"Prussian blue is reported to have been successfully used in a number of cases, and it seems to meet the general requirements of oil field conditions. It is a cyanide of iron, and the writer is not fully advised as to the effect that sulphur compounds may have upon it. It is, how ever, probably the most reliable and available dye for present use.
"It is understood that the underground flow of water has been traced by colorless substances which the water did not contain except in com paratively small quantities. For instance one of the rare elements, such as lithium, may be added in small quantity and samples from some other point tested by spectroscopic analyses. Or an excess of a com pound, such as chlorides or sulphates, may be added and the samples tested by measuring the resistance to an electric current, or by chemical means.
Oil and Emulsion.—"The underground association of oil and water presents some interesting phenomena. Water is less viscous than crude oil and therefore finds easier passage to a well, with the effect of holding the oil back in the formations. If water and oil are intimately mixed
And agitated, an emulsion is formed. If a well produces emulsion and it be known to originate not by leak-back in the pump or by excessive working of gas in the well, the conclusion may be drawn that the water has entered the well through an oil stratum, or that the oil and water have passed a defective plug or bridge." Final Test of Well by Pumping.—The final test to determine whether water has been excluded from a well, and also to prove its productiveness, is made by pumping. However, a careful operator will not fail to apply the other tests that may be made before completion, for the reason that the pumping test covers all the possible elements of improper construction, and in the event of adverse results it may be difficult to determine the exact cause of failure.
The details of a pumping test are important, and are set forth in a succeeding chapter under the head of "gauging."