The Jurassic fractured sandstone gas reservoir has low permeability and strong heterogeneity in northern Kuqa depression in Tarim Basin. Therefore, the use of highly deviated well will be an effective way to improve efficiency of natural gas development. But the drilling of highly deviated well was faced challenges associated with complex geologic conditions: ① How to optimize the trajectory of highly deviated well to penetrate permeable fractures? ②How ensure the wellbore stability during drilling? ③ How to optimize fracturing stimulation program after drilling?
To demonstrate the feasibility of highly deviated well and optimize its drilling program, an integrate research combined geology, geomechanics and petroleum engineering was conducted. The stress regime, magnitude and orientation were defined by using borehole information, including log data, lab tests, leak off and drilling experience. The distribution characteristics of in-situ stress were described by structural architecture, 3D seismic data and numerical simulation. By analyzing the relation-ship between in-situ stress and development of natural fractures, the key geological factors controlling reservoir quality were further defined. Based on the estimation of stress state acting on natural fractures, the orientation of effective fractures in gas reservoir space was predicted. The wellbore stability was analyzed by considering stress variation, borehole deviation and in-clination orientation. Finally, the reservoir fracability was evaluated by incorporating stress, shear-to-normal stress ratio of fractures, brittleness and fracture toughness.
It is shown that in-situ stress not only affects density and orientation of natural fractures, but also indirectly affects the quality of reservoir by controlling the mechanical behavior of natural fractures. Therefore, the geomechanical model is a useful supplementary geological attribute in gas well planning and wellbore trajectory optimization, and then it is also an important basis for optimization of drilling program and design of fracturing stimulation. Based on this concept, the necessity and feasibility of highly deviated well drilling were proved in Jurassic fractured sandstone reservoir of northern Kuqa depression. Then, two conditions of low in-situ stress magnitude and weak horizontal stress anisotropy were used as supplementary basis for gas well placement optimization. Finally, the wellbore trajectory of highly deviated well was optimized by combining borehole drilling across permeable fractures, wellbore stability and fracturing stimulation post-drilling. The optimized well site and wellbore trajectory effectively minimized cost and risk, while maximizing the well performance.
The first highly deviated well was successfully drilled under complex geologic conditions in Kuqa depression. Its wellbore trajectory was relatively stable and was conducive to fracturing. Theoretical calculations and actual drilling experience were consistent, which indicated that it is feasible to develop natural gas using highly deviated well, and it also proves that the geomechanical approach is effective in optimizing well location and wellbore trajectory in Kuqa depression.