Gas Generation from natural oils -- the details matter in resource assessment

Abstract

It is generally accepted that the kinetics of cracking reactions within the kerogen – bitumen – petroleum transformation control the thermal evolution of hydrocarbons in the subsurface. Pyrolysis experiments analyzing gas generation from natural oils and model compounds have been conducted to ascertain the kinetic parameters and estimate gas yield under varying conditions (Tsuzuki et al., 1999 Fusetti, Behar et al., 2010). Results from these kinetic studies are utilized in petroleum systems modeling to generate estimates of gas yields.

Unfortunately, depending upon the experimental method, composition of oil used, and whether ramped or isothermal heating is used to crack the oil to gas the kinetic parameters vary dramatically. As an example, a comparison between 7 different gas generation pyrolysis experiments produced Ea (activation energy) parameters that vary between 48 – 88 kcal/mol and A (frequency factor) values that range from 4.5 × 1010 to 3.2 × 1022 s-1 (Willette, et al., in prep). Application of these various kinetic parameters in modeling gas yields will produce estimates that differ significantly.

It may be more useful to look directly at experimental gas yields and use those results for ‘ground truth’ in the application of kinetic parameters in petroleum systems modeling.

Pyrolysis experiments were conducted on a natural oil under varying conditions: anhydrous, hydrothermal (with water in reactor), quartz sand in reactor, illitic shale in reactor, dolomite in reactor, and limestone in reactor. The gas generated from cracking the oil at 380° C ranged between 216mg/gOo to 166mg/gOo.

This difference may appear small, but if the difference in yields from just the anhydrous and hydrothermal experiments are converted to gas yield from a barrel of oil, an additional 2.1 – 2.4 mcf is produced from a barrel of oil using the hydrothermal experimental yield. Using experimental yield data to constrain kinetic parameters will result in more realistic assessment of gas saturations in unconventional reservoirs, of original oil/gas in place, and provide insight into the common problem of initially under-estimating the amount of gas produced during field development.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016