An experimental and modelling investigation of the rheological properties of water/oil/gas hydrate mixtures

In the search for new conventional oil and gas reserves, operators are moving into more challenging reservoirs. The move of the oil and gas industry into increasingly deeper and colder locations and/or production from mature reservoirs, in which water cut can be relatively high, has faced the industry with a major challenge, because the traditional hydrate prevention methods are very expensive (i.e., high CAPEX and/or OPEX) and even, in some cases, unfeasible. In this context, hydrate management may be more economical than hydrate avoidance. Forming dispersed hydrate particles using Anti-agglomerants (AAs) is currently an attractive option for overcoming hydrate blockage problems, especially for long tieback and high subcooling systems. This study mainly focuses on the rheological behaviour of hydrate slurry in high water-cut systems (from 60 to 80%), as these are probably the most difficult conditions for managing flow assurance issues using conventional techniques. To engineer a controlled formation of slurry flow made up of hydrate particles in high water cut systems, it is critical that the flow characteristics of water-oil emulsions and the hydrate slurries are well understood. In high water cut systems, by converting a certain amount of water phase into hydrate particles, the behaviour of hydrate slurry will be strongly dependent on the water-oil emulsion which acts as a carrier fluid to the hydrate particles. In this case, the behaviour of water-oil emulsions can depend on many parameters, such as the presence of natural surfactants, AAs, salt and even hydrate particles, might affect the morphology of the emulsions. So far, however, in terms of hydrate slurries, there has been very little research on the morphology of water-oil emulsions in the presence of AAs and hydrate particles. This work is initially focused on the effect of these parameters on the stability of water-in-oil (W/O) emulsions, oil-in-water (O/W) emulsions and phase inversion from W/O to O/W and vice versa. The rheological study of hydrate slurries is a difficult subject and to date there has been little study on this issue. Most of these studies have focused on low water cut systems and there is a lack of data specifically relating to hydrate slurries in high water cut systems. In this research work, which concentrates on the rheological behaviour of hydrate slurries in iii high water cut systems, the agglomeration of hydrate particles has been shown to be responsible for the rheological behaviour of water/oil/hydrate mixtures through viscosity measurements from an in-house high pressure Helical Tube Impeller (HTI) viscometer and pressure drop measurements from a pilot-scale flow loop. The effect of oil composition, AA concentration, water cut, shear rate and salt concentration has been investigated on the rheological behaviour of hydrate slurries in high water cut systems. Existing models to predict viscosity of hydrate slurry do not consider the effect of water-oil emulsion, which acts as a carrier fluid for transporting hydrate particles. It will lead to deviations between model and experimental data specifically relating to hydrate slurry in high water cut systems. In this study a model has been developed to predict the viscosity of water-oil emulsion in the presence of hydrate particles in high water cut systems using the concept of a bimodal mixture. In the model, water-oil emulsion and hydrate aggregates in the liquid continuous phase are treated separately as unimodal models. A new modification of Mills’ (1985) equation has been applied to describe the viscosity of unimodal hydrate suspension. The model has been validated using experimental data acquired by the HTI viscometer for water/oil/hydrate mixtures in the presence of different AA concentrations and different oil compositions. The predictions of the proposed model are in good agreement with experimental data for both experiments performed with oil-in-water and water-in-oil emulsion