Keywords: 
• drilling fluid /
• leakage /
• leakage layer depth /
• leakoff pressure /
• computation model

Abstract: Leakage creates extreme damage to oil and gas drilling and well cementation. Aiming at the problem that the leakage layer information cannot be figure out through existing calculation model with the on-site data, starting from stretching back leakage process, calculation model of permeable leakage layer depth and pressure was analyzed and established by utilizing non-Newtonian fluid mechanic and seepage mechanics. Then we established functional relation between leakage depth and total amount of liquid leakage by analyzing the relation of leakage occurrence time, leakage amount of drilling fluid and bits footage; and figure out the function relationship among leakage pressure, leakage flow and thickness of leakage layer according to the permeability loss mechanism. Therefore, a new computation model which calculates the leakage depth and pressure was put forward and was applied to a particular block in some oil field to analyze the leakage problems. This method provides the basis not only for determining the leakage layer information accurately, but also for selecting and designing drilling fluid system, cement slurry and parameters of drilling and cementing reasonably.