Conditions prevailing in the mobile oil zone (MOZ) in the THAITM thermal recovery process are known only to a limited degree, yet such information is important for understanding how catalytic upgrading occurs in the associated CAPRI ™ process.
The phase behavior of THAI pilot oil with process gas mixtures under proposed catalytic cracking conditions has been studied using WinProp (CMG). Two and three phase boundary calculations were carried out by solving an equation of state (EOS) to generate lines of constant phase mole fraction or lines of constant volume fraction (quality lines). The results are presented in pressure-temperature (P-T) diagrams, or as P-T envelope, which represent a thermodynamic boundary separating the two-phase conditions (inside the envelope) from the single-phase region (outside). The results provide some new insight into the conditions existing in the MOZ, under specific operational constraints.
In the THAI ™ process in Fig. 1, the zone immediately ahead of the combustion front is a high temperature region in which coke formation occurs. Coke is laid down on the reservoir sand surface due to thermal cracking of the heavy residua resulting from vaporization of the lighter oil components. The coke zone grades into what is termed the MOZ, a region where thermal upgrading, including deposition of heavy metals, is considered to take place. It is very important to understand what the properties of this zone are, since the partially upgraded THAI oil flowing through this zone, becomes the feedstock entering the exposed layer of catalyst surrounding the horizontal producer well. The resulting state of the oil and gas mixture is critical to understanding operation of the CAPRI ™ ‘reactor’. Fig. 2 shows the expected temperature profile ahead of the combustion front, through the MOZ, and downstream into the cooler regions of the sandpack/reservoir. Observed experimental temperatures provide some indication of the possible extent of the MOZ. The actual lower temperature cutoff point in the MOZ for catalytic reactions to be significant in CAPRI ™ is not known precisely, but is probably ∼300 ° C. Catalytic reaction conversion of the partially upgraded THAI oil also depends critically on the phase state in this zone. Whether it is all gas, liquid and gas, and in what proportions?
In low pressure, 3D laboratory experiments at the University of Bath, significant thermal and catalytic upgrading of Athabasca Oil Sands bitumen (7.9 °API) was observed, up to 10 to 12 °API(1). Typically, with heavy crude oils, like Wolf Lake (10.6 °API) and Lloydminster (12.2 °API) (2), there was additional catalytic upgrading of around 4–6 °API. In some experiments, the upgraded, produced oil, using standard refinery HDS catalysts, reached as high as 26 ∼28 °API, almost a light oil. In Petrobank's THAI ™ field pilot trials at WHITESANDS, Christina Lake, Alberta, thermally upgraded bitumen of 12.4 ° API has been continuously produced during the last 3 years – ever since the pilot was first started. Additionally, about 10 per cent of light condensables are also produced and these are captured separately.
