The wellbore stability during well test has always been a primary concern, especially the determination of the test pressure difference which links drilling with production processes. If the pressure difference is too low, the reservoir capacity will be difficult to release. Conversely, excessive pressure difference will bring the risk of borehole collapse and sand production. Therefore, designing a critical differential pressure that maintains the wellbore stability becomes an important part during well test. In this paper, a series of indoor mechanics experiments were carried out for tight sandy conglomerate formation in the Bohai Oilfield, China. The results show that tight sandy conglomerate is highly heterogeneous. The elastic modulus of rock ranges from 6 GPa to 13 GPa, and the uniaxial compressive strength ranges from 16 MPa to 48 MPa. A wellbore-formation plane strain finite element model is established to simulate the expansion of the near-wellbore plastic zone during well test, and a criterion for determining critical pressure difference is proposed. The effects of different pore pressure levels, drilling directions, and rock mechanical properties on the extent of plastic zone expansion are explored. The simulation result show that with the test pressure difference increasing, the area of plastic zone in the near-wellbore region increased exponentially. The higher the pore pressure, the lower the critical test pressure difference. When drilling horizontal wells, the azimuth should be as close as possible to the direction of the minimum horizontal in-situ stress, which can guarantee that the critical test pressure difference is as large as possible. The pressure difference for weak formation is only 60% of that for the high-strength formation. If the pressure difference is set too high, the plastic zone in the near-wellbore formation will rapidly expand and communicate, and eventually a V-shaped damage zone will be formed around the wellbore. Finally, the simulation result has been applied to analyze the critical test pressure difference in Bohai oilfield of China. The prediction result is consistent with the actual data on field, which proves the reliability of this method.

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