A new model of predicting the optimal mud density for maximizing the open-hole extension capacity in offshore extended-reach horizontal well drilling

The pressure fluctuation caused by the axial movement of the string easily causes the bottomhole pressure to exceed the safe pressure window, which seriously restricts the horizontal open-hole extension capacity (OHEC) of the offshore extended-reach horizontal wells (ERHWs). In this work, in order to improve the horizontal OHEC for ERHWs, an optimal mud density model in extended-reach horizontal drilling (ERHD) was established based on the prediction and analysis of horizontal OHEC of ERHWs under string tripping operations. The results show that with the increase of mud density, the horizontal OHEC has a trend of first increasing and then decreasing in both tripping operations and casing tripping operations, and there exists an optimal mud density to maximize the horizontal OHEC. The optimal mud density of the two operations is obtained when the horizontal OHEC of the two operations is equal, and it is positively correlated with pull out of hole (POOH) speed, diameter of the wellbore and drill string, whereas it is negatively correlated with diameter of the casing and casing running speed. Controlling the tripping speed and optimizing the rheological properties are crucial to improve the horizontal OHEC. This model can provide scientific basis for optimization design of ERHD and the improvement of horizontal OHEC of ERHWs.