Optimum drilling fluid displacement design of large displacement horizontal well considering extension limit

Drilling fluid displacement has the direct effect on the extension capacity of extended-reach horizontal wells.To maximize the extension limit of extended-reach horizontal wells, the displacement optimization design principle was put forward from the aspect of drilling fluid displacement, and the design method of optimal drilling fluid displacement was proposed.The displacement range of drilling fluid was determined by taking into consideration the demand of hole cleaning.According to open-hole extension limit principle and hydraulic extension limit principle of extended reach well comprehensively, the optimal displacement of drilling fluid was determined with the maximization of horizontal section extension limit as the ultimate target.This method was used to analyze one extended-reach horizontal well.It is shown that the allowed displacement is in the range of 26.2-42.9 L/s, and optimal displacement is 31.8 L/s, corresponding to the maximum horizontal section extension limit of 6 251 m.Based on further studies, several understandings were obtained.First, for any displacement, the minimum extension limit of two horizontal sections which are obtained based on the above mentioned two principles can be taken as the horizontal section extension limit of extended-reach horizontal well.Second, the value in the occasion the extension limit of both horizontal sections is equal is the maximum extension limit of horizontal sections, and its corresponding displacement is the optimal displacement of drilling fluid.And third, at the optimal displacement, the extension limitof extended-reach horizontal well can be maximized, the demand of hole cleaning can be satisfied and the subsurface pressure is kept below the maximum bearing capacity of wellbore stability and drill pump.