1. Field of the Invention
This invention relates to the drilling of an oil or gas well with a subsurface positive displacement motor that is driven by drilling fluid under pressure. The positive displacement motor is connected to a drill bit at one end and a tubular drilling string at the other end.
2. Description of Related Art
When a positive displacement motor is used to drill an oil or gas well, it is known to position what is referred to as a dump valve between the drill string and the motor. The dump valve is in an open position when the drill string is being run in or out of the well. This establishes a flow path from the interior of the drill string to the annulus between the drill string and the casing or well bore which allows the drill string to either fill with drilling fluid as the drill string is run into the well or to empty as the drill string is run out of the well. When, however, drilling fluid is pumped down the drill string to the dump valve and motor, the dump valve is moved to a closed position by a pressure drop across a movable piston. This movement of the piston closes the flow path between the interior of the drill string and the annulus. See for example U.S. Pat. No. 4,574,894 to Jadwin.
Because the operation of the dump valve requires a pressure drop across a restriction in the flow path to keep it in the closed position, the efficiency of the positive displacement motor is reduced due to the pressure loss. Also at lower flow rates the valve may open altogether. See Hydraulic Downhole Drilling Motors: Turbodrills and Positive Displacement Rotary Motors by Wladimir Tiraspolsky (November 1985) pages 265-267, the contents which is expressly incorporated herein by reference thereto.