Abstract

A preliminary numerical simulation study of THAI - ‘Toe-to-Heel’ Air Injection, is described. The reference physical model experiments are one dry and one wet combustion test carried out using heavy Wolf Lake Oil. The experiments operated in a completely stable manner and it was of primary interest to investigate via the numerical model, whether this behaviour could be predicted. The experiments were three-dimensional and so a corresponding 3-D numerical model was constructed using the STARS reservoir simulator. Initially, because of insufficient fuel laydown in the path from the injector well to the horizontal producer well, the predicted result was unstable because of oxygen breakthrough into the toe of the horizontal well. Using a sleeve-back modification to the horizontal well gave a stable result for dry combustion. Sleeve-back was also used to initiate the wet combustion simulation, but it was then removed and the combustion front propagation continued in a stable manner. A third component - coke was added to the oil pseudo-component description in a further simulation of the dry combustion experiment. This was set equal to the measured coke residue in the gas swept zone in the sandpack. The simulation result was completely stable, implying that an improved cracking reaction model was needed in the numerical model. The 3-D simulation result predicts a very stable combustion front profile, which propagated stably and completely to the end of the sandpack. Overall oil production was 85% OOIP, which was in good agreement with the experimental results.

Introduction

Numerical simulators have been developed in order to increase our understanding of the complex processes involving fluid flow, heat transfer and chemical reactions occurring during in situ combustion. Most simulators reported in the literature can be divided into two categories depending on the way they represent the chemical reactionsin the combustion process. Reactant controlled models, such as those of Youngren (1) and Davies (2) assume that oxygen reacts instantaneously with the oil on contact. However, the great majority of models are of the kinetic-controlled category, which use Arrhenius-type reaction rate expressions to represent the temperature-dependent reactions. These models include among others the work of Crookston, Culham and Chen (3), Grabowski, Vinsome, Behie and Rubin (4), Coats (5), Rubin and Vinsome (6) and Vaughn (7). A comprehensive review has also been given by Islam, Chakma and Farouq Ali (8).

There are no published numerical modelling studies of the THAI process - ‘Toe-to-Heel’ Air Injection, described recently by Greaves et al (9,10). This new air injection process achieves very stable combustion performance by controlling adverse gas override. The main desirable benefits of THAI are given below.

  • Gas override is controlled, upright in situ combustion front

  • All mobilised liquids and combustion gases are drawn down into exposed section of horizontal producer well

  • High sweep efficiency, related to the absence of any gas coning (channelling) in the producer

  • Significant environmental benefits due to in situ removal of sulphur and heavy metals

  • Unique enhanced mobility oil zone downstream of the in situ combustion front reduces sensitivity (preferential advancement) to reservoir heterogeneity in the virgin zone, mainly for extra-heavy reservoirs

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