(305b) Gelation of Agglomerating Slurries As a Mechanism for Pipeline Flow Blockage

Transportation of particulate matter as suspensions or slurries is a widely-used industrial operation. Examples are found in waste water treatment and mineral transport. It is also being evaluated for the transport of hydrate particles in the Oil and Gas industry. Colloidal particles with sizes between 1 nm and 10 µm can undergo agglomeration (or aggregation) leading to the formation of large fractal aggregate clusters. This leads to an increase in the effective particle aggregate volume fraction and consequently in the viscosity of the flowing suspension. Pronounced aggregation can lead to a dramatic increase in the suspension viscosity once the effective volume fraction of the particles approaches a gelation transition. This can lead to an increase in the pressure drop across the pipe by two orders of magnitude, effectively plugging the flow. This mechanism is one of the many proposed mechanisms for subsea pipeline blockage due to hydrates. The plugging of subsea pipelines due to hydrate particle agglomeration is considered in this work. The industry spends about US$ 200 million/year on hydrate plug mitigation efforts. The hydrate management paradigm is shifting towards risk management where hydrate particles are to be transported as a flowable dispersion or slurry. For implementing this risk management framework, a detailed understanding of the possible plugging mechanisms is needed. Gelation of the flowing slurry, where the particle volume fraction reaches a freezing transition due to agglomeration is proposed as one such possible mechanism. Particulate aggregation can occur by three possible mechanisms – diffusion-induced or perikinetic, shear-induced or orthokinetic, and gravity-induced or differential settling, or a combination of them. For flowing hydrate suspensions, only the second mechanism has received attention [1,2]. A new framework which considers all the aggregation mechanisms for the prediction of the gelation time, location, and the evaluation of plugging risk has been developed. The model is limited to two-phase laminar flow of Newtonian slurries. Models for particulate aggregation in concentrated systems are used for obtaining rate constants rigorously. The models account for entropic packing effects, non-ideal particle diffusion, and unsteady-state effects in colloidal aggregation. These rate constants are then used in a population balance framework for predicting the effective particle aggregate volume fraction as a function of time, which in turn is used to estimate the gelation time and thus the location of a potential plug. The risk of plugging is evaluated by comparing the gelation time with the residence time of the particles in the flowing system. We provide a regime map to evaluate the relative importance of each of the aggregation mechanisms. When dispersants are used for preventing agglomeration, they result in repulsive thermodynamic forces between particles, the effects of which are also included in the models.

[1] A. Sinquin, T. Palermo, and Y. Peysson. Oil & Gas Science and Technology-Revue d'IFP Energies Nouvelles, 59(1):41-57, 2004.

[2] E. Colombel, P. Gateau, L. Barre, F. Gruy, and T. Palermo. Oil & Gas Science and Technology- Revue d'IFP Energies Nouvelles, 64(5):629-636, 2009.