New Considerations for Handling Gas in a Deepwater Riser

Abstract

As the drilling industry advances to drilling wells in deeper waters, technology must also advance so that we understand and manage the associated risks. We must be careful not to extrapolate existing conventional procedures. The rules that we apply to gas migration behaviour in shallow water drilling risers may not be adequate when the riser is significantly deeper and new factors start to dominate. Factors such as bubble fragmentation and suspension, which have little effect in a shallow water situation, begin to have a significant effect as the riser depth increases. Under certain conditions, the gas bubbles stop migrating. The gas is then held static by the yield stress of the mud, and must be circulated out of the riser. This paper reviews the theory on the behaviour of gas migration through water based drilling fluids and shows how this can affect operations in a deep waterwell. The operations following gas getting above the BOP stack and into the riser during a well control incident are considered. Computer model simulations are presented to define a range of conditions. Field test data is compared to simulator data to validate the predictions. If the amount of gas that gets above the BOP stack is limited by successful detection, the gas can be safely removed from the riser using controlled circulating procedures, without the need for much specific equipment. These proposed procedures are also presented. The use of advanced kick detection equipment is evaluated as a method of minimizing the risk of gas volumes in the riser. Introduction In a deepwater drilling situation, where the productive horizons tend to be relatively shallow below the mud line and where operators are using horizontal and extended-reach well profiles, there is a greater chance that an influx of gas will be above the subsea BOP stack before it is detected. Indeed, most of the volume of the circulating system is contained within the riser of many deep water wells. Much has been written about the potential problems that can arise from gas getting above a subsea BOP stack and into the riser. However, there is not much data quantifying the effect that this gas could have. There is a shortage of full-scale tests covering the broad range of operating conditions. One test that has been very useful to the industry was conducted jointly by Amoco, Shell, and Exxon in 1986 using the Discoverer Seven Seas in 3118 ft of water. The use of advanced well control simulators has become very beneficial inexpanding the industry's understanding of the complexity of gas migration. Over 300 simulations covering a broad range of operating depths and conditions wereconducted while preparing for this paper. Others, too, are using simulators to increase their understanding in this area. The industry will continue to learn and would benefit from more field tests. Advanced hardware is available to handle gas at the surface by allowing back pressure to be held on the riser. However, there are additional things to consider before using this equipment. There is a possibility of creating a pressure inversion, where bottom hole pressure is brought to surface, if the gas migrates and is not allowed to expand while the annular element is closed at the surface. Risers are usually not designed to handle this pressure, so could burst. Also, this additional equipment can be expensive.

Experimental Study on the Suspension Amount of Invasion Gas in Drilling Fluid with Yield Stress

“…The smaller the Bi value of the suspended bubble is, the closer its volume is to the critical suspended bubble volume and the stress field around the suspension is more prone to the plastic flow under the disturbance of the transporting bubble. When the allowable distance between the suspended bubble and the trajectory of the moving bubble is large enough [27][28][29], the suspension can be maintained. Then, with the increase of the Bi value, the stronger the antidisturbance ability of the fluid around the suspended bubble, the smaller the allowable distance between the suspended bubble and the trajectory of the moving bubble and the smaller the distance between the bubble groups at this time.…”

Mitigating Gas in Riser Rapid Unloading for Deepwater Dual Gradient Well Control

“…Using water based drilling fluids and a computer model, Lloyd et al (2000) studied gas migration in riser depths of 500 ft to 7,500 ft. The test showed once gas is held in suspension, it is possible to safely remove riser gas using existing rig equipment with minor modifications.…”

Ultra-Deepwater Hydraulics and Well Control Tests with Extensive Instrumentation: Field Tests and Data Analysis

“…During ultra-deep water drilling the chances of having gas passing through the BOP and into the riser prior to detecting a kick is larger than for traditional offshore wells [2]. In order to prepare for the gas arrival at surface, it is very important to take into account the gas migration velocity in the drilling mud.…”