Silty-clayey Sediments Research Articles

Geophysical Properties of Hydrate-Bearing Silty-Clayey Sediments with Different Compaction Patterns

Geophysical Properties of Hydrate-Bearing Silty-Clayey Sediments with Different Compaction Patterns

Theoretical investigation of threshold pressure gradient in hydrate-bearing clayey-silty sediments under combined stress and local thermal stimulation conditions

Due to the characteristics of smaller grain size and higher clay mineral content, the threshold pressure gradient (TPG) exists in multi-phase flow within hydrate-bearing clayey-silty sediments (HBCSS), which significantly affects the gas production from hydrate reservoirs. However, multi-phase flow through HBCSS relates to thermal-hydrological-mechanical coupling, leading to the understanding of TPG in HBCSS with complex pore structures and hydrate distribution is unclear. In this study, a theoretical TPG model of HBCSS is developed by considering the combined influences of effective stress, temperature increase, pore structures, hydrate saturation and its growth patterns. The proposed TPG model has been thoroughly validated using available experimental data. Moreover, the parameter sensitivity analysis is conducted based on this derived model, revealing a positive correlation between TPG and both effective stress and temperature increase. While TPG generally increases with higher hydrate saturation when other parameters are held constant, the relationship between TPG and hydrate saturation is non-monotonic. This observation suggests that TPG is impacted not only by hydrate saturation but also by other factors, including hydrate growth patterns and pore structures. The findings of this study establish a theoretical foundation for characterizing the nonlinear flow behavior during hydrate exploitation.

Triaxial tests on hydrate-bearing silty-clayey sediments concerning pore-filling habit

Recovering massive hydrates in pore-filling morphology deposited in silty-clayey sediments is significant for global long-term clean energy requirements. However, hydrate dissociation leads to sediment strength attenuation and reservoir deformation, thereby endangering reservoir and wellbore safety. To ensure safe and sustainable hydrate production, more efforts need be made to comprehensively investigate the mechanical behavior of such reservoirs. In this study, a series of triaxial shearing tests were launched on artificial hydrate-bearing silty-clayey sediments (HBSCSs) containing hydrates in pore-filling morphology, and the relevant mechanical behaviors of HBSCSs were obtained and discussed. The strength indication of HBSCSs is revealed to elevate evenly with hydrate saturation from results, while the strength growth amplitude of this material due to pore-filling hydrate (PFH) is much smaller than that due to cementing hydrate reported in previous studies. The strength increment of silty-clayey sediments induced by PFH presence is mainly due to the enhancement in the friction property of this material. The deformation behaviors of HBSCSs are affected by the combination of the initial consolidation degree, the hydrate pore-filling effect and the effective stress. Further advances are awaited to be made about the studies of silty-clayey reservoirs deposited with hydrates in pore-filling morphology.

Effect of marine clay minerals on the thermodynamics of CH4 hydrate: Evidence for the inhibition effect with implications

Clay minerals are widely distributed in hydrate-bearing sediments worldwide, especially in the clayey-silty sediments in the South China Sea. The debate on the influence of clay on the thermodynamics of natural gas hydrate (NGH) remains and impedes the fundamental understanding of hydrate-clay interactions and the design of effective production strategies for energy recovery. Specifically, the phase equilibria of MH in the presence of clay and how different types of water associated with clay particles affect NGH thermodynamics and stability are not fully elucidated and warrant investigation. In this study, the phase equilibria of methane hydrate (MH) were measured in the presence of typical clays from the South China Sea, i.e., montmorillonite, kaolinite, and illite. The shift of MH phase equilibria curve to a more stringent pressure was identified with Na-montmorillonite showing the most significant inhibition. The leftward shift (inhibition) of phase equilibrium temperature (Teq) is up to 3.9 K at a pressure of 10.40 MPa for Na-montmorillonite compared with pure water. Furthermore, we develop a thermodynamic model based on the classical Chen-Guo model first accounting for the change in water activity (aw) due to cation exchange of Na-montmorillonite. The developed model exhibits good predictability with the measured phase equilibira data. The inhibition on the phase equilibria of MH is linked to the reduction of aw due to the clay-bound water for clays with a swelling nature. The findings of this study offer direct evidence and explanations on how clay minerals thermodynamically inhibit CH4 hydrate. It provides valuable insights into the resource estimation of clayey-silty NGH in nature and facilities the design of suitable production strategies targeting clay-rich hydrate-bearing sediments.

Mechanical Behavior of Hydrate-Bearing Clayey-Silty Sediments in the South China Sea: Strain-Rate Dependency

Mechanical Behavior of Hydrate-Bearing Clayey-Silty Sediments in the South China Sea: Strain-Rate Dependency

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

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

Permeability properties of natural gas hydrate-bearing sediments considering dynamic stress coupling: A comprehensive experimental investigation

As a key exploitative parameter, permeability affects the extraction efficiency and program for natural gas hydrate (NGH). However, few studies have considered the effect of dynamic stress (continuously changing stress) on permeability, let alone permeability evolution during reservoir instability. Given the dynamic stress-seepage coupling process of actual NGH exploitation, this paper upgrades the triaxial apparatus to new equipment that can realize the unified tests of dynamic stress and water permeability. Subsequently, the seabed clay recovered from the South China Sea and quartz sand were used to remold fine-grained hydrate-bearing clayey-silty sediments (HBCSS). Finally, a series of stress-seepage coupling tests were carried out to reveal the permeability characteristics during sediment destabilization and to explain the coupling mechanism. The results show that seepage can reduce the strength and stiffness, and enhance the softening or weaken the hardening of the stress-strain curve. The dynamic stress-influenced permeability exhibits a non-monotonic evolution, which is essentially an expansion or contraction of seepage channels. Stress-strain relationship, volume deformation, and permeability are coupled and have good correspondence. The shear-seepage coupling process can be divided into four stages: initial compression, plastic strain development, plastic expansion and fracture development, plastic failure, and rapid expansion. This paper enriches the existing research from three aspects: permeability properties affected by dynamic stress, mechanical behavior affected by seepage, and stress-seepage coupling mechanism. It has played a significant reference for the safe and efficient exploitation of fine-grained NGH.

Enhanced gas production from silty clay hydrate reservoirs using multi-branch wells combined with multi-stage fracturing: Influence of fracture parameters

Natural gas hydrates are typically deposited in low-permeability clayey-silty sediments with low gas production. The two main methods for enhancing gas production are complex structural wells and hydraulic fracturing. However, using only one of those methods does not significantly increase gas production. Herein, a method of combined multi-branch well with multi-stage fracturing was proposed. A novel evaluation method for increased gas production under fracturing was first proposed to reveal the effects of fracturing domain area and number. Then five fracturing distribution models were designed and analyzed. Results indicate that the combined method increased the gas recovery ratio by 829.13% and the hydrate dissociation ratio by 710.34% compared to branching wells because of enlarging pressure propagation and communicating deep thermal fluids. The dissociation of hydrates is enhanced by an increase in the area and the number of fractures, but the rangeability of the increased stimulating effect declines. Different fracture distributions influence production performances at different stages. Short-term production is beneficial when fractures are in the branch well centers. Long-term gas production can be improved when fractures are mainly located at the branch distal end. This novel study has important implications for commercial exploitation at different production stages.

Acoustic characteristics on clayey-silty sediments of the South China Sea during methane hydrate formation and dissociation

Acoustic wave propagation can be used to detect hydrate reservoirs due to their unique characteristics in different media. Gas hydrate solids in reservoir pores exhibit higher wave velocities compared to pore fluids like seawater and free gas. Accurately predicting the abundance of hydrate reservoirs under different effective stress conditions is crucial for evaluating the exploitation value of hydrate reservoirs. In this study, the wave velocity of sediments was measured using two orthogonal S-waves for the first time, and the differences in S-wave velocities were used to characterize the anisotropy of the sediments. The results show that 1) a threshold value of hydrate saturation exists in clayey-silty sediments of the South China Sea, causing the wave velocity to show a trend of slow-then-fast, a new hydrate growth model was proposed to explain this change. 2) the growth pattern and occurrence type of hydrate play a dominant role in wave velocity. 3) hydrate can change sediment anisotropy, with higher hydrate saturation leading to a more isotropic sediment. 4) effective confining pressure can enhance wave velocity by altering the sediment skeleton. Finally, based on experimental data, an empirical formula was established and used to estimate hydrate saturation under different effective confining pressures.

Permeability damage and hydrate dissociation barrier caused by invaded fracturing fluid during hydrate reservoir stimulation

Hydraulic fracturing is a promising stimulation technology for enhancing the gas productivity and energy efficiency of low-permeability hydrate reservoirs. Presently, most studies focus on the fracturing feasibility of hydrate-bearing sediments and recovery efficiency after fracturing. Although the fracturing fluid inevitably invades the hydrate reservoirs during fracturing, the effects of invaded fracturing fluid on sediment permeability and hydrate dissociation have not been investigated yet. In this study, the invasion of water-based fracturing fluid into hydrate-bearing sediments was experimentally studied to clarify the dynamic characteristics of the fracturing fluid invasion, including its effects on sediment permeability and hydrate dissociation. The results revealed that the fracturing fluid loss rate was initially high but abruptly became extremely small, potentially because of the filter cake deposition on the fracture surface and the secondary hydrate formation in the invaded zone. However, the invaded fracturing fluid increased the dissociated gas flow resistance and inhibited the hydrate dissociation and gas production that yielded a two-stage gas production process, including slow hydrate dissociation in the invaded zone followed by relatively fast hydrate dissociation in the uninvaded zone. Furthermore, the degree of permeability damage in the invaded zone gradually decreased with the increasing invasion distance that was caused by the variations in the microscopic filtrate–sediment reactions. Interestingly, the water saturation measured by NMR gradually decreased from about 90% to 60% (initial water saturation) along the invasion distance. This phenomenon indicates that water sensitivity and water lock acted as the primary sources of permeability damage to the clayey–silty sediments after fracturing fluid invasion, suggesting that inhibiting clay swelling and increasing the flowback rate of the invaded fluid were essential for reducing permeability damage. Noted that using the fracturing fluid with breaker and KCl exhibited the lowest degree (25%) and shortest distance (<15 cm) of sediment permeability damage. The present results are vital for applying hydraulic fracturing in the field tests of hydrate reservoirs to optimize the fracturing fluid and improve the fracturing stimulation effect.

Deformation characteristics on anisotropic consolidated methane hydrate clayey-silty sediments of the South China Sea under heat injection

Natural gas hydrate is a new energy source with huge reserves, and the carbon content of hydrates is twice the carbon content of all proven fossil fuels combined, but the natural gas hydrate exploitation involves solid-liquid-gas phases change, which could induce a potential geological and environmental risk. Accurate description of the deformation behavior of sediments during hydrate dissociation is the key to predict the long-term stability of hydrate reservoirs. Nowadays, related studies mainly focus on the deformation behavior of hydrate-bearing sediment without dissociation. In this study, the deformation behaviors of remolded cores during thermal dissociation under anisotropy stress states were studied. The results show that 1) hydrate dissociation can lead to sediment deformation, the deformation rate of the sediments during the process of hydrate dissociation is 3 orders of magnitude larger than that before the dissociation. 2) the sediments under larger effective confining pressure (burial depth) present less deformation. 3) the sediments under isotropic stress conditions exhibit smaller deformation than that under anisotropic stress conditions. 4) dynamic permeability evolution during hydrate dissociation process can lead to effective confining pressure changes of the sediment, which can exacerbate the deformation.

Experimental investigation on the permeability characteristics of methane hydrate-bearing clayey-silty sediments considering various factors

Permeability is a key physical parameter of hydrate-bearing sediments (HBS), which is an important basis for evaluating reservoirs and developing exploitation plans. However, few permeability studies of HBS conducted so far have involved effective stress and seepage pressure difference, which is inconsistent with the stress-seepage coupling process in actual hydrate exploitation. In this study, marine clay taken from the South China Sea and quartz sand were used to synthesize hydrate-bearing clayey-silty sediments (HBCSS). A series of water permeability tests were conducted to investigate the effects of effective stress, hydrate saturation and seepage pressure difference on the permeability of HBCSS. The results show that an increase in effective stress can cause a significant decrease in permeability, and this effect is particularly pronounced at the beginning of stress increase. The decreasing effective stress can only cause a slight rebound in permeability, as only part of the elastic strain can be recovered. The negative exponential relationship between hydrate saturation and permeability is controlled by the blockage mechanism of hydrate particles. The change in effective stress is the essence of the effect of seepage pressure difference on permeability, and this effect is attenuated with increasing hydrate cementation. Based on the experimental results, permeability models considering the coupling effect of effective stress and hydrate saturation are proposed.

Mechanical characteristics of overconsolidated hydrate-bearing clayey–silty sediments with various confining pressures

Mechanical characteristics of overconsolidated hydrate-bearing clayey–silty sediments with various confining pressures

Undrained Shear Properties of Shallow Clayey-Silty Sediments in the Shenhu Area of South China Sea

Suction piles are used to ensure wellhead stability during natural gas hydrate production in the Shenhu area of the South China Sea (SCS). Undrained shear properties of clayey-silty sediments play a critical role in the stability analysis of suction piles. However, it has not been fully studied. This study conducts a series of undrained triaxial shear tests on shallow clayey-silty sediments in the Shenhu area of SCS, and stress–strain curves under different overconsolidation ratio (OCR) conditions are obtained. OCR effects on undrained shear properties of clayey-silty sediments are discussed, and a model to predict the pore pressure coefficient at failure is proposed. Results show that the isotropic compression index is 0.175, and the isotropic swelling index is 0.029. The undrained shear strength is proportional to the effective confining pressure, and the proportionality coefficient is 0.42 for normally consolidated specimens, while the undrained shear strength of OC specimens nonlinearly increases with OCRs increasing. The proposed model aptly predicts the pore pressure coefficient at the failure of clayey-silty sediments of SCS with different OCRs.

Pore-Scale Investigation of CH4 Hydrate Kinetics in Clayey-Silty Sediments by Low-Field NMR

Clayey-silty sandy media have been widely discovered in naturally occurring hydrate-bearing sediments in the South China Sea. However, the phase change behavior of CH4 hydrate (MH) and the resulting pore structure change and the migration of fluid in clayey-silty sediments remain less known and warrant investigation. In this study, we examine the pore-scale behavior of MH formation and dissociation in clayey-silty sediments and the associated fluid migration by low-field nuclear magnetic resonance (NMR). Based on T2 spectra measurement, MH starts to grow in small pores (pore size <1 μm) first and in large pores (pore size >10 μm) subsequently. The presence of clay, i.e., Na-MMT, practically retards the overall growth kinetics of MH evidenced by the low H2O conversion (<10%) to MH in clay-associated small pores. During depressurization, MH starts to dissociate in sand-associated large pores first. Free water migrates to clay-associated small pores and partially converts to clay-bound water. MRI visualization depicts the heterogeneous spatial distribution of both MH and the residual water in the process. The experimental results provide possible explanations on the spatial heterogeneity of MH in clay-silty sediments in nature and on the multiphase fluid migration during energy recovery from MH reservoirs.

Hydrate phase equilibria in natural sediments: Inhibition mechanism and NMR-based prediction method

The hydrate phase equilibrium condition in natural sediments is an important prerequisite for gas hydrate exploitation and subsea sequestration of CO2. However, its rapid and accurate prediction remains a challenge. In this work, we proposed a novel method to predict the hydrate phase equilibria in various natural sediments using low-field NMR technology. Also, its potential inhibition mechanism was clarified from the perspective of different types of pore water. The results indicated the peaks of T2 spectra for all types of pore water shifted to lower transverse relaxation times during the hydrate formation, meaning that the formed hydrates act as solid matrices in sediments and thus reduce pore size. Furthermore, it was observed that hydrate formation from bound water was much more difficult than capillary water, even at the same pore size. More importantly, not all bound water participated in hydrate formation. During the hydrate dissociation, montmorillonite exhibited the greatest dissociation temperature depression, followed by SA sediments, while illite and kaolin showed a relatively low inhibition on hydrate phase equilibria. This difference in dissociation temperature depression is attributed to water adsorption on the mineral surface and interlayer cation hydration, in addition to the capillary effect. Among them, the capillary-induced phase equilibrium inhibition is the lowest, followed by interlayer cation hydration, and mineral surface adsorption shows the most strong inhibition. Finally, the feasibility and validity of the NMR-based method in predicting the hydrate phase equilibrium condition in natural sediments were verified. These findings not only add further insights into the hydrate phase behavior in clayey-silty sediments but also provide an effective tool for rapid and accurate predictions of hydrate phase equilibria.

Strength behaviors of hydrate-bearing clayey-silty sediments with multiple factors

As a kind of new clean energy, natural gas hydrate has attracted the attention of the world. A full understanding of the physical properties that determine the strength behaviors of hydrate-bearing sediments (HBS) is hence required prior to the safe and efficient exploitation of the hydrates. Due to the lack of research work on fine-grained HBS, clay collected from the South China Sea and quartz sand were used to remold hydrate-bearing clayey-silty sediments (HBCSS). A series of consolidated-drained triaxial tests were carried out to determine the strength behaviors related to relationships between the physical properties of HBCSS. The results showed that higher saturation facilitates strain softening, while a higher confining pressure tends to result in strain hardening. The higher the clay content, the lower the failure strength and secant modulus E50 value. Clay can lubricate particles, hence an increase in clay content could lead to an increase in cohesion, but a slight decrease in the internal friction angle. The confining pressure and hydrate saturation were the main factors influencing the failure strength, while clay content had a limited influence on resistance to failure. The factors influencing the secant modulus E50 value could be arranged in a descending order of significance, namely hydrate saturation > clay content > confining pressure.

Hydrate-bearing sediment of the South China Sea: Microstructure and mechanical characteristics

The successful offshore hydrate production tests in the South China Sea proved the high productivity of clayey-silty hydrate reservoirs. However, the mechanical characteristics of these clayey-silty sediments have not been comprehensively investigated since the concerns about the productivity of such low-permeability reservoirs. In this study, based on the sediment matrix from the South China Sea, specimens were remolded with a porosity of 41% and hydrate saturation of 30% to perform a comprehensive mechanical investigation from micro to macro. Microcosmically, the hydrate in the clayey-silty sediment presents patchy distribution, being particle-displacive and particle-wrapped segregated hydrate morphology. Macroscopically, the compression and swelling indices of hydrate-bearing sediments in the South China Sea are obtained for the first time, which are larger than that of the hydrate reservoir in Krishna-Godavari Basin. The increasing effective confining pressure can slightly reduce the strain-softening and dilation characteristics of the hydrate-bearing sediments in the South China Sea. What's more, the failure strength and stiffness increase with an increasing effective confining pressure, while the failure volumetric strain firstly increases and subsequently decreases with an increasing effective confining pressure. These results are essential input parameters to establish a constitutive model and numerical simulations.

Experimental and modeling investigations of hydrate phase equilibria in natural clayey-silty sediments

Rapid and accurate acquisition of hydrate phase equilibria in sediments is crucial for gas hydrate resource assessment and exploitation and hydrate-based technological applications. However, current models have significant limitations and deficiencies in predicting hydrate phase equilibria in sediments, especially clayey-silty sediments prevalent in nature. To this end, a unified hydrate phase equilibrium model was developed, which considers the combined inhibitions of surface adsorption, capillary effect, and salt solution. It is derived that water content and salinity can be used as direct input parameters to the model to estimate the hydrate dissociation temperature depression. Using the stepwise heating method, the hydrate dissociation conditions in various natural sediments were measured. It suggests that montmorillonite has the greatest dissociation temperature depression, followed by SA sediments and kaolin, while pore hydrates in silts exhibit almost the same thermodynamic equilibrium conditions as bulk hydrates unless the water content is below 15%. Special attention was also paid to the additional inhibition of surface adsorption on hydrate phase equilibria in sediments with salt solutions. This inhibition was found to be quite pronounced in clay-rich sediments, especially expansive clays, but essentially negligible in silty or sandy sediments. The maximum average error of 3.29% was observed between model predictions and measured data. Additionally, the amounts of clathrated water (CW), non-equilibrium unclathrated water (NEUW), and equilibrium unclathrated water (EUW) in sediments after the end of hydrate formation were determined using the model. It reveals that CW has the highest proportion in silts, reaching 68.9%, while only 0.231 in montmorillonite; in contrast, EUW is dominant in montmorillonite; NEUW seems to be independent of lithology but shows a strong dependence on the water content. It implies that silty sediments are more conductive to hydrate-based gas storage and CO2 sequestration than clayey sediments. This study provides some practical insights and implications for hydrate phase behaviors in natural sediments and can potentially be employed as a tool for phase equilibrium prediction with universality and accuracy.

Permeability change with respect to different hydrate saturation in clayey-silty sediments

The successful pilot trial of methane hydrate production in the South China Sea has proven the technical feasibility of natural gas hydrates (NGHs) exploitation in clayey-silty reservoirs. As a key issue of NGHs exploitation in clayey-silty sediments, the variation of permeability with hydrate saturation (Sh) has not been thoroughly studied. In this study, a triaxial core holder was designed to simulate the synthesis and dissociation of gas hydrates in a clayey-silty core plug sample. In the process of hydrate synthesis and multi-step dissociation, the water content and distribution in different pores were quantitatively determined by a nuclear magnetic resonance (NMR) spectrometer, and the variation of water content under different Sh was successively monitored. Meanwhile, in each depressurization phase, the gas permeabilities k(Sh) with respect to different Sh were measured under constant effective stress. Results highlight that the change of k(Sh) is more prevalent in clayey-silty sediments than that in sandstones. As Sh increases from 0 to 10.43%, the permeability ratio (k(Sh) to sedimentary permeability ks) in clayey-silty sediments decreases to 0.01–0.02; whereas in sandstones, the permeability ratio falls between 0.4 and 0.8. To predicate the permeability change in clayey-silty sediments, the previous Cubic model is improved and a Power-exponential model is presented. In the updated model, the variable of power parameter n can characterize different reservoir types. Specifically, in artificial sandstones, sandpacks or sandy reservoirs, the n values range from 3.1 to 8.3, while in clay packs or clayey-silty sediments, the n values are from 19.1 to 30.7. The Power-exponential model provides reference values for the prediction of different types of gas hydrate reservoirs such as permafrost and marine sediments.