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A well can flow exclusively through the casing or exclusively through atubing but can also flow partly through a casing and at the top part through atubing. The main principles of the flowing of wells under these conditions wereexplained in the author's former paper. 1 Practically all factors thatinfluence the regular flow are dealt with therein. The question of irregularflow, however, will be discussed briefly here. As a rule, an irregular flowmust be avoided, as it may be assumed that the efficiency is thereby verysmall, while on many fields it is injurious to the sand of the oil-bearinglayer. Periodical flowing often occurs with rising mixtures of gas and liquid. The phenomenon can be explained if it is accepted that two conditions arepossible in the mixture; namely, the foam condition and the mistcondition. If there is no stabilizer in the liquid, the foam condition can exist only whenthe liquid forms a greater portion of the volume than the gas. The mistcondition, on the other hand, can exist only when the liquid occupies a smallerpart of the volume than the gas. The foam condition occurs when there are gasbubbles in the liquid mass, while in the mist condition drops of liquid aredisseminated in a gas-filled space. For sake of simplicity, it has been assumed that the mist condition prevailswhen 50 per cent. or more of the volume is occupied by gas, while with lessthan 50 per cent. gas the foam condition will arise. For distilled water andfor oils which do not tend to form stable foams this is probably approximatelytrue. If this does not hold good, however, two cases are possible:Thefields of the two conditions overlap orthey do not meet. If the fields of the conditions overlap there is an intermediate field whereeither of the two conditions can exist, depending on the history of themixture.

Some Principles Governing the Choice of Length and Diameter of Tubing in Oil Wells