Controlling hydrate slurry transportability by optimizing anti-agglomerant usage in high water cut systems

Abstract

For mature fields with high increasing water cut, gas hydrate is a serious problem because hydrate inhibition with thermodynamic inhibitors, such as methanol and glycols, often become economically and logistically unpractical. The potential of transporting hydrate particles using anti-agglomerant (AA) in high water cuts systems could be an attractive alternative to the traditional approaches. AAs have long been considered for improving transportability of hydrate slurry flow in pipelines. Although many pilot tests and a few field tests have been conducted, the economics of AA injection have rarely been favourable. Most AA studies focused on the injection of AAs at concentrations higher than 1% to create transportable hydrate slurry. The operational cost of such a process can be only favourable if water cut is very low and oil prices are high. From the economic point of view, it would be more profitable to decrease as much as possible the AA concentration, in particular in high water cut systems. Low AA concentration restriction is expected to benefit not only the economical viability of the oil and gas production but also minimize the potential environmental impact of the AAs. In this communication, systematic experiments were conducted to determine the effect of AA on apparent viscosity of water/oil/hydrate mixture at different AA concentrations ranging from 0.15 to 2% (w/w) of aqueous phase. The water cut in the emulsion was varied from 60% to 80%. Hydrate formation was controlled, by injecting gas, until apparent viscosity of hydrate slurry increased sharply at a certain hydrate volume fraction. The results based on the systems and AA tested show that rheology of hydrate slurry strongly depends on the AA concentration, type of emulsion before hydrate formation and the nature of the liquid hydrocarbon phase. It was also concluded that hydrate transportability depends on the amount of hydrates in the system (not the water cut as generally believed). Therefore, for high water-cut systems where gas is the limiting reactant, it is possible to form transportable hydrate slurries at much lower AA concentrations. This could result in economical application of AA in high water cut systems. Copyright 2011, Offshore Technology Conference.