Permeable Sediments Research Articles

Effect of particle size distribution variability on the permeability of hydrate-bearing sediments: A CFD study

Natural gas hydrates (NGH) are considered as a future clean energy in the era of carbon neutrality. The seepage behaviour of hydrate-bearing sediment (HBS) and its controlling factors are significant for developing effective production strategies. In this study, we aim to elucidate how the heterogeneous spatial distribution of clayey-silty sediment particles, particularly the non-uniform particle size distribution (PSD) affects the permeability of HBS. An algorithm was developed to generate the heterogeneous spatial distribution of sediment particles based on the PSD of hydrate-bearing cores extracted from Japan Nankai Trough (D50 = 122 μm). A series simulation of fluid flow in porous media based on CFD was conducted for the HBS with varying hydrate saturations (SH). The watershed segmentation algorithm was employed for the identification of pore structure, including pore space and pore throat with a detailed analysis on the pore structure evolution with SH. Host sediments with higher uniformity result in initial high permeability but the permeability reduction with increasing SH is more significant compared with that of lower uniformity. Pore throat width reduced to less than 5 μm with SH above 50% compared with its original width of ∼30 μm. Tortuosity exhibits a linear increase with SH, while permeability decreases exponentially with increasing tortuosity. We propose an improved permeability model (KCpro) based on the classical Kozeny-Carman model accounting for the change in specific surface area for non-uniform PSD in HBS with good agreement. This study confirmed the applicability of CFD modelling as a promising tool to study the seepage behaviour of HBS complementary to classical permeability experimental tests. Results of this study provide fundamental understanding on how permeability evolves in HBS with significant PSD variations in natural gas hydrate reservoirs in nature.

Distribution, source identification, and contamination level of trace metals in the sediment of the Shari-Goyain River in Bangladesh: Implications for ecological health risks

The major issue associated with coal mine drainage is its contribution to river pollution, which occurs at the local, regional, and global levels. The pollution of surface sediment by heavy metals is a major environmental and health concern in coal mining and downstream areas. This study explored the concentrations of eight metal components in the sediments from the Shari-Goyain River of Bangladesh. The trend of decreasing metal concentration was identified as Fe > Mn > Ni > Zn > Cu > Cr > Pb > Cd in sediment. The level of metal pollution in the study area was assessed by using various indicators like geo-accumulation index (Igeo), pollution load index (PLI), contamination factor (CF), potential ecological risk factor, and risk index (RI). The sediment in the river exhibited PLI values ranging from 0.133 to 0.543, suggesting a low level of pollution from the evaluated heavy metals (PLI < 1). The mean Igeo showed that the study area was unpolluted whereas Ni showed unpolluted to moderately polluted status. For most of the metals, the sediment samples recorded a low degree of contamination (CF < 1) except Pb, Cd, and Ni which exhibited moderate degree of contamination (1 ≤ CF < 3). Through the application of various statistical analyses, coal mine drainage has been identified as the possible source of pollution of the analyzed metals in the Shari-Goyain River. However, the risk index and RI suggested a low risk of metal pollution in the studied areas. To improve the environmental conditions of the Shari-Goyain River, it is crucial to construct permanent sediment quality monitoring stations and conduct extensive ecological investigations.

Permeability of surface clay-bearing sediments in Shenhu Area of South China Sea

The migration of fluids, such as the dissociation of natural gas hydrates, can affect the stability of the seabed and even induce submarine geological disasters and change the marine environment. The migration of fluids in sediments depends on its permeability properties. Therefore, it is crucial to investigate the sediment permeability and its influencing factors. This paper investigates the particle size, porosity, pore size distribution, and permeability of surface clay-bearing sediments in the Shenhu Area of the South China Sea by physical property tests, computed tomography (CT) scanning tests, constant rate of strain (CRS) consolidation tests, and nuclear magnetic resonance (NMR) tests. The permeability of the surface sediments in the Shenhu Area ranges from 3.32 × 10−15–6.76 × 10−14 m2. The pore size mainly ranges between 10−5 and 10−1 mm, exhibiting a bimodal distribution with peaks from 10−4 and 10−2 mm. The increase in clay content leads to enhanced permeability due to the bimodal distribution of pore sizes, which will cause the enlargement of pores. As the pore size increases, the sediment permeability also increases. Sediment permeability linearly decreases with increasing effective axial stress and deformation due to compression of the pore space. Additionally, a new model for calculating permeability with different sediment properties is established, which improves the accuracy of permeability calculations for clay-bearing sediments compared to previous research. The findings of our study can provide a reference for exploring the influence of sediment parameters on fluid migration mechanisms in clay-bearing sediments.

The influence of hydrocarbons on the physical properties of sea bottom sediments of the Black Sea of different stages of diagenesis under conditions of high pressures and temperatures and the degree of saturation

The influence of hydrocarbons on the physical properties of sea bottom sediments of the Black Sea of different stages of diagenesis under conditions of high pressures and temperatures and the degree of saturation

Bottom simulating reflections across the northern Gulf of Mexico slope

Bottom simulating reflections (BSRs) are widely used indicators for natural gas hydrate in reflection seismic data. BSRs, however, are typically studied on a local scale at a single site and less frequently on a large, regional scale. Herein, we map and describe BSRs over a 68,000 km2 area across the northern Gulf of Mexico slope, using publicly released industry-acquired 3D seismic data. We map 74 BSR zones that cover a total area of 2060 km2. To understand the characteristics of these BSR zones, we classify them based on their BSR characteristics in the seismic data (continuous, discontinuous, and clustered) and the geological settings in which they appear (structural, stratigraphic, and venting). The most common type of BSRs are discontinuous BSRs; such BSRs were mapped over ∼60% of the total BSR area. Discontinuous BSRs occur most commonly in structural or stratigraphic settings. Discontinuous BSRs indicate that gas and hydrate are likely concentrated in discrete layers crossing the base of the hydrate stability zone (HSZ). Continuous BSRs occur in 27% of the total BSR area and they primarily occur in venting and stratigraphic settings. At vent sites, hydrate formation likely reduces sediment permeability and focuses fluid flow laterally underneath the base of the HSZ, likely resulting in a continuous BSR along the area of lateral gas flow. Clustered BSRs, chaotic sequences of high amplitude reflections, occur in only 13% of the total BSR area and are only found within structural settings. In addition, the majority of clustered BSR zones are close to paleochannels, suggesting that clustered BSRs may be more common where coarser-grained sediments exist.

Net-zero produced water strategy on gas production in marine gas hydrate bearing sediment

Gas hydrates, commonly found in deep marine sediments, have been considered a potential long-term energy source. Recent field trials demonstrated that well depressurisation, converting solid gas hydrates into fluid in situ, is the most efficient method for gas production. However, this process involves continuously pumping a large volume of water from the ocean that cannot be simply disposed of due to environmental concerns. The produced water, with temperature, salinity and residues different from those of shallower seawater, could impact marine species, habitats and ecosystems if directly discharged into the sea. Storing or transporting this water for treatment is economically impractical. Furthermore, as hydrate dissociation increases sediment permeability, water production rates rise over time. Therefore, re-injecting produced water into gas hydrate reservoirs emerges as a promising net-zero produced water strategy. This study investigates the effects of produced water re-injection on gas productivity and sediment deformation through coupled thermo-hydro-chemo-mechanical numerical analysis. The results indicate that re-injecting the water could improve gas productivity but may cause heave and differential displacement near the injection well. This highlights the importance of considering thermo-hydro-chemo-mechanical processes in comprehensively evaluating energy production strategies and suggests that further studies are necessary for future commercial gas production from gas hydrate reservoirs.

Thermal stratification and meromixis in four dilute temperate zone lakes

Abstract. Four adjacent lakes (Arco, Budd, Deming, and Josephine) within Itasca State Park in Minnesota, USA, are reported to be meromictic in the scientific literature. However, seasonally persistent chemoclines have never been documented. We collected seasonal profiles of temperature and specific conductance and placed temperature sensor chains in two lakes for ∼1 year to explore whether these lakes remain stratified through seasonal mixing events and what factors contribute to their stability. The results indicate that all lakes are predominantly thermally stratified and are prone to mixing in isothermal periods during spring and fall. Despite brief, semi-annual erosion of thermal stratification, Deming Lake showed no signs of complete mixing from 2006–2009 and 2019–2022 and is likely meromictic. However, the other lakes are not convincingly meromictic. Geochemical data indicate that water in Budd Lake, which contains the most water, is predominantly sourced from precipitation. The water in the other three lakes is of the calcium–magnesium–bicarbonate type, reflecting a source of water that has interacted with the deglaciated landscape. δ18OH2O and δ2HH2O measurements indicate the lakes are supplied by precipitation modified by evaporation. Josephine, Arco, and Deming lakes sit in a valley with likely permeable sediments and may be hydrologically connected through wetlands and recharged with shallow groundwater, as no streams are present. The water residence time in meromictic Deming Lake is short (100 d), yet it maintains a large reservoir of dissolved iron, indicating that shallow groundwater may be an additional source of water and dissolved ions. All four lakes develop subsurface chlorophyll maxima layers during the summer. All lakes also develop subsurface oxygen maxima that may result from oxygen trapping in the spring by rapidly developed summer thermoclines. Documenting the mixing status and general chemistry of these lakes enhances their utility and accessibility for future biogeochemical studies, which is important as lake stratification and anoxia are becoming more prevalent due to changes in climate and land use.

Gas production from a promising reservoir of the hydrate with associated and shallow gas layers in the low permeable sediments

The gas production rate from hydrate trials is still below commercial criterion. To increase the production rate, a promising geological system consisting of hydrate, associated and shallow gas layers was explored recently. Herein, the gas production of this complex geological system through depressurization was numerically investigated. The result showed a high gas production rate in the first tens day, and a subsequently slow decrease with an increasing water production rate under the immediate depressurization condition. The free gas layer contributed the most of gas production while the least is from hydrate reservoir. For water production, the most was from the associated gas layer and a moderate from hydrate layer. Among all factors, the depressurization rate affected greatly the initial gas rate, and slight depressurization caused a high and stable rate. The permeability, gas saturation and irreducible water saturation of the shallow gas layer affected dramatically the gas rate and obviously the corresponding water rate. Special attention should be paid to the hydrate forming in the shallow gas layer. This hydrate barrier caused a decrease of gas rate when reducing production pressure. The production of the complex system enhances the economic benefit, and it becomes a prospective system of exploration.

Nonempirical fractal permeability model and experimental verification of hydrate-bearing sediments based on heterogeneous distribution of particles

The one of the factors limiting production of gas-water is affected by sediments' permeability. Evaluating the normalized permeability (Kn) of sediments containing hydrates is necessary for analyzing the saturation (decomposition level) and flow capacity. The mistakes in Kn prediction are frequently brought about by the assumption such as homogenous particles distribution and ignoring microscopic pore structure. To address the issue, a fractal-based nonempirical prediction model of Kn was developed by modeling, parameter acquisition using SEM and Analyzer, experiment validation, error comparison with published data, and sensitivity analysis for evolution, respectively. Assuming that sediment skeleton particles were misaligned spherical particles of equal diameter. The heterogeneity of sediments was described by the vertical and horizontal distance ratio (m) and offset angle (θ) of the particles. This led to a novel approach for determining average tortuosity (τav,0) and tortuosity fractal dimension (Dτ,0). The viability of the model was confirmed by permeability measurement experiments. Additionally, using the measuring function of SEM, the maximum pore diameter of sediments was obtained. Furthermore, a comparison with earlier data demonstrated the effect. Considering heterogeneity, the average accuracy of Geometric Mean Variance in Kn has increased to 9.66%. Finally, each parameter's impact on prediction was examined. At m = 1.357, Dτ,0 grows symmetrically. Kn,GC varies 15 times more with an increase in Dτ,0 than Kn,PF. All findings indicate that the new model is more likely to be forecasted Kn for a variety of systems, including clayey-silt, sand, and silty-sand system, and especially in reflecting the heterogeneity.

Experimental study on the impact of effective stress and cyclic loading on permeability of methane hydrate clayey sediments

Permeability evolution directly determine gas production and its efficiency of gas hydrates in the South China Sea. Due to existence of submarine overlying rocks and change of soil internal mechanics during the hydrate exploitation, permeability evolution is significantly affected. Therefore, in this paper, influence of effective stress on the porosity and permeability of sediment with different hydrate saturation was studied, and functional relationship between them was revealed. Effect of cyclic loading and unloading stress on the permeability recovery and damage degree was also discussed. The results show that when hydrate saturation increases, the relation of permeability and effective stress are power function rules, while porosity and effective stress are exponential function rules in clayey sediments. An empirical prediction equation for gas permeability of clay sediments with effective stress was presented, which takes into account hydrate saturation and porosity. In addition, during cyclic loading experiments, the results showed a linear increase in permeability during unloading. The permeability recovery rate and permeability damage rate are completely opposite with change of hydrate saturation. Permeability damage rate of kaolin is different from that of other two kinds of sediment. With the increase of cycle times, the proportion of plastic deformation of sediment decreases gradually.

Modeling of synthesized hydrate-bearing sands: Effect of hydrate morphology on absolute and relative permeabilities

Absolute and relative permeability of hydrate-bearing sediments are crucial flow parameters to hydrate exploitation processes. However, up to now, grain-coating hydrate-bearing sediments (GC-HBS) with high hydrate saturation (Sh) and pore-filling hydrate-bearing sediments (PF-HBS) are still prohibitive to be prepared in laboratory. In this work, three-dimensional pore structures of GC-HBS with high Sh and PF-HBS were numerically synthesized, based on the CT scanning of porous samples. Then, a state-of-the-art quasi-static pore-network model (PNM) was used to study the absolute and relative permeability of these HBS. Results show that the absolute permeability of both of the GC-HBS and PF-HBS declined rapidly with increasing of Sh, they can be well fitted by the Tokyo model with exponential parameter of 3.5 and 5.3, respectively. And the absolute permeability of PF-HBS is smaller than that of GC-HBS with same Sh for a bigger specific surface area of PF-HBS which increases the flow resistance for flows. It shows that the wetting and non-wetting relative permeability of GC-HBS decreases with the increase of Sh. But the gas-water two-phase relative permeability of PF-HBS has nonmonotonic changes with the increase of Sh.

High methane ebullition throughout one year in a regulated central European stream

Ebullition transports large amounts of the potent greenhouse gas methane (CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document}) from aquatic sediments to the atmosphere. River beds are a main source of biogenic CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document}, but emission estimates and the relative contribution of ebullition as a transport pathway are poorly constrained. This study meets a need for more direct measurements with a whole-year data set on CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document} ebullition from a small stream in southern Germany. Four gas traps were installed in a cross section in a river bend, representing different bed substrates between undercut and slip-off slope. For a comparison, diffusive fluxes were estimated from concentration gradients in the sediment and from measurements of dissolved CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document} in the surface water. The data revealed highest activity with gas fluxes above 1000 ml m-2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-2}$$\\end{document} d-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document} in the center of the stream, sustained ebullition during winter, and a larger contribution of ebullitive compared to diffusive CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document} fluxes. Increased gas fluxes from the center of the river may be connected to greater exchange with the surface water, thus increased carbon and nutrient supply, and a higher sediment permeability for gas bubbles. By using stable isotope fractionation, we estimated that 12-44% of the CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document} transported diffusively was oxidized. Predictors like temperature, air pressure drop, discharge, or precipitation could not or only poorly explain temporal variations of ebullitive CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document} fluxes.

Impact of hydrate spatial heterogeneity on gas permeability in hydrate-bearing sediments

The permeability characteristic of hydrate-bearing sediments (HBSs) is an essential factor for assessing the exploiting potential of target area. The two trial tests in the South China Sea have demonstrated the production feasibility of clayey hydrate reservoirs, and further clarifying main controlling factors of the permeability characteristic is an essential prerequisite for commercial development. However, the current studies mainly focus on homogeneous HBSs with different saturations and porosities, ignoring the natural spatial heterogeneity of hydrate since the sedimentary and accumulation history. In this study, spatial heterogeneous hydrate (0–23.1%) was generated in montmorillonite specimens, and effective stress (1–5 MPa) was applied. The results show that 1) HBSs permeability with heterogeneity hydrate occurrence presents an exponential-like relationship with saturation. 2) Heterogeneous hydrate occurrence would reduce the permeability sensitivity to saturation. 3) The permeability of heterogeneous HBSs decreases in a power function trend with increasing effective stress. 4) The heterogeneity reduces compression effect and decreases permeability damage rate. 5) Increasing hydrate spatial heterogeneity would decrease HBSs permeability.

Characterization of methane hydrate extraction influenced by hydraulic fractures using a coupled thermo-hydro-mechanical-chemical model

The low permeability of the methane hydrate-bearing sediment limits the methane gas extraction. To enhance methane hydrate extraction, hydraulic fracturing can be a promising approach to improve the hydrate reservoir permeability by creating a fracture network in the reservoir. In this study, a coupled thermo-hydro-mechanical-chemical mathematical model and its numerical implementation based on finite element technology are introduced to analyze the methane hydrate extraction through fractured methane hydrate-bearing sediment considering methane hydrates dissociation, gas-water two-phase flow, heat transfer, dynamic changes of the sediment permeability, and deformation of both sediment matrix and fractures as well as capturing the interplay between them. The coupled thermo-hydro-mechanical-chemical numerical model is verified by reproducing a methane hydrates dissociation laboratory test. Finally, we conduct a series of simulations for the methane gas depressurization extraction through the sediments with the DFNs assigned as diverse geometrical characteristics. The influence of hydraulic fracture network geometrical and hydraulic characteristics on methane hydrate extraction are discussed. The results can offer a reference for enhancing the methane hydrate extraction efficiency.

Experimental Study on the Permeability of Hydrate-Bearing Silty-Clayey Sediments with Grain-Cementing and Pore-Filling Hydrate Morphologies

Experimental Study on the Permeability of Hydrate-Bearing Silty-Clayey Sediments with Grain-Cementing and Pore-Filling Hydrate Morphologies

Seasonality of bioirrigation by the maldanid polychaete Clymenella torquata and related oxygen dynamics in permeable sediments

Benthic organisms in coastal sediments affect elemental cycling and control benthic-pelagic coupling through particle reworking and ventilation of their burrows. Bioirrigation and associated porewater advection create intermittently oxic regions within sediments. The spatio-temporal patterns of such biogenic redox oscillations likely respond to seasonal factors, but quantitative information on the seasonality of bioirrigator behaviors and associated redox dynamics is scarce. We examined bioirrigation by the maldanid polychaete Clymenella torquata and its impacts on sediment oxygenation patterns in permeable sediments using high-resolution planar optode oxygen imaging. In sediment mesocosms with reconstructed summer-collected sediment, the durations of pumping and resting varied inversely with temperature. The average durations of pumping and resting increased from 4 min/4 min at 21 °C, to 6 min/6 min at 12 °C, to 15 min/14 min at 5 °C. In intact cores collected in summer, irrigation patterns (3.5 min/3.5 min) were similar to those observed at 21 °C during the temperature ramp. Pumping and resting durations in intact cores collected in winter at 6 °C averaged 9 min/26 min, significantly different from patterns at comparable temperatures in the temperature ramp. Pumping patterns strongly affected the temporal patterns of redox dynamics in surrounding sediments. In addition, temperature strongly affected burrow irrigation depth (exclusively within the top ∼10 cm at 21 °C, and down to ∼20 cm at 5–6 °C with an apparent transition at ∼15 °C), indicating that the zone with dynamic redox conditions migrates vertically on a seasonal basis. The differences in pumping patterns between in- and out-of-season experiments and the effect of temperature on irrigation depth underscore the importance of conducting experiments with bioturbators in-season and at field temperatures. The observed seasonal differences in bioirrigation patterns and associated spatio-temporal redox dynamics suggest that rates and pathways of redox-sensitive diagenetic processes and benthic chemical fluxes in permeable sediments likely show considerable seasonal variation.

The influence of in-stream structures on temperature dynamics via induced hyporheic exchange

In-stream restoration structures, such as woody weirs, are intended to enhance hyporheic exchange, thus altering temperature dynamics in restored reaches. However, the relative effects of various in-stream structure types and streambed settings (i.e. the size, spacing, number and sediment hydraulic conductivity) remain poorly understood. Here, we conducted a field experiment with a channel-spanning weir in a stream to study its impact on hyporheic exchange and streambed and surface water temperatures. We subsequently applied numerical models to explore the effects of height, spacing and number of weirs and sediment permeability on hyporheic exchange and thermal distribution characteristics. Based on the field experiments and numerical models, the results showed that the addition of a weir enhanced hyporheic exchange and produced a characteristic temperature heterogeneity pattern in sediment that varied with height, spacing, number of weirs and sediment permeability. The spatial extent and rate of hyporheic exchange and temperature heterogeneity near the weir primarily increased with weir height and sediment permeability. With increasing spacing, the exchange flux and thermally heterogenous area increased, but the exchange intensity decreased, and the sediment temperature readily stabilized. With increasing number of weirs, the exchange flux first decreased and then increased, and the region exhibiting temperature heterogeneity increased. The interaction between the introduction of in-stream structures introduction and streambed settings must be understood to improve vertical connectivity in rivers, increase the thermal heterogeneity in the hyporheic zone and create localized but potentially valuable thermal refuge areas.

Experimental Study on Permeability and Gas Production Characteristics of Montmorillonite Hydrate Sediments Considering the Effective Stress and Gas Slippage Effect

Summary Gas permeability in hydrate reservoirs is the decisive parameter in determining the gas production efficiency and gas production of hydrate. In the South China Sea (SCS), the gas flow in tight natural gas hydrate (NGH) silty clay reservoirs is significantly affected by the gas slippage effect and the effective stress (ES) of overlying rock. To improve the effectiveness of hydrate exploitation, it is necessary to understand the influence of gas slippage in hydrate reservoirs on the permeability evolution law. For this paper, the gas permeability characteristics and methane production of hydrate montmorillonite sediments were studied at different pore pressures and ESs. Experimental data revealed that the gas permeability of montmorillonite samples before methane hydrate (MH) formation is seriously affected by the Klinkenberg effect. The gas permeability of montmorillonite sediments before hydrate formation is generally smaller than that after hydrate formation, and the gas slippage effect in the sediments after hydrate formation is weaker than that before hydrate formation. With the change in ES, the intrinsic permeability of sediment has a power law relationship with the simple ES. The ES law coefficient n was determined using the response surface method to eliminate the influence of gas slip on gas permeability. As pore pressure decreases and MH decomposes, montmorillonite swelling seriously affects gas permeability. However, the gas slippage effect has a good compensation effect on the permeability of montmorillonite sediments after MH decomposition under low pore pressure. The multistage depressurization-producing process of MH in montmorillonite sediments is mainly 3 MPa depressurization-producing stage and 2 MPa depressurization-producing stage. In this paper, the influence mechanism of gas slippage effect of hydrate reservoir is studied, which is conducive to improving the prediction accuracy of gas content in the process of hydrate exploitation and exploring the best pressure reduction method to increase the gas production of hydrate in the process of exploitation.

Microorganisms oxidize glucose through distinct pathways in permeable and cohesive sediments.

In marine sediments, microbial degradation of organic matter under anoxic conditions is generally thought to proceed through fermentation to volatile fatty acids, which are then oxidized to CO2 coupled to the reduction of terminal electron acceptors (e.g. nitrate, iron, manganese, and sulfate). It has been suggested that, in environments with a highly variable oxygen regime, fermentation mediated by facultative anaerobic bacteria (uncoupled to external terminal electron acceptors) becomes the dominant process. Here, we present the first direct evidence for this fermentation using a novel differentially labeled glucose isotopologue assay that distinguishes between CO2 produced from respiration and fermentation. Using this approach, we measured the relative contribution of respiration and fermentation of glucose in a range of permeable (sandy) and cohesive (muddy) sediments, as well as four bacterial isolates. Under anoxia, microbial communities adapted to high-energy sandy or bioturbated sites mediate fermentation via the Embden-Meyerhof-Parnas pathway, in a manner uncoupled from anaerobic respiration. Prolonged anoxic incubation suggests that this uncoupling lasts up to 160h. In contrast, microbial communities in anoxic muddy sediments (smaller median grain size) generally completely oxidized 13C glucose to 13CO2, consistent with the classical redox cascade model. We also unexpectedly observed that fermentation occurred under oxic conditions in permeable sediments. These observations were further confirmed using pure cultures of four bacteria isolated from permeable sediments. Our results suggest that microbial communities adapted to variable oxygen regimes metabolize glucose (and likely other organic molecules) through fermentation uncoupled to respiration during transient anoxic conditions.

Study on the production of gas hydrates and underlying free gas by horizontal well under different directions of hydraulic fracturing

In most of gas hydrates reservoirs, the low permeability of sediments limits the commercial production of gas hydrates. Hydraulic fracturing is an efficient way to improve the permeability in hydrate reservoirs. However, how to choose a suitable hydraulic fracturing direction during hydrate production in a 3D hydrate reservoir is still unknown. In this study, the production of gas hydrates and underlying free gas within 10 years by a depressurization horizontal well (along X-axis) under different directions (XY, YZ, XZ plane) of hydraulic fracturing was numerically studied in a 3D model. The simulation results showed that, compared with no hydraulic fracturing, the final increase ratios of the hydrate dissociation volume and gas production volume for the fracturing in XY, YZ, and XZ plane were 188.16 %, 218.96 %, 84.95 %, and 170.29 %, 253.66 %, 102.64 %, respectively, and the final increase ratios of water production amount for the fracturing in XY, YZ, and XZ plane were 140.88 %, 787.25 %, and 381.07 %, respectively. For the hydraulic fracturing in YZ plane, the connection of the production well with the upper and lower strata was earlier, and the underlying free gas was mainly produced in the early period of hydrate production, leading to the higher gas production rate and gas to water ratio in the early period. However, the gas production rate and the gas to water ratio were reduced in the later period of hydrate production due to the consumption of the underlying free gas. For the hydraulic fracturing in XY plane, the connection of the production well with the upper and lower strata was delayed, and the underlying free gas was mainly produced in the later period of hydrate production, leading to the higher gas production rate and gas to water ratio in the later period. By comprehensive consideration, for the depressurization induced hydrate production by a horizontal well along the X-axis, it's suggested that the most suitable fracturing direction should be in YZ plane for a short production period (within 5 years), and it should be in XY plane for a long production period (exceeds 5 years). This study proposes a new point that the suitable hydraulic fracturing direction is related to hydrate production period, which could provide some guidance for gas hydrates production by hydraulic fracturing.