Abstract:

The directional well trajectory is an important approach to efficiently produce ultra-deep fractured oil and gas reservoirs by drilling through effective natural fracture development zone. However, the complex downhole accidents, such as overflow, collapse and leakage, occur frequently during drilling process due to the high pressure, high temperature and high in-situ stress of ultra-deep gas reservoir. In addition, the combination of fracture development and stress concentration leads to the extremely high anisotropy of the target formation, as a result, various drilling risks exist in borehole with different azimuths of well trajectory. In this paper, Bozi A gas reservoir in Kelasu structural belt of Kuqa Depression is studied as an example. By considering both safe drilling and intersecting with fracture zone of the directional well as far as possible, a 3D geomechanical model for the target reservoir has been established in the study area, so as to analyze the dominant orientation and occurrence of active fractures, evaluate strong in-situ stress and fracture weak plane, determine the mud density window of safe drilling for preventing downhole collapse and leakage, and provide an optimized well trajectory scheme for pre-drilling design of directional well. The results indicate that: (1) The fracture effectiveness is better when the angle between natural fracture strike and maximum horizontal principal stress is low and the ratio of shear stress to normal stress on fracture plane is high; (2) The fluid conductivity is the largest when borehole is perpendicular to the effective fracture plane; (3) When reservoir is in strike slip stress field, borehole is generally stable within a certain fan-shaped area along the maximum horizontal principal stress, and the larger the deviation angle is, the safer the drilling operation is; (4) When natural fractures are developed, the borehole stability decreases as a whole. While the borehole is relatively stable when it is perpendicular to the effective fracture plane; (5) The development position and occurrence of effective fractures can be determined based on shear deformation capability of fractures. Finally, the optimized wellbore trajectory can be quantitatively predicted by considering both wellbore stability and leakage pressure in fractured reservoir. The practice shows that the prediction is consistent with drilling results, which proves the reliability of the optimized borehole trajectory in penetrating high-quality fractured reservoir and safe drilling, providing geomechanical foundation for the development of highly deviated wells in ultra-deep fractured reservoir.

Key words:

 , ultra-deep fractured reservoir, geomechanics, borehole trajectory, quantitative optimization, Kelasu structural belt