THERMODYNAMIC AND KINETIC STUDIES OF METHANE HYDRATES FORMATION AND DISSOCIATION

Abstract

The ever-increasing global energy demand, accompanied by the depletion of fossil fuels, besides their large carbon footprint, has shifted the attention of researchers towards alternative, sustainable, unconventional energy resources such as natural gas hydrates (NGHs). NGHs are primarily found along continental margins of sea and permafrost areas such as Arctic region. These are discerned as potential energy source owing to their abundance. On the basis of preliminary estimations, worldwide hydrate-bound methane is almost twice the size of conventional fossil fuel reserves and can adequately substitute the world's rapidly dwindling conventional oil and gas resources. Furthermore, the high methane fraction in NGHs makes them a cleaner source of energy compared to conventional fossil fuels, petroleum and coal, which are the main causes of global warming due to their high carbon footprint. However, the production of gas from these hydrates is affected by various factors such as: i) Method of hydrate dissociation (depressurization, thermal stimulation, chemical inhibitor injection, CH4-CO2 replacement, etc.,) ii) Type of hydrate reservoir (marine or permafrost; class I, II, III, or IV) iii) Properties of host sediment (sediment lithology, rheology, mineralogy, porosity, intrinsic permeability, permeability anisotropy, mechanical properties, etc.) iv) Production well configuration (vertical, horizontal, or inclined well; single or multiple wells; well spacing in case of multiple wells, etc.) Thus far, numerous experimental and numerical studies have been conducted around reservoir-scale NGH dissociation and gas production in order to analyze its potential future commercialization and it is anticipated that the effective exploration and profitable exploitation of these hydrate reserves will occur in the near future. The present study has been divided into five sections, namely i) Experimental Investigation of CH4 hydrates formation at various sub-cooling degrees, ii) Experimental Investigation of CH4 hydrates dissociation by thermal stimulation, iii) Numerical simulation of gas production ability of reservoir for varying depressurization magnitude and production interval, iv) Numerical simulation of gas production ability of reservoir for varying permeability anisotropy conditions, and v) Effect of hydrate bearing sediment characteristics‘ on hydrates formation and dissociation. i) Experimental Investigation of CH4 hydrates formation at various sub-cooling degrees: Experiments on hydrate formation were conducted in a high-pressurized reactor vessel. Methane gas of 99.9 % purity and Millipore water were used for forming hydrates. The experiments were conducted at various temperatures in the range of 24 °C to 1 °C and initial system pressures between 15 MPa to 13 MPa, to study the effect of degree of sub-cooling on percent(%) conversion of gas to hydrate and rate of hydrate formation. In 1st case, the experiments were conducted at a constant cooling rate of 1°C/h from initial temperature of around 24 °C and at stirring rate of 500 rpm. The initial pressure in the system varied between 14.2 MPa to 13.9 MPa. The induction time for hydrate formation was obtained somewhat in between 10 to 12 hrs for different trails. In 2nd case, the experiments were conducted at constant temperature of 1 °C and at 500 rpm stirring rate. The initial pressure was around 13.1 MPa to 13.5 MPa. An overlapping between gas dissolution and hydrate nucleation was observed, indicating faster hydrate formation during initial stages of reaction.