Apparatus and methods 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 positioned 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 the
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 valve applied to preferably equalize pressure above and below the valve and the drill string run into the hole for further drilling.