Geochemical alteration of crude oils during thermal recovery processes

Abstract

Thermal recovery methods are designed to reduce the viscosity of the remaining oil in place by increasing its temperature. During thermal oil recovery the elevated reservoir temperature and pressure in combination with the possible catalytic effect of rock matrixes could lead to significant changes in the composition of the phases and their physical and chemical properties. This can modify the capillary forces in the oil/rock matrix and the oil displacement mobility and the sweep efficiency. The objective of this study was to improve the understanding of the basic mechanisms responsible for the geochemical alteration of crude oils during thermal recovery processes and to use that understanding to identify the conditions under which the thermal process can be applied to improve the recovery of medium and heavy oils. Using reservoir rocks of varying mineralogical composition and crude oils with different geochemical composition, pyrolysis (PY), thermocatalytic conversion and oxidation tests were performed to examine the influence of thermocatalytic reactions on the geochemical conversion of crude oil compounds. From the results of this study, it appears that the controlling mechanism of alteration of crude oil compounds is their radical polymerization. The rate of conversion is influenced by many factors such as rock lithology, metal compounds and clay minerals in reservoir rock and process conditions. The thermal cracking of heavy- and heterocompounds of crude oils leads to significant changes in the oil composition. The labile compounds crack from resins and asphaltene and produce saturate and aromatic hydrocarbons. This induces important modifications in the oil composition. The amount and properties of hydrocarbons formed during the thermal reactions is influenced by the nature of the initial oil. Significant differences in geochemical composition of crude oils may lead to differences in their reactivity. The results of this study provide a new insight on the mechanisms of the reactions involved in thermal recovery processes and also a basis for a better material characterization of various crude oils in numerical models describing PY and oxidation behavior.