Kick Mechanisms and Unique Well Control Practices in Deepwater Vugular Carbonates
Authors: Dupriest Fred E. 
Published: May 2016

The standard well control training drillers receive in the industry prepares them to respond to an influx that occurs during underbalanced conditions. The mechanism through which hydrocarbon may enter the wellbore following a vugular loss can differ from this. One potential result is that the influx may not be detected as early as that which occurs during a conventional underbalanced condition. The purpose of this paper is to provide a model to explain the unique mechanism through which kicks may occur following vugular losses, as well as effective recognition and response practices that are consistent with that model.

When a loss occurs in a highly vugular zone the vugs offer no resistance to flow and the bottomhole pressure (BHP) will immediately fall to equal the pore pressure within the vug. The general belief that the BHP will not drop if drill weight mud is pumped continuously down the annulus is incorrect. Following the loss, the BHP then controls the pressure at any point in the mud column above the loss zone. The pressure at any point above will be approximately equal to BHP (pore pressure) minus the head of mud up to that point. In contrast, the pressure outside the wellbore in the carbonate will equal the same BHP minus a hydrocarbon gradient. Because the mud gradient in the wellbore will always be higher then the formation fluid gradient, the borehole pressure at all points above the loss zone will always become less than the formation pressure in the exposed carbonate, with the maximum underbalance being at the top. The influx that will occur will travel downward, displacing the mud to the loss zone. Once the interval is displaced to hydrocarbon the pressure gradients inside and outside the wellbore become the same and flow stops as there is no longer any differential to drive it. The entry of hydrocarbon at the top and the downward flushing of mud is referred to as a flow cell because it is initially a self-contained process. An important conclusion from understanding the manner in which the flow cell works is that any complete loss of returns in a carbonate may result in an influx and that this cannot be prevented by filling the annulus. The unique well control response in deepwater carbonates should then be to shut in as soon as any full loss of returns is observed.

This paper describes the flow cell and subsequent swapping mechanisms and recognition and response procedures to prevent an influx from traveling above the subsea stack before detection. Also described are unique challenges that the flow cell introduces and operational techniques that will allow the continued safe drilling of a subsea well in deep water with conventional marine riser and subsea BOP equipment.

Introduction

When massive losses occur in vugular formations the well's behavior does not follow the conventional well control scenario. Gains are not seen in pit volumes despite hydrocarbon entry and kicks can go undetected until they have traveled some distance up the annulus. Once the kick is detected, backpressure cannot be held effectively to prevent further influx while circulating out the initial kick and annulus pressure trends and values appear to be unpredictable. It is also difficult to control the placement of fluids or pills. The most significant challenge is the inability to detect an influx as soon as it occurs. In deepwater wells the distance from the vugular zone to the subsea BOPs may be short. The challenge is illustrated in Fig 1.