A Simple Kinetic Model for Coke Combustion During an In-Situ Combustion (ISC) Process
Authors: Ren Y. , Freitag N.P. , Mahinpey N. 
Published: March 2010

Although coke combustion studies have long been conducted, the literature is still lacking an accurate understanding of reaction kinetics. To this end, the thermo-oxidative behaviours of Neilburg oil and its asphaltene fraction were examined in the presence of core sand. Thermo gravimetric analysis (TGA) was performed in a flowing atmosphere at the heating rate of 10 °C/min up to 750 °C. Both nitrogen and air were used at a flow rate of 45 ml/min in the experiments. As earlier researchers have observed, at least two main regions of reactions were identified by the thermo gravimetric (TG) and derivative thermo gravimetric (DTG) thermo grams. Various effects, including distillation, low-temperature oxidation (LTO), thermal cracking, middle-temperature oxidation (MTO), high-temperature oxidation (HTO) or combustion, and even mineral decomposition, were observed.

In this study, Neilburg oil and asphaltenes were completely cracked in a nitrogen atmosphere at 425 ° C to produce coke. Subsequently, the fresh coke was subjected to isothermal combustion at several temperatures from 374 °C to 519 °C. A two-step oxidation reaction model was applied to describe this combustion process. The chemical reactions were simplified into two oxidations occurring in series. In the first reaction, coke was partially oxidized to form an intermediate product, which was then burned in the second reaction. Based on the TGA data, kinetic parameters were estimated with the aid of custom written software. For comparison, the one-step oxidation reaction model was also employed to predict the combustion process. The two-step oxidation reaction model gave a better fit to the experimental data. It was also confirmed that the coke derived from the Neilburg asphaltenes is reasonably representative of the coke derived from the whole oil.

Introduction

It has long been recognized that in situ combustion (ISC) might be a potentially effective enhanced oil recovery (EOR) technique, particularly suitable for medium and heavy crude oil-bearing reservoirs, as well as for steam depleted reservoirs. However, the fact that the fundamental reaction mechanisms of the ISC process have not been completely understood makes its field performance prediction unreliable.

It is generally believed that there are three major reactions during an ISC process: thermal cracking, low temperature oxidation (LTO) and high temperature oxidation (HTO). Thermal cracking of crude oil, including visbreaking and coking, produces a solid residue on the surface of the reservoir sand grains. The solid residue, or so-called coke, is consumed as the fuel for combustion. LTO reactions, which are typically heterogeneous reactions between the gas and condensed phases, involve the formation of oxygenated hydrocarbons. During an ISC process, LTO is possible if oxygen is present ahead of the combustion front, either due to the oxygen flux being relatively high or due to bypassing of the front by some of the injected oxygen(1). HTO is the heterogeneous (gas-solid) reactions that consume the coke deposited by thermal cracking to produce carbon oxides and water.

Some efforts to investigate the oxidation reaction mechanisms during ISC can be found in the literature. An early extensive study was carried out by Dart et al.(2) who tested the combustion rate for oxidation of carbonaceous residues on clay catalyst pellets.