THE writer gives in a condensed form his principles concerning the locations recommended for wells with respect to the structure. These principles are based on the author's conception of the effects of the various sources of energy available in an oilfield. The phenomena which cause production are mainly "depletion,". "general motion" and drainage by "the direct action of gravity on the oil." Invasion by oil of that portion of the structure which was originally occupied by gas has to be avoided as much as possible because only part of the oil which enters the gas cap can be recovered. For this reason care should be taken not to tap gas from the gas cap in the first stage of the producing life of a field. When, however, pressure has dropped in the field on account of production, conservation of the gas in the gas cap need not be made a hard-and-fast rule. Parts of the field which have been drained by depletion and by the direct action of gravity have yielded only a small percentage of their oil and more oil can still be recovered if water is admitted afterwards, and preferably slowly. The general motion causes the gas cap to expand and the edge-water to encroach. Their edges advance the more regularly the more evenly the wells are distributed around the crest, as they are farther away from these edges, and produced more gently, and as their number is greater. The old practice of stepping out in a zigzag line on one side of a structure should be abandoned. The most favourable method of developing an oilfield would be to drill one or two rows of wells on one or two contours in the oil, so that they should not be invaded too soon by edge-water or by gas from the gas cap. Considered from a merely theoretical standpoint such wells should be drilled very closely and opened all at the same time, but for various reasons this is not commercially feasible. The structure should, therefore, be encircled with wells as soon as possible, so that it is better to space them widely in order to establish drainage within a short time on all sides with the available drilling strings. If necessary, intermediate wells on the same contours can be drilled later on. As more oil can be set' free from the sand when it is invaded by water than when it is depleted or invaded by gas, it may, in many cases, be tried whether it pays to recover the oil which remains in the sand up-dip of the original wells, after these wells have gone to water, i.e. by drilling crestal stripping wells. There are fields in which the edge-water encroaches so slowly as to be negligible, and such fields could be drained by wells on the periphery.. When the limit of economic production of these wells is reached, artificial flooding from the periphery and production through crestal wells may be profitable. All measures to ensure regular transgression of the edges of the edge-water

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