STUDIES ON THE FORMATION AND DISSOCIATION OF GAS HYDRATES IN POROUS SEDIMENTS

Abstract

Gas hydrates are a nonstoichiometric crystalline form of solids formed by the amalgamation of methane gas molecules with water molecules at low temperature and high pressure. For oil, gas, chemical, and other industries, forming gas hydrates has been a problem for many years because hydrates may block the pipelines or valves. Hydrate formation in a pipeline may also cause a blowout in the drilling operations. The knowledge of the equilibrium conditions of gas hydrate is obligatory for the economical and safe plan of operations in oil, gas, and chemical industries where hydrate nucleation and formation occur. The physio-chemical and mineralogical analysis of hydrate-bearing sediments can provide valuable data for the methane production from Indian hydrate reservoir by adjusting the natural sediment properties and can help design a viable technology for the commercial production of gas from the hydrates. The characteristics of the hydrate-bearing sediments affect the formation and dissociation of gas hydrate in sediments. The mineral composition, dispersion, and chemical composition of hydrate-bearing sediment samples play a dominant role in the hydrate stability condition and economic development. In the present study, the physical properties of hydrate-bearing sediment of India are compared with each other. The sediment samples are taken from the Krishan-Godavari basin (Depth—127.5 and 203.2 mbsf), Mahanadi basin (Depth—217.4 mbsf), Andaman Basin (Depth- mbsf), and Kerala-Konkan basin (Depth—217.4 mbsf). The gas hydrate saturation observed at these sites is between 3 and 50%. Particle size is an essential parameter of the sediments because it provides information on the transportation and deposition of sediment and the deposition history. The particle size of sand is large (0.1-2 nm), while the particle size of the clay is extremely fine (<0.002 mm), and the particle size of the silt lies in between clay and sand (0.002-0.05 mm). In the present study, the mineralogy of hydrate-bearing sediments was investigated by chemical analysis and X-ray Diffraction. XRD, FTIR, and Raman Spectroscopy distinguished the mineralogical behavior of sediments. Quartz is the main mineral (66.8% approx.) observed in the gas hydrate-bearing sediments. The specific surface area was higher for the sediment sample from the Mahanadi basin, representing the sediments' dissipation degree. The order of specific surface area and CEC for hydrate-bearing sediments from different natural deposits of India are Mahanadi>KG(1b)>Andaman >KG(1a)>KK. This characterization will give important information for the possible recovery of gas from Indian hydrate reservoirs by controlling the behavior of host sediment. SEM analysis shows the morphology of the sediments, which can affect the mechanical properties of the hydrate-bearing sediments. The EDS graph confirmed the existence of O, Na, Mg, Si, Al, S, Cl, K, Ca, Au and Fe. These properties can become the main parameters for designing suitable and economic dissociation techniques for gas hydrates formed in sediments. The fugacity-based thermodynamic model for hydrate has been used to determine the equilibrium pressures of hydrate formation. This fugacity-based model uses the PRSV equation of state, which is used to represent the gas phases in the hydrate. The model's parameters are fitted to the experimental data of binary guest hydrates. The present study investigates binary mixtures of CH4–H2S, C3H8–N2, N2–CO2, CH4–i-butane, C3H8–i-butane, CH4–n-butane, C3H8–n-butane, i-butane–CO2, and n-butane–CO2 hydrates, which have not been modeled before. Unlike previous studies, the Kihara potential parameters were obtained using the second virial coefficient correlation and the viscosity data for gases. The fugacity-based model provides reasonably good predictions for most binary guest hydrates (CH4–C3H8). However, it does not yield a good prediction for hydrates of (CO2–C3H8). The sII hydrates are formed at high concentrations of nitrogen (>0.8) and temperatures higher than 277 K. At higher concentrations of nitrogen, N2 can occupy small cages with higher occupancy and large cages with low occupancy. On the other hand, CO2 gas is observed to occupy small and large cages with low occupancy. The transitions of hydrate structure from sI to sII and from sII to sI have also been predicted by this model for binary guest hydrates. The AAD % calculated using the experimental data of natural gas hydrates is only 10 %, which is much lower than the AAD % estimated for the equilibrium data predicted by the VdP-w model.