EXPERIMENTAL-DETERMINATION OF THE RATE CONSTANTS OF THE N-C-25 THERMAL-CRACKING AT 120, 400, AND 800 BAR - IMPLICATIONS FOR HIGH-PRESSURE HIGH-TEMPERATURE PROSPECTS/

The aim of this work is to determine the apparent rate constants for t he n-C-25 thermal cracking and, then, to study pressure effect on thes e rate constants so as to evaluate n-alkane stability in high-pressure /high-temperature deep pro spe cts. Pyrolyses were carried out in anhy drous do sed system (gold vessels) during times ranging from 1 to 360 h under isothermal conditions (325-425 degrees C) and various pressure s (120, 400, and 800 bar) in the range of those occurring in petroleum systems. Results show that degradation of n-C-25 follows a first-orde r kinetics under isothermal conditions at 120 bar. The main chemical c lass produced along increasing severity is the saturated hydrocarbons ranging from C-10 to C-22 (80 wt %). The gas contribution is very low (<2 wt %) up to 85 wt % conversion. On the basis of the linearity of t he Arrhenius plot, accurate kinetic parameters were calculated (E = 68 .2 kcal/mol and A = 6.1 x 10(17) s(-1)). An extrapolation of these kin etic parameters to geological conditions shows that n-C-25 begins to b e measurably degraded above 180 degrees C for residence times higher t han 10 million years. This means that a reservoired oil enriched in n- alkanes will be stable in very high temperature conditions. When this oil begins to degrade, condensate is mainly produced and will be measu rably degraded only above 200 degrees C during similar residence times . Pressure effect increases the thermal degradation rate of n-C-25 bet ween 120 and 400 bar then decreases it between 400 and 800 bar down to its value at 120 bar. These results are in a good agreement with thos e already published (Fabuss, B. M.; et al. In Advances in Petroleum Ch emistry and Refining; Me Ketta, J., Jr., Ed.; Wiley and Sons: New York , 1964; Vol. 5, Chapter 4, pp 156-201) for thermal degradation of n-C- 6 and n-C-7 saturated hydrocarbons in the same range of pressure. Neve rtheless, the relative increase of the degradation rate for n-C-25 rea ching a maximum around 1.7 for 400 bar, i.e., for an average depth of 4000 m and a temperature of 140 degrees C, the pressure effect is not sufficient to allow a measurable degradation in these geological condi tions.