An Experimental Study of Well Control Procedures for Deepwater Drilling Operations
Authors: Bourgoyne Adam T. , Holden William R. 
Published: December 2007

A research facility has been designed and constructed to model thewell-control flow geometry present on a floating drilling vessel operating in 3,000 ft [914 m] of water. The main feature of thisfacility is a highly instrumented 6,000-ft [1829-m] well equipped with a packer and triple parallel flow tube at 3,000 ft [914 m] tomodel a subsea blow out preventer (BOP) stack with connecting subseachoke and kill lines. Several types of experiments were conductedin which gas kicks were simulated by the injection of nitrogen intothe bottom of the well. Alternative procedures studied andevaluated included techniques to compensate for chokeline frictional pressure loss during pump startup and techniques forhandling rapid gas-zone elongation when the kick reaches theseafloor. It was found that the demands placed on a choke operator during well-control operations in deep water were not as severe asanticipated from computer simulator studies and could be managedwith existing equipment by an experienced choke operator.

Introduction

In the late 1940's, the search for oil and gas accumulations firstmoved offshore to the shallow marine environment. Since that time, drilling operations have been extended steadily across thecontinental shelf. More recently, developments in the technologyfor drilling from floating drilling vessels have allowed exploratory drilling beyond the limits of the continental shelf andinto the relatively deep water of the continental slopes. In 1974, the first well was drilled in water deeper than 2,000 ft [610 in].1 By 1979, the water depth record was extended to 4,876 ft [1486in] on a well drilled offshore Newfoundland. More recent exploration offthe U.S. east coast extended the water depth record to 6,848 ft[2087 m]. Future plans in the Natl. Science Foundation's Ocean MarginDrilling Program call for scientific ocean drilling during the nextdecade in water depths of 13,000 ft [3962 m].

Like many other aspects of drilling operations, the problemof blowout prevention increases in complexity for floating drillingvessels operating in deep water. Several special well-control problems stem from greatly reduced fracture gradients and the use problems stem from greatly reduced fracture gradients and the use of long subsea choke and kill lines. Fig. 1 shows the approximateeffect of water depth on fracture gradients, expressed in terms ofthe maximum mud density that can be, sustained during normal drillingoperations. 4 Note that the maximum mud density that can be used withcasing penetrating 3,500 ft [1067 m] into the sediments decreases fromabout 13.9 lbm/gal [1666 kg/m3] on land to about 10.7 lbm/gal [1282 kg/m3]in 4,500 ft [1372 m] of water, and to about 9.8 lbm/gal [1174 kg/m3] in13,000 ft [3962 m] of water. These lower fracture gradients resultprimarily because the open hole must support a column of drilling fluid primarily because the open hole must support a column of drilling fluid that extends far above the mudline to the rig floor. This additionalcolumn weight is only partially offset by the seawater. An additional contributing factor is the relatively low bulk density of unconsolidated shallow marine sediments.

The required vertical subsea choke and kill lines extendingfrom the BOP stack at the seafloor have two detrimental aspects. One difficulty arises because of the increased circulating frictional pressure loss caused by the great length of these lines. This frictional pressure loss can cause a significant increase inthe pressures occurring in the wellbore. The combination of highcirculating pressure losses in the chokeline and low wellborefracture gradients reduces the tolerance for error by the chokeoperator.

A second difficulty results from rapid changes in hydrostaticpressure in the vertical chokelines when circulating a gas kick. pressure in the vertical chokelines when circulating a gas kick. Hydrostatic pressure falls quickly when gas exits the large casingand displaces mud from the relatively small-diameter chokeline. To maintain the bottomhole pressure (BHP) constant, there must be acorresponding increase in surface choke pressure to make up forthis decrease in hydrostatic pressure. Choke operation becomesmuch more difficult during this period, as rapid changes in controlpressure are required. This difficulty tends to increase with well pressure are required. This difficulty tends to increase with well depth because choke manipulation is based on surface drill pipepressure whose unsteady-state readjustment time increases with well pressure whose unsteady-state readjustment time increases with well depth and gas-kick volume.

Evaluations of anticipated well-control problems for a givenset of deepwater conditions are often conducted by computer studiesthat predict the pressure response of a well for variousalternative procedures being evaluated. These studies have raisedquestions about the severity of the control problem faced by the choke operator.

JPT

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