Water Intrusion and Methods of Prevention in California Oil Fields

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In order that the conditions which obtain in an oil well may be readilyunderstood, a brief description of a typical California well and a number ofthe phenomena accompanying same will be given. That the necessity for theexclusion of water be realized, the effects of water intrusion, a descriptionof methods of prevention, and a discussion of causes of failure will bepresented, followed by a summary and recommendations. Fig. 1 shows a section of a typical well which illustrates the relation of oilsands to overlying strata. It indicates a deep water sand separated from theoil sand by shale, and the proper place for the final shutting off of water. Inthis well a comparatively thick stratum of compact shale protected the oil fromoverlying water so that the operation of landing the water string was a simplematter. In some wells the protecting stratum maybe only from 3 to 5 ft. thickand very careful operation is necessary in order to land the water string ofcasing properly and not puncture through to the oil sand. The cases just citedare common, but occasionally, after excluding overlying water, productive oilsands are drilled through in the expectation of finding more productiveunderlying sand, and a water-bearing stratum is tapped. In this case theproblem is the reverse of the normal; water must be prevented from ascendingrather than descending. A discussion of well-drilling methods is not within the scope of this paper, except such operations as pertain to shutting off of water. A brief outline ofthe several conditions which may accompany drilling follows. Some or all may bepresent in any particular case. The direct relation of these to prevention ofintrusion will be discussed in detail further on. In drilling, the hole becomes more or less irregular in shape, and rough, andas the casing follows the bit into the well it does not fit snugly against thewalls. As a result, water may descend between the casing and walls of the well.If a circulation pump is not used during drilling, the hole is simply punchedthrough and the pulverized material removed from the well by means of a bailer.If circulation is maintained there is a continual flow of water down inside thecasing and up around the outside between the pipe and the walls of hole. Thiskeeps an open space around the pipe. AIME 048–49