Abstract

THAI - 'Toe-to-Heel' Air Injection is an integrated horizontal wells process, which enables the propagation of a stable combustion front through an oil layer. The THAI process operates via a 'short-distance displacement' mechanism, so that the oil which is mobilized ahead of the combustion front is drawn-down into the exposed section of the horizontal producer well, immediately below. Thermally cracked heavy oil, produced during the process, makes an important contribution to oil recovery and also in situ upgrading. A number of 3-D combustion cell tests were performed to investigate the effect that oil layer thickness has on the dynamics of the THAI process. For the experiments, 10.5 API gravity Wolf Lake heavy oil was used. The 3-D experiments exhibited very stable combustion front propagation through the sandpack, at all oil layer thicknesses, 0.1m, 0.2m and 0.4m. The combustion front peak temperature increased with increasing oil layer thickness, reaching 700 ∼ 800 °C. However, increasing the oil layer thickness had little effect on oil recovery, or the API gravity of the produced oil. Oil recovery was consistently very high, at 80 ∼85 % OOIP. There was also an increase of 8 API points in the gravity of the produced oil and a corresponding reduction in the viscosity, from 24,400 mPas (20 °C) for the original Wolf Lake crude oil, down to 30 mPas (20 °C).

Introduction

In situ combustion (ISC) is an air injection process in which a combustion front is propagated through the oilbearing formation, mobilizing the oil ahead of the front to a production well. In principle, a small fraction of the oil in place (5–10%) is burned, in order to mobilise the unburned fraction (1). The lowering of the heavy oil viscosity and consequent increase in oil mobility, is very dramatic in ISC, due to the high temperatures generated. Conventional forward combustion uses vertical injection and vertical production wells (VIVP), either in a patternflood, or direct line drive configuration. The mobilized oil ahead of combustion front is banked-up in the colder, downstream regions of the reservoir, before it is produced. This is an inevitable consequence of the longdistance operation of the conventional process, which can contribute to major operational problems, such as gas overriding, channeling and loss of air injectivity (2). Any one of these effects can seriously impair the performance of the process. Reduction of air injectivity can lead to the process becoming locked in a LTO (low temperature oxidation) mode. If this happens, the process is virtually irrecoverable (3).

THAI - 'Toe-to-Heel' Air Injection is an integrated horizontal wells process that achieves stable combustion front propagation, once the combustion front has become 'anchored' on to the horizontal well (4, 5). It then propagates from the 'toe' position to the 'heel' (Figure 1). Because of the stable, nearly vertical movement of the combustion front, very high oil recovery and substantial in situ upgrading of heavy oil and Tar Sands bitumen can be achieved (5–8). THAI operates in a radically different way to that of the conventional ISC process. This is because it operates as a short-distance displacement process, not long-distance.

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