All of the enhanced oil recovery (EOR) methods are outlined in a systematic and balanced manner, and the current impact of these methods is assessed in the light of the field experience. As such, this paper is a timely and comprehensive description of EOR methods, and where they stand today, and examines the reasons for the lack of success. 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. It is shown that certain EOR methods, such as chemical floods have inherent limitations, explaining the lack of success in the field, in contrast to laboratory.
Enhanced oil recovery (EOR) is usually taken to mean oil recovery beyond primary production and waterflooding. In the case of very viscous oils and oil sands, with little or no primary and/or secondary productivity, EOR may even refer to the recovery techniques employed from the start We are thus concerned with a whole range of unconventional - usually costly - oil recovery methods, few of which have been shown to be commercially successful, and that too in specific reservoirs.
The interest in EOR, and particularly the field activity, rises and falls with the prospect of increasing or decreasing oil prices, and also with the perceived foreign oil supply situation and government incentives. Thus economics dominate much of the EOR activity reflected by the extensive field project surveys published by the Oil & Gas Journal every two years (e.g. see Aslund (I)). The large number of field projects for a given EOR method does not necessarily mean that the method is technically successful Similarly, very few field tests of a particular process do not imply that the process is technically ineffective. This simple-sounding statement has far reaching implications in the choice and application of a particular EOR method. As an example, Table I lists the number of projects and the oil production for several major EOR categories, based upon Ref. 1.
Quite a different reason for interest in the EOR methods is the simple fact that oil recovery by primary and secondary methods (pressure maintenance, waterflooding) is less than 50% of the oil in a reservoir, and much less than that in many reservoirs. For the past fifty years, researchers have been consistently looking for methods to increase this figure. The volume of such research is again determined by the factors mentioned previously for field activity.
Our objective is to discuss the EOR methods from a mechanistic point of view. What are the technical limitations on each method as far as the oil recovery is concerned? And why some of the recovery methods, which look so good in the laboratory, fail in the field? Oil recovery is not a good criterion for field applications, where economics and cash flow (viz. oil production rate) are the key factors. Nevertheless, it is a worthwhile target to consider.