Shale Pore Research Articles

Types and microstructures of pores in shales of the Ordovician Wulalike Formation at the western margin of the Ordos Basin, China

Shale samples from the Ordovician Wulalike Formation at the western margin of the Ordos Basin are studied to define the types, microstructures and connectivity of pores as well as the relationships between the pore structures and gas content of the samples by using experimental techniques such as high-resolution field emission scanning electron microscopy (FESEM), mercury injection capillary pressure (MICP), low-temperature nitrogen adsorption (LTNA), CO2 adsorption, and focused ion beam scanning electron microscopy (FIB-SEM). The results show that the shale has 10 different lithofacies, typical mixed sedimentary characteristics, and poorly developed pores. The reservoir space mainly consists of intercrystalline pores, dissolution pores, intergranular pores, and micro-fissures, with organic pores occasionally visible. The pore size is mostly within 0.4–250 nm range but dominated by micropores and mesopores less than 20 nm, with pore numbers peaking at pore sizes of 0.5 nm, 0.6 nm, 0.82 nm, 3 nm, and 10 nm, respectively. The pores are poorly connected and macropores are rarely seen, which may explain the low porosity and low permeability of the samples. Samples with high content of organic matter and felsic minerals are potential reservoirs for oil and gas with their favorable physical properties and high connectivity. The pores less than 5 nm contribute significantly to the specific surface area and serve as important storage space for adsorbed gas.

Accumulation Conditions and Pattern of Tight Oil in the Lower Submember of the Fourth Member of the Shahejie Formation in the Damintun Sag, Bohai Bay Basin

To determine the accumulation conditions and pattern of tight oil in oil shales in the Damintun Sag, Bohai Bay Basin, this study investigated the basic geological conditions of the source rocks and reservoirs in the sag using methods such as organic carbon analysis, whole-rock XRD analysis, and field emission scanning electron microscopy. The results show that: (1) The high-quality source rocks in the lower submember of the fourth member of the Shahejie Formation (E2S42) in the Damintun Sag have high organic matter abundance, favorable organic matter types, high hydrocarbon expulsion efficiency, and high fluidity. Therefore, they provide sufficient oil sources for tight oil accumulation.; (2) During the burial of organic-rich shales, the thermal degradation of organic matter produces large amounts of organic acids, which can dissolve carbonate minerals. In this way, secondary pores are formed.; (3) The special microscopic pore structure that connects fractures to pores is the key to the enrichment of tight oil is a key factor for the high oil saturation of pores in oil shales; (4) The breakthrough pressure (up to 100 MPa) and specific surface area of dolomitic mudstones in the E2S42 submember are significantly higher than those in other horizons. As a result, the dolomitic mudstones can effectively seal the underlying tight reservoirs; (5) Compared with the tight oil in tight sandstones, the tight oil in the oil shales in the study area has significantly superior geological conditions for reservoir formation, such as the favorable arrangement of hydrocarbon expulsion channels, low filling resistance, and the presence of reservoir spaces.

EVALUATION OF SHALE GAS EXPLORATION BY MICROSTRUCTURE BEHAVIOR AND SHALE PERMEABILITY BASED ON FRACTAL THEORY AND UNDER MULTI-FIELD EFFECTS

The key to shale gas exploration is the characterization of gas migration under the combination of multiple factors. To address the long-standing energy challenge of rapidly and accurately quantifying the behavior of natural fractures and matrix pores in shale at an engineering scale in interaction with gas migration. This study proposes an interdisciplinary model for shale gas extraction by adopting fractal theory. Five innovative microstructural parameters are developed to characterize the size and scale of natural matrix pores/fractures in shale, so as to investigate the contributions of fractal distributed pores and fractal power-law distributed fractures to shale gas extraction. The present results of the proposed model are consistent with the exploitation state of the UK Bowland Shale #114 well. The evolution of the shale microstructure will lead to changes in gas migration behavior throughout the reservoir and in turn affect shale stress, temperature and gas adsorption–desorption effect, and finally have a significant impact on permeability. It is found that in the present analysis of the entire Bowland Shale, the overall permeability changes by 10.8% with the evolution of fractal distributed pores and by 41.3% with the evolution of fractal power-law fractures. This work provides a new approach for rapidly exploring the behavior of shale fractures and matrix pores at engineering scales. This work also offers a new and practical baseline for shale gas extraction assessment and fossil energy management.

Effects of Supercritical CO2 Injection on the Shale Pore Structures and Mass Transport Rates

Characterizing the pore structures and transport properties of low-permeability shales is critical for evaluating these formations as potential seals or storage sites for geological CO2 sequestration. Here, we use low-pressure gas adsorption in conjunction with nuclear magnetic resonance (NMR) to characterize the pore-size distribution of shales before and after injection of supercritical CO2. Nitrogen gas was used as the detecting phase for the adsorption experiments and pentane liquid was used for the NMR experiments. We also performed time-resolved NMR and gravimetric microbalance measurements to observe mass transport during desaturation. We use these data to estimate the self-diffusion coefficient of pentane and changes in the saturation state of the pore network. We analyzed samples with a range of compositions from the Wolfcamp shale before and after exposure to supercritical CO2 for 3 days. Integrating the gas adsorption and NMR data shows how supercritical CO2 injection alters the pore-size distribution for pore sizes <1 nm to 1 mm. Our results provide insights on how the pore structure and mass transport properties of different shale lithologies may evolve during storage of supercritical CO2.

Study on the pore structure and fractal characteristics on shale rock and isolated organic matter in lacustrine shales from the Changling fault depression in the Songliao Basin, China

The stock of shale gas in the Shahezi shale reservoir in the Changling fault depression, Songliao Basin, is believed to be worth exploring. In-depth study on the pore structure and fractal characterization of organic matter (OM) can help better understand the pore system of shale reservoir, which has implications for the exploration of lacustrine shale. To demonstrate the nanoscale pore structure and irregularity of the isolated OM, we collected a large number of samples and then conducted a series of laboratory experiments, such as X-ray diffraction, scanning electron microscopy, and [Formula: see text] and [Formula: see text] adsorption experiments to determine the pore structure parameters and reveal their heterogeneity according to Frenkel-Halsey-Hill theory. As suggested by the experimental results, the pore size distribution curve of the [Formula: see text] and [Formula: see text] exhibits similar pore peaks, whereas the trends of change in bulk shale and isolated OM samples are basically paralleled. The pore volume of the isolated OM ranges between 0.034 and [Formula: see text], which is approximately 0.90–3.06 times that of bulk shale samples. As for the fractal dimensions [Formula: see text] (2.594 on average) and [Formula: see text] (2.657 on average) of bulk shale, they are larger as compared with isolated OM, indicating that inorganic minerals can make a significant difference in the heterogeneity of shale pores. The fractal dimensions ([Formula: see text] and [Formula: see text]) of bulk shales indicate a close correlation with the parameters of pore structure, whereas there is no significant correlation observed between the dimensions of isolated OM and its parameters. In addition, thermal maturity and solid bitumen have only limited impact on the OM pore structure of isolated OM samples. Then, we conducted further research to reveal that the insoluble OM macerals derived from terrestrial higher plants can be used to explain the difference in pore structure and heterogeneity between isolated OM samples. Therefore, we arrived at the conclusion that the composition of macerals depends on the exact pore structure and fractal characteristics of isolated OM samples with similarity in thermal maturity.

Investigation of induced change in pore structure by the reaction of shale with fracturing fluid

To study the change in the shale pore structure during the reaction between shale and fracturing fluid, a soaking experiment was conducted on shale from the Longmaxi Formation in the Sichuan Basin, China. The changes in the pore structure before and after the experiment were analyzed. The results show that the number of mesopores, macropores, and microfractures increased after the shale reacted with fracturing fluid. The analysis reveals that the dissolution of salts is the main reason for the change in the shale's pore structure. In addition, the dissolution of salt promotes the hydration of clay minerals and changes the pH of the fracturing fluid to produce 500–1000 nm corrosion pores.

Impacts of different matrix components on multi-scale pore structure and reservoir capacity: Insights from the Jurassic Da’anzhai member in the Yuanba area, Sichuan Basin

Pore structure is an important parameter for the gas-bearing evaluation of lacustrine shale. However, the characteristics and influencing factors of pore structure and the shale reservoir capacity remain unclear due to the heterogeneity and complexity of shale reservoirs. This study focused on Da’anzhai shale to research the pore structure characteristics using a combination of field emission scanning electron microscopy, low pressure gas adsorption, and mercury intrusion porosimetry. The effects of different matrix components on multi-scale pore structure and the reservoir capacity are discussed. The results show that the total organic carbon (TOC) and vitrinite reflectance (Ro) are 0.7%–1.68% (average of 0.98%) and 1.42%, respectively. The organic macerals are vitrinite, inertinite, and solid bitumen, with the vitrinite predominating. The main minerals are clay minerals, quartz, and calcite. Matrix pores, including mineral-associated pores and organic matter pores, and microfractures jointly constitute the pore system of the Da’anzhai shale. Clay-mineral pores dominate, followed by organic matter pores. Pore sizes mainly range from 0.3 nm to 20 nm, reaching up to 10μm. Mesopore volumes are dominant, followed by micropores and macropores. The pore specific surface areas are 12.76–21.24 m2/g (average of 17.43 m2/g), of which micropores and mesopores account for 48.7% and 51.3%, respectively. TOC, organic macerals, and clay minerals are the controlling factors of the pore structure. In contrast, the impacts of Ro and the framework mineral content on shale pore structure are relatively weak. Da’anzhai shale has sufficient pore volume and specific surface area but the shale is dominated by clay mineral-associated large-size pores and microfractures with few organic micropores, resulting in limited adsorption capacity for the shale. A greater understanding of the pore formation mechanism of shale reservoirs is obtained through this study, which is of great significance for gas-bearing evaluations.

Simulation of water self-imbibition in nanometer throat-pore structure filled with oil

Water self-imbibition potentially serves as a driving force for shale oil recovery due to the sizeable capillary pressure in the nanometer pores of shale. Understanding the water self-imbibition mechanism at the nanometer pore-scale helps promote shale oil recovery. In this work, we performed the direct numerical simulation using the phase-field model to study water self-imbibition dynamics in the nanometer throat-pore structure filled with oil. The effects of throat/pore morphologies (throat size, coordination number, and pore shape) and fluid properties (interfacial tension, contact angle, and viscosity) on the water imbibition dynamics were studied in detail. We distinguished two types of imbibition processes and found that the throat/pore morphologies significantly affect the final oil displacement ratio. Meanwhile, the interfacial tension and water contact angle mainly influence the imbibition rate (i.e., the oil displacement rate), while the water viscosity influences both the imbibition rate and the final oil displacement ratio. A qualitative correlation of the imbibition time and fluid properties is deduced by comparing classical theory predictions and simulation results. The quantitative deviations caused by local confinement are delineated as well.

Impact of cyclic liquid nitrogen treatment on the pore and fracture structures of shale at different scales and its effect on permeability enhancement

The ultra-low permeability of shale reservoirs limits shale gas production. Liquid nitrogen (LN2) fracturing is a promising reservoir stimulation technology that has attracted considerable attention in the unconventional natural gas industry. However, the evolution of pore and fracture structures in shale subjected to cyclic LN2 treatment and its effect on permeability enhancement have not been fully elucidated, especially with the presence of pore or fracture water. In this study, the physical response of Longmaxi Formation shale to cyclic LN2 treatment was carefully investigated. Scanning electron microscopy (SEM) and computed tomography (CT) were used to investigate the alteration of the pore and fracture structures in water-saturated and dry shale samples at different scales before and after cyclic LN2 treatment. Based on digital image processing and the fractal theory of pore structure, the variation in pore and fracture structures in shale matrices at the microscale was quantitatively analyzed. The experimental results indicate that cyclic LN2 treatment can effectively enhance pore volume and induce microfracture initiation and propagation. These phenomena were more significant in the water-saturated shale samples. Different correlations between the areal porosity of matrices and overall porosity/permeability in dry and water-saturated samples indicate that thermal stress and frost-heave force have different effects on shale pore and fracture structures. Owing to the different generation mechanisms, thermal stress was mostly applied to shale matrices, whereas frost-heavy force only existed near water-rich minerals, which were mainly clay in shale natural fractures. These two driving forces also had different responses to cyclic LN2 treatment. Because the new generated pores and microfractures have increased the distance among mineral particles, thermal stress gradually decreased with increasing LN2 treatment cycles. However, frost-heavy force could still induce macro-fractures along the bedding face of shale samples after several LN2 treatment cycles, owing to water migration during water-ice phase change.

Geological controls on the pore system of lacustrine unconventional shale reservoirs: The Triassic Chang 7 member in the Ordos Basin, China

The complex pore system of lacustrine organic-rich shales poses challenges to the assessment of shale oil and gas resources. To better understand the pore system and its controls of the lacustrine unconventional shale reservoirs, 12 oil mature shale samples with varying organic matter contents from the Triassic Chang 7 Member in the Ordos Basin were analyzed using elemental, petrographical, petrophysical, and fractal methods. The results showed that macro- and meso-pores are the main pore space in the lacustrine Chang 7 Shale. Pore diameter peaks are dominantly distributed around 0.7 and 37 nm. The intragranular pores in clays and intergranular pores between organic matter (OM) and minerals are well developed. Whereas the pores in OM are rarely observed. The fractal dimensions of the lacustrine shale's pore structure suggest that D2 is better than D1 in reflecting pore heterogeneity. OM shows negative effects on the total pore volume, while quartz promotes the development of macro- and meso-pores. Total pore volume shows an oscillation with clay mineral content (pore volume increases first up to 45% clay content and decreases thereafter). Both quartz and clays show negative effects on the heterogeneity of meso- and macro-pores, and positive effects on the micro-pore heterogeneity. A new critical maturity point of 0.8% Ro is observed for the change of the pore system of the lacustrine shale reservoirs during the early oil window. Both the fractal dimensions (D1 and D2) and pore structure proxies show the inverse trends before and after this critical value. These observations are probably controlled by the compaction processes coupled with hydrocarbon generation and expulsion, which helps better understand the pore system evolution in the lacustrine shales. The positive role of quartz, 45% clay mineral content and 0.8% Ro as the critical points contribute to the high-quality reservoir or sweet spot identification in lacustrine shales.

Insight into Water Occurrence and Pore Size Distribution by Nuclear Magnetic Resonance in Marine Shale Reservoirs, Southern China

The exploitation of shale gas has been a hot topic, and the understanding of water occurrence and pore size distribution in shale reservoirs plays a crucial role in efficiently developing this type of resource. In this study, we target moderate-shallow marine shales from the Lower Cambrian Niutitang Formation and deep marine shales from the Lower Silurian Longmaxi Formation. Low-field nuclear magnetic resonance is applied to investigate the water occurrence and pore size distribution together with complementary experiments including mineral component analysis, scanning electron microscopy (SEM), contact angle measurement, and high-speed centrifugation. The SEM analysis shows that the moderate-shallow shales have some inorganic nanopores and cracks but almost no organic pores, while the deep shales have abundant organic nanopores. The average movable water saturation of moderate-shallow and deep shales is only 15.5% and 15.2%, respectively. It implies that the high irreducible water saturation is a typical characteristic for both marine shales, which is ascribed to the strong capillary force of the nanopore inhibiting water flowing out of the pore space. By means of high-speed centrifugation and contact angle measurement, the converting factor of the T2 value into pore size was obtained. The average pore size in moderate-shallow shales is 19.4, 21.8, and 26.8 nm, respectively, while the average pore size in deep shales is 109, 57, and 68.4 nm, respectively, for the tested samples. This indicates that organic pores in deep shales contribute to the development of abundant nanopores. This study brings insight into characterizing the water occurrence and pore size distribution of moderate-shallow and deep marine shales.

Effect of Supercritical Carbon Dioxide Fracturing on Shale Pore Structure

Supercritical carbon dioxide (SC-CO2) fracturing technology has the potential for shale reservoir stimulation. Most studies have predominantly focused on the fracture morphology of shales after SC-CO2 fracturing, while the alterations in shale pore structure have rarely been investigated. Here, CO2 adsorption, liquid nitrogen (N2) adsorption, and mercury intrusion porosimetry (MIP) tests were used to quantitatively characterize the changes in the pore shape, volume, and area as well as fractal characteristics of shales fractured by water and SC-CO2. The results show that the changes in micro-, meso-, and macropores are controlled by the injection pressure, axial-confining pressure, and infiltration range of SC-CO2 and water. However, both hydraulic fracturing and SC-CO2 fracturing do not alter the dominance of the plate-shaped and slit-type pores in the shales. For samples away from the induced fracture, the extent of SC-CO2 infiltration is greater than that of water, which is documented by the increase in total CO2 adsorption, cumulative intrusion, incremental/cumulative pore volumes of macropores, and porosity. After hydraulic/SC-CO2 fracturing, the proportions of micropores and mesopores reduce sharply, while the proportion of macropores increases significantly, reaching above 70%. Both hydraulic/SC-CO2 fracturing operations result in more regular pore structures and smoother pore surfaces for meso- and macropores near the induced fractures. However, by comparing the average fractal dimension of the samples near the induced fractures after hydraulic/SC-CO2 fracturing, it is found that the treatment of SC-CO2 makes the mesopores structure more complex and the pore surface of mesopores and macropores rougher.

Occurrence and Migration Mechanisms of Methane in Marine Shale Reservoirs

The occurrence mechanism of methane is very important as evaluating the gas-bearing properties of marine shale reservoirs, and the evaluation of the development effect of shale gas wells need to focus on the migration mechanism of methane. In this study, LTNA technology and NMR technology were used to analyze the pores and methane of shale. The results show that inorganic pores have better connectivity, larger pore size, and micro–nano cracks between pores compared to organic pores. Most of the pores in shale are micropores and mesopores, which provide most of the specific surface area, but the contribution of macropores to pore volume cannot be ignored. Adsorbed gas volume depends on the pore surface area and gas pressure, while free gas volume depends on pore volume and gas pressure. The pore structure of micropores and mesopores is complex, and the specific surface area is large. The dispersion force between pore surface molecules and methane molecules is firm, which makes the pore wall an ideal enrichment space for adsorbed gas. Macropores have larger pore volumes and can store more free gas. In the process of gas well development, free gas is first discharged from pores under the action of the pressure gradient. As the pore pressure is lower than the critical desorption pressure, adsorbed gas begins to desorb in large quantities. It should be noted that the desorption process of adsorbed gas is slow and persistent, which makes it impossible for gas wells to achieve higher recovery in a shorter production cycle. Therefore, improving the recovery rate of adsorbed gas is the key to future research on shale gas development effects. This study is helpful in clarifying the occurrence and migration mechanism of methane in marine shale reservoirs and guiding the development of gas wells.

Structural deformation of shale pores in the fold-thrust belt: The Wufeng-Longmaxi shale in the Anchang Syncline of Central Yangtze Block

The gas-rich Wufeng-Longmaxi shale has been intensely deformed within the fold-thrust belt of the Yangtze Block. To better understand the impact of structural deformation on the shale pore system, this paper systematically investigated the matrix components, porosity and pore structures in core samples from theWufeng-Longmaxi shale, newly collected from various structural domains in the first commercial shale gas field of the Central Yangtze Block, the Anchang Syncline. The shale porosity generally showed a positive relationship with total organic carbon content. Nevertheless, even at a constant total organic carbon content, the shale porosity decreased from the syncline limb to the syncline hinge zone and with a decreasing interlimb angle in the syncline hinge zone, which aligned with the structural deformation strain during folding. The artificial axial compression of shale samples also confirmed that the decrease in shale porosity was stronger at an elevated axial compression stress and was relatively higher in samples with higher total organic carbon content. The organic pore size decreased with higher structural deformation strain, but the aspect ratio of the pore shape increased. Even quartz failed to resist the effective stress under the intensive structural deformation, changing the correlation between porosity and quartz from positive to negative. In contrast, pore spaces generated by the slipping between clay flakes under intensive deformation accounted for a positive relationship between clay content and bulk porosity. Considering the shale porosity reduction caused by the intensive structural deformation of shale pores, the Wufeng-Longmaxi shale, that is rich in fracture networks between roof and floor layers, may still be an excellent exploration target in the fold-thrust belt of the Yangtze Block. Cited as: Guo, X., Liu, R., Xu, S., Feng, B., Wen, T., Zhang, T. Structural deformation of shale pores in the fold-thrust belt: The Wufeng-Longmaxi shale in the Anchang Syncline of Central Yangtze Block. Advances in Geo-Energy Research, 2022, 6(6): 515-530. https://doi.org/10.46690/ager.2022.06.08

The Phase Behaviors of Confined Shale Fluids Considering Adsorption Effect

AbstractThe molecular adsorption in nano-scale shale pores results in the reduction of effective pore volume and further strengthens the confinement effect. This study aims at examining the adsorption effect coupled with confinement effects on phase behaviors of shale fluids. First, a modified extended Langmuir formula was developed to calculate the adsorption amount for a multi-component shale mixture. A modified cubic Peng–Robinson equation of state was proposed, and the occupied volume by the adsorbed phase was taken into account. The saturation pressures and fluid properties under the confinement effects and adsorption isotherms were examined. In order to examine the change of phase properties during a gas injection process in a shale condensate reservoir, we gradually increase the mole fractions of N2 or CO2 in shale condensate mixtures by coupling with confinement effects. We found that the thickness of the adsorption film reduces the effective pore throat, leading to intensified confinement effects and smaller bubble point pressures. When the gas adsorption layer is considered, a more significant decrease in density and viscosity is observed. The critical pressure of the condensate fluids increases and the critical temperature decreases with the continuous N2 injection. Contrary to N2 injection, the critical pressure decreases and the critical temperature moves upwards with CO2 injection. For condensate that accumulates in nano-pores (e.g., r ≤ 6 nm), the condensate fluid always exists in gas status during the gas injection and the subsequent production processes.

New Model of Oil Migration in Shale Nanopores Considering Microscopic Deformation Induced by Stress and Pore Pressure

During the exploitation of shale oil, the stress and pore pressure of the reservoir generally change, resulting in the deformation of the shale pore structure, which affects the oil migration in shale nanopores. Establishing an oil migration model and a permeability model that take into account the effects of reservoir stress and pore pressure is important for the numerical simulation shale oil development. In this paper, a new oil migration model in inorganic (IM) and organic (OM) nanopores of shale and a new shale apparent liquid permeability (ALP) model that consider the effects of reservoir stress and pore pressure are established. The molecular dynamics simulation data and experimental data are used to verify the validity of the proposed model. The results show that the model can reasonably describe the transport process of oil in IM and OM nanopores and calculate the ALP. The flow enhancement factor in inorganic and organic nanopores and the shale ALP are negatively correlated with the mean compressive stress and positively correlated with the pore pressure. When the shale bulk modulus is small, the flow enhancement factor and ALP are more sensitive to the stress and pore pressure. Moreover, the effects of stress or pore pressure on shale microscopic flow capacity which was evaluated by the flow enhancement factor and macroscopic flow capacity which was evaluated by ALP are consistent.

Study on the distribution and connectivity of organic matter pores in Longmaxi shale based on 2D and 3D FIB‐SEM

AbstractKnowledge of shale pore structure characteristics is crucial to understand gas storage and seepage mechanisms. Organic matter (OM) pores are considered the most important pore type in shale, and one of the currently significant research questions focuses on the spatial distribution and connectivity of OM pores. To answer this question, typical OM‐rich siliceous shale samples from the Lower Silurian Longmaxi Formation were comprehensively investigated using focused ion beam scanning electron microscopy. A three‐dimensional model of the OM‐rich region of interest was segmented and reconstructed based on numerous two‐dimensional slices. The types of OM were found to control the development of organic pores, and OM pores including honeycomb‐shaped pores, spongy‐shaped pores, and slit‐like irregular pores are mainly formed in the pyrobitumen. The pore structure parameters of the OM‐rich ROI revealed that the pore size distribution of honeycomb‐shaped OM pores formed in the pyrobitumen was mainly distributed in the range of 10–50 and 80–100 nm, while the throat equivalent diameter distribution demonstrated a unimodal curve with the main peak located at approximately 30 nm. Pore connectivity analysis further indicated that pyrobitumen also contained several isolated nano‐pores, and pores with diameters smaller than 40 nm were poorly connected. Furthermore, permeability simulation revealed clear discrepancies in different directions owing to the heterogeneity of the OM pores. These findings provide experimental evidence for the assessment of shale gas resources and their development potential.

Pore Characteristics and Influencing Factors of Marine and Lacustrine Shale in the Eastern Sichuan Basin, China

Although almost all the shale gas in China is exploited from marine shale (Wufeng–Longmaxi Formation) in Sichuan Basin and several prolific wells, it has also been obtained in Jurassic lacustrine shale. However, the reservoir conditions of the lacustrine shale are not well understood, which has impeded a breakthrough regarding lacustrine shale gas in Sichuan Basin. To probe the reservoir conditions of the lacustrine shale in Sichuan Basin, we take the Wufeng–Longmaxi shale and Dongyuemiao shale sampled from wells and outcrops as examples. A series of experiments were conducted, including TOC, XRD, FE-SEM, N2 adsorption, Micro-CT, vitrinite reflectance and bitumen reflectance. The results show that the pores in marine shale are mainly composed of organic-matter-hosted pores (OM pores). However, in the lacustrine shale, the pores are mainly composed of dissolution pores and intergranular pores. The marine shale is characterized by small-caliber and large-volume pores in which cluster pores are levitated in the shale as kites and connected by past channels. However, in the lacustrine shale, the cluster pores and the past channels are mainly arranged according to the flow channels in the vertical direction. The arrangement of the pores in the marine shale is obviously deformed by compaction. The lacustrine shale is characterized by under-compaction. It can be deduced that the sweet spots for lacustrine shale gas are likely located at the areas characterized by under-compaction resulting from fluid pressure conducted upward, such as the hinge zone of syncline or the core of anticline overlap on the gas reservoirs.

Shale Microstructure Characteristics under the Action of Supercritical Carbon Dioxide (Sc-CO2)

Supercritical carbon dioxide (SC-CO2) is suitable to extract low-polar organics and to assist in the dissolution of pores and fractures in shale. In this work, we investigate the effect of temperature on the structure of five shale samples via high pressure reaction assisted with SC-CO2. Shale samples were analyzed using X-ray diffraction, field emission scanning electron microscopy, and ImageJ software. Due to the extraction of CO2, after Sc-CO2 treatment, carbonate and clay content decreased, while quartz and plagioclase increased slightly, which improved gas and oil flow in microscopic pores and shale cracks. Shale samples showed an increase in surface fracture area as experimental temperature increased. Since Sc-CO2 fluid density and solubility increase with temperature, more organics can be extracted from shale pores and fractures, resulting in newly formed pores and fractures. As a result, the threshold temperature for shale high-temperature Sc-CO2 cracking was confirmed to be 400 °C, and the fracture area increased by more than 45% at this temperature. Based on the findings of this study, Sc-CO2 technology can be used to potentially recover low-maturity shale oil efficiently.

Modified Laponite synthesized with special wettability as a multifunctional additive in oil-based drilling fluids

In this study, Modified Laponite with lyophobic wettability was synthesized as a multi-functional additive to solve instability of emulsions, rheological deterioration, and wellbore instability of oil-based drilling fluid. Properties and its influence on emulsion stability, rheological behavior and plugging property were investigated. The results showed that Modified Laponite was in a shape of sheet layer and had micro-meter length and nano-meter thickness which was in match with the diameter of shale pores. And it was proved to be resistant to approximately 450 °C by thermogravimetric analysis measurement. Mechanism analysis revealed that Modified Laponite could stable the oil-based drilling fluid through four sections. Firstly, Modified Laponite with lyophobic wettability could promote its existence in the interface of brine/mineral oil. Secondly, Modified Laponite could decrease the interfacial tension from 32.8 to 13.5 mN/m to maintain the emulsion particles in a small size for a long time then to ensure the emulsion stability. Thirdly, Modified Laponite could enhance the shear-thinning rheological behavior which could maintain the rheological stability and enhance the sedimentation stability in ultra-deep reservoirs with high temperature and high pressure conditions. Fourthly, Modified Laponite could plug the shale pores effectively by preventing pressure transmission and enhancing shale compressive strength, and decrease the filtration volume of oil-based drilling fluid from 7.4 mL to 2.6 mL after aging at 180 °C for 16 h to stable the wellbore. Therefore, Modified Laponite was a multifunctional additive to stable the emulsion, enhance the rheological stability and strength the wellbore stability, which could ensure the application of oil-based drilling fluid in drilling process of ultra-deep reservoirs.