Leakage mechanism is the basis to study the formation leakage phenomenon and quantitatively analyze the characteristics of the leakage formation. The leakage phenomenon is divided into differential pressure leakage and fractured leakage in this paper. By simulating the U-tube, we analyze the leakage mechanism of different lithology. The calculation formulas of leakage volume and leakage velocity on differential pressure leakage are brought forward by making use of water injection model.

The analysis applied to the leakage of Sinkiang Baiquan-1 Well, Laoyemiao region of Eastern Hebei oilfield and TABNAK gas field of Iran. Based on the investigation, it is clearly incorrect that the leakage of Sinkiang Baiquan-1 Well is attributed to fractured leakage or rupture leakage, but should be considered as differential pressure leakage. The lost circulation in Eastern Hebei oilfield is mainly caused by the permeable leakage based on declined pressure. Because the connectivity and porous fracture development of carbonate formation in TABNAK gas field of Iran are good, the serious leakage happened in the formation above sea when there were positive pressure between the mud and the formation.

Investigation results show that in fracture formation with good permeability or in other thief formation which has fracture- cavity, the leakage type is mostly differential pressure leakage and rarely fractured leakage. So when analyzing the safe drilling density window, differential pressure leakage should be regarded as the main object to prevent.

It will be helpful to further analyze the leakage mechanism, and essential to formulate the leakage prevention and plugging program that the leakage is divided into differential pressure leakage and fractured leakage and the concept of differential pressure leakage is put forward.

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