Abstract: On the basis of the theory of drilling fluid hydrodynamics，we analyzed the stress situation of horizontal section in the
horizontal well，and find that the pressure loss in the annulus space of the horizontal well and the total circulating pressure loss
are of the two main factors that restricted horizontal section extension ability. In this paper，pressure loss in the annulus space
of the horizontal well and the total circulating pressure loss in the whole borehole are calculated，and the horizontal hydraulic
extensions ability value was obtained，and then the drilling fluid density，debris bed height，pump pressure and drilling fluid
delivery rate and the effect on the horizontal section extension ability was analyzed. In order to enhance the horizontal hydraulic
extensions ability，we can use low mud density and the lower height of cutting bed is kept. We can draw a conclusion that we
can enhance the horizontal hydraulic extensions ability by choosing drilling pump with higher rated pump pressure.

Key words: horizontal well, hydraulic extensions ability, hydraulic parameter calculation, circulation pressure loss, cuttingbed