Abstract: When a high pressure zone is encountered while drilling a fractured formation,a serious well kick will occur as overpressured formation fluids flow into the borehole.Most of the existing overflow mechanism models are based on the seepage theory of permeable formations,but they are snot applicable to fractured formations.Therefore,it is necessary to study the process of gas flowing into wellbore from the fractures under normal conditions.Based on the Hertz elastic contact theory and the binomial seepage theory,both the fracture microscopic closure model and the fracture gas flow model were established.The two models were coupled by taking the fracture pressure as the node.Then a single fracture underbalanced overflow intrusion model was established simultaneously by considering the fracture closure conditions.The finite difference method was used to find the numerical solution of the model.Then influences of fracture geometry(opening,radius)and pressure difference on the overflow laws were obtained under the conditions with/without considering the closure conditions.The simulation results showed that the fracture pressure drop clearly slowed down compared with the model without considering fracture closure,and in this case,the overflow duration lasted longer.So,the overflow rate increased,and the total volume of overflow was greater.The research results indicated that the overflow in fractured formations was more severe than that in conventional permeable formations.