Circulating Out Gas Kicks In Deepwater Floating Drilling Operations
Authors: Ilfrey W.T. , Alexander C.H. , Neath R.A. , Tannich J.D. , Eckel J.R. 
Published: December 2007

Friction and hydrostatic effects in the long, small-diameter choke line used in deepwater floating drilling operations can cause large and rapid wellbore pressure changes when normal techniques are used for pressure changes when normal techniques are used for circulating out a gas kick. These changes can be sufficient to cause secondary kicks or loss of returns. Use of easily collected additional onsite test information and minor modifications in conventional wellcontrol equipment and procedures can minimize or eliminate these problems.

Introduction

Conventional floating drilling operations use a relatively small-diameter choke line to connect the ocean floor blowout preventers to the choke manifold aboard the drill vessel. Frictional and hydrostatic effects resulting from use of the choke line complicate the conventional process of circulating out a gas kick. The usual process of circulating out a gas kick described by Goins, and generally used in industry, involves:Shutting-in the well after a kick is detected.Recording shut-in stabilized drill-pipe and annulus pressures.Establishing a constant kick pump-out rate and the related constant dynamic drill-pipe pressure, while holding the mud weight and the annulus pressure constant at the original shut-in value, through choke manipulation.Circulating out the gas kick at this constant pump rate while holding bottom-hole pressure constant by choke manipulation, to either increase or decrease annulus pressure as necessary to produce the required drill-pipe pressure.

If the mud used to circulate out the kick is original weight mud, the procedure is known as the "Driller's Method." In this method, drill-pipe pressure is held constant throughout the process. pressure is held constant throughout the process. If heavier mud is introduced during the circulation, the procedure must be modified and drill-pipe pressure must procedure must be modified and drill-pipe pressure must be decreased systematically as the heavy mud fills the drill pipe.

Use of the procedure for establishing dynamic drill-pipe pressure causes the initial pressure drop resulting from friction in the annulus to be added to the bottom-hole pressure. For wells with blowout preventers (BOPS) at the surface or for shallow-water preventers (BOPS) at the surface or for shallow-water floating drilling, this annular pressure drop is negligible at usual pump-out rates. As choke-line length increases, however, the initial friction pressure drop in the choke line can be an appreciable addition to both casing-seat and bottom-hole pressures. This could be a cause of formation fracturing at the casing seat.

When the gas kick reaches the base of the choke line and displaces mud in the choke line, or later when mud displaces gas in the choke line, rapid hydrostatic pressure changes can occur. These changes will cause pressure changes can occur. These changes will cause (respectively) either a rapid decrease or a rapid increase in drill-pipe pressure that must be compensated by equally rapid increases or decreases in annulus pressure through choke manipulation to hold bottom-hole pressure constant.

Even if a circulation rate as low as 2 bbl/min is used, the required rates of annulus pressure change may exceed the ability of the choke and operator to react. The operator may either fail to detect drill-pipe pressure changes in a timely fashion or be unable to pressure changes in a timely fashion or be unable to manipulate the choke quickly enough to compensate for the rapidity of those changes and thus fail to prevent a secondary kick or formation fracturing.

These changes in pressure are the principal reason the "Driller's Method" is recommended for deepwater well control. Drill-pipe pressure decreases required when heavier mud is being introduced may be impossible to distinguish from drill-pipe pressure decreases that signal arrival of gas at the sea floor, if kick size and open-hole annulus capacity are not known.