Well integrity/control

How To Develop a Well-Specific Capping-Stack Blowout Contingency Plan

Developing a well-specific subsea-capping contingency plan involves assessing the feasibility of deploying a capping stack from a floating vessel, determining the weight and stability, and performing dynamic-flow simulations of closing the capping stack outlets.

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Fig. 1—In the CFD model, the capping stack is lowered onto the jet of the blowout, in this case for a partially bent wellhead.

The planning process for developing a well-specific subsea-capping contingency plan involves assessing the feasibility of deploying a capping stack from a floating vessel, determining the weight and stability of the capping stack to overcome the force of the blowout jet, and performing dynamic-flow simulations of closing the capping stack outlets without loss of well integrity. This process not only assesses the feasibility, complexity, and risk exposure of the capping operation but also may justify further planning, studies, or expenditure.

Introduction

Blowout-kill techniques can be classified as either surface-intervention or relief-well methods. Relief wells aim to kill a blowout from the bottom by injecting fluid into the wellbore until the influx of formation fluid is stopped and the well is static when pumping stops. In many cases, a relief well will be the safest intervention and have the highest likelihood of success, and often it is considered the last line of defense against a blowout. Surface interventions are kill attempts aimed at controlling the discharge at the wellhead or at the fluid-exit point. An example is to install a capping stack on top of the wellhead and close its openings, which can be gate valves, chokes, or rams.

Blowout Contingency Planning. Blowout contingency plans ensure that people understand and can manage risks, thus ensuring safe and sustainable operations at all times.

All blowouts are inherently different; thus, it would be both impractical and impossible to attempt to cover every possible blowout scenario in a contingency plan. For source-control planning, an operator should, at a minimum, address a worst credible scenario to demonstrate how the well will be brought under control. This scenario may be different from the worst-case discharge, which is used to estimate spill potential and necessary resources for containment and cleanup. As an example, if there is uncertainty about the gas/oil ratio (GOR), a low-case GOR may cause more oil spill, while a high-case GOR may make source control more challenging.

Relief-well contingency planning has been standard practice in parts of the world for decades, and there are many available guidelines detailing a planning process. In comparison, the post-­Macondo subsea-capping stacks are a relatively new technology and source-control plans that cover the associated equipment and operations are a recent requirement. Industry standards and guidelines cover a lot of general information on the equipment, connections, and interfaces needed for capping and containing a blowing subsea well. However, there is little or no information available on how to develop a well-specific capping plan that covers engineering analysis of the feasibility of deploying a capping stack from a floating vessel, the weight and stability of the capping stack to overcome the force of the blowout jet, and dynamic-flow simulations of closing the capping-stack outlets without loss of well integrity.

Computational-Fluid-Dynamics (CFD) Analysis of Capping-Stack Landing

The weight of capping stacks can vary from 30 to 170 t with connectors, but, typically, the stacks with higher pressure and temperature ratings will be heavier. When a stack is lowered onto the jet from a prolific blowout, the effective weight and stability of the stack must be sufficient to overcome the upward hydrodynamic forces from the flow stream exiting the wellbore. Depending on the worst-credible-blowout scenario and the stack that has been identified as likely to be mobilized, it may be necessary to conduct CFD modeling to determine the feasibility of landing and installing the capping stack successfully.

For the CFD modeling, a simplified computer-aided-drafting geometry is prepared to represent the capping stack. The geometry must be of sufficient detail to ensure that the results are not distorted but also simple enough to keep computational time to a minimum. The model has an open bore through the center, which reduces the hydrodynamic forces significantly because it allows the jet to pass through the stack without obstruction as the stack is moved close to the casing. The stationary capping stack will be placed with a vertical offset with respect to the well before one end of the cable is lowered toward the well. During this process, the capping stack is allowed to move in all six degrees of freedom (three rotations and three translations) in response to hydrodynamic forces. The capping stack is then landed on the fluid-exit point, as illustrated in Fig. 1 above.

CFD simulations may be used to determine if a capping stack can be landed and installed for a specific well and blowout scenario. They may also be run for a wide range of blowout scenarios to identify an acceptable operating envelope, which will be unique for every subsea capping stack, depending on its weight and configuration. If it is found that a capping-stack landing may be challenging, it may be possible to use devices such as funnels and guide wires to aid in centering the stack and keeping it stable.

Cap-and-Contain Analysis

After assessing the feasibility of subsea-capping-stack deployment and landing, analysis of the well-flow situation and development of a safe shut-in procedure are necessary. All subsea capping stacks will be designed with several independent outlets that can be opened or closed by use of rams, gate valves, or chokes. The stack will also have a pressure sensor directly below the stack outlets that is used to monitor the pressure in the wellbore. During the closing sequence, the pressure readings, in combination with multiphase-flow simulations, will be used to diagnose downhole conditions. If the pressure response indicates a potential for loss of well integrity, the closing sequence will be halted and, in some cases, the stack will be opened up again to relieve pressure. This reduces the risk of a potential broach scenario, which often is considered a worst-case ­scenario because it would reduce or completely eliminate any further chance of a surface intervention.

A hard shut-in, where all capping-stack openings are closed instantly, may, in some cases, result in unnecessary ­liquid-hammer effects. Furthermore, it reduces the ability to conduct downhole diagnostics while closing the capping stack. For these reasons, a hard shut-in will not be recommended for most ­scenarios. Instead, the capping stack should be closed in a carefully planned sequence—a soft shut-in—while using a multiphase-flow-simulation tool to estimate downhole conditions.

If well-integrity analysis supports the feasibility of capping the well, then the decision will likely be to attempt a cap-and-contain operation. In this operation, all the openings of the stack will be closed in sequence until the well is completely shut in. The expected pressure-response ranges should be developed and a safe soft-shut-in procedure made to ensure that the stack openings can be closed without risking loss of well integrity.

Cap-and-Flow Analysis

On the basis of the risk of losing wellbore integrity or broaching to the seabed, the blowout task force may decide to initiate a cap-and-flow operation instead of cap and contain. That is, the capping stack will be partially or completely open and the flow from the capping stack will be diverted through a riser system and coflex hoses to a floating production, storage, and offloading unit or a flowback vessel that would house the well-test kit. The main challenge will be to investigate the effect of the hose length and inner diameter that are  required to achieve the rated capacity of the well-test kit, which will require computation of the required backpressure for choking the well back and staying within the rate limit of the line or surface equipment. The main goal is to confirm that the well would have sufficient integrity to handle the backpressure for controlled flowback.

Well-Kill Intervention

After a capping stack has been used to cap and contain or cap and flow, a well-kill technique to bring the well to static conditions must be performed. For cap and contain, the choice may be to bullhead or wait for a relief well to intersect. For cap and flow, a relief well may be the only practical kill solution because the blowout well likely cannot be shut in.

Bullheading may increase pressures at the wellhead above the shut-in pressure and increase risk of a burst failure. Furthermore, the condition of the blowout wellbore may be unknown after days of flowing hydrocarbons that could include gas and sand at high velocities.

Unlike in bullheading, the wellhead pressure will likely never exceed the shut-in pressure for a relief-well intervention. Although a relief well may take a long time before making connection with the blowout well, it is considered a safer approach than bullheading. With a capping stack installed, a slow circulation of kill mud can be pumped from the relief well into the blowout wellbore and up through the capping stack. By controlling the choke opening on the capping stack, a constant bottomhole pressure can be maintained above the formation pore pressure.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 181393, “How To Develop a Well-Specific Blowout Contingency Plan That Covers Engineering Analysis of the Deployment, Installation, and Soft Shut-In of a Subsea Capping Operation,” by Ray T. Oskarsen, SPE, Morten H. Emilsen, SPE, and Amir S. Paknejad, Add Energy; Mike Cargol, Trendsetter Engineering; and Kwee Choong See, SPE, Shell International Exploration and Production, prepared for the 2016 SPE Annual Technical Conference and Exhibition, Dubai, 26–28 September. The paper has not been peer reviewed.