We recently attended a spacing hearing in a relatively new oil state. The hearing was one of the longest ever held before a state conservation commission. It covered, among other things, many of the aspects of petroleum engineering. Many of those attending the hearings as witnesses, attorneys and commission representatives appeared to have the idea that water injection was almost always successful in increasing oil recovery. They also had the impression that the proper time to start water injection was "immediately". A review of many industry magazines, technical publications and petroleum engineering textbooks shows that, while there are many articles and papers on water injection, waterflooding and pressure maintenance through water injection, few of the publications discuss projects which were failures. It is easy to see how non-technical, intelligent people can get the erroneous impression that water injection is almost always successful. If one reviews the mass of articles in trade publications on pressure maintenance and notes that they are almost always justified by large increases in recovery, one can also get the impression that water injection should start as soon as possible.

The experience and knowledge we gained while planning, operating and analyzing the results of over a hundred water injection projects proves that the above two impressions are not always correct.

Predicting the optimum time to start water injection is a very complex problem because it depends on many factors. To simplify this paper, we assume the operator wishes to recover the maximum ultimate oil regardless of time. Also, in this paper, we are ignoring other methods of increasing recovery such as gas injection, insitu combustion, etc.

The total ultimate recovery can be divided into recovery before water injection plus recovery after water injection. Often we can do little as engineers to affect total primary recovery other than to use the reservoir energy which is available as efficiently as possible. However we can, as engineers, decide when and in what manner water injection should be started to gain the optimum ultimate recovery. In practice, with a good reservoir where such factors as permeability distribution, oil viscosity, water saturation, etc. are favorable, a good rule of thumb is "water injection should be started at the time the reservoir reaches the bubble point".

When studying the problem of when water injection should be started in a specific reservoir, we find that many factors are involved so the optimum time for water injection into a particular reservoir may range from immediately to never.

Some of the factors affecting optimum time for water injection are discussed below. They are all inter-related, however, for simplicity we will attempt to discuss the factors as though they are independent variables not in any way related. Each factor can to some degree be evaluated to determine the optimum time to start water injection.

Recovery Mechanism:

The forces which enable us to recover hydrocarbons from reservoirs to a very large extent govern the adaptability of the reservoir to water injection.

One would usually expect a reservoir with a strong natural water drive not to be subjected to water injection unless there were some very unusual circumstances such as tremendous reservoir size or to increase the rate of production.

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