A multiscale approach for gas hydrates considering structure, growth kinetics, agglomeration, and transportability under multiphase flow conditions

Abstract

A worldwide problem reported by oil companies is the plugging of flowlines because of gas hydrates, a crystal that forms and agglomerates causing partial or complete obstructions. This incurs in revenue losses because of production stop, and also relates to safety and environmental risks. The main production strategy consists in avoiding gas hydrates by, e.g., injecting a high volume of chemical inhibitors. In order to reduce production costs, a new strategy called hydrate management is at research, where hydrates are let form, but its stable flow needs to be assured. In this sense, a deep knowledge on off-equilibrium processes such as growth kinetics, agglomeration and transportability is required to design and manage pipelines. This thesis quantitatively describes part of these processes. Several multiscale concepts are gathered from multidisciplinary fields (heat and mass transfer, crystallization, porous media, multiphase flow), leading to new interpretations. Hydrates are porous, hydrophilic particles that act as sponges entrapping water, where crystallization occurs mainly in the capillary walls (1st new assumption). Permeation through the porous particles furnishes water to its outer surface, promoting liquid bridge formation after particles’ collision, which leads to agglomeration (2nd new assumption). Higher subcoolings are shown to promote faster sealing-up of the particles, decreasing permeation rates and causing the particles to be inert in the agglomeration-sense (called dry particles). Furthermore, additives with surfactant properties decrease the permeation rate, which explains their anti-agglomerant effects. Several mechanisms are discussed upon modeling growth kinetics and agglomeration and by further coupling with a steadystate multiphase flow model. The model sensitivity evidences that a general classification of the system can be done in four distinct types of limiting phenomena: active surface-limited, dissolution-limited, heat transfer-limited, and pressure drop-limited. For engineering purposes, the model is simplified into a dimensionless criterion that determines stable production in oil-dominant systems, having the shape of Ba ∞ Da Re -n , which relates the Damköhler and Reynolds dimensionless groups graphical abstract). This expression still needs future testing in order to retrieve the exact shape of the dimensionless groups. An absolute form that depends on subcooling, water cut, mixture velocity and interfacial properties is nevertheless proposed and preliminary test shows agreement with experimental data. This criterion evidences that, once hydrates form in oil-continuous systems, the faster the particles seal-up, the quicker the particles turn dry, and the smaller the stable agglomerate size, thus requiring smaller mixture velocities in order for particles to remain suspended. Another dimensionless group La = f (Da,Re) is proposed to further explain particle-wall interactions into predicting deposition for future studies. If ever these two new dimensionless groups show consistent in future testing and fitting against larger databases, they will represent an important advance on how engineers design flowlines using the hydrate management strategy.