The formation of gas hydrates from water-in-oil emulsion was investigated on two different flow loops: a laboratory scale flow loop (Archimede flow loop: 30 m long, and 1 cm diameter, St-Etienne School of Mines) and a pilot scale flow loop (Lyre flow loop: 150 m long, 5 cm diameter, IFP Solaize). Both flow loops are equipped with a focused beam reflectance measurement (FBRM) probe for in situ particle size analysis. These FBRM probes were used to monitor chord length distribution (CLD) during the crystallization process of water-in-oil emulsions into gas hydrate slurries. When water droplets crystallize into hydrate particles, an agglomeration phenomenon is evidenced by pressure drop measurements. This agglomeration phenomenon is also detected by the FBRM probe and is highlighted by a sharp change in the mean chord length and a spread of the CLD to larger chord length. In order to better interpret the chord length distribution measurements, a modelling work has been made. This paper gives a description of the algorithm used for building 3D fractal aggregates and simulating CLD measurements on them. Aggregates are constructed from a monodisperse spherical particle. The influence of different parameters (fractal dimension, number of particles in the aggregate, diameter of primary particles) on the simulated CLD is also discussed. Some comparisons between experimental and simulated CLD are finally used to describe the physical properties of aggregates during an experiments. (C) 2009 Elsevier Ltd. All rights reserved.