Oxidation of Heavy Oil and Their SARA Fractions: Its Role in Modeling In-Situ Combustion

Abstract

Abstract In situ combustion is a thermal recovery technique where energy is generated by a combustion front that is propagated along the reservoir by air injection. Most of the previously conducted studies report thermal and fluid dynamics aspects of the process. Modeling in situ combustion process requires extensive knowledge of reservoir data as well as reaction kinetics data. Unfortunately, limited kinetic data are available on the rates and the nature of partial oxidation reactions and the high-temperature combustion reactions of crude oils and their saturate, aromatic, resin, and asphaltene (SARA) fractions. Moreover, the impact of such data on the modeling of the in situ combustion process has not been investigated thoroughly. Thus, we modeled in situ combustion experiments conducted on a 3D semi-scaled physical model that represents one fourth of a repeated five spot pattern. In all experiments a vertical injector is employed whereas, both vertical and horizontal producers have been installed to recover two different crude oils (heavy and medium). Several locations for the producers have been tried while keeping the length of the wells constant: vertical injector-vertical producer, vertical injector-horizontal side producer, and vertical injector-horizontal diagonal producer. In these experiments diagonal producers performed better than the others. We first simulated the experiments by incorporating a kinetic model that is based on grouping the products of cracking into six pseudo components as heavy oil, medium oil, light oil, two non-condensable gases and coke using a commercial thermal simulator (CMG's STARS). Four chemical reactions were considered: cracking of heavy oil to light oil and coke, heavy oil burning, light oil burning, and coke burning. Most of the experiments were history matched successfully with the exception of ones where a diagonal horizontal producer was used. We then repeated the simulations using SARA kinetic parameters. We observed that all matches were somewhat improved. We finally present a discussion of application of the models to field scale. Introduction In-situ combustion is an important enhanced oil recovery process that has been studied extensively the past 45 years. This process has been considered particularly applicable for in-situ recovery of medium and heavy oil reservoirs. In in-situ combustion, heat is generated within the reservoir by igniting the formation oil and then propagating a combustion front through the oil reservoir. The fuel necessary to sustain the combustion front is supplied by the heavy residual material or "coke" that deposits on the sand grains during distillation, thermal cracking, pyrolysis etc. of the crude oil ahead of the combustion front. Sweep efficiency during in-situ combustion is one of the most important process parameters, but which has not been extensively evaluated and is least understood. Most laboratory investigations are conducted in combustion tubes, which essentially use a vertical well arrangement and which, of course, because of their basically one-dimensional geometry, cannot provide information on either areal or vertical sweep. Information on the combustion sweep efficiency is very important for comparing process variations and also for predicting performance. 3-D scaled physical models can provide much valuable insight into the areal and vertical sweep processes and stability of the combustion front over a range of operating conditions. Results from such experiments may be used in conjunction with those from combustion tube tests to predict performance in the field, and also to validate numerical simulator models.