The specific gravity test is simple, being made by a special hydrometer on the Baume scale. The percentage of water mixed with oil is also readily determined by a "centrifuge."' The specific gravity of a mixture of oil and water depends upon their relative amounts, and also upon the specific gravity of the oil. Where either of the two factors is known the other is readily computed, or read directly from a diagram such as shown in Figure 25.
The foregoing diagram is useful in the study and comparison of production reports dealing with varying grades of oil.
Use of Production Reports.—To be useful, production reports must be so designed that a study of them will lead to the cause of conditions which are either profitable or unprofitable. A concrete example of how production reports may be used to discover the source of water in a group of wells is afforded in the following description by M. J. Kirwan.' "All of the wells were drilled several years before the period covered by these figures. Some of the dates and figures have been slightly changed from those shown in the original reports, so that it is possible to show the use of records with a limited number of wells and figures. There are, of course, many factors which influence the production of wells which do not show on a production report, such as their physical condition, and these factors must be taken into consideration when dealing with any given problem.
"The figures appearing on the regular monthly production reports show the total amounts of oil and water produced and the number of days the wells pumped, as follows: Methods for the Determination of Water in Petroleum and Its Products. I. C. ALLEN and W. A. JACOBS. Technical Paper No. 25, U. S. Bureau of Mines, 1912.
Second Annual Report of State Oil and Gas Supervisor, California State Mining Bureau, Bulletin 82, pp. 27-30, 1918.
A careful study of the above figures would reveal many striking features, but in order to most clearly bring out their meaning diagrams are necessary. The diagrams here presented (Figs. 26 and 27) are almost self-explanatory, and show that well 10 was the first in the group to produce water, which quickly increased in amount accompanied by an equally rapid decrease in the amount of oil. It will be noted that the other wells in the group later followed a similar course. This fact indicates that well 10 is the source of the water trouble.
Diagrams based only on the water precentage, such as Fig. 27, can be more easily made and sometimes are sufficient to point out the origin of the trouble. However, such a diagram might be misleading for the reason that a well reporting a high percentage of water, but making only a small amount of oil, would be given equal prominence on the diagram with a more productive well.
It will be readily seen that figures giving percentage mean but little unless accompanied by another figure which shows the true volume of water. The percentage of water in the wells under con sideration is shown as follows in a diagram (Fig. 27).
The accompanying drawing, shown in Fig. 28, illustrates a form which has proved very useful. The record here shown is that of well 3B referred to in the two preceding figures.
The specially ruled cross-section form is printed on ordinary letter sized paper which simplifies filing.
The record can be quickly made with colored pencil or pen, and the process is so simple that no particular skill is required. An ordinary clerk can do the work.
In the particular instance illustrated the record so clearly showed the relation between the amounts of oil and of water that no doubt was left as to the cause of the decline in oil production.
Maps Showing Productiveness of Wells.—The productiveness of wells in various portions of a field will sometimes change on account of conditions which may be purely local. Water may be flooding a portion of a field, gas pressure may be exhausted, or some other local causes may affect the productiveness of the wells.
Intelligent control of such varying conditions demands that maps be periodically prepared so as to show the conditions at each well. The period of time elapsing between revisions of such maps will, of course, depend entirely upon the problem involved. Some work may require a new map each week, while in other cases annual maps may be sufficient.
Blue line prints of the regular field maps may serve as a basis for such work, and three or four symbols can be chosen so that no great time or skill is required in posting a map. A circle, drawn with a colored pencil, around a well may be used to indicate daily production of oil, gas or water, between certain limits. Two maps, one for oil and another for water, will clearly show the nature and extent of the damage to productive formations. The manner of enlargement of a flooded area will frequently indi cate whether the damage is radiating from some improperly drilled well, or is gradually encroaching along the edge of an entire pool. The first case is susceptible of complete cure, while the second may be so controlled that the maximum amount of oil may be obtained. Examples of such maps will be found in the following chapter.