Petrographic characterization to build an accurate rock model using micro-CT: Case study on low-permeable to tight turbidite sandstone from Eocene Shahejie Formation

Investigation on multi-parameter of hydrate-bearing synthetic sediment during hydrate replacement process: A in-situ X-ray computed tomography study

Carbon sequestration can be achieved by carbon dioxide replacement in natural gas hydrate exploitation, which reducing greenhouse gas emissions and providing an effective solution to address climate change, while simultaneously protecting the environment and promoting sustainable energy development. Gas replacement can achieve gas exploitation, gas storage, and stability enhancement simultaneously. However, time-varying microstructure evolution of the hydrate-bearing sediment (HBS) during this process remain a large amount of uncertainty. In this study, with microfocus computer tomography, hydrate replacement process is realized using xenon gas to replace krypton hydrate. During this period, the initial hydrate saturation and effective confining pressure were 63 % and 1 MPa respectively, the results were obtained as follows: 1. Hydrate occurrence dynamically adjusted during replacement process due to the "barrier effect" and "diffusion effect". 2. Dissociated water migration occurred in the sediment, and this induced local hydrate enrichment temporarily and blockages, but the blockages were eventually dredged with the dissociation of the Kr hydrate. 3. The sphericity and surface roughness of the hydrate particles were slightly improved, the pore space connectivity was well enhanced, and both tortuosity and absolute permeability was better strengthened after replacement process, where the absolute permeability was increased by 225.23 %, though the blockage occurrence temporarily weakened this strengthener.

Pore Structures and Evolution Model of Reservoirs in Different Secondary Structural Zones in the Eocene Shahejie Formation, Chezhen Sag, Bohai Bay Basin

Although abundant unconventional oil resources have been discovered in conglomerate and sandstone reservoirs in rift basins, the mechanism of differential pore evolution in conglomerates and sandstone reservoirs within different secondary structural zones of rift basins is not yet clear. The pore structures of conglomerate and sandstone reservoirs in the distinct secondary structural zones in the Chezhen Sag were quantified in three dimensions using high-resolution microcomputed tomography (micro-CT). Thin section and scanning electron microscopy observations were used to investigate the differential evolution mechanisms of conglomerate and sandstone reservoirs. Micro-CT analysis of the pore structures of conglomerate and sandstone reservoirs revealed that sandstone reservoirs are superior to conglomerate reservoirs with regard to the pore number and pore connectivity and that sandstone reservoirs are more heterogeneous than conglomerate reservoirs. Triangles dominate the pore and pore throat geometries of sandstone and conglomerate reservoirs, while the sandstone reservoir pores are more regular than conglomerate reservoir pores. The depositional environment, mineral composition, and diagenetic intensity jointly control the quality of the reservoirs. Because of the lengthy transportation distance of their parent rocks, the compositional maturity and sorting behavior of sandstone reservoirs in depression and gentle slope zones are better than those of conglomerate reservoirs in steep slope zones, and thus sandstone reservoirs have a higher initial porosity than conglomerate reservoirs. The rapid compaction experienced by the conglomerate reservoirs in steep slope zones in their early stages creates a closed diagenetic environment, making it difficult to effectively improve reservoir porosity through dissolution. However, the widely developed microfractures in the reservoirs provide channels for fluid migration, promote the development of dissolution pores, and form a tight reservoir dominated by secondary pores. With weak compaction and an open diagenetic environment, the primary pores in sandstone reservoirs in the gentle slope zone are preserved in large quantities. Meanwhile, dissolution expands the secondary pores of the reservoir, resulting in a high-quality reservoir having both primary and secondary pores. In addition, an approach based on primary, secondary, and total porosity was proposed in the study to efficiently evaluate reservoir quality and identify reservoir evolution mechanisms.

Potentialities of iodine-enhanced micro-CT imaging in the morphological study of adult Aedes (Stegomyia) aegypti (Linnaeus, 1762) mosquitoes

X-ray micro-computed tomography (CT) produces three-dimensional images of samples on a micrometer scale. This technique has several advantages, such as its nondestructive character and low measurement time, compared with other techniques. However, when applied to biological samples of soft tissue, the low attenuation and low effective contrast between structures pose difficulties in creating appropriate images for morphological studies. Diffusible iodine-based contrast-enhanced CT (DICE-CT), which uses iodine solutions to enhance contrast, is a viable alternative for addressing the aforementioned challenges. Given the variety of biological samples, an appropriate methodology must be adapted depending on the dimensions and morphological characteristics of the investigated object. A specimen that has not been morphologically studied by micro-CT and is of high sanitary importance is the adult Aedes aegypti mosquito. This study investigated the stage of iodine staining in the treatment of the A. aegypti mosquito to determine the most suitable staining time for the morphological study of this mosquito in adulthood. After determining the appropriate staining time, we discuss the potential of applying DICE-CT and methodology to mosquito studies. Seven A. aegypti females were treated using fixation steps with Bouin's solution, dehydration in a graded ethanol series, staining with iodine solution (1%), and washing in absolute ethanol. Only the staining step was different between samples. Each mosquito spent a varying amount of time (6-72 h) in the iodine solution (1%). For comparison, one of the mosquitoes was not stained. After treatment, the samples were scanned using the Bruker SkyScan 1172 micro-CT scanner. The reconstructed volumes and histograms were compared to determine the most suitable time. In addition, a quantitative analysis was performed based on a comparison of the attenuation profiles of the mosquito brains. Thereafter, the most suitable treatment process was selected, and two other samples were scanned after applying the selected process. Although fewer than 18 h was insufficient for an effective increase in attenuation and effective contrast, surpassing 24 h proved unnecessary and resulted in saturating the gray tones visualized through the histograms, leading to information loss. Therefore, a time of approximately 24 h was the most suitable staining time for studying adult A. aegypti. It was possible to isolate the organs of the digestive and reproductive systems of the mosquito stained for 24 h. Thus, micro-CT was confirmed to be an excellent technique in studies of individual structures of adult A. aegypti mosquitoes.

Identification of Diagenetic Facies Logging of Tight Oil Reservoirs Based on Deep Learning—A Case Study in the Permian Lucaogou Formation of the Jimsar Sag, Junggar Basin

As a typical tight oil reservoir in a lake basin, the Permian Lucaogou Formation of the Jimsar Sag in the Junggar Basin has great potential for exploration and development. However, at present, there are few studies on the identification of the diagenetic facies of tight oil reservoir logging in the study area, and the control effect of diagenesis on tight oil reservoirs is not clear. The present work investigates the diagenesis and diagenetic facies logging of the study area, making full use of core data, thin sections, and logs, among other data, in order to understand the reservoir characteristics of the Permian Lucaogou Formation in the Jimsar Sag. The results show that the Lucaogou Formation has undergone diagenetic activity such as compaction, carbonate cementation, quartz cementation, and clay mineral infilling and dissolution. The diagenetic facies are classified according to mineral and diagenetic type, namely, tightly compacted facies, carbonate-cemented facies, clay mineral-filling facies, quartz-cemented facies, and dissolution facies. The GR, RT, AC, DEN, and CNL logging curves were selected, among others, and the convolutional neural network was introduced to construct a diagenetic facies logging recognition model. The diagenetic facies of a single well was divided and identified, and the predicted diagenetic facies types were compared with thin sections and SEM images of the corresponding depths. Prediction results had a high coincidence rate, which indicates that the model is of a certain significance to accurately identify the diagenetic facies of tight oil reservoirs. Assessing the physical properties of the studied reservoirs, dissolution facies are the dominant diagenetic facies in the study area and are also the preferred sequence for exploration—to find dominant reservoirs in the following stage.

Microscopic Conductivity Mechanism and Saturation Evaluation of Tight Sandstone Reservoirs: A Case Study from Bonan Oilfield, China

Core samples of the tight sandstone reservoir in the Bonan Oilfield were analyzed by using multiple petrophysical experimental techniques, then a multi-scale three-dimensional digital rock model was constructed. The pore structure parameters of tight sandstone and homogeneous Berea sandstone were compared. The electrical simulation method based on the digital rock model was utilized to quantitatively reveal the influence of five micro-pore structure parameters (pore size, throat size, pore-throat size, coordination number, and shape factor) on the rock’s electrical properties. In addition, the saturation of tight sandstone reservoirs was evaluated in combination with the three-component automatic mixed-connection conductivity model. The results show that the “non-Archie” phenomenon in sandstone is obvious, which is mainly caused by the small radius of the maximum connected pore throat and the complex structure of the pore throat. We noted that: with an increase in pore radius, throat radius, and coordination number, the formation factor decreases and tends to be stable; the pore-throat size increases and the formation factor decreases in the form of power function; the shape factor increases, and the formation factor increases; the larger the pore–throat ratio and shape factor, the greater the resistivity index; with an increase in coordination number, the resistivity index decreases; and the pore-throat size has no effect on the resistivity index. The calculation accuracy of oil saturation is improved by 6.54% by constructing the three-component automatic mixed-conductivity saturation model of tight sandstone.

Pore scale image analysis for petrophysical modelling

Sedimentary rocks are known for their complex pore system with varying morphology due to intricate diagenetic processes. The present study demonstrates the applicability of image analysis in analysing and defining reservoir rock properties. Conventional techniques provide quantitative results but fail to give information about the internal microstructure of the rock. On the other hand, digital image techniques reveal the micro and macro-pore types and their connectivity across multiple scales. Hence, we performed the digital image analysis on Field Emission Scanning Electron Microscopy (FESEM) images of sandstone and carbonate samples collected from the upper Assam and Bombay offshore basins. FESEM derived image analysis was used exclusively due to its several unique features over contemporary techniques involving lesser data acquisition, simulation time and performing analysis even on a rock chip obtained while drilling the borehole. Porosity was evaluated based on the percentage of pores available within the image, and permeability was evaluated using the Kozeny-Carman equation. Further, we developed statistical equations to understand the existence of coherence amongst these parameters. Our study shows that we could determine both open and closed porosities by this method. In addition, there is an agreement between the conventional porosity measurement and image-derived porosity for most rock samples, especially for very low and high porosity. Further, this study highlights the importance of thresholding, an essential component in evaluating porosity using digital images. We propose that the methodology developed can accurately characterise reservoirs based on pore networks using high-resolution imaging techniques. The developed methodology may be adopted to promote best practices. Since we used digital images obtained from small chip size rock samples, this method is advantageous to quickly calculate the porosity and permeability from rock chips retrieved from the sieve shaker while drilling. Digital datasets extracted from this analysis will be helpful for reservoir description and characterisation based on image-derived petrophysical parameters.

A review of high-resolution X-ray computed tomography applied to petroleum geology and a case study

High-resolution X-ray computed tomography (micro-CT) has been widely used as a non-destructive technique, allowing 3D imaging and analysis of internal features of various objects. This paper briefly describes this technique used in the field of petroleum geology, with an example of its applications for tight-gas sandstone reservoirs. Reservoir quality controls the storage, distribution, and flow of fluids within tight-gas sandstone reservoirs, however, it remains difficult to predict. Although our earlier work has reported that reservoir quality of non-marine tight-gas sandstones in the Lower Cretaceous Denglouku Formation in the Songliao Basin, China is controlled by sedimentary texture, depositional facies, and diagenesis, there is lack of information on the effectiveness of pore-lining clay minerals on pore preservation and pore connectivity. Micro-CT is performed on two representative sandstone samples from the studied Denglouku sandstones, (1) to quantify the internal architecture in 3D and (2) to carry out fluid-flow modelling to determine the effective transport capacity. The quantitative 3D imaging and modelling by means of micro-CT provides a better insight of pore network connectivity and pore preservation characteristics at the pore scale. Here, we develop a procedure that integrates pore-scale imaging and modelling with depositional and diagenetic processes in the context of non-marine tight-gas sandstones. Micro-CT data are compared with results derived from core and wireline log interpretation as well as laboratory measurements on the same core samples, which results in good agreement and supports the prediction of pore preservation effectiveness of the sandstones containing pore-lining chlorite and illite with relatively high grain-surface coverage. It opens a potential opportunity for a pore-to-core upscaling approach to predict reservoir quality for tight-gas sandstones.

Numerical Research on Energy Evolution in Granite under Different Confining Pressures Using Otsu’s Digital Image Processing and PFC2D

Research on energy accumulation and releasing in the rock plays a key role on revealing its failure mechanism. This paper establishes a microscopic structure model of granite using Otsu digital image processing (DIP) technology and particle flow code software (PFC2D). A series of numerical compression tests under different confining pressures were conducted to investigate the macro and micro characteristics of energy evolution in granite. The results showed that the energy evolution of granite is divided into three stages: stable accumulation, slow dissipation, and rapid release. With increasing confining pressure, the strain energy accumulation ratio decreased exponentially and the peak value of strain energy increased linearly. It was found that the energy accumulation speed in the pre-peak stage increased as a linear function, while the energy release speed in the post-peak stage decreased as an exponential function. In addition, the feldspar is the main microstructure which played a major part in accumulating energy in granite. However, the unit mineral energy of mica particles was bigger than that of feldspar and quartz. When subjected to increasing confining pressure, the feldspar’s total energy growth rate was fastest. Meanwhile, the mica’s unit energy growth rate was fastest.