Where rotary tools are used the samples are usually obtained as the drill cuttings appear at the surface with the drilling water which rises alongside the tools. After the drill enters a stratum some time will elapse, varying from 15 to 40 minutes or even longer, before the cuttings will appear at the surface. The sample will therefore be thoroughly mixed and washed. A skilled driller can frequently recognize a change in underground formation by the behavior of the tools. When a change in formation is noted by the driller, it is sometimes advisable merely to rotate the drill, without gaining depth, until cuttings of the new formation appear at the surface.
The time required for rotary cuttings to reach the surface will vary with the size and depth of the hole and the volume of water furnished by the pump. An approximate determination of the time can be made by a test which consists of adding some dis tinctive coloring matter, such as brick dust, to the drilling water, and noting the time elapsed before its reappearance, due con sideration being taken of the pump speed and the inside diameter of the drill stem.
When the rotary-bit is withdrawn from the well a sample of formation will sometimes be found upon it. However, such an occurrence is exceptional, and as considerable time is required to withdraw and replace the bit, little practicable dependence can be placed upon such a method.
Sampling devices are sometimes attached to rotary tools for the purpose of gathering a sample at the bottom of the hole. This procedure can be adopted for special samples of particular importance, but it has not yet been developed or perfected so as to gather samples economically of all formations penetrated. A distinct field of usefulness for new and Improved sampling devices exists.
Experimental drilling with core drills indicates that the process may be developed to usefulness.
Where either rotary or cable tools are used, it is advisable to save small samples of cuttings and place them in separate bottles or boxes which must be accurately labeled. A complete collec tion of such samples may be readily examined and compared with others so that identification is more likely to be correct.
If considerable drilling work is being carried on, one man should be detailed to visit regularly the various wells for the purpose of collecting the samples and classifying them. Such a classification by one man will tend to uniformity in the names which will be applied. If the person who collects the samples
has had experience and training in geological work, the value of the classification of samples would be enhanced. If each driller merely notes his individual observations, using various names for formations, the records will vary so widely that geological comparison may be difficult or impossible.
Recent work by J. A. Udden and other scientists has proved that microscopic examination of drill cuttings is economically useful in many cases.
After several wells have been drilled and the logs carefully compared, some striking and unmistakable strata usually develop which may thereafter be carefully watched for as markers, and the remainder of the log may be given only incidental attention.
Water Encountered during Drilling.—The characteristics of water encountered during the process of drilling an oil well should be carefully noted. An important reason for such care is that the water must be excluded from the oil bearing formations in order to secure the maximum production and profit. Further more, the water originally found in certain strata is frequently different from that found in overlying or underlying strata and therefore helps to identify the strata.
Some of the more important characteristics of water to be noted are volume, head, purity and temperature.
The volume of water encountered will usually be measured by bailing, when cable tools are used. Sometimes the water will flow out of the top of the well in which case it is possible to meas ure it by tank, meter or weir, regardless of the kind of drilling tools in use.
The head of water, or the level to which it will rise if undis turbed, is an important fact to be determined because it affects the volume or flow in some cases, and, furthermore, it sometimes indicates the source of the water. It must be remembered that the level to which water will rise in a well may vary if neighbor hood conditions are changed so as to either raise or lower the general fluid level. The accompanying sketch (Fig. 14) by M. J. Kirwan' shows such a change in fluid level at a group of wells. The level shown by line "B" prevailed about a year after that shown by line "A." The highest point in the more recent line was caused by the condition of a single well into which water found entrance, which raised the fluid in some of the nearby wells.