Subsea Capping-Stack Usage Evaluated for CO2 Blowouts
Authors: Carpenter Chris 
Published: January 2025

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 35246, “Evaluating Subsea Capping-Stack Usage for CO2 Blowouts,” by Lei Zhou, SPE, SLB, Eric R. Upchurch, SPE, Chevron, and Yaxin Liu, SPE, SLB, et al. The paper has not been peer reviewed.

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A dynamic multiphase-flow simulator was used to investigate a capping operation for CO2 well blowouts. Following the typical sequence of a capping procedure and applying a soft shut-in, different primary bore sizes and choke-line configurations were considered. Additionally, different reservoir flow rates, fluid types [CO2 and methane (CH4)] and water depths were investigated, with the intention of understanding what differentiates a CO2 blowout from that of CH4 under varying conditions.

Introduction
In offshore CO2 storage operations targeting saline aquifers, the depth and pressure gradient of such storage zones dictate that stored CO2 will almost always be in a supercritical state. Given that reality, the complexities of supercritical-to-gaseous phase transition, varying compressibilities (of both gas and supercritical fluid), and the formation of hydrates all must be addressed accurately in simulation if a path to using capping stacks in response to subsea CO2 blowouts is to be charted. Blowouts with CO2 have complications that CH4 blowouts may not exhibit, given the phase change of supercritical CO2 with significant volume expansion.
Capping stacks have been the oil and gas industry’s chosen first line of response to subsea blowouts. The adaptation of subsea capping stacks for halting CO2 blowouts is a logical next step for their application. The unique properties of CO2, combined with conditions of the subsea environment, however, can lead to the formation of CO2 hydrates, frozen sea water, and dry ice. Thus, a thorough assessment of the design, deployment, and operation of capping stacks in a CO2 environment is required to ensure their effectiveness. To the authors’ knowledge, no analogous well-control modeling or case studies are available in the literature.

Capping-Stack Configurations
In this study, the authors consider multiple capping stacks with dimensions and ratings summarized as follows:
- Stack 1: 18.75 in., two rams, pressure rating of 15,000 psi, temperature rating of 250°F, four chokes, choke inner diameter (ID) of 5.125 in.
- Stack 2: 18.75 in., three rams, pressure rating of 15,000 psi, temperature rating of 250°F, two chokes, choke ID of 3.063 in.
- Stack 3: 7.063 in., two gate valves, pressure rating of 10,000 psi, temperature rating of 302°F, four chokes, choke ID of 5.125 in.

Well and Reservoir Scenarios
Fig. 1 shows the well and reservoir scenarios used in this evaluation. The configuration in Fig. 1a is used for shallow water (984 ft), while that in Fig. 1b is used for deep water (2,500 ft). In shallow water, reservoir and mudline temperatures were assumed to be 210 and 59°F, respectively, and reservoir pressure was set at 5,000 psi. In deep water, reservoir and mudline temperatures of 167 and 40°F, respectively, were used and reservoir pressure was set at 5,500 psi. By using these static variables and varying reservoir permeability, various blowout rates were simulated ranging from 150 to 370 MMscf/D.
Each scenario’s steady-state blowout condition was used as the starting point for modeling the operational steps that led to full closure of the capping stack’s primary bore and choke lines. For simulation simplicity, it was assumed that the well unloaded and stabilized with the capping stack already installed on the well. This assumption had no effect on the final simulation results that reflected how fluid behaved during the process of shutting in the well.