The Promise And Problems of Enhanced Oil Recovery Methods

Abstract

Abstract Enhanced oil recovery (EOR) methods are outlined in a systematic and balanced manner, and the current impact of these methods is assessed in light of field experience. The main EOR methods are described briefly, their current status is described, and the reasons for the lack of success are examined. The paper does not rely on voluminous statistics, nor does it give another set of screening criteria. Rather it develops the process discussion in terms of mobility ratio and capillary number. Successful EOR methods such as SAGD are modifications of standard methods to unusual reservoir conditions. Certain EOR methods, such as chemical floods, have inherent limitations, explaining the lack of success in the field in contrast to laboratory. Introduction Primary oil recovery can vary from zero to over 50% of the original oil in place (OOIP), depending on the type of hydrocarbon and the reservoir drive. Primary recovery from oil sands is zero, whereas that for a water drive in a light oil reservoir can reach 50%, or even more in the case of an efficient gravity drive. Secondary recovery is usually taken to mean waterflooding, and can again vary from zero (oil sands) to a few per cent (heavy oils) to 20 - 50% of the initial oil in place (IOIP) for a light oil. Enhanced oil recovery usually implies recovery beyond the secondary stage. In the case of very viscous oils and oil sands, with little or no primary and/or secondary productivity, EOR may refer to the recovery techniques employed from the start. We are thus concerned with a whole range of unconventional-usually costly oil recovery methods, only a few of which have been shown to be commercially successful, under favourable conditions. Improved oil recovery (IOR) sometimes signifies measures taken during-not after-the primary and secondary stages to increase oil recovery. This is often the case in offshore oil reservoirs where the economics would not permit operation beyond the secondary stage. As the conventional oil reserves in U.S.A. and Canada decline, EOR is gaining increasing importance. The extensive survey published every two years by the Oil & Gas Journal (Moritis(l)) shows that the EOR production in Canada and U.S.A. is about 25% and 10%, respectively, of the total oil production, and is increasing. Table 1, based on the survey, gives the oil production rates for various EOR method, for 1994. Miscible displacement is the dominant recovery method in Canada, with steam injection gaining fast. The latter is the principal recovery method in U.S.A., with miscible carbon dioxide flooding a distant second. In Canada, steam injection will continue to increase in importance, as the heavy oil/bitumen resources are exploited. In U.S.A., steam is limited to California heavy oil reservoirs, and carbon dioxide flooding to areas with inexpensive sources. Chemical flooding methods have declined with the drop in oil prices over the last decade. These methods have the promise of recovering oil from depleted light oil reservoirs. Hundreds of EOR projects are being operated. Many are experimental.