Norman P. Freitag

Low-Temperature Oxidation of Oils in Terms of SARA Fractions: Why Simple Reaction Models Don't Work

The low-temperature oxidation (LTO) reactions of the SARA fractions separated from two crude oils were studied in the presence of their reservoir sands at temperatures between 130 and 230° C. The results indicated that the usual approach to modelling LTO-the use of a very few single-step Arrhenius-rate equations-could not be made to reflect the observed reaction kinetics. Instead, this investigation found that the following reaction characteristics were needed for accurate reaction modelling: 1) a change in the order of reaction with respect to oxygen concentration from to 1 as temperature rises; 2) the repression of a saturates oxidation reaction by other fractions; and, 3) a prominent induction period exhibited by the saturates fraction. The compositions and yields of the ultimate LTO reaction products were measured, and these included relatively stable residues with high oxygen contents. Because the LTO reactions play an important role in enhanced oil recovery by air injection methods, the above information is valuable for the simulation and prediction of these processes.

A SARA-based model for simulating the pyrolysis reactions that occur in high-temperature EOR processes

Although there is a need for forecasting the performance of enhanced oil recovery processes involving air injection, the capability to do so is still modest. One of the limitations to such forecasting is the lack of knowledge of the reaction chemistry, which leaves questions as to which or how many reactions are needed, and how to obtain values for the associated rate parameters. A model is presented to describe one of the three major categories of reaction that must be considered when simulating air injection: the heat-induced cracking of oil components. The model is well suited for the numerical simulation of air-injection EOR processes with commercial simulators. It is based on the measured rates of pyrolysis/coking reactions of purified SARA fractions separated from two very different sources: a Lloydminster heavy oil, and a Cold Lake bitumen. Most of the results for the two oils were fairly similar, which suggested that the model might apply readily to a broad range of oils. This paper also outlines a modified SARA analytical procedure that proved to be more reliable for this type of study than conventional methods of SARA analysis.

A Simple Kinetic Model for Coke Combustion During an In-Situ Combustion (ISC) Process

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. Thermogravimetric 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 thermogravimetric (TG) and derivative thermogravimetric (DTG) thermograms. 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.

Evidence That Naturally Occurring Inhibitors Affect the Low-Temperature Oxidation Kinetics of Heavy Oil

The so-called induction period, the time delay between the initial exposure to oxygen of an oil or oil fraction and the start of rapid oxidation, was examined experimentally for the saturates fraction separated from a Lloydminster heavy oil. The observed kinetics could be explained by assuming that the saturates contained a small amount of naturally occurring oxidation inhibitors, which repressed the oxidation rates by rapidly consuming an essential intermediate in the reaction chain, but which were also gradually consumed in the process. This observation explains some of the complexity that has been seen in the oxidation rates that control combustion front development during in-situ combustion, and provides some added direction in the development of a comprehensive reaction model for this process.

Experimental Investigation of Capillarity and Drainage Height Roles in the Vapor Extraction Process

The authors present the results of a novel comprehensive experimental program to investigate the roles of capillary pressure and drainage height on the performance of the VAPEX process. Measured stabilized drainage rates at lower permeability range (5.1–6.4 D) in this study show no linear relationship with square root of permeability, as reported for experiments at high permeabilities (>200 D). In these tests stabilized drainage rates are a function of drainage height to the power of 1.1–1.3.

Capillarity and Drainage Height Effects on the Diffusion/Dispersion Coefficient in the VAPEX Process

This study is conducted to investigate the roles of capillarity and drainage height on mass transfer phenomena during the VAPEX process in heavy oil reservoirs. A comprehensive experimental study was conducted in two 2-D slab models using Plover Lake oil from west-central Saskatchewan and n-butane as the solvent. This study revealed significant effects of drainage height and capillarity on oil drainage rates in contrast to previous studies conducted at unrealistically high permeabilities (i.e., >200 D). A new correlation was developed to predict the effective diffusion/dispersion coefficient from solvent concentration, drainage height, and total pore surface area of the porous media.

Prediction of fluid phase behaviors in a CO2-EOR process in Weyburn field, Saskatchewan, Canada

Publisher Summary The objective of the multidisciplinary “Weyburn CO2 Miscible Flooding Project” is to develop a Pressure-Volume-Temperature (PVT) model for the CO2-Weyburn oil system that can be coupled with compositional reservoir models for the short- and long-term field-scale reservoir simulations. This project is operated by Encana Corporation in southeastern Saskatchewan, Canada, provides a unique opportunity for the “lEA GHG Weyburn CO2 Monitoring and Storage Project” to add to the knowledge and understanding of the process mechanisms of enhanced oil recovery (EOR) and CO2, a greenhouse gas (GHG), storage in oil depleted reservoir. The model is continuously modified as the field process proceeds to capture the dynamic change in fluid properties, including minimum miscibility pressure (MMP) and the effect of contaminates in the injecting CO2. Accurate prediction of the CO2 distribution in different phases (that is, aqueous, oleic, and gaseous) in the reservoir after the CO2-EOR process is essential for long-term risk assessment that is relied on the understanding of the fluid phase behaviors. For example, estimations of mineral trapping, ionic trapping, and solubility trapping of CO2 are based on the amount of CO2 stored in the aqueous phase. On the other hand, the amount of CO2 stored in the gaseous phase, which is the most mobile phase of CO2 in the reservoir, is essential in the estimation of CO2 leakage.