Flow Assurance in Deep-Water Gas Pipelines: Locating Hydrate Plugging Position in a Fast Way

Hydrate-associated issues are of great significance to the oil and gas sector when advancing the development of offshore reservoir. Gas hydrate is easy to form under the condition featuring depressed temperature and elevated pressure within deep-water gas pipeline. Once hydrate deposition is formed within the pipelines, the energy transmission efficiency will be greatly reduced. An accurate prediction of hydrate-obstruction-development behavior will assist flow-assurance engineers to cultivate resource-conserving and environment-friendly strategies for managing hydrate. Based on the long-distance transportation characteristics of deep-water gas pipeline, a quantitative prediction method is expected to explain the hydrate-obstruction-formation behavior in deep-water gas pipeline throughout the production of deep-water gas well. Through a deep analysis of the features of hydrate shaping and precipitation at various locations inside the system, the advised method can quantitatively foresee the dangerous position and intensity of hydrate obstruction. The time from the start of production to the dramatic change of pressure drop brought about by the deposition of hydrate attached to the pipe wall is defined as the Hydrate Plugging Alarm Window (HPAW), which provides guidance for the subsequent hydrate treatment. Case study of deep-water gas pipeline constructed in the South China Sea is performed with the advised method. The simulation outcomes show that hydrates shape and deposit along pipe wall, constructing an endlessly and inconsistently developing hydrate layer, which restricts the pipe, raises the pressure drop, and ultimately leads to obstruction. At the area of 700m-3200m away from the pipeline inlet, the hydrate layer develops all the more swiftly, which points to the region of high risk of obstruction. As the gas-flow rate increases, the period needed for the system to shape hydrate obstruction becomes less. The narrower the internal diameter of the pipeline is, the more severe risk of hydrate obstruction will occur. The HPAW is 100 days under the case conditions. As the concentration of hydrate inhibitor rises, the region inside the system that tallies with the hydrate phase equilibrium conditions progressively reduces and the hydrate deposition rate slows down. The advised method will support operators to define the location of hydrate inhibitor injection within a shorter period in comparison to the conventional method. This work will deliver key instructions for locating the hydrate plugging position in a fast way in addition to solving the problem of hydrate flow assurance in deep-water gas pipelines at a reduced cost.

Methane hydrates achieve both thick oceans and thick lithospheres in Enceladus and Mimas

The difference between the inactive surface of Mimas and the active surface of Enceladus is puzzling. We investigate the conditions under which both have a thick subsurface ocean and the thermal lithosphere of Mimas is thicker than that of Enceladus by using a one-dimensional simulation of thermal evolution. We adopt the initial core temperature, initial methane concentration, and tidal heating rate as free parameters in the calculation. The initial methane concentration and tidal heating rate greatly affect the current ocean thickness, although the initial core temperature does not affect the thickness. Methane hydrate forms at the base of the icy shell if the initial methane concentration is not 0. The methane hydrate layer plays an insulative role in an icy shell. When the initial methane concentration is 1000 , ∼2 GW is needed to achieve more than 50 km of the subsurface ocean on Mimas and ∼7.5 GW is needed to achieve more than 25 km of the subsurface ocean on Enceladus. These values are smaller than those needed when the initial methane concentration is 0 . The existence of the methane hydrate layer promotes the survival of the subsurface ocean because it insulates internal heat. In addition, it is found that the surface heat flux is depressed if the methane hydrate layer exists, which is consistent with the unrelaxed craters in Mimas. Methane hydrate may explain the thick oceans in Mimas and Enceladus and the inactive shell of Mimas.

A formation and growth model of CO2 hydrate layer based on molecular dynamics

This study develops a model to predict the CO2 hydrate layer thickness. As to achieve this, we need the mass transfer coefficients at the interface between water phase and CO2 hydrate layer and the diffusion coefficients in CO2 hydrate. Firstly, we conducted the visualization experiment of CO2 hydrate layer dissolution behavior. From the experiment, we obtain the mass transfer coefficient on the CO2 hydrate layer. The experimental results show good agreement with the existing empirical equation. Secondly, we conducted the molecular dynamics simulation of CO2 hydrate to obtain the self-diffusion coefficients of CO2 and H2O molecules. As to calculate the self-diffusion coefficients, we identified inter-cage hopping and intra-cage movement of molecules based on each molecule travel distance. Finally, the results indicate that the kinetic model we proposed reproduce the layer thickness on the order.

Methane hydrate creates the thick oceans in Mimas and Enceladus

The difference between the inactive surface of Mimas and the active surface of Enceladus is puzzling. We investigate the conditions under which the both have a thick subsurface ocean and the thermal lithosphere of Mimas is thicker than that of Enceladus by using a one-dimensional simulation of thermal evolution. We adopt the initial core temperature, initial methane concentration, and tidal heating rate as free parameters in the calculation. The initial methane concentration and tidal heating rate greatly affect the current ocean thickness, although the initial core temperature does not affect the thickness. Methane hydrate forms in a subsurface ocean if the initial methane concentration is not 0. The methane hydrate layer plays an insulative role in an icy shell. When the initial methane concentration is 1000 \(\text{m}\text{o}\text{l}\hspace{0.17em}{\text{m}}^{-3}\), ∼3 GW is needed to achieve more than 50 km of the subsurface ocean on Mimas and ∼10 GW is needed to achieve more than 25 km of the subsurface ocean on Enceladus. These values are smaller than those needed for when the initial methane concentration is 0 \(\text{m}\text{o}\text{l}\hspace{0.17em}{\text{m}}^{-3}\). The existence of the methane hydrate layer promotes the survival of the subsurface ocean because it insulates internal heat. In addition, it is found that the surface heat flux is depressed if the methane hydrate layer exists, which is consistent with the unrelaxed craters in Mimas. Methane hydrate may explain the thick oceans in Mimas and Enceladus and the inactive shell of Mimas.

Improved Thermal Model for Hydrate Formation Drilling Considering Multiple Hydrate Decomposition Effects

Hydrate is ice-like solid non-stoichiometric crystalline compound, which is stable at favorable low temperature and high-pressure conditions. The predominant gas component stored in naturally-occurring hydrate bearing sediment is CH4 and is estimated about 3000–20000 trillion cubic meter worldwide. Thus, it has attracted significant research interests as an energy source from both academic and industry for the past two decades. Ensuring drilling safety is much important to realize efficient exploitation of hydrate source. Additionally, accurate prediction of wellbore temperature field is of great significance to the design of drilling fluid and cement slurry and the analysis of wellbore stability. However, the heat transfer process in wellbore and hydrate layer during drilling through hydrate formation is a complex phenomenon. The calculation method used in the conventional formation cannot be fully applied to hydrate reservoir drilling, largely due to the complex interactions between the hydrate decomposition, multiphase flow and heat transfer behaviors. In this study, an improved thermal model of wellbore for hydrate layer drilling process is presented by coupling the dynamic decomposition of hydrate, the transportation of hydrate particles in cuttings and heat transfer behaviors in multiphase flow. The distribution of temperature field and rules of hydrate decomposition both in wellbore and hydrate layers are thoroughly analyzed with case study, which is very helpful for the designing drilling parameters, avoiding the gas kick accidents. As well as making a detailed guidance of wellbore stability analysis. This proposed mathematical model is a more in-depth extension of the conventional temperature field prediction model of wellbore, it can present some important implications for drilling through gas–hydrate formation for practical projects.