Investigation Of The Mechanisms Involved In The Steam-Air Injection Process
Authors: Ivory J. , De Rocco M. , Paradis N. 
Published: March 2010

Primarily as a result of low temperature oxidation (LTO) reactions, the utilization of air as a steam additive results in an increase in both oil recovery and the role at which it is recovered. The individual mechanisms of the steam-air injection process are complicated and difficult to isolate. This paper presents results which were obtained from an ongoing mechanistic investigation into the LTO process.

In particular the experiments were designed to determine the effect of air on residual oil saturation pressure drop (i.e. ability of air to divert steam), bitumen properties (asphaltene content, viscosity, and acid number), produced aqueous phase properties (pH, sulphate, acetate, and total inorganic carbon), and coke formation during both steam and water injection experiments.

Introduction

In physical simulator experiments at the Alberta Research Council(1) the addition of air or O2 to a saturated steam injection stream markedly increased the bitumen recovery rate. These 3-D experiments were performed at 216 °C and the higher recovery rate and ultimate recovery were primarily due to the effects of LTO reactions on the horizontal high-permeability communication path between the injection and production wells. This path was a channel of clean 20 to 40 frac sand which had been soaked in water. Air also increased the final bitumen recovery when it was coinjected with water(2).

Air increases the bitumen recovery rate as a result of one or all of the following factors:Steam diversion due to increased flow resistance in the communication Path. The increased resistance is caused by (a) rapid movement of cold bitumen into the communication path, (b) reduced absolute permeability of the communication path due to coke formation and asphaltene precipitation, (c) change in sand wettability (from water-wet to oil-wet) due to the low pH conditions produced by low-temperature oxidation (LTO) reactions, and (d) increase in the volumetric flow rate due to the presence of non-condensible gases (N2, O2, CO, CO2, H2S).Heat produced by oxidation reactions.In situ production of CO and CO2.

In addition, air may affect gas saturations and relative permeabilities.

The purpose of the present study was to examine some of the above mechanisms and determine their relative importance.

Literature Review

Low-temperature oxidation reactions increase bitumen resistance to fluid flow both by increasing bitumen viscosity and by forming coke.

Bitumen Viscosity

Babu and Cornack(3) observed that the viscosity of Athabasca bitumen (measured at 78 °C) increased. with extent of oxidation up to 35 g O2/kg of bitumen. As shown in Figure 1, the bitumen viscosity increased even more dramatically at higher O2 consumption. This viscosity increase is caused by the increased asphaltene content which results from oxidation of maltenes.

Coke Formation

Millour et al.(4) defined the following three LTO regions:

Region 1

(O2 uptake less than 40 g/kg bitumen): Maltenes are converted to asphaltenes. The asphaltene content rises from an initial value of 20% by mass to 42%. Essentially no coke is formed.