The Interaction of Talc, Montmorillonite, and Silica Sand with H₂O Influences Methane Hydrate Formation

Abstract

Methane hydrates in natural geological settings are commonly distributed within sediments, with a variety of minerals (such as silica sand, talc, and montmorillonite). The mechanisms that control the influence of sediments on methane hydrate formation remain poorly understood. In this study, we performed experiments on methane hydrate formation in pure H₂O with the addition of 3% sediments (montmorillonite, talc, and silica sand). A large-volume stirred reactor (80 mL) and a small-volume unstirred reactor (20 mL) were used. The results show that montmorillonite and talc severely inhibit methane hydrate formation. For experiments in the stirred reactor with pure H₂O, normalized gas consumption is 30 (mmol/mol) after 1000 min. In contrast, normalized gas consumption in experiments with the addition of 3% montmorillonite and talc decreases greatly to <5 (mmol/mol) over the same period. The inhibiting effect of montmorillonite and talc is closely associated with the release of cations (Mg²⁺, Ca²⁺, K⁺, and Na⁺) into fluids, with higher concentrations of cations for slower rates of methane hydrate formation. The interaction of montmorillonite and talc with H₂O consumes hydrogen ions (H⁺), resulting in alkaline solutions. It was found that alkaline solutions may not be favorable for methane hydrate formation. In contrast, silica sand slightly promotes methane hydrate formation in the unstirred reactor, which may be related to acidic solutions formed during the interaction of silica sand with H₂O. The phase equilibrium temperatures and pressures of methane hydrate in the presence of 3% montmorillonite, talc, and silica sand are essentially the same as those in pure H₂O, excluding the thermodynamic effect of minerals. The experiments of this study are important for understanding the formation of massive methane hydrates with low amounts of sediment (e.g., ≤3%). They suggest that methane hydrates may not be highly concentrated in sediments with abundant talc and montmorillonite. The experiments of this study may explain the close association of methane hydrates with silica sand.

1. Introduction

Gas hydrates are clathrate compounds that are formed from water and gases (e.g., methane, hydrogen, and carbon dioxide) at relatively low temperatures and high pressures, where gas molecules are trapped inside hydrogen-bonding water cages [1].Gas hydrates are mostly distributed in oceanic sediments as layered, massive, nodular, and dispersed forms, with the amounts of sediments ranging from <6% to >90% [2,3,4].Studying the formation of gas hydrate in sediments and understanding the interactions between hydrates and the host sediments play an important role in estimating the amounts of hydrates in natural hydrate reservoirs.

Previous experiments and numerical modeling have focused on hydrate formation in sediments, mostly silica sand and silica gel, and the influence of sediments on the formation kinetics of gas hydrates remains controversial [5,6,7,8,9,10,11,12,13]. Numerical modeling suggests that hydrate growth is significantly impeded within fine-grained sediments, due to capillary inhibition effects and a decrease in water activity [9].Consistently, the equilibrium temperatures and pressures of gas hydrate formation indicate that sediments inhibit gas hydrate nucleation and growth [5,6,7,8,13].Experiments on the formation kinetics of gas hydrates in sediment were performed under a variety of conditions, e.g., initial water contents ranged from very low (water saturation of <25%) to much higher [10,11,12,13,14,15,16,17,18].Some studies show that silica sand increases the kinetics of gas hydrate formation [10,11,12,13,16,17,18], while some experiments suggest that the effect of silica sand depends on its starting grain sizes, i.e., silica sand with starting grain sizes of 1 μm inhibits methane hydrate formation, and silica sand with larger grain sizes (50 μm) promotes hydrate formation [19]. In contrast, some experiments show that silica sand (with starting grain sizes of 50, 150, and 250 μm) and bentonite have a promoting effect at the early stage of hydrate formation and an inhibiting effect with progressive reactions [20]. Some experiments show a negligible influence of silica sand on the formation of gas hydrates [14]. The inconsistency in previous kinetic experiments may be associated with different experimental set-ups, such as the volume of reaction vessels, the cooling rate and initial water contents, and the inhomogeneity of methane hydrate due to the ultra-slow diffusion of methane in H₂O [21]. In addition, submarine sediments are composed of not only silica sand but also other minerals, such as talc and montmorillonite. The effect of other minerals and the mechanisms that control their influence on methane hydrate formation is still unclear.

In this study, we performed experiments at an initial pressure of 11.40 MPa to investigate the influence of montmorillonite, talc, and silica sand on methane hydrate formation. Two types of reaction vessels were used: a large-volume stirred reactor (80 mL) and a small-volume unstirred reactor (20 mL). The targets of this study are to (1) investigate the influence of montmorillonite, talc, and silica sand on methane hydrate formation, and (2) explore the mechanisms that control the effect of montmorillonite, talc, and silica sand on methane hydrate formation.

2. Materials and Methods

2.1. Apparatus

The formation of methane hydrate was studied in reaction vessels of two sizes: 20 mL (inner diameter, 2.0 cm; height, 101 cm) and 80 mL (inner diameter, 3.8 cm; height, 101 cm). Figure 1 shows the schematic of the apparatus used in the experiments of this study, which consists of a reaction vessel made of 316 stainless steel, a thermostatic bath, and a data acquisition system. The reaction vessels were immersed in a temperature-controlled water bath (GDH-2015, accuracy: ±0.01 °C). The 80 mL reaction vessel was equipped with two Pt100 temperature transducers (WZP-236, WIKA, Shanghai, China, accuracy: ±0.1 °C) to measure the temperatures of the liquid and gas phases, and a pressure transducer (DG1300-GE, SENEX, Guangzhou, China, accuracy: 0.01 MPa). A magnetic stirrer was used to agitate the test fluid. The 20 mL reaction vessel was equipped with a Pt100 temperature transducer and a pressure transducer without magnetic stirring. Temperature and pressure were recorded by the data acquisition system, and data were collected every 10 s.

2.2. Materials

All experiments were performed with 99.99% pure methane (supplied by Huatepeng Gas Co., Ltd., Foshan, China) and deionized water with a resistivity of 18.2 MΩ.cm was provided using an Eco-S15UVF water purifier. Talc powders (325 and 1250 mesh), montmorillonite (specific area of 240 m²/g), and silica powders (600–800 mesh) were purchased from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China.

2.3. Experimental Procedure

Experimental conditions are illustrated in

Table 1. Experimental conditions.

Exp. No.	T (°C)	P (MPa)	Vvessel (ml) a	Sediment Type	Sediment (g)	Pure H2O(g)	Normalized Gas Consumption (mmol/mol)
G-80-3	0.0	11.40	80	—	—	40.0	39.2
G-80-13	0.0	11.40	80	talc (1250 mesh)	1.3482	40.0	1.1
G-80-14	0.0	11.40	80	talc (325 mesh)	5.0369	40.0	1.5
G-80-15	0.0	11.40	80	talc (325 mesh)	1.3591	40.7	3.5
G-80-16	0.0	11.40	80	talc (325 mesh)	1.3867	40.6	1.3
G-80-18	0.0	11.40	80	talc (325 mesh)	1.3827	40.6	1.1
G-80-20	0.0	11.40	80	talc (325 mesh)	1.3902	40.0	1.5
G-80-21	0.0	11.40	80	talc (1250 mesh)	1.3956	40.0	5.4
G-80-26	0.0	11.40	80	montmorillonite	1.3678	40.4	8.8
G-80-28	0.0	11.40	80	silica (600-800 mesh)	1.2921	40.0	4.3
G-80-29	0.0	11.40	80	silica (600-800 mesh)	2.0187	40.0	4.5
G-20-2	0.0	11.40	20	—	—	10.0	14.1
G-20-13	0.0	11.40	20	talc (1250 mesh)	0.3287	10.6	0.5
G-20-14	0.0	11.40	20	talc (325 mesh)	1.0165	10.0	1.5
G-20-16	0.0	11.40	20	talc (325 mesh)	0.3417	10.0	2.4
G-20-17	0.0	11.40	20	talc (325 mesh)	0.4864	10.9	4.4
G-20-18	0.0	11.40	20	talc (1250 mesh)	0.4917	10.0	0.36
G-20-25	0.0	11.40	20	silica (600-800 mesh)	0.5241	10.0	16.9
G-20-26	0.0	11.40	20	silica (600-800 mesh)	0.5297	10.0	16.0

^a The volume of reaction vessels.

. Considering the amounts of sediments in methane hydrate range from <6% to >90%, we performed experiments using pure H₂O with 3% talc, montmorillonite, or silica sand. Around 10 Ml of pure H₂O was filled into the small reaction vessel, with 40 mL pure of H₂O in the large reaction vessel. Before charging with methane, the reaction vessels were evacuated with a vacuum pump. The vessels were then pressurized with methane gas up to the target pressure (11.40 MPa). After the diffusion of methane gas into water for around 24 h at room temperature, pressure has a slight decrease, which is comparable with the solubility of methane in water. Then, the reaction vessels were immediately immersed in a temperature-controlled bath at a temperature of 2 °C. The liquid in the bath is a mixture of water and ethylene glycol (50%). For experiments carried out in the large reaction vessel (80 mL), the stirring rate is 520 rpm; experiments in the small reaction vessel were performed without magnetic stirring. After hydrate formation, the reaction vessels were heated slowly (1 °C/h), and the dissociation of methane hydrate was observed with an increase in pressure. The dissociation of methane hydrate is completed at the temperature where the formation-dissolution loop is closed.

2.4. Observation of XRD and SEM for Sediments

X-ray diffraction (XRD) patterns were collected using Cu-Kα radiation, 45 kV voltage, and 200 mA current on a Rigaku Smartlab X-ray diffractometer at the Southern University of Science and Technology, Shenzhen, China. X-ray data were acquired in a 2θ range of 5–70° with a step size of 0.01° and a counting time of 10 s per step. Figure 2 shows the X-ray diffraction patterns of montmorillonite, talc, and silica sand used in the experiments of this study. Observations of silica, talc, and montmorillonite under a scanning electron microscope were also performed using a Zeiss Ultra 55 field emission gun scanning electron microscope at the Southern University of Science and Technology, Shenzhen, China. An accelerating voltage of 3 kV was used.

2.5. Chemical Compositions of Fluids during Methane Hydrate Formation

The concentrations of cations (Na⁺, K⁺, Mg²⁺, and Ca²⁺) in fluids during methane hydrate formation were determined using a Dionex ICS-1100ion chromatography (IC) with a conductometric detector at Southern University of Science and Technology, Shenzhen, China. The sampling of water samples is close to the bottom of the reaction vessel, very far away from methane hydrates (Figure 1), in order to minimize the influence of sampling on thermodynamic equilibrium and the kinetics of hydrate formation. The separation column used was Dionex Ion Pack AS22 RFIC (4 × 250 mm). The eluent was 2.6 mM methyl sulfonic acid at a flow rate of 0.5 mL/min. The injected sample volume was 25 μL for each probe. Each sample was analyzed at least three times. Quantification of the concentrations of Na⁺, K⁺, Mg²⁺, and Ca²⁺ in fluid was according to a standard curve established based on standard solutions, with the amounts of Na⁺, K⁺, Mg²⁺, and Ca²⁺ ranging from 0.5 to 80 mg/L.

2.6. Gas Consumption

The number of moles of gas consumed during methane hydrate formation is the difference between the amounts of gases initially introduced into the reactor and the amounts of gases at time t:

$$∆\mathrm{n}=\frac{\mathrm{V}{\mathrm{P}}_0}{{\mathrm{Z}}_0{\mathrm{R}\mathrm{T}}_0}-\frac{\mathrm{V}{\mathrm{P}}_{\mathrm{t}}}{{\mathrm{Z}}_{\mathrm{t}}\mathrm{R}{\mathrm{T}}_{\mathrm{t}}}$$

(1)

where ∆n is the amount of gas consumed when methane hydrate forms, V is the volume of the gas phase in the reactor, and P₀ and T₀ are the pressure and temperature in the initial condition, respectively. P_t and T_t are the pressure and temperature at time t in the reactor, respectively. R is the universal gas constant, and Z₀ and Z_t are the compressibility factors at the beginning and at time t. Compressibility factors were calculated using Pitzer’s correlations [22].

Normalized gas consumption (NG) at any given time t, was calculated according to the following equation:

$$\mathrm{N}\mathrm{G}=\frac{∆\mathrm{n}}{{\mathrm{n}}_{{\mathrm{H}}_2\mathrm{O}}}$$

(2)

where n_H2O is the total number of moles of water in the system.

3. Results

Methane hydrate formation was studied in pure H₂O with the addition of 3% montmorillonite, talc, and silica sand. After charging reaction vessels with methane, the reaction vessels were maintained at room temperature for around 2 days to enhance methane concentrations in H₂O. Then the reaction vessels were put into a 0 °C thermostatic bath, and the temperature of the reaction vessels decreased quickly. The fast cooling rate results in a very short induction time, and methane hydrate in most experiments was formed immediately after the destination temperature has been reached (Figure 3). Consistently, previous experiments show that a fast cooling rate induces a short nucleation time during methane hydrate formation [23].

Figure 4 shows normalized gas consumption during methane hydrate formation. For experiments performed in a stirred reaction vessel (80 mL), methane hydrate formation in pure H₂O has much higher gas consumption compared to gas consumption in experiments with the addition of 3% montmorillonite and talc (Figure 4a). For experiments with pure H₂O, normalized gas consumption is 28 (mmol/mol) after an experimental duration of 1000 min; for experiments with 3% montmorillonite and talc, normalized gas consumption decreases to <5 (mmol/mol) over the same period (Table 1). For experiments carried out in the unstirred reaction vessel (20 mL), a decrease in normalized gas consumption was also observed with the presence of 3% talc and montmorillonite. This suggests that methane hydrate formation is inhibited in the presence of talc and montmorillonite.

For experiments in the stirred reaction vessel, silica sand inhibits methane hydrate formation (Figure 4a, Table 1). In contrast, silica sand in the unstirred reaction vessel (20 mL) slightly promotes methane hydrate formation (Figure 4b). This suggests that the mechanisms controlling the effect of talc and montmorillonite on methane hydrate formation may differ from those controlling the influence of silica sand. Our experimental results are consistent with thermodynamic models [10] and kinetic experiments [14,15]. They disagree with many experiments showing that sediments greatly enhance methane hydrate formation [11,13,19].

4. Discussion

4.1. The Effect of Talc, Montmorillonite, and Silica Sand on Hydrate Formation

Previous experiments show that talc and montmorillonite enhance methane absorption [24]. For the experiments of this study, the amount of dissolved methane in H₂O is essentially the same as that in the presence of montmorillonite, talc, and silica sand, possibly due to their low abundance. Therefore, the effect of montmorillonite, talc, and silica sand on methane hydrate formation may not be attributed to methane absorption. In order to illustrate the thermodynamic influence of montmorillonite, talc, and silica sand on methane hydrate formation, we performed formation and dissociation experiments to obtain methane hydrate phase equilibrium data. As suggested by the pressure-temperature profile of methane hydrate formation in the presence of 3% montmorillonite and talc, a sub-cooling (5–8 °C) was required for hydrate nucleation, and methane hydrate formation is associated with a significant pressure drop (Figure 5). After that, the reaction vessels were heated at a very slow rate (~1 °C/h), and the pressure in the reaction vessel increased almost linearly with increasing temperature (Figure 5). Methane hydrate dissociation results in a dramatic increase in the pressure of the reaction vessel. As shown in Figure 6, the phase equilibrium temperatures and pressures in the experiments of this study using pure H₂O agree well with the equilibrium data reported by previous experimental studies [25,26]. It also shows that the equilibrium data are unchangeable in the presence of 3% montmorillonite, talc, and silica sand (Figure 6), which is consistent with previous experimental data [27]. This suggests that the effect of montmorillonite, talc, and silica sand on methane hydrate formation is not attributed to equilibrium changes.

When montmorillonite and talc interact with pure H₂O, these minerals can be dissolved, releasing cations (e.g., Na⁺, K⁺, Mg²⁺, and Ca²⁺) into fluids. The dissolution of talc into H₂O can be described using the following reaction:

Mg₃Si₄O₁₀(OH)₂ + 6H⁺ = 3Mg²⁺ + 4SiO_2(aq) + 4H₂O

(3)

Fluids equilibrated with montmorillonite and talc during methane hydrate formation were sampled, and their chemical compositions were determined using ion chromatography. It shows that the dissolution of talc leached magnesium (Mg²⁺) and calcium (Ca²⁺) cations into fluids. For experiments with 3% talc, fluids have 15.0 mg/L Mg²⁺, 10.3 mg/L Ca²⁺, and 2.3 mg/L Na⁺ (

Table 2. Chemical compositions of fluids equilibrated with sediments.

Exp. No.	Na+ (mg/L)	K+ (mg/L)	Mg2+ (mg/L)	Ca2+ (mg/L)
G-80-14	4.2	1.5	22.0	11.0
G-80-15	2.0	0.7	15.7	10.6
G-80-16	2.1	0.7	19.2	10.0
G-80-20	2.3	0.7	15.5	10.3
G-80-26	28	2.4	0.4	16.0
G-20-16	2.6	1.1	3.2	9.8

). The interaction of montmorillonite with H₂O results in releases of sodium (Na⁺) and calcium (Ca²⁺) into fluids, e.g., for experiments with 3% montmorillonite, fluids contain 18 mg/L Na⁺, 7.6 mg/L Ca²⁺, 1.4 mg/L K⁺, and 0.3 mg/L Mg²⁺. As indicated by Reaction (3), the hydrolysis of minerals consumes hydrogen ions (H⁺), resulting in alkaline fluids. The final pH of fluids was determined using a pH meter, which is ~8.81. Thermodynamic models have been carried out to calibrate the pH of fluids using thermodynamic data from SUPCRT 92 [28]:

Mg²⁺ + 2H₂O = Mg(OH)₂ + 2H⁺

(4)

Ca²⁺ + 2H₂O = Ca(OH)₂ + 2H⁺

(5)

The pH of fluids can be calibrated according to Equation (6):

pH = −(logK₄ + logK₅ + log[Mg²⁺] + log[Ca²⁺])/4

(6)

where K₄ and K₅ are equilibrium constants of Reaction (4) and (5), respectively. The pH of fluids is ~9.1, which agrees well with the measured pH.

The pH of fluids equilibrated with montmorillonite and talc was also measured before experiments. Montmorillonite and talc powders, together with pure H₂O, were loaded into polypropylene bottles, and the mass ratios of minerals and pure H₂O are essentially the same as those in experiments about methane hydrate formation. The experimental durations ranged from 2 to 10 days. Compared to experiments about methane hydrate formation, fluids contain lower concentrations of Ca²⁺, Mg²⁺, K⁺, and Na⁺, and the pH value is slightly lower (~8.02). This indicates that the hydrolysis of talc and montmorillonite leaches cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) into fluids, resulting in alkaline solutions. This also indicates that the hydrolysis of talc and montmorillonite is enhanced during methane hydrate formation, supported by higher concentrations of cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) in fluids and more alkaline solutions. Consistently, previous experiments suggest that the rates of talc dissolution increase with increasing temperatures at 25–150 °C [29], and the concentration of Mg in fluids at 150 °C is around one order of magnitude lower compared to that of this study.

Cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) greatly influence the kinetics of methane hydrate formation. Experiments were carried out in the stirred reactor (80 mL) with 0.2 mol/L NH₄Cl and 3% talc, and concentrations of Mg²⁺ and Ca²⁺ in fluids are 70 mg/L and 67 mg/L, respectively. They are several times higher compared to concentrations of Mg²⁺ and Ca²⁺ in experiments with talc and pure H₂O. Normalized gas consumption decreases to 4.9 (mmol/mol), around five times lower compared to that in experiments with pure H₂O/NH₄Cl solutions. This indicates that an increase in concentrations of Mg²⁺ and Ca²⁺ is associated with a great decrease in normalized gas consumption. This also indicates that the negative effect of talc and montmorillonite on methane hydrate formation is closely related to the release of cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺).

The negative effect of talc and montmorillonite on methane hydrate formation may be associated with alkaline solutions. Experiments were performed in the stirred reactor (80 mL)using acidic solutions (0.2 mol/L NH₄Cl, 40 mL) and alkaline solutions (0.2 mol/L (NH₄)₂CO₃, 40 mL).The normalized gas consumption in experiments with acidic solutions is 20 (mmol/mol) after 200 min, which is comparable to that in experiments with pure H₂O. In contrast, a great decrease in normalized gas consumption was observed for experiments using alkaline solutions, 5 mmol/mol over the same reaction period. Carbonate ion (CO₃²⁻) and Cl⁻ can interact with H₂O:

CO₃²⁻ + H₂O=HCO₃⁻ + OH⁻

(7)

Cl⁻ + H₂O = HCl + OH⁻

(8)

Thermodynamic calibrations suggest that the equilibrium constants of Reaction (7) under the T-P conditions investigated are around nine orders of magnitude higher compared to the equilibrium constants of Reaction (8). As a result, the interaction of carbonate ion (CO₃²⁻) with H₂O produces alkaline solutions. This indicates that methane hydrate formation is greatly impeded in alkaline solutions (0.2 mol/L (NH₄)₂CO₃), possibly due to the interaction of carbonate ion (CO₃²⁻) with H₂O.

As suggested by previous experiments, a stable clathrate structure (H₂O)₂₁H⁺ can be produced during methane hydrate formation, where H₃O⁺ ion is encaged inside the clathrate structure (H₂O)₂₀ [30,31]. This indicates that hydrogen ions (H⁺) may be favorable for methane hydrate formation. The interaction of montmorillonite and talc with CH₄-H₂O fluids consumes hydrogen ions (H⁺), producing alkaline solutions. The decrease in concentrations of H⁺ in fluids might prohibit the formation of the clathrate structure (H₂O)₂₁H⁺. Consequently, the kinetics of methane hydrate formation decrease greatly.

The interaction of silica sand with pure H₂O at room temperature and pressure produces slightly acidic fluids. For experiments in an unstirred reaction vessel with 3% silica sand, fluids after methane hydrate formation are slightly acidic, which is associated with a slight increase in the rates of methane hydrate formation. For experiments in the stirred reaction vessel with the addition of 3% silica sand, the pH of fluids after methane hydrate formation is 7.18. These neutral fluids may not have a great effect on methane hydrate formation. This indicates that the observed decrease in normalized gas consumption with the addition of 3% silica sand in the stirred reaction vessel may be associated with other factors, such as the formation of much finer particles after the dissolution of silica sand.

Scanning electron microscope imaging of montmorillonite, talc, and silica after experiments shows much finer particles and a hexagonal talc crystal, indicating that minerals were dissolved and re-precipitated during methane hydrate formation (Figure 7). Consistently, X-ray diffraction patterns also suggest the peak broadening of montmorillonite, indicating dissolution of montmorillonite during methane hydrate formation. Moreover, the peaks of talc are shifted to slightly higher peak positions (Figure 8). The re-precipitation of talc was not observed in previous dissolution experiments [29], indicating that the dissolution of minerals can be greatly enhanced in CH₄-H₂O fluids.

4.2. Comparison with Previous Studies

Methane hydrate formation in sediments has been experimentally studied with different experimental set-ups, including the volume of reaction vessels, initial water contents, and methane transport into the liquid [5,6,7,8,12,14]. In experiments performed with relatively low water contents (e.g., 10–18.4%), silica sand was found to promote methane hydrate formation [10,23,32,33]. However, the influence of grain size remains controversial [10,23,32,33]. Some experiments suggest that the kinetics of methane hydrate formation in silica sand with smaller grain sizes (<74 μm) are much faster compared to those in silica sand with larger grain sizes (210–297 μm) [10]. In contrast, some experiments show that the kinetics of methane hydrate formation in coarse sand are fastest, followed by fine sand, and the kinetics in loess are slowest [23,33]. For experiments performed with much larger water contents (e.g., >80%), the rates of methane hydrate formation differ greatly during the experiment, and sediments have a contrasted effect at the early and late stages of the reaction [16,17]. Silica sand and bentonite have a promoting effect at the early stage of hydrate formation, and they have an inhibiting effect with progressive reaction [19,20].Normalized gas consumption in the H₂O-CH₄ system is essentially the same as that in experiments with sediments [12,19], e.g., for H₂O-CH₄ experiments, normalized gas consumption is 63.21 mmol/mol H₂O, compared to 62.41 mmol/mol H₂O for experiments with the addition of 11.7% bentonite [19].This may be associated with the spatial heterogeneity of hydrate formation, possibly due to the ultra-slow diffusion of methane in H₂O [21].All these suggest that methane hydrate formation in experiments with low initial water contents may differ greatly from that in experiments with much higher water contents.

For the experiments in this study, essentially the same experimental set-ups were applied, including initial pressure (11.40 MPa), water contents (97%), and cooling rates. The reaction vessels were maintained for around 2 days at room temperature after charging with methane to increase the homogeneity of methane distribution. It shows that the influence of silica sand depends on experimental setups, especially the volume of reaction vessels and stirring or non-stirring conditions. For the large reaction vessel (80 mL, stirred), the addition of 3% silica significantly inhibits methane hydrate formation; for the small reaction vessel (20 mL, unstirred), silica slightly enhances methane hydrate formation with progressive reaction. Consistently, previous experiments performed in a 7 mL reaction vessel show that silica sands have a negligible influence on the formation of gas hydrates [14]. Laponite, however, impedes methane hydrate formation [14], which agrees well with the experimental results of this study. For experiments performed in the large- and small-reaction vessels, talc and montmorillonite greatly inhibit methane hydrate formation, which is attributed to the release of cations (e.g., Na⁺, K⁺, and Ca²⁺) from talc and montmorillonite during their interaction with H₂O.

5. Conclusions

Experiments on methane hydrate formation were performed in pure H₂O with the addition of 3% sediments (montmorillonite, talc, and silica sand). A fast cooling rate was applied, which resulted in a very short induction time. In most experiments, methane hydrate forms immediately after the destination temperature has been reached. Methane hydrate formation was significantly inhibited in the presence of talc and montmorillonite, which are closely associated with the release of cations (e.g., Na⁺, K⁺, and Ca²⁺) during their interaction with H₂O. The interaction of talc and montmorillonite with H₂O consumes hydrogen ions (H⁺), resulting in the formation of alkaline solutions. In contrast, silica sand slightly enhanced methane hydrate formation in the unstirred reaction vessel, and the interaction of silica sand with H₂O produced slightly acidic solutions. Equilibrium changes due to the presence of montmorillonite, talc, and silica sand are excluded, as indicated by essentially the same phase equilibrium temperatures and pressures in pure H₂O as those in experiments with 3% montmorillonite, talc, and silica sand. The experiments of this study suggest that minerals (montmorillonite, talc, and silica sand) greatly influence the kinetics of methane hydrate formation, which is important for understanding the formation of massive methane hydrates with low amounts of sediments (e.g., ≤3%). The effect of other minerals (such as serpentine, olivine, and brucite) on methane hydrate formation will be investigated in future studies.