Experimental Study on the Distribution Characteristics of CO₂ in Methane Hydrate-Bearing Sediment during CH₄/CO₂ Replacement

Abstract

CH₄/CO₂ replacement is of great significance for the exploitation of natural gas hydrate resources and CO₂ storage. The feasibility of this method relies on our understanding of the CH₄/CO₂ replacement efficiency and mechanism. In this study, CH₄/CO₂ replacement experiments were carried out to study the distribution characteristics of CH₄ and CO₂ in hydrate-bearing sediments during and after replacement. Similar to previously reported data, our experiments also implied that the CH₄/CO₂ replacement process could be divided into two stages: fast reaction and slow reaction, representing CH₄/CO₂ replacement in the hydrate-gas interface and bidirectional CH₄/CO₂ diffusion caused replacement, respectively. After replacement, the CO₂ content gradually decreased, and the methane content gradually increased with the increase of sediment depth. Higher replacement percentage can be achieved with higher replacement temperature and lower initial saturation of methane hydrate. Based on the calculation of CO₂ consumption amounts, it was found that the replacement mainly took place in the fast reaction stage while the formation of CO₂ hydrate by gaseous CO₂ and water almost runs through the whole experimental process. Thus, the pore scale CH₄/CO₂ replacement process in sediments can be summarized in the following steps: CO₂ injection, CO₂ diffusing into sedimentary layer, occurrence of CH₄/CO₂ replacement and CO₂ hydrate formation, wrapping of methane hydrate by mixed CH₄-CO₂ hydrate, continuous CO₂ hydrate formation, and almost stagnant CH₄/CO₂ replacement.

1. Introduction

Natural gas hydrate (mainly methane hydrate) widely occurs in the seabed on the continental margin and permafrost area [1]. The huge reserves of natural gas hydrate make it an alternative energy source. Thus, hydrate production method research and field pilot production tests are attracting worldwide attention [2]. At present, the main hydrate exploitation methods include depressurization, heat stimulation, chemical additive injection, and CO₂ replacement [3,4,5,6,7]. Among the above methods, depressurization has long been considered as the most feasible method in field exploitation, even though there are still risks of sand production, formation instability, and production stagnation for long-term mining. In recent years, a CH₄/CO₂ replacement method has attracted worldwide interests as a win-win method for simultaneous CH₄ recovery and CO₂ storage, captured by absorption, adsorption, and membrane separation [8,9,10,11]. The feasibility of gas production from hydrate by CH₄/CO₂ replacement was first confirmed by Ohgaki et al. [12]. The replacement of CH₄ from methane hydrate by CO₂ is thermodynamically and kinetically feasible by obtaining −3.49 kJ/mol for the Gibbs free energy [13,14,15,16]. As CO₂ hydrate is more stable than CH₄ hydrate at the same temperature and pressure, the CH₄/CO₂ replacement method for hydrate production can maintain the mechanical stability of the sediments [17,18]. In 2012, a CH₄/CO₂ replacement method was adopted in a test production of natural gas hydrate conducted in Alaska [19,20]. The production results proved the theoretical feasibility of this method. Unfortunately, the low replacement efficiency restricted the practical application.

Owing to the advantages in stabilizing the hydrate reservoir and CO₂ storage and the low efficiency in the practical production test of CH₄/CO₂ replacement, studies have been extended to enhance the replacement rate and efficiency and to explore the replacement mechanism in the laboratory. In order to improve the replacement efficiency of CO₂ replacement, factors such as replacement conditions (pressure and temperature), CO₂ injection state, and sediment characteristics that may affect the replacement efficiency have been studied by many researchers [5,9,21,22,23,24]. Chen et al. [25] suggested that temperature had a great influence on CH₄/CO₂ replacement. At higher temperature, decomposition and replacement may exist simultaneously [26]. Moreover, the influence of temperature is related to the freezing point of water. For example, if the temperature is higher than the freezing point, the replacement efficiency increases with the increase of temperature; while if the temperature is lower than the freezing point, an ice layer may form on the hydrate surface, which may result in a relatively slow replacement process [27,28]. The state of CO₂ is also an essential factor affecting the replacement efficiency. At the same P-T condition, injection of gaseous CO₂, liquid CO₂, CO₂ emulsion, and supercritical CO₂ into methane hydrate results in the following replacement efficiency ranking: supercritical CO₂ > emulsion CO₂ > liquid CO₂ > gaseous CO₂ [22,29,30,31,32]. Ota et al. [33] found that the replacement reaction was promoted by an increase in pressure from 3.2 to 3.6 MPa. However, no change was observed at higher pressures because of CO₂ liquefaction. Several studies also demonstrated that injecting a certain proportion of N₂–CO₂ gas mixture could improve the replacement rate [34,35,36]. It was proposed that the external N₂ molecules specifically would attack the CH₄ molecules already entrapped in a small cage and play a significant role in substantially increasing the CH₄ recovery rate [34]. It was also found that minerals with small particle size (for example clay minerals) could significantly reduce the permeability of sediments, and result in lower CH₄/CO₂ replacement efficiency [9]. These studies have shown that optimizing CO₂ injection conditions can improve the displacement efficiency effectively. Moreover, researchers also studied the replacement efficiency of the combined production methods, for example, CH₄/CO₂ injection and depressurization, CH₄/CO₂ injection and heat stimulation, and injection of both CH₄/CO₂ and chemical additives (such as NaCl, surfactant, etc.). The combined production methods were found to be favorable in improving the CH₄/CO₂ replacement efficiency [37,38,39,40,41,42,43].

When discussing the reasons for the low replacement efficiency, it is generally believed that the CH₄/CO₂ replacement starts at the surface of methane hydrate, which hinders further diffusion of CO₂ and the occurrence of replacement. Lee et al. [16] and Bai et al. [44] proposed that the replacement rate gradually decreased as the CO₂-CH₄ mixed hydrate gradually formed, which decreased the diffusivity of the CO₂ through the preformed hydrate layer. However, this may not be the case for hydrate reservoirs. With the injection of CO₂ in sediments, they may quickly form a CO₂ hydrate layer which prevents the further penetration of gaseous CO₂. Therefore, the vertical diffusion of CO₂ in CH₄ hydrate bearing sediments is essential in determining the vertical displacement degree of CO₂, which is of theoretical significance for further understanding the displacement process. Although a number of previous CO₂ replacement experiments have been carried out to examine the factors affecting the replacement efficiency, there is still a lack of theoretical discussion on the CO₂ replacement efficiency with regard to the vertical diffusion process of CO₂ in a methane hydrate reservoir. In this study, experiments were conducted in a pressure vessel with sea sand as sediment media. Different from previous works, the sediment samples were carefully examined layer by layer after replacement, aiming to study the vertical distribution and diffusion characteristics of CO₂ in the process of replacement, as well as the influence of factors that affect the replacement efficiency including replacement temperature and initial CH₄ hydrate saturation.

2. Materials and Methods

2.1. Experimental Apparatus

The experimental system (Figure 1) consists of a cylindrical pressure vessel, a reactant-supply module for gas injecting, a thermostatic chamber, a gas chromatograph (GC), and a data-acquisition module. The main body of the pressure vessel is made of stainless steel with a pressure tolerance of 30 MPa and equipped with two temperature sensors and one pressure sensor. The effective volume of the vessel is 120 mL. The GC (ThermoGC1100) is connected to the pressure vessel through two valves to measure the gas components in the vessel during the replacement. After replacement, the recovered gas of each sample slice is also analyzed by GC. The pressure vessel is situated in the thermostatic chamber with a temperature controlling range of −30 °C to 25 °C. The temperature fluctuation during experiments is lower than 0.1 °C.

2.2. Experimental Procedure

2.2.1. Sample Preparation

The cylindrical pressure vessel and the associated inner cylinder sample holder (inner diameter 35 mm) were carefully washed with deionized water and dried. Then selected sea sand (grain size ranges from 0.15 mm to 0.25 mm) were evenly mixed with a certain amount of deionized water (water saturation is 30–35%). After that, water bearing sea sand of a total volume of 60 mL was added into the sample holder and the sample holder was placed in the pressure vessel. The pressure vessel was then sealed and put under vacuum. Based on the required hydrate saturation of a single experiment, CH₄ gas was injected into the pressure vessel to a certain pressure and maintained for 24 h. The pressure vessel was then cooled to 1.5 °C to form CH₄ hydrate until the pressure remained unchanged for at least 12 h. To ensure the reliability of the experimental results, the distribution uniformity of methane hydrate in the sample before replacement was examined in the following steps. First, a methane hydrate-bearing sample was prepared with exactly the same procedure as described above. Then the sample was sliced into two pieces horizontally with the protection of liquid nitrogen, and the upper and lower parts were scanned by nano-focus X-ray computed tomography (X-CT, Phoenix X-ray v|tome|xs, GE Sensing & Inspection Technologies, Wunstorf, Germany). A self-developed container with controllable temperature and pressure was used for X-CT analysis, which can maintain the stability of the hydrate during CT scanning. The X-CT data were analyzed with VG Studio MAX. The detailed X-CT measurement and analytical method were followed exactly as that reported in Li et al. [45,46].

2.2.2. CH₄/CO₂ Replacement

(1)

CO₂ injection: After the formation of CH₄ hydrate, the pressure vessel was cooled down to −20 °C and held for at least 1 h. CH₄ gas in the vessel was released within 10 s and CO₂ was re-injected until the pressure reached ~3.5 MPa. To avoid hydrate dissociation, CO₂ was precooled in the cold-water buffer tank before injection. Then the vessel was heated to a certain temperature, and this specific point in time was recorded as the start of the replacement process.

(2)

In situ analysis: Straight after the re-injection of CO₂, the valves between the pressure vessel and the GC were opened and closed in turn to allow a small volume of gas (less than 0.2% v/v of the pressure vessel) in the pressure vessel to be sampled, so that the initial gas composition in the pressure vessel could be determined. The same method was conducted to obtain the gas composition in the pressure vessel every 24 h until the end of the experiments, which lasted for 12 days.

(3)

Sample slicing: After replacement, hydrate-bearing sample in the pressure vessel was taken out and sliced into four pieces horizontally under the protection of liquid nitrogen (Figure 2). The composition of the decomposed gas for each sample piece was collected and analyzed by GC.

The replacement conditions are summarized in

Table 1. Experimental conditions of individual CO₂ replacement.

	Run1	Run2	Run3	Run4	Run5	Run6	Run7	Run8
Replacement Temperature (T)	1.5	2.5	3.5	5.0	2.5	2.5	2.5	2.5
Initial hydrate saturation (Sh)	46.5	46.9	45.8	46.2	10.8	26.7	42.1	55.2
Pressure of CO2 injection	3.0	3.0	3.0	3.0	3.0	3.0	3.0	3.0

below.

2.3. Calculation Methods

2.3.1. Hydrate Saturation

The amount of formed methane hydrate in our experiments was determined according to the amount of methane gas consumed in the process of hydrate formation. Assuming that the density of methane hydrate is the same as the value reported previously [47], then the saturation of hydrate can be calculated by using the porosity of the sediments. The formula to calculate the saturation of hydrate is shown in Equation (1):

$$S_h=\frac{\left(\frac{P_1}{Z_{1}T}-\frac{P_2}{Z_{2}T}\right)\times \frac{V_g}{R}\times M_h}{{\rho }_h{V}_p}$$

(1)

where $S_h$ is the saturation of gas hydrate (the volume ratio of gas hydrate and pore space) in our sample prepared in the pressure vessel; $M_h$ is the molar mass of gas hydrate, taking CH₄ hydrate for example, the value of $M_h$ is 122.02 g/mol if the hydration index of methane hydrate is 5.89; ${\rho }_h$ is the density of gas hydrate, which is 0.91 g/mL in the current calculation; $V_p$ is the total volume of pore space of the sediment sample; $V_g$ is the volume of gas phase in the reactor; $T$ is the replacement temperature; $P_1$ is the initial pressure of the system; $P_2$ is the pressure after hydrate formation; $R$ is the ideal gas constant, which is 8.314 J/(mol·K); $Z_1$ and $Z_2$ are the gas compressibility factors at $P_1$ and $P_2$, respectively.

2.3.2. Replacement Efficiency

The replacement efficiency $\mathsf{\gamma }$ is calculated by the following formula:

$$\mathsf{\gamma }=\frac{n_{C{H}_4}}{n_{C{H}_{4}Hydrate}}$$

(2)

where $n_{C{H}_4}$ is the mole number of methane in the gaseous phase in the pressure vessel; $n_{C{H}_4,Hydrate}$ is the mole number of methane in the hydrate phase before replacement.

3. Results and Discussion

3.1. Distribution of Methane Hydrate before Replacement

Since the CO₂/CH₄ replacement can be highly affected by the spatial distribution of methane hydrate in sediments, the study of the vertical distribution of CO₂ requires an even distribution of methane hydrate in the sediment sample. Therefore, to ensure the reliability of the experimental results, the distribution of methane hydrate in our sample before replacement was examined. The spatial distribution of methane hydrate in the prepared sediment sample is shown in Figure 3. Since the methane hydrate was formed in the selected sea sand, it was mainly formed in the pore space of the sample and was unlikely to form relatively larger clumps (Figure 3). We used the sediment porosity and hydrate saturation as criteria to examine the distribution uniformity of methane hydrate. Through three-dimensional data reconstruction and gray threshold segmentation of the scanned CT image, the gray ranges of gas, water, hydrate, and sediment were obtained [46]. Then, by calculating the volume proportion of each component, the porosity and hydrate saturation of the sample were estimated. The X-CT image analysis results showed that the porosity and the hydrate saturation of the upper and lower slice were 38.2%, 44.6% and 38.4%, 43.2%, indicating uniform spatial distribution of methane hydrate in the sediment sample. Therefore, it was reasonable for us to use the hydrate-bearing sample to study the influence factors of CH₄/CO₂ replacement and the distribution characteristics of CO₂.

3.2. Influencing Factor of Replacement Efficiency

3.2.1. Temperature

Based on the results of GC measurement; the CO₂/CH₄ replacement percentages during experiments with similar initial hydrate saturation were calculated and are shown in Figure 4. For all the four experiment runs; the replacement process can be divided into two stages according to the replacement rate. The first stage starts from the beginning of the experiment and lasts for about 3 days. The replacement percentage reached over 80% of the final replacement percentage. The second stage begins from Day 4 to Day 12; when the replacement percentage showed quite a slight increase. This two-stage replacement process has been reported in many previous studies [21,48]. The rapid reaction stage refers to fast surface reactions; and the second stage with low reaction rate was due to the hinderance of CO₂ diffusion by the formation of the mixed CO₂-CH₄ hydrate layer on the surface of methane hydrate.

At the end of the replacement experiment, the overall replacement percentages were 11.1%, 18.9%, 24.1%, and 29.0% at 1.5 °C, 2.5 °C, 3.5 °C, and 5.0 °C, respectively. Higher temperature results in relatively higher replacement percentage. Our results are consistent with previous studies of the effect of thermal stimulation on CH₄/CO₂ replacement [25,26]. Noting that the replacement P-T conditions of all our experiments were below the equilibrium point of methane hydrate, the replacement reaction would inevitably be accompanied by the dissociation of methane hydrate. Previous experiments [16] have shown that CO₂/CH₄ replacement always occurred with the dissociation of methane hydrate, then the released methane gas could mix with CO₂ and reform CO₂-CH₄ hydrate. Since higher temperature is favorable for methane hydrate to dissociate and release methane gas, the higher dissociation rate resulting by higher temperature would benefit CO₂/CH₄ replacement and eventually lead to higher replacement percentage.

3.2.2. Initial Hydrate Saturation

The variation of CO₂ replacement percentage with time for different initial methane hydrate saturations is shown in Figure 4. Similar with the temperature-controlled experiments, the replacement process can also be divided into fast stage and slow stage at the same time point (72 h). The replacement percentage of the experiment also reached about 80% of the final value. Meanwhile, the increasing rate of the replacement percentage is negatively corelated with the initial hydrate saturation (Figure 5). Samples with lower initial hydrate saturation result in greater replacement percentage. After 12 days’ replacement, the CO₂ replacement percentages corresponding to 10.8%, 26.7%, 42.1%, and 55.1% initial hydrate saturation are 27.2%, 22.0%, 18.9%, and 10.1%, respectively. Such a trend in replacement percentage has also been reported by Yuan et al. [49]. Although they performed replacement experiments with liquid CO₂ at relatively higher pressure, the experimental results also showed an increasing replacement percentage of CH₄ hydrate with the decrease of hydrate saturation and the increase of water saturation.

Despite the highest replacement percent, the CO₂/CH₄ replacement experiment conducted with lowest hydrate saturation generated the lowest amount of CH₄ production. Previous studies [16] have shown that the fast reaction stage of replacement is closely related with the surface reaction. Therefore, surface area (i.e., hydrate–CO₂ contact area) would probably act as an important factor in determining the amount of produced CH₄ in the fast reaction stage. In the current study, methane hydrate was formed in the porous sand media. With increasing hydrate saturation in the sediment sample, there must be a bottleneck in the growth of the hydrate surface area with this increasing hydrate saturation (shown in Figure 6). This is probably the reason why the amount of recovered CH₄ gas did not linearly increase with the initial hydrate saturation. When the initial methane hydrate saturation exceeded ~42% in our experiments, the overall area of hydrate–gas interface probably became smaller owing to the agglomeration of hydrate particles in the pore space. As a result, the total amount of recovered CH₄ gas in the experiment with highest initial hydrate saturation was not the largest.

3.3. Vertical Distribution of CO₂

As discussed in Section 3.1, methane hydrate in our samples before replacement was evenly distributed in the pore space. In addition, the injection of CO₂ was performed on top of the sediment sample. Therefore, the pore structure of the hydrate-bearing sample may affect the distribution of CO₂ and CH₄ along the direction of CO₂ permeation. The vertical distribution characteristics of CO₂ and CH₄ for each sample after replacement are shown in Figure 7 and Figure 8. For most experiments, the CO₂ molar percentage gradually decreased and the CH₄ molar percentage gradually increased with the increase of sediment depth. When comparing the gas contents in slices at the same sample location in different experiments, the contents of CO₂ increase with higher temperature and lower initial hydrate saturation, whereas the contents of CH₄ decrease with higher temperature and lower initial hydrate saturation. The above distribution character is consistent with the penetration order of CO₂ through the sample. With the injection of CO₂, hydrate in the upper part of the sample started to react first. The dissociation of methane hydrate and reformation of the mix CO₂/CH₄ have a chance to reduce the sediment porosity and permeability, and subsequently slow down the CH₄/CO₂ replacement rate in the lower sediment layers. However, such a hinderance seems not strong enough to prevent the penetration of CO₂ through the sediments completely. This phenomenon is probably caused by both the relatively larger pore size and the smaller vertical height of the sediment sample.

3.4. The Excess Consumption of CO₂

For CH₄/CO₂ replacement experiments, the consumed gaseous CO₂ and the recovered CH₄ should be equal in molar number if the effect of gas solubility is excluded. In such circumstance, the pressure in the vessel should remain constant throughout the replacement process if the compressibility factors of CH₄ and CO₂ are assumed equal. However, we observed an obvious pressure drop for all our experiments. Taking the saturation-controlled experiments for example, the pressure decreased about 0.1–0.3 MPa after replacement (Figure 9). Although the CH₄/CO₂ replacement is an exothermic process, the relatively slow reaction rate and the small amount of released heat would probably result in a tiny heat fluctuation. Moreover, the discrepancy in the compressibility factor of CO₂ and CH₄ is quite small. Therefore, it can be deduced that the distinct pressure drop during the replacement was probably caused by the excess consumption of CO₂ in the gaseous phase.

The process of methane hydrate formation in the sample preparation was inadequate in consuming all the water in the pore space. Moreover, the replacement P-T condition used in our experiments was above the equilibrium condition of CO₂ hydrate. Thus, the formation of CO₂ hydrate could inevasibly occur at the same time of CH₄/CO₂ replacement. This implication can be further verified by the sharp pressure decrease in the experiment with 10.8% initial hydrate saturation (red line in Figure 9). Lower initial hydrate saturation indicates higher contents of free water in the sample, which is favorable in CO₂ hydrate formation. Thus, the pressure drop right after the onset of the particular replacement experiment was relatively fast in comparison with other experiments. Based on the GC data, the relative percentage of hydrate forming-CO₂ and replacement-CO₂ were calculated. Figure 10 shows the variation of CO₂ consumed in the two processes in one particular experiment. It is shown that the consumption of CO₂ in CH₄ recovery almost stopped after three days reaction, whereas the consumption rate of CO₂ by CO₂-hydrate formation was almost steady. The continuous formation of CO₂ hydrate throughout the whole experiment implied that the migration of CO₂ gas probably could not be interrupted by either the formation of CO₂ hydrate or the CH₄/CO₂ replacement. Under current experimental conditions and duration, it is deduced that the permeability of the sediments during replacement remained feasible for the diffusion of the gaseous phase. Therefore, the major hinderance of CH₄ recovery should be the diffusion of CO₂ through the mix CH₄-CO₂ layer formed at the surface of methane hydrate.

Based on the above discussion, the CH₄/CO₂ replacement process in our experiments can be summarized by the following steps (Figure 11): (1) after the injection of gaseous CO₂, CO₂ diffused into the sediment sample; (2) replacement occurred at the surface of methane hydrate particles in the pore space with a relatively fast rate, meanwhile, CO₂ gas hydrate also formed in the pore space with diffused CO₂ gas and pore water; (3) after 3 days replacement, almost all the methane hydrate particles were isolated by both the mixed CH₄-CO₂ hydrate and CO₂ hydrate, so that the replacement was nearly stopped. However, the formation of CO₂ hydrate in the pore space was unlikely to prevent the diffusion of CO₂ under our experimental conditions; as a result, CO₂ hydrate was synthesized at a steady rate throughout the whole experiment.

4. Conclusions

In this work, we performed CH₄/CO₂ replacement experiments in hydrate-bearing sediments. The distribution characteristics of gaseous phases and the replacement percentages during the experiments were examined. Our simulation experiments yielded the following results:

(1)

Based on the replacement rates, the replacement process can be divided into a fast stage and a slow stage, which represent two different reaction processes. At the fast stage, replacement mainly occurred at the gas-hydrate interface, while at the slow stage, the occurrence of the replacement relied on the diffusion of CO₂ and CH₄ through the hydrate phase. Although the fast stage only lasted for about 3 days, the overall replacement percentage was mainly determined at this stage.

(2)

Higher replacement temperature and lower initial methane hydrate saturation resulted in higher overall CO₂ replacement percentage. Meanwhile, the CO₂ content decreased, and the methane content increased with the increase of sediment depth. At the same sediment depth, the variation of the relative CO₂/CH₄ contents showed an increasing trend with the increase of experimental temperature/initial methane hydrate saturation.

(3)

The CH₄/CO₂ replacement reaction mainly occurs in the fast reaction stage while the CO₂ hydrate formation by CO₂ and pore water almost runs through the whole experimental cycle at almost the same rate. Under current experimental conditions and duration, the permeability of the sediments during replacement remained feasible for the diffusion of the gaseous phase. The major hinderance of CH₄ recovery should be the diffusion of CO₂ through the mix CH₄-CO₂ layer formed at the surface of methane hydrate.

It is worth noting that, before being applied, the CH₄/CO₂ replacement process must be carefully evaluated and all the variables which affect the overall efficiency must be considered and analyzed in future investigations.