| 摘要 |
<p>A monostable, solenoid-operated valve in a passage which bypasses the drilling fluid pressure drop across a drill bit (34) at the lower end of a drill string (28) in a well opens and closes in response to downhole conditions to create pressure pulses in the drilling fluid. The valve (166) is urged toward a closed position by drilling fluid pumped through the drill string. A larger force is required to open the valve (166) than to hold it open. Current supplied to the solenoid (67, 70) to open the valve is thereafter reduced to a value sufficient to hold the valve open. The solenoid is de-energized to close the valve.</p> |
