Free Pore Water Research Articles

Comparative Laboratory Wettability Study of Sandstone, Tuff, and Shale Using 12-MHz NMR T1-T2 Fluid Typing: Insight of Shale

Summary Wettability is a fundamental parameter significantly influencing fluid distributions, saturations, and relative permeability in porous media. Despite the availability of several wettability measurement techniques, obtaining consistent wettability index results, particularly in tight reservoirs, remains a challenge. Nevertheless, obtaining accurate wettability indices is crucial for gaining a more profound understanding of rock properties and precisely identifying and evaluating oil recovery processes. This study adapts T1-T2 nuclear magnetic resonance (NMR) in twin plugs (cores cut in half from the middle) style wettability measurement for different reservoirs. The fluid typing in different lithologies by T1-T2 NMR is proved to be effective by introducing D2O with a modified pressurization saturation process. Therefore, demarcating the regions requires multiple experiments, including sole brine, sole oil phase, and D2O imbibition processes, to define oil and water distribution regions. Such fluid typing ability enables better accuracy in wettability characterization. The weighing method shows good agreement with the T2 spectrum but lacks the ability to differentiate fluids. It is observed that the same fluid in various porous media displays different divisions of T1/T2 ratios. The wettability index of sandstone, tuff, and shale measured by weighing and T1-T2 NMR method are compared and studied to demonstrate the applicability of different methods. The weighing method and the NMR method, as modified-Amott methods, share the same fundamental principle but differ in their measurement techniques. This study’s T1-T2 NMR wettability indices are −0.52, 0.06, and 0.14, whereas the weighing wettability indices are −0.63, 0.07, and 0.34 of sandstone, tuff, and shale, respectively. In addition to the difference in shale wettability index, there are also differences in shale porosity measured by methods with/without the ability to differentiate the fluid types. The T1-T2 NMR method is more accurate in measuring the wettability of shale because it can distinguish among free water in pores, structural water, and clay-bound water in smectitic clay minerals. If the clay-related water is not treated properly, the hydrophilicity of the shale will be overestimated. Ultimately, four types of pores (water-wet, oil-wet, mixed-wet, and unconnected pores) are classified and quantified by the proposed NMR method.

Frequency shifts of free water signals from compact bone: Simulations and measurements using a UTE-FID sequence.

Free water in cortical bone is either contained in nearly cylindrical structures (mainly Haversian canals oriented parallel to the bone axis) or in more spherically shaped pores (lacunae). Those cavities have been reported to crucially influence bone quality and mechanical stability. Susceptibility differences between bone and water can lead to water frequency shifts dependent on the geometric characteristics. The purpose of this study is to calculate and measure the frequency distribution of the water signal in MRI in dependence of the microscopic bone geometry. Finite element modeling and analytical approaches were performed to characterize the free water components of bone. The previously introduced UTE-FID technique providing spatially resolved FID-spectra was used to measure the frequency distribution pixel-wise for different orientations of the bone axis. The frequency difference between free water in spherical pores and in canals parallel to B0 amounts up to approximately 100 Hz at 3T. Simulated resonance frequencies showed good agreement with the findings in UTE-FID spectra. The intensity ratio of the two signal components (parallel canals and spherical pores) was found to vary between periosteal and endosteal regions. Spatially resolved UTE-FID examinations allow the determination of the frequency distribution of signals from free water in cortical bone. This frequency distribution indicates the composition of the signal contributions from nearly spherical cavities and cylindrical canals which allows for further characterization of bone structure and status.

Mobility of connate pore water in gas shales: A quantitative evaluation on the Longmaxi shales in the southern Sichuan basin, China

Flowback and produced water is causing water environment pollution and elevated disposal costs during the shale gas development. Due to high salinity and variously hazardous substances in the connate pore water (i.e., initial water storing in shale matrix pores), whether the connate pore water is produced in company with fracking water flowback has long been concerned. The key is to determine the mobility of connate pore water, which is closely associated with the occurrence of adsorbed and free water in shale pores. In this study, five fresh shale samples were obtained from the Longmaxi Formation shale gas reservoirs in the southern Sichuan basin of China to evaluate the mobility of connate pore water. Theoretical models of describing the non-equilibrium and equilibrium expulsion processes of connate pore water from shale matrix were proposed. On this of basis, the amounts and microscopic distributions of adsorbed and free water in the shales were determined by using centrifugation and nuclear magnetic resonance (NMR) tests under atmospheric pressure and 30 °C temperature condition. Results show that: (1) the amounts of adsorbed and free water in the shales varies from 5.9132 to 8.3442 mg/g (mean 7.3842 mg/g) and 2.1224–5.7931 mg/g (mean 3.9507 mg/g), respectively. The amount of free water (i.e., maximum mobile water amount) accounts for 23.69–41.37 wt% (mean 34.12 wt%) of total water, indicating that about one-third of the connate pore water in gas shales is potentially recoverable. (2) The connate pore water in the adsorbed and free states mainly distributes in the pores of 0.5–10 nm, of which the adsorbed water mainly distributes in the pores of 0.5–3 nm and the free water (i.e., mobile water) mainly stores in the pores of 3–10 nm. (3) The mobility index, that is the ratio of free water amount to mid-value pressure difference, was introduced to quantify the mobility of connate pore water by considering the equilibrium expulsion process of pore water. The mobility index of connate pore water in fresh shales is negatively related to the water-adsorbed ratio. This study contributes to identifying the source of flowback and produced water and is also significant to water environment.

Chloride accessible porosity fractions across the Jurassic sedimentary rocks of northern Switzerland

The Jurassic clay-rich sedimentary sequence in northern Switzerland was investigated in the context of nuclear waste disposal. Tracer profiles across this sequence play an important role when assessing transport processes and long-term evolution of the environment. Negatively charged clay-mineral surfaces cause a depletion of anions in the porewater close to the surfaces compared to free porewater further away. The chloride accessible porosity fraction (fCl) is an important parameter to scale Cl bulk porewater concentrations from aqueous extraction to free porewater concentrations, which are required to establish anion profiles used for transport modelling of anionic tracers. A vast set of fCl data from advective displacement (AD), squeezing (SQ) and through-diffusion (DI) experiments allowed for method comparison and yielded insight into controlling factors of fCl across the sedimentary sequence. Two observations were made: (1) fCl varies strongly depending on lithology and partly on ionic strength; (2) at high ionic strength, the three methods yield consistent results, but at low ionic strength, fCl from DI are lower than those from AD and SQ. The observations are discussed in terms of influencing factors, including structural charge, ionic strength, rock texture, pore sizes and the fact that the different methods sample different parts of the pore space. The stronger variation of fCl from DI experiments with the ionic strength of the experimental porewater is in agreement with theoretical concepts. The volume fraction of small pores (r < 2 nm) inversely correlates with fCl; it accounts for ∼50% of the anion depleted pore volume. Two empirical approaches were tested to derive functions for fCl valid for the lithologically diverse sequence: the clay-content model uses a constant fCl at a clay-mineral content >30% and extrapolates to a fCl of 1 at zero clay content. The formation model applies formation- or member-specific average values of experimental fCl for each unit. Both approaches work reasonably well, with the formation model producing less apparent outliers of free porewater concentrations than the clay-content model. Still some scatter remains, particularly in heterogeneous and low-porosity formations. This work results in a comprehensive and coherent fCl dataset for all studied Jurassic formations and serves as a basis to interpret the profiles of anions with regard to transport across the entire low-permeability sequence.

Porewater composition in clay rocks explored by advective displacement and squeezing experiments

Argillaceous rocks are foreseen in many countries as the potential hosts for nuclear waste repositories. The knowledge of the chemical composition of the free porewater in these formations is required for the understanding of the paleo-hydrogeological evolution, for the assessment of radionuclide solubilities and migration parameters and for assessing the long-term stability of the technical barrier system. High pressure squeezing and advective displacement are two methods that aim at direct sampling of this porewater fraction while minimizing experimental artefacts. Within the framework of a recent deep drilling campaign in Switzerland, targeting the Opalinus Clay as the designated host rock, a substantial dataset of porewater compositions was obtained by these two methods. It included 51 squeezing and 30 advective displacement experiments on drillcore samples from the Opalinus Clay and confining units distributed over 8 boreholes in 3 study areas. Porewater compositions obtained by either method reflect the geochemical characteristics of each siting region, such as different salinities and water types as well as depth gradients informing on the diffusive exchange with bounding aquifers. An in depth comparison of the Opalinus Clay porewater compositions obtained by both methods shows a high degree of consistency with regard to the ion ratios or mineral equilibria. The pH/pCO2 system was found to be prone to experimental artefacts, but applying a correction, a fairly consistent dataset was obtained. Porewaters acquired by squeezing exhibit systematically lower salinities by 10–40% when compared to those from advective displacement. It is concluded that this is due to the mobilization of a higher fraction of an anion depleted porewater, either due to the higher mobilization of water from the diffuse layer or due to the expulsion of water from interlayer (-like) pores. The comparison with a geochemical model indicates that the experimental data from both methods can be considered as proxies for in-situ major-ion porewater compositions. It also confirms the robustness of the geochemical model predicting porewaters of the Opalinus Clay and confining units. Differences between in-situ, sample storage and extraction temperatures need to be taken into account when interpreting the porewaters obtained by either method and modelling in-situ porewater compositions. Combining different laboratory and analytical methods for porewater investigations in clay rocks provides an added value as it enables a detailed assessment of natural heterogeneities and experimental uncertainties.

Experimental study on water transfer mechanism of quicklime modified centrifugal dewatering clay

This study investigates the water transfer mechanism of quicklime modified centrifugal dewatering clay. A small-scale centrifuge device was used to investigate the change of bound water content in modified clay under different laboratory test conditions including quicklime content and curing age, and analyze the water transfer mechanism in the process of quicklime treatment, and its impact on clay properties. The results show that the content of pore water decreased by 6.2% ∼ 16.9%, the content of bound water increased by 4.2% ∼ 12.9%, and the content of hydration water increased by 6.1% ∼ 14.1% due to hydration reaction caused by the quicklime. The increase of quicklime content facilitates the conversion of free water into bound water and hydration water in a relatively short time. The pozzolanic reaction occurs on the surface of the clay particles to absorb free water and convert it into bound water, while part of Ca (OH)2 crystals generated by hydration reaction adhere to the surface of clay particles, grow into needle-shape and fibrous structures, and absorb free pore water and convert it into hydration water. For field applications, the mixing percentage of quicklime is recommended to be below 9% by dry weight. From the cost perspective, 7-day curing age is recommended in the field. The essence of quicklime modification is to transform the free water with relatively low pF potential energy into the bound water and hydration water with relatively high pF potential energy. A schematic diagram of water transfer mechanism was also proposed in this study for quicklime modified clay and validated using the aforementioned experimental data.

Thermodynamic Feasibility of the Black Sea CH4 Hydrate Replacement by CO2 Hydrate

There is an international consensus that reductions of CO2 emissions are needed in order to reduce global warming. So far, underground aquifer storage of CO2 is the only commercially active option, and it has been so since 1996, when STAOIL started injecting a million tons of CO2 per year into the Utsira formation. Storage of CO2 in the form of solid hydrate is another option that is safer. Injection of CO2 into CH4 hydrate-filled sediments can lead to an exchange in which the in situ CH4 hydrate dissociates and releases CH4. Two types of additives are needed, however, to make this exchange feasible. The primary objective of the first additive is related to hydrodynamics and the need to increase injection gas permeability relative to injection of pure CO2. This type of additive is typically added in amounts resulting in concentration ranges of additive in the order of tens of percentages of CO2/additive mixture. These additives will, therefore, have impact on the thermodynamic properties of the CO2 in the mixture. A second additive is added in order to reduce the blocking of pores by new hydrates created from the injection gas and free pore water. The second additive is a surfactant and is normally added in ppm amounts to the gas mixture. A typical choice for the first additive has been N2. The simple reasons for that are the substantial change in rheological properties for the injection gas mixture and a limited, but still significant, stabilization of the small cavities of structure I. There are, however, thermodynamic limitations related to adding N2 to the CO2. In this work, we discuss a systematic and consistent method for the evaluation of the feasibility of CO2 injection into CH4 hydrate-filled reservoirs. The method consists of four thermodynamic criterions derived from the first and second laws of thermodynamics. An important goal is that utilization of this method can save money in experimental planning by avoiding the design of CO2 injection mixtures that are not expected to work based on fundamental thermodynamic principles. The scheme is applied to hydrates in the Black Sea. Without compositional information and the knowledge that there is some verified H2S in some sites, we illustrate that the observed bottom hydrate stability limits are all with hydrate stability limits of hydrates containing from 0 to 3 mole% H2S. A limited number of different injection gas mixtures has been examined, and the optimum injection gas composition of 70 mole% CO2, 20 mole% N2, 5 mole% CH4, and 5 mole% C2H6 is feasible. In addition, a surfactant mixture is needed to reduce blocking hydrate films from injection gas hydrate.

Sediment Microstructure in Gas Hydrate Reservoirs and its Association With Gas Hydrate Accumulation: A Case Study From the Northern South China Sea

Exploration and pilot production have confirmed that gas hydrates in the Shenhu area on the northern continental slope of the South China Sea have enormous resource potential. However, a meticulous depiction of gas hydrate reservoirs based on sediments is limited. The distributed low-flux gas hydrates are mainly deposited in the Shenhu area, and the gas hydrate saturation exhibits extreme vertical heterogeneity. In this study, we focused on the sediment microstructure of gas hydrate reservoirs. Based on the variation in gas hydrate saturation, the study interval was divided into non-gas hydrate (non-GH) as well as I-, II-, and III-gas hydrate reservoir layers. We analyzed the relationship between sediment microstructure and gas hydrate reservoirs based on computed tomography scans, specific surface area analysis, and scanning electron microscopy observations. The results showed that the sediment in gas hydrate reservoirs had three types of pores: 1) intergranular pores between coarse grains (CG-intergranular pores), 2) intergranular pores between fine grains (FG-intergranular pores), and 3) biologic grain pores (BG-pores). The CG- and FG-intergranular pores were mainly formed by the framework, which consisted of coarse minerals (such as quartz and feldspar) and clay minerals, respectively. The BG-pores were mainly formed by the coelomes of foraminifera. CG-intergranular pores and BG-pores can provide effective reservoir space and increase the permeability of sediment, which is conducive to gas hydrate accumulation. The FG-intergranular pores reduce permeability and are not conducive to gas hydrate accumulation. Clay minerals and calcareous ultramicrofossils with small grain sizes and complex microstructures fill the effective reservoir space and reduce the permeability of sediment; additionally, they improve the adsorption capacity of sediment to free gas or pore water, which is not conducive to gas hydrate formation and accumulation. The results of our study explicitly suggest that the microstructure of sediment is an important controlling factor for gas hydrate accumulation and reveals its underlying mechanism.

Study on the immobile water of saturated compacted clay-rich materials by oil seepage

Immobile water is defined as a firmly bound thin film of water surrounding clay particles that somehow resembles solid features. The characteristics and amount of immobile water depend on pore water chemistry, the type of clay, and the contact area of water with particles. Seepage can occur only when the hydraulic gradient increases sufficiently. To date, no accurate method is available to determine the immobile water content. Therefore, an approach is proposed for direct measurement of the immobile water content, which applies vegetable oil to displace water in saturated compacted clay-rich materials. Under a high hydraulic gradient, the vegetable oil completely displaces the free porewater in the clay-rich materials, and the remaining residual water is assumed to be immobile water. By using carbon tetrachloride (CCl4) as an extracting agent, the oil in the permeated sample was subjected to extraction and distillation, and the total liquid content was determined by the oven-drying method. As a final step, the immobile water content in the sample was calculated. A high-speed centrifugation method was used to determine the immobile water content from the corresponding soil water potential, which accounted for approximately 634.3 kPa. It was found that the immobile water of the clay-rich materials was essentially equal to the bound water. The effective porosity of clay was obtained based on the immobile water content. By comparing the relationship between porosity, effective porosity and hydraulic conductivity, the effective porosity revealed an improved linear relationship with log (k). The test results show the oil seepage method to be useful and reliable for the quantification of the immobile water content of saturated compacted clay-rich materials.

A zero emission scheme for producing energy from natural gas hydrates and conventional natural gas

The global amount of energy contained in natural gas hydrates is huge, maybe as much as twice all known conventional fossil fuel resources. Unlike the worldwide distribution of conventional fossil fuels, hydrocarbons trapped in water as hydrate is also available in countries with limited conventional hydrocarbon resources. The development towards lower global emissions of greenhouse gases requires new strategies for the use of hydrocarbons, whether they are available as conventional resources, or in the form of natural gas hydrates. In this work we outline some possible strategies of utilizing hydrocarbon energy resources in a clean and environmentally friendly way. The use of carbon dioxide for producing hydrates is not new. Results from several experimental studies are available. In this work we shed more light on thermodynamic limitations and the need for additives in order to make the approach technically efficient. Through thermodynamic analysis we show that up to 20 mol per cent N2 is feasible in a CO2/N2 injection gas based on ability to form a new hydrate with free pore water and the released enthalpy needed to dissociate in situ CH4 hydrate. Surfactant is also needed in order to keep the injection gas front free of blocking hydrate films. Small alcohols like methanol and ethanol have surfactant properties. It is demonstrated that even 10 wt% ethanol in liquid pore water still makes it feasible to create a new hydrate from injection gas containing 20 mol per cent N2. Consequences of other components than CH4 in the hydrate are also discussed. And in practical cases with a well-defined source of CO2 it is also important to investigate impact of other components like for instance H2S on the stability of injection gas hydrate, as well as changes in enthalpy of hydrate formation. Another key element of this work is the conversion of produced hydrocarbons over to hydrogen and carbon dioxide using steam cracking. The technology for this is very old and in daily use.

Small Alcohols as Hydrate Promoters

Many methods for the production of natural gas from hydrate reservoirs have been proposed during the latest 3 decades. Reducing pressure, thermal stimulation, or injection of thermodynamic hydrate inhibitors are three examples. A typical problem is, however, that different methods for producing hydrates are not evaluated thermodynamically prior to planning expensive experiments or even more expensive pilot tests. This can be due to the lack of a thermodynamic toolbox for the purpose. On a macroscopic level, there are two criteria that need to be met: the Gibbs free energy change has to be favorable and sufficient heat must be supplied in order to fulfill the first law of thermodynamics. Another challenge is the lack of focus on the limitations of the hydrate phase transition itself. The interface between the hydrate and liquid water is a kinetic bottleneck that requires efficient breaking of water hydrogen bonds. Reducing pressure does not address this problem. Injection of CO2, however, will lead to the formation of a new CO2 hydrate. This released heat from this hydrate formation is an efficient heat source for dissociating the in situ CH4 hydrate. Adding limited amounts of N2 increases the permeability for the injection gas. Addition of a surfactant increases the gas/water interface dynamics and promotes heterogeneous hydrate formation. In this work, we demonstrate a residual thermodynamic scheme that opens up for detailed thermodynamic analysis of different routes to hydrate formation and dissociation. It is demonstrated that addition of 20 mol % N2 to CO2 is thermodynamically feasible for generating a new hydrate in the pores. The available hydrate formation enthalpy when adding N2 is reduced as compared to pure CO2 but still considered as sufficient. Up to 3 mol % ethanol in the free pore water is also thermodynamically feasible. The addition of alcohol will not significantly disturb the ability to form a new hydrate from the injection gas. The released enthalpy from the formation of the new hydrate is also considered as sufficient compared to what is needed for dissociation of the in situ CH4 hydrate. Homogeneous hydrate formation from dissolved CH4 and/or CO2 is limited in amount and is not important. However, the hydrate stability limits related to the concentration of the hydrate former in the surrounding water are important. Mineral surfaces can act as hydrate promoters through direct adsorption, or adsorption in water, which is structured by the mineral surface charges. Examples from theoretical studies are discussed.

Small Alcohols as Surfactants and Hydrate Promotors

Many methods to produce hydrate reservoirs have been proposed in the last three decades. Thermal stimulation and injection of thermodynamic hydrate inhibitors are just two examples of methods which have seen reduced attention due to their high cost. However, different methods for producing hydrates are not evaluated thermodynamically prior to planning expensive experiments or pilot tests. This can be due to lack of a thermodynamic toolbox for the purpose. Another challenge is the lack of focus on the limitations of the hydrate phase transition itself. The interface between hydrate and liquid water is a kinetic bottle neck. Reducing pressure does not address this problem. An injection of CO2 will lead to the formation of a new CO2 hydrate. This hydrate formation is an efficient heat source for dissociating hydrate since heating breaks the hydrogen bonds, directly addressing the problem of nano scale kinetic limitation. Adding limited amounts of N2 increases the permeability of the injection gas. The addition of surfactant increases gas/water interface dynamics and promotes heterogeneous hydrate formation. In this work we demonstrate a residual thermodynamic scheme that allows thermodynamic analysis of different routes for hydrate formation and dissociation. We demonstrate that 20 moles per N2 added to the CO2 is thermodynamically feasible for generating a new hydrate into the pores. When N2 is added, the available hydrate formation enthalpy is reduced as compared to pure CO2, but is still considered sufficient. Up to 3 mole percent ethanol in the free pore water is also thermodynamically feasible. The addition of alcohol will not greatly disturb the ability to form new hydrate from the injection gas. Homogeneous hydrate formation from dissolved CH4 and/or CO2 is limited in amount and not important. However, the hydrate stability limits related to concentration of hydrate former in surrounding water are important. Mineral surfaces can act as hydrate promotors through direct adsorption, or adsorption in water that is structured by mineral surface charges. These aspects will be quantified in a follow-up paper, along with kinetic modelling based on thermodynamic modelling in this work.

The influence of solar radiation on plastic shrinkage cracking in concrete

Plastic shrinkage cracking (PSC) in concrete is related to the amount and rate of free pore water loss due to evaporation. This study investigates the influence of solar radiation on evaporation, the concrete temperature, plastic shrinkage and cracking. Literature suggests that exposure to solar radiation can either: increase the concrete temperature and, consequently, increase the rate of tensile strength gain resulting in less severe PSC or can increase the concrete temperature, resulting in a higher amount and rate of pore water loss to produce more severe PSC. The results of this study indicate that exposure to solar radiation significantly increases the amount and rate of pore water loss as well as plastic shrinkage and severity of PSC. To study PSC, many authors have estimated the evaporation rate from wind speed, air and concrete temperature and humidity which would result in significantly underestimating the actual evaporation rate if the concrete is exposed to moderate or high solar radiation. The accuracy of several radiation-related evaporation estimation models was evaluated by comparing the models to the actual rate of evaporation from specimens placed in the sun. A more accurate model was identified to estimate the evaporation in concrete specimens when exposed to solar radiation.

Residual water content of excavated soil in EPB tunnelling

The water content of the excavated soil in EPB (Earth Pressure Balance) tunnel construction controls the amount of conditioning of the excavated material in the excavation chamber. The water content influences crucial performance parameters (conditioning, wear, support pressure, etc.). With regard to foam conditioning, this means that the less water enters the excavation chamber, the drier the soil-foam mixture with constant foam properties. Especially in cohesionless soils below the groundwater level, the amount of free pore water is quite high. The cutting process, the effective pressure state and the use of conditioning agents result in flow processes of pore water and foam, which can lead to the displacement of water from the pore space at the tunnel face. Therefore, only residual moisture is present when the soil enters the excavation chamber. However, how much water enters the excavation chamber is still not determinable and not considered designing soil conditioning for the TBM drive.In this study, the initial water content was linked to an analysis of the residual water content entering the excavation chamber during EPB advance. Foam Penetration tests with different cohesionless soils were performed on the laboratory scale to analyze the residual water content. From the laboratory tests, water contents for the design of soil conditioning concepts can be determined for specific tunnelling situations.

Coupled nitrification and N2 gas production as a cryptic process in oxic riverbeds

The coupling between nitrification and N2 gas production to recycle ammonia back to the atmosphere is a key step in the nitrogen cycle that has been researched widely. An assumption for such research is that the products of nitrification (nitrite or nitrate) mix freely in the environment before reduction to N2 gas. Here we show, in oxic riverbeds, that the pattern of N2 gas production from ammonia deviates by ~3- to 16-fold from that predicted for denitrification or anammox involving nitrite or nitrate as free porewater intermediates. Rather, the patterns match that for a coupling through a cryptic pool, isolated from the porewater. A cryptic pool challenges our understanding of a key step in the nitrogen cycle and masks our ability to distinguish between sources of N2 gas that 20 years’ research has sought to identify. Our reasoning suggests a new pathway or a new type of coupling between known pathways in the nitrogen cycle.

Polychaete Invasion May Lead to Biogeochemical Change in Host Marine Environment

Marine invasive species may modify their host environment by altering ecosystem biogeochemistry. We hypothesized that the invasive polychaete Marenzelleria viridis in Baltic Sea areas increases sulfate reduction (SR) in sediment micro-zones surrounding its burrow. Consequently, higher free porewater sulfide (H2S) is expected in sediments dominated by M. viridis than in corresponding sediments inhabited by the native polychaete Hediste diversicolor. In a thin-aquaria experiment, we found high SR rates (220 to 539 nmol cm−3 d−1) around the burrow walls of M. viridis as well as in surface and subsurface sediments with overall rates 2-fold higher than in defaunated control sediment. Similarly, an in situ survey revealed subsurface porewater H2S peaks moving upward towards the sediment surface in M. viridis inhabited areas. Accordingly, 50–85% higher porewater H2S was found almost year-round in these areas compared with H. diversicolor inhabited areas, suggesting that the invasion of M. viridis probably led to a substantial change in sediment biogeochemistry. In conclusion, M. viridis stimulates SR in sediment micro-zones and increases H2S in coastal sediments. Such change to more reducing conditions after the invasion may have critical environmental implications on, e.g., the distribution of H2S intolerant flora and fauna species.

Crack-resistant cement–bentonite cut-off wall materials incorporating superabsorbent polymers

Preserving the integrity of cement–bentonite cut-off walls, particularly in aggressive environments, is critical to their serviceability in polluted sites. The hardened cement–bentonite material in cut-off walls is highly susceptible to desiccation and wet–dry cycles, commonly leading to cracking. The objective of the work presented in this paper was to develop crack-resistant cement–bentonite cut-off wall materials subject to wet–dry cycles. Superabsorbent polymers (SAPs), which are cross-linked polymers that can absorb and retain a large amount of water and swell as a result, were employed for this purpose. It is found that the added SAPs increased the compressive strength by decreasing the water-to-cement ratio and that the strain at failure also increased due to energy dissipative and reinforcement effects. In addition, crack resistance was greatly improved under the imposed wet–dry cycles as the matrix suction was reduced as a result of the reduction of the contact between the free pore water and cement–bentonite particles and the increase of the pore size in the matrix. The morphology and microstructure of the interconnected foam network formed by the SAP films in the matrix were identified with SEM-EDX and micro-CT scan analyses. The results demonstrated the significant potential for SAPs in the development of crack-resistant cement–bentonite cut-off wall materials.

Discrete element modeling of concrete under high stress level: influence of saturation ratio

The discrete element model proposed in this paper addresses the macroscopic behavior of concrete taking into account the presence of free water in pores, thanks to a new interaction law between spherical discrete elements (DE). When concrete structures are subjected to a severe loading, e.g., an impact, material exhibits high triaxial compressive stresses which are highly influenced by the saturation ratio. In this new constitutive model, cracking and compaction are modeled at the interaction level between DEs and free water effects are taken into account by introducing a dependency between the water saturation ratio and the inelastic deformation due to the pore closure. The present numerical model has been implemented in the Yet Another Dynamic Engine code in order to deal with extreme loading situations leading to stress states characterized by a high mean stress level.

Swelling of natural fibre bundles under hygro- and hydrothermal conditions: Determination of hydric expansion coefficients by automated laser scanning

The effect of humidity conditions on the moisture content and dimensional variations of natural fibre bundles from several botanical origins with contrasting biochemical and structural characteristics is investigated. Results highlight wide variations in water uptake and swelling behaviour of fibre bundles according to plant species. Two main swelling mechanisms are identified: (i) a microscopic swelling due to the sorption of bound water in the cell walls and the middle lamella, and (ii) a macroscopic swelling related to the formation of free water in pores and lumens, which induces anisotropic deformation of bast fibre bundles cross section. Cross-sectional hygro- and hydroexpansion coefficients are determined and studied in relation with the structural features of plant fibre bundles. These results constitute key data for the predictive modelling of “in-service” mechanical behaviour of biocomposites.

High‐resolution seismic velocity field estimation techniques and their application to geohazard, lithology and porosity prediction

ABSTRACTSeismic velocity is an attractive parameter for geohazard interpretation, pore pressure analysis, play and prospect evaluations, and other geological studies, but ordinary seismic processing velocities often do not have a good enough resolution. We adapt a dynamic time warping algorithm to estimate geologically reasonable high‐resolution velocities from average‐quality seismic data that can be used for geohazard analysis based on 3D seismic data. To predict free gas and/or excess pore water pressure in thin shallow layers, we use velocity inversion. It is a method for simultaneous inversion of velocity data to geological attributes. It runs in the depth domain, uses a background velocity model for balancing of input velocities to wells and a normal compaction trend model to simultaneously estimate lithology, pore pressure and net apparent erosion attributes, while porosity is calculated from a sandstone–porosity relationship. The overall workflow is applied for geohazard analysis at two marine sites. The first example is a deep‐water one from the Norwegian Sea, where thin and possibly overpressured or gas‐filled layers are identified in the Pleistocene section. The second example is in a region with limestone‐dominated lithology, where thin overpressured shales can cause severe drilling problems. In both examples, the thicknesses of the layers prone to geohazards are estimated to be about half of the wavelength. Dedicated high‐resolution velocity estimations, such as those obtained through the proposed workflow with dynamic time warping, applied to standard 3D seismic data and followed by dedicated velocity inversion routines, are, therefore, a necessity for proper geohazard assessment.