(124a) High Pressure Phase Behavior of Long Chain Alkanes: Solid-Solid and Solid-Liquid Transitions

An example of precipitation of undesired solid phases is the production of petroleum fluids from hyperbaric reservoirs which are found at increasing depths. This higher depths lead to a very asymmetric composition of the reservoir fluids, with a mole fraction of methane up to 0.6 and presence of heavy alkanes with carbon number up to 60. It is very common that a solid phase precipitates because of a change in the fluid pressure and temperature. A well known phenomenon is wax precipitation, the precipitation of solid phases made up of long chain heavy alkanes.

To get a better understanding of the phase behaviour of these hyberbaric reservoirs we are studying the phase behaviour of binary and ternary model reservoir fluids composed of methane and long-chain n-alkanes.

In this contribution, the thermal behavior, upon heating, was studied for 4 pure alkanes: eicosane, tetracosane, triacontane and tetracontane at pressures from 10 MPa to 150 MPa. The experiments were performed using the transitiometer developed and described by Randzio et al. [1].The calorimetric signal, pressure and temperature was measured using a very slow heating rate to guarantee thermal equilibrium.

It was found that from the compounds studied, eicosane was the only one that did not present a solid-solid transition; the other compounds show a solid-solid transition a few Kelvin below the solid-liquid transition temperature. This solid-solid transition disappears when the pressure is increased in a triple point of type: solid-solid-liquid.

1. S.L. Randzio, J. Therm. Anal. 46 (1996) 335-336.