**Study on Mechanisms of Borehole Instability in Naturally Fractured Reservoir During Production Test for Horizontal Wells**

**Yan, J.; Zili, Q.*; Mian, C.; Fuxiang, Z.; Yunhu, L**

**Abstract**

During the production test for horizontal wells, down-hole pressure will drop down significantly to induce the crude oil flowing out of the wellbore, which always leads to wellbore collapse problems, especially in naturally fractured reservoirs. Hence, some special researches on this problem were carried out. Based on the mechanisms of wellbore stability and failure criterion of weak plane, the authors established the stress state around the horizontal wellbore and mathematical model of minimum down-hole pressures required to maintain the wellbore stable. This model is complex nonlinear equations, which involves several variables, including natural fracture occurrences, wellbore orientation, in situ stress, and rock strength. The combination impacts of these variables on the minimum down-hole pressure were quantified by solving this nonlinear model. The study helps deepen the understanding of the mechanisms of wellbore stability in naturally fractured reservoir.