Variation Of Pore Structure Research Articles

Petrophysics and sediment variability in a mixed alluvial to lacustrine carbonate system (Miocene, Madrid Basin, Central Spain)

AbstractThis study evaluates variations in petrophysical properties within a mixed alluvial to lacustrine carbonate system (Miocene, Madrid Basin, Central Spain). The transition from alluvial environments to lake margins settings displays a shift from alluvial, siliciclastic red sandstones and mudstones to palustrine–lacustrine mudstones to packstones. Fluctuations in lake‐water level enabled land plants to occupy the lake margins during periods of low lake levels. The palustrine carbonates include features like pseudo‐microkarst, nodular and mottled limestones; the lacustrine deposits include enlarged root cavities, desiccation cracks and channel bodies. Scarce fresh water biota comprises charophytes, gastropods and ostracods. The sediments possess high natural, irregular varying, gamma‐ray values at the alluvial–lacustrine transition, and low, but constant values at full lacustrine sites. Acoustic properties agree with lithological variations within individual facies. Porosity is the most important parameter influencing P‐wave and S‐wave velocities. The scatter in the velocity–porosity relationship links to the porosity type; macro‐porosity or microporosity. The wide range of pore types and pore sizes results in a weak porosity to permeability relationship for the carbonate‐dominated rocks with low permeability for microporous and high permeability for macro‐porous carbonates. The sandstones (probably only inhibiting interparticle porosity), and to a lesser extent the sandy wackestones to packstones, show quite a strong relationship between porosity and permeability. Elastic properties of mixed alluvial–lacustrine deposits (this study) and marine deposits (literature data) overlap as variations in pore structures and porosity values are similar. Only 16% of the marine and lacustrine carbonate sediments display Equivalent Pore Aspect Ratio values above 0.2. In travertine deposits, 83% of the samples exceed Equivalent Pore Aspect Ratio values above 0.2, which highlights that travertine deposits are dominated by larger Equivalent Pore Aspect Ratios compared to lacustrine and marine carbonate deposits. Travertines display other rock frameworks with different pore types, pore distribution and amount of porosity.

Variation of Organic Pore Structure With Maceral Types in the Longmaxi Shale, Sichuan Basin

Organic matter (OM), composed of various macerals, has a strong influence on the enrichment of shale gas. Nevertheless, the connection between OM-hosted pore structure and maceral type is not yet fully understood because of the difficulty to identify the maceral types by traditional scanning electron microscope (SEM). Using a combination of the reflected light microscopy, focused ion beam SEM (FIB-SEM), and Raman spectrum, three maceral types, including alginite, graptolite, and solid bitumen, are identified in the Longmaxi Shale of the Sichuan Basin. The alginate is characterized by the linear arrangement of OM-hosted pores due to the inherited biological structure of benthic algae. Pores in the structureless solid bitumen are randomly distributed with the highest abundance. The graptolite containing pore rarely is unfavorable for the pore generation but can be a good proxy for thermal maturity. Variation in thermal maturity levels accounts for the change of total pore volume in a given marcel type in the Longmaxi Shale obtained from different shale gas fields.

Effect of the Mechanical Load on the Carbonation of Concrete: A Review of the Underlying Mechanisms, Test Methods, and Results.

As one of the major causes of concrete deterioration, the carbonation of concrete has been widely investigated over recent decades. In recent years, the effect of mechanical load on carbonation has started to attract more attention. The load-induced variations in crack pattern and pore structure have a significant influence on CO2 transport which determines the carbonation rate. With different types of load, the number, orientation, and position of the induced cracks can be different, which will lead to different carbonation patterns. In this review paper, the carbonation in cracked and stress-damaged concrete is discussed first. Then, literature about the effect of sustained load during carbonation is compared in terms of load type and load level. Finally, the advantages and disadvantages of possible test methods for investigating the effect of sustained load on carbonation are discussed with respect to loading devices, load compensation, and specimen size.

Variation of pore structure in Zhundong coal particle with stepped K2CO3 loading during supercritical water gasification

Supercritical water gasification (SCWG) is a promising technology for coal utilization and has drawn lots of attention. K2CO3 is an outstanding catalyst for coal gasification and can significantly improve pore structure. The evolution of pore structure in coal particles affects heat and mass transfer efficiency, but little research has investigated the effect of K2CO3 on gasification results from the perspective of pore structure. Hence, this study revealed the variation of pore structure of coal chars with stepped K2CO3 loading during SCWG of coal by conducting experiment in a high-capacity autoclave reactor. Gas products and coal chars were characterized by GC, SEM, and BET to obtain the gasification result, surface morphology, and pore structure characteristics of coal chars. The results show that K2CO3 improved gasification efficiency, gas percentage, and gas yield of H2 and CO2. The time when K2CO3 changes pore structure is during the gasification process rather than before gasification. Macropores are dominant in coal chars. 0 wt%-8 wt% K2CO3 favors the formation of mesopores and macropores, while 10 wt% K2CO3 blocks the pore structure. 0 wt%-4 wt% K2CO3 loading makes a pore structure of the ink-bottle type, and 6 wt%-10 wt% K2CO3 loading makes the narrow-slit pore structure. When K2CO3 increases from 0 wt% to 8 wt%, production rate of C-containing gases has a good linear relationship with specific surface area, and pore volume also has a good linear relationship with carbon gasification efficiency. 10 wt% K2CO3 destroys the linear relationship by the blocking effect.

Triple combination of natural microbial action, etching, and gas foaming to synthesize hierarchical porous carbon for efficient adsorption of VOCs

Fungi residue, vinasse, and biogas residue differ from general biomass waste due to natural microbial action. Microbial fermentation helps create natural channels for the permeation of activators and produces proteins for natural nitrogen doping. Inspired by these advantages on porous carbon synthesis, this study adopted dual activators of KOH and KHCO3 to synthesize porous carbon with different pore ratios for efficient adsorption of volatile organic compounds (VOCs). The fungi residue possessed the least lignin due to the most severe microbial action, contributing to the best pore structures after activation. The etching effect from potassium compounds and gas foaming from the carbonate decomposition contributed to creating hierarchical porous carbon with ultra-high surface area, ca. 1536.8–2326.5 m2/g. However, KHCO3 addition also caused nitrogen erosion, such that lower adsorption capacity was attained even with a higher surface area when the mass ratio of KOH/KHCO3 decreased from 2.5:0.5 to 2:1. The maximum adsorption capacities of chlorobenzene (CB) and benzene (PhH) reached 594.0 and 394.3 mg/g, respectively. Pore structure variations after adsorption were evaluated by freeze treatment to discover the adsorption mechanism. The surface area after CB and PhH adsorption decreased 40.3% and 34.5%, respectively. Most of the mesopores might transform into micropores due to the mono/multilayer stacking of adsorbates. The VOC adsorption kinetics were simulated by the Pseudo-first- and -second-order models and Y–N model. This paper provides a new approach for high-value biomass waste utilization after microbial action to synthesize efficient adsorbents for VOCs.

Laboratory investigation on pore characteristics of coals with consideration of various tectonic deformations

Coal structure is highly related to paleo-stress variation and the pore structure of coals can be modified through tectonic activities. This study characterizes the pore structure variations for various coal samples with different tectonic deformation intensities. The low temperature N2 and pressure CO2 adsorption analyses were conducted to quantify the pore structures, and high pressure CH4 adsorption measurements were carried out for the sorption analysis. The pores were classified into ultramicropores (<2 nm), micropores (2–10 nm), mesopores (10–100 nm) and macropores (>100 nm). The results show that with the increase of coal deformation intensity, the proportion of pore size of 50–300 nm decreased, showing that more macropores and mesopores were deformed to the smaller pores (<50 nm). Micropores (2–10 nm) in granulated and mylonitic coals obviously increased. The predominant ultramicropore in four kinds of coal structures were distributed at the range of 0.45–0.6 nm and 0.80–1.0 nm. Tectonic coals formed more ultramicropores lower than 0.65 nm. The adsorption of CH4, N2 and CO2 increased as the coal deformation degree increased, following the order: mylonitic coal > granulated coal > cataclastic coal > intact coal. Fractal dimensions show that tectonic coals characterized the rougher pore surface (higher D1) and more homogeneous pore structures (lower D2), which led to the higher adsorption capacity. The development of micropores was positive with Langmuir VL, but the ultramicropore SSAs were significantly larger than the contribution to SSAs of micropores and mesopores. Thus, ultramicropore provided the main adsorption sites for CH4. The variations of adsorption potential increased with the coal deformation intensity increased, and the smaller the adsorption space volume was, the larger adsorption potential would be, illustrating that adsorption in micropores was higher than mesopores and macropores. Tectonic coals have the higher reduced rate of cumulative surface free energy, which shows that tectonic damages promote the adsorption potential and surface free energy in coals.

Pore-Structure Characterization of a Complex Carbonate Reservoir in South Iraq Using Advanced Interpretation of NMR Logs

The Rumaila Field is in southeast Iraq and contains multiple reservoir intervals, including the Upper Cretaceous Mishrif carbonate reservoir, one of the major reservoirs in the world, that has been producing for more than 50 years. One of the key challenges in the Mishrif is to characterize the pore-structure distinction between primary and secondary porosity. The secondary porosity in the form of large pores, if present, dominates the petrophysical properties, especially permeability. Advanced logs, e.g., nuclear magnetic resonance (NMR) and image logs, can be used to understand the variations in pore structure, both qualitatively and quantitatively. In this paper, we focused primarily on four new wells with very comprehensive logging and coring programs. NMR logs were acquired using different tools and pulse sequences. This resulted in uncertainty in porosity and T2 distributions and, consequently, complications in the NMR interpretation. We observed two key issues: porosity deficit due to lack of polarization and T2 distribution truncation due to the low number of echoes. We used a single pore model to reproduce the NMR response in different pore sizes and fluid types for different pulse sequences. The results showed that the NMR response, especially in water-filled (water-based-mud filtrate) large pores, is sensitive to polarization time, echo spacing, and tool gradient strength. NMR log data confirmed the modeling results. We recommended an optimum pulse sequence and tool characteristics to fully capture the heterogeneous rock and fluid system in this carbonate reservoir. NMR logs, when available, were the primary tools to identify the large pores. We present a consistent workflow for NMR log analysis that was developed to identify and quantify large pores and extended to all wells in the field. We used advanced NMR interpretation techniques, e.g., factor analysis (NMR FA) (Jain et al., 2013), in a series of oil wells drilled with water-based mud. Using factor analysis, we identified a cutoff value of 847 ms for large pore volumes. In this manuscript, we also present an integration of laboratory measurements, e.g., NMR, mercury intrusion capillary pressure (MICP) data, whole-core CT scanning, and thin-section analysis, in our interpretation workflow. We also compared the large pore volume from image logs with NMR logs and other laboratory data and observed very consistent results. All the available information was integrated to build an “NMR-based” petrophysical model for porosity, rock type, permeability, and saturation determination. The NMR-based model was very comparable with the classic flow zone indicator (FZI) rock typing. The results of this study were used to modify the NMR acquisition program in the field and to build a petrophysical model based on only NMR and image log measurements for carbonate reservoirs. In this paper, we will discuss NMR modeling and corresponding log data from various wells to confirm the results. Furthermore, we will present a novel interpretation workflow integrating laboratory measurements and log data, which led to the modification of the NMR acquisition program in the field and the creation of a data-driven petrophysical model based on only NMR and image log measurements for carbonate reservoirs.

Dissolution of Different Reservoir Rocks by Organic Acids in Laboratory Simulations: Implications for the Effect of Alteration on Deep Reservoirs

Organic acids are important agents in the alteration of deep reservoirs. It is difficult, however, to assess the impact of organic acid alteration on deep reservoirs because different dissolution processes may occur during diagenesis. This study simulated the dissolution of three different types of reservoir rocks by acetic acid in a closed system and compared the mineral and elemental composition, surface morphology, pore structure, and water chemistry variations of the initial and altered samples. The study demonstrated that both micrite and sucrosic dolostone are strongly dissolved, losing about 20%–30% of their initial rock sample weights. Observation under SEM showed that the limestone dissolved homogenously, whereas the dolostone showed honeycomb-like dissolution. Both carbonate samples showed the development of large voids, including holes and cavities of micrometer scale, but nanopores of various sizes were blocked. In contrast, lithic arkose was heterogeneously altered, losing a weight proportion of about 13% by dissolution of calcite cement. These micrometer-scale microfissures were developed, but those nanometer-scale pores just varied in a narrow range of sizes. The volume increase in all three reservoir types is mainly attributed to the dissolution of carbonate minerals. In deep reservoirs, in situ generated organic acids can enlarge existing cavities in carbonates and develop microfissures in sandstones. The microfissure porosity in sandstone is limited but can increase through other geological processes such as overpressure. More importantly, these acids can maintain the acidity of pore waters, inhibit the precipitation of dissolved minerals, and help to preserve reservoir porosity. Although temperature plays an insignificant role in laboratory simulations, it influences both the generation and destruction processes of organic acids in deep reservoirs on geologic time scales and, thus, warrants further attention. The results provide a basis for recognizing the typical patterns of organic acid dissolution on different reservoir rocks and further suggest the potential role of organic acids in the formation and preservation of secondary porosity in deeply buried reservoirs.

Reservoir operation-induced hydrodynamic disturbances affect the distributions of Cd, Cu, and Pb in the riparian soil of the water-level-fluctuation zone

This study aimed to relate the altitudinal and vertical distributions and occurrences of major heavy metals (HMs) to soil physicochemical properties in the riparian soils subject to periodic and intense wet–dry cycles Cadmium (Cd), copper (Cu), and lead (Pb) and their chemical fractions were investigated in the riparian soil (0–40 cm) of the upland and within the water-level-fluctuation zone (WLFZ) at two tributary backwaters of the Three Gorges Reservoir (TGR) in China. Selected soil basic properties and pore structures determining the distribution of HMs and their fractions were identified using correlation and redundancy analysis Cd showed moderate–strong contamination and prevailed in non-residual fraction (>73.2%) in the riparian soil of the tributary backwaters in the TGR. In contrast, Cu and Pb exhibited slight contamination and prevailing in residual fraction (>50.3%). Enrichment of HMs in the lower end relatively flat WLFZ where flow rate decreased abruptly and dramatically was observed. Cd exhibited the largest coefficient of variation (CV, up to 0.263) across soil profiles while Cu showed a relatively homogeneous profile distribution in the riparian soil. Soil organic matter (SOM), cation exchange capacity (CEC), and pH were identified to be the primary factors determining the distributions of Cd, Cu, Pb, and their chemical fractions, respectively, in the riparian soil with a high macroporosity. Nevertheless, soil porosity was the dominant factor (>65.0%) determining the distributions of all the three HMs and their fractions in the non-drainable micropore-dominated riparian soil Soil basic property and pore structure variations due to periodic hydrodynamic disturbances from reservoir operation are closely related to the redistribution of HMs in the riparian soil. The coupling of these two variations is strongly suggested to assist reliable evaluation of various contaminants, including but not limited to HMs, regarding their spatial redistributions and mobilization potentials in the vadose zone at larger scales in response to climate change.

Effects of ammonium ion and bentonite content on permeability of bentonite-clay mixture

Sanitary landfill is a common method for the treatment of municipal solid waste. The landfill leachate contains a large amount of NH4+, which will affect the physicochemical properties of bentonite, and subsequently change the anti-seepage function of the bentonite enhanced cutoff wall. The permeability tests were carried out on bentonite-clay mixed soil infiltrated by the ammonium salt solutions using a head pressure control permeameter. The results show that saturated permeability coefficient of the bentonite-clay mixture increases with the decrease of bentonite content and with the increase of NH4+ concentration. Moreover, the reasonable mass content of bentonite is about 10% if the studied bentonite-clay mixture are used in vertical cutoff wall. The scanning electron microscope (SEM) and the nuclear magnetic resonance (NMR) measurements were conducted on the bentonite-clay mixed soil samples with different bentonite contents, and samples infiltrated by the ammonium salt solutions with different concentrations, and afterwards the variations of the micro pore structure and pore-size distribution were obtained. Connecting the Donnan osmotic pressure with saturated permeability of bentonite-clay mixed soils, the effect mechanism of bentonite content and ammonium solution concentration on the permeability of the bentonite-clay mixture was analyzed.

Effect of supercritical CO2 extraction on CO2/CH4 competitive adsorption in Yanchang shale

Competitive adsorption characteristics of CO2/CH4 on shales before and after supercritical CO2 (ScCO2) extraction are closely related to CO2 injection in shale reservoir for CO2 sequestration and enhanced shale gas recovery (CS-ESGR). Using a self-developed PCTPro-type automatic high-pressure gas adsorption instrument, the adsorption behaviors of CH4, CO2 and their mixtures with different ratios were investigated on raw and ScCO2-treated (10 day/16 MPa/40 °C) shale collected from the Ordos Basin. The influences of ScCO2 extraction, pressure and CO2/CH4 cratios on adsorption capacity and selective adsorption factor of CO2/CH4 (SCO2/CH4) of shale have been analyzed. To interpret the experimental data, total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, low-pressure gases adsorption and mercury intrusion were performed to evaluate the variations of mineral compositions and pore structure of shale exposure to ScCO2. It is found that SCO2/CH4 and the adsorption capacity of CO2/CH4 with different ratios decreased after ScCO2 extraction and are mainly attributable to the decrease of TOC, carbonates, clay minerals and specific surface area in ScCO2-treated shale. The feasibility of CS-ESGR cannot be affected by ScCO2 due to the values of SCO2/CH4 are always greater than 2.5 after ScCO2 extraction. Additionally, SCO2/CH4 decreased initially and then tended to be stable with increasing pressure, and it decreased slightly with increasing CH4 concentration in gas mixtures, suggesting that the reservoir pressure should be lowered before the injection of CO2 into shale reservoir, and the efficiency and engineering cost of CS-ESGR should be considered comprehensively. This study provides a reference for future optimization design of CS-ESGR.

Characterization of the Pore Structure of Well Cement under Carbon Capture and Storage Conditions by an Image-Based Method with a Combination of Metal Intrusion.

To more quantitatively and subtly analyze effects of carbonation on the pore structure of well cement by supercritical CO2 under carbon capture and storage (CCS) conditions, a digital scanning electron microscopy-backscattered electron (SEM-BSE) image analysis with a combination of nontoxic low-melting point metal intrusion is used to characterize the exposed cements to humid supercritical CO2 for 10 and 20 days. The porous area fraction (PAF) and pore size distribution (PSD) profiles obtained by slicing operation are used to describe the pore structure variation along the corrosion direction in a two-dimensional (2D) plane. The results show that the image-based method with the combination of metal intrusion is an effective method for characterizing the layer structure of exposed cement and getting quantitative information about the pore structure. From the surface to the core, the main altered layers in exposed cement for 10 days include the partially leached layer, the carbonated layer, and the calcium hydroxide (CH)-dissolved layer. For the exposed cement for 20 days, the main altered layers include the porous leached layer, the partially leached layer, the carbonated layer, and the carbonated transition layer. The nonporous carbonated layer can effectively block the flow parallel to the corrosion direction, while the porous leached layer can facilitate the flow perpendicular to the corrosion direction. Findings from this study will provide valuable information for understanding the effects of carbonation on the pore structure of well cement under CCS conditions.

Investigation on the Structure and Fractal Characteristics of Nanopores in High-Rank Coal: Implications for the Methane Adsorption Capacity.

The structure and fractal characteristics of nanopores of high-rank coal were investigated using an approach that integrates N₂ adsorption and field emission scanning electron microscopy (FE-SEM). The results indicated that the high-rank coal of the Shanxi Formation has a complex pore-fracture network composed of organic matter pores, mineral-related pores, and microfractures. The pore type of high-rank coal tends to be complicated, and the main pore types are inkbottle pores and open pores, which are more conducive to methane enrichment. The Ro,max has a negative relationship with the total pore volume. In addition, the ash and inertinite contents show a positive correlation with the average pore size (APS), while the fixed carbon content exhibits a negative relationship with the APS. The pore structure of high-rank coal is controlled not only by the degree of metamorphism but also by coal composition, which leads to the variation in pore structure becoming more complicated. With the increase in coal metamorphism, high-rank coal with high amounts of fixed carbon content generally possesses a higher irregularity in pore structure. No obvious relationship was observed between D₂ and the coal components, which indicates that the pore structure, ash content, moisture content and other factors controlled by the metamorphism of coal have different effects on D₂ that lead to this inapparent relationship. A negative relationship exists between adsorption volume and D1, which indicates that the high irregularity of the pore structure is not conducive to methane absorption and that no obvious correlation exists between the adsorption volume and D₂. In the high-rank coal, the high D1 value represents the complexity and heterogeneity of the pore structure and represents a low adsorption affinity for methane molecules; in addition, D₂ has no effect on the methane adsorption capacity.

Influence of Supercritical CO &lt;sub&gt;2&lt;/sub&gt; Exposure on Water Wettability of Marine and Continental Shale

To investigate the influence of supercritical CO2 (ScCO2) exposure on water wettability of shale, a systematic contact angle experiments considering ScCO2 exposure time and pressure were conducted on shale samples collected from Sichuan Basin (marine) and Ordos Basin (continental), respectively. In addition, X-ray diffraction analysis and low-pressure N2 adsorption were performed to evaluate the variations of mineral compositions and pore structure of shale. Results indicate that the tested shale samples are both hydrophilic rocks, and the shale-water contact angles generally increased with increasing ScCO2 exposure time and pressure, which is primarily caused by the the decrease of clay minerals and carbonates in shale. The water wettability of shale weakened after ScCO2 exposure, indicating that the interaction between shale and water molecules decreased, which may reduce the resistance of water to flow in pores and fractures of shale. Accordingly, it can be inferred that the alteration of shale water wettability after ScCO2 injection is beneficial to the seepage of gas in shale pore channels, but may exist a negative influence on the stability of CO2 geological sequestration. This study provides a theoretical reference for CO2 sequestration and CO2-enhanced shale gas recovery (CS-ESGR).

Moisture dependency of variation of pore structure and property of cementitious materials due to high speed impact

In some numerical simulations of collisions on concrete, the densification of the pore structure has been assumed to be attributed to collisions. However, a previous experimental study conducted by the authors revealed that the pore structure can be coarsened by impact. The conditions of this coarsening have not yet been clarified. In this study, impact tests were conducted on cementitious materials with and without sand, admixture materials, and with different moisture contents, to understand the influences of these factors on the cracking, pore structure, and compositional changes caused by impacts. In the water-saturated cement paste specimens, mercury intrusion porosimetry showed that the pore structure of the region near the impact point was coarsened. Conversely, the pore structure of the cement paste specimens with low-water content was densified. In the mortar, the pore structure was coarsened in both the saturated and dried specimens. To explain the mechanism of incurred pore structure changes due to collision, mechanical and thermal effects were proposed. These include compaction due to mechanical action, the expansion force generated by liquid-water pressure upon impact, and vapor pressure changes due to heat, respectively. The study’s findings improve our understanding of the responses of cementitious materials subjected to impact loads and allow us to obtain more accurate damage estimations. The findings revealed the necessity of the investigation on the cause of the high-stiffness response at impact other than compaction. If the liquid or vapor pressure caused the pore coarsening at impact, it may be reduced by the effective release of these pressures.

Variations in pore structure of marine shale from the same horizon of the Longmaxi Formation with changing position in a small-scale anticline: Implications for the influence of structural deformation

Nanopores are well developed in shale, and are influenced by many rock properties, such as mineral composition, TOC (Total Organic Carbon) and thermal maturity. However, the influence of structural deformation on pore properties of shale is still not well understood. In this study, 19 samples from the same horizon of the lower Silurian Longmaxi shale, but from different positions in a small-scale anticline of Wuping Village, Chongqing (China), were collected. Low pressure N2 and CO2 adsorption experiments were performed to study the influence of structural deformation on the pore structure. The results show that the mineral composition, TOC and thermal maturity of all samples are similar, but the specific surface areas and pore volumes exhibit great differences in different positions of this anticline. The mean N2-BJH pore volumes, the mean N2-BET specific surface areas, the mean CO2 D-R pore volumes, as well as the mean CO2 D-R specific surface areas exhibit decreasing trends for the samples from the core part to the sub-core part, and to the limb part of the studied anticline, respectively. Structural deformation has shown a considerable influence on the micropores and mesopores, especially for the pores lower than 10 nm because that tectonic stress increases from the core part to the sub-core part and the limb part of the anticline. Moreover, the structural curving degree characterized using curvature shows good correlation with the pore volume and specific surface area. Compared to the limb part, the core part of the anticline with high curvature is located in the tension environment, and it is beneficial for pore development and gas accumulation. Our study implies that moderate structural deformation can provide more specific surface area for absorbed gas and pore volume for free gas at the core part of the structure on a larger scale.

Evaluation of the Pore Structure Variation During Hydraulic Fracturing in Marine Shale Reservoirs

Abstract Hydraulic fracturing becomes a necessary method to exploit shale gas, and the imbibition behavior of fracturing fluid can cause the pore structure variation. At present, the evaluation of this variation has less been investigated, and repeated imbibition was conducted to research the variation of pore structure. First imbibition was conducted as simulating the pore structure variation during hydraulic fracturing, and repeated imbibition was carried out to appraise this kind of variation. Two significant parameters were proposed to estimate the pore structure variation, which are the differential value of initial imbibition rate and the utmost differential value of normalized imbibed volume. Initial imbibition rate is closely related to pore connectivity, and normalized imbibed volume reflects the pore volume. Reservoirs with a higher value of these two parameters have advantage for developing its resources. Sichuan Longmaxi formation has the highest value in both parameters among all formations, which showed that Sichuan Longmaxi formation has advantage for exploiting its resources by hydraulic fracturing. Baojing Longmaxi formation, Sichuan Niutitang formation, and Cengong Niutitang formation have a lower value in both two parameters which indicated that these formations have less potential to develop its resources by hydraulic fracturing. These two parameters are mainly influenced by wettability and initial permeability, and have no obvious relationship with clay content. Overall, our research is conducive to screening the ideal formation for exploiting shale gas by hydraulic fracturing.

Pore structure response of sedimentary cycle in coal-bearing strata and implications for independent superposed coalbed methane systems

ABSTRACT Coalbed methane (CBM)-bearing systems are often superposed in coal-bearing strata with multiple coal seams. The formation and development of independent superposed CBM-bearing systems is based on the vertical variation of pore structure in coal-bearing strata. This study examines the potential of the pore structure cycle of the intervening rock formation in coal-bearing strata that compartmentalize the CBM system, which, in turn, assist in the production of the coal gas. The pore structure is characterized using lithology, sequence stratigraphy, and mercury injection porosimetry data generated from 25 samples of the intervening formation. The samples originate from a representative drilling operation located in the Bide-Santang basin, western Guizhou, China. We divided the pore structures into 4 types (I, II, III and IV): type I, “well sorted and fine slanting degrees;” type II, “well sorted and coarse slanting degrees with extremely low mercury saturation;” type III, “poorly sorted and uniform distribution” and type IV, “poorly sorted and polarization of micropores and macropores.” Pore structure changes cyclically in accordance with the sedimentary cycle: pores developed poorly within rocks that formed in deep water bodies with low energy and reducing sedimentary environments, such as lagoons and limited subtidal and mire facies; pores are better developed within rocks formed in shallow water bodies with high energy and oxidizing environments, such as supratidal flats, intertidal zones, and distributary channel facies. Third-order sequence boundaries (SB) favor the development of secondary porosity, whereas the maximum flooding surface (MFS) possesses a superior condition for forming regional impermeable barriers between different CBM-bearing systems. The sequence stratigraphic framework limits the variation trend of the pore structure and therefore limits the vertical distribution of independent superposed CBM-bearing systems. The findings are of great significance for enriching the geological theory of multi-seam CBM accumulation, optimizing the co-mining methods and improving the co-mining efficiency of superposed CBM systems.

Evolution of pore structure and fractal characteristics of marine shale during electromagnetic radiation.

Electromagnetic radiation has been proposed to non-aqueously stimulate shale formations, which can generate fractures and enhance the porosity of the matrix. The proposed method consumes electricity and thereby possesses significant advantages for sustainable and environmental hydrocarbon production. In this study, we investigate the pore structure variations of marine shale during electromagnetic radiation. First, the prepared marine shale samples are exposed to electromagnetic radiation for different times; an infrared thermometer monitors the temperatures. Then, the nitrogen adsorption/desorption technique is applied to examine the evolutions of the pore structure. Next, a scanning electron microscope is adopted to reveal the morphology and identify newly developed pores. Lastly, fractal analyses are performed to quantify pore structure variations. The sample exhibits quick temperature rises, whose temperature reaches about 300 °C after 5 min of electromagnetic radiation. The elevated temperature causes clay dehydration, thermal expansion, and organic matter decomposition, leading to significant changes in pore structures. The nitrogen adsorption/desorption characteristics demonstrate enhancements in pore spaces, including volume, size, and surface area. Fractal analyses show that the pores become rougher and exhibit less heterogeneity after electromagnetic radiation. The obtained results demonstrate a great potential of using electromagnetic radiation to enhance the porosity of shale rocks.

Variations of Pore Structure in Organic-Rich Shales with Different Lithofacies from the Jiangdong Block, Fuling Shale Gas Field, SW China: Insights into Gas Storage and Pore Evolution

The pore structure of gas shale reservoirs strongly influences gas occurrence, migration and enrichment, which has become an important fundamental issue for shale gas exploration and production. To...