Gas Overflow Model and Analysis in a Fractured Formation

At present, the wellbore overflow in a fractured gas reservoir is mainly characterized by qualitative methods. The development of quantitative methods would need a thorough theoretical study of the correlation between the overflow phenomenon and the complex drilling characteristics of the formation. In this paper, the authors have established a mathematical model of the deep-wellbore multiphase flow considering the parameters of wellhead pressure, annular equivalent diameter, wellhead gas flow, and drilling fluid displacement. The model has been solved by the Runge-Kutta method. The results show that the increase in the surface overflow increment correlates with the increase in the well depth. With increase in the surface overflow, the wellbore gas volume increases. When the local surface overflow is Qg = 0.2 m3 and the well bottom depth is 6000 m, the overflow volume at the bottom of the well increases from 0.000505 to 0.000649 m3. When the ground overflow increment is doubled, the gas volume at the depth of 6000 m increases by 34.72%.