Laboratory Combustion Behaviour Of Countess B Light Oil
Authors: Tzanco E.T. , Moore R.G. , Belgrave J.D.M. , Ursenbach M.G. 
Published: March 2010

In-situ combustion behavior in light oil reservoirs (i.e. &gt;25 ° API) is not as well-documented as combustion of heavy oil and bitumen's. The usual concerns expressed are that the light oil will be swept away by the gas flood to a residual level that is too low to sustain combustion, or that the process will be unable to deposit sufficient amounts of coke (i.e. fuel) to make the process self sustaining. In this study, 15 laboratory in-situ combustion tube tests were performed on 28 ° API oil from the Countess B pool located in southeastern Alberta using both normal (21 % oxygen) and enriched air (38% and 95% oxygen) injection. A wide range of water injection rates allowed observation of combustion under dry, normal wet and superwet conditions.

The primary result of this study is the discovery that Countess B does not appear to be burning a coke-like fuel. Rather, it appears to be burning an oxidized asphaltenes fraction. The low rate of coke deposition observed for this oil provides an important explanation of the difficulties reported in sustaining high-temperature combustion in a light oil reservoir.

Introduction

In-situ combustion as an enhanced recovery technique is often associated with thin reservoirs containing heavy to medium oils. The process has enjoyed mixed success in field applications, however, most of the problems can be traced to a lack of understanding of the controlling mechanisms. One requirement, often overlooked, is that the oil be mobile at the reservoir conditions. Showalter(1) has stated that in-situ combustion should be viewed as a displacement, rather than a thermal, process, as the mobilized oil is displaced into the cold regions of the reservoir. This statement recognizes that much of the energy generated by the process remains in the swept portion of the reservoir.

The application of in-situ combustion in light oil reservoirs has considerable technical merit, as oil mobility is not a limiting factor. The main concern with light oils is whether the residual oil saturation available to the combustion zone and the oxidation kinetics are such as to provide sufficient fuel to make the process self-sustaining. A review of the in situ combustion literature shows that several technically-successful field operations have involved light oils (Table 1). While a full review of the reported field data is beyond the scope of this paper, it is of interest to look at the combustion characteristics based on the apparent atomic H/C ratios and the combustion temperatures.

Moss et al.(2) reported laboratory and field-measured apparent H/C ratios of 2.89 and 2.73 respectively for the Pontotoc pool of southern Oklahoma. Hardy et al.(3) described the May-Libby. Delhi project that involved a 40 ° API oil. They reported an apparent H/C ratio of 3.1 for the field project. The H/C ratios for both the May-Libby and Pontotoc projects were significantly higher than the actual atomic H/C ratios of the native oils. This suggests that either significant oxygen was consumed by liquid phase oxygen-addition reactions or that the fuel for the process was not a hydrogen deficient "coke" fraction.