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Controlled gradient drilling technology was put forward to solve the problem of narrow pressure margins in deep water drilling. In this paper, an optimization model for optimal drilling parameters design is established for controlled gradient drilling. The established optimization model aims at achieving the longest single drilling depth, while reducing pressure difference between wellbore pressure and formation pore pressure as much as possible. It is beneficial to increase the ROP and reduce formation damage. The SQP (sequential quadratic programming) algorithm is presented to solve the optimization model. Simulation results indicate that longer single drilling depth and better drilling parameters (control variables) can be obtained by use of optimization model when the range of control variables widens. Furthermore, in case study, small bottom-hole pressure difference can be obtained by use of optimization model. In addition, the established optimization model ensured the dynamic wellbore pressure within narrow pressure margins during optimization process. It is conducive to safe and efficient drilling.

Transient fluctuation pressure is extremely harmful during the drilling process. Fluctuation pressure can cause problems such as kicks or blowouts and lost circulation. Mastering and calculating fluctuation pressure is of great significance for reducing drilling accidents, increasing drilling speed and reducing drilling costs. In this paper, a mathematical model of downhole transient fluctuation pressure is established for the deepwater dual gradient drilling process. The mathematical model is solved by the characteristic line method and the finite difference method. The effects of drilling fluid performance (density, consistency coefficient, fluidity index), drilling speed and drilling acceleration on transient fluctuation pressure were analyzed. The results show that the transient fluctuation pressure increases with the increase of drilling fluid density, and the greater the penetration speed, the more obvious the increase. The consistency coefficient K and the fluidity index n have the same effect on the transient fluctuation pressure. The performance is as follows: the maximum transient fluctuation pressure increases with the increase of K(n), and when the acceleration changes greatly, the amplitude of the fluctuation decreases with the increase of K(n) value. The greater the drilling speed or the drilling acceleration, the greater the transient fluctuation pressure. The calculation results have a certain reference function for better control of the drilling speed and drilling fluid performance at the drilling site.