Water standing in a well will frequently prevent oil from enter ing, even though oil bearing formations have been penetrated and are actually standing exposed in the well. In some localities, such as most of the California fields, the possibility of a stratum producing oil cannot be definitely determined until water is entirely excluded from the well and a bailing or pumping test conducted. These precautionary tests doubtless will prove use ful in many localities where they have not been used.
Hard or compact formations frequently retain oil until after the well has been shot, and in some cases the wells must be swabbed in order to start a profitable flow of oil.
Where rotary tools are used in drilling a well the presence of oil will be indicated on the surface of the water or mud which returns from the bottom of the well outside the drill stem. Heavy mud may obscure a very good showing of oil so that the oil bearing formation may be passed through without recognition. Many wells have passed through oil sands without their being noted, and their true nature has subsequently been disclosed during repair or redrilling work. Some wells have finally flowed oil at the rate of several thousand barrels per day although the original drilling developed an apparently "dry hole." The utmost vigilance therefore is necessary when rotary tools are used. Upon the appearance of the slightest showing of oil the downward progress of the tools should be stopped and the cir culating mud made as thin as possible. Furthermore, clean water and mud should be introduced into the tools. Even with these precautions it may, in doubtful cases, be necessary to withdraw the tools, set a string of casing and test the well by bailing. This last step is, of course, an extreme measure because in some formations the casing can not be again removed, and the diameter of the hole is permanently reduced so that further drill ing may be difficult if not impossible.
Under such adverse conditions as have just been cited, it is absolutely necessary to have a definite knowledge of underground conditions as shown by correct logs of neighboring wells.
Gas Encountered in Drilling.—Gas, like water and oil, is frequently a characteristic of certain strata and should be care fully noted during drilling so that the stratigraphic relations between wells may be accurately compared or correlated. Furthermore, the development of gas itself will frequently be profitable.
Gas usually manifests its presence, where cable tools are used, either by blowing the water out of the hole or rising through the water in unmistakable manner. A low pressure of gas sometimes makes its presence manifest only when the well is bailed nearly dry.
Every evidence of the presence of gas should be fully recorded in the daily drilling reports.
With rotary tools the presence of gas is manifested by bubbles in the circulating mud as it comes from the well. However, it is readily obscured by heavy mud, and may be passed through without recognition. All the precautions taken to observe traces of oil must also be applied if gas is to be discovered. Well authenticated instances are known where gas under considerable pressure has been drilled through without recognition, notably in the case of the Elk Hills field of California' with a pressure of more than 400 lbs. per square inch, and a volume exceeding thirty million cubic feet per day.
Permanent Record of Drilling.—When all the necessary care above outlined has been taken, and the advised precautions observed, the well-owner will possess a fairly complete record or inventory of the condition of his property.
The daily drilling reports, when filed in regular order, will serve as a guide to intelligent repair or maintenance of the well and will also furnish information as to how neighboring wells can be properly and most economically drilled. The daily reports will be voluminous, and should therefore be condensed for the purpose of ready reference and easy comprehension. This end can be best achieved by keeping a summarized record-book in conjunction with the daily reports.
In addition to this record-book, a complete log should be written, with duplicate copies sufficient in number to supply each individual or department concerned. The written log should be revised whenever any additional work is performed at the well.
The details of a form for a written log may of necessity vary slightly with local conditions. The following form has been found adequate for some ten thousand wells and may therefore serve as a suggestion for others: For convenience of filing, all the forms should be on letter size paper (11 X inches), with the exception of Form 1, which may be double length, and by folding will serve as a cover or container for the others. The paper should be thin enough to permit making several carbon copies.