In the last two years, there has been increased interest in low pressure SAGD operations, in part due to the gas over bitumen resource conflict. In an earlier paper it was predicted that low reservoir steam pressures had the effect of sharply reducing hydrogen sulphide and carbon dioxide production per barrel of oil. Likewise, particularly troublesome scales, such as the magnesium and calcium silicates, were less thermodynamically favoured than at high steam pressures.
Further investigations into the fate of gases in SAGD have been undertaken, especially for the case where the virgin pressure of the reservoir is low. In such cases, the amount of solution gas is limited, and the bitumen at steam zone pressure is often undersaturated with respect to the available methane. The polar gases, hydrogen sulphide and carbon dioxide, on the other hand, are almost entirely produced via dissolution in the produced water. In the low virgin pressure case, the methane is entirely produced via dissolution in bitumen. This contrasts with initial reservoir pressures of, for example, 3500 kPa, where more than half of the methane is also produced via the produced water. The other major difference is that, in the low virgin pressure case, very little gas is predicted to accumulate in the steam zone.
The implications of this work include an effect on simulation studies of bitumen production, and on the possible effect of cap gas production via the produced water. This may be an equity issue in the Athabasca area where cap gas has been shut in by regulatory intervention.
There has been considerable interest in the issue of SAGD steam zone pressure (or temperature) in recent years. The issue revolves around the range of pressures under which a given SAGD project might be operated, without serious adverse economic effects on the project. Chhina1 is reported to have advocated the use of lower pressures in order to reduce steamoil ratios (SOR), and therefore production costs. The issue has been further explored by Kisman2, who defines low pressure SAGD as involving "pressures below what steam/gas lift can accommodate, and above pressures where low-pressure consequences such as reduced oil rates and artificial lift difficulties affect the viability of SAGD operations". One of Kisman's conclusions was that operation at reasonably low pressures provides savings due to reduced SOR, and the smaller production well liners and tubulars that are needed. While there are limited field data on low pressure SAGD at present, the issue has received more detailed attention in the last few months owing to the ongoing gas-bitumen resource conflict.
A previous paper3 explored a number of aspects of the engineering chemistry of SAGD, which are also dependent on the pressure (i.e. temperature) of the steam zone. In the author's experience, these aspects have had significant impacts on some field operations. The engineering chemistry considerations pointed to potential benefits of operating SAGD at low pressure. The benefits included a possibility for avoiding the need for sulphur recovery, and reduced probability of mineral scales that have plagued some SAGD projects.
