High Fines Content Research Articles

Technology Focus: Sand Management (October 2024)

With the resurgence of drilling activities post-COVID-19, there is a growing interest in refining existing technologies and exploring new ones to discover cost-effective and reliable solutions for developing aged and more-challenging new reservoirs. Although approaches may vary slightly depending on the unique characteristics of reservoirs in different regions, the general trends are conceptually similar. Drilling longer wells, increasing the number of fracture stages, and completing multiple zones in complex stacked reservoirs, along with optimizing multilateral wells with sand control to enable long laterals in stacked reservoirs, have emerged as key focuses of the industry. Drilling longer wells introduces several challenges, including managing installation loads, ensuring proper wellbore cleanout, and ultimately producing effectively and uniformly from extended laterals that drill through heterogeneous reservoirs with varying sand facies. While liner flotation is effective in reducing installation loads and improving wellbore cleaning, it presents challenges when used in conjunction with conventional sand-control screens. Sand control and flow control are becoming more integrated to offer solutions for effective production from longer laterals. Flow segmentation helps prevent premature failures caused by gas or water breakthroughs or localized high rates causing erosion. Paper SPE 218074 is a good example of using flow-control devices to enhance the reliability and longevity of sand-control systems. Stacked reservoirs have always posed challenges in terms of zonal isolation and sand-control design. Higher fines content and silty sand with complex reservoir conditions leads to less interest in standalone screens because of the high risk of plugging and failure in such developments. Even though gravel packing is effective, it poses several technical and implementation challenges. The complexity of sandface completion, with varying sand sizes, pressures, and fluid properties, makes these wells particularly difficult to complete. Paper SPE 214914 offers a good case study of such a complex completion successfully implemented in the field. Despite all this, the fundamentals of screen design and implementation remain the same. Screen plugging remains one of the major challenges in the sand-control industry. Rigorous testing of sand-control media to verify retention properties, erosional resistance, integrity requirements for the installation-loading and lifelong-loading scenarios, and plugging resistance are critical aspects of any sand-control qualification. I strongly recommend reading paper IPTC 23690 for a detailed understanding of the screen-qualification process and the design of screens for target sand facies. Also, it is crucial to remember that no screen is completely immune to plugging. The unintended consequences of a change in drilling/completion fluid or cleanout process could lead to screen plugging. It is always important to test and validate the effect of any change in fluids or completion practices on sand-control performance. Recommended additional reading at OnePetro: www.onepetro.org. SPE 215070 A Comprehensive Review of Screen Plugging During Openhole Sand-Control Completions Installation: Causes, Consequences, and Best Practices by Maye Beldongar, SLB, et al. SPE 214934 Shunted Gravel-Pack Failures and How To Prevent Them by Carolina Latini, DuneFront, et al. SPE 215411 Performance of Bonded Bead Sand Screen as Remedial Sand Control in Highly Erosive Environment by Ashvin Avalani Chandrakant, Petronas, et al.

The influence of fines content on ground collapse due to internal erosion of sand-fines mixtures around defective pipes

Ground collapse caused by defective pipelines has become a global issue with the continuous advancement of urbanization. This study reproduced the internal erosion process of sand-fines mixtures around defective pipes through a series of model tests considering a relatively wide range of fines content and three hydraulic conditions. The erosion characteristics and the evolution of the cavity were analyzed to determine the triggering mechanisms of ground collapse. The results indicate that the formation and evolution of the cavity predominantly depend on fines content under hydraulic loading, which is replaced by ground settlement at lower fines content. Moreover, the scouring of groundwater amplifies the impact of internal erosion, which means the marked ground settlement at lower fines content or the expansion of the cavity even to ground collapse at higher fines content. To prevent sudden ground collapse, it is not recommended to use sand-fines mixtures with a fines content of more than 10% as embedment materials.

A New Shear Wave Velocity-Based Liquefaction Probability Model Using Logistic Regression: Emphasizing Fines Content Optimization

A new liquefaction probability model based on shear wave velocity (Vs) was developed through a detailed comparative analysis of existing evaluation methods. Publicly available shear wave velocity liquefaction data were used to evaluate multiple existing liquefaction probability assessment methods under various probability contours and fines content levels. Significant performance differences were observed among the formulae under varying fines content levels. To construct the new model, the random forest feature importance ranking algorithm was employed to select the key parameters, including the effective stress-normalized shear wave velocity (Vs1), corrected cyclic resistance ratio (CSR7.5), magnitude (MW), depth (Z), and fines content (FC). Using these parameters, a new liquefaction probability assessment formula was developed utilizing the logistic regression model to predict the liquefaction probability. The new formula’s performance was subsequently evaluated through a detailed case analysis and validation. The results demonstrate that the new formula achieves a higher accuracy (3–11%) for the liquefaction assessment compared to the existing formulae, performing consistently well across different probability contours and fines content levels, especially in areas with high fines content. This study provides theoretical support and empirical evidence for optimizing the shear wave velocity-based liquefaction probability assessment methods.

Internal erosion in granular soils with different microstructures under cyclically increased hydraulic gradients

Internal erosion is one of the leading causes of failures and accidents of embankment dams, dikes, and slopes. The hydraulic loading acts as the driving force to detach the soil particles, while the initial soil microstructure determines the susceptibility of particle loss. In engineering practices, the soil may be subjected to cyclic hydraulic loadings due to water level fluctuations by extreme weather of intensive rainfall and drought. Under such conditions, the soil internal erosion process will be different from that under the steady unidirectional seepage, which has not been fully studied and needs urgent investigation. Thus, in this study, the internal erosion process and hydraulic characteristics of soils with different microstructures were investigated by both laboratory seepage tests and the discrete element method (DEM) simulation. The results indicate that the soil with a higher fine content was more structurally stable, and required a larger hydraulic gradient for erosion initiation. Once the internal erosion occurred, the particle loss and the soil hydraulic conductivity increased with increasing fine content. Additionally, when the applied hydraulic gradient was essentially the same, the soil experienced a server erosion under the cyclically than monotonically increased hydraulic gradients, and the amount of eroded soils increased with the increasing gradient amplitude. The results of this study will expand our understanding of the physical mechanism and hydraulic behaviors of soils subjected to cyclically increased hydraulic gradients and with different microstrures.

Electrochemical Characteristics of Lignin in CTMP for Paper Battery Electrodes.

Lignin has been extensively researched as a cathode active material in secondary batteries. In the present work, the energy storage potential of lignin naturally present in papers made of softwood chemi-thermomechanical pulp (CTMP) is explored. More specifically, effects from softwood CTMP fines on the electrochemical characteristics have been studied. Compared to pulp fibers, fines are higher in lignin content and have higher specific surface area. It was expected that this would be positive for the electrode performance; however, the result points to the opposite. The fines do not significantly contribute to a higher lignin specific capacity, and they deteriorate the cycling stability. Higher fines content was found to result in a higher oxidative activity as well as more abundant competing reactions. These competing reactions are believed to be linked to the cycle stability. Therefore, we hypothesize that the electrochemical stability of lignin can be better understood by studying differences between fines and fiber lignin. As the theoretical specific capacity of this material is about 20 times larger than obtained here, identification of the reasons for this capacity discrepancy is needed to realize the full potential of lignin-based paper batteries.

Impact of pozzolanic and inert powders on the microstructure and thermal chemistry of cement mortars

The aim of this work is the investigation of the microstructure of two types of white, high-workability cement mortars: type 1 (Am) with a relatively high content of marble powder (Binder:Aggregate = 1:2) containing fine particles, and type 2 (Mz) with a high content of marble powder (Binder + Pozzolan:Aggregate = 0.9 + 0.1:2) and up to 10 wt% clinoptilolite as a pozzolan from the white cement content. The density of the composites’ structures at 1, 28, and 120 days of water curing was evaluated by measurements of their physical-mechanical properties - density, compressive strength, and porosity. Crystal microstructure and sample morphology were investigated with mercury intrusion porosimetry (MIP), X-ray powder diffraction analysis (PXRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermal analysis (TG/DTG-DSC). The conducted studies established that a high content of fines makes the structure denser. The formed structure has open porosity, which allows for the transportation of water, with slow continuous hydration resulting in the growth of crystals and the formation of various products, especially with the addition of clinoptilolite. The formation of cement hydrated minerals (CSH and CASH) such as ettringite, hillebrandite, yugawaralite, wairakite, hibschite, together with hemi- and mono-carboaluminate, etc., has been proven. The composition of the cement mortar with a high content of marble powder and up to 10 wt% clinoptilolite has the potential for further improvement of the process of hydration by forming a greater variety of calcium silicate hydroxyl phases. Despite the complexity of the reactions in the hydration of mortars, a generalized scheme of the reactions describing the main steps of the process is presented in correlation with the specified experimental conditions. The results allow for the development of sustainable building material strategies using waste and natural materials to reduce resource dependence and energy for production. Widespread adoption of sustainable building materials is seen as one of the most promising approaches to increasing the efficiency of the construction sector in a sustainable way and ensuring continued economic growth and a circular economy.

A comparative study assessing the validity of water absorption methods when applied to secondary fine aggregates

The valorization of secondary sands is crucial for fostering a circular economy within the construction sector. One significant obstacle in this endeavor stems from the high water absorption (WA) rates commonly reported for secondary sands. This high WA leads to increased water and cement demand in cementitious mixes and elevates costs. The only widely standardized method to measure the WA of fine aggregates is the cone method, which has proven ill-suited for fine aggregates with angular particles or a high content of fines. This study undertakes a comparative evaluation of the cone method and seven alternative methods: immersion, centrifugation, paper sheet, colorimetric, volumetric flask, continuous drying, and electrical conductivity. Tests were performed on natural river sand and three types of secondary sand. Findings reveal that the continuous drying and electrical conductivity methods demonstrates the greatest potential in terms of reliability and applicability. Also the volumetric flask and centrifugation methods show some potential, provided that critical refinements are made to the corresponding test protocols.

A resolved CFD-DEM investigation of near-wellbore fine sand migration and production during methane hydrate extraction

Methane hydrate extraction from unconsolidated reservoirs can face challenges due to excessive sand production in the wellbore. Sand production has long been a concern in petroleum engineering and has been extensively studied by researchers. This study investigates sand production in gas-water two-phase flow through numerical simulations. The simulations incorporate the discrete element method (DEM) and resolved computational fluid dynamics (CFD) to model the solid-fluid interaction，which allows for simulating the particle movements and capturing the variations in hydraulic properties of the granular sample at a particle scale. Additionally, a volume of fluid (VOF) method is employed to simulate the two-phase flow. The numerical model provides insights into the gas movement process within the granular matrix and visually depicts the microscopic mechanisms of particle migration during methane hydrate extraction. The results of the study demonstrate that the model incorporating gas injection, which involves injecting a predetermined volume of gas at the inlet to the fluid model, yields a higher mass of produced sand compared to the model without gas injection. Furthermore, as the volume of gas injection increases, the produced mass initially rises and then declines. In addition, parameter analysis shows that the pattern of sand production differs between the model with a higher fines content and the model with a lower fines content. With the increase of hydraulic gradient, the produced mass increase.

Research on construction parameters of stone columns for liquefaction mitigation in coral reef sand areas

For loose coral reef sand with high fines content located in strong earthquake zones, it is necessary to choose stone columns as a liquefaction mitigation ground improvement method that can consider both drainage and densification functions. Taking the Tibar Bay Port project in East Timor as an example, the bottom-feed technology of vibrating stone columns was developed, and typical coral reef sand sites with different fines contents were selected as trial test areas. Experimental research was performed to study the construction parameters for onshore and offshore stone column processes. Tests showed that the vibratory process control parameters, quality acceptance standards, and control procedures proposed through research meet the anti-liquefaction requirements for both drainage and densification effectiveness. The different construction characteristics of onshore and offshore stone columns were also analyzed. Considering the particularity of underwater stone column construction, the research results can be used for stone columns reinforced grounds similar to coral sand onshore and offshore.

Progressive internal erosion during triaxial shearing: an X-ray micro-computed tomography study

Fines removal during internal erosion results in the permeability increase and strength reduction of granular soils. This study examines internal erosion during shearing of granular soils under triaxial loading conditions using a specially designed apparatus and X-ray computed tomography scans. Two gap-graded soil samples with different fines content are tested and scanned multiple times to characterise sample- and pore-scale volume change and fines content using image-processing techniques. Results show that internal erosion increases gradually with axial strain and has a complex impact on permeability increase due to the spatial variation of fines content. Triaxial shearing induces both pore contraction and dilation, destabilises the clogging of fine particles and promotes internal erosion. Also, the erosion of force-supporting fine particles at a high fines content facilitates pore contraction. The progressive internal erosion due to mechanical disturbance leads to the formation of a flow channel from the upstream loading platen throughout the sample, resulting in a dramatic increase in permeability. Overall, this study provides novel insights regarding the interplay between the hydro-mechanical processes during shearing under constant seepage.

Diatom-induced impact on shear strength characteristics of fine-grained soils

Diatomaceous soils, composed of diatom microfossils with biological origins, have geotechnical properties that are fundamentally different from those of conventional non-diatomaceous fine-grained soils. Despite their high fines content, diatomaceous soils typically exhibit remarkably high shear resistance, approaching that of sandy soils. However, the exact role that diatoms play in controlling the mechanical properties of fine-grained soils and the underlying mechanisms remain unclear. In light of this, the shear strength response of diatomaceous soils was systematically investigated using consolidated undrained triaxial compression tests on diatom–kaolin mixtures (DKMs) with various diatom contents and overconsolidation ratios. The micro- and nano-scale structures of the soil samples were characterized in detail using scanning electron microscope (SEM) and atomic force microscope (AFM) to interpret the abnormal shear strength parameters of diatomaceous soils. The results indicated that the presence of diatoms could contribute to significantly higher strength, e.g. the friction angle of DKMs was improved by 72.7% to 37° and the value of undrained shear strength tripled with diatom content increasing from 20% to 100%. Such significant improvement in soil strength with diatom inclusion could be attribute to the hard siliceous skeleton of diatoms and the interlocking between particles with rough surfaces, which were quantitatively analyzed by the surface roughness parameters with AFM. Furthermore, a conceptual model established based on the macro-mechanical tests and microscopic observations portrays a microstructural evolution of soils with increasing diatoms. The microstructure of soils was gradually transformed from the matrix-type to the skeletal one, resulting in a continual augmentation in shear strength through mutual interactions between diatom microfossils. This paper provides new insights into the multi-scale structural properties of diatoms and significantly advances our understanding of the mechanical behavior of diatomaceous soils.

Micromechanism and mathematical model of stress sensitivity in tight reservoirs of binary granular medium

Research on reservoir rock stress sensitivity has traditionally focused on unary granular structures, neglecting the binary nature of real reservoirs, especially tight reservoirs. Understanding the stress-sensitive behavior and mathematical characterization of binary granular media remains a challenging task. In this study, we conducted online-NMR experiments to investigate the permeability and porosity evolution as well as stress-sensitive control mechanisms in tight sandy conglomerate samples. The results revealed stress sensitivity coefficients between 0.042 and 0.098 and permeability damage rates ranging from 65.6% to 90.9%, with an average pore compression coefficient of 0.0168–0.0208 MPa−1. Pore-scale compression occurred in three stages: filling, compression, and compaction, with matrix pores playing a dominant role in pore compression. The stress sensitivity of binary granular media was found to be influenced by the support structure and particle properties. High stress sensitivity was associated with small fine particle size, high fines content, high uniformity coefficient of particle size, high plastic deformation, and low Young's modulus. Matrix-supported samples exhibited a high irreversible permeability damage rate (average = 74.2%) and stress sensitivity coefficients (average = 0.089), with pore spaces more slit-like. In contrast, grain-supported samples showed low stress sensitivity coefficients (average = 0.021) at high stress stages. Based on the experiments, we developed a mathematical model for stress sensitivity in binary granular media, considering binary granular properties and nested interactions using Hertz contact deformation and Poiseuille theory. By describing the change in activity content of fines under stress, we characterized the non-stationary state of compressive deformation in the binary granular structure and classified the reservoir into three categories. The model was applied for production prediction using actual data from the Mahu reservoir in China, showing that the energy retention rates of support-dominated, fill-dominated, and matrix-controlled reservoirs should be higher than 70.1%, 88%, and 90.2%, respectively.

Rock fragmentation size distribution control in blasting: a case study of blasting mining in Changjiu Shenshan limestone mine

Deep-hole bench blasting is the primary method for aggregate extraction in mines. However, factors such as complex geological conditions and suboptimal blasting parameters often result in uneven rock fragmentation and high fines content. This not only increases the cost and energy consumption of subsequent aggregate processing but also has adverse environmental implications. In this study, based on the Changjiu Shenshan limestone aggregate mining project in China, large-scale blasting experiments were conducted to investigate the influence of rock properties and blasting parameters on the size distribution of post-blast fragments and fines content. The results of the blasting experiments indicate that by controlling the size of the crushing zone and adjusting explosive performance, it is possible to significantly reduce fines content while improving mining efficiency. Recommended values for drilling and blasting parameters have been proposed based on geological conditions to more effectively control the generation of fines. The results highlight the importance of optimizing blasting parameters and charge structure for large-scale mining operations to achieve uniform rock fragmentation and low fines content. By adopting explosive performance adjustment methods based on reasonable control of the crushing zone, improving explosive performance can improve the economic benefits of mining operations, reduce energy consumption, and contribute to environmental protection.

Properties governing the flow of solution through crushed ore for heap leaching: Part III – Low-permeability ores

Heap leaching is used extensively for the processing of low-grade copper and gold ores. If the ore has low permeability, it may lead to underperformance or even project failure. Poor permeability is caused by a high fines content, resulting in a high steady-state moisture hold-up which causes the bed to operate close to saturation. The problem of low permeability can be addressed by either desliming (removal of the fines by wet screening), or by agglomeration (binding the fines together) with a cementitious binder. Fines removal by desliming is shown to reduce the steady-state moisture hold-up, resulting in the bed operating further away from saturation and increasing the maximum solution application rate. Agglomeration with Ordinary Portland Cement modified with additives resulted in fine particles binding together as lumps or coatings onto coarser rocks. These bonds were not destroyed after prolonged exposure to dilute sulphuric acid, due to the pozzolanic properties of the additives (e.g. fly ash), which cause cementitious Ca-bonds to be substituted with more stable Al- or Mg-bonds. Agglomeration with cementitious binder was also found to reduce slumping in leach column tests. This resulted in a higher bed porosity, higher limits on the solution application rate and higher copper dissolution rates. Agglomeration with modified cement may provide a novel, cost-effective method for the treatment of low-permeability ores in acid medium (e.g. copper, uranium and nickel), since cement agglomeration has only been applied commercially in alkali medium (e.g. cyanide solutions for gold ores) to date. Desliming is less attractive due to the high losses of the target metal to the fines.

Rheological behavior of cellulose nanofibril suspensions with varied levels of fines and solid content

The rheological behavior of mechanically disk refined cellulose nanofibril (CNF) aqueous suspensions with 50, 60, 70, 80, 90, 100 % fines contents at 1 and 3 wt% solid concentrations was studied by conducting various dynamic and steady-state rheological experiments, including oscillatory shear, flow sweep, startup, flow loop, and three-step oscillation experiments. All the samples exhibited a gel-like behavior in oscillatory shear tests and samples, with 50 % fines showed the highest levels of dynamic moduli for both solid contents. Suspensions with higher concentrations of CNFs showed higher values for viscosity, complex viscosity, and dynamic moduli. A critical shear rate of 10 s−1 independent of the solid contents of the suspensions was found for varying fines levels, where viscosity measurements above this critical shear rate converged and viscosity measurements below it diverged. All samples exhibited yield in shear flow and yield stresses exhibited a decreasing trend followed by a plateau as fines levels increased. The level of yield stress for 3 wt% suspensions was higher than that of 1 wt% suspensions. The lowest structure recovery was observed for samples containing 50 % fines content. Moreover, a rheopectic-thixotropic transition was observed for samples with high fines content (90 and 100 %) at low solid content (1 wt%).

Microscopic mechanism of the combined effects of confining pressure and fines content on suffusion in gap-graded underfilled soils

Suffusion is a typical form of internal erosion, which is the primary cause of seepage failures in hydraulic geo-structures. Revealing the underlying mechanism of suffusion is crucial for designing hydraulic geo-structures and preventing associated failures. Extensive studies have investigated the combined effects of fines content and confining pressure on suffusion in gap-graded soils. However, few of them have focused on underfilled soil, which may be largely different from overfilled soil. This study aims to study microscopic mechanism of suffusion in underfilled soil using the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM). Three fines content (15 %, 20 % and 25 %) and three confining pressures (50, 100 and 200 kPa) are considered for investigating their combined effects on suffusion. The results show two distinct patterns of macroscopic response for the underfilled soils. That is, the cumulative eroded fines mass decreases with increasing confining pressure, especially for samples with high fines content. Additionally, samples with low fines content exhibit negligible suffusion-induced volumetric deformation, while those with high fines content display significant deformation. Particle migration analysis shows that the samples are subjected to non-uniform erosion along the seepage direction, with minimal changes in the number of particles in the middle section, but a notable reduction near the inlet and outlet sections. Moreover, greater confining pressure results in fewer migrating fine particles, particularly in samples with high fine content. Furthermore, particle contact analysis indicates that in samples with low fines content, effective stress is mainly transmitted through coarse particles, resulting in a reduced impact of confining pressure on the internal stability of fine particles. Conversely, in samples with high fines content, a greater proportion of fine particles and coarse particles jointly form the load-bearing skeleton, making them more prone to confining pressure. Hence, two distinct microscopic mechanisms are proposed to well explain the suffusion behavior of underfilled soil from particle-scale perspectives.

Soil strength improvement by reinforcing soil with geotextiles

In recent times, geosynthetic products are widely utilized in the construction sector and particularly in the field of Geotechnical Engineering. Geosynthetics are the synthetic products used mainly within the soil to perform various functions. In this paper, an attempt has been made to utilize Woven geotextiles (WG) and Non-woven geotextiles (NWG) as reinforcement within soil to enhance the strength of subgrade soil. 2 types of coarse-grained soil, soil with high fines content and soil with low fines content were utilized in the experimental work. WG and NWG were placed at varying depths of H/5, 2H/5, 3H/5 and 4H/5 from top of the soil specimen (where H is the height of soil specimen), to find the enhancement in soaked & unsoaked California Bearing Ratio (CBR) strength of soil with respect to that of unreinforced soil. Based on the experimental results, improvement in CBR strength of soil reinforced with geotextile and the ideal location of placement of geotextile within the soil thickness varied according to the soil type and this may be attributed to the presence of varied percentages of fines within soil. Also, the soil reinforced with WG was observed to be more effective in terms of strength gain as compared to that of soil reinforced with NWG. Improvement in CBR strength of subgrade soil reinforced with geotextiles leads to reduced pavement thickness thus resulting in a cost-effective construction.

Modification and mechanism of polymer in high-performance concrete with full aeolian sand: An integrated macro-to-micro perspective

Producing high-performance concrete with full aeolian sand (FA-HPC) can achieve efficient utilization of desert resources to combat the shortage issue of aggregates. However, low water–cement ratio and high fine content result in a high risk of brittleness and durability in this concrete. Herein, polyacrylamide (PAM) and styrene-butadiene rubber latex (SBR) were incorporated into the FA-HPC to optimize its performance. Effects and mechanisms modifying the FA-HPC with polymers were compared and studied from macro-to-micro perspectives, with respect to workability, mechanical properties, durability, adsorption, and hydration. The results revealed a negative correlation with the PAM content (contrary to the SBR content) in fluidity and the content-dependent extension in setting time. For strength and shrinkage, the best performance was observed in the PAM-modified FA-HPC at 0.05% content presenting the substantial mitigation in brittleness. Moreover, regarding chloride resistance, SBR showed a more positive effect than PAM, with a 40.9% improvement at 10% content. From the micro-perspective observation, the retarding effect at early stages and acceleration during the late hydration process were verified by phase analysis. The polymer film and polymer bridge as well as the refined pore structure were the primary contributing factors to the performance improvement. Overall, compared to SBR modification, PAM modification with stronger adsorption capacity balances the comprehensive performance improvement of FA-HPC better at 0.05% content. Additionally, the specificity of the FA-HPC and the difference in modification behavior were discussed. This research allows the material-composition optimization of FA-HPC and further promotes the wide application of aeolian sand-based cementitious materials.

Experimental study of fines migration in gap-graded soils with gas production: Implications to hydrate production from unconsolidated reservoirs

Methane hydrate production from unconsolidated reservoirs could be hampered by excessive sanding, which has been particularly noticed during gas production from hydrate-bearing sediments in the deep ocean. The sanding process involves the interaction of liquid, gas, particles, and phase change. Here we present an experimental study of production-induced fines migration under K0 pressure in a gap-graded sediment by using a transparent horizontal permeameter that allows the sand production process to be recorded. Effervescent powders, which release carbon dioxide when exposed to water, are used to simulate hydrate, producing gas in unconsolidated sediments to reproduce the phase change process. Our results show that permeability decreases due to pores being filled by gas in the initial stage of pressure variation, which further leads to the sudden collapse of the specimen, manifested by a burst period on the erosion curve. For the gap-graded soil with low fines content, gas fills the pores and accelerates the flow to wash away more fine grains. As the fines content increases, the effect of gas on the finial erosion weight decreases. The erosion localization is more severe, and the formation time of the flow channel is shorter in the soil with gas generation and high fines content. Once the flow channel is formed, erosion in the remaining areas is reduced as water flows converge into the flow channel. Sensitivity analysis shows that the final erosion weight increases with increasing grain size ratio, fine grain content, and effervescent powder content. As the size of effervescent powder increases, the final erosion weight also increases.

Shear stiffness of sand-fines binary mixtures: Effects of sand gradation and fines content

In this study, a comprehensive experimental programme was carried out on five clean sands of different gradations mixed with low plastic fines using an apparatus that incorporates both resonant column (RC) and bender element (BE) functions. The sand gradation is characterized by uniformity coefficient of host sand (Cus) and grain size ratio (Rd), which is defined as the ratio of the mean grain size of sand to that of fines. The effects of fines content (FC) and sand gradation on small strain shear modulus G0 are analyzed from the experimental results. Both RC and BE test results show that G0 continuously decreases in a non-linear manner as FC increases to 30% and becomes as low as nearly 50% of that of clean sand at the highest FC. The reduction rates with respect to FC are similar regardless of the testing type and the stress dependence of G0 is insensitive to FC. Meanwhile, G0 decreases as Cus or Rd increases at a given FC and void ratio or at a fixed equivalent skeleton void ratio e*. A new predictive model in terms of e* is proposed to evaluate the G0 of sand-fines mixtures to account for the effects of void ratio, confining pressure, FC, and sand gradation within ±20% deviation. Moreover, the rate of shear modulus degradation with shear strain and the damping ratio are higher for sand-fines mixtures with higher FCs.