Underbalanced Drilling (UBD) is becoming more widely used and accepted because of the many benefits it provides such as:Limitations to the formation damage common in regular overbalanced drilling.Increased rates of penetration.Increased production rates and in some cases production while drilling which can provide early cash flow and enhance economics. However with these advantages come some drawbacks, the chief among which is the safety of the operation while tripping the drill pipe into and out of the well. Since the well is allowed to flow in traditional UBD operations a flowing or shut in pressure results in the well.Any significant pressure at surface requires that special precautions be taken during tripping operations to control formation pressure.Several techniques can be used among which are:Killing the well, this provides for safe tripping but can obviate the very reason for using UBD - formation damage can result.Flowing the well, this can lead to a dangerous situation if a sloughing formation forms a temporary bridge in the annulus.A Snubbing unit can be employed, this enhances safety but can add considerably to the operational cost. It is clear that a better and safer method is needed, since all the above carry disadvantages, and to this end a new method involving a downhole valve was conceived.In this approach a full opening valve which can be closed below the drill string while tripping, is deployed downhole below the "pipe light" depth and operated from surface by way of a hydraulic control bundle. This paper will review the concept of the Downhole Deployment Valve (DDV), its design and operation.The authors will go on to provide details of the first field trial of the a 7" 26lbs/ft full opening valve in a multilateral well which was drilled underbalanced in the James Lime play in East Texas/Northwest Louisiana. They will conclude that this device can provide major enhancement to a UBD operation without in any way compromising its advantages. Introduction Increased awareness of the degree of formation damage caused by fluid invasion into the reservoir during conventional overbalanced drilling techniques has resulted in a growing interest in the benefits offered by UBD.These techniques, though not necessarily suitable for all reservoirs, can have a considerable positive impact in such instances as:Depleted reservoirs where in-fill wells can be drilled with little or no damage.Highly permeable and fractured reservoirs where fluid invasion can be greatly limited and consequent degradation of permeability reduced or eliminated.Hard formations where greatly increased penetration rates can be achieved. However along with the many benefits offered by UBD techniques come some disadvantages, such as higher associated costs and increased perceived risk and safety issues.Perhaps the most significant such risk is during the normal process of tripping the drill string to change the bottomhole assembly (BHA).Since the formation is allowed to flow during UBD operations a surface pressure is ever present in the annulus which is controlled by a rotating control head.Once tripping begins and the pipe is being stripped through the wellhead, this pressure must be handled in some manner before a "pipe light" situation is reached.

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