Abstract: Directional well trajectory intersecting with cross effective natural fracture is an important approach to effi cient development in ultra-deep fractured reservoirs. However, ultra-deep gas reservoir commonly appear high pressure, high temperature and high in-situ stress magnitude, which cause frequently occurred overfl ow, collapse and leakage accidents during drilling process. In addition, the combination of fracture development and in-situ stress concentration leads to extremely strong anisotropy of the target formation, and different drilling risks in different directions. In this study, with Bozi A gas reservoir in Kelasu structural belt of Kuqa depression as a case study, considering both directional well intersecting with fracture zone and safe drilling, a three-dimensional geomechanical model of the target reservoir in the study area was established, the azimuth position and occurrence characteristics of dominant fracture permeability were analyzed, and the safe drilling density window relating to collapse and leakage was evaluated by taking account of strong in-situ stress and fracture development. All above works provide an optimized well trajectory scheme for pre-drilling design of directional wells. The results indicate that: fracture effective is better when the angle is low between natural fracture strike and horizontal maximum principal stress orientation and the ratio of shear stress to normal stress on fracture surface is high;  fl uid conductivity is the largest when drilling hole is perpendicular to the effective
fracture surface; when the reservoir is under strike-slip stress pattern, drilling hole is generally stable t along a certain fan-shaped area in the direction of horizontal maximum principal stress and larger deviation angle commonly produces safer drilling; rock strength decreases,anisotropy increases and borehole stability decreases when natural fractures develop within reservoirs, but if drilling hole is perpendicular to the effective fracture surface, it is generally stable; the development position and occurrence of effective fractures can be determined based on shear deformation ability of fractures. The optimal wellbore trajectory can be quantitatively predicted when considering both wellbore stability evaluation and leakage pressure prediction. Actual practice shows that prediction results match well with the drilling process, whichproves the feasibility of considering both high-quality reservoir exploration and safe drilling, which provides geomechanical foundation for the development of highly deviated wells in ultra-deep heterogeneous fractured reservoirs

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