Patent ID: 8196678

Claim:
A method of downlinking to a downhole tool located in a borehole, wherein the downhole tool detects transitions in the flow velocity of fluid circulating in the borehole at the downhole tool, the method including the steps of: (a) pumping fluid into a drillstring to circulate fluid in the borehole at the downhole tool; (b) increasing the pumping rate of fluid into the drillstring to a rate which overshoots a steady state pumping rate needed to produce a transition which the downhole tool will detect, or decreasing the pumping rate of fluid into the drillstring to a rate which undershoots the steady state pumping rate needed to produce a transition which the downhole tool will detect, wherein prior to increasing or decreasing the pump rate said steady state pumping rate and said increased or decreased pumping rate are calculated based on a characteristic time for the circulating fluid to respond to changes in pumping rate and any one or any combination of: a compliance per unit length of the fluid circulating within the drillstring, a frictional pressure drop in the drillstring, and a ratio of the frictional pressure drop at the downhole tool, and wherein said characteristic time is determined by temporarily stopping the pumping of fluid into the drillstring; and (c) subsequently adjusting the pumping rate of fluid into the drillstring to approach or achieve said steady state pumping rate; wherein steps (b) and (c) produce a transition which is detected by the downhole tool.