NUMERICAL AND EXPERIMENTAL STUDIES ON THE FORMATION AND DISSOCIATION OF METHANE GAS HYDRATE

Abstract

The exploration of methane gas hydrates is important since natural gas hydrates (NGH) happen to be one of the most abundant energy resources on the planet. Minimal estimates put offshore and onshore gas hydrate estimates to be twice of all carbon fossil fuels on earth. The depletion of petroleum, and coal reserves all over the world has led the world to search for alternative energy sources. Moreover, petroleum and coal are known to be key contributors to global warming. On the hand, burning natural gas does cause global warming however, it is known to be a much cleaner fuel than the former. However, recovering methane from NGH requires knowledge of understanding the hydrate formation and dissociation process. In this research work, we explore methane hydrate formation and dissociation through following experimental and numerical studies. 1. Thermodynamic modeling using updated UNIFAC model with cage occupancies and generalized Correlation The thermodynamic modeling work include modeling of hydrate formation of pure hydrates formed under conditions of pure water without any promoters, inhibitors or salts. For this purpose, hydrates of CH4, C2H6, CO2, H2S and N2 are investigated. For this purpose, we implemented the Vander Waals Platteeuw (vdPW) model available in literature. In the I-H-V phase regime, for CH4 hydrates the minimum Average Absolute deviation (AAD %) is found to be 1.16 using Vander Waals equation (VDW). The performance of other equations namely Peng Robinson (PR), Soave Redlich Kwong (SRK), Redlich Kwong (RK), Patel Teja (PT), Esmaeilzadeh-Roshanfekr (ER) also yield excellent matching with experimental results with error values less than 2%. Also, genetic programming algorithm is adopted to develop a generalized correlation. Overall, the correlation yields quick estimation with an average deviation of less than 1%. The accurate estimation yields a minimal AAD of 0.32% for CH4, 1.93% for C2H6, 0.77% for CO2, 0.64% for H2S, and 0.72% for N2. In conjunction with determining equilibrium hydrate formation pressure, fractional small- and large-cage occupancies were determined and compared with those available in the literature. The values were found to be in close agreement with experimental data. 2. Energy analysis and determination of controlling mechanism of hydrate dissociation. In the hydrate dissociation mechanism study, the different hydrate dissociation mechanisms like Heat transfer, Flow-controlled and hydrate dissociation are investigated. The overall dissociation mechanism is actually a complex function of different parameters like Overall thermal conductivity (K), intrinsic Permeability (k), Wellbore heating Temperature (Tb), Bottom Hole Pressure (Pb) and Rate constant (kd0). Here, it must be noted that K represents thermal conductivity under (liquid) saturated conditions. The intrinsic permeability of porous media is measured prior to the hydrate formation. For the Base Case, the values are thermal conductivity (K=3.1W/m.K), intrinsic Permeability (k=2.96x10-14), Wellbore heating Temperature (Tb=278.15K), Bottom Hole Pressure (Pb=2.7MPa) and Rate constant (kd0=3x104 mol/m2.Pa.Sec). In the investigative study using different parameters, it is found that depressurization and thermal conductivity fare among the most sensitive parameters which influence the hydrate dissociation rate. Although thermal stimulation plays a crucial role in determining the rate of hydrate dissociation, its influence is found to be localized in a sense that it leads to development of thermal gradients which are dependent on thermal conductivity of sediments. Moreover, it is observed that these thermal gradients tend to flatten out over a small range within the reservoir. This adds to ineffectiveness of thermal stimulation for lab scale as well as on a practical (field) scale. The role of permeability is found to be far more complex and interesting. Since, a broad range of permeability cases have been investigated, it is found that a higher permeability does not necessarily implicate that a higher percentage of natural gas can be recovered from the hydrate. Simply put, a high value of K showcases a potential for efficient extraction. The actual efficiency of extraction is linked with depressurization. An optimum value of depressurization must be used which prevents “secondary hydrate formation”. Intrinsic hydration rate constant is not found to be a sensitive parameter, but rather a more intimately coupled one with different parameters like intrinsic permeability. 3. Numerical simulation of depressurization under suppressed heat transfer from surroundings. The Class IV reservoir study describes 2-D numerical investigation of a gas hydrate reservoir under conditions of suppressed heat transfer from surroundings. We carried out numerical simulation of a field scale hydrate reservoir of dimensions (150m x 150m) to determine its response to depressurization. For this purpose, we employed a single well for depressurization of the system. Five different scenarios of hydrate-bearing media, with different phase saturation of gas, hydrate, and water are considered for simulation. The system is depressurized with a single well placed at the center of the domain. The depressurization is conducted for different magnitude with withdrawal rates in the range of 0.01-0.6kg/sec. In case I WR=0.04kg/sec yields a maximum cumulative production of 4.7 x 105 kg (From hydrate). The estimated amount of methane gas produced from hydrate dissociation (Both m3 and kg) is 7 x 105 m3 or 4.7 x 105 kg. The overall gas produced at the well is found to be 7.7 x105 kg. In case II, the maximum methane production is observed for WR=0.06 kg/sec. The net amount of methane produced from hydrate is found to be 7.42 x105m3 and 5.02 x105 kg, respectively. The net quantity of methane produced from hydrate dissociation is found to be 4.91 x 105 kg and 7.25 x 105 m3. The overall production is estimated at 5.5x105 kg. In case IV, with WR=0.05, maximum cumulative methane production is found to be 5.48 x 105 kg and 8.1 x 105 m3, respectively. The aggregate production at the well is found to be 7.41 x 105 kg. In case V, the cumulative production for the maximum depressurization rate yielded, an aggregate mass and volume produced from hydrate dissociation is estimated to be 5.18 x 105kg and 7.66 x105 m3. The overall mass of methane produced at the well is estimated to be 8.14 x105kg. Regarding causes of stopping of gas production. An increasing water and gas saturation makes hydrate depressurization more selective to dissociate hydrate effectively. For systems with abundant free gas, initially, depressurization leads to the release of free gas. Ice formation near the well vicinity is one of the major causes of stopping of gas production from the well. Secondary hydrate formation near the well vicinity leads to restriction of gas flow and consequently causes stopping of gas production. The hydrate reservoir can also reach a steady-state, which leads to the stopping of gas production after a consistent production period. 4. Numerical simulation of gas hydrate reservoir NGHP-01-10D in Krishna Godavari basin, India to determine viability of long term production. In this study, the NGHP-01-10D gas hydrate reservoir site which was discovered as one of the richest gas hydrate accumulation in the world is numerically explored. The reservoir response when subjected to different magnitude of depressurization is investigated. For this purpose, depressurization using a constant bottom hole pressure of 6MPa, 5MPa, 4MPa, and 3MPa was used to depressurize the hydrate using a single vertical well. The cumulative methane gas production mass and volume were found to be 4.6 x 106 m3, 3.4 x 106 m3, 2.37 x 106 m3, and 1.47 x 106 m3 in order of increasing bottomhole pressure. The peak methane gas production rats were also found to be a function of the bottomhole pressure, i.e., lower the bottomhole (higher magnitude) pressure, higher the peak gas production rates. A stable gas production rate of 300m3/d was achieved in case of 3MPa, which reduced to 1900m3/d in the case of 4MPa. It was concluded that depressurization efficiency is limited by the high ratio of water to gas production. Hence different well strategies must be employed to raise efficiency of depressurization 5. Experimental studies on hydrate formation and dissociation (using microwaves) in pure water and silica sediments. Experimental studies on hydrate formation were carried out in a high-pressure autoclave of volume 3000ml. Methane hydrate formation was carried out in pure water (milli-pore) and sediments. Methane gas with industrial-grade purity of 99.9% was used to conduct hydrate formation experiments. In the case of sediments, the particle size was found to be in the range of 150-210um. The formation experiments achieved a maximum water-to-hydrate conversion efficiency of 32.4%, while the average conversion efficiency was found to be 27.4%. Following hydrate formation, the hydrate dissociation was carried out using microwave. For conducting the hydrate dissociation experiments, a microwave generator was switched on, and different power levels were used for the different set of experiments. For this purpose, microwave frequency, 𝜈=2.45𝐺𝐻𝑧 was used to dissociate the methane hydrate. Different microwave power levels (Q) were used to conduct hydrate dissociation experiments, i.e., 100W, 300W, 500W, 700W, 900W, and 1100W. The different experiments conducted indicate a wide variation in the behavior owing to the different power magnitude of microwave radiation. In the case of heating microwave with sediments, it was observed that there is much rapid dissociation of hydrate compared to pure water. When more water is heated due to its dipolar nature, more heat is retained by the water. This results in water providing additional heat to the hydrate, ice and contributing towards their dissociation. The time scale of heat transfer is obviously smaller than the timescale of mass transfer. This inference is made owing to the rapid evolution of temperature, while the pressure increase occurs at a much slower pace. The heat diffusion occurs within the hydrate with tH. However, the release of methane gas from the hydrate cages occurs at a much slower rate owing to a much larger tM. 5. Experimental and analytical study of methane hydrate formation in partially water-saturated sand using Time-Lapse 4-D Synchrotron X-Ray Computed Tomography In this study, In this study, we used previous experimental data from NOC researchers to investigate methane hydrate formation within partially water-saturated sand using Time-Lapse 4-D Synchrotron X-Ray Computed Tomography. The study focused on determining the distribution of sand, water, methane, and hydrate within the system to identify the different stages of hydrate formation. The experimental findings of hydrate film thickness was validated using a kinetic-diffusion model employing concentration difference as a driving force. The gas distribution within the system is tracked to determine the bubble population size distribution in order to correlate with the observed patterns of hydrate growth. For this purpose, time stamps of 16h 42min, 17h 25min, 19h 10min, 21hr 10min, 23h 30min, and 26h 45min were used to measure the hydrate film thickness. Initially, at t= 16h 42min, we observed discrete bubbles of methane gas sparsely distributed within the pore space. The size of the bubbles/slugs was found to be comparatively larger at initial time instants. Based on the following study it is conclude that initially a thin hydrate shell is formed around the gas bubbles, which can be described as “hydrate encapsulated gas bubbles”. Owing to internal gas consumption, the pressure difference causes hydrate shell morphology to undergo shrinking, wrinkling and rupture, which leads to secondary gas bubbles production. The gas distribution within the system varies widely with elapsed time. The initial mean gas bubble size was found to be 34.2um (t=1hr 42min) while the final mean bubble size reduced to 9.65um (26hr 45min). The kinetic-diffusion model employing concentration difference as a driving force for hydrate formation provides decent predictions against the experimental data. The absolute average deviation was found to be 3.3%.