ASSEMBLING OF INFORMATION RELATIVE TO UNDER GROUND CONDITONS Underground conditions affecting the flow of oil into wells can only be conjectured after a careful consideration of the facts dis closed above ground. In the preceding chapter were outlined some of the more important facts which should be observed, and the methods to be followed in their collection were indicated. It will be noted that the information deemed necessary to a compre hensive understanding of this subject is composed of a great mass of apparently insignificant facts.
The information collected, step by step, as oil wells are drilled, soon becomes so voluminous that it is incomprehensible unless properly systematized. The method of systematizing the facts, as in all branches of scientific work, must be such that it will lead to the disclosure of their causes. This chapter presents methods which have proved useful in digesting and interpreting all avail able information.
Maps.—The first step which must be taken in the study of a tract of oil land is the preparation of a complete and accurate map. The elementary training of an engineer deals with the subject of surveying and mapping, and it is, therefore, unnecess ary to here recount the details of such work.
However, the most necessary features of an oil field map deserve some notice because many carefully prepared maps fail to meet entirely the needs of the work at hand.
The scale of a detailed working-map should ordinarily not be smaller than 500 ft. to the inch. The largest scale required will seldom exceed 100 ft. to the inch. Maps compiled to cover large tracts of land containing many wells should not ordinarily be drawn to a smaller scale than 2000 ft. to the inch.
The position of all wells, irrespective of depth, degree of com pletion or operating condition, should be shown on the map. The number or name by which each well is known should also be shown, as well as the properly lines and ownerships, so far as they help to describe or identify wells. The elevation of each well at the derrick floor, should also be shown. Elevations should be as accurate as it is possible to measure the depths of wells; that is, within less than one foot. If sites for additional wells have been
definitely chosen they should also be indicated, together with the ground elevation. Both the location and elevation of well sites shown on the map must be verified after the rig is built.
Topography shown by contours will frequently be useful, but is not necessary in all cases.
Symbols should be chosen to show the degree of completion of wells. Considerable latitude exists in the choice of symbols. In some localities certain symbols have come into common use and, unless they are entirely inappropriate, the local usage should be followed. It will be found most convenient if symbols are chosen so as to show the progress of drilling by additional mark ings, thus making erasures unnecessary. The symbols shown in (Fig. 18) have been found practicable in extensive work, and may serve as suggestions.
The degree of accuracy required in oil field maps, or rather the permissible percentage of error, is governed by the uses to which the map is to be put. For instance, when a map is to serve ts a basis for making cross sections or peg models, the distances between wells may frequently be determined by measurement on the map. If the cross section or peg model is made on a scale larger than the original map, serious errors will be introduced by magnifying the original errors.
Graphic Logs of Wells.—After all the available information is recorded in a complete and carefully written log, it usually fails to present a clear and comprehensible mental picture of actual conditions, while a detailed sketch or graphic log, accurately drawn to scale, makes the information fully comprehensible. To obtain the greatest benefit from the use of graphic logs a number of details must be given careful consideration. In the previous chapter were stated numerous instances of how the mechanical or drilling conditions of a well affect the recorded observations of geological conditions. Therefore a sketch designed to present all available information must show the mechanical features as well as the geological record.