Phase behavior and the interaction of multiple gas molecules in hydrate-dominated geological flow processes

Hydrate is a non-stoichiometric, ice-like solid compound of water and gas molecules that forms at low temperatures and high pressures. The stability of a particular hydrate is affected by the molecular composition of the environment in which it forms. For example, salt causes freezing point depression of hydrate much like it does for ice. In addition, a gas molecule, such as methane, that ordinarily forms hydrate at one pressure-temperature condition, may not form hydrate if the gas is mixed with another molecule, such as nitrogen, that requires increased pressure or decreased temperature to form hydrate. Here, I develop a modeling framework that incorporates the phase stability of gas mixtures to understand the coupling of equilibrium thermodynamics and fluid flow that governs hydrate-dominated geological flow processes. I first present a benchmark study that utilizes standard hydrate models to demonstrate the complex phase stability that occurs when salt and only methane are considered. The results show the impact that three-phase equilibrium, or the co-existence of a gas phase, a liquid water phase, and a hydrate phase, has on the evolution of hydrate systems. I then develop compositional phase diagrams for systems composed of water, methane, carbon dioxide, and nitrogen that elucidate how multiple hydrate-forming components interact to alter the composition of hydrate, completely de-stabilize hydrate, or create three-phase equilibrium conditions. I finally incorporate these compositional phase diagrams into a mathematical framework that describes multi-phase fluid flow that I use to simulate a subsurface injection strategy designed to simultaneously sequester carbon dioxide as hydrate and produce methane gas. The modeling framework illuminates the processes that govern the dynamic behavior of multiple hydrate-forming components. Simulations of subsurface injection demonstrate behaviors that support field and laboratory observations and clarify how composition impacts internal reservoir dynamics. The modeling framework developed here is general and flexible, so it can be modified to model additional components or to include additional physics. In particular, the modeling framework presented here is well-suited to simulate the buoyant ascent of thermogenic gas mixtures through marine sediments or the out-gassing of hydrate layers within the interior of icy planetary bodies like Enceladus.Geological Science

Numerical modelling of multiphase diesel fuel properties using the PC-SAFT equation of state and its effect on nozzle flow and cavtation under extreme pressurisation

The present work investigates the influence of properties variation of Diesel fuel in the range of injection pressures from 60MPa to 450MPa on nozzle flow and cavitation. The PC-SAFT equation of state is utilised to derive physical property predictions of a grade no.2 Diesel emissions certification fuel. Four candidate multicomponent Diesel surrogates are modelled. Density, viscosity and volatility predictions are compared to experimental data from several other Diesel fuels and against Peng Robinson. PC-SAFT calculations are performed using different sources for the pure component parameters, namely LC and GC methods. An eight-component surrogate yields the best match for Diesel properties with a combined mean absolute deviation of 7.1% from experimental data found in the literature for conditions up to 373 K and 500 MPa. The vapour-liquid equilibrium of this surrogate is then calculated with a novel algorithm, which uses as independent variables the mixture composition, density and temperature. This algorithm is based on unconstrained minimisation of the Helmholtz Free energy via a combination of the successive substitution iteration and Newton-Raphson minimisation. The reliability of two different methods presented in the existing literature is assessed for 7 different cases. The properties of the eight component surrogate are derived and put onto tables to be used in simulations. These simulations are performed on a tapered heavy-duty Diesel engine injector at a nominal fully open needle valve lift of 350μm. Two approaches have been followed: (i) a barotropic evolution and (ii) the inclusion of wall friction-induced thermal effects. Results indicate a significant increase in the mean vapour pressure of the fuel and an unprecedented decrease of cavitation volume inside the fuel injector with increasing injection pressure. This has been attributed to the shift of the pressure drop from the feed to the back pressure inside the injection hole orifice as fuel discharges. The study links friction-induced thermal effects to the preferential cavitation of the fuel components. Lighter fuel components are found to cavitate to a greater extent than heavier ones, independently of the initial fuel composition. Moreover, the final vapour cloud composition was found to differ with injection pressure, as the components within vaporise at their respective rhythm according to their molecular structure and global pressure/temperature conditions

An integrated model for optimizing production of marginal oil fields

For the optimization of production in an operating marginal oil field, it is necessary to consider the reservoir inflow, the artificial lift systems, as well as the surface facilities. Since most reservoir simulation software does not include detailed facility modeling, an integrated model of an entire field has been developed including the surface facilities, to allow detailed modeling of the entire field operation. This model is useful for optimizing production and for use in field surveillance activities, as well as investigating the applicability of simplified engineering assumptionsPetroleum and Geosystems Engineerin

Numerical Aspects of Phase Equilibrium Calculations with the Cubic and Association Models

The isobaric–isothermal phase equilibrium (PT Flash) calculation has been an active research topic of thermodynamics for decades. In this work, the conventional framework of the PT Flash calculation, consisting of stability analysis and phase-split calculation, is briefly reviewed by giving the key working equations of the first- and second-order methods. With different type of equations of state, the numerical aspects of the PT Flash calculation have been systematically investigated for various systems over a wide range of conditions: the significance of the first-order methods, volume based versus pressure based second-order methods, a safe-unstable criterion in stability analysis, comparisons of different models and modeling approaches, as well as the converged volume as an initial guess in the volume root solver. Moreover, the same numerical algorithm is used in the second-order methods for both volume and pressure based stability analysis as well as pressure based phase-split calculation for fair comparisons to the largest possible extent. The results reveal that a few iterations of the pressure based first-order method will significantly improve the efficiency of stability analysis, and it is not more efficient to use a volume based second-order method from an overall point of view. A volume based second-order method can improve the efficiency of phase-split calculation, of which the extent depends on the systems and models. This study also shows that the efficiency deterioration of using association models compared to cubic ones is moderate

Mixing and Demixing Processes in Multiphase Flows With Application to Propulsion Systems

A workshop on transport processes in multiphase flow was held at the Marshall Space Flight Center on February 25 and 26, 1988. The program, abstracts and text of the presentations at this workshop are presented. The objective of the workshop was to enhance our understanding of mass, momentum, and energy transport processes in laminar and turbulent multiphase shear flows in combustion and propulsion environments

Modelling cavitation during drop impact on solid surfaces

The impact of liquid droplets on solid surfaces at conditions inducing cavitation inside their volume has rarely been addressed in the literature. A review is conducted on relevant studies, aiming to highlight the differences from non-cavitating impact cases. Focus is placed on the numerical models suitable for the simulation of droplet impact at such conditions. Further insight is given from the development of a purpose-built compressible two-phase flow solver that incorporates a phase-change model suitable for cavitation formation and collapse; thermodynamic closure is based on a barotropic Equation of State (EoS) representing the density and speed of sound of the co-existing liquid, gas and vapour phases as well as liquid-vapour mixture. To overcome the known problem of spurious oscillations occurring at the phase boundaries due to the rapid change in the acoustic impedance, a new hybrid numerical flux discretization scheme is proposed, based on approximate Riemann solvers; this is found to offer numerical stability and has allowed for simulations of cavitation formation during drop impact to be presented for the first time. Following a thorough justification of the validity of the model assumptions adopted for the cases of interest, numerical simulations are firstly compared against the Riemann problem, for which the exact solution has been derived for two materials with the same velocity and pressure fields. The model is validated against the single experimental data set available in the literature for a 2-D planar drop impact case. The results are found in good agreement against these data that depict the evolution of both the shock wave generated upon impact and the rarefaction waves, which are also captured reasonably well. Moreover, the location of cavitation formation inside the drop and the areas of possible erosion sites that may develop on the solid surface, are also well captured by the model. Following model validation, numerical experiments have examined the effect of impact conditions on the process, utilizing both planar and 2-D axisymmetric simulations. It is found that the absence of air between the drop and the wall at the initial configuration can generate cavitation regimes closer to the wall surface, which significantly increase the pressures induced on the solid wall surface, even for much lower impact velocities. A summary highlighting the open questions still remaining on the subject is given at the end

Design of a general architecture for the integration of process engineering simulation and computational fluid dynamics

Imperial Users onl