A study of the formation morphology and phase equilibrium of fractured methane hydrates

ENERGY EXPLORATION & EXPLOITATION Volume 33 Number 5 2015 pp. 745 754 745 A study of the formation morphology and phase equilibrium of fractured methane hydrates Youhong Sun, Guobiao Zhang, Wei Guo *, Bing Li, Shengli Li, Kai Su and Rui Jia College of Construction Engineering, Jilin University, No. 973 Ximinzhu Str., Chaoyang District, Chang Chun, 130000, China * Author for corresponding. E-mail: guowei6981@126.com (Received 2 May 2015; accepted 18 July 2015) Abstract Natural gas hydrates mostly fill in the pores or fractures of host sediments and fractured gas hydrates are the main reservoir type in the Qilian mountain region of China. The process of gas production from hydrates can be influenced by the pore structures, fracture properties and mineral compositions of the host sediments. To determine whether the formation and phase equilibrium of hydrates are affected by the properties of the fractures, including the angle and width, formation experiments on methane hydrates were conducted using fracture media composed of artificial sandstone and natural mudstone. The phase equilibrium points for the methane hydrates hosted in fracture mudstone were measured using multi-stage heating. The experimental results indicate that the gas hydrates hosted within the fracture media grew faster. However, there were obvious differences in the methane hydrate formation morphologies for the two types of fracture media: the main occurrences of gas hydrates in sandstone were massive formations, and layered gas hydrates were formed along the fracture surfaces in mudstone. Additionally, a comparison of the phase equilibrium between the fractured CH 4 and bulk phase CH 4 hydrates indicated that the fracture scale did not have a significant influence on the thermodynamic equilibrium of the CH 4 hydrates for the experimental conditions and also implied that fracture properties had an insignificant impact on the hydrate stability zone. Keywords: Fracture media, Methane hydrate, Formation, Phase equilibrium 1. INTRODUCTION Natural gas hydrates primarily fill the pores and fractures of their host sediments (Salehi et al., 2014). In a recent international marine gas hydrate exploration, fractured gas hydrates have been found in the Ulleung Basin of Korea (Park et al., 2008), the Krishna-Godavari Basin offshore India (Collett et al., 2008 ) and the Gulf of Mexico in the USA (Cook et al., 2010). Natural fractured gas hydrates are the most prolific gas distribution type throughout the world, second only to polar and marine sandstone reservoirs (Boswell and Collett, 2006), and are the main reservoir type in the Qilian

746 A study of the formation morphology and phase equilibrium of fractured methane hydrates mountain region of China (Zhang et al., 2013). Natural hydrate-bearing sediments have characteristics including: complex mineral components and irregular distributions of fractures and pores, which affect the properties of the hydrates. Many researchers are aware of the influence that the storage media has on the properties of the pore-filling hydrate reservoirs (Salehi et al., 2014). Experimental and theoretical studies have been conducted using different media to simulate sediments to examine the formation and decomposition mechanisms and phase equilibrium of different types of gas hydrates. Handa and Stupin (1992), Seshadri et al. (2001) and Uchida et al. (2004) studied the formation process, decomposition kinetics and phase equilibrium of gas hydrates using porous media, such as silica gel, sandstone, glass beads, sand and clay. Zang et al. (2013), Hu et al. (2008), Jiang et al. (2013) and Liu et al. (2010) used fine-grained sediments, unconsolidated sediments, sand, porous sediments from the South China Sea, respectively, as porous media to investigate gas hydrate formation and decomposition processes. In these studies, different types of porous media were used to simulate the porosity of hydrate-bearing sediments to study the influences of porous structural properties on hydrate formation, decomposition and phase equilibrium. These are considered less relevant studies with regards to fractured gas hydrates. Studies on the formation and development, decomposition and phase equilibrium of fractured hydrates are still inadequate, which affects the exploration and development of fractured hydrates. In this work, the formation and phase equilibrium experiments of CH 4 hydrates in fracture media were conducted to determine the influences that natural fractures in sediments have on the formation and phase equilibrium of methane hydrates and to provide basic experimental data for potential use in possible exploitation studies. 2. EXPERIMENTAL MATERIALS AND METHODS 2.1. Selection of fracture media In the Qilian mountain region of China, the formation lithology of the fractured gas hydrate reservoir is mainly siltstone, oil shale, mudstone and fine sandstone (Wang et al., 2011), which are compact and fractured sedimentary rocks. Therefore, sandstone and mudstone were selected as the fracture media for these experiments. Sandstone was artificially synthesized using fine sand, cement and deionized water, which provided a material similar to the characteristics of sandstone, including the particle size and friction. Natural mudstone cores were directly selected for use. In nature, gas hydrates filling fractures have a certain thickness, ranging from 1 mm to several centimetres (Zhu et al., 2010; Ryu et al., 2013). The angles of the fractures in hydratebearing sediments are high, as much as 60 to 90, with an average occurring between 43 and 63 (Lee and Collett, 2009; 2013). Based on these sediment fracture characteristics, two types of fracture media were examined: a cylindrical-type with a diameter of 55 mm and a height of 65 mm and a penetrating fracture, which had an angle of 90 and a width of approximately 1 mm. In general, the moisture content of these rocks is generally high. Therefore, to simulate the moisture content characteristic of these sediments, the fracture media was saturated with deionized water. During the process, fracturing mudstone produced tiny fractures due to swelling after the material absorbed water, which was consistent with natural conditions.

ENERGY EXPLORATION & EXPLOITATION Volume 33 Number 5 2015 747 2.2. Experimental apparatus A diagram of the experimental apparatus and arrangement is shown in Figure 1 and is divided into four parts: a high pressure reactor, a low temperature thermostat, supplying gas lines and the data collection system. The primary component of the experimental apparatus is the high pressure reactor, which has an internal radius of 60 mm, a height of 90 mm and a maximum operating pressure of 20 MPa. The reactor is placed within the low temperature thermostat, which can maintain the temperature range from -25 to 90 C. Methane gas, supplied by the Beijing AF BaiF Gases Industry Corporation with a purity of 99.99%, is injected into the high pressure reaction through high pressure gas lines. In the experiment, the pressure and temperature were measured by PTX1517 pressure transducers and PT100 thermocouples, respectively. The thermocouples were inserted into the fracture medium. The experimental pressure and temperature data were recorded and stored in real time. Figure 1. Schematic of the experimental apparatus. 1-gas cylinder, 2-reducing valve, 3-pressure gauge, 4-valves, 5-reactor, 6-Low temperature thermostat, 7-thermometer, 8-pressure transducer, 9-paperless recorder and 10-computer. 2.3. Experimental methods First, the fracture media saturated with deionized water was placed in the reaction chamber and the experimental device was assembled according to the schematic. After checking the tightness of the apparatus, the reaction chamber was evacuated, and the atmosphere was replaced with methane. This process was repeated three times. Then, the thermostat temperature was set and methane gas was injected into the reactor until the gas pressure of the reactor was higher than the equilibrium pressure of the bulk hydrates at a constant reactor temperature. During the experiment, the reactor pressure higher was maintained higher than the equilibrium pressure by injecting methane gas repeatedly to promote the formation of hydrates. After the reactor pressure changes are observed to be small for a long period, the following actions were taken: 1) open the

748 A study of the formation morphology and phase equilibrium of fractured methane hydrates reactor and observe the morphology of the hydrates that form in the fracture media and 2) determine the fractured hydrate equilibrium conditions for pure methane. The phase equilibrium was measured using a multi-step heating method (Sun et al., 2010; Mahboobeh et al., 2013). First, the reactor temperature is decreased to -3.5 C, and the valve to remove methane gas from the reactor was opened until the gas pressure in the reactor was lower than the pressure of the bulk hydrates. The pressure increased to a constant value, indicating an equilibrium state based on the gas expansion or hydrate dissociation, which consequently was the hydrate equilibrium pressure at -3.5 C. Then, a rise in the reactor temperature by 1 C promoted hydrate decomposition and pressure increase. When the reactor pressure was kept constant for 12 hours, another gas hydrate phase equilibrium point was acquired. Following this method, the temperature was increased from -3.5 to 7.5 C by a step-heating path to record the reactor pressure at each step and acquire the phase equilibrium curve of the fractured methane hydrates over a certain temperature range. 3. RESULTS AND DISCUSSION 3.1. Formation morphology of the CH4 hydrates in the fracture media The morphology of the CH 4 hydrates after formation at low temperatures in fractured sandstone and mudstone is shown in Figure 2. There were obvious differences in the methane hydrate formation morphologies in the two types of fracture media. In the fractured sandstone, CH 4 hydrates mainly formed on the surface of the medium (Fig. 2 (A1)), most of which were distributed in the gap between the medium and the reactor wall, with a small part amount distributed on the upper and lower surfaces around the fracture, and a very small amount of forming on the bottom edge of the fracture surface. In the fractured mudstone, CH 4 hydrate formed on the surface and in the fracture medium. The hydrate on the surface of the medium was mainly distributed like a belt along the fracture, and the fracture was filled with a large amount of gas hydrate, which was thicker along the edge of the fracture. The main reason for this phenomenon is the difference in the water adsorptions of the sandstone and mudstone. For the fractured sandstone, the adsorption force of the water is smaller due to the differences in the mineral components and pore properties. Saturated water gradually seeped out of the pores and fractures in the sandstone and enriched the areas around the fracture and on the wall of the reactor, corresponding with the methane hydrates that formed on lower edge of the fracture and in the gap between the reactor wall and the sandstone. There was not a significant amount of hydrates that formed on the fracture surface due to less water being attached to the fracture. After the fractured mudstone was saturated with water, the pores and tiny fractures filled and the clay minerals in the fracture surface adsorbed enough water to from a layer of water film (Fig. 3 (A)) due to the strong water adsorption. Therefore, the water could react with the CH 4 gas to form crystal nuclei (Fig. 3 (B)) because of a sufficient amount of gas-liquid contacts, which was followed by the formation of hydrates. The induction time was very short, meaning that a mudstone fracture surface would form and be covered with a layer of hydrates along the direction of the fracture (Fig. 3 (C)). Simultaneously, a small amount of water absorbed in a pore will slowly form a small amount of pore-filling gas hydrate (Fig. 3 (D)).

ENERGY EXPLORATION & EXPLOITATION Volume 33 Number 5 2015 749 3.2. Characteristics of the temperature and pressure in hydrate formation process Figure 4 displays the reaction temperature and pressure changes versus time during CH 4 hydrate formation in the fracture media. The formation process for fractured hydrates was similar to that of pore-filling hydrates (Hu et al., 2008; Zang et al., 2013). According to the changes in the curve slope, the fractured hydrate formation process can be divided into three periods: induction, fast formation and slow formation. (A1) (A2) (B1) (B2) Figure 2. The morphology of the CH4 hydrates in the fractures of sandstone and mudstone.(a1) Image of hydrates in sandstone in the reactor, (A2) image of hydrates on the fracture surface of the sandstone, (B1) image of hydrates in mudstone in the reactor and (B2) image of hydrates on the fracture surface of the mudstone. The induction period occurs at the beginning of methane gas injection. The methane gas pressures in the two types of fracture media drop as a result of both the temperature drop and the rapid dissolution of the methane gas in water. The induction time for fractured hydrates was as short as 10-20 min, which may be due to the large gas-liquid contact area, which results from the rough surface structure of the fractured medium and the numerous mineral particles. These properties provide favourable conditions for hydrate nucleation and speed up the nucleation rate. The fast formation period occurs when the reactor temperature is lower than the phase equilibrium temperature and the reactor pressure decreases greatly from 4 MPa to 3 MPa in sandstone and from 3.9 MPa to 3.3 MPa in mudstone, indicating the fast formation of hydrates. This was due to the good dispersion of fracture surface water. Methane hydrate

750 A study of the formation morphology and phase equilibrium of fractured methane hydrates formed rapidly along the gas-liquid interface of the fracture and covered the fracture or medium surface, as seen from the morphology of hydrate shown in Fgures 2 (B1) and 2 (B2). Correspondingly, the reactor temperature suddenly increased by 0.7 C to 0.9 C in the sandstone (Fig. 4 (A)) and by 0.3 C to 0.6 C in mudstone (Fig. 4 (B)), which was because the hydrate formation released a significant amount of heat. The slow formation period occurs in two types of fracture media. The reactor pressure decreased slowly and small temperature fluctuates occurred, indicating the slow formation of CH 4 hydrates. This is mainly because after hydrate formation at the gas-liquid interface of the fracture, the formation is controlled by a process in which the gas transfers mass to the liquid through the hydrate layer, resulting in the slow growth of hydrates due to fracturing with a certain width and a water film with a certain thickness. Hydrates in fracture media also include a small part of pore-filling hydrates, which are slowly formed in a small amount of pores in the fracture media, such as the pore-filling hydrates shown in Figure 2. 3.3 Phase equilibrium of CH4 hydrates in fractured media In this experiment, the phase equilibrium conditions of the fractured methane hydrates were measured under an isochoric process with a step-heating path. The data obtained (A) (B) (C) (D) Figure 3. Schematic of the fractured methane hydrate formation process. (A) Water film formation, (B) Crystal nuclei formation, (C) Fractured hydrate formation and (D) Fractured hydrate derivation.

ENERGY EXPLORATION & EXPLOITATION Volume 33 Number 5 2015 751 (A) Sandstone (B) Mudstone Figure 4. Pressure and temperature changes versus time in the reaction are shown in Table 1, and a comparison of the phase equilibrium between the fractured and bulk phase methane hydrates (Mei et al., 1997; Zhao, 2005) is shown in Figure 5. The deviation between the equilibrium conditions of the fractured methane hydrates measured in this experiment and the bulk phase methane hydrates was very small. The analysis on the morphology of the hydrates (shown in Fig. 2) indicated that the methane hydrate occurring in fractured mudstone exhibited three different occurrences due to the large size of the fracture, including nodular, massive and layered distributions, which were similar to the methane hydrate morphology of pure water. Therefore, the fracture scale does not affect the thermodynamic conditions of the gas hydrates. Also, when researching the formation and distribution of fractured gas hydrates, an analysis of the formation and distribution of hydrates in the fracture reservoirs should be able to be performed macroscopically and roughly follows the laws and characteristics of porefilling hydrate reservoirs. However, in terms of the formation kinetics, differences in the gas-liquid contact may lead to differences in the rate of hydrate formation, the length of accumulation time and the amount of reserves available within a hydrate reservoir at a certain stage for two types of different hydrate types. 4. CONCLUSIONS Methane hydrate formation experiments were conducted in fractured saturated sandstone and mudstone media. The phase equilibrium points of the methane hydrates in fracture mudstone were measured using multi-step heating. The following conclusions were drawn from the results: (1) There were obvious differences in the methane hydrate formation morphologies for the two types of fracture media. The main occurrences of gas hydrates in sandstone were massive and layered, whereas the hydrates formed along the fracture surfaces in mudstone. Differences in the water adsorptions of the sandstone and mudstone were the primary reason for this phenomenon and were due to their different mineral compositions.

752 A study of the formation morphology and phase equilibrium of fractured methane hydrates Figure 5. Comparison of equilibrium conditions between fractured methane hydrate and bulk phase methane hydrate. Table 1. Measured data of CH 4 hydrate equilibrium conditions in fractured mudstone. Temperature of low First equilibrium experiment(e1 Second equilibrium experiment(e2) Pressure (MPa) Pressure (MPa) -3.5-3.12 2.23-3.18 2.28-2.5-2.18 2.29-2.18 2.31-1.5-1.24 2.36-1.24 2.45-0.5-0.30 2.48-0.32 2.66 0.5 0.82 2.76 0.90 2.90 1.5 1.80 3.04 1.88 3.14 2.5 2.77 3.35 2.75 3.39 3.5 3.78 3.71 3.75 3.73 4.5 4.80 4.11 4.70 4.08 5.5 5.76 4.50 5.70 4.47 6.5 6.65 4.92 6.73 4.99 7.5 7.45 5.31

ENERGY EXPLORATION & EXPLOITATION Volume 33 Number 5 2015 753 (2) The fractured hydrate formation processes can be divided into three periods: induction, fast formation and slow formation, which is the same for hydrates formed in porous media. (3) The deviation between the equilibrium conditions of the fractured methane hydrates measured in this experiment and bulk phase methane hydrates was very small, indicating that the fracture scale in the experiments does not affect the thermodynamic conditions of the gas hydrate. Therefore, the exploration and exploitation of hydrates in fracture reservoirs can be observed macroscopically and roughly follows the laws and characteristics of a pore-filling hydrate reservoir. However, differences in the rate of hydrate formation, the length of accumulation time and the amount of reserves available in a hydrate reservoir at a certain stage for the two types of different types hydrate may be caused by different formation kinetics. 5. ACKNOWLEDGEMENT This study has been supported by National Natural Science Foundation of China (Grant No.51304079, Grant No. 51474112) and China Geological Survey Project (GZH201400307, GZHL20110326). REFERENCES Boswell R. and Collett T.S., 2006. The gas hydrates resource pyramid: Fire in the ice. Methane Hydrate Newsletter 6(3), 5-7. Collett T.S., Riedel M., Cochran J., Boswell R., Presley J., Kumar P., Sathe A., Sethi A., Lall M., Sibal V. and the NGHP Expedition 01 Scientists, 2008. Geologic Controls on the Occurrence of Gas Hydrates in the Indian Continental Margin: Results of the Indian National Gas Hydrate Program (NGHP) Expedition 01 Initial Reports, pp. 1-135. Cook A., Guerin G., Mrozewski S., Collett T. and Boswell R., 2010. Gulf of Mexico gas Hydrate Joint Industry Project Leg II: Walker Ridge 313 LWD Operations and Results, pp. 1-24. Handa Y.P. and Stupin D.Y., 1992. Thermodynamic properties and dissociation characteristics of methane and propane hydrates in 70-?-Radius silica gel pores. The Journal of Physical Chemistry 96(21), 8599-8604. Hu G.W., Ye Y.G., Zhang J., Diao S.B., Liu C.L., Wang H.X. and Wang J.S., 2008. Study on gas hydrate formation-dissociation and its acoustic responses in unconsolidated Sands. Geoscience 22(3), 465-474. Jiang G.L., Wu Q.B., Yang Y.Z. and Zhan J., 2013. Formation and dissociation of methane hydrate in different occurrences in sand. Nature Gas Geoscience 24(6), 1305-1310 (in Chinese with an English abstract). Lee M.W. and Collett T.S., 2009. Gas hydrate saturations estimated from fractured reservoir at Site NGHP-01-10, Krishna-Godavari Basin, India. Journal of Geophysical Research. Part B: Solid Earth 114(B7), 13. Lee M.W. and Collett T.S., 2013. Characteristics and interpretation of fracture-filled gas hydrate-an example from the Ulleung Basin, East Sea of Korea. Marine and Petroleum Geology 47, 168-181.

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