General approach for the calculation and optimal control of the extended-reach limit in horizontal drilling based on the mud weight window

- A general calculation methods of open-hole extended-reach limit is presented.
- Drilling, Tripping in and tripping out process are considered simultaneously.
- A modified MWW is presented for horizontal ERW.
- Optimal control methods are proposed to obtain maximum horizontal-section limit.

There are two shortcomings in the previous open-hole extended-reach limit model: 1) only the drilling process is analysed, while the whole processes should also include the tripping process; 2) the optimal control methods have not been fully considered.To overcome these shortcomings, a set of calculation and optimal control methods of the open-hole extended-reach limit in horizontal drilling are presented in this paper. First, the horizontal section limit expressions of a horizontal extended-reach well (ERW) for different drilling processes are obtained. A modified mud weight window (MWW) for a horizontal ERW is determined according to those expressions, and the modified MWW can be used to solve the problem that the conventional MWW may fail to satisfy the operation window of horizontal extended-reach drilling (ERD). Second, the horizontal section limit values of the drilling process (sliding drilling mode and rotating drilling mode) and the tripping in and tripping out processes are calculated, and the minimum of these values is taken as the maximum horizontal section limit. Finally, several optimal control methods of obtaining the greater horizontal section limit are also provided from three aspects: drilling modes, drilling parameters and mud performance parameters.The results of the case study show that the modified MWW can change with the different operating parameters, rather than remain constant range. Moreover, the modified MWW is narrower than the conventional one. The maximum horizontal section limit should be simultaneously determined by the processes of drilling, tripping in and tripping out. The tripping velocity has little effect on the optimal mud density, whereas the maximum horizontal section limit decreases with the increase in tripping velocity. In addition, both the optimal mud density and the maximum horizontal section limit decrease with the increase in drill string rotation speed. Moreover, the sliding drilling mode can provide greater extension ability than rotating drilling mode. Furthermore, methods of optimizing the drilling parameters and mud performance parameters are also given in detail.