Given that the conventional five-arc trajectory design of the double-step horizontal wellbore may have no feasible solution in the case of limited horizontal section spacing，this study proposed a wellbore trajectory design method based on constrained optimization. By establishing a constrained optimization model to minimize the horizontal section length，an efficient solving algorithm was constructed，which combines the quasi-analytic solution and the bisection method. Given the failure problem that the singularity polynomial of the proposed quasi-analytic solution is always zero when the inclination and azimuth angles of the two horizontal sections are the same，a modified analytical calculation formula was proposed to deal with this theoretical defect of the traditional method. Compared with the traditional iterative method，the proposed method reduces the computational complexity to the order of solving quadratic equations via the characteristic polynomial dimension reduction strategy. Examples showed that this method can converge rapidly in cases of three-dimensional stepped wells，two-dimensional wells and simplified cases. By optimizing the adjustment of the horizontal section through the bisection method，the control of the minimum trajectory length under the constraint of wellbore curvature was achieved. This method can effectively deal with the trajectory design in cases of narrow spacing，like fault-separated reservoirs，and provide theoretical support for the development of drilling software. The research findings significantly improve the reliability and engineering applicability of the trajectory design for stepped horizontal wells.

Well shut-in after fracturing is of great importance for increasing the oil recovery and production of shale oil reservoirs. The key is to determine the shut-in time. At present，there is no systematic and effective method for determining the time of shut-in either in China or across the world. The pressure transmission of the fracture system，the ion diffusion of the produced fluid and the capillary force imbibition are the key problems to be solved for addressing the above challenges. With laboratory experiments and field analysis，a comprehensive determination method for the shut-in time for Jimsar shale oil reservoirs was developed by clarifying the microscopic pore characteristics，wettability，laminae，and imbibition flooding potential of Jimsar shale oil reservoirs and analyzing the existing methods used and characteristics of shut-in time determination. It is concluded that the Jimusar shale oil reservoir provides strong imbibition and diffusion ability，and well shut-in after fracturing contributes to the displacement of oil. The shut-in time can be determined comprehensively in accordance with the required equilibrium time of the wellhead pressure，imbibition，and production fluid salinity. To determine the post-fracturing shut-in time for Jimsar shale oil reservoirs，the synergy of micro-fracture fluid charging and matrix imbibition should be taken into consideration. Specifically，with the second turning point of wellhead pressure decline taken as the lower limit and the turning point for the stable rising of salinity as the upper limit，the optimal shut-in time is determined，considering the imbibition equilibrium. The shut-in time after fracturing for Well Q was optimized based on laboratory experiments and field data. The suggested shut-in time for Well Q is about 55 days. This study provides an important reference for optimizing the shut-in time after fracturing.

Bohai Oilfield is an important support for increasing offshore oil and gas reserves and production in China. In view of the complex geological conditions， the coexistence of multiple pressure systems， poor wellbore stability， highly risky drilling operation， and the great difficulty in comprehensively improving inefficient wells in Bohai Oilfield， an analysis and study of the system of innovative technologies to improve drilling speed and efficiency are conducted with safe drilling operation as the criterion. These technologies have been applied successfully in field， which include the wellbore stability study-based extreme extending techology for ERW， the technology of twin holes in monobore to use the slot more efficiently， the well structure optimization technology to minimize casing stages and sizes， the dynamic comprehensive analysis and optimization technology for coated tubulars， and the decision-tree model-based refined sidetracking technology. The future development trend of reducing cost and increasing efficiency for drilling in Bohai Oilfield is also looked into in order to provide technical reference for the efficient development of Bohai Oilfield. 

At present，extended-reach well（ERW）drilling has become an important means for oil and gas exploration and development，and trajectory design optimization is the key to the successful drilling of ERW and the improvement of operating efficiency. With the continuous increase in horizontal displacement and horizontal length to vertical depth ratio，ERW drilling is facing many serious challenges. Therefore，it is necessary to comprehensively analyze and evaluate the factors affecting trajectory design，such as trajectory control ability and wellbore stability，so as to form an integrated technical thinking. In this study，a matrix of trajectory evaluation indicators was first established by analyzing the characteristics of ERW drilling technique，and then scores and weights of all indicators were determined based on expert experience. Consequently，quantitative evaluation was used to judge the selection of ERW trajectory，and applied to the ERW design in an oilfield. Drilling operations were carried out in the field strictly following the selected trajectory plan，and finally a high-quality trajectory was achieved for the whole well. The results show that the evaluation method is practicable and feasible，and provides a technical basis and guiding method for selecting the design plan of ERW trajectory.