Experimental analysis of hydrate deposition under multiphase flow conditions

Abstract

One of the challenges faced during deep-water oil and gas production operations is the hydrate formation and accumulation throughout the flowline since it may cause either production impairment or pipeline blockage, leading to temporary revenue losses, whilst threatening the system’s structural integrity. Historically, hydrate formation in offshore operations has been prevented through heating, insulation or injection of thermodynamic inhibitors. However, these approaches might be inefficient and involve high costs; thence, the industry has been moving from hydrate avoidance towards hydrate management strategies, which allow hydrate formation whilst preventing its accumulation. In this context, a solid understanding of hydrate formation and accumulation mechanisms is key to safely implementing hydrate management strategies. For this purpose, the interdependence of multiphase flow conditions and the process of hydrate formation and accumulation was investigated in a high-pressure rock-flow cell with viewing windows. Several experiments were performed under different conditions of subcooling, temperature gradients, liquid loadings and water contents to analyse the influence of these parameters on the mechanism of hydrate formation, wall deposition and agglomeration. Two different system compositions were considered: water/gas and water/liquid hydrocarbon/gas. To focus on hydrate wall deposition, a gradient temperature inside the rocking cell was forced to promote upper wall deposition. Phenomena such as hydrate agglomeration, wall deposition, sloughing and washing were identified. The footage of the experiments allowed the identification of the flow conditions before and after the beginning of the hydrate formation and the development of a phenomenological model for the process of hydrate formation and accumulation under multiphase flow conditions. It was found that the hydrodynamics of the system before the beginning of the hydrate formation impacts the hydrate accumulation process. The flow conditions promote the dispersion of the phases and consequently influence the location of the hydrate deposits. These findings suggest that the presence of a free water phase after the beginning of the hydrate formation represents severe concerns in flowlines in terms of transportability and plug formation as it favours the hydrate wall deposition no matter the amount of water and gas consumed, as well as the presence or absence of an oil phase. These outcomes might help in the development of hydrate management strategies as well as in the improvement of multiphase hydrate flow models, contributing to increasing operational safety and field optimisation.