Pore Fluid Salinity Research Articles

Pore fluid salinity effects on physicochemical-compressive behaviour of reconstituted marine clays

It has been well recognized that pore fluid salinity significantly affects physical and mechanical properties of clays. However, how to correlate the pore fluid salinity effects on mechanical behaviour with physicochemical variations induced by pore fluid salinity changes is still pending. This study investigates the changes in liquid limit, oxide compositions and compressibility with pore fluid salinity, based on experimental data obtained from reconstituted specimens of two marine clays in China. It is found that oxide compositions vary little with the change in pore fluid salinity, indicating that no chemical reaction occurs during pore salt changing for the investigated clays. Such a finding means that the pore fluid salinity effects on compression behaviour of reconstituted clays can be attributed to the variations in physical properties. For the investigated marine clays with illite as predominant clay mineral, the void ratio at liquid limit is the crucial physical index of assessing the pore fluid salinity effects on the compression behaviour. The intrinsic compression concept is also introduced to compare the compression behaviour of reconstituted clays with and without pore salt. A quantitative approach is proposed to assess the pore fluid salinity effects on the compression behaviour of marine clays reconstituted at different initial water contents.

Development of a laboratory-scale numerical model to simulate the mechanical behaviour of deep saline reservoir rocks under varying salinity conditions in uniaxial and triaxial test environments

Abstract The maintenance of the long-term mechanical stability of the reservoir rock mass is essential in CO 2 sequestration in deep saline aquifers. However, it cannot be confirmed without predicting the worst-case scenarios in saline aquifers, including high salinity conditions and the complexities caused by surrounding factors such as reservoir depth. Laboratory experiments to identify all such situations are difficult due to the advanced facilities required, and the associated cost and time. Therefore, numerical models play an important role in extending laboratory measurements for such complex and extreme situations. Although numerous numerical studies have been performed to date on field-scale conditions in saline aquifers, less consideration has been given to simulating laboratory data, which is important for up-scaling the data to field conditions. This study therefore aims to develop a laboratory-scale numerical model to simulate the mechanical behaviour of brine-saturated reservoir rock under triaxial stress laboratory conditions. The model validation was performed by measuring uniaxial and triaxial laboratory test data under 10–25 MPa confining pressures and the model was then used to investigate the influence of pore fluid salinity percentage on reservoir rock strength by considering various possible salinity levels (5%, 10%, 15%, 20%, 25% and 30% NaCl) and the influence of depth using a range of confining pressures from 10 to 100 MPa. The proposed numerical model based on the stiffness degradation mechanism of reservoir rock can accurately simulate salinity-dependent stress-strain behaviour under any stress environment (uniaxial/triaxial). According to the model, both pore fluid salinity and confining stress add additional strength to the reservoir rock mass due to NaCl crystallization and pore shrinkage. Importantly, the model clearly shows a reduction of the effect of pore fluid salinity on reservoir rock strength characteristics with increasing reservoir depth or confinement, mostly due to the more significant effective stress at such extreme depths. This provides an important finding on CO 2 sequestration in saline aquifers: salinity-dependent strength alteration is not very important for extremely deep aquifers compared to shallow aquifers. Although this model has the capability to simulate the failure of reservoir rock under extreme pressure conditions, the simulation results show a small fluctuation near the post-peak stage due to the complexity of the damage mechanism caused by strain localization.

Effects of pore fluid salinity on the collapsibility of low plasticity clay

This paper presents the results on the influence of pore fluid salinity on the Collapsibility of low plastic clay from Hashemite town, Jordan. Double odometer tests were carried out on natural specimens using pore fluids with different salt concentrations. The results indicate a considerable amount of collapse takes place for the sample with increasing the initial applied pressure on the sample regardless of the initial molding water content. However, the dry side of optimum sample shows larger collapse than that on wet side of optimum sample and vice versa. At low applied initial stresses, the higher the salts percentage in the sample, the larger the collapse occurs. Moreover, the amount of collapse tends to increase with increasing the applied pressure to reach a certain value for a given salts percentage, then the amount of collapse asymptotes to its maximum value or level off with increasing the pressure on the sample.

Monitoring shale gas resources in the Cooper Basin using magnetotellurics

The magnetotelluric (MT) method is introduced as a geophysical tool to monitor hydraulic fracturing of shale gas reservoirs and to help constrain how injected fluids propagate. The MT method measures the electrical resistivity of earth, which is altered by the injection of fracturing fluids. The degree to which these changes are measurable at the surface is determined by several factors, such as the conductivity and quantity of the fluid injected, the depth of the target interval, the existing pore fluid salinity, and a range of formation properties, such as porosity and permeability. From an MT monitoring survey of a shale gas hydraulic fracture in the Cooper Basin, South Australia, we have found temporal and spatial changes in MT responses above measurement error. Smooth inversions are used to compare the resistivity structure before and during hydraulic fracturing, with results showing increases in bulk conductivity of 20%–40% at a depth range coinciding with the horizontal fracture. Comparisons with microseismic data lead to the conclusion that these increases in bulk conductivity are caused by a combination of the injected fluid permeability and an increase in wider scale in situ fluid permeability.

Impact of pore fluid salinity on the mechanical behavior of unsaturated bentonite–sand mixture

High salinity of local groundwater and the thermal gradient present within the deep geological repository environment make it necessary to investigate the effect of the pore fluid salinity on the hydro-mechanical behavior engineered barriers at different degrees of saturation. This study examines the impact of CaCl2 salt solution at two concentrations, i.e., 100 and 250 g/L on the compressibility and shear strength behavior of the bentonite–sand buffer (BSB) within a range of saturations. Both as-compacted and desiccated BSB specimens were studied. Desiccated specimens were prepared in a constant-suction environment using the vapor equilibrium technique, and high-pressure cells were used to carry out the triaxial shear experiments. Experimental results were compared with the available data from the literature for BSB materials prepared with distilled water in order to get a better understanding of the impact of the salinity. The comparison shows that the existence of salt in the specimens has notable effect on the total suction levels, compressibility and shear strength of the material.

Archie's law – a reappraisal

Abstract. When scientists apply Archie's first law they often include an extra parameter a, which was introduced about 10 years after the equation's first publication by Winsauer et al. (1952), and which is sometimes called the “tortuosity” or “lithology” parameter. This parameter is not, however, theoretically justified. Paradoxically, the Winsauer et al. (1952) form of Archie's law often performs better than the original, more theoretically correct version. The difference in the cementation exponent calculated from these two forms of Archie's law is important, and can lead to a misestimation of reserves by at least 20 % for typical reservoir parameter values. We have examined the apparent paradox, and conclude that while the theoretical form of the law is correct, the data that we have been analysing with Archie's law have been in error. There are at least three types of systematic error that are present in most measurements: (i) a porosity error, (ii) a pore fluid salinity error, and (iii) a temperature error. Each of these systematic errors is sufficient to ensure that a non-unity value of the parameter a is required in order to fit the electrical data well. Fortunately, the inclusion of this parameter in the fit has compensated for the presence of the systematic errors in the electrical and porosity data, leading to a value of cementation exponent that is correct. The exceptions are those cementation exponents that have been calculated for individual core plugs. We make a number of recommendations for reducing the systematic errors that contribute to the problem and suggest that the value of the parameter a may now be used as an indication of data quality.

Preliminary Studies of Hydraulic and Mechanical Behavior of Nanoparticle-Based Light Backfill Exposed to Pore Fluid Salinity

AbstractBecause of its high swelling pressure and low hydraulic conductivity, compacted bentonite–sand is widely accepted as a barrier/backfill material used in deep geological repositories to safely contain highly toxic radioactive waste. The majority of research conducted in this field shows that the pore fluid salinity present in the repository’s environment reduces the swelling capacity and increases the hydraulic conductivity of backfill material. However, research has yet to determine how to improve the material properties of backfill under saline water conditions. In this study, two different percentages of nanoparticles derived from bentonite (1 and 2% with respect to dry weight of bentonite) were carefully mixed with a bentonite–sand mixture to prepare a nanoparticle-based light backfill. Next, the effect of different concentrations of NaCl and CaCl2 on the hydraulic and mechanical behavior of a 50∶50 bentonite–sand mixtures was examined, at a maximum dry density of 1.24 Mg/m3. The results showe...

Geophysical delineation of acidity and salinity in the Central Manitoba gold mine tailings pile, Manitoba, Canada

Surface electrical and electromagnetic geophysical methods can map enhanced electrical conductivity caused by acid mine drainage in mine tailings piles. In this case study, we investigate quantitative relationships between geophysical responses and the electrical conductivity, acidity and salinity of tailing samples at the Central Manitoba Mine tailings in Manitoba, Canada. Previous electromagnetic surveys at the site identified zones of enhanced conductivity that were hypothesized to be caused by acid mine drainage. In the present study, high-resolution EM31 and DC-resistivity measurements were made on a profile through a zone of enhanced conductivity and laboratory measurements of salinity and pH were made on saturation paste extracts from an array of tailing samples collected from the upper 2m of tailings along the profile.Observed spatial correlation of pH and pore-fluid salinity in the tailings samples confirms that the enhanced conductivity in the Central Manitoba Mine tailings is due to acid mine drainage. Contoured cross-sections of the data indicate that the acid mine drainage is concentrated near the base of the oxidized zone in the thicker parts of the tailings pile. The zone of increased acidity extends to the surface on sloping margins causing an increase in apparent conductivity in shallow penetrating geophysical responses. The quantitative relationship between measured pH and salinity shows that the conductivity increase associated with the acid mine drainage is due only in part to conduction by ions produced from dissociation of sulfuric acid. Comparison of the observations with fluid conductivity estimates based on statistical relationships of pH and ion concentrations in water samples from across the tailings pile shows that Ca2+ and Mg2+ ions also make significant contributions to the conductivity at all values of pH and Cu2+, Al3+ and Fe3+ ions make additional contributions at low pH. Variability in the measured conductivity at constant values of pH is explained by variations in the contribution of the SO42− to the measurements. The results of the study demonstrate the feasibility of using electromagnetic geophysical surveys to map acid mine drainage, and related ionic concentration, in mine tailings.

Pore fluids and the LGM ocean salinity—Reconsidered

Pore fluid chlorinity/salinity data from deep-sea cores related to the salinity maximum of the last glacial maximum (LGM) are analyzed using estimation methods deriving from linear control theory. With conventional diffusion coefficient values and no vertical advection, results show a very strong dependence upon initial conditions at −100 ky. Earlier inferences that the abyssal Southern Ocean was strongly salt-stratified in the LGM with a relatively fresh North Atlantic Ocean are found to be consistent within uncertainties of the salinity determination, which remain of order ±1 g/kg. However, an LGM Southern Ocean abyss with an important relative excess of salt is an assumption, one not required by existing core data. None of the present results show statistically significant abyssal salinity values above the global average, and results remain consistent, apart from a general increase owing to diminished sea level, with a more conventional salinity distribution having deep values lower than the global mean. The Southern Ocean core does show a higher salinity than the North Atlantic one on the Bermuda Rise at different water depths. Although much more sophisticated models of the pore-fluid salinity can be used, they will only increase the resulting uncertainties, unless considerably more data can be obtained. Results are consistent with complex regional variations in abyssal salinity during deglaciation, but none are statistically significant.

Theoretical study on the amplitude ratio of the seismoelectric field to the Stoneley wave and the formation tortuosity estimation from seismoelectric logs

SUMMARY Seismoelectric logging has potential applications in exploring the porous formation characteristics. In this study, we theoretically address how the amplitudes of the Stoneley wave and its accompanying borehole seismoelectric field depend on the porous formation parameters such as porosity, permeability and tortuosity. We calculate the ratio of the electric field amplitude to the pressure amplitude (REP amplitude) for the low-frequency Stoneley wave of different formations and find that the REP amplitude is sensitive to porosity and tortuosity but insensitive to permeability. To confirm this conclusion, we derive the approximate expression of the REP amplitude in the wavenumber–frequency domain which shows that the REP amplitude is dependent on tortuosity but independent of permeability. This contradicts the result concluded by previous researchers from experiments that the REP amplitude is directly proportional to the permeability. The reason is probably attributed to the fact that the rock samples with different permeabilities typically have different tortuosities. Since the REP amplitude is sensitive to tortuosity, we propose a method of estimating formation tortuosity from seismoelectric logs. It is implemented by using the REP amplitude and the wavenumber of the Stoneley wave at a given frequency when the porosity and the pore fluid viscosity and salinity are known. We test the method by estimating the tortuosities from the synthetic seismoelectric waveforms in different formations. The result shows that the errors relative to the input tortuosities are lower than 5.0percent without considering the uncertainties of other input parameters.

Effects of Salinity on the Geotechnical Characterization of Fine-Grained Soils and Mine Tailings

The objectives of this study were to evaluate the effects of soluble salts (i.e., salinity) on the geotechnical characterization of fine-grained soils and mine tailings and to identify applicable test methods to characterize high-saline soils. Three fine-grained soils were used in this study: soda ash mine tailings, kaolin clay, and bentonite clay. The soda ash (sodium carbonate) mine tailings contained high-saline pore fluid and, predominantly, sodium on the exchange complex, whereas kaolin and bentonite clay were used for comparison with the soda ash tailings. Soluble salts were removed from the tailings using dialysis to create specimens with different pore fluid salinity, whereas sodium chloride (NaCl) was added to kaolin and bentonite. The 1:5 electrical conductivity (EC1:5) of the as-collected soda ash tailings was 19.3 dS/m (deciSiemens per meter), and five additional specimens were prepared with EC1:5 ranging between 1.12 and 17.3 dS/m. The EC1:5 of kaolin specimens ranged from 0.14 to 27.4 dS/m and bentonite specimens ranged from 3.78 to 116 dS/m. The effects of pore fluid salinity were evaluated on Atterberg limits, specific gravity, and particle-size distribution via hydrometer tests. Liquid limit (LL), plastic limit, and clay content decreased for all three materials with increasing pore fluid salinity. Temporal evaluations of soil plasticity suggest that hydration times of at least 2 days are required to solubilize salts and capture salinity effects on LL. Additionally, experimental methods were developed for correcting errors in hydrometer and specific gravity tests that may originate from the presence of soluble salts.

Effect of pore water salinity on the coefficient of earth pressure at rest and friction angle of three selected fine-grained materials

This study investigates the effects of pore fluid salinity on the shear strength and earth pressure coefficient at rest (K0) of three fine-grained soils: kaolin, bentonite and a marine clay found in Hong Kong. The K0 coefficient and critical state friction angle (ϕ') are evaluated from triaxial stress path testing of remolded normally consolidated samples. The pore fluid salinity has little effect on the K0 and ϕ' of kaolin and the studied marine clay, whereas an increase in pore fluid salinity noticeably increases ϕ' and decreases the K0 of bentonite. These phenomena are explained by the microstructure of the fundamental minerals of the materials. The K0 of each of the studied materials is estimated by two commonly used empirical formulas in which the internal friction angle (Jaky's formula) or Atterberg limits are used, respectively. Jaky's formula satisfactorily predicts K0 for kaolin regardless of its salinity, whereas it slightly but consistently underestimates that of the studied marine clay. Moreover, Jaky's formula gives a reasonable prediction of K0 for bentonite at higher salinity. Prediction using the Atterberg limits fails in all cases, particularly for bentonite at low salinity.

Effect of Changing Water Content on the Properties of Kuttanad Soil

This paper brings out various mechanisms that control the changes in the properties of Kuttanad soil due to removal of water upon drying. The detailed studies conducted include microfabric imaging, analysis of pore fluid and evaluation of the index properties of the soil before and after drying. The role of pore fluid salinity and organic matter was evaluated by studying the index properties of the soil after subjecting it to different treatments. Although several factors such as dehydration due to mineralogical changes, presence of sesquioxides and oxidation of pyrites are responsible for changes in soil properties, it has been brought out that the drying up of organic matter further causing cementation and aggregation in the soil is a major cause, influencing the soil properties upon drying.

Effect of water salinity on Atterberg limits of El-Hodna sabkha soil

Sabkha soils are recent saline sediments widely distributed in Algeria and other countries. As they are characterised by low strength and high compressibility, they are considered to be difficult foundation soils that pose special problems for design and construction engineering. This study deals with the effect of salts on the determination of the water content and Atterberg limits of sabkha soils. The tests were performed using distilled water, natural sabkha brine and saline solutions with different salt concentrations. The results indicate that the liquid and plastic limits decrease with pore fluid salinity when the conventional water content procedure is used, but increase when the fluid content method is used. Also, the results revealed that the oven-drying time of saline soils is highly affected by the size of soil clods and surface salt precipitation.

Comment on “Examination of a Theoretical Model of Streaming Potential Coupling Coefficient”

Recently, Luong and Sprik published an article that compared measurements that had been made on 20 samples of saturated rock with a number of empirical models and Glover et al.’s 2012 theoretical model for zeta potential and streaming potential coefficient. They found that none of the empirical models could reproduce the streaming potential coefficient measurements which had been made in the presence of low pore fluid salinities, and the theoretical method could only do so if a constant zeta potential was invoked. This contribution in the form of a comment (i) indicates at least three possible errors in modelling that contribute to the mismatch between the theoretical model and the data at low salinities and (ii) carries out individual modelling on all of samples of Luong and Sprik’s 2014 dataset, showing that Glover et al.’s 2012 theoretical model matches the data well when the zeta potential is allowed to vary and good match can only be obtained with a constant zeta potential if an unrealistic value of zeta potential offset is used.

Fluid Inclusions and Their Application in Hydrocarbon History and Genesis

The fluid inclusions characteristics and pore fluid salinity of 56 samples from clastic reservoir of the Lower Cretaceous were analyzed in the Tanzhuang Sag. The authors also discuss the hydrocarbon charge history and the genesis of secondary pores in the Lower Cretaceous. The multipeak distribution of homogenous temperature of the brine inclusions showed that the inclusions captured should be multiperiod fluids at various depths. Comparison of the homogeneous temperature of brine inclusions with the temperature on the burial history curve suggested that the source rocks had entered the stage of large-scale hydrocarbon generation. However, only individual brine inclusions containing methane were found, and no oil-bearing inclusions were seen. The absence of oil-bearing inclusions indicated that no large-scale oil charges and limited favorable mature source rocks were present in the study area. The paleosalinity varied with depth and was divided into three zones, namely two low-value zones in the shallow and deep burial and one high-value zone in the medium burial. The secondary pores were relatively developed in the high-salinity fluid zone. Secondary pores are chiefly related to the inorganic acid associated with dehydration and mutual transformation of clay materials, and to the organic acid released from limited mature organic matter.

Spectral induced polarization of clay-sand mixtures: Experiments and modeling

Spectral induced polarization or complex conductivity is a promising electric method in hydrogeophysics because of its sensitivity to water saturation, permeability, and particle size distribution (PSD). However, the physical and chemical mechanisms that generate the low-frequency complex conductivity of clays are still debated. To explain these mechanisms, the complex conductivity of kaolinite, smectite, and clay-sand mixtures was measured in the frequency range 1.4 mHz–12 kHz with various clay contents (100%, 20%, 5%, and 1% in volume of the clay-sand mixture) and salinities (distilled water, [Formula: see text], [Formula: see text], and [Formula: see text] of NaCl in solution). The results indicated the strong impact of the cation exchange capacity of smectite upon the complex conductivity of the material. The quadrature conductivity increased steadily with the clay content and was fairly independent of the pore fluid salinity. A mechanistic induced polarization model was also developed. It combined a Donnan equilibrium model of the surface electrochemical properties of clays and sand, a conduction model of the Stern and diffuse layers, a polarization model of the Stern layer, and a macroscopic conductivity model based on the differential effective medium theory. It also included the effect of the PSD. Our complex conductivity model predicted very well the experimental data, except for very low frequencies ([Formula: see text]) at which membrane polarization may dominate the observed response.

Different effects of temperature and salinity on permeability reduction by fines migration in Berea sandstone

Hot water injection into geothermal aquifers is considered in order to store energy seasonally. Berea sandstone is often used as a reference formation to study mechanisms that affect permeability in reservoir sandstones. Both heating of the pore fluid and reduction of the pore fluid salinity can reduce permeability in Berea sandstone. These effects could be caused by mobilisation of fines by increasing the repulsive electrical double layer forces among sandstone grains and the fines. We investigated the reversibility and the dependence on flow velocity and flow direction of the permeability change by means of flow through experiments and examined thin sections of samples prior to and after tests. A permeability reduction at 20°C with decreasing salinity was not reversed by restoring the salinity, whereas a permeability reduction due to heating to 80°C was reversible by restoring the temperature to 20°C. A reversible permeability increase with increasing flow rate was observed at 80°C but not at 20°C. We observed no difference in the distribution of kaolinite clay minerals in thin sections of untested and tested samples. Dissolution of iron bearing carbonates and precipitation of iron hydroxides was observed but no effect on permeability was found. The experimental results suggest that different mechanisms are responsible for permeability reduction depending on temperature and salinity.

The role of fluid pressure and diagenetic cements for porosity preservation in Triassic fluvial reservoirs of the Central Graben, North Sea

Anomalously high porosities and permeabilities are commonly found in the fluvial channel sandstone facies of the Triassic Skagerrak Formation in the central North Sea at burial depths greater than 3200 m (10,499 ft), from which hydrocarbons are currently being produced. The aim of our study was to improve understanding of sandstone diagenesis in the Skagerrak Formation to help predict whether the facies with high porosity may be found at even greater depths. The Skagerrak sandstones comprise fine to medium-grained arkosic to lithic-arkosic arenites. We have used scanning electron microscopy, petrographic analysis, pressure history modeling, and core analysis to assess the timing of growth and origin of mineral cements, with generation, and the impact of high fluid pressure on reservoir quality. Our interpretation is that the anomalously high porosities in the Skagerrak sandstones were maintained by a history of overpressure generation and maintenance from the Late Triassic onward, in combination with early microquartz cementation and subsequent precipitation of robust chlorite grain coats. Increasing salinity of pore fluids during burial diagenesis led to pore-filling halite cements in sustained phreatic conditions. The halite pore-filling cements removed most of the remaining porosity and limited the precipitation of other diagenetic phases. Fluid flow associated with the migration of hydrocarbons during the Neogene is inferred to have dissolved the halite locally. Dissolution of halite cements in the channel sands has given rise to megapores and porosities of as much as 35% at current production depths.

Seismic facies analyses as aid in regional gas hydrate assessments. Part-II: Prediction of reservoir properties, gas hydrate petroleum system analysis, and Monte Carlo simulation

A new strategy to assess occurrences of marine gas hydrate has been implemented for the study area of the Ulleung Basin, East Sea, where two comprehensive gas hydrate drilling programs completed logging and coring at 18 sites. In this study we introduce multi-attribute analyses using five drill sites along a regional 2D seismic line to predict physical properties of P-wave velocity, density, and porosity required for gas hydrate saturation calculations. The five well-sites allow a reasonable cross-validation of the final multi-attribute-based prediction and to derive statistical parameters used in a Monte Carlo simulation of total gas hydrate volume in the study area. A similar analysis was completed for a 3D seismic volume around Site UBGH1-4. The crucial boundaries of the top of gas hydrate occurrence zone (TGHOZ) and base of the gas hydrate stability zone (BGHSZ) that are part of the gas hydrate petroleum system were calculated using constraints on geothermal gradient, pore-fluid salinity, type of hydrocarbon gases present, sedimentation rates, and total organic carbon content based on results from the two drilling expeditions carried out in the Ulleung Basin as well as other regional studies. Gas hydrate saturations were determined following the effective medium theory and using the sedimentological data obtained at the drill sites to define required mineral compositions. Seismic facies classification allowed the definition of occurrence of certain sediment types along the seismic data used in this study, and physical properties from the multi-attribute analyses and mineral compositions were assigned to each facies class. Statistical parameters of mean and standard deviation were estimated for each input parameter (P-wave velocity, bulk density, mineral compositions, grain density, density-porosity, TGHOZ, BGHSZ) in the gas hydrate saturation calculations based on individual uncertainties in the measurements. In the final step, a Monte Carlo simulation is used to calculate the total amount of gas hydrate (in m3) present in the study region for the two example data set of a 2D seismic line and one 3D seismic volume. The Monte Carlo simulation draws on the various statistical distributions of the input parameters and we have implemented 150,000 simulation runs in this study. The final histogram distributions of total volume of gas hydrate (in m3) allow determination of the mean, median, and mode values, as well as the 95% and 5% probability thresholds for gas hydrate volumes being present in the two study regions, though they are not meant as estimates of possible commercial amounts of gas associated with gas hydrates in the Ulleung Basin.