Drilling fractured formations by overbalanced method causes lost circulation of drilling fluid in fractures. The lost circulation in drilling fractured zones has been traditionally accepted due to inability to control its occurrence. Most of drilled wells in fractured oil zones after completion are conducted on acid job in order to obtain well flowing. If the radius of skin due to lost circulation of drilling fluid through fractured oil zones be determined, the volume of acid needed to conduct any acid job can be calculated. The main goal of this work is to obtain a mathematical model for determination of skin radius in naturally fractured oil reservoir wells.

In this study, one vertical fracture is assumed to symmetrically intersect a wellbore. During a lost circulation drilling fluid enters the fracture and displaces formation fluid. The mechanism of lost circulation through the modeled porous medium is formulated using mass transfer phenomena. Obtained equations give concentration of the lost fluid with respect to time and position. Filtration of the drilling fluid at walls of the fracture is considered to be exponential which is ceased after mud cake is formed. A simulator is developed using derived equations in this work. The concentration of the drilling fluid could now be plotted against the position in the fracture and also against the position in lateral direction for each point in the fracture at a given time. These curves tend to be S-shaped.

The position in the fracture at which the curve of concentration reaches zero could be considered to represent skin radius caused by drilling operation. Similarly the position in lateral direction at which the concentration reaches zero represents lateral penetration of the drilling fluid in the matrix from its corresponding position in the fracture.

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