In Situ Combustion Performance In Steam Flooded Heavy Oil Cores

Abstract

Abstract In situ combustion is well suited as a follow-up method to steam-based recovery processes in heavy oil reservoirs. In general, during steam stimulation, these reservoirs develop high mobility, low oil saturation channels which could facilitate the movement of a combustion front. Although there may be little oil in these channels, the energy and CO2 generated by the combustion reactions can be used to mobilize additional oil from the cold regions. Accordingly, a series of four in situ combustion tube tests were carried out to examine the combustion behaviour which could be expected in the low oil saturation channels. Each test involved flooding, first by water and then by high quality steam, to near-residual oil saturations, followed by in situ combustion. The effects of pressure and degree of oxygen enrichment on the combustion characteristics were examined. Also determined were the incremental oil recovery during the water flood, steam flood, and in situ combustion phases of the tests. It was found that combustion operated very efficiently in the pre-steamed channels, although the overall oil recovery was low during this stage of the test due to low initial oil saturation. Introduction An estimated 400 billion tonnes (2.7 trillion barrels) of hydrocarbon are contained in Canada's tarsands and heavy oil deposits(1). While the economic climate of the 1990's does not appear favourable for the recovery of these reserves, most forecasts predict a steadily increasing requirement for heavy oils. In order to ensure security of supply to meet this demand, efficient recovery strategies are necessary as only a small fraction of these deposits is accessible by surface mining techniques. Owing to their simplicity and effectiveness, steam based processes have been the most successful heavy oil recovery techniques to date. In situ combustion, while theoretically more energy efficient, is an alternative method which has enjoyed very limited success in Canada. Although in situ combustion is classified as a thermal process, it might better be described as a displacement process, since it is often overlooked that the energy generation and its associated hot zone is located upstream of the combustion front. As the displaced oil flows into unheated regions of the reservoir, its viscosity increases, which renders it immobile and plugs the communication channels. The entire process stalls as a result, and oxygen is no longer consumed by high temperature reactions. This scenario, which explains the probable cause for the failure of many in situ combustion projects, is supported by laboratory observations that oil mobility is critical to the success of in situ combustion(2). BP Canada Resources Limited, in their Marguerite Lake in situ combustion pilot (and Phase A semi-commercial project at Wolf Lake)(3, 4), attempted to avoid this scenario by using in situ combustion as a follow-up to cyclic steam stimulation in a pattern of wells where the steam chambers had become linked to a sufficient extent that the bitumen was mobile. Their combustion process was designed to operate in the low oil saturation steamed channels, and it was generally considered to be technically successful.