Hydrate Production Philosophy and Thermodynamic Calculations

Abstract

The amount of energy in the form of natural gas hydrates is huge and likely substantially more than twice the amount of worldwide conventional fossil fuel. Various ways to produce these hydrates have been proposed over the latest five decades. Most of these hydrate production methods have been based on evaluation of hydrate stability limits rather than thermodynamic consideration and calculations. Typical examples are pressure reduction and thermal stimulation. In this work we discuss some of these proposed methods and use residual thermodynamics for all phases, including the hydrate phase, to evaluate free energy changes related to the changes in independent thermodynamic variables. Pressures, temperatures and composition of all relevant phases which participate in hydrate phase transitions are independent thermodynamic variables. Chemical potential and free energies are thermodynamic responses that determine whether the desired phase transitions are feasible or not. The associated heat needed is related to the first law of thermodynamics and enthalpies. It is argued that the pressure reduction method may not be feasible since the possible thermal gradients from the surroundings are basically low temperature heat that is unable to break water hydrogen bonds in the hydrate–water interface efficiently. Injecting carbon dioxide, on the other hand, leads to formation of new hydrate which generates excess heat compared to the enthalpy needed to dissociate the in situ CH4 hydrate. But the rapid formation of new CO2 hydrate that can block the pores, and also the low permeability of pure CO2 in aquifers, are motivations for adding N2. Optimum mole fractions of N2 based on thermodynamic considerations are discussed. On average, less than 30 mole% N2 can be efficient and feasible. Thermal stimulation using steam or hot water is not economically feasible. Adding massive amounts of methanol or other thermodynamic inhibitors is also technically efficient but far from economically feasible.

Why Should We Use Residual Thermodynamics for Calculation of Hydrate Phase Transitions?

“…Similar for industrial systems, like processing and transport of hydrocarbon systems, which also has the same situation of not being able to reach equilibrium due to all the possible routes to hydrate formation, including the impact of solid surfaces like rusty pipelines [22,[25][26][27]29,30,[79][80][81]. Even if hydrate forms in a pipeline it can re-dissociate if the flow surrounding the hydrate results in contact with liquid water under saturated with hydrate former or hydrocarbons which is under saturated with water.…”

“…All the thermodynamic properties involved in various nucleation theories are available from the concept demonstrated here; (6) Present stage of modeling hydrate production was very limited by lack of consistent thermodynamic tools that is able to address the variety of calculations needed for all the phase transitions involved. The non-equilibrium nature of hydrates in sediments [79][80][81] requires a residual thermodynamic scheme that is able to address competing phase transitions for hydrate formation and hydrate dissociation. Work is therefore in progress [76][77][78][79][80] on the development of a new hydrate reservoir simulator, which is fundamentally.…”

“…The non-equilibrium nature of hydrates in sediments [79][80][81] requires a residual thermodynamic scheme that is able to address competing phase transitions for hydrate formation and hydrate dissociation. Work is therefore in progress [76][77][78][79][80] on the development of a new hydrate reservoir simulator, which is fundamentally. Different from any other hydrate reservoir simulators because in utilize a reactive transport platform in which all hydrate phase transitions are treated as pseudo reactions.…”

“…Different from any other hydrate reservoir simulators because in utilize a reactive transport platform in which all hydrate phase transitions are treated as pseudo reactions. Each of the thesis in references [76][77][78][79][80] contain 6 to 12 Journal publications. The thesis can be downloaded from University of Bergen for free or they can be sent from the leading author of this study; (7) The residual thermodynamic scheme described in this work was applied to discussion on maximum water that can be tolerated in various hydrate forming systems during transport in pipelines [22,[25][26][27][28][29][30].…”

Wellbore Temperature and Pressure Field in Deep-water Drilling and the Applications in Prediction of Hydrate Formation Region

“…The parameters need to be adjusted according to the actual drilling process in calculation, such as drilling parameters, wellbore structure and casing parameters, basic thermophysical parameters, and so on. If the gas in the reservoir is not only methane, the phase equilibrium model of mixed-gas hydrate should be considered, which can be referred to (Kvamme and Tanaka, 1995;Kvamme et al, 2020;Mohammad et al, 2020). In addition, this method can be used separately to accurately predict temperature and pressure in onshore or offshore drilling without considering hydrate, and the calculation parameters should be adjusted according to the actual basic parameters and boundary conditions.…”