UNDERBALANCED DRILL STRING DEPLOYMENT VALVE METHOD AND APPARATUS

Apparatus and methods are disclosed for a deployment valve used with an underbalanced drilling system to enhance the advantages of underbalanced drilling. The underbalanced drilling system may typically comprise elements such as a rotating blow out preventer and drilling recovery system. The deployment valve is position ed in a tubular string, such as casing, at a well bore depth at or preferably substantially below the string light point of the drilling string. When the drilling string is above the string light point then the upwardly acting forces on the drilling string become greater than downwardly acting forces such that the drilling string begins to accelerate upwardly. The deployment valve has a bore sufficiently large to allow passage of the drill bit therethrough in the open position. The deployment valve may be closed when t he drill string is pulled within the casing as may be necessary to service the drill string after drilling into a reservoir having a reservoir pressure. To allow the drill string to be removed from the casing, the pressure produced by the formation can be bled off and the drill string removed for servicing. The drill string can then be reinserted, the pressure in the casing above the deployment applied to preferably equalize pressure abov e and below the valveand the drill string run into the hole for further drilling.