Decreasing Water Saturation Research Articles

Interactions Between Imbibition and Pressure-Driven Flow in a Microporous Deformed Limestone

Neutron imaging is used for direct observation of evolving water–air and deuterated water–normal water exchanges in flow experiments performed on a laboratory-deformed, microporous laminated limestone, an extremely fine-textured rock altered by arrays of superposed fractures generated in a rock mechanics apparatus. The neutron images document significant, evolving, water speed and flow direction variability at the deci-micron scale and spatially complex patterns of both increasing and decreasing water saturation. We infer that capillarity-driven and pressure-driven water movement occurs concurrently, in close proximity and in competition, and that as local and global water saturations evolve these two drivers can change their dominance in both matrix and deformed elements. Thin sections are used to obtain sub-micron resolution SEM images that provide multi-scale information on the textural features’ spatial arrangements. The textural characteristics are consistent with the inferences made from the coarser flow imaging. Alternating lamina types provide the primary lithological heterogeneity, while the experimentally created deformations lead to quasi-planar zones of highly comminuted matrix and fracture-like voids, each with lengths ranging from sub-mm to cm. Together deformation features delineate a partially connected array. The interplay between fluid movement through deformation features, and flow into (and out of) the laminae, implies near-equivalence of local driving pressure- and capillary-related energies, with subtle shifts in this balance as water saturation increases. The insights gained invite a re-examination of common rules-of-thumb for multi-phase fluid flow often adopted in fractured, low-permeability microporous rocks.

Improved Permeability Model of the Binary Gas Interaction within a Two-Phase Flow and its Application in CO2-Enhanced Coalbed Methane Recovery.

In this paper, we develop a dual-porosity dual-permeability model for binary gas migration to explore the permeability evolution in the matrix and fracture in the process of a gas–water two-phase flow during CO2-enhanced coalbed methane (CO2-ECBM) recovery in coal reservoirs. This mechanistic model accommodates the effects of elastic deformation caused by the effective stress change in the matrix and fracture, the swelling/shrinkage deformation of the matrix caused by adsorption/desorption, the convection and diffusion of gas, and the discharge of water. Specifically, the time-dependent matrix swelling, from initially completely reducing the fracture aperture to finally affecting the coal bulk volume, is considered by the invaded volume fraction involving binary gas intrusion. The model is validated through laboratory data and applied to examine the permeability evolution of CO2-ECBM recovery for 10 000 days. Furthermore, we analyze the sensitivity of some selected initial parameters to capture the key factors affecting CO2-ECBM recovery. Our modeling results show that the permeability evolution can be divided into two stages during the process, where stage I is dominated by effective stress and stage II is dominated by adsorption/desorption. Increasing the injection pressure or initial permeability advances the start of stage II. The decrease in initial water saturation causes the permeability to change more drastically and the time of stage II to appear earlier until a time long enough, after which little effect is seen on the permeability results.

Investigation of saturation effects on vibrations of nearly saturated ground due to moving train loads using 2.5D FEM

An improved two-and-a-half dimensional finite element method (2.5D FEM) model is developed to predict ground-borne vibrations in coastal areas from passing high-speed trains (HSTs). The model treats the ground as nearly saturated layered half-space, and describes the pore water/air mixed fluid by the equivalent fluid approach. Reliability of the proposed model is verified by comparing with the field measurements for X-2000 train and published analytical solutions. On this basis, the ground vibrations and excess pore water pressure under different train speeds and on various soil cases are analyzed in detail. In particular, three kinds of water saturation are discussed to show the sensitivity of vibration responses to the minor change of saturation. Results show that the saturation would cause pronounced influence on vibration displacements and excess pore pressure, and a minor decrease in water saturation would lead to a much higher vibration displacement level and a big difference of excess pore pressure distribution along depth. The influence of saturation is dependent both on soil configurations and on train speeds. It is highlighted that the vibration level on coastal ground would be underestimated by using fully saturated model instead of the nearly saturated model.

Mathematical model and field application of aqueous phase trapping release

ABSTRACT Aqueous phase trapping (APT) is a kind of mechanical formation damage, which occurs in tight sandstone gas reservoirs. The occurrence of APT has a dramatically adverse effect on the productivity of gas wells due to the decrease of water saturation and gas permeability near wellbore. In this research, a spontaneous imbibition experiment was carried out. The Lucas–Washburn model was used to describe the imbibition behavior. Then, a mathematical model of release judgment conditions was established based on the imbibition model, taking the threshold pressure gradient into consideration. Finally, combined with the two-phase flow mechanism of gas and water, the release time of APT was deduced. This model was used to calculate the releasing time of APT in actual producing wells, and the difference between the actual release time and predicted release time is only 0.1%, which indicates that the model established in this paper can be applied to predicting release judgment condition and release time in tight sandstone gas reservoirs precisely. The proposed model along with the findings of this research can pave the way for the continuous development of water breakthrough wells in tight sandstone gas reservoirs.

Pore-Scale Investigation of the Electrical Property and Saturation Exponent of Archie’s Law in Hydrate-Bearing Sediments

Characterizing the electrical property of hydrate-bearing sediments is essential for hydrate reservoir identification and saturation evaluation. As the major contributor to electrical conductivity, pore water is a key factor in characterizing the electrical properties of hydrate-bearing sediments. The objective of this study is to clarify the effect of hydrates on pore water and the relationship between pore water characteristics and the saturation exponent of Archie’s law in hydrate-bearing sediments. A combination of X-ray computed tomography and resistivity measurement technology is used to derive the three-dimensional spatial structure and resistivity of hydrate-bearing sediments simultaneously, which is helpful to characterize pore water and investigate the saturation exponent of Archie’s law at the micro-scale. The results show that the resistivity of hydrate-bearing sediments is controlled by changes in pore water distribution and connectivity caused by hydrate formation. With the increase of hydrate saturation, pore water connectivity decreases, but the average coordination number and tortuosity increase due to much smaller and more tortuous throats of pore water divided by hydrate particles. It is also found that the saturation exponent of Archie’s law is controlled by the distribution and connectivity of pore water. As the parameters of connected pore water (e.g., porosity, water saturation) decrease, the saturation exponent decreases. At a low hydrate-saturation stage, the saturation exponent of Archie’s law changes obviously due to the complicated pore structure of hydrate-bearing sediments. A new logarithmic relationship between the saturation exponent of Archie’s law and the tortuosity of pore water is proposed which helps to calculate field hydrate saturation using resistivity logging data.

Calculation Model of Relative Permeability in Tight Sandstone Gas Reservoir with Stress Sensitivity

During the development of tight gas reservoir, the irreducible water saturation, rock permeability, and relative permeability change with formation pressure, which has a significant impact on well production. Based on capillary bundle model and fractal theory, the irreducible water saturation model, permeability model, and relative permeability model are constructed considering the influence of water film and stress sensitivity at the same time. The accuracy of this model is verified by results of nuclear magnetic experiment and comparison with previous models. The effects of some factors on irreducible water saturation, permeability, and relative permeability curves are discussed. The results show that the stress sensitivity will obviously reduce the formation permeability and increase the irreducible water saturation, and the existence of water film will reduce the permeability of gas phase. The increase of elastic modulus weakens the stress sensitivity of reservoir. The irreducible water saturation increases, and the relative permeability curve changes little with the increase of effective stress. When the minimum pore radius is constant, the ratio of maximum pore radius to minimum pore radius increases, the permeability increases, the irreducible water saturation decreases obviously, and the two-phase flow interval of relative permeability curve increases. When the displacement pressure increases, the irreducible water saturation decreases, and the interval of two-phase flow increases. These models can calculate the irreducible water saturation, permeability and relative permeability curves under any pressure in the development of tight gas reservoir. The findings of this study can help for better understanding of the productivity evaluation and performance prediction of tight sandstone gas reservoirs.

A novel technique to estimate water saturation and capillary pressure of foam in model fractures

Foam is applied in enhanced oil recovery to improve the sweep of injected gas and increase oil recovery, by greatly reducing the mobility of gas. In the laboratory, X-ray computed tomography is commonly used to evaluate the performance of foam in core plugs. However, foam properties, such as bubble size and capillary pressure, are much more difficult to measure. In recent years, microfluidic models have gained much attention because they easily facilitate the imaging study of in-situ foam. However, it is still challenging to estimate capillary pressure, in a model with a uniform depth of etching. In this paper, we report a novel technique to estimate water saturation and capillary pressure of foam in two 1-meter-long model fractures. Both model fractures are made of glass plates. They have different roughness and hydraulic apertures. Unlike microfluidics with uniform depth of etching, our model fractures each has a variation of aperture. We characterize the roughness and represent the aperture distribution of the fracture as a network of pore bodies and pore throats. In this study, foam is pre-generated and then injected into the fractures. The inlet and outlet valves are closed for 24 hr after foam reaches steady-state. We use a high-speed camera to visualize foam in the fractures. We use ImageJ software to analyze foam texture and quantify bubble density, average bubble size and polydispersivity. In addition, we estimate water saturation and capillary pressure by analyzing images in terms of fracture geometry. We found that water in foam resides in locations of narrow aperture, Plateau borders, lamellae between bubbles, and water films on glass walls. Water-filled zones of narrow aperture and Plateau borders account for almost all the water. During the re-distribution of water and gas in static foam, in-flow and out-flow of water must take paths along the network of Plateau borders and water-occupied zones, as they are the only continuous paths for water flow. In both model fractures, the decrease in water saturation coincides with an increase in capillary pressure, as expected. This novel technique of estimation of water saturation and capillary pressure of foam provides insights for studies of foam in naturally fractured reservoirs with complex geometry, where measuring such foam properties is challenging. This analysis is possible because aperture varies along our model fractures, unlike most microfluidic devices. Our technique would also have an application to foam aquifer remediation and CO2 sequestration.

A Laboratory Approach to Measure Enhanced Gas Recovery from a Tight Gas Reservoir during Supercritical Carbon Dioxide Injection

Supercritical carbon dioxide injection in tight reservoirs is an efficient and prominent enhanced gas recovery method, as it can be more mobilized in low-permeable reservoirs due to its molecular size. This paper aimed to perform a set of laboratory experiments to evaluate the impacts of permeability and water saturation on enhanced gas recovery, carbon dioxide storage capacity, and carbon dioxide content during supercritical carbon dioxide injection. It is observed that supercritical carbon dioxide provides a higher gas recovery increase after the gas depletion drive mechanism is carried out in low permeable core samples. This corresponds to the feasible mobilization of the supercritical carbon dioxide phase through smaller pores. The maximum gas recovery increase for core samples with 0.1 mD is about 22.5%, while gas recovery increase has lower values with the increase in permeability. It is about 19.8%, 15.3%, 12.1%, and 10.9% for core samples with 0.22, 0.36, 0.54, and 0.78 mD permeability, respectively. Moreover, higher water saturations would be a crucial factor in the gas recovery enhancement, especially in the final pore volume injection, as it can increase the supercritical carbon dioxide dissolving in water, leading to more displacement efficiency. The minimum carbon dioxide storage for 0.1 mD core samples is about 50%, while it is about 38% for tight core samples with the permeability of 0.78 mD. By decreasing water saturation from 0.65 to 0.15, less volume of supercritical carbon dioxide is involved in water, and therefore, carbon dioxide storage capacity increases. This is indicative of a proper gas displacement front in lower water saturation and higher gas recovery factor. The findings of this study can help for a better understanding of the gas production mechanism and crucial parameters that affect gas recovery from tight reservoirs.

Effect of dry-wet alternations on the mechanical strength of deep tight sandstone

The working fluid used for gas drilling and the excessive differences in production pressure during the gas production process causes repeated fluctuations between increasing and decreasing water saturation of the rock. These repeated fluctuations result in dry-wet state alternations of the rock, which weakens the rock strength. Weakening rock strength will lead to sand shedding, which damages reservoirs because of particle migration, solid blockage, and sand production. This will also change the stress sensitivity of deep tight sandstone, thus affecting gas well production. Therefore, this study conducted both a dry-wet alternation experiment and a stress sensitivity experiment after dry-wet alternation using deep tight sandstone. During the experiment, the changes of the rock dynamic Young's modulus, Poisson's ratio, and stress sensitivity coefficient were monitored. The experimental results show that the dynamic Young's modulus and Poisson's ratio of rock samples decreased after dry-wet alternations. Furthermore, these alternations increase the stress sensitivity of the rock matrix and weaken the stress sensitivity of fractured rock samples. • The mechanism of weakening the strength of deep tight sandstone by dry-wet alternation is analyzed. • The influence of dry-wet alternation on stress sensitivity of deep tight sandstone is analyzed. • Dry-wet alternation may be one of the reasons for sand production in deep tight sandstone.

NAPL-water interfacial area as a function of fluid saturation measured with the interfacial partitioning tracer test method

The presence of organic immiscible liquids such as chlorinated solvents and fuels continues to be a primary source of risk for many hazardous waste sites. In this study, the standard miscible-displacement interfacial partitioning tracer test (IPTT) method was used for the first time to measure NAPL-water interfacial areas for a range of saturations. Multiple measurements were conducted for a natural quartz sand, with tetrachloroethene as the representative NAPL. The interfacial areas increased with decreasing water saturation. The measurements compared well to interfacial areas measured for the same sand with two alternative tracer methods, the mass-distribution batch method and the two-phase flow method. Measurements obtained with all three tracer-based methods exhibit a relatively large degree of variability. Thus, it is important to employ replication when using these methods. In contrast, interfacial areas measured with x-ray microtomography exhibit very small variability. However, the measured interfacial areas do not capture the contribution of surface-roughness to film-associated interfacial area. Each method has associated advantages and disadvantages, and it is important to be cognizant of them during their application.

Substantiating the choice and optimizing the parameters for the technology of hot recycling of asphalt concrete road coating

The method of expert assessments has been used to substantiate that the most significant factors that significantly influence the mechanism and kinetics of technological processes of the regenerated mixture preparation are the temperature of its preparation and the content of the regenerating additive. We have investigated their influence on the physical-mechanical parameters of regenerated asphalt concrete, obtained using the technology of hot recycling of asphalt concrete pavement applying the «in place» method. The experimental research has established the dependences of the physical and mechanical parameters (water saturation, compressive strength at 0 °С and 50 °С) of regenerated asphalt concrete on the content of an additive and stirring temperature. The mathematical models have been constructed in the form of second-degree polynomials that describe the dependence of water saturation and strength at the compression of regenerated asphalt concrete on the temperature of preparation and the content of a regenerating additive. It has been established that increasing the stirring temperature increases compressive strength at 0 °С; at the same time, compressive strength at 50 °С and the measure of water saturation decreases. The increase in the regenerating additive reduces all the examined physical and mechanical indicators. Based on an analysis of the results obtained, we have determined the regions of rational values of the stirring temperature of loose asphalt-concrete crumbs (125‒135) °С, as well as the content of an additive (0.26‒0.28) %, when preparing hot asphalt concrete mixture. In terms of physical-technical indicators, the resulting mixture meets the requirements set by DSTU B V.2.7-119:2011 «Asphalt concrete mixes and road and airfield asphalt concrete. Technical specifications» for hot fine-grained asphalt mixtures. The reported results could prove useful when devising a technology of hot recycling of asphalt concrete pavement by the «in place» method and for studies related to its improvement

Effects of wettability of conductive particles on the multifrequency conductivity and permittivity of fluid-filled porous material

Existing mechanistic models for electromagnetic characterization of porous materials filled with two fluids (e.g. oil- and water-filled subsurface geological formation) assume the conductive solid components (e.g. graphite and pyrite particles) and the non-conductive solid components (e.g. clay and sand grains) are completely wetted by only one fluid. However, the wettability of particles constituting the porous material have strong influence on the electromagnetic properties of the fluid-filled porous materials. We developed a new mechanistic model to quantify the effects of wettability (i.e. contact angle) of conductive particles on the multi-frequency complex conductivity/permittivity of the fluid-filled porous material containing both non-conductive and conductive particles. The new mechanistic model accounts for the interfacial polarization phenomena at the interface of the conductive particles having any wettability (ranging from strongly water wet to strongly oil wet) and the pore-filling two-phase fluid for various fluid saturations as a function of the operating frequency of the externally applied electromagnetic field. The newly developed model shows that the frequency dispersions of complex conductivity/permittivity of porous material filled with oil and water reduce with the decrease in water-wetness of the conductive particle and with the decrease in water saturation. Notably, frequency-dependent complex conductivity/permittivity of porous material is more sensitive to change in water wetness (i.e. contact angle) of the conductive particles as compared to change in water saturation. This work has significance and relevance for electromagnetic-based characterization of fluid-filled porous materials; for example, estimation of hydrocarbon pore volume in oil/gas reservoirs and estimation of water saturation in aquifers.

Resilience of the sorption capacity of soil organic matter during drying-wetting cycle

The sorption capacity of soil organic matter (SOM) for hydrophobic organic chemicals (HOCs) is affected by various environmental factors, such as soil water saturation and drying. In this study, we used passive sampling to investigate the changes in the sorption capacity of SOM during a drying-wetting cycle using batch sorption experiments. Dried and non-dried peat mosses were used to observe the effect of the drying process on the sorption capacity of SOM at various levels of water saturation in soil pores. At soil with non-dried peat moss, the partition coefficient between the sampler and the soil (Ksampler/soil) slightly increased with decreasing water saturation. At soil with dried peat moss, however, there were almost no differences in the Ksampler/soil among different water saturations except for 100%. The soil organic carbon-water distribution coefficients (KOC) for dried peat moss were consistently larger than those for non-dried peat moss at all water saturation levels. However, the KOC values obtained at 100% water saturation for both non-dried and dried peat mosses differed only by 18-29%. For fluoranthene, there was only an 18% difference between the two KOC values at 100% water saturation, whereas it was 91% at 10% water saturation. This finding suggests that wetting SOM returns mostly its sorption capacity for HOCs after the increase in KOC caused by extreme drying. The range in sorption capacity obtained in this study showed the resilient margin of the sorption capacity of SOM for HOCs according to microclimatic changes that would occur constantly under environmental conditions.

A model of fully coupled two-phase flow and coal deformation under dynamic diffusion for coalbed methane extraction

To further explore the mechanism of fluid migration during long-term CBM recovery, a fully coupled model of two-phase flow and coal deformation is developed in this investigation based on gas dynamic diffusion. Although the coal mass is considered to be a dual-porosity single-permeability (DPSP) geological model, multiscale fractal characteristics of multistage pores are considered to illustrate the decrease in the gas diffusion coefficient over time. In addition, the model also considers water saturation, relative permeability, Klinkenberg effects and the changes in permeability caused by the fracture pressure (including the water pressure and gas pressure), matrix pressure (only the gas pressure), and desorption-induced shrinkage. Considering the combined impact of the abovementioned parameters, we established four governing equations and two supplementary equations and solved these problems with COMSOL multiphysics software. Subsequently, the model is validated via field test data, and then, through a comparison between our model and two typical coupled models for CBM extraction, we observed that the model that simplified CBM extraction as a single-phase flow process could overestimate gas production at the early stage, while the model that disregarded the impacts of the dynamic diffusion coefficient could overestimate gas production at the later stage. Finally, the coupled model was used to model long-term CBM and water production. The numerical results show that the gas pressure in the fracture and matrix and the water saturation decrease with time. And matrix gas pressure obtains a higher value than the fracture gas pressure in the later stage of extraction. In the area far from the wellbore, the permeability is lower than its initial value, while in the near-wellbore region, the permeability shows the opposite behavior. A sensitivity analysis of the current model illustrates that increases in the Langmuir strain constant, Young's modulus, initial permeability diffusion coefficient could promote the rate of gas production. Overall, our modified model for CBM extraction can enhance the gas and water migration rules and provide more accurate predictions for gas production.

Experimental study of the effective stress coefficient for coal permeability with different water saturations

The effect of effective stress on coal reservoir permeability is inevitable during coalbed methane (CBM) production. Studying the effective stress coefficient for permeability (ESCK) is crucial because it determines the effective stress and stress sensitivity of a coal reservoir. Considering the actual drainage process of a CBM well, a series of stress sensitivity experiments with different water saturations were designed and implemented. The experimental results show that the gas measured permeability of the coal sample decreased with increasing water saturation. The change in permeability with different water saturations had a significant power relationship with increasing net pressure. Using the response surface method (RSM), ESCK values with different water saturations were obtained. The ESCK value, regardless of the stress conditions, is a constant at a certain water saturation, and it shows an increasing trend with increasing water saturation. The ESCK values with different water saturations indicate that the effective stress decreases with increasing water saturation. The stress sensitivity and dynamic change in coal reservoir permeability were also analyzed using ESCK values with different water saturations. The stress sensitivity coefficient decreased with increased water saturation, and the reservoir permeability decreased more obviously with an ESCK value of 0.7675 than that with a value of 1.0. During CBM well production, water saturation decreases, and the stress sensitivity of coal reservoir permeability increases.

Investigating ground vibration induced by moving train loads on unsaturated ground using 2.5D FEM

A two-and-a-half-dimensional finite element method (2.5D FEM) is applied to investigate the dynamic response of an unsaturated ground subjected to moving loads caused by high-speed train. The partial differential equations of unsaturated porous medium in frequency domain are deduced based on the equations of motion and mass conservation of three phases, with consideration of the compressibility of solid grain and pore fluid. Governing equations of unsaturated soil in 2.5D FE form are derived by using the Fourier Transform with respect to the load moving direction. The track structure is simplified as an Euler beam resting on the unsaturated porous half-space and the viscous-elastic artificial boundaries are used to avoid the energy reflection from the boundary. Numerical simulations demonstrate effects of the degree of water saturation and train speed to the ground vibration and the excess pore water pressure. It is concluded that the degree of water saturation has a different influence on the ground displacement and acceleration. The gas phase has varied influence to the ground displacement amplitude at different train speed level at the track center. A very small amount of gas in the saturated ground largely increases the ground acceleration amplitude at a given train speed. Ground displacements attenuate rapidly with almost the same rate for both high and low train speeds near the track center. The maximum amplitude of excess pore water pressure is located at 1.5–2.0 m beneath the ground surface and decreases significantly as the degree of water saturation decreases.

Effect of oil-contamination and water saturation on the bearing capacity and shear strength parameters of silty sandy soil

This study investigates the effect of oil contamination and water saturation on the geotechnical properties of silty sandy soil. The silty sandy soil (SM) in this study was obtained from Prachuap Khiri Khan Province, Thailand, and the contaminant product was 95 octane gasoline. The SM soils were prepared in a tank model under the conditions of dry soil, soil with a water level, gasoline-contaminated soil of 2%, and gasoline-contaminated soil of 4%. A laboratory experiment (direct shear test) and a field experiment (lightweight penetration test) were performed on SM soil for all conditions. The lightweight penetration test results indicate that the ultimate bearing capacity in the case of soil with water saturation decreases by 90% on average compared to the dry soil condition. The ultimate bearing capacity for the gasoline-contaminated SM soils of 2% and 4% decrease by 30% and 52%, respectively, on average compared to the dry soil condition. Moreover, the internal friction angle obtained from the direct shear test linearly decreases with the gasoline content and reduces by at least half in the case of water saturation. The cohesion of the oil-contaminated SM soils increases with increasing oil content as according to the second-order polynomial regression, and the dry soil and saturated soil have similar cohesions. These results reveal that the oil contamination and water saturation cause the loss of shear strength and reduce the ultimate bearing capacity of SM soil.

Effects of water saturation on the strength and loading-rate dependence of andesite

Water saturation conditions in the rock masses around underground openings vary with place and time. For the long-term stability assessment of underground structures, it is essential to understand the effect of water on the time-dependent behavior of rocks. In this study, the effects of water saturation on the loading-rate dependence of strength, which is one of the time-dependent characteristics of rocks, were investigated both theoretically and via experiments. First, the effects of loading rate and water saturation on rock strength and its variability were clarified by means of a theory that is based on both stochastic and rate processes. Next, alternating loading-rate tests were conducted on Sanjome andesite using uniaxial compression testing under various water saturation conditions. The test results showed that the rock strength increases with a decrease in water saturation and that an increase in strength accompanied by a tenfold increase in loading rate is almost constant under various water saturation conditions. All of the test results were consistently explained by our theory of the dependence of rock strength and its variability under different loading rates and water saturation conditions. It was also found that the dependence of strength on the loading rate is equivalent to that on the water saturation, and that the strength obtained from a test could be converted to one under an arbitrary loading rate and water saturation condition. In addition, we proposed a constitutive equation based on these theoretical and experimental results that could reproduce the loading-rate dependence of stress-strain curves under various water saturation conditions using a single set of constants.

Influence of water saturation and particle size on methane hydrate formation and dissociation in a fixed bed of silica sand

In this work, the effects of water saturation and particle size on methane hydrate formation and dissociation in a fixed bed of silica sand were investigated. The experiments were carried out at 277.15 K with the initial pressure fixed at 8 MPa. Three water saturations (50%, 70%, and 90%) and two particle sizes (0.18-0.25 mm and 0.3-0.9 mm) of silica sand were used. The results indicated that water conversion to methane hydrates increased with the decrease of water saturation. 70% was found to be an optimum water saturation due to the largest hydrate amount and the highest gas uptake obtained using the small particle size of silica sand (0.18-0.25 mm). In addition, hydrate formation rate and gas uptake obtained using small particle size of silica sand were larger, and gas recovery and hydrate dissociation rate were higher in the dissociation experiments using the small particle size of silica sand. Therefore, the medium water saturation (70%) and small particle size (0.18-0.25 mm) are favorable for methane hydrate formation and dissociation in the fixed bed of silica sand.

Experimental and Theoretical Study of Water Retention Effects on Elastic Properties of Opalinus Shale

Understanding of shales elastic properties behaviour with saturation changes is important for geological storage of nuclear waste, CO2 sequestration as well as for development of conventional and unconventional shale oil and gas reservoirs. Existing data describing effects of saturation on elastic properties of shales are sparse and contradictory. To improve understanding of the effects of changing water content on elastic properties in shales, we conduct an experimental study on Opalinus shale samples. We measure vertical and horizontal ultrasonic P- and S-wave velocities of the same set of samples with controlled water content. The measured velocities are used to calculate components of elastic stiffness tensor in the shale at different saturations assuming its vertical transverse isotropy. Obtained results show increasing C11 and decreasing C33 with drying of the samples. Moreover, we observe 80% and 60% increase of shear moduli C44 and C66, respectively, with reduction of the water content from 5.5% weight in the preserved state to 0.3%. Conventional rock physics models are not designed to explain the observed dynamics. Here we perform a theoretical investigation of the influence on the shale moduli of following factors: (1) mechanical softening of the rock with the decrease of water saturation; (2) shrinkage of clay leading to reduction of porosity; (3) chemical hardening of clay particles; and (4) enhancing stiffness of contacts due to removing of water between clay particles.