Low Pressure SAGD Operations

Abstract

Abstract In the last few 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, 3,500 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. This paper examines the scaling tendencies of minerals, and the fate and compositions of produced gases in both low pressure SAGD and SAGD in low virgin pressure reservoirs. Use is made of recent advances in gas-liquid solution thermodynamics. The implications of this work include a possible effect on simulation studies for bitumen production, as well as on cap gas production via the produced water. This may also be an equity issue in the Athabasca area where cap gas has been shut in by regulatory intervention. Introduction 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. Chhina(1) is reported to have advocated the use of lower pressures in order to reduce steam-oil ratios (SOR), and therefore production costs. The issue has been further explored by Kisman(2), 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 paper(3) 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.