, permeability, and tortuosity of sandstone.

Comparison of fractal models using NMR and CT analysis in low permeability sandstones

Fractal geometry provides an effective method for characterization of the complex and irregular pore structure of Eocene Shahejie low permeability sandstones in the Raoyang Sag, the Bohai Bay Basin, China. Laboratory measurements including porosity, permeability, scanning electron microscope (SEM), thin sections, nuclear magnetic resonance measurements (NMR) and X-ray computed tomography (CT) technology are used to provide insights into the fractal characteristics of pore structure in sandstones of Eocene Shahejie Formation in the Bohai Bay Basin, China. Quantitative CT analysis reveals the pore radius is not always linear to T2 (transverse relaxation time) value obtained from the NMR tests, but instead power function of T2. Fractal analysis was performed on the T2 distribution using various fractal models, and the related fractal dimensions are calculated. The fractal dimensions calculated using various fractal models are correlated with NMR parameters and permeability. The fractal curves break into two segments at the T2cutoff (T2 separating the immovable and movable fluids) value or smaller when using fractal model Ⅰ and fractal model Ⅱ, and only the large-scale pore networks can be described by the fractal geometry. Mostly the entire pore size distributions (micro-pores to large-scale pore networks) can be described by the fractal model Ⅲ, and the calculated fractal dimensions are in accordance with the CT scanning and thin section data, and are strongly correlated with the T2gm (geometric mean of T2), permeability and BVI (bulk volume of immovable fluids). The fractal behaviors of pore size distributions from NMR analysis have implications for pore structure evaluation in low permeability sandstones with similar geological settings.

Diagenesis and diagenetic facies distribution prediction of Chang 8 tight oil reservoir in Maling area, Ordos Basin, NW China

In this study, characteristics of Chang 8 tight sandstone pore structure and diagenesis are analyzed. A classification standard for sandstone diagenetic facies is established. Combined with the probabilistic neural network method, logging curves are used to predict the distribution of diagenetic facies. The following results are found: the Chang 8 reservoir has low porosity-low permeability characteristics, and the pore-throat structures have strong microscopic heterogeneity; both compaction and siliceous and carbonate cementation promote reservoir densification; chlorite cement lining, hydrocarbon emplacement, and the dissolution of feldspar and rock fragments have constructive effects on reservoir development; and two favorable diagenetic facies, a weakly compacted and chlorite-authigenic quartz cementation facies and an intensely compacted-dissolution facies, are developed in the middle of the thick sandstone and are a good match in their horizontal distribution with distributary channel sands. The Chang 81 member has a much larger area that is more favorable for the development of reservoir diagenetic facies than the Chang 82

Fractal Analysis of Pore Structure Development of Sandstone: A Nuclear Magnetic Resonance Investigation

The pore structure of rock has a great influence on its physical and mechanical properties. Factors such as chemical corrosion and temperature changes affect the pore structure evolution. In this paper, the pore structure of sandstone was investigated under rapid freeze-thaw (F-T) cycles and chemical corrosion. A nuclear magnetic resonance (NMR) testing system is used to study the pore structure of tight sandstone samples immersed in different chemical solutions after 10, 20, and 30 F-T cycles. Permeability is determined by using empirical method. Results found that permeability is strongly affected by the erosion of NaOH and NaCl solutions. The pores in the rock were divided into three categories based on the pore size, i.e., minipores, mesopores, and macropores. The results showed that the amount of mini-pores and mesopores both decreased with an increase in the number of F-T cycles while the amount of macropores increased for groups of NaOH, NaCl, and pure water. No conclusive trend can be found in the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> group. Fractal analysis of the pore structure revealed that no conclusive trend was observed for fractal dimension of mini-pores D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> . Fractal dimension of mesopores D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ranged from 2.79 to 2.93, indicating a medium complexity pore structure of the mesopores. Fractal dimension of macropores D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> was over 2.9, implying that the pore structure of the macropores is the most complex. The fractal dimension of the T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> spectrum DNMR ranged from 2.55 to 2.77. Correlations between the fractal dimensions and porosity are also presented. Results showed that D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> can be good indicators for the pore size volume of sandstone samples immersed in H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , NaOH and NaCl solutions, while DNMR is a good indicator for the pore size volume of sandstone samples immersed in NaOH solution and pure water.

Pore-scale study of the pressure-sensitive effect of sandstone and its influence on multiphase flows

The pressure-sensitive effect on the pore structure of sandstone was investigated using X-ray computed micro-tomography and QEMSCAN quantitative mineral analysis. In a physical simulation study, we extracted the pore network model from digital cores at different confining pressures and evaluated the effect of pressure sensitivity on the multiphase displacement process. In both the pore network model and QEMSCAN scanning, the pore structure was observed to be damaged under a high confining pressure. Due to their different scales, the pores and throats exhibited inhomogeneous changes; further, the throats exhibited a significant variation compared to that exhibited by the pores. Meanwhile, the heterogeneity of the pore structure under the two aforementioned activities was aggravated by the elastic–plastic deformation of the pore structure. The pressure-sensitive effect increased the proportion of mineral particles, such as quartz (the main component of the core skeleton), and reduced the proportion of clay minerals. The clay minerals were originally attached to the pore walls or interspersed in the pores; however, as the pressure increased, the clay minerals accumulated in the pores resulting in blockage of the pores. While simulating the multiphase displacement process, increasing the confining pressure was observed to severely restrict the flowability of oil and water. This study promises to improve the efficiency of reservoir development in terms of oil and gas exploitation.

Experimental investigations on the effects of ambient freeze-thaw cycling on dynamic properties and rock pore structure deterioration of sandstone

The issue of rock pore structure deterioration and dynamical mechanical properties has drawn much attention in recent years in the rock engineering community. In this study, a series of 140 freeze-thaw cycling tests are conducted on sandstone samples. The sandstone pore structure after freeze-thaw cycles was detected by Nuclear Magnetic Resonance (NMR), and the dynamic load test was carried out by Split Hopkinson Pressure Bar system (SHPB). The results are: with the increase of freeze-thaw cycles, the saturated mass and porosity of sandstone increase, the dynamic peak stress of sandstone decreases; while the peak strain and total strain increase gradually, and the macroscopic damages of rocks increase. The results of T2 distribution and pore size changes are that the pore sizes of sandstones increase, especially that of micro-pores and macro-pores increase obviously after 140 freeze-thaw cycles. Under the effect of freeze-thaw cycles, the sandstone pores become connected with each other, some of the micro-pores and mini-pores are changed into meso-pores and macro-pores. The number of marco-pores increases evidently, and the largest increase is 197.23%. The rock pore structures have changed, which would lead to the change of mechanic properties. As the number of freeze-thaw cycles increases, the rock dynamical peak stress gradually decreases and the peak strain and the overall strain increase gradually.

A tight sandstone trapezoidal pore oil saturation model

Abstract “Non-Archie” phenomena are common in tight sandstone reservoirs with complicated pore structure, bringing challenges to the logging evaluation of tight sandstone. Based on the characteristics of tight sandstone pore structure, a new trapezoidal pore saturation model considering the effect of pore structure on rock conductivity is presented, in which the pore in tight sand is divided into straight pore with constant cross-sectional area and trapezoidal pore with variable cross-sectional area, and the total rock resistance is taken as the parallel resistance of these two parts to compressively consider the influence of pore structure and conductive volume on rock conductivity. The model parameters are studied by reconstructing the trapezoidal pore structure. Based on the trapezoidal pore reconstruction, characterization methods of model parameters were studied under the constraint of rock electrical properties test, and the variation was revealed of tortuosity, straight pore proportion and trapezoidal factors. The model was used in logging evaluation of tight sandstone of several wells, the results show the oil saturation obtained from the new model considering the effect of pore structure on electrical properties from the aspects of pore length and pore cross-section is in good accordance with the real regularity and petrophysical characteristics of the reservoir, and much closer to the formation oil-bearing conditions than that from the Archie model.

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.

Variations of Micropores in Oil Reservoir Before and After Strong Alkaline Alkaline-surfactant-polymer Flooding

Enhanced oil recovery (EOR) provides a significant contribution for increasing output of crude oil. Alkaline-surfactant-polymer (ASP), as an effective chemical method of EOR, has played an important role in advancing crude oil output of the Daqing oilfield, China. Chemical flooding utilized in the process of ASP EOR has produced concerned damage to the reservoir, especially from the strong alkali of ASP, and variations of micropore structure of sandstones in the oil reservoirs restrain output of crude oil in the late stages of oilfield development. Laboratory flooding experiments were conducted to study sandstones’ micropore structure behavior at varying ASP flooding stages. Qualitative and quantitative analysis by cast thin section, scanning electric microscopy (SEM), atomic force microscopy (AFM) and electron probe X-Ray microanalysis (EPMA) explain the mechanisms of sandstones’ micropore structure change. According to the quantitative analysis, as the ASP dose agent increases, the pore width and pore depth exhibit a tendency of decrease-increase-decrease, and the specific ASP flooding stage is found in which flooding stage is most affective from the perspective of micropore structures. With the analysis of SEM images and variations of mineral compositions of samples, the migration of intergranular particles, the corrosions of clay, feldspar and quartz, and formation of new intergranular substances contribute to the alterations of sandstone pore structure. Results of this study provide significant guidance for further application to ASP flooding.